Somatostatin Depresses Excitatory but not Inhibitory Neurotransmission in Rat CA1 Hippocampus
Melanie K. Tallent and
George R. Siggins
Department of Neuropharmacology, The Scripps Research Institute, La Jolla, California 92037
 |
ABSTRACT |
Tallent, Melanie K. and George R. Siggins. Somatostatin depresses excitatory but not inhibitory neurotransmission in rat CA1 hippocampus. J. Neurophysiol. 78: 3008-3018, 1997. In rat CA1 hippocampal pyramidal neurons (HPNs), somatostatin (SST) has inhibitory postsynaptic actions, including hyperpolarization of the membrane at rest and augmentation of the K+ M-current. However, the effects of SST on synaptic transmission in this brain region have not been well-characterized. Therefore we used intracellular voltage-clamp recordings in rat hippocampal slices to assess the effects of SST on pharmacologically isolated synaptic currents in HPNs. SST depressed both (R,S)-
-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate and N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs) in a reversible manner, with an apparent IC50 of 22 nM and a maximal effect at 100 nM. In contrast, SST at concentrations up to 5 µM had no direct effects on either
-aminobutyric acid-A (GABAA) or GABAB receptor-mediated inhibitory postsynaptic currents (IPSCs). The depression of EPSCs by SST was especially robust during hyperexcited states when polysynaptic EPSCs were present, suggesting that this peptide could play a compensatory role during seizurelike activity. SST effects were greatly attenuated by the alkylating agent N-ethylmaleimide, thus implicating a transduction mechanism involving the Gi/Go family of G-proteins. Use of 2 M Cs+ in the recording electrode blocked the postsynaptic modulation of K+ currents by SST, but did not alter the effects of SST on EPSCs, indicating that postsynaptic K+ currents are not involved in this action of SST. However, 2 mM external Ba2+ blocked the effect of SST on EPSCs, suggesting that presynaptic K+ channels or other presynaptic mechanisms may be involved. These findings and previous results from our laboratory show that SST has multiple inhibitory effects in hippocampus.
 |
INTRODUCTION |
Somatostatin (SST) is a peptide widely distributed throughout the periphery and brain. SST was originally characterized as an inhibitor of growth hormone release from the anterior pituitary (Brazeau et al. 1972
), and SST function as a hormone in the periphery is well-established (Reisine 1995
). By contrast, the role of SST in the extrahypothalamic brain is less clear. This peptide is abundant in nearly all brain regions (Crawley 1985
) and is thought to play a neuromodulatory role (Epelbaum 1986
). However, studies examining the specific actions of SST on neurotransmission have been sparse, and determining the endogenous function of the peptide has been hampered by the lack of receptor antagonists.
High expression levels of SST and its receptors are found in the hippocampus. Extensive SST immunoreactivity is found in extrinsic fibers and interneurons of the stratum oriens, hilus, and subiculum, where it is colocalized with
-aminobutyric acid (GABA) (Esclapez and Houser 1995
; Feldman et al. 1982
; Kohler et al. 1987
; Kosaka et al. 1988
; Morrison et al. 1982
). SST receptors are also discretely localized in the hippocampus, with four of the five cloned SST receptors (sst1-4) expressed (sst5 shows no significant expression anywhere in the brain). In situ hybridization shows that the mRNAs for sst1-4 occur primarily in principal cells of the CA1, CA3, dentate gyrus, and subiculum, with little expression in the stratum oriens, stratum radiatum, or molecular layers (Breder et al. 1992
; Kong et al. 1994
; Perez et al. 1994
; Thoss et al. 1996
). An immunocytochemical study using antibodies selective for sst2 showed that the receptor protein is expressed largely in the soma as well as the basal and apical dendrites of CA1 hippocampal pyramidal neurons (HPNs), with no expression in CA3 (Dournaud et al. 1996
). Interestingly, autoradiographic studies using iodinated SST ligands show little binding in the principal cell layers of CA1 and CA3; instead binding is limited to the plexiform layers, mostly the stratum oriens of CA1 and CA3, the hilus, and the subiculum (Holloway et al. 1996
; Leroux et al. 1993
; Perez et al. 1995
). These studies suggest that SST receptor mRNAs are mostly exported out of the soma and into dendritic layers, where they would be more proximate to terminals of SSTergic interneurons.
The majority of studies examining the effects of SST in hippocampus have been done in CA1. Much is known about postsynaptic effects of SST on HPNs. SST was first reported to depolarize CA1 HPNs (Dodd and Kelly 1978
) but was later found to hyperpolarize these neurons (Pittman and Siggins 1981
). Subsequent studies demonstrated that SST augments the voltage-sensitive K+ M-current in these cells (Moore et al. 1988
), an action mediated by an arachidonic acid metabolite (Schweitzer et al. 1990
, 1993
). SST may also augment a voltage-insensitive K+ leak current in HPNs (P. Schweitzer and G. R. Siggins, unpublished observations), a mechanism that would account for much of the SST hyperpolarizing effect at rest. Although earlier studies on hippocampal slices showed no effect of SST on Ca2+ currents in HPNs (Schweitzer et al. 1993
), recent studies in acutely isolated HPNs showed that SST inhibits anN-type Ca2+ current in these cells (Ishibashi and Akaike 1995
). At the cellular level, these postsynaptic actions of SST on ionic currents are inhibitory, in that they decrease the probability of the neuron firing an action potential. These data suggest that SST acts similarly to GABA, with which it is colocalized (Crawley 1990
; Kosaka et al. 1988
).
The actions of SST on CA1 synaptic transmission are less clear. Two studies found that SST attenuates inhibitory postsynaptic potentials (IPSPs) in rabbit (Scharfman and Schwartzkroin 1989
) and guinea pig (Xie and Sastry 1992
) HPNs. This reduction of inhibitory input would result in a net excitatory effect of SST that contrasts with its direct postsynaptic inhibitory actions. However, these earlier studies were done on compound postsynaptic potentials (PSPs), under conditions where excitatory PSPs (EPSPs) and IPSPs could shunt each other. Furthermore, only current-clamp recordings were performed in these studies, possibly introducing confounding influences caused by SST postsynaptic effects on input resistance and membrane potential.
We show here, using voltage-clamp recordings on pharmacologically isolated synaptic components, that SST does not significantly affect inhibitory postsynaptic currents (IPSCs) in rat CA1 HPNs. However, SST in low concentrations robustly attenuates both (R,S)-
-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate andN-methyl-D-aspartate (NMDA) excitatory postsynaptic currents (EPSCs). SST actions appear to be enhanced under conditions of reduced inhibition and increased excitability. Thus SST may play a homeostatic role in the hippocampus and could compensate for increased excitatory neurotransmission during pathological states such as epilepsy. A preliminary report of some of these results has been published in abstract form (Tallent and Siggins 1996
).
 |
METHODS |
Slice preparation
We prepared hippocampal slices from male Sprague-Dawley rats as described previously (Pittman and Siggins 1981
; Schweitzer et al. 1993
). Briefly, we cut transverse hippocampal slices 350 µm thick on a brain slicer, incubated them in an interface configuration for ~30 min, and then completely submerged and continuously superfused the slices with warm (31-33°C), gassed (95% O2-5% CO2) artificial cerebral spinal fluid (ACSF) at a constant flow rate of 2-4 ml/min. The ACSF was of the following composition (in mM): 130 NaCl, 3.5 KCl, 1.25 NaH2PO4, 1.5 MgSO4·7H2O, 2.0 CaCl2, 24 NaHCO3, and 10 glucose. Other ions and drugs were added to the ACSF in known concentrations. For studies usingN-ethylmaleimide (NEM), temperature was lowered to 30°, and flow rate during NEM application was reduced to 1-1.5 ml/min.
Electrophysiology
We used the discontinuous single-electrode voltage-clamp technique with standard sharp intracellular micropipettes (3 M KCl except for experiments depicted in Fig. 9 in which 2 M CsCl was used; tip resistance 60-90 M
with either solution). The switching frequency was 3-4 kHz; we continuously monitored electrode settling time and capacitance neutralization at the headstage on a separate oscilloscope (Finkel and Redman 1985
). We acquired selected data with an Axoclamp-2B amplifier (Axon Instruments) and stored them on a PC for data analysis using pClamp software (Axon Instruments).

View larger version (29K):
[in this window]
[in a new window]
| FIG. 9.
Inhibition of EPSCs by SST is not sensitive to internal Cs+ but is attenuated by external Ba2+. A: in the presence of 15 µM bicuculline, SST inhibits the EPSCs recorded with a 3 M KCl electrode. Top panel: representative cell with a robust inhibition by SST (4.5 min) of the EPSC evoked by a stimulus intensity 70% of maximal (VH was 75 mV; RMP was 70 mV). Bottom panel: mean data from 4 cells show the inhibition of the EPSC area by SST across a range of hyperpolarized potentials. B: inhibition of the EPSC (70% maximal) by SST using an electrode filled with 2 M CsCl. Top panel: EPSCs recorded with the Cs+ electrode are strongly inhibited by SST (3 min) in a representative cell (VH was 81 mV; RMP was 49 mV). Bottom panel: averaged data from 5 cells show that SST robustly inhibited the mean EPSC area. No significant difference was found between SST inhibition in K+ vs. Cs+ electrodes at any potential. C: inhibition of the EPSC by SST is largely blocked by 2 mM external Ba2+. SST inhibition of EPSC area is shown for 7 neurons before and after treatment with Ba2+. SST reduces the EPSC area to 41 ± 12% of control before addition of Ba2+, but to only 87 ± 7% of control after superfusion of Ba2+. EPSCs were recorded in the presence of 15 µM bicuculline and elicited with a stimulus 70% of maximal.
|
|
EPSCs were evoked by orthodromic stimulation (0.05-ms stimulus duration; 0.1-Hz frequency) of Schaeffer collaterals (SCs) with a bipolar tungsten electrode placed in the stratum radiatum with the orientation of the tips parallel to the stratum pyramidale. We generated IPSCs by local stimulation with the electrode tips placed on either side of the stratum pyramidale. We obtained input/output curves using three different stimulus intensities: threshold, half-maximal, and maximal. Two traces were averaged for each stimulus intensity. We measured both the amplitude and the area (time-integral) of the PSCs using Clampfit software (Axon Instruments). The area of the PSC reflects the total charge crossing the membrane and thus the synaptic strength. Current traces shown for representative cells were normalized to a common baseline for easier comparison of amplitudes. We usually voltage clamped the neurons slightly negative to the resting membrane potentials (RMPs). The various problems associated with voltage clamping neurons with extended processes are addressed elsewhere (Finkel and Redman 1985
; Halliwell and Adams 1982
; Johnston and Brown 1983
); these problems may be less acute when dealing with relative changes after drug application (Madison et al. 1987
).
To isolate synaptic components, we superfused specific antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 20 µM) to block AMPA/kainate receptors, DL-2-amino-5-phosphonovaleric acid (APV, 30 µM) to block NMDA receptors, bicuculline (15-30 µM) to block GABAA receptors, and/or CGP 55845A (1 µM) to block GABAB receptors. At the end of some experiments, we applied the specific blocker of the isolated synaptic component to verify its identity. SST was superfused at 1 µM.
To determine whether the inhibition of EPSCs by SST was mediated by a G-protein, slices were treated with 150 µM NEM for 15-20 min. We prepared NEM daily in dimethylsulfoxide (DMSO); the concentration of DMSO in the bath was 0.03%. No GABAB antagonists were used in the NEM studies, and NEM treatment was terminated when the GABAB-IPSC was eliminated. We detected no changes in membrane properties using this treatment paradigm, nor did NEM treatment significantly alter the amplitudes of the evoked EPSCs (146 ± 20 vs. 150 ± 18 pA before and after NEM treatment, respectively). An increase in background noise associated with NEM treatment returned to pretreatment levels by addition of CNQX (20-40 µM), indicating that NEM caused an increase in spontaneous glutamate release (see Fig. 7A), as reported for GABA release in this preparation (Morishita et al. 1997
). After NEM application, slices were washed with normal ACSF for 5 min before continuing the experiment. To determine the efficacy of NEM treatment to inactivate pertussis toxin (PTX)-sensitive G-proteins, we superfused baclofen before and after NEM treatment and measured its ability to elicit an outward current, shown to be a PTX-sensitive process (Andrade et al. 1986
). In addition, we also examined the ability of baclofen to inhibit EPSCs before and after NEM treatment. Previous studies have shown that GABAB-induced depression of EPSCs (but not IPSCs) is PTX insensitive (Colmers and Pittman 1989
; Dutar and Nicoll 1988
; Thompson and Gähwiler 1992
).

View larger version (24K):
[in this window]
[in a new window]
| FIG. 7.
Treatment with 150 µM N-ethylmaleimide (NEM) reduces the ability of SST to inhibit the AMPA/kainate-EPSCs. A: AMPA/kainate-EPSCs from a representative cell. SST (1 µM; 3 min) and the GABAB receptor agonist baclofen (10 µM; 2 min) both attenuate the EPSC before treatment with NEM. After treatment with 150 µM NEM for 15 min, neither SST (3 min) nor baclofen (2.5 min) measurably affect the EPSC. However, the -adrenergic agonist isoproterenol (4 min) still potentiates the EPSC after NEM treatment. Note also that the late outward current representing the GABAB-IPSC is lost after NEM treatment. The stimulation intensity (normalized to 70% maximal) was increased between the SST wash out and the baclofen control in this cell (VH was 79 mV; RMP was 73 mV). B: mean data from 5 neurons showing that NEM greatly attenuates inhibition by SST and baclofen of the AMPA/kainate-EPSC area.
|
|
We obtained SST from BaChem (Torrance, CA) or Penninsula Laboratories (Belmont, CA), tetrodotoxin (TTX) from Calbiochem (San Diego, CA), CNQX from Tocris Cookson (St. Louis, MO), APV from Research Biochemicals International (Natick, MA), and all other drugs from Sigma (St. Louis, MO). CGP 55845A was generously provided by W. Fröstl and A. Suter (Novartis Pharma). All measures are reported as means ± SE. We determined statistical significance by two-way analysis of variance (ANOVA) for repeated measures, unless otherwise indicated. Statistical significance is considered to be P < 0.05 and is indicated in the figures by an asterisk.
 |
RESULTS |
SST does not directly modulate GABAA or GABAB receptor-mediated IPSCs
We evoked isolated GABAA-IPSCs in the presence of 30 µM APV, 20 µM CNQX, and 1 µM CGP 55845A. We recorded GABAA IPSCs from eight neurons at a holding potential (VH) of
77 ± 1 (SE) mV. Mean RMP was
68 ± 1 mV, mean input resistance was 65 ± 7 M
, and mean spike amplitude was 104 ± 2 mV for these eight cells. When tested, these IPSCs were blocked by 15-30 µM bicuculline (Fig. 1A).

View larger version (17K):
[in this window]
[in a new window]
| FIG. 1.
Somatostatin (SST) does not significantly alter -aminobutyric acid-A-inhibitory postsynaptic currents (GABAA-IPSCs). A: GABAA-IPSCs (half-maximal stimulus intensity) from a representative cell recorded in the presence of 30 µM DL-2-amino-5-phosphonovaleric acid (APV), 20 µM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and 1 µM CGP 55845A. A slight increase in the size of the IPSC occurs after 1 µM SST superfusion (6 min), an effect not reversible on wash out. Bicuculline (BMI; 20 µM) completely blocked the evoked IPSC (VH was 78 mV; resting membrane potential (RMP) was 65 mV). B: mean data from 8 cells showing peak amplitude of the GABAA-mediated IPSCs before, during, and after superfusion of SST. The small increase in amplitude that occurred during and after SST application was not significantly different from control (P > 0.05). C: mean area of the IPSC for the same 8 cells. A small increase in the area is seen during and after SST application at half-maximal and maximal stimulus intensities, but this increase was not significantly different from control (P > 0.05).
|
|
Bath application of 1 µM SST caused no significant change in the amplitude or area of the GABAA-mediated IPSCs (Fig. 1), although we observed a trend toward increased amplitude (124 ± 12% and 125 ± 7% of control; Fig. 1, A and B) and area (114 ± 9% and 120 ± 6%; Fig. 1, A and C) at both half-maximal and maximal stimulus intensities. This effect was not reversible on wash out of SST nor was it significantly different from control [amplitude F(2,14) = 0.87; P > 0.1; area F(2,14) = 2.68; P > 0.1]. This small potentiation of the GABAA-IPSC could result from a slow drift of the equilibrium potential for Cl
ions during the experiments, due to the use of a KCl-filled recording electrode. We therefore measured GABAA-IPSCs over time in three cells without adding SST and also observed modest increases in IPSC amplitude (101-113% of control) and area (101-107%). Further statistical comparison showed no significant difference of the GABAA-IPSC increase between control (n = 3) and SST-exposed (n = 8) neurons (unpaired Student's t-test, P > 0.05). We also tested 5 µM SST on GABAA-IPSCs, and found no significant effect on the IPSC, although a trend toward an increase in IPSC area was again seen [n = 3; F(2,4) = 2.39; P > 0.2].
GABAB-IPSCs were isolated using 30 µM APV, 20 µM CNQX, and 15-30 µM bicuculline. These IPSCs were blocked by 1 µM CGP 55845A (Fig. 2A). We studied eight neurons with a mean RMP of
70 ± 1 mV, mean input resistance of 55 ± 3 M
, and mean spike amplitude of110 ± 4 mV; mean VH was
73 ± 1 mV. Superfusion of 1 µM SST caused no significant change in the amplitude or area of the mean GABAB-mediated IPSC (Fig. 2), as depicted by the representative current traces in Fig. 2A. In the presence of SST, the mean IPSC amplitudes were 103 ± 5, 99 ± 6, and 97 ± 3% of control at threshold (n = 5), half-maximal (n = 8), and maximal (n = 8) stimulus intensities, respectively [Fig. 2B; F(1,3) = 0.162; P > 0.5]. SST also did not alter significantly the area of the mean GABAB-IPSCs [F(1,3) = 0.042; P > 0.5]. The average IPSC areas after SST superfusion were 94 ± 4, 89 ± 7, and 92 ± 4% of control at threshold, half-maximal, and maximal stimulus intensities, respectively (Fig. 2C). We also tested the effect of 5 µM SST on three cells and found no significant change in GABAB-IPSC area [F(1,2) = 0.394; P > 0.5].

View larger version (18K):
[in this window]
[in a new window]
| FIG. 2.
SST has no effect on GABAB-IPSCs. A: synaptic currents from a representative cell evoked by local stimulation across stratum pyramidale (maximal stimulus intensity), in the presence of 20 µM CNQX, 30 µM APV, and 30 µM bicuculline (VH was 70 mV; RMP was 67 mV). Superfusion of 1 µM SST (7 min) did not alter the GABAB-IPSC. CGP 55845A (CGP; 1 µM) blocked the evoked response. B: plot of the mean peak amplitudes of the GABAB-IPSC before, during, and after bath application of 1 µM SST at 3 different stimulus intensities: threshold (n = 5), half-maximal (n = 8), and maximal (n = 8). SST did not affect the peak amplitudes of the GABAB-elicited IPSC. C: mean area of the GABAB-IPSCs plotted for the same cells. SST (1 µM) had no significant effect (see text for statistics).
|
|
SST depresses AMPA/kainate-EPSCs
We isolated AMPA/kainate-EPSCs by applying 30 µM APV, 1 µM CGP 55845A, and 15-30 µM bicuculline in 26 cells. These cells had a mean RMP of
69 ± 1 mV, mean input resistance of 64 ± 4 M
, and mean spike amplitude of 107 ± 1 mV; mean VH was
78 ± 1 mV. SST markedly depressed the AMPA/kainate-EPSCs in 14 of 21 cells. Figure 3A shows a representative cell where 1 µM SST robustly inhibited the AMPA/kainate-EPSCs. SST superfusion significantly reduced the mean amplitude of the EPSC to 78 ± 6 (n = 19), 73 ± 5 (n = 21), and 81 ± 5% (n = 21) of control at threshold, half-maximal, and maximal stimulus intensities, respectively [Fig. 3B; F(1,20) = 22.4; P < 0.0001]. The degree of inhibition of individual EPSC amplitudes by SST ranged from 0 to 74%. SST significantly reduced [F(1,20) = 35.6; P < 0.0001] the mean area of the AMPA/kainate EPSCs to 58 ± 11% (n = 19), 61 ± 7% (n = 21), and 80 ± 6% (n = 21) of control at threshold, half-maximal, and maximal stimulus intensities, respectively (Fig. 3C). The SST-induced inhibition of individual EPSC areas ranged from 0 to 99%. Peak inhibition of the EPSCs occurred 2-4 min after beginning SST superfusion and was reversible upon wash out. Responses to a second SST application after a 5- to 10-min wash out were not attenuated, implying that no desensitization occurred under our recording conditions.

View larger version (20K):
[in this window]
[in a new window]
| FIG. 3.
SST depresses AMPA/kainate-excitatory postsynaptic currents (EPSCs) in the presence of 30 µM APV, 30 µM bicuculline, and 1 µM CGP 55845A. A: synaptic currents from a representative cell (maximal stimulus). SST superfusion (4 min) inhibits the AMPA/kainate-EPSCs in a reversible manner. 50 µM CNQX (6 min) blocks the EPSC (VH was 75 mV; RMP was 67 mV). B: the amplitude of the AMPA/kainate EPSC is decreased by 1 µM SST. Mean data showing that SST diminishes the EPSC amplitude across the range of stimulus intensities: threshold (n = 19), half-maximal (n = 21), and maximal (n = 21). C: 1 µM SST attenuated the area of the AMPA/kainate-EPSC in the same neurons. These input-output curves show that inhibition by SST occurs across the range of stimulation intensities.
|
|
In the presence of bicuculline and CGP 55845A to block inhibitory GABAergic inputs, the slices sometimes became hyperexcitable, whereby a single stimulation of the SCs would evoke AMPA/kainate-EPSCs that appeared polysynaptic, even at submaximal stimulus intensities (Fig. 4A); they were blocked by superfusion of 20 µM CNQX. These polysynaptic EPSCs probably resulted from activation of recurrent excitatory connections between HPNs normally masked by inhibitory inputs (Christian and Dudek 1988
; Crepel et al. 1997
; Gereau and Conn 1994
; Meier and Dudek 1996
). In five cells that showed a clearly separate first and second EPSC, SST preferentially inhibited the second EPSC (Fig. 4, B and C; paired t-test, P < 0.001). At maximal stimulus intensity, SST strongly reduced the amplitude of the second EPSC to 20 ± 2% of control, whereas the first component was only reduced to 90 ± 3% of control (Fig. 4, B and C). The area of the second EPSC was reduced to 8.4 ± 3% of control by SST, whereas the first EPSC area was reduced to only 86 ± 8% of control (Fig. 4C).

View larger version (24K):
[in this window]
[in a new window]
| FIG. 4.
Polysynaptic EPSCs are highly sensitive to SST. In the presence of GABA receptor antagonists, AMPA/kainate EPSCs sometimes display multiple components, most likely representing polysynaptic EPSCs. In cells where a 1st and 2nd component are clearly distinct, the 2nd component is especially sensitive to SST. A: isolated AMPA/kainate-EPSCs recorded from a representative cell (maximal stimulus), as in Fig. 3. SST (1 µM, 4 min) preferentially attenuates the 2nd component (VH was 78 mV; RMP was 72 mV). B: mean peak amplitude from 5 neurons where 2 components were clearly separate; data shown are from the maximal stimulus intensity. C: mean area of the 2nd EPSC in the same cells is especially sensitive to SST (n = 5).
|
|
SST also reduces NMDA-EPSCs
In 11 neurons superfused with 20 µM CNQX, 1 µM CGP 55845A, and 15-30 µM bicuculline, we evoked NMDA-EPSCs that were subsequently blocked by 30 µM APV. These cells had an average RMP of
67 ± 1 mV, a mean spike amplitude of 106 ± 1 mV, and mean input resistance of 62 ± 7 M
; mean VH was
74 ± 1 mV. At hyperpolarized membrane potentials and in the presence of external Mg+, high stimulation intensities were used to overcome Mg+ block of the NMDA receptors, as reported for the hippocampus (Madamba et al. 1996
) and nucleus accumbens (Martin et al. 1997
). This EPSC is a CNQX-, bicuculline-, and CGP 55845A-resistant synaptic component elicited by strong SC stimulation, is voltage dependent in current- and voltage-clamp mode, and is totally blocked by superfusion of the specific NMDA receptor antagonist, D-APV (30-60 µM) (Madamba et al. 1996
).
SST reduced the amplitude and area of the NMDA-EPSCs in 9 of 11 cells (Fig. 5). For these 11 cells the mean EPSC amplitude was reduced to 84 ± 5, 83 ± 3, and 81 ± 2% of control at threshold, half-maximal, and maximal stimulus intensities [F(1,6) = 14; P < 0.01], respectively (Fig. 5B). SST reduction of the EPSC amplitude did not reach statistical significance at the threshold stimulus intensity (P > 0.05). SST also significantly attenuated the mean NMDA-EPSC area in these 11 cells to 67 ± 6% at threshold, 70 ± 4% at half-maximal, and 73 ± 3 % of control at maximal intensities [Fig. 5C; F(1,6) = 55.7; P < 0.0001]. The degree of inhibition produced by SST across cells ranged from 0 to 33% for the amplitude and from 0 to 56% for the area. Peak inhibition by SST was rapid in onset, occurring 2-4 min after application began, and reversed upon wash out. We observed no desensitization of this response upon a second SST application after a 5- to 10-min wash out.

View larger version (18K):
[in this window]
[in a new window]
| FIG. 5.
SST inhibits the NMDA-EPSCs in the presence of 20 µM CNQX, 30 µM bicuculline, and 1 µM CGP 55845A. A: NMDA-EPSCs (maximal stimulus) from a representative cell are depressed reversibly by SST (1 µM, 4 min). APV (30 µM, 9 min) blocks the NMDA-EPSC (VH was 76 mV; RMP was 72 mV). B: SST inhibits the mean peak amplitude of the NMDA-EPSCs. Data shown are the mean from 11 (half-maximal and maximal) or 7 (threshold) neurons. C: mean data from the same cells showing that SST attenuated the NMDA-EPSC area across the range of stimulation intensities.
|
|
SST inhibition of compound EPSCs is dose and time dependent
We examined the dose-response characteristics of the SST effect on compound EPSCs, recorded in the presence of bicuculline and CGP 55845A only (Fig. 6A). The EPSCs recorded probably contained both NMDA and AMPA/kainate components, because when tested they were sensitive to both APV and CNQX (data not shown). We normalized the SST effect at each concentration to the percent of the effect on EPSC area elicited by 1 µM SST, which inhibited EPSC area to 27.5 ± 11% of control (n = 5, Fig. 6A). Up to two concentrations of SST were tested per cell, with no observable desensitization. The apparent IC50 was calculated to be 22 nM (see Fig. 6A legend for details of the dose-response fitting routine).

View larger version (19K):
[in this window]
[in a new window]
| FIG. 6.
Inhibition of EPSCs by SST is dose and time dependent. A: dose-response curve for SST inhibition of compound EPSC area, as percent of maximal SST effect. The data are normalized relative to the maximal effect. For each concentration n = 4 except for 1 µM where n = 5. Data for 1 µM are also shown in Fig. 9A. SST concentration is plotted on a log scale; dose-response curve was fitted by software (Origin; Microcal Software) to a logistic curve: y = (A1 A2)/{1 + (x/x0)p} + A2, where A1 is the initial Y value (6.2%), A2 is the estimated final (maximum; 103%) Y value, x0 is the center x value (apparent IC50; 22 nM), and p is the power (1.4). Confidence interval was set at 95%. B: time course of the SST (1 µM) effect. Data from a representative neuron are plotted as percent of control EPSC area vs. time. Time of SST superfusion is shown by the shaded area. RMP in this cell was 64 mV; VH was 76 mV. EPSCs of this cell only recovered to ~85% of the control EPSC area.
|
|
A more detailed time course of the SST response in a representative cell is shown in Fig. 6B as the percent of the control compound EPSC area over time. SST superfusion leads to a reduction in the EPSC area that peaks 2-3 min after the slice is exposed to SST. There is no attenuation of the SST effect during the 4-min course of the application. The EPSC area returns to ~85% of control ~2 min after the onset of the wash out where it persists throughout the rest of the experiment.
Depression of EPSCs by SST is mediated by the Gi/Go family of G-protein
We assessed the involvement of G-proteins in SST effects using the alkylating agent NEM. This agent has been shown to inactivate the same family of G-proteins (Gi/Go) as PTX does (Choi and Lovinger 1996
; Morishita et al. 1997
; Shapiro et al. 1994
). Our attempts to use long-term incubation of PTX to determine G-protein involvement in SST effects were considered unsuccessful because of the inconsistent and incomplete block by PTX of the outward current induced by the GABAB agonist baclofen (used as a Gi/Go-linked control) (Andrade et al. 1986
). Furthermore, control experiments showed that long-term incubation of slices (6-10 h) often resulted in a loss of the SST response. Such incubation time is necessary for PTX to be effective.
NEM treatment significantly attenuated the effects of both baclofen and SST. Before treatment, 10 µM baclofen induced a mean outward current of 114 ± 10 pA in five neurons held at
75 ± 2 mV. After NEM treatment, the baclofen-induced current was reduced to 22 ± 11 pA. Surprisingly, NEM exposure also significantly attenuated the baclofen-induced depression of the AMPA/kainate-EPSCs (Fig. 7). Before NEM, 10 µM baclofen inhibited the AMPA/kainate-EPSC area (elicited by a 70% maximal stimulus) to 14 ± 3% of the control EPSC (n = 5; mean VH,
75 ± 2 mV). After NEM, baclofen only reduced the AMPA/kainate-EPSC area to 70 ± 7% of control, representing a 66% reduction in the baclofen-induced inhibition (Fig. 7B). In the same five neurons SST reduced the area of the mean AMPA/kainate-EPSC to 44 ± 8% of control before NEM, and to 81 ± 7% of control after NEM; thus the response to SST was significantly attenuated by 65% [Fig. 7B; F(1,4) = 27.2; P < 0.01]. NEM treatment also blocked the SST-induced outward current (34 ± 7 pA before NEM,
14 ± 4 pA after NEM; P < 0.05; n = 5). In four cells, we assessed the degree of potentiation of the AMPA/kainate-EPSC area by the
-adrenergic receptor agonist isoproterenol (10 µM) after NEM treatment. Isoproterenol significantly increased the area of the EPSC to 187 ± 3, 148 ± 2, and 163 ± 3% of control at
75,
85, and
95 mV, respectively (Fig. 7A).
NEM also significantly reduced the ability of baclofen and SST to inhibit the pharmacologically isolated NMDA-EPSCs (Fig. 8). Before NEM, baclofen superfusion reduced the area of the mean NMDA-EPSC to 23 ± 3% of control (mean VH,
73 ± 2 mV; n = 5). After NEM, baclofen depressed the mean NMDA-EPSC area only to 75 ± 5% of control. Thus baclofen inhibition of the mean NMDA-EPSC was attenuated by 68%. NEM also reduced the baclofen-induced outward current in these cells from 118 ± 16 pA to 28 ± 6 pA. As for SST effects on NMDA-EPSCs, before NEM treatment 1 µM SST attenuated the NMDA-EPSC to 64 ± 3% of control. Subsequent to NEM treatment, SST reduced the mean NMDA-EPSC only to 90 ± 4% of control [F(1,4) = 96.6; P < 0.0001; Fig. 8B], representing a 72% reduction in SST efficacy. The SST-induced outward current again was completely blocked by NEM treatment in these neurons: the mean SST-induced current was 42 ± 12 pA before NEM and
2 ± 5 pA after NEM.

View larger version (28K):
[in this window]
[in a new window]
| FIG. 8.
NEM treatment attenuates the depression of NMDA-EPSCs by SST. A: NMDA-EPSCs (70% maximal) from a representative cell are decreased by SST (1 µM; 3 min) and baclofen (10 µM; 2.5 min) before NEM treatment. After treatment, both SST (3 min) and baclofen (3 min) are ineffective in depressing the NMDA-EPSCs. NEM also blocked the GABAB IPSC in this cell (VH was 78 mV; RMP was 69 mV). Calibration bar applies to both panels. B: mean data from 5 cells showing that NEM treatment greatly attenuates the depression of the NMDA-EPSC area by SST and baclofen.
|
|
Inhibition of EPSCs by SST is insensitive to internal Cs+ but sensitive to external Ba2+
To determine whether postsynaptic K+ currents were involved in SST effects on EPSCs, we used 2 M Cs+-filled recording electrodes to block the postsynaptic K+ currents known to be affected by SST. In these experiments we superfused only bicuculline (15 µM) in order to ascertain the effectiveness of Cs+ in blocking the K+-mediated GABAB-IPSC. The SST-induced outward current was blocked using Cs+-containing electrodes. The mean SST-induced current was 6 ± 13 pA (n = 5), compared with an average current of 38 ± 8 pA using a K+ electrode (n = 4). We also tested four neurons for the presence of the SST-sensitive M-current (Moore et al. 1988
; Schweitzer et al. 1990
). As expected when using Cs+ electrodes, the inward current relaxation characteristic of the closing of M-channels was not present (data not shown), indicating that the postsynaptic effects of SST on K+ currents were effectively blocked.
Despite the block of postsynaptic K+ currents, SST robustly inhibited the EPSCs in five of five cells. Figure 9 shows the mean inhibition of compound EPSCs by SST for both K+- (Fig. 9A) and Cs+- (Fig. 9B) containing electrodes. There was no significant difference in the SST effect mean EPSCs between Cs+ (mean VH,
80 ± 2 mV) and K+ (mean VH,
77 ± 1 mV) electrodes [F(1,25) = 0.048, P > 0.5]. Moreover, the SST inhibition of EPSCs did not vary significantly when tested at three different holding potentials (Cs+ or K+ electrodes, P > 0.1; Fig. 9, A and B). Thus the effect of SST was voltage independent over the voltage range tested (
80 to
100 mV).
We tested the sensitivity of the SST-induced inhibition of EPSCs to external Ba2+ by adding 2 mM BaCl in the superfusate. Such a Ba2+ concentration blocks the postsynaptic effects of SST on K+ currents (Moore et al. 1988
). Additionally, Ba2+ should block most presynaptic K+ currents (Thompson and Gähwiler 1992
) and may also interfere with synaptic transmission by hindering Ca2+-mediated processes (Medina et al. 1994
).
Superfusion of slices with 2 mM Ba2+ made it necessary to decrease the stimulus intensity to normalize the EPSC, because blocking postsynaptic K+ currents increased the excitability of the neurons. However, after 3-5 min of Ba2+ application, a higher stimulus intensity was required (often higher than before Ba2+ application), probably because of interference by Ba2+ of glutamate release, as previously reported in CA1 neurons (Medina et al. 1994
). In five of seven cells, we established a more hyperpolarized holding potential after addition of Ba2+ (mean VH in control,
74 ± 1 mV; in Ba2+,
77 ± 2 mV) to help prevent unclamped dendritic Na+ and/or Ba2+ spikes. In cells that previously responded to SST, addition of 2 mM Ba2+ greatly attenuated the SST-induced inhibition of EPSCs (Fig. 9C). Before superfusion of Ba2+, SST reduced the EPSC area in seven cells to 41 ± 12% of control [F(1,6) = 22; P < 0.005], and to only 87 ± 7% of control after Ba2+ treatment [F(1,6) = 1.09; P > 0.1].
 |
DISCUSSION |
Previous studies from our laboratory aimed to assess the postsynaptic actions of SST in HPNs, and these studies revealed that SST augments an M-type K+ current through an arachidonic acid pathway (Moore et al. 1988
; Schweitzer et al. 1990
, 1993
). SST also enhances a voltage-insensitive leak K+ current in these neurons (P. Schweitzer and G. R. Siggins, unpublished observations). Both of these effects contribute to the SST-induced hyperpolarization at resting potential reported for HPNs (Pittman and Siggins 1981
; Schweitzer et al. 1993
; Watson and Pittman 1988
). Such hyperpolarizations reduce HPN excitability by moving the cell away from its threshold for firing. In contrast, other laboratories have reported excitatory actions of SST in CA1 (Dodd and Kelly 1978
; Scharfman and Schwartzkroin 1989
). It is possible that depression by SST of inhibitory interneurons could lead to excitation of HPNs, as has been reported for opiates in this region (Madison and Nicoll 1988
; Siggins and Zieglgänsberger 1981
; Zieglgänsberger et al. 1979
). Indeed, previous current-clamp studies have suggested that SST reduces compound IPSPs but not EPSPs in rabbit (Scharfman and Schwartzkroin 1988
, 1989
) and guinea pig CA1 neurons (Xie and Sastry 1992
), suggesting that a disinhibitory action of SST might be in play here. However, there were several potential confounds (e.g., shunting effects, summation of opposite effects, postsynaptic SST effects) in these previous studies that led us to reexamine SST modulation of synaptic transmission in the rat CA1, now using isolated synaptic components and voltage-clamp recording.
We have shown here that SST in low concentrations(IC50 = 21.9 nM) reduces both AMPA/kainate- and NMDA-EPSCs. Inhibition of both EPSC components was rapid in onset, with peak inhibition occurring within 2-4 min after beginning SST superfusion. We saw little attenuation of the SST response, either over the time of the application or when a second SST application was introduced, suggesting a relative lack of desensitization. In the presence of the GABAergic blockers bicuculline and CGP 55845A, a single stimulation of the SC sometimes evoked AMPA/kainate EPSCs with multiple components. Although these recurrent polysynaptic EPSCs were thought to be driven by afferent input from CA3 (Traynelis and Dingledine 1988
), more recent results suggest that they result from reciprocal excitatory connections within CA1 that are normally masked by inhibitory inputs (Crepel et al. 1997
; Deuchars and Thomson 1996
; Meier and Dudek 1996
). These polysynaptic responses are present even after cutting the connection between the CA1 and CA3, further supporting the likelihood that they are locally driven (Crepel et al. 1997
; Gereau and Conn 1994
). Recently, these polysynaptic EPSCs have been characterized in depth; they were found to be driven by either NMDA or AMPA/kainate receptors and regulated by inhibitory synapses located within the stratum radiatum and lacunosum-moleculare region of the CA1 (Crepel et al. 1997
). In our study, in neurons where two components of the AMPA/kainate EPSC were clearly distinct, the second component was especially sensitive to SST. This could reflect an additive property of SST across multiple synapses or a specific role for SST at recurrent excitatory synapses between HPNs. The latter hypothesis is supported by the close anatomic proximity of the processes of SSTergic interneurons to the local axon collaterals of CA1 pyramidal neurons (Freund and Buzsaki 1996
). These considerations suggest that SST effects may develop largely under conditions of increased excitability, such as epileptiform activity.
The levels of both SST peptide and receptors are altered in models of epilepsy (Perez et al. 1995
); thus it has been speculated that SST may be involved in this pathology. Indeed, several studies have suggested a role for SST in depressing hippocampal seizure activity. SST injected locally into the hippocampus, as well as the amygdala, reduced tonic-clonic seizures induced by picrotoxin, a GABAA receptor antagonist, whereas they were augmented by intrahippocampal injection of anti-SST antiserum (Mazarati andTelegdy 1992). An sst2-selective agonist infused into the hippocampus reduced tonic-clonic seizures in kainate-treated rats (Perez et al. 1995
). Similarly, the SST analogue SMS 201-995 (octreotide), which also shows sst2 selectivity, decreased kainate-induced seizures in rats when injected into the stratum radiatum (Vezzani et al. 1991
). Finally, SST release was enhanced in hippocampus of kindled rats, suggesting that endogenous SST may play a role in this model of epilepsy (Vezzani et al. 1992
). These studies point to alterations of SSTergic systems as a consequence of epilepsy and suggest that SST may reduce the hippocampal hyperexcitability characteristic of epilepsy. Our results suggest a homeostatic mechanism by which SST may reduce seizures: depression of glutamatergic excitatory neurotransmissionin CA1.
The inhibition of EPSCs by SST appears to be mediated by the Gi/Go family of G-proteins, because NEM greatly reduced the ability of SST to inhibit EPSCs. NEM effects have been highly correlated with those of PTX, an agent that ADP-ribosylates G-proteins of the Gi/Go family (Choi and Lovinger 1996
; Morishita et al. 1997
; Shapiro et al. 1994
). The majority of SST effects in the brain and periphery have been reported to be PTX sensitive (Reisine and Bell 1995
; Schindler et al. 1996
). SST effects in the hippocampus, however, have yet to be characterized in terms of G-protein coupling. We have shown here that both SST inhibition of EPSCs and the postsynaptic SST-induced outward current near rest are greatly attenuated by NEM treatment. NEM did not change membrane properties of HPNs or alter synaptic transmission per se, and isoproterenol, likely acting through Gs-coupled presynaptic
-adrenergic receptors (Gereau and Conn 1994
), still enhanced EPSCs after NEM treatment, providing evidence that NEM did not cause nonspecific damage to the slice.
To determine the efficacy of NEM treatment in inactivating PTX-sensitive G-proteins, we also examined the effects of the GABAB receptor agonist baclofen. The baclofen-induced outward current is known to be PTX sensitive (Andrade et al. 1986
). We show here that the baclofen-induced outward current is also blocked by NEM, as previously reported (Morishita et al. 1997
). Interestingly, NEM treatment also greatly reduced the ability of baclofen to depress EPSCs. This effect of baclofen was previously reported to be mediated by presynaptic GABAB receptors and to be PTX insensitive (Colmers and Pittman 1989
; Dutar and Nicoll 1988
; Thompson and Gähwiler 1992
). That NEM but not PTX attenuated this presynaptic effect of GABAB receptors suggests that some G-proteins may have different sensitivities to these agents. The relatively slow onset of ADP-ribosylation of G-proteins by PTX, however, and its slow accessibility to certain cellular regions, could also lead to misinterpretation of negative data. Indeed, it has been reported that in hippocampus postsynaptic G-proteins are more accessible to block by PTX than presynaptic ones (Thompson and Gähwiler 1992
). Such problems may be less acute with the rapidly acting NEM.
The inhibition of EPSCs by SST was insensitive to intracellular Cs+. The use of Cs+-filled recording electrodes blocked the SST-induced postsynaptic effects on K+ currents, as reflected by the absence of both the SST-induced outward current near rest and the M-current. However, SST still robustly inhibited EPSCs in Cs+-filled cells, with no significant difference between Cs+ and K+ electrodes. Thus the mechanism by which SST inhibits EPSCs does not appear to involve postsynaptic K+ channels.
The reduction of EPSCs by SST was blocked by 2 mM external Ba2+. This Ba2+ concentration blocks several types of K+ currents in CA1 HPNs, including those that mediate SST (Moore et al. 1988
) and baclofen (Gähwiler and Brown 1985
; Newberry and Nicoll 1984
) postsynaptic effects. Ba2+ also may interfere with synaptic transmission, because this ion fails to support neurotransmission in the absence of Ca2+ in CA1 (Medina et al. 1994
). The inhibition of presynaptic K+ channels by Ba2+ could also interfere with repolarization of the presynaptic membrane after an action potential, increasing the amount of Ca2+ influx. This appears to be the mechanism by which the K+ channel blocker tetraethylammonium interferes with presynaptic inhibition of EPSCs by neuropeptide Y in CA1 (Colmers et al. 1993
). Ba2+ blocks GABAB receptor-mediated presynaptic inhibition of IPSPs but not EPSPs in hippocampus (Misgeld et al. 1989
; Thompson and Gähwiler 1992
). It is as yet unclear whether Ba2+ acts on presynaptic K+ currents or some other aspect of synaptic transmission (e.g., Ca2+-mediated processes) to block baclofen- and SST-induced inhibition of synaptic events.
In contrast to previous studies examining the effects of SST on compound PSPs in CA1 (Scharfman and Schwartzkroin 1988
; Xie and Sastry 1992
), we show here that SST has no direct effects on GABAergic transmission. Although SST had no direct effect on GABAB-IPSCs, we sometimes observed an indirect reduction by SST of polysynaptically driven GABAB-IPSCs, probably a consequence of SST attenuation of the driving glutamatergic EPSC (see Fig. 7A). Therefore attenuation of EPSCs by SST could lead indirectly to reduced feed-forward and feedback inhibitionin CA1.
We also found no depression by SST of GABAA-mediated IPSCs, in contrast to previous studies (Scharfman and Schwartzkroin 1989
; Xie and Sastry 1992
). In fact, we saw a trend toward an increase in the amplitude of the GABAA-IPSCs during SST application, but this increase never reached statistical significance, nor was it reversible. We did not examine SST actions on spontaneously released GABA, although it seems unlikely that SST would have an effect on GABA release because it does not alter evoked events even at low stimulation intensities. The contradiction between our results and earlier studies could be due to species differences. Also, in the Scharfman and Schwartzkroin studies, the biologically inactive cleavage product of SST 28, SST(1-12), had an effect similar to SST (Scharfman and Schwartzkroin 1988
). SST(1-12) does not bind to SST receptors (Raynor and Reisine 1989
; Reisine and Bell1995) and has been shown by other groups to have no effect in the hippocampus (Araujo et al. 1990
; Greene andMason 1996; Watson and Pittman 1988
). Therefore some of the SST effects seen by Scharfman and Schwartzkroin (Scharfman and Schwartzkroin 1988
, 1989
) may have been nonspecific.
Our experiments suggest that SST actions on synaptic activity are distinct from its postsynaptic effects on K+ conductances. No other postsynaptic effects for SST have yet been characterized using the slice preparation, suggesting that SST may be acting presynaptically. The fact that SST acts similarly on both NMDA and AMPA/kainate glutamatergic EPSCs also suggests a presynaptic effect. Nevertheless, further studies will need to directly address the site of action of SST. A very recent study on cultured hippocampal neurons reports similar findings to ours, that SST reduces excitatory but not inhibitory autaptic events, and that this effect appears to be presynaptic, because SST reduced the frequency but not the amplitude of miniature EPSCs (Boehm and Betz 1997
). Somatostatin may be released from CA1 interneurons under conditions of high-frequency activation (as might be expected during epileptiform activity) (Vezzani et al. 1992
) and act by multiple mechanisms to reduce the excitatory drive onto CA1 principal neurons. Thus SST appears to act in concert with GABA, with which it is colocalized, to play a major homeostatic role in regulating hippocampal excitability.
 |
ACKNOWLEDGEMENTS |
We thank S. J. Henriksen, P. Schweitzer, and S. G. Madamba for helpful comments and technical assistance, W. Fröstl and A. Suter (NovartisPharma) for the gift of CGP 55845A, and Dr. P. L. Herrling (Novartis Pharma) for the gift of various drugs.
This work was supported by National Institutes of Health Grants MH-44346 and AA-07456.
 |
FOOTNOTES |
Address for reprint requests: G. R. Siggins, Dept. of Neuropharmacology, CVN-12, The Scripps Research Institute, 10550 North Torrey Pines Rd., La Jolla, CA 92037.
Received 5 May 1997; accepted in final form 28 August 1997.
 |
REFERENCES |
-
ANDRADE, R.,
MALENKA, R. C.,
NICOLL, R. A. A G
protein couples serotonin and GABAB receptors to the same channels in hippocampus.
Science
234: 1261-1265, 1986.[Medline]
-
ARAUJO, D. M.,
LAPCHAK, P. A.,
COLLIER, B.,
QUIRION, R.
Evidence that somatostatin enhances endogenous acetylocholine release in the rat hippocampus.
J. Neurochem.
55: 1546-1555, 1990.[Medline]
-
BOEHM, S.,
BETZ, H.
Somatostatin inhibits excitatory transmission at rat hippocampal synapses via presynaptic receptors.
J. Neurosci.
17: 4066-4075, 1997.[Abstract/Free Full Text]
-
BRAZEAU, P.,
VALE, W.,
BURGUS, R.,
LING, N.,
RIVIER, J.,
GUILLEMIN, R.
Hypothalamic polypeptide that inhibits the secretion of immunoreactive pituitary growth hormone.
Science
129: 77-79, 1972.
-
BREDER, C. D.,
YAMADA, Y.,
YASUDA, K.,
SEINO, S.,
SAPER, C. B.,
BELL, G. I.
Differential expression of somatostatin receptor subtypes in brain.
J. Neurosci.
12: 3920-3934, 1992.[Abstract]
-
CHOI, S.,
LOVINGER, D. M.
Metabotropic glutamate receptor modulation of voltage-gated Ca2+ channels involves multiple receptor subtypes in cortical neurons.
J. Neurosci.
16: 36-45, 1996.[Abstract]
-
CHRISTIAN, E. P.,
DUDEK, F. E.
Electrophysiological evidence from glutamate microapplications for local excitatory circuits in the CA1 of rat hippocampal slices.
J. Neurophysiol.
59: 110-123, 1988.[Abstract/Free Full Text]
-
COLMERS, W. F.,
MCQUISTON, A. R.,
KOMBIAN, S. B.,
KLAPSTEIN, G. J.
Presynaptic inhibition mediated by neuropeptide Y in the mammalian CNS: possible physiological implications.
In: Presynaptic Receptors in the Mammalian Brain,
edited by
T. V. Dunwiddie,
and D. M. Lovinger
. Boston, MA: Birkhäuser, 1993, p. 87-103
-
COLMERS, W. F.,
PITTMAN, Q. J.
Presynaptic inhibition by neuropeptide Y and baclofen in hippocampus: insensitivity to pertussis toxin treatment.
Brain Res.
498: 99-104, 1989.[Medline]
-
CRAWLEY, J. N.
Comparative distribution of cholecystokinin and other neuropeptides.
Ann. NY Acad. Sci.
448: 1-8, 1985.
-
CRAWLEY, J. N.
Co-existence of classical neuropeptides and classical neurotransmitters.
Ann. NY Acad. Sci.
579: 233-244, 1990.[Medline]
-
CREPEL, V.,
KHAZIPOV, R.,
BEN-ARI, Y.
Blocking GABA(A) inhibition reveals AMPA- and NMDA-receptor-mediated polysynaptic responses in the CA1 region of the rat hippocampus.
J. Neurophysiol.
77: 2071-2082, 1997.[Abstract/Free Full Text]
-
DEUCHARS, J.,
THOMSON, A. M.
CA1 pyramid-pyramid connections in rat hippocampus in vitro: dual intracellular recordings with biocytin filling.
Neuroscience
74: 1009-1018, 1996.[Medline]
-
DODD, J.,
KELLY, J. S.
Is somatostatin an excitatory neurotransmitter in the hippocampus.
Nature
273: 674-675, 1978.[Medline]
-
DOURNAUD, P.,
GU, Y. Z.,
SCHONBRUNN, A.,
MAZELLA, J.,
TANNENBAUM, G. S.,
BEAUDET, A.
Localization of the somatostatin receptor SST2A in rat brain using a specific anti-peptide antibody.
J. Neurosci.
16: 4468-4478, 1996.[Abstract/Free Full Text]
-
DUTAR, P.,
NICOLL, R. A.
Pre- and postsynaptic GABAB receptors in the hippocampus have different pharmacological properties.
Neuron
1: 585-591, 1988.[Medline]
-
EPELBAUM, J.
Somatostatin in the central nervous system: physiology and pathological modifications.
Prog. Neurobiol.
27: 63-100, 1986.[Medline]
-
ESCLAPEZ, M.,
HOUSER, C. R.
Somatostatin neurons are a subpopulation of GABA neurons in the rat dentate gyrus: evidence from colocalization of pre-prosomatostatin and glutamate decarboxylase messenger RNAs.
Neuroscience
64: 339-355, 1995.[Medline]
-
FELDMAN, S. C.,
DREYFUSS, C. F.,
LICHTENSTEIN, E. S.
Somatostatin neurons in the rodent hippocampus: an in vitro and in vivo immunocytochemical study.
Neurosci. Lett.
33: 29-34, 1982.[Medline]
-
FINKEL, A. S.,
REDMAN, S. J.
Optimal voltage clamping with single microelectrodes.
In: Voltage and Patch Clamping With Microelectrodes,
edited by
T. G. Smith,
H. Lecar,
S. J. Redman,
and P. W. Gage
. Baltimore, MD: Williams and Wilkins, 1985, p. 95-120
-
FREUND, T. F.,
BUZSAKI, G.
Interneurons of the hippocampus.
Hippocampus
6: 347-470, 1996.[Medline]
-
GÄHWILER, B. H.,
BROWN, D. A.
GABAB-receptor activated K+ currents in voltage-clamped CA3 pyramidal cells in hippocampal cultures.
Proc. Natl. Acad. Sci. USA
82: 1558-1562, 1985.[Abstract]
-
GEREAU, R. W.,
CONN, P. J.
Presynaptic enhancement of excitatory synaptic transmission by
-adrenergic receptor activation.
J. Neurophysiol.
72: 1438-1442, 1994.[Abstract/Free Full Text] -
GREENE, J.R.T.,
MASON, A.
Effects of somatostatin and related peptides on the membrane potential and input resistance of rat ventral subicular neurons in vitro.
J. Pharmacol. Exp. Ther.
276: 426-432, 1996.[Abstract]
-
HALLIWELL, J. V.,
ADAMS, P. R.
Voltage-clamp analysis of muscarinic excitation in hippocampal neurons.
Brain Res.
250: 71-92, 1982.[Medline]
-
HOLLOWAY, S.,
FENIUK, W.,
KIDD, E. J.,
HUMPHREY, P.P.A. A
quantitative autoradiographical study on the distribution of somatostatin sst2 receptors in the rat central nervous system using [125I]-BIM-23027.
Neuropharmacology
35: 1109-1120, 1996.[Medline]
-
ISHIBASHI, H.,
AKAIKE, N.
Somatostatin modulates high-voltage-activated Ca2+ channels in freshly dissociated rat hippocampal neurons.
J. Neurophysiol.
74: 1028-1036, 1995.[Abstract/Free Full Text]
-
JOHNSTON, D.,
BROWN, T. H.
Interpretation of voltage-clamp measurements in hippocampal neurons.
J. Physiol. (Lond.)
50: 464-486, 1983.
-
KOHLER, C.,
ERIKSSON, L. G.,
DAVIES, S.,
CHAN-PALAY, V.
Co-localization of neruopeptide tyrosine and somatostatin immunoreactivity in neurons of individual subfields of the rat hippocampal region.
Neurosci. Lett.
78: 1-6, 1987.[Medline]
-
KONG, H.,
DEPAOLI, A. M.,
BREDER, C. D.,
YASUDA, K.,
BELL, G. I.,
REISINE, T.
Differential expression of messenger RNAs for somatostatin receptor subtypes SSTR1, SSTR2, and SSTR3 in adult rat brain: analysis by RNA blotting and in situ hybridization histochemistry.
Neuroscience
59: 175-184, 1994.[Medline]
-
KOSAKA, T.,
WU, J.-Y.,
BENOIT, R.
GABAergic neurons containing somatostatin-like immunoreactivity in the rat hippocampus and dentate gyrus.
Exp. Brain Res.
71: 388-398, 1988.[Medline]
-
LEROUX, P.,
WEISSMANN, D.,
PUJOL, J.-F.,
VAUDRY, H.
Quantitative autoradiography of somatostatin receptors in the rat limbic system.
J. Comp. Neurol.
331: 389-401, 1993.[Medline]
-
MADAMBA, S. G.,
SCHWEITZER, P.,
ZIEGLGÄNSBERGER, W.,
SIGGINS, G. R.
Acamprosate (calcium acetylhomotaurinate) enhances the N-methyl-D-aspartate component of excitatory neurotransmission in rat hippocampal CA1 neurons in vitro.
Alcohol Clin. Exp. Res.
20: 651-658, 1996.[Medline]
-
MADISON, D. V.,
LANCASTER, B.,
NICOLL, R. A.
Voltage-clamp analysis of cholinergic action in the hippocampus.
J. Neurosci.
7: 733-741, 1987.[Abstract]
-
MADISON, D. V.,
NICOLL, R. A.
Enkephalin hyperpolarizes Interneurones in the rat hippocampus.
J. Physiol. (Lond.)
398: 123-130, 1988.[Abstract]
-
MARTIN, G.,
NIE, Z.,
SIGGINS, G. R.
Mu-opioid receptors modulate NMDA receptor-mediated responses in nucleus accumbens neurons.
J. Neurosci.
17: 11-22, 1997.[Abstract/Free Full Text]
-
MAZARATI, A. M.,
TELEGDY, G.
Effects of somatostatin and anti-somatostatin serum on picrotoxin-kin led seizures.
Neuropharmacology
31: 793-797, 1992.[Medline]
-
MEDINA, J. I.,
BARDEN, S. D.,
DAVIES, M. S.,
NEWELL, B.,
SHAW, S. A.,
WILLIS, M.,
HALLIWELL, J. V.
Barium ions fail to support neurotransmission at a central synapse.
Neurosci Lett.
168: 106-110, 1994.[Medline]
-
MEIER, C. L.,
DUDEK, F. E.
Spontaneous and stimulation-induced synchronized burst afterdischarges in the isolated CA1 of kainate-treated rats.
J. Neurophysiol.
76: 2231-2239, 1996.[Abstract/Free Full Text]
-
MISGELD, U.,
MULLER, W.,
BRUNNER, H.
Effects of baclofen on inhibitory neurons in the guinea pig hippocampal slices.
Pflügers Arch.
414: 139-144, 1989.[Medline]
-
MOORE, S. D.,
MADAMBA, S. G.,
JOELS, M.,
SIGGINS, G. R.
Somatostatin augments the M-current in hippocampal neurons.
Science
239: 278-280, 1988.[Medline]
-
MORISHITA, W.,
KIROV, S. A.,
PITLER, T. A.,
MARTIN, L. A.,
LENZ, R. A.,
ALGER, B. E.
N-ethylmaleimide blocks depolarization-induced suppression of inhibition and enhances GABA release in the rat hippocampal slice in vitro.
J. Neurosci.
17: 941-950, 1997.[Abstract/Free Full Text]
-
MORRISON, J. H.,
BENOIT, R.,
MAIGISTRETTI, P.,
LING, N.,
BLOOM, F. E.
Immunohistochemical distribution of pro-somatostatin-related peptides in hippocampus.
Neurosci. Lett.
34: 137-142, 1982.[Medline]
-
NEWBERRY, N. R.,
NICOLL, R. A.
Direct hyperpolarizing action of baclofen on hippocampal pyramidal cells.
Nature
308: 450-452, 1984.[Medline]
-
PEREZ, J.,
RIGO, M.,
KAUPMANN, K.,
BRUNS, C.,
YASUDA, K.,
BELL, G. I.,
LUBBERT, H.,
HOYER, D.
Localization of somatostatin (SRIF) SSTR-1, SSTR-2, and SSTR-3 receptor mRNA by in situ hybridization.
Naunyn Schmeidebergs Arch. Pharmacol.
349: 145-160, 1994.[Medline]
-
PEREZ, J.,
VEZZANI, A.,
CIVENNI, G.,
TUTKA, P.,
RIZZI, M.,
SCHUEPBACH, E.,
HOYER, D.
Functional effects of D-Phe-c[Cys-Tyr-D-Trp-Lys-Val-Cys]-Trp-NH2 and differential changes in somatostatin receptor messenger RNAs, binding sites and somatostatin release in kainic acid-treated rats.
Neuroscience
65: 1087-1097, 1995.[Medline]
-
PITTMAN, Q. J.,
SIGGINS, G. R.
Somatostatin hyperpolarizes pyramidal cells in vitro.
Brain Res.
221: 402-408, 1981.[Medline]
-
RAYNOR, K.,
REISINE, T.
Analogs of somatostatin selectively label distinct subtypes of somatostatin and receptors in rat brain.
J. Pharmacol. Exp. Ther.
251: 510-517, 1989.[Abstract]
-
REISINE, T.
Somatostatin.
Cell. Mol. Neurobiol.
15: 597-614, 1995.[Medline]
-
REISINE, T.,
BELL, G. I.
Molecular biology of somatostatin receptors.
Endocr. Rev.
16: 427-442, 1995.[Medline]
-
SCHARFMAN, H. E.,
SCHWARTZKROIN, P. A.
Further studies of the effects of somatostatin and related peptides in area CA1 of rabbit hippocampus.
Cell. Mol. Neurobiol.
8: 411-429, 1988.[Medline]
-
SCHARFMAN, H. E.,
SCHWARTZKROIN, P. A.
Selctive depression of GABA-mediated IPSPs by somatostatin in area CA1 of rabbit hippocampal slices.
Brain Res.
493: 205-211, 1989.[Medline]
-
SCHINDLER, M.,
HUMPHREY, P.P.A.,
EMSON, P. C.
Somatostatin receptors in the central nervous system.
Prog. Neurobiol.
50: 9-37, 1996.[Medline]
-
SCHWEITZER, P.,
MADAMBA, M.,
SIGGINS, G. R.
Arachidonic acid metabolites as mediators of somatostatin-induced increase of neuronal M-current.
Nature
346: 464-467, 1990.[Medline]
-
SCHWEITZER, P.,
MADAMBA, S.,
CHAMPAGNAT, J.,
SIGGINS, G. R.
Somatostatin inhibition of hippocampal CA1 pyramidal neurons: mediation by arachidonic acid and its metabolites.
J. Neurosci.
13: 2033-2049, 1993.[Abstract]
-
SHAPIRO, M. S.,
WOLLMUTH, L. P.,
HILLE, B.
Modulation of Ca2+ channels by PTX-sensitive G-proteins is blocked by N-ethylmaleimide in rat sympathetic neurons.
J. Neurosci.
14: 7109-7116, 1994.[Abstract]
-
SIGGINS, G. R.,
ZIEGLGÄNSBERGER, W.
Morphine and opioid peptides reduced inhibitory synaptic potentials in hippocampal pyramidal cells in vitro without alteration of membrane potential.
Proc. Natl. Acad. Sci. USA
78: 5235-5239, 1981.[Abstract]
-
TALLENT, M.,
SIGGINS, G.
Somatostatin depresses excitatory but not inhibitory postsynaptic currents in rat CA1 hippocampal neurons.
Soc. Neurosci. Abstr.
22: 1999, 1996.
-
THOMPSON, S. M.,
GÄHWILER, B. H.
Comparison of the action of baclofen at pre- and postsynaptic receptors in the rat hippocampus in vitro.
J. Physiol. (Lond.)
451: 329-345, 1992.[Abstract]
-
THOSS, V. S.,
PEREZ, J.,
PROBST, A.,
HOYER, D.
Expression of five somatostatin receptor mRNAs in the human brain and pituitary.
Naunyn Schmiedebergs Arch. Pharmacol.
354: 411-419, 1996.[Medline]
-
TRAYNELIS, S. F.,
DINGLEDINE, R.
Potassium-induced spontaneous electrographic seizures in the rat hippocampal slice.
J. Neurophysiol.
59: 259-276, 1988.[Abstract/Free Full Text]
-
VEZZANI, A.,
MONNO, A.,
RIZZI, M.,
GALLI, A.,
BARRIOS, M.,
SAMANIN, R.
Somatostatin release is enhanced in the hippocampus of partially and fully kindled rats.
Neuroscience
51: 41-46, 1992.[Medline]
-
VEZZANI, A.,
SERAFINI, R.,
STASI, M. A.,
VIGANO, G.,
RIZZI, M.,
SAMANIN, R. A
peptidase-resistant cyclic octapeptide analogue of somatostatin (SMS 201-995) modulates seizures induced by quinolic acid and kainic acids differently in the rat hippocampus.
Neuropharmacology
30: 345-352, 1991.[Medline]
-
WATSON, T.W.J.,
PITTMAN, Q. J.
Somatostatin(14) and -(28) but not somatostatin(1-12) hyperpolarize CA1 pyramidal neurons in vitro.
Brain Res.
448: 40-45, 1988.[Medline]
-
XIE, Z.,
SASTRY, B. R.
Actions of somatostatin on GABA-ergic synaptic transmission in the CA1 area of the hippocampus.
Brain Res.
591: 239-247, 1992.[Medline]
-
ZIEGLGÄNSBERGER, W.,
FRENCH, E. D.,
SIGGINS, G. R.,
BLOOM, F. E.
Opioid peptides may excite hippocampal pyramidal neurons by inhibiting adjacent inhibitory interneurons.
Science
205: 415-417, 1979.[Medline]