Effect of Adrenalectomy on Membrane Properties and Synaptic Potentials in Rat Dentate Granule Cells

M. Joëls, C. Stienstra, and Y. Karten

Institute for Neurobiology, University of Amsterdam, 1098 SM Amsterdam, The Netherlands


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Joëls, M., C. Stienstra, and Y. Karten. Effect of Adrenalectomy on Membrane Properties and Synaptic Potentials in Rat Dentate Granule Cells. J. Neurophysiol. 85: 699-707, 2001. Adrenalectomy is known to accelerate both neurogenesis and cell death of granule cells located in the suprapyramidal blade of the rat dentate gyrus. Three days after adrenalectomy, some granule cells have already died by apoptosis while newly formed cells are not yet incorporated in the cell layer, resulting in a temporary loss of granule cells. Concomitantly, the field response to stimulation of perforant path afferents is reduced. While the temporary cell loss is likely to attenuate synaptic field responses, adrenalectomy-induced changes in properties of the surviving cells may also contribute to the reduction in field response amplitude. To address this possibility, we here investigated the membrane properties and synaptic responses of dentate granule cells, 3 days after adrenalectomy. We found that passive and most of the active membrane properties of granule cells in adrenalectomized rats were not significantly different from the cell properties in sham-operated controls. However, intracellularly recorded synaptic responses from surviving granule cells were markedly reduced after adrenalectomy. The N-methyl-D-aspartate (NMDA)- and the non-NMDA receptor-mediated components were reduced to a similar extent, suggesting that the attenuation of synaptic transmission after adrenalectomy could be partly of presynaptic origin. The data indicate that the earlier observed attenuation of synaptic field responses after adrenalectomy may be partly due to a diminished glutamatergic input to the dentate gyrus and not exclusively to a loss of granule cells participating in the synaptic circuit.


    INTRODUCTION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Granule cells in the rat dentate gyrus have a relatively short life cycle (Gage et al. 1998; Gould and Cameron 1996; Gould and McEwen 1993). New cells are continuously formed, even in adulthood, through proliferation from progenitor cells in the hilus; some of these cells enter the granule cell layer and develop into neurons. While cells mature, they migrate through the cell layer. Aged cells at the outer border of the granule cell layer die by apoptosis (Gage et al. 1998; Gould and Cameron 1996).

The turnover of cells is under control of adrenal steroids. It was shown that adrenalectomy (ADX) accelerates both neurogenesis and apoptosis of dentate granule cells (Cameron and Gould 1994; Gould et al. 1990; Hornsby et al. 1996; Hu et al. 1997; Jaarsma et al. 1992; Sapolsky et al. 1991; Sloviter et al. 1989, 1993a,b). In particular, older granule cells, located in the outer part of the cell layer, are liable to ADX-induced apoptosis (Cameron and Gould 1996). These apoptotic cells can be discerned already at 3 days after ADX (Hu et al. 1997). Incorporation of newly formed granule cells in the cell layer, however, occurs with a longer delay (Cameron and Gould 1996), so that temporarily fewer viable granule cells are present in the granule cell layer. Replacement with a low dose of corticosterone, which is sufficient to occupy the high-affinity mineralocorticoid receptors (Reul and de Kloet 1985) in dentate granule cells, prevents the acceleration of cell turnover after ADX; compounds acting on the lower affinity glucocorticoid receptor are less effective (Gould et al. 1991; Hornsby et al. 1996; Sloviter et al. 1995; Woolley et al. 1991).

Extracellular recording studies showed that, concurrent with the ADX-induced acceleration of cell turnover, field excitatory postsynaptic potentials (fEPSPs) in the outer molecular layer in response to perforant path stimulation are reduced, with respect to amplitude and slope (Stienstra et al. 1998). In corticosterone-replaced ADX animals (which lack apoptotic cells) the reduction in field responses was not observed. Also, in those animals (~20% of the total population) that display no apoptotic cells after ADX despite corticosterone levels below the detection limit, field responses were comparable to those in sham-operated control animals. The reduction in synaptic responses, however, did not always correlate with the appearance of apoptotic cells. Thus 1 day after ADX, apoptotic cells have not yet appeared, while the field response in the granule cell layer is already significantly reduced (Stienstra et al. 1998).

The reduced fEPSP amplitude 3 days after ADX can be explained in several ways. One possibility is that fewer mature cells participate in the field response, since apoptotic cells do no longer respond to synaptic activation, while newly formed cells do not yet participate in the synaptic circuit. A second possibility is that passive membrane properties of surviving granule cells are changed after ADX. Thus shifts in resting membrane potential or resistance are likely to affect synaptic responses, due to changes in driving force or shunting of the signal. A third possibility is that ADX alters the synaptic transmission between perforant path afferents and dentate granule cells, regardless of the properties of the postsynaptic granule cells. To investigate these possibilities, we used intracellular recording methods to monitor the passive and active membrane properties as well as synaptic responses of dentate granule cells, 3 days after ADX, sham operation, or ADX in combination with corticosterone treatment. Of all animals, extracellularly recorded responses to synaptic stimulation were obtained concomitant with the intracellular signals.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Experimental animals and tissue processing

Forty-one male Wistar rats (100-200 g: Harlan CPB, The Netherlands) were housed under standard conditions with an alternating light/dark cycle (12 h cycle, lights on at 8.00 a.m.). Three days before the experiment, rats were bilaterally adrenalectomized (ADX, n = 26) or sham-operated (sham, n = 15) under halothane anesthesia, as described earlier (Ratka et al. 1989). One-half of the ADX animals received a low dose of corticosterone (20 µg/ml in saline, 0.1% ethanol) in their drinking bottle. The other half of the ADX rats and the sham-operated control group only received the vehicle. ADX rats that were not treated with corticosterone received a bottle of tap water in addition to saline; sham-operated animals only received tap water. Food and water were provided ad libitum. Body weight of all three experimental groups at the moment of operation was comparable (averages between 140 and 170 g). The gain in bodyweight in the 3-day period between operation and the experiment was significantly (P < 0.05) attenuated in the untreated ADX group (3.2 ± 2.7 g, mean ± SE) compared with the corticosterone-replaced group (15.0 ± 2.0 g) and the sham-operated control group (15.0 ± 1.4 g). All efforts were made to minimize animal suffering, to reduce the number of animals used, and to utilize alternatives to in vivo techniques, if available. The experiments were carried out with prior, written consent of the local "Animal Experiments Committee" (DED12).

On the morning of the experiment, the rat was moderately stressed by exposure to a novel environment as described earlier (e.g., Joëls and de Kloet 1993) and decapitated 30-45 min later. Trunk blood was collected by exsanguination; subsequently, serum corticosterone levels were analyzed by means of a radioactive immunoassay. Plasma corticosterone levels in the sham group (21.4 ± 3.6 µg/dl) indeed indicated that the animals were moderately stressed by the experimental procedure. All untreated ADX rats incorporated in the present study displayed plasma corticosteroid levels below 1 µg corticosterone/100 ml plasma (0.04 ± 0.03 µg/dl). The corticosterone-replaced ADX rats also exhibited low corticosterone levels (0.70 ± 0.29 µg/dl), due to the breakdown of corticosterone consumed during the active period of the animals, i.e., 6-12 h before the decapitation.

After decapitation the brain was rapidly taken out of the skull; frontal lobes and cerebellum were removed. Horizontal slices (400 µm) containing the middle third of the hippocampal lobes (along the septo-temporal axis) were cut on a Vibroslicer (Campden Instruments) and kept at room temperature in carbogenated (95% O2-5% CO2) artificial cerebrospinal fluid (ACSF) containing (in mM) 124 NaCl, 3.5 KCl, 1.25 NaH2PO4, 1.5 MgSO4, 2 CaCl2, 25 NaHCO3, and 10 glucose. After an equilibration period of at least 1 h, one slice at a time was transferred to the recording chamber, which was continuously perfused with warm (32-34°C), carbogenated ACSF (pH 7.4). The slice was fixed between two nylon meshes and kept fully submerged.

For histological analysis, two slices per rat were kept overnight in phosphate-buffered paraformaldehyde (4% paraformaldehyde in 0.1 M phosphate-buffered saline; pH 7.4). After dehydration in phosphate-buffered sucrose (30% sucrose in 0.1 M phosphate buffer; pH 7.4), the slices were frozen in liquid nitrogen and cut into thinner sections (30 µm). These sections were stained for Nissl with a cresyl violet stain to visualize nuclear chromatin. After mounting, dehydration, and embedding, the degree of apoptotic-like degeneration in the tip of the dorsal blade of the dentate gyrus was determined in a qualitative way. ADX animals with no or only one pyknotic nucleus within a 500 × 500 µm grid were regarded as nonapoptotic and excluded from the study.

Stimulation and recording

A bipolar stimulation electrode was placed in the outer molecular layer of the dentate gyrus to activate the fibers of the perforant pathway through constant current pulses (150 µs; intensity ranging from 0 to 525 µA). The evoked field potentials (fEPSPs) were recorded with glass ACSF-filled microelectrodes (1-8 MOmega ) placed in the outer molecular layer of the tip of the dentate gyrus (see Fig. 1). Intracellular recordings were routinely (see, e.g., Joëls and de Kloet 1993) made through 4 M KAc-filled glass microelectrodes (impedances: 80-130 MOmega ), placed in either the tip of the suprapyramidal blade or the curvature of the dentate gyrus (see Fig. 1). In earlier patch-clamp studies under visual control, in which pyknotic cells could be recognized by their condensed nuclear chromatin, we were unable to record any signals from pyknotic cells (unpublished observation), probably due to disintegration of the plasma membrane, which is a prominent feature of apoptotic degeneration. This supports that recording in the present study was confined to nonapoptotic cells.



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Fig. 1. A1: Nissl-stained horizontal section of the dentate gyrus from a 3-day adrenalectomized rat, showing (as squares) the 2 areas from which granule cells were recorded, i.e., the tip of the suprapyramidal blade and the curvature of the granule cell layer. A2: higher magnification of the tip of the suprapyramidal blade, showing many pyknotic cells with condensed nuclear chromatin. A3: at the curvature, fewer pyknotic cells were encountered. B: in tissue from a 3-day sham-operated control rat, no pyknotic cells were seen. C: similarly, in tissue from a 3-day adrenalectomized rat that received a low dose of corticosterone in the drinking water, pyknotic cells were not encountered. Calibration bar: 100 µm for A1 and 60 µm for A2, A3, B, and C.

Voltage signals were passed to an NPI SEC 1 l/H (Germany) amplifier. The membrane potential and current injections were continuously registered on a chart recorder. Neurons incorporated in the present study displayed stable membrane potentials of at least -55 mV. The stimulation protocols, data acquisition, and the analysis of the intracellular responses and fEPSPs were performed with a Falcon computer (C-lab Falcon MKI) and in-house developed software. From all animals included in the present study, stable fEPSPs were recorded as well as intracellularly recorded properties of at least one cell per animal. The evoked fEPSPs had to be constant for at least 15 min before the stimulation protocol was started. Although both the fEPSP slope and amplitude were determined as parameters for further analysis, we here only report the data obtained with regard to the fEPSP amplitude; earlier data showed that the fEPSP amplitude is a very reliable parameter with these relatively small signals (Stienstra et al. 1998). In one slice from each animal an input-output curve was constructed, consisting of 15 stimuli ranging from 0 to 525 µA (interval between stimuli 10 s).

In another slice from the same animal, neurons were impaled with microelectrodes. No more than two cells were recorded per animal. After impaling, the input resistance was determined by passing hyper- and depolarizing currents of increasing magnitude through the microelectrode. In off-line analysis, input resistance was estimated from the inverted slope of the linear part of the current-voltage plot. For a current step of -0.3 nA, the time constant of the membrane was determined. The first action potential evoked by a depolarizing current step was used to determine the firing threshold, the action potential amplitude (from the firing threshold), width (at the threshold), and rise time (from threshold to peak, see also RESULTS). After applying hyper- and depolarizing current pulses, granule cells were subjected to perforant path stimulation with increasing intensity (between 200 and 350 µA). In off-line analysis we determined the EPSP amplitude, the area under the EPSP up to 50 ms after stimulation, and the time constant for the decay of the EPSP (provided synaptic stimulation did not evoke an action potential). Moreover, the slope of the rising phase of the EPSP (between 0.1 and 1.5 ms after the start of the EPSP) was analyzed. In a limited number of experiments, D(-)-2-amino-5-phosphonovaleric acid (APV, Tocris; 50 µM) was added to the medium for at least 20 min to investigate the participation of N-methyl-D-aspartate (NMDA) receptors in the synaptic response. If cells still displayed stable responses after synaptic responses in the presence of APV were established, the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; disodium salt, Tocris, 10 µM) was added to the perfusion medium to determine contribution of transmitters other than glutamate to the synaptic response.

Analysis

All data are presented as means ± SE. Statistical analysis of the input-output curves for the fEPSP amplitude and the slope of the rising phase of the EPSP was performed by ANOVA for repeated measurements (MANOVA), using treatment as between-subjects factors. The remaining parameters were statistically analyzed with a Mann-Whitney U test (level of significance P < 0.05).


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Extracellular recording

Field EPSPs in response to perforant path stimulation were recorded through microelectrodes, positioned in the outer molecular layer of the tip of the suprapyramidal blade (see examples in Fig. 2A). Figure 2B illustrates the averaged input-output relationship for the fEPSP amplitude in animals incorporated in the present study. As described before (Stienstra et al. 1998), the fEPSP amplitude was significantly reduced 3 days after ADX compared with the amplitude recorded from the sham-operated control group. Substitution with a low dose of corticosterone in the drinking water not only prevented the appearance of apoptotic cells in all animals (see example in Fig. 1), but also normalized the ADX-induced reduction of the fEPSP amplitude (Fig. 2B).



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Fig. 2. A: typical examples of field excitatory postsynaptic potentials (fEPSP) evoked by stimulation of the perforant path, in the molecular layer of the dentate gyrus. The recording electrode was placed in the tip of the suprapyramidal blade, the area indicated in Fig. 1A. The maximal responses displayed here demonstrate that the fEPSP amplitude was reduced after adrenalectomy (ADX; A2), compared with the response in a sham-operated control rat (A1) and the corticosterone (Cort) replaced ADX rat (A3). On the right, the 3 traces are superimposed (black = sham = 1; dark gray = ADX = 2; light gray = ADX/Cort = 3). B: averaged input-output curves (+SE) for the fEPSP amplitude recorded in the molecular layer of the dentate gyrus as a function of the stimulus intensity. The input-output curve for the amplitude of the fEPSP was significantly reduced in slices from ADX rats (n = 12) compared with sham-operated controls [n = 15; F(1,25) = 4.49, P < 0.05] and the ADX rats that received corticosterone [n = 12; F(1,22) = 7.10, P < 0.05].

Membrane properties

Intracellular recording techniques were employed to investigate membrane properties and synaptic potentials of dentate granule cells in the above set of animals that showed a clear reduction of synaptic field responses after ADX.

Resting membrane potential (RMP) and input resistance (Rin) of dentate granule cells in the tip of the suprapyramidal blade from sham-operated controls was -77 mV and 82 MOmega , respectively. Table 1 summarizes the average values for RMP and Rin in the three experimental groups. No significant differences between groups were observed for either parameter, although the Rin in the sham group tended to be somewhat lower than in the ADX group and particularly the ADX group receiving corticosterone (P < 0.11). Granule cells recorded in the curvature of the dentate gyrus, i.e., a region where fewer apoptotic cells are observed (see Fig. 1A3), exhibited comparable membrane properties as cells in the tip of the suprapyramidal blade. On average, the RMP of neurons in the curvature of the dentate gyrus was equal for the ADX and control group [-76 ± 2 and -76 ± 1 mV for the ADX (n = 14 animals) and sham group (n = 13 animals), respectively].


                              
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Table 1. Passive membrane and action potential (AP) properties of dentate granule cells after sham operation (Sham), adrenalectomy (ADX), or ADX with in vivo corticosterone replacement (ADX/Cort)

We next investigated the action potential properties in the three experimental groups (see example in Fig. 3). In some dentate granule cells (Table 1), no action potential could be evoked with the presently used current injections (up to 0.5 nA) or stimulation intensities (up to 350 µA). In the remaining cells, the action potential amplitude, width, rise time, and threshold were determined, as illustrated in Fig. 3C. No significant differences between the three experimental groups were observed with respect to the amplitude or duration of the action potential (Table 1). However, the rise time of the action potential was significantly slower in cells from ADX compared with sham animals. Values for the ADX group treated with corticosterone were generally intermediate, between the untreated ADX and sham groups. The threshold for generation of an action potential was nearly equal for all groups (Table 1).



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Fig. 3. A: typical examples of the action potential evoked by a depolarizing current step in dentate granule cells from a sham (A1), adrenalectomized (A2), and corticosterone-replaced (A3) animal. B: when superimposed and normalized to the amplitude of the sham signal, the differences between the action potential characteristics for the sham (black, 1), ADX (gray, 2), and ADX/Cort (stippled, 3) animal become apparent. Action potentials in ADX rats tend to have a longer duration and display a slower rise time. C: for further statistical analysis, the action potential amplitude, width, and rise time were calculated as indicated in this example.

Synaptic potentials

In nearly all cells, reliable synaptic potentials were recorded in response to stimulation of perforant path afferents. With a stimulus intensity of 200 µA, none of the cells responded to synaptic stimulation with an action potential.

With this stimulus intensity, the amplitude of the synaptic response was significantly reduced in ADX animals compared with the sham-operated controls (Table 2; example in Fig. 4). The area under the EPSP (up to 50 ms after stimulation) was also significantly smaller in ADX rats, in comparison to the sham group. The decay of the EPSP was fitted with a single exponential. Although the time constant was on average larger in cells from ADX rats compared with sham-operated controls, this difference was not statistically significant. Synaptic responses recorded in cells from ADX animals that received corticosterone were generally quite similar to the responses from the sham control group.


                              
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Table 2. Intracellularly recorded synaptic responses induced in dentate granule cells by stimuli via the perforant path



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Fig. 4. A: typical examples of the synaptic responses evoked by a 200-µA stimulus to the perforant path, in a sham (A1), adrenalectomized (ADX, A2), and ADX rat receiving corticosterone (A3). B: superimposed traces showing that after ADX (gray, 2), synaptic potentials were very much reduced compared with the signal in the sham-operated control rat (black, 1) and the ADX rat that received corticosterone (3). C: normalizing the synaptic responses from the ADX- and corticosterone-treated rats to the amplitude of the signal in the sham-operated control supports that amplitude and kinetic properties are similarly affected after ADX.

At higher stimulus intensities, i.e., 245 µA, the EPSP amplitude was still markedly reduced in the ADX group compared with the sham-operated controls (i.e., by 33%, data not shown). However, these data were somewhat less reliable since at higher stimulus intensities, some of the cells responded to synaptic stimulation with an action potential superimposed on the EPSP. In these cells, EPSP amplitude and area could not be determined correctly, so that they were excluded from the analysis, thereby introducing a bias in the cell population. At this intensity, however, the slope of the rising phase of the EPSP could still be determined, even if an action potential was superimposed on the EPSP. We observed that over a stimulation range of 200-245 µA, the slope of the EPSP was significantly [MANOVA, F(1,29) = 4.27, P < 0.05] reduced in the ADX versus the sham-operated control group. The EPSP slope over this range observed in the corticosterone-treated ADX group was not different from the slope seen in the sham control group [F(1,25)= 0.51, P = 0.48]. Over this stimulation range, the slope of the EPSP in the ADX group was significantly smaller than in the sham group for all intensities, including the highest intensity of 245 µA (see Table 2).

In a limited number of cells, we investigated whether ADX reduced the NMDA- and non-NMDA receptor-mediated synaptic responses to an equal extent. To this end, the effect of the NMDA receptor blocker APV was tested on the synaptic responses of four cells from ADX rats and seven cells from sham controls. The averaged synaptic responses before APV application in these ADX and sham cells (4.2 and 7.4 mV, respectively) were comparable to the average of all ADX and sham cells (4.3 and 8.4 mV, respectively). In the seven cells from sham-operated control animals, the EPSP amplitude was reduced by 35% (from 7.4 to 4.9 mV) in the presence of the NMDA receptor blocker APV. The area under the EPSP was even more reduced, by approximately 50% (from 147 to 80 mV × ms), in the presence of APV. As shown in Fig. 5, the EPSP was nearly completely blocked when the non-NMDA receptor blocker CNQX was added to the perfusion medium, in addition to APV. On average (n = 6 cells), the area under the EPSP in the presence of APV and CNQX amounted to only 3% (4.8 mV × ms) of the total area before the blockers were applied, indicating that non-glutamate receptors (e.g., GABAa receptors) contributed little to the synaptic response, under the present recording conditions. In cells from ADX rats, APV reduced the amplitude of the synaptic response by approximately 25% (from 4.2 to 3.1 mV) and the EPSP area by 45% (from 91 to 49 mV × ms), indicating that the contribution of NMDA receptors to the synaptic response was roughly comparable between granule cells in sham and ADX rats.



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Fig. 5. Typical example of synaptic potentials recorded in a cell from a sham-operated rat, showing that compared with the control situation (top left), the response was reduced in the presence of bath applied D(-)-2-amino-5-phosphonovaleric acid (APV; bottom left). When 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was also added to the perfusion medium (top right), the response was nearly abolished. The effect was particularly clear when the traces under control conditions (black), in the presence of APV (gray) and with both APV and CNQX (stippled) were superimposed (bottom right).

The ratio between the average response obtained in ADX versus sham-operated animals was comparable for the NMDA- and non-NMDA receptor-mediated components (see example in Fig. 6). Thus on average, the area under the EPSP before application of APV in ADX rats (91 mV × ms, n = 4 cells) amounted to 57% of the response seen in the sham control group (147 mV × ms, n = 7 cells). A similar ratio (62%) was found for the NMDA receptor-mediated synaptic component (42 and 67 mV × ms, in ADX and sham-operated rats, respectively). Also, for the non-NMDA receptor component of the synaptic response, a comparable reduction (52%) was seen between ADX (49 mV × ms) and sham cells (80 mV × ms). These data indicate that after ADX, NMDA- and non-NMDA receptor-dependent components of the EPSP area were reduced to a similar extent. This was also found with respect to the EPSP amplitude (57, 49, and 64% reduction after ADX, for the total synaptic response, the NMDA receptor-mediated component and the non-NMDA receptor-mediated component, respectively).



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Fig. 6. A: typical example of the total response (A1), the non-NMDA receptor-mediated component, recorded in the presence of APV (A2) and the NMDA-dependent component [A3 = (A1 - A2)] of the synaptic response in a granule cell from a sham-operated control rat. B: similar example for a cell from an adrenalectomized rat. C: superimposed traces for the sham (black) and ADX (gray) cell shows that the relative reduction after ADX is comparable for the total response (C1), the non-NMDA receptor-mediated component (C2) and the NMDA receptor-evoked component (C3) of the synaptic response. In C1, the amplitude of the ADX response was normalized to the sham response. The ADX responses depicted in C2 and C3 were expressed relative to the normalized response in C1.


    DISCUSSION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

All ADX animals incorporated in the present study showed clear signs of apoptosis in the suprapyramidal blade of the dentate gyrus. Earlier it was demonstrated that, with such short delays after ADX, part of the granule cells (particularly older cells) have already gone into apoptosis while newly formed cells are not yet fully incorporated into the granule cell layer (Cameron and Gould 1996). We therefore assume that in the presently used ADX animals, temporarily fewer cells participated in the synaptic circuit in the suprapyramidal blade of the dentate gyrus. While this in itself could partly explain the attenuated synaptic field response amplitude that was reported earlier (Stienstra et al. 1998) and confirmed in the present study, it is also possible that ADX-induced changes in intrinsic membrane properties and synaptic responses of nonapoptotic cells contribute to the attenuated field response.

The presently observed membrane characteristics of granule cells in the sham group were similar to the properties described earlier in studies conducted under comparable experimental conditions (Blaxter and Carlen 1988; Gao et al. 1998; Lambert and Jones 1990; Scharfman and Schwartzkroin 1990; Staley et al. 1992). Although no extensive changes in intrinsic membrane properties after ADX were earlier found in CA1 pyramidal cells (Joëls and de Kloet 1989; Kerr et al. 1989), this could nevertheless happen in dentate granule cells. It was shown that particularly the "old" dentate granule cells are prone to ADX-induced apoptosis (Cameron and Gould 1996). This may temporarily cause a shift in the age of the surviving granule cells, toward a somewhat younger age. Earlier it was shown that the maturation of granule cells is associated with a gradually more negative resting membrane potential, lower input resistance, larger spike amplitude, faster spike kinetics, and more hyperpolarized spike threshold (Liu et al. 1996). Young adult rats (as we presently used) mostly exhibited mature, i.e., hyperpolarized neurons, although neurons with immature characteristics were also observed (Liu et al. 1996). It may be expected therefore that an ADX-induced shift in the age of the granule cells is associated with an apparent change in the intrinsic properties of the granule cells. On average, after ADX granule cells exhibited a slightly more depolarized resting membrane potential, higher input resistance, smaller action potential amplitude, and a longer duration and slower rise time of the action potential, which is consistent with properties of less mature cells. However, with the exception of the action potential rise time, none of the differences reached statistical significance, indicating that the putative shift in the age of the granule cells 3 days after ADX must be limited. Apparently, neither the absence of corticosterone nor the process of apoptosis in neighboring cells seems to largely affect basal membrane characteristics of nonapoptotic granule cells after ADX. In agreement, membrane properties of cells in the curvature of the dentate gyrus, which also lack corticosterone but are surrounded by much fewer apoptotic cells, displayed similar membrane characteristics. We conclude that the attenuation of synaptic responses observed after ADX is not caused by changes in membrane properties of nonapoptotic granule cells.

We next addressed the possibility that synaptic transmission between perforant path afferents and nonapoptotic granule cells is attenuated after ADX. Synaptic responses were recorded at resting membrane potential. In the sham-operated control group, synaptic responses were comparable to responses reported in earlier studies under comparable recording conditions (Lambert and Jones 1990). In sham-operated rats, granule cells exhibited a synaptic response that was mediated for one-third by NMDA receptors and for the remainder almost completely by alpha -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or kainate receptors. The GABAa receptor-mediated component was apparently very limited, when cells were recorded at their resting membrane potential, i.e., around -75 mV.

We observed that the intracellularly recorded EPSPs evoked by perforant path stimulation were reduced after ADX to a similar extent as the field response recorded in the same group of animals. Given the marked attenuation of intracellularly recorded EPSPs after ADX, we conclude that a putative loss of a specific cell population is certainly not the only cause of the reduced fEPSP amplitude; apparently, the synaptic transmission in the neurons that do participate in the synaptic circuit is also changed. The fact that synaptic responses were to a large extent normalized by corticosterone treatment supports that the attenuated synaptic responses seen after ADX are caused by the absence of corticosterone, rather than other deficiencies associated with ADX.

The reduction was very comparable for the NMDA and the non NMDA receptor-mediated components, indicating that the attenuation is probably not due to an altered functionality of specific glutamate receptors. This largely agrees with the limited changes in NMDA and AMPA receptor binding or mRNA expression in dentate granule cells shortly after ADX, as observed in earlier studies (Clark and Cotman 1992; Joëls et al. 1996; Watanabe et al. 1995). Rather than an altered glutamate receptor function, a 1) reduced density of glutamatergic afferents, 2) attenuated glutamate release, and/or 3) diminished dendritic surface (assuming equal density of glutamate receptors) of postsynaptic granule cells seems to underlie the impaired synaptic transmission seen after ADX. Support for the first possibility comes from an earlier study (Cameron et al. 1995) showing that entorhinal lesions result in acceleration of dentate granule cell turnover comparable to what is seen after ADX. This may point to a common factor in the two experimental conditions, supporting that afferent projections to the dentate gyrus are compromised after ADX. The second explanation (substantial changes in the process of neurotransmitter release after ADX) is less likely, since earlier extracellular findings showed that paired pulse response ratios in the dentate gyrus are not altered after ADX (Stienstra et al. 1998).

The present data are inconclusive whether the attenuation of synaptic responsiveness in the dentate gyrus is a consequence of apoptosis, or rather is caused independently by modulatory actions of corticosteroid hormones on glutamate-mediated transmission. Such modulatory actions independent from apoptosis have been described earlier for CA1 pyramidal neurons (Birnstiel et al. 1995; Joëls and de Kloet 1993; Joëls and Fernhout 1993; Reiheld et al. 1984; Rey et al. 1987, 1989; Vidal et al. 1986). Selective activation of high-affinity corticosteroid receptors in CA1 neurons was found to enhance synaptic transmission (Birnstiel et al. 1995; Joëls and de Kloet 1993; Reiheld et al. 1984; Rey et al. 1989), while additional activation of lower affinity receptors markedly reduced synaptic responses (Joëls and de Kloet 1993; Joëls and Fernhout 1993; Rey et al. 1987, 1989; Vidal et al. 1986). If corticosteroids modulate synaptic responses similarly in the dentate gyrus, loss of synaptic function may occur independent from the appearance of apoptotic cells. In this respect it is interesting that 1 day after ADX, when no apoptosis can be discerned, synaptic field responses are already impaired (Stienstra et al. 1998). Moreover, entorhinal lesions or blockade of NMDA receptors in the dentate gyrus was shown to increase the number of newborn cells as well as apoptotic cells (Cameron et al. 1995; Stienstra et al. 2000). The latter observations suggest that impaired synaptic function may be a cause rather than consequence of the presence of apoptotic cells.

In conclusion, 3 days after ADX, intrinsic properties of dentate cells were not markedly altered compared with the sham control group. Importantly, a considerable reduction of EPSP amplitude, area, and rise time was observed after ADX in nonapoptotic granule cells. The reduction in amplitude and area was nearly equal for the NMDA and non-NMDA receptor-mediated components, pointing to a possible presynaptic impairment. We tentatively conclude that loss of synaptic input to dentate granule cells is a prominent feature following ADX and may even contribute to the observed instability in cell turnover.


    ACKNOWLEDGMENTS

We thank S. Maslam for histological support, E. Velzing for photography, and L. van Essen for assistance with the corticosterone radioimmunoassay.

This work was supported by Grants 903-42-017 and 900-95-312 of the Dutch Organization for Scientific Research (Nederlandse Organisatie voor Wetenschappelyk Onderzoek) and Stichting Glaxo Research Nederland Grant GRN 94-005.


    FOOTNOTES

Address for reprint requests: M. Joëls, Inst. Neurobiology UvA, Kruislaan 320, 1098 SM Amsterdam, The Netherlands (E-mail: joels{at}bio.uva.nl).

Received 19 June 2000; accepted in final form 24 October 2000.


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0022-3077/01 $5.00 Copyright © 2001 The American Physiological Society




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