NMDA-Independent LTP by Adenosine A2 Receptor-Mediated Postsynaptic AMPA Potentiation in Hippocampus

Kofi Kessey1 and David J. Mogul1, 2

1 Department of Neurobiology and Physiology and 2 Department of Biomedical Engineering, Northwestern University, Evanston, Illinois 60208

    ABSTRACT
Abstract
Introduction
Methods
Results
Discussion
References

Kessey, Kofi and David J. Mogul. NMDA-independent LTP by adenosine A2 receptor-mediated postsynaptic AMPA potentiation in hippocampus. J. Neurophysiol. 78: 1965-1972, 1997. The role of adenosine A2 receptors in normal synaptic transmission and tetanus-induced long-term potentiation (LTP) was tested by stimulation of the Schaffer collateral pathway and recording of the field excitatory postsynaptic potential (EPSP) in the CA1 region of rat transverse hippocampal slices. Activation of adenosine A2 receptors with the A2 agonist N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA; 20 nM) enhanced synaptic transmission during low-frequency test pulses (0.033 Hz). Paired stimulation before and during DPMA exposure indicated no paired-pulse facilitation as a result of A2 activation, suggesting that enhancement was not a result of presynaptic modulation. DPMA enhanced the early phase alpha -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) component of the EPSP. In contrast, DPMA had no effect on the N-methyl-D-aspartate (NMDA) component isolated using low extracellular Mg2+ and the AMPA receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione (20 µM), indicating that the effects of A2 activation on synaptic transmission were mediated by a postsynaptic enhancement of the AMPA response. Activation of adenosine A2 receptors during a brief tetanus (100 Hz, 1 s) increased the level of LTP by 36% over that seen in response to a tetanus under control conditions. DPMA exposure after prior induction of LTP showed no additional potentiation, indicating that the mechanisms that contribute to both types of increases in synaptic transmission share a common mechanism. A slow onset NMDA-independent LTP could be induced by application of a tetanus during perfusion of DPMA with the NMDA blocker AP5 (50 µM). Blockade of L-type Ca channels with nifedipine (10 µM) had no effect on normal synaptic transmission but reduced NMDA-independent LTP by 32%. Very little NMDA-independent LTP could be induced after prior saturation of NMDA-dependent LTP via multiple tetani spaced 10 min apart, indicating that both forms of LTP are eventually convergent on a common mechanism, presumably the postsynaptic AMPA receptor response. Because extracellular adenosine levels are modulated by cellular activity throughout the brain and because adenosine receptor activation can markedly alter levels of synaptic transmission independent of NMDA receptors, adenosine may play an important and complex role as a modulator of synaptic transmission in the brain.

    INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References

Long-lasting changes in synaptic transmission, such as LTP, can be induced in the hippocampus by repetitive stimulation of excitatory pathways (Bliss and Lomo 1973; Collingridge et al. 1983; Harris et al. 1984; Morris et al. 1984). Because levels of extracellular adenosine in the brain rise in response to increased rates of neuronal firing as well as during ischemia, the possibility exists of a significant role for adenosine receptors in the modulation of neurotransmission. Adenosine has been shown to have a potent inhibitory effect on neurotransmission in the hippocampus through activation of adenosine A1 receptors (Dunwiddie et al. 1981; Greene and Haas 1985), resulting in a decrease in transmitter release as well as modulation of transmembrane potassium and calcium currents. In addition to its inhibitory effect, adenosine also has been shown to have an excitatory influence on synaptic transmission via activation of adenosine A2 receptors (Nishimura et al. 1990; Okada et al. 1992; Sebastiao and Ribeiro 1996).

The A1 and A2 adenosine receptors were first differentiated based on their ability to down- or upregulate adenylyl cyclase (Londos and Wolff 1977; VanCalker et al. 1979), respectively. The A2 receptor has been subdivided further into an A2a and A2b adenosine receptor (Bruns et al. 1986; Ukena et al. 1986) based on its affinity for 5'-N-ethylcarboxamidoadenosine with the A2a receptor having the higher affinity. Activation of A2 receptors during tetanic stimulation has been shown to modulate the level of LTP achieved in a adenosine 3',5'-cyclic monophosphate (cAMP)-dependent manner (Kessey and Mogul 1996), whereas A2 receptor blockade significantly reduces tetanus-induced LTP (Mogul et al. 1994; Sekino et al. 1991). These effects appeared to be a function of A2b receptors because neither selective A2a activation nor blockade had any effect (Mogul et al. 1994) although a role for A2a-mediated increases in hippocampal synaptic transmission has been reported (Cunha et al. 1994). Elevated levels of cAMP appear to be critical for both the early and late phases of LTP (Blitzer et al. 1995; Frey et al. 1993) and can be blocked by the N-methyl-D-aspartate (NMDA)-receptor antagonist AP5 (Chetkovich et al. 1991), which also blocks LTP. Although induction of LTP in CA1 is thought to depend primarily on Ca2+ influx through NMDA channels, voltage-sensitive Ca channels (VSCCs) have been shown to contribute to an NMDA-independent LTP (Grover and Teyler 1990; however, see Hanse and Gustafsson 1995). In CA3 neurons, activation of A2 receptors has been shown to augment Ca currents through VSCC in a cAMP-dependent manner (Mogul et al. 1993). Collectively, these results suggest that A2 receptors, via their stimulatory influence on cAMP and possibly VSCC, maybe able to induce LTP independent of NMDA-receptor activation.

LTP has traditionally been divided into induction and maintenance phases, which are believed to proceed along separate pathways. Although it generally is agreed that in the Schaffer collateral/CA1 pathway, induction of LTP depends on postsynaptic activation of NMDA-receptor-gated ion channels and a subsequent influx of Ca2+, the mechanism underlying the maintenance phase of LTP is less well understood. In the hippocampus, studies using quantal analysis (Bekkers and Stevens 1990; Malinow and Tsien 1990) suggested that the maintenance phase of LTP is a result of presynaptic changes in transmitter release. In contrast, an alternative hypothesis based on recent experiments suggests that previously silent synapses, without functional postsyna p t i c   alpha  - a m i n o - 3 - h y d r o x y - 5 - m e t h y l - 4 - i s o x a z o l e p r o p i o n i cacid (AMPA) receptors but possessing NMDA receptors, potentiate their response to glutamate after a tetanus by the addition of functional postsynaptic AMPA channels (Isaac et al. 1995; Liao et al. 1995). Here we present evidence that A2 receptor-mediated augmentation of both normal synaptic transmission and tetanus-induced LTP at the Schaffer collateral/CA1 synapse occurs through a postsynaptic potentiation of AMPA responses. Furthermore, a tetanus applied during elevated A2 activation induces a novel form of NMDA-independent LTP in these synapses that partially involves VSCC but the pathway of which is convergent with that of NMDA-mediated LTP. Because levels of extracellular adenosine increase with increased neuronal firing rates as well as during ischemia, these purinergic receptors may play a critical role in both the down- and upmodulation of synaptic transmission.

    METHODS
Abstract
Introduction
Methods
Results
Discussion
References

Hippocampal slices were obtained from 22- to 35-day-old male Sprague-Dawley rats (Harlan Sprague Dawley; Indianapolis, IN). Animals were anesthetized with isoflurane by inhalation and then decapitated. The brain was excised rapidly, hemisected at the interhemispheric fissure, and placed in cold (4°C) artificial cerebrospinal fluid (ACSF) containing (in mM) 124 NaCl, 24 NaHCO3, 10 D-glucose, 1.3 MgSO4, 1.25 NaH2PO4, 3 KCl, and 2.4 CaCl2 and gassed with 95% O2-5% CO2. Both hippocampi were dissected free, and 350 mm transverse slices were cut using a vertical tissue chopper (Stoelting). Slices were stored in the bubbled ACSF at room temperature (~22°C) and transferred as needed to a submersion chamber maintained at 30°C.

Extracellular recordings were made using glass microelectrodes (2-4 MOmega ) pulled from borosilicate glass capillaries and filled with NaCl (2 M). Recording electrodes for field excitatory postsynaptic potentials (EPSPs) were placed in stratum radiatum of the CA1 region. Orthodromic stimulation was provided by placement in stratum radiatum near the CA3/CA1 border of a twisted Teflon-coated platinum bipolar electrode. Test pulses were delivered at 0.033 Hz. Tetanic stimulation consisted of a 1-s train at 100 Hz delivered at the monitoring intensity. Constant current stimulation (100-400 µA) pulses were delivered by a current stimulus isolator. Stimulus response curves were constructed by varying pulse width (50-250 µs), and the test stimulus duration for each slice experiment was selected as that required to achieve ~50% of the maximum EPSP slope response (usually 60-90 µs). The test duration was not changed within an experiment. Recording signals were amplified using a DAM80 amplifier with active headstage (World Precision Instruments, Sarasota, FL), filtered between 0.1 Hz and 3 kHz, and were sampled at 10 kHz by a Digidata 1200 (Axon Instruments, Foster City, CA). Experiments were controlled and analyzed using the Axobasic software (Axon Instruments). The maximum slope of the initial negative deflection for each extracellular field measurement was used to quantify each dendritic EPSP. The level of LTP attained was calculated as the averaged response for the last 10 min of each experiment. Statistical comparisons were made by t-tests unless the normalcy of samples could not be assumed; alternate tests were used as indicated.

    RESULTS
Abstract
Introduction
Methods
Results
Discussion
References

Adenosine A2-receptor activation potentiates synaptic transmission via increases in the postsynaptic AMPA response

The effect of adenosine A2 receptor activation was tested on synaptic transmission during low-frequency test pulses (0.033 Hz) using the A2 agonist DPMA. Bath applicationo f  N6 - [2 - (3, 5 - d i m e t h o x y p h e n y l) - 2 - (2 - m e t h y l p h e n y l)ethyl]adenosine (DPMA; 20 nM) caused an increase of 30% ± 2.2% (mean ± SE, n = 5) in the maximum slope of the EPSP elicited over that of the baseline response recorded during perfusion with control ACSF (Fig. 1). This increase reverses back to baseline within ~5 min after washout. It has been shown previously that A2 receptors stimulate cAMP accumulation (Londos and Wolff 1977) and that the effects of these receptors on normal synaptic transmission and LTP are mediated, at least partly, by a cAMP-dependent cascade (Kessey and Mogul 1996). Because cAMP is known to regulate both transmitter release (Chavez-Noriega and Stevens 1992, 1994) and postsynaptic receptors and channels (Greengard et al. 1991; Wang et al. 1991, 1993), it is possible that A2 receptors could increase synaptic transmission through either a pre- or postsynaptic mechanism and the changes in cAMP that it evokes. It is unlikely to exert its effects on inhibitory GABAergic synapses because adenosine and adenosine agents do not appear to modulate gamma -aminobutyric acid release in the hippocampus (Dolphin and Archer 1981; Yoon and Rothman 1991). To determine if activation of A2 receptors alters transmitter release, we first monitored paired-pulse facilitation (PPF) during perfusion of the A2 agonist DPMA (20 nM). The DPMA-induced increase in synaptic transmission, however, was not associated with any significant differences in either the response of the first and second pulses (Fig. 2Ai) or changes in the PPF ratio (measured as percent) (Fig. 2Aii) at any interstimulus interval tested.


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FIG. 1. Activation of A2 receptors with N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) during low-frequency test pulses enhances synaptic transmission. A: slices were first perfused with control artificial cerebrospinal fluid (ACSF). Exposure of slices to DPMA (20 nM) for 15 min resulted in an increase in synaptic responses of 130 ± 2.2% (n = 5) compared with baseline responses taken as 100%. B: extracellular field recordings during superfusion of control buffer and DPMA.


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FIG. 2. DPMA potentiates neurotransmission by selectively increasing postsynaptic alpha -amino-3-hydroxy-5-methyl-4-isoxa z o l e p r o p i o n i c   a c i d   ( A M P A ) - r e c e p t o rfunction. A, i and ii: activation of A2 receptors with DPMA during paired-pulse facilitation (PPF) did not result in any significant difference between the first and second responses at all interstimulus intervals (ISI) tested. For clarity, SE bars have been omitted in Ai. Note that, as in Fig. 1, there was an increase in synaptic responses during DPMA exposure when compared with control. For Aii, each data point represents averaged data for 5 slices and 6 continuous responses at each ISI. Aiii: typical EPSP recordings made in control ACSF or DPMA at ISI = 50, 150, or 300 ms.- - -, amplitude of the initial downward deflection in control ACSF. Bi: to determine the effect of DPMA on isolatedN-methyl-D-aspartate (NMDA) responses, slices were perfused with low Mg2+ buffer and AMPA receptor blockade with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 µM). DPMA did not alter the slope or amplitude of the NMDA-mediated response. Bii: NMDA-mediated responses to DPMA after AMPA blockade with CNQX.

Increases in the early phase of the EPSP slope has been attributed previously to augmentation of the postsynaptic AMPA-receptor component of the synaptic response to glutamate. To further examine a possible presynaptic locus of A2 receptor activity and because of possible difficulties in interpreting PPF protocols (Schulz et al. 1994), the effects of DPMA perfusion on the NMDA-receptor-mediatedcomponent of the EPSP were explored. If the action of A2 receptors involved an increase in glutamate release, then comparable changes in both the postsynaptic AMPA- and NMDA-receptor-mediated components of the EPSP would be expected. Conversely, a change in only one component of the synaptic response would suggest selective modification of postsynaptic sensitivity. The NMDA-receptor-mediated component of the EPSP was isolated pharmacologically by bath application of the AMPA-receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 µM) in low Mg2+ (100 µM) buffer. Perfusion of DPMA during continued AMPA-receptor blockade for 15 min did not alter the size (slope or amplitude) of the isolated NMDA-mediated synaptic response (n = 6; Fig. 2B). Together, these results show that activation of A2 receptors enhanced synaptic transmission through selective postsynaptic modulation of AMPA receptors.

DPMA modulates the level of tetanus-induced LTP during but not several minutes after tetanic stimulation

Because it has been suggested that the maintenance of LTP may involve selective modulation of AMPA receptors (Davies et al. 1989; Isaac et al. 1995; Liao et al. 1995), we determined the effects of activating A2 receptors with DPMA on the level of expression of LTP in response to tetanic stimulation. The level of LTP induced in the presence of DPMA was compared with the level of LTP obtained under control conditions. For the control experiment, tetanic stimulation (100 Hz, 1 s) applied after a stable baseline of synaptic responses (defined as 100%) yielded a level of LTP of164 ± 8% (n = 6) measured 30-40 min posttetanus (Fig. 3A). When the same tetanus was applied in the presence of DPMA (Fig. 3B), the level of LTP achieved was 223 ± 5% (n = 6), significantly higher (P < 0.0001) than that recorded during control conditions.


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FIG. 3. Activation of A2 receptors before long-term potentiation (LTP)-inducing tetanus (100 Hz, 1 s) increases the level of potentiated responses. A: at least 10 min after a baseline of stable responses in control ACSF, slices were tetanized (right-arrow). Under such conditions, the level of LTP obtained after 40 min was 164 ± 8% (n = 6). Bi: slices were first perfused with DPMA for 15 min. Increase in synaptic responses during DPMA perfusion is consistent with Fig. 1A. When slices were tetanized with the same LTP-inducing stimulation protocol as above, the subsequent level of LTP achieved was 223 ± 5.7% (n = 6) over the original baseline. The rationale for using the level in ACSF as control baseline is shown in C. Bii: field potentials recorded during protocol in Bi. C: LTP was induced in slices bathed with control ACSF, and the subsequent changes in excitatory postsynaptic potentials (EPSP) monitored for 15 min. Slices then were superfused with DPMA for 20 min with no net change in the EPSP level. Because the increase in synaptic transmission normally observed during DPMA perfusion was occluded by LTP, this suggests that the mechanisms that contribute to both types of increases in synaptic transmission share a common mechanism and that the critical period for LTP modulation is during induction.

In both the dentate gyrus and CA1 neurons, cAMP levels are elevated immediately after tetanus but not 30 min after (Chetkovich et al. 1991; Stanton and Sarvey 1992). Although A2 receptors modulate the level of LTP expression in a cAMP-dependent manner, bath application of forskolin, the direct activator of adenylate cyclase, does not alter the level of LTP expression when applied several minutes after tetanic stimulation (Pockett et al. 1993). Assuming that the level of expression is determined by cAMP accumulated immediately after tetanus, then activation of A2 receptors several minutes posttetanus should not alter the expression of LTP. To test for this, LTP was induced in control ACSF, and the potentiated EPSP allowed to stabilize. A2 receptor activation after addition of DPMA did not result in a significant change in the level of LTP expression when compared with control conditions within the same time period (172 ± 25% in DPMA vs. 180 ± 9% in ACSF; P = 0.763) (Fig. 3C). In control experiments, it took at least three separate tetani to saturate the level of LTP (n = 8), indicating that the lack of response of DPMA was not due to prior saturation of synaptic potentiation. Because the increase in synaptic transmission observed during DPMA superfusion (Figs. 1 and 2A) is occluded by LTP expression (Fig. 3C), these results indicate that the mechanisms that contribute to both types of synaptic enhancement are convergent.

A2 activation induces an NMDA-independent LTP

NMDA-receptor activation is correlated with increases in intracellular cAMP (Chetkovich et al. 1991), which is required for both the early and late phases of LTP (Blitzer et al. 1995; Frey et al. 1993). We addressed the question of whether activation of A2 receptors could induce NMDA-independent LTP given their stimulatory influence on cAMP and VSCCs. Consistent with previous reports, the NMDA channel receptor antagonist AP5 (50 µM) completely blocked induction of tetanus induced LTP (Fig. 4A). However, when A2 receptors already were activated by DPMA for 10 min (Fig. 4B), tetanic stimulation in the presence of AP5 resulted in a slowly developing potentiation, during a 10- to 20-min period, which attained a level of 231 ± 13%(n = 9). This form of LTP was not accompanied by posttetanic potentiation (PTP). Although NMDA receptor-ion channels are important for the induction of tetanus-induced LTP in CA1, our results suggest that alternative routes of Ca2+ influx or cAMP upregulation in postsynaptic dendrites may be sufficient to induce this novel form of LTP.


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FIG. 4. Activation of A2 receptors prior to tetanic stimulation induces an AP5-resistant LTP. A: consistent with previous reports, activation of NMDA receptors is critical for the induction of tetanus-induced LTP in area CA1. A single tetanus was delivered to each slice in the presence of AP5 (50 µM). Using this protocol, LTP was blocked completely for the duration of the experiment. Bi: in contrast, LTP could be induced during coperfusion of AP5 and DPMA. Tetanic stimulation resulted in an AP5-insensitive LTP that developed during a 10- to 20-min period to a final level of potentiation of 231 ± 13% (n = 9). This novel form of LTP persisted for >= 40 min and lacked posttetanic potentiation (PTP), which is believed to be a result of an increase in transmitter release. Because the actions of DPMA appear to be postsynaptic (Fig. 2), both the induction and expression of this AP5-insensitive LTP have a postsynaptic locus involving selective enhancement of the AMPA-receptor response to stimulation. Bii: extracellular recordings from CA1 showing the changes in the size of the EPSP after different drug treatments and after tetanus.

A2-mediated NMDA-independent LTP depends partly on Ca influx through VSCC and is convergent with NMDA-dependent LTP

L-type Ca channels that are blocked by the dihydropyridine, nifedipine, have been shown to contribute to NMDA-independent LTP (Grover and Teyler 1990). These VSCCs are present on CA1 neurons (Cortes et al. 1983) and could serve as alternative routes of Ca2+ influx (Jones and Heinemann 1987; Regehr et al. 1989). We therefore examined the possible involvement of these channels in A2-receptor-induced LTP. In preliminary experiments, nifedipine (10 µM) had no effect on the control EPSP (data not shown). When nifedipine and DPMA (20 nM) were added to the perfusion medium, the level of NMDA-independent potentiation obtained after tetanic stimulation in AP5 (Fig. 5) was reduced significantly to 157 ± 3.7% (P < 0.001).


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FIG. 5. AP5-resistant LTP was reduced by inclusion of the dihydropyridine Ca2+ channel blocker nifedipine. A: because nifedipine-sensitive L-type Ca2+ channels have been shown to reduce the level of NMDA-independent LTP, a role of these ion channels in A2-mediated LTP was tested. In separate experiments, nifedipine (10 µM) did not have any effect on either the slope or amplitude of the EPSP in control ACSF. Slices were superfused with ACSF containing both DPMA and nifedipine for 10 min to establish a baseline of stable synaptic responses. Under these conditions, the inclusion of nifedipine in the perfusion medium with AP5 (50 µM) in the perfusing medium, standard tetanus reduced the level of potentiation to 157 ± 3.7% (n = 6). B: typical field EPSPs during exposure to different extracellular reagents.

Because nifedipine did not completely block induction of NMDA-independent LTP, it is possible that either alternate routes of Ca2+ influx contribute to this form of LTP or other factors besides Ca2+ influx are critical for its induction. Although this novel form of LTP and NMDA-dependent LTP appear to use different routes for Ca2+ entry into CA1 neurons, the question remains whether these Ca2+ currents initiate a common pathway leading to LTP. If the two forms of LTP share a common pathway, then one might expect that saturation of one process would occlude the other. To test this possibility, NMDA-dependent LTP was first brought to near-saturation in ACSF by giving three high-frequency tetani each separated by 10 min. Twenty minutes after the third tetanus, slices were perfused with buffer containing both DPMA and AP5. The saturation of NMDA-dependent LTP significantly reduced (111 ± 1.4%; P = 0.001; Mann-Whitney Rank Sum test) induction of NMDA-independent LTP (Fig. 6), suggesting that the two forms of LTP interact either via the convergence of prior parallel pathways or that the two forms proceed along identical routes.


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FIG. 6. A2-induced NMDA-independentL T P  s h a r e s  a  c o m m o n  p a t h w a y  w i t hNMDA-dependent LTP. A: 3 consecutive tetani spaced 10 min apart achieve a nearly saturating level of NMDA-dependent LTP. Twenty minutes after the third tetanus, slices were perfused with buffer containing both AP5 and DPMA for 15 min. A 4th tetanus-induced LTP (111 ± 1.4%), which was reduced significantly compared with Fig. 4B. B: final 55 min of A in expanded scale. LTP in control buffer was near-saturated by administering 3 tetani, 10 min apart. C: extracellular field potentials during experimental protocol as indicated.

    DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References

The experiments reported here demonstrate that adenosine A2 receptors increase synaptic transmission through selective modulation of the postsynaptic AMPA-receptor-mediated response. Activation of A2 receptors during the induction of LTP enhances its expression, however, activation 20 min after inducing LTP has no significant effect. This is consistent with previous findings that changes in cAMP levels in response to high-frequency tetanus may determine the final level of LTP expression and that this rise is most critical during or shortly after the initiation of the induction process. The loci and mechanisms that contribute to the expression of LTP have been the subject of intense debate (Bliss and Collingridge 1993; Nicoll and Malenka 1995). Recent evidence does suggest a postsynaptic mechanism involving either an increase in the number of functional AMPA receptors (Isaac et al. 1995; Liao et al. 1995) or a change in the phosphorylation state of these receptors (Greengard et al. 1991; Wang et al. 1993). Because the site of action of A2 receptors appears to be postsynaptic, our data supports a postsynaptic site for the long-term modulation underlying LTP expression. Furthermore, the mechanisms that are involved in expression of LTP appear to result in an increase in the AMPA-receptor-mediated synaptic response, consistent with Liao et al. (1995), and similar in form to metabotropic receptor-mediated LTP (Bortolleto and Collingridge 1995). With these experiments, however, we cannot distinguish between an increase in either the sensitivity or number of functional AMPA receptors at each synapse.

A2 receptor activation during a tetanus increased the level of LTP; however, A2 activation several minutes after the tetanizing protocol had no effect; this is consistent with the hypothesis that the level of expression may be determined during or immediately after the tetanization process. This finding also further explains the result that A2 antagonism during a tetanus blocked or significantly reduced LTP induction but not expression (Mogul et al. 1994; Sekino et al. 1992). We also show that A2 receptors induce a NMDA-independent tetanus-induced LTP that shares a common pathway with NMDA receptors and is dependent, at least in part, on Ca2+ influx through nifedipine sensitive VSCCs.

Because the concentration of adenosine released during stimulation depends on the frequency of neuronal firing, stimulation at 100 Hz would be expected to greatly increase the extracellular concentration of adenosine, enough perhaps to activate A2 receptors and subsequently elicit LTP in the presence of NMDA-receptor blockade. If this is the case, then the question arises as to why there is the need to prime A2 receptors with DPMA to induce NMDA-independent LTP. A possible rationale is that, in addition to being rapidly degraded by ectonucleotidases present in the synaptic region, adenosine has a low affinity for A2b receptors, the probable A2 receptor subtype involved here. Thus either not enough A2 receptors are activated to be able to induce LTP during perfusion of AP5 alone or the effect of A1 receptor-induced inhibition of cellular excitability may predominate. Because the process of synaptic plasticity in the hippocampus appears to involve a complex series of biochemical responses, including metabotropic receptor activation (Bortolotto and Collingridge 1993), a number of factors modulate the process of LTP induction that is dominated by NMDA activation. Thus because adenosine is found in synapses throughout the CNS and its extracellular concentration is a function of electrical and metabolic activity, this purine may play a critical role in the modulation of neurotransmission by differential activation of the various adenosine receptors, and these effects may vary throughout the brain partly as a consequence of differential adenosine receptor localization.

    FOOTNOTES

  Address for reprint requests: D. J. Mogul, Dept. of Biomedical Engineering, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208.

  Received 30 September 1996; accepted in final form 9 June 1997.

    REFERENCES
Abstract
Introduction
Methods
Results
Discussion
References

0022-3077/97 $5.00 Copyright ©1997 The American Physiological Society