Ethanol differentially enhances hippocampal GABA A receptor-mediated responses in protein kinase C gamma (PKC gamma) and PKC epsilon null mice.

Ethanol intoxication results partly from actions of ethanol at specific ligand-gated ion channels. One such channel is the GABA(A) receptor complex, although ethanol's effects on GABA(A) receptors are variable. For example, we found that hippocampal neurons from selectively bred mice and rats with high hypnotic sensitivity to ethanol have increased GABA(A) receptor-mediated synaptic responses during acute ethanol treatment compared with mice and rats that display low behavioral sensitivity to ethanol. Here we investigate whether specific protein kinase C (PKC) isozymes modulate hypnotic and GABA(A) receptor sensitivity to ethanol. We examined acute effects of ethanol on GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) in mice lacking either PKCgamma (PKCgamma(-/-)) or PKCepsilon (PKCepsilon(-/-)) isozymes and compared the results to those from corresponding wild-type littermates (PKCgamma(+/+) and PKCepsilon(+/+)). GABA(A) receptor-mediated IPSCs were evoked in CA1 pyramidal neurons by electrical stimulation in stratum pyramidale, and the responses were recorded in voltage-clamp mode using whole-cell patch recording techniques. Ethanol (80 mM) enhanced the IPSC response amplitude and area in PKCgamma(+/+) mice, but not in the PKCgamma(-/-) mice. In contrast, ethanol markedly potentiated IPSCs in the PKCepsilon(-/-) mice, but not in PKCepsilon(+/+) littermates. There was a positive correlation between ethanol potentiation of IPSCs and the ethanol-induced loss of righting reflex such that mice with larger ethanol-induced increases in GABA(A) receptor-mediated IPSCs also had higher hypnotic sensitivity to ethanol. These results suggest that PKCgamma and PKCepsilon signaling pathways reciprocally modulate both ethanol enhancement of GABA(A) receptor function and hypnotic sensitivity to ethanol.

alpha7 nicotinic acetylcholine receptors on GABAergic interneurons evoke dendritic and somatic inhibition of hippocampal neurons.

GABAergic interneurons in the hippocampus express high levels of alpha7 nicotinic acetylcholine receptors, but because of the diverse roles played by hippocampal interneurons, the impact of activation of these receptors on hippocampal output neurons (i.e., CA1 pyramidal cells) is unclear. Activation of hippocampal interneurons could directly inhibit pyramidal neuron activity but could also produce inhibition of other GABAergic cells leading to disinhibition of pyramidal cells. To characterize the inhibitory circuits activated by these receptors, exogenous acetylcholine was applied directly to CA1 interneurons in hippocampal slices, and the resulting postsynaptic responses were recorded from pyramidal neurons or interneurons. Inhibitory currents mediated by GABA(A) receptors were observed in 27/131 interneuron/pyramidal cell pairs, but no instances of disinhibition of spontaneous inhibitory events or GABA(B) receptor-mediated responses were observed. Two populations of bicuculline-sensitive GABA(A) receptor-mediated currents could be distinguished based on their kinetics and amplitude. Anatomical reconstructions of the interneurons in a subset of connected pairs support the hypothesis that these two populations correspond to inhibitory synapses located either on the somata or dendrites of pyramidal cells. In 11 interneuron/interneuron cell pairs, one presynaptic neuron was observed that produced strong inhibitory currents in several nearby interneurons, suggesting that disinhibition of pyramidal neurons may also occur. All three types of inhibitory responses (somatic-pyramidal, dendritic-pyramidal, and interneuronal) were blocked by the alpha7 receptor-selective antagonist methyllycaconitine. These data suggest activation of these functionally distinct circuits by alpha7 receptors results in significant inhibition of both hippocampal pyramidal neurons as well as interneurons.

Regulation of the activity of hippocampal stratum oriens interneurons by alpha7 nicotinic acetylcholine receptors.

GABAergic interneurons have been shown to be a major target of cholinergic inputs to the hippocampus. Because these interneurons project to pyramidal neurons as well as other interneurons, activation of the cholinergic system is likely to produce a complex modulation of local inhibitory activity. To better understand the role of post-synaptic alpha7 nicotinic acetylcholine receptors in the hippocampus, we have characterized the effects of nicotinic agents on local interneurons of the rat CA1 stratum oriens in terms of activation, desensitization, and region of axonal termination. Fast application of acetylcholine onto stratum oriens interneurons during whole-cell recordings from hippocampal slices activated the majority of cells tested, and these responses were mediated almost entirely by alpha7 nicotinic acetylcholine receptors. Anatomical reconstructions showed no clear relationship between the acetylcholine responsivity of interneurons and the regions to which their axons project. Currents mediated by alpha7 receptors declined markedly during repetitive activation in the theta rhythm range (4-12 Hz) when activated by either pressure application or synaptic release of acetylcholine. However, the decay of alpha7 receptor-mediated currents was unaffected by treatment with the cholinesterase inhibitor neostigmine (10 nM-10 microM), suggesting that hydrolysis of acetylcholine is not a rate-limiting step in the termination of these responses. From these findings we suggest that nicotinic receptor activity in this region has an extensive and complex impact on local inhibitory circuits that is mediated by activation of several classes of intrinsic GABAergic cells. In addition, desensitization of the alpha7 nicotinic acetylcholine receptor is likely to contribute to the decay of individual responses to pressure application of agonist, and may also act in a cumulative fashion to impair the ability of these receptors to support repetitive activity during trains of activation. If applicable to alpha7 receptor responses in vivo, we suggest it may be difficult to enhance these responses for therapeutic purposes with cholinesterase inhibitors.

Hyperalgesia, anxiety, and decreased hypoxic neuroprotection in mice lacking the adenosine A1 receptor.

Caffeine is believed to act by blocking adenosine A(1) and A(2A) receptors (A(1)R, A(2A)R), indicating that some A(1) receptors are tonically activated. We generated mice with a targeted disruption of the second coding exon of the A(1)R (A(1)R(-/-)). These animals bred and gained weight normally and had a normal heart rate, blood pressure, and body temperature. In most behavioral tests they were similar to A(1)R(+/+) mice, but A(1)R(-/-) mice showed signs of increased anxiety. Electrophysiological recordings from hippocampal slices revealed that both adenosine-mediated inhibition and theophylline-mediated augmentation of excitatory glutamatergic neurotransmission were abolished in A(1)R(-/-) mice. In A(1)R(+/-) mice the potency of adenosine was halved, as was the number of A(1)R. In A(1)R(-/-) mice, the analgesic effect of intrathecal adenosine was lost, and thermal hyperalgesia was observed, but the analgesic effect of morphine was intact. The decrease in neuronal activity upon hypoxia was reduced both in hippocampal slices and in brainstem, and functional recovery after hypoxia was attenuated. Thus A(1)Rs do not play an essential role during development, and although they significantly influence synaptic activity, they play a nonessential role in normal physiology. However, under pathophysiological conditions, including noxious stimulation and oxygen deficiency, they are important.

Changes in hippocampal adenosine efflux, ATP levels, and synaptic transmission induced by increased temperature.

Previous studies have demonstrated that when the temperature of hippocampal brain slices is increased, there is a corresponding depression of synaptic potentials mediated by an increased activation of presynaptic adenosine A(1) receptors. The present experiments demonstrate that when the temperature of hippocampal slices is raised from 32.5 degrees C to either 38.5 degrees C or 40.0 degrees C there is a marked, temperature-dependent increase in the efflux of endogenous adenosine and a corresponding decrease in excitatory synaptic responses. The increase in efflux is rapidly reversible on lowering the slice temperature and the temperature-induced efflux is repeatable. Control experiments suggest that this increased efflux of adenosine is not the result of hypoxia or ischemia secondary to a temperature-induced increase in the metabolic rate of the slice. The increase in adenosine efflux was not accompanied by any significant change in the ATP levels in the brain slice, whereas a hypoxic stimulus sufficient to produce a comparable depression of excitatory transmission produced an approximately 75% decrease in ATP levels. These experiments indicate that changes in brain slice temperature can alter purine metabolism in such a way as to increase the adenosine concentration in the extracellular space, as well as adenosine efflux from hippocampal slices, in the absence of significant changes in ATP levels.

The role and regulation of adenosine in the central nervous system.

Adenosine is a modulator that has a pervasive and generally inhibitory effect on neuronal activity. Tonic activation of adenosine receptors by adenosine that is normally present in the extracellular space in brain tissue leads to inhibitory effects that appear to be mediated by both adenosine A1 and A2A receptors. Relief from this tonic inhibition by receptor antagonists such as caffeine accounts for the excitatory actions of these agents. Characterization of the effects of adenosine receptor agonists and antagonists has led to numerous hypotheses concerning the role of this nucleoside. Previous work has established a role for adenosine in a diverse array of neural phenomena, which include regulation of sleep and the level of arousal, neuroprotection, regulation of seizure susceptibility, locomotor effects, analgesia, mediation of the effects of ethanol, and chronic drug use.

Correlation between molecular volume and effects of n-alcohols on human neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes.

Nicotinic acetylcholine receptors (nAChRs) are neurotransmitter-gated ion channels and like most such channels, ethanol and longer chain alcohols modulate their activity. In the present studies, the effects of alcohols were characterized on defined combinations of human neuronal nAChR subunits heterologously expressed in Xenopus oocytes. Short-chain alcohols, such as ethanol, propanol, and butanol potentiated ACh-induced currents in both alpha(2)beta(4) and alpha(4)beta(4) nAChRs. Longer chain alcohols, however, inhibited these receptor subtypes. Small increases in alcohol chain length were sufficient to produce a "crossover" from potentiation to inhibition. For the alpha(2)beta(4) receptor subunit combination, butanol clearly potentiated while pentanol inhibited ACh-induced current, whereas for alpha(4)beta(4) nAChR, propanol potentiated, butanol had no discernable effect, and pentanol inhibited receptor function. Fluorinated analogs of ethanol, propanol, and butanol were used to determine whether the effects of the alcohols were dependent upon chain length or whether another related attribute, such as molecular volume, was the defining characteristic. The experimental results support the hypothesis that for both alpha(2)beta(4) and alpha(4)beta(4) receptor subtypes, molecular volume appears to be the most important determinant of both the potency as well as the direction of modulation of nAChR function by n-alcohols and related compounds. Although it has been suggested that the inhibitory and facilitatory effects of alcohols are mediated by actions at different sites on the receptor molecule, the present data suggest the possibility that there may be a single site of alcohol action and that the nature of this action is dependent upon the physical properties of the molecule.

Electrophysiological analysis of G protein-coupled receptors in mammalian neurons.

This unit describes general techniques that are useful for recording electrophysiological responses that are mediated via the activation of G-protein coupled receptors (GPCRs). It includes a brief description of preparations, but focuses primarily on experiments using hippocampal brain slice preparations. Techniques for the preparation of brain slices, electrodes, filling solutions, and the recording protocols that are suitable for assessing the activity of GPCRs using electrophysiological techniques are summarized, and various protocols for the activation of these receptors are discussed.

A transient increase in temperature induces persistent potentiation of synaptic transmission in rat hippocampal slices.

Previous studies have shown that increasing the temperature of rat hippocampal brain slices from 32.5 to 38.5 degrees C initiates a profound, adenosine-mediated decrease in excitatory synaptic transmission in the CA1 region. Here we found that upon lowering the temperature back to 32.5 degrees C, the amplitude of the field excitatory postsynaptic potential often recovers to a level that is significantly potentiated with respect to the initial baseline. This potentiation is rapid in onset (< 5min following return to 32.5 degrees C) and long lasting (>60min following the termination of the increase in temperature). Similar effects could not be induced by superfusion with adenosine alone, and adenosine receptor antagonists did not block the potentiation. Therefore, although an adenosine-mediated decrease in excitatory synaptic transmission occurs during the temperature increase, it is unrelated to the potentiation. Likewise, N-methyl-D-aspartate receptor activation is not required, as N-methyl-D-aspartate receptor antagonists do not influence this form of potentiation. In summary, we propose that transiently increasing brain slice temperature represents a novel way to induce synaptic plasticity in the hippocampus, and may provide a paradigm to elucidate additional cellular mechanisms involved in functional plasticity.

The in vitro ethanol sensitivity of hippocampal synaptic gamma-aminobutyric acid(A) responses differs in lines of mice and rats genetically selected for behavioral sensitivity or insensitivity to ethanol.

Previous work has demonstrated that in the hippocampal CA1 region of Sprague-Dawley rats, there are ethanol-sensitive and ethanol-insensitive populations of GABAergic synapses on pyramidal neurons. The present experiments characterized the ethanol sensitivity of these pathways in lines of rats and mice genetically selected for sensitivity or insensitivity to the behavioral effects of ethanol. In ethanol-sensitive inbred long sleep mice, GABA(A) IPSCs induced by stimulation of proximal (probably somatic) synapses were enhanced by 80 mM ethanol, whereas the distal (i.e., dendritic) pathway was unaffected. Thus, the relative sensitivity of these pathways (proximal > distal) is the same in both Sprague-Dawley rats and in inbred long sleep mice. However, in the ethanol-insensitive inbred short sleep mice, neither proximal nor distal IPSCs were affected by 80 mM ethanol. The ethanol sensitivity of the proximal pathway was also examined in replicate lines of rats selected for either high ethanol sensitivity or low ethanol sensitivity. GABA(A) IPSCs in the high ethanol sensitivity lines were significantly enhanced by 80 mM ethanol, whereas IPSCs in the low ethanol sensitivity lines were unaffected. Thus, IPSCs evoked via the proximal pathway were enhanced by ethanol in all the sensitive mouse and rat lines, and unaffected in all the insensitive lines. These experiments demonstrate that GABA(A) synapses in brain differ in their sensitivity to enhancement by ethanol, and the sensitivity to such enhancement is under the control of genes that can be selected for using classical genetic selective breeding based on a behavioral phenotype.

Adenosine receptor antagonists induce persistent bursting in the rat hippocampal CA3 region via an NMDA receptor-dependent mechanism.

Adenosine receptor antagonists initiate repetitive bursting activity in the CA3 region of hippocampal slices. Although some studies have suggested that this effect is irreversible, this has been difficult to establish because many adenosine antagonists wash out of brain slices extremely slowly. Furthermore the cellular mechanism that underlies persistent bursting is unknown. To resolve these issues, we studied the effects of nonselective (8-p-sulfophenyltheophylline, 8SPT, 50-100 microM), A(l)-selective (8-cyclopentyl-1, 3-dipropylxanthine, 100 nM; xanthine carboxylic acid congener, 200 nM), and A(2A)-selective (chlorostyryl-caffeine; 200 nM) adenosine antagonists in the CA3 region of rat hippocampal slices using extracellular recording. Superfusion with all of the adenosine antagonists except chlorostyryl-caffeine induced bursting, and the burst frequency after 30 min drug superfusion did not differ for the different antagonists. Most slices showed a period of rapid initial bursting, followed either by stable bursting at a lower frequency or a pattern of oscillating burst frequency. In either case, the bursting continued after drug washout. Virtually identical patterns of long-term bursting activity were observed when 8SPT was washed out or applied continuously. Control experiments using exogenous adenosine to characterize the persistence of 8SPT in tissue demonstrated >95% washout at 60 min, a time when nearly all slices still showed regular bursting activity. When the N-methyl-D-aspartate (NMDA) antagonists DL-2-amino-5-phosphonovaleric acid (AP5; 50 microM) or dizocilpine (10 microM) were applied before and during 8SPT superfusion, bursting occurred in the presence of the NMDA antagonists but did not persist once the 8SPT was washed out. AP5 had no effect on persistent bursting when applied after the initiation of spiking. The selective calcium/calmodulin-dependent protein kinase inhibitor 1-[N, O-bis-(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62; 3 microM), which has been shown to block NMDA receptor-dependent synaptic plasticity in the CA1 region, also significantly decreased the long-term effect of 8SPT. Thus adenosine antagonists initiate persistent spiking in the CA3 region; this activity does not depend on continued occupation of adenosine receptors by antagonists, and can be blocked by treatments that prevent NMDA receptor-dependent plasticity.

Regulation of extracellular adenosine in rat hippocampal slices is temperature dependent: role of adenosine transporters.

While a great deal is known about stimuli that can induce the release of adenosine from brain tissue, relatively little is known about the regulation of the basal extracellular concentration of adenosine that is present in the absence of stimulation. Under normal conditions, enough adenosine is present to tonically activate a significant portion of the high-affinity adenosine A1 receptors. The present experiments demonstrated that the estimated basal concentration of extracellular adenosine in rat hippocampal slices maintained at 21 degrees C (430 nM) is approximately twice that at 32 degrees C (220 nM). The sensitivity of presynaptic modulatory adenosine A1 receptors was not significantly different at 21 degrees C or at 32 degrees C. Slices maintained at 21 degrees C also showed a reduced ability to inactivate extracellular adenosine, which reflects a reduction in adenosine transport across cell membranes. This effect appears to be primarily due to a reduction in the function of the equilibrative, dipyridamole-sensitive (ei) adenosine transporter; the nitrobenzylthioinosine-sensitive equilibrative transporter (es transporter) appears to be relatively less affected by temperature than is the ei transporter. These experiments demonstrate that extracellular concentrations of adenosine in the brain are sensitive to temperature, and suggest that some of the neurological effects of hypothermia might be mediated via increased concentrations of adenosine in the extracellular space.

N-methyl-D-aspartate receptor responses are differentially modulated by noncompetitive receptor antagonists and ethanol in inbred long-sleep and short-sleep mice: behavior and electrophysiology.

BACKGROUND: Short-sleep (SS) mice exhibit higher locomotor activity than do long-sleep (LS) mice when injected with low doses of ethanol or the noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist MK-801 (dizocilpine). SS mice also have higher densities of brain NMDARs. However, two strains of LS X SS recombinant inbred (RI) mice also show differential activation to ethanol and MK-801, but have similar numbers of NMDARs. Here we used inbred LS (ILS) and SS (ISS) mice to investigate further the relationship between NMDARs and sensitivity to the stimulant effects of low doses of ethanol. METHODS: Open field activity and spontaneous alternations were measured after saline or drug injection. [3H]MK-801 binding parameters were determined in hippocampus, cortex, striatum, and nucleus accumbens. Extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded in the CA1 region of hippocampal slices. RESULTS: Systemic injection of either ethanol or MK-801 increased locomotor activity to a greater extent in ISS mice than in ILS mice. The competitive NMDAR antagonist 2-carboxypiperazin-4-yl-propyl-1-lphosphonic acid (+/- CPP) depressed activity of ILS, but not ISS, mice. No strain differences were observed in spontaneous alternations or in the number or affinity of NMDARs in the brain regions examined. Likewise, the magnitudes of hippocampal NMDAR-mediated fEPSPs were similar in ILS and ISS mice and were inhibited to the same extent by a competitive NMDAR antagonist. However, both ethanol and the NMDAR NR2B receptor antagonist ifenprodil inhibited the late component of hippocampal NMDAR fEPSPs to a greater extent in ISS, than in ILS, mice. CONCLUSIONS: Differential ethanol- and MK-801-induced behavioral activation in ILS and ISS mice was not associated with differences in NMDAR number. Nonetheless, pharmacological differences in hippocampal NMDAR responsiveness suggest that ISS mice express NMDARs that have a greater sensitivity to noncompetitive, but not competitive, NMDAR antagonists. These differences, which may reflect differences in NMDAR subunit composition, could underlie the differential responsiveness to low doses of ethanol in ILS and ISS mice.

Acute peroxide treatment of rat hippocampal slices induces adenosine-mediated inhibition of excitatory transmission in area CA1.

Brief exposure to conditions that generate free radicals inhibits synaptic transmission in hippocampal slices, most likely via a presynaptic mechanism. Because other physiologically stressful conditions that generate free radicals, such as hypoxia or ischemia, stimulate the release of adenosine from brain slices, we determined whether increases in extracellular adenosine mediate the presynaptic inhibition of excitatory transmission induced by peroxide treatment. Simultaneous addition of hydrogen peroxide (0.01%) and ferrous sulfate (100 microM) resulted in a >80% decrease in synaptic potentials recorded in the CA1 region of hippocampal slices of adult male rats. Treatment with theophylline (200 microM), a non-selective adenosine receptor antagonist, or 8-cyclopentyl-1,3-dipropylxanthine (100 nM), a selective adenosine A1 receptor antagonist, prior to and during hydrogen peroxide superfusion prevented the inhibition. These results demonstrate that acute exposure to hydrogen peroxide induces an adenosine-mediated decrease in synaptic transmission in hippocampal slices.

Allosteric control of gating and kinetics at P2X(4) receptor channels.

The CNS abundantly expresses P2X receptor channels for ATP; of these the most widespread in the brain is the P2X(4) channel. We show that ivermectin (IVM) is a specific positive allosteric effector of heterologously expressed P2X(4) and possibly of heteromeric P2X(4)/P2X(6) channels, but not of P2X(2), P2X(3), P2X(2)/P2X(3,) or P2X(7) channels. In the submicromolar range (EC(50,) approximately 250 nM) the action of IVM was rapid and reversible, resulting in increased amplitude and slowed deactivation of P2X(4) channel currents evoked by ATP. IVM also markedly increased the potency of ATP and that of the normally low-potency agonist alpha, beta-methylene-ATP in a use- and voltage-independent manner without changing the ion selectivity of P2X(4) channels. Therefore, IVM evokes a potent pharmacological gain-of-function phenotype that is specific for P2X(4) channels. We also tested whether IVM could modulate endogenously expressed P2X channels in the adult trigeminal mesencephalic nucleus and hippocampal CA1 neurons. Surprisingly, IVM produced no significant effect on the fast ATP-evoked inward currents in either type of neuron, despite the fact that IVM modulated P2X(4) channels heterologously expressed in embryonic hippocampal neurons. These results suggest that homomeric P2X(4) channels are not the primary subtype of P2X receptor in the adult trigeminal mesencephalic nucleus and in hippocampal CA1 neurons.

Ethanol inhibition of synaptically evoked kainate responses in rat hippocampal CA3 pyramidal neurons.

Many studies have demonstrated that intoxicating concentrations of ethanol (10-100 mM) can selectively inhibit the component of glutamatergic synaptic transmission mediated by N-methyl-D-aspartate (NMDA) receptors while having little or no effect on excitatory synaptic transmission mediated by non-NMDA receptors [i.e., alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and/or kainate (KA) receptors]. However, until the recent development of highly selective AMPA receptor antagonists, it was not possible to assess the relative contribution of AMPA and KA receptors to non-NMDA receptor-mediated synaptic transmission or to determine whether these glutamate receptor subtypes differed in their sensitivity to ethanol. In the present experiments, we used the highly selective AMPA receptor antagonist LY 303070 to pharmacologically isolate KA receptor-mediated excitatory postsynaptic currents (EPSCs) in rat hippocampal CA3 pyramidal neurons and tested their sensitivity to ethanol. Concentrations of ethanol as low as 20 mM significantly and reversibly depressed KA EPSCs. Ethanol also inhibited KA currents evoked by direct pressure application of KA in the presence of LY 303070, suggesting that this inhibition was mediated by a postsynaptic action. In contrast, ethanol had no effect on AMPA EPSCs in these cells, even at the highest concentration tested (80 mM). Ethanol significantly inhibited NMDA EPSCs in these neurons, but these responses were less sensitive to ethanol than KA EPSCs. These results suggest that in addition to its well-described depressant effect on NMDA receptor-mediated synaptic transmission, ethanol has an even greater inhibitory effect on glutamatergic synaptic transmission mediated by KA receptors in rat hippocampal CA3 pyramidal neurons.

NMDA and non-NMDA receptors mediate responses in the primary gustatory nucleus in goldfish.

Primary gustatory afferents from the oropharynx of the goldfish, Carassius auratus, terminate in the vagal lobe, a laminated structure in the dorsal medulla comparable to the gustatory portion of the nucleus of the solitary tract in mammals. We utilized an in vitro brain slice preparation to test the role of different ionotropic glutamate receptor subtypes in synaptic transmission of gustatory information by recording changes in field potentials after application of various glutamate receptor antagonists. Electrical stimulation of the vagus nerve (NX) evokes two short-latency postsynaptic field potentials from sensory layers of the vagal lobe. 6,7-Dinitroquinoxaline-2,3-dione and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione, two non-N-methyl-D-aspartate (NMDA) ionotropic receptor antagonists, blocked these short-latency potentials. Slower potentials that were revealed under Mg2+ -free conditions, were abolished by the NMDA receptor antagonist, D(-)-2-amino-5-phosphonovaleric acid (APV). Repetitive stimulation produced short-term facilitation, which was attenuated by application of APV. These results indicate that the synaptic responses in the vagal lobe produced by stimulation of the gustatory roots of the NX involve both NMDA and non-NMDA receptors. An NMDA receptor-mediated facilitation may serve to amplify incoming bursts of primary afferent activity.

Temperature-dependent modulation of excitatory transmission in hippocampal slices is mediated by extracellular adenosine.

Although extracellular adenosine concentrations in brain are increased markedly by a variety of stimuli such as hypoxia and ischemia, it has been difficult to demonstrate large increases in adenosine with stimuli that do not result in pathological tissue damage. The present studies demonstrate that increasing the temperature at which rat hippocampal brain slices are maintained (typically from 32.5 to 38.5 degrees C) markedly inhibits excitatory synaptic transmission. This effect was reversible on cooling, readily repeatable, and was blocked by A1 receptor antagonists and by adenosine deaminase, suggesting that it was mediated by increased activation of presynaptic adenosine A1 receptors by endogenous adenosine. This increase in adenosinergic inhibition was not a response to hyperthermia per se, because it could be elicited by temperatures that remained entirely within the hypothermic range (e. g., from 32.5 to 35.5 degrees C). The increased activity at A1 receptors appeared to be attributable to the direct release of adenosine via nucleoside transporters; the release of adenine nucleotides, linked to either the activation of NMDA receptors or the increased efflux of cAMP, appeared not to be involved. These results suggest that changes in brain temperature can alter the regulation of extracellular adenosine in rat brain slices and that increased adenosine release may be an important regulatory mechanism for countering increased excitability consequent to increased brain temperature.