An antidepressant-induced decrease in the responsiveness of hippocampal neurons to group I metabotropic glutamate receptor activation

Imipramine, a serotonin and noradrenaline uptake inhibitor, is the prototypical tricyclic antidepressant. The effects of imipramine on neuronal responsiveness to the group I glutamate metabotropic (mGlu) receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) were studied ex vivo, in the CA1 area of rat hippocampus, using extracellular and intracellular recording. DHPG increased the population spike amplitude, depolarized CA1 cells and decreased the slow afterhyperpolarization. Imipramine (20 microM) administered acutely in vitro did not change the effect of DHPG on population spikes. Repeated treatment with imipramine (10 mg/kg, twice daily, for 14 days) significantly attenuated the enhancing effect of DHPG (2.5 and 5 microM) on population spikes, as well as the DHPG-induced depolarization and the decrease in the slow afterhyperpolarization. Repeated treatment with imipramine had no effect on passive or active membrane properties of CA1 pyramidal cells. The results of the time-course experiment demonstrated that the imipramine-induced decrease in the responsiveness of CA1 cells to DHPG was apparent after a 7-day treatment; there was a further decrease after 14 days of treatment to a level which was not changed by longer (21-day) administration of imipramine. The attenuation of neuronal responsiveness to DHPG induced by a 14-day treatment was still detectable 7 days after imipramine withdrawal. It is concluded that repeated treatment with imipramine induces a decrease in the responsiveness of rat CA1 hippocampal neurons to group I mGlu receptor activation with a time course which correlates with the delayed onset of the therapeutic effect of antidepressants in humans. This suggests that alterations in mGlu receptors may contribute to antidepressant efficacy.     

Nitric oxide mediates interactions between GABAA receptors and adenosine A1 receptors in the rat hippocampus

Adenosine and gamma-aminobutyric acid (GABA) are both major inhibitory neuromodulators/neurotransmitters in the CNS. We now investigated if endogenous GABA modulates adenosine A(1)-mediated action on synaptic transmission in the hippocampus. Field excitatory postsynaptic potentials (fEPSP) were recorded from the CA(1) area of rat hippocampal slices. The adenosine analogue 2-chloroadenosine (0.15-1 microM) inhibited synaptic transmission with an EC(50) of 398 nM. Blocking GABA(A) receptors with the specific antagonists, bicuculline (10 microM) or picrotoxin (10 microM) potentiated the inhibitory effect of 2-chloroadenosine. The concentration-response curve for 2-chloroadenosine was displaced to the left by a factor of 2 (EC(50)=210 nM) in the presence of bicuculline (10 microM). GABA(A) receptor blockade also potentiated the action of N(6)-cyclopentyladenosine (CPA, 10 nM), a specific adenosine A(1) receptor agonist. Prevention of adenosine accumulation with adenosine deaminase (1 U/ml) did not influence bicuculline-induced potentiation of the effect of 2-chloroadenosine. The potentiation of adenosine A(1)-mediated response by bicuculline was abolished when nitric oxide (NO) synthase was inhibited with nitroarginine (100 microM), and when guanylyl cyclase was inhibited with 1H-[1,2,4]Oxadiazolo[4,3-a] quinoxalin-1-one (ODQ, 20 microM). The NO donors, (+/-)-S-nitroso-N-acetylpencillamine (SNAP, 300 microM) and diethylamine NONate diethylammonium salt (DEA/NO, 100 microM), significantly enhanced the inhibitory action of 2-chloroadenosine (150 nM). It is concluded that the blockade of GABA(A) receptors induces a potentiation of adenosine A(1) receptor-mediated inhibitory action, an effect that involves NO acting through guanylyl cyclase. Therefore, endogenous GABA might exert an inhibitory effect over adenosine A(1)-mediated responses in the hippocampus, which may represent a physiologic regulatory mechanism between the two inhibitory mediators.     

Differential effects of repeated imipramine on hippocampal responsiveness to adenosine and serotonin.

Imipramine-induced enhancement of the inhibitory action of 5-HT(lA) receptor activation in hippocampal pyramidal neurons has been attributed to alterations in the transduction mechanism that involves G protein-dependent opening of K(+) channels. Postsynaptic 5-HT(lA) and adenosine Al receptors may share that transduction pathway. We investigated the influence of repeated imipramine administration on 5-HT(lA) and adenosine A1 receptor-mediated effects in rat hippocampal slices. Repeated imipramine selectively enhanced the postsynaptic effects of 5-HT(1A) receptor activation, including hyperpolarization and reduction of input resistance of neurons and reduction of the population spike amplitude. In contrast, after imipramine treatment only the presynaptic effect of adenosine receptor agonists, a decrease of the field excitatory postsynaptic potential, was enhanced. The data demonstrate that alterations in the presumed common transduction mechanism that was postulated for the 5-HT(lA) and adenosine A1 receptor-mediated activation of K(+) channels are not involved in the effect of repeated imipramine administration.     

5-hydroxyindole causes convulsions and increases transmitter release in the CA1 region of the rat hippocampus

1 5-hydroxyindole (5-OHi) is a proposed tryptophan metabolite able to cause convulsions when systemically injected into rodents. We studied its effects using microdialysis in vivo and electrophysiological approaches in vitro. 2 Local administration of 5-OHi into the CA1 region of the rat hippocampus, via a microdialysis probe, significantly increased glutamate concentrations in the dialysates. 3 In rat hippocampal slices, using extracellular recordings in the CA1 region, 5-OHi (30-300 microM) increased the amplitude of population spikes and fEPSPs. 4 In the same preparation, using intracellular recordings in CA1 pyramidal neurons, 5-OHi reduced the latency of firing induced by direct depolarization and increased both evoked excitatory and slow inhibitory postsynaptic potential amplitudes, without affecting the resting membrane potential, the after-hyperpolarization or the neuronal input resistance. It also altered GABA(A)-mediated neurotransmission by increasing the frequency and the amplitude of pharmacologically isolated spontaneous inhibitory postsynaptic currents (sIPSC). 5 In separate experiments, performed by measuring AMPA or NMDA-induced depolarization in cortical wedges, 5-OHi did not modify glutamate receptor agonist responses. 6 Our results show that 5-OHi causes convulsions, modifies the properties and the function of the hippocampal circuitry, and facilitates the output of both excitatory and inhibitory transmitters.     

Opposite effects of antidepressants and corticosterone on the sensitivity of hippocampal CA1 neurons to 5-HT1A and 5-HT4 receptor activation

Using extracellular and intracellular ex vivo recording techniques we studied changes in the reactivity of hippocampal pyramidal CA1 neurons to serotonin (5-HT) and to the 5-HT1A- and 5-HT4 receptor agonists (+/-)-2-dipropylamino-8-hydroxy- 1,2,3 ,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT) and zacopride, respectively, evoked by repeated electroconvulsive shock (ECS), imipramine and corticosterone treatments. Rats were subjected to ECS for 1 or 10 days, treated with imipramine for 1, 7, 14 or 21 days (10 mg/kg p.o., twice daily) and with corticosterone for 7 days (10 mg/kg s.c., twice daily). Hippocampal slices were prepared 2 days after the last treatment. Activation of 5-HT1A receptors decreased the amplitude of population spikes evoked by stimulation of the Schaffer/collateral-commissural pathway and hyperpolarized CA1 cells. Activation of 5-HT4 receptors increased the population spike amplitude and decreased the amplitude of slow afterhyperpolarization. Both repeated ECS and imipramine enhanced the effects related to 5-HT1A receptor activation and attenuated the effects of 5-HT4 receptor activation. The action of imipramine was significant after a 7-day treatment and reached a maximum after 14 daily applications, remaining at the same level in a group of animals treated for 21 days. Repeated corticosterone attenuated the inhibitory effect of 5-HT and 8-OH-DPAT on the population spike amplitude and enhanced the increase in population spike amplitude induced by zacopride. These findings indicate that antidepressant treatments and repeated corticosterone have opposite effects on hippocampal responsiveness to 5-HT1A and 5-HT4 receptor activation. In consequence, antidepressants enhance, whereas corticosterone reduces the 5-HT-mediated inhibition of hippocampal CA1 cells, which may be relevant to the antidepressant and pro-depressant effects of either treatment, respectively.     

Repeated treatment with antidepressant drugs induces subsensitivity to the excitatory effect of 5-HT4 receptor activation in the rat hippocampus

The effect of repeated treatment with various antidepressant drugs on the reactivity of CA1 neurons to the 5-HT₄ receptor agonist zacopride was examined. Zacopride decreased the calcium-activated afterhyperpolarization and adaptation, it also elicited a slow membrane depolarization associated with an increase in input resistance. All those effects may have contributed to the zacopride-induced increase in the amplitude of population spikes, evoked in the CA1 cell layer by stimulation of the Schaffer collateral/commissural pathway. The later effect of zacopride was concentration-dependent and was antagonized by the 5-HT₄ receptor antagonist DAU 62805. Repeated (14 days, twice daily), but not single, administration of the antidepressant drugs imipramine, citalopram, fluvoxamine and paroxetine (10 mg/kg) attenuated the effect of zacopride on population spikes. Because inhibitory 5-HT_1A and excitatory 5-HT₄ receptors are colocalized on pyramidal neurons, and our previous data demonstrated an increase in the 5-HT_1A receptor-mediated inhibition after repeated treatment with antidepressants, we conclude that treatment with antidepressant drugs may enhance the inhibitory effect of 5-HT directly, by increasing the 5-HT_1A receptor responsiveness, and indirectly, by inducing subsensitivity to the 5-HT₄ receptor activation. Received: 24 June 1996 / Accepted: 28 August 1996     

Imipramine treatment ameliorates corticosterone-induced alterations in the effects of 5-HT1A and 5-HT4 receptor activation in the CA1 area of rat hippocampus.

This study tested whether imipramine reverses adaptive modifications in the function of hippocampal 5-HT_1A and 5-HT₄ receptors induced by repetitive administration of corticosterone. Rats received corticosterone for 1 or 3 weeks or imipramine for 2 weeks. The fourth experimental group was treated with corticosterone for 3 weeks and additionally with imipramine, beginning on the eighth day of corticosterone administration. Hippocampal slices were prepared 48 h after the last drug administration. 5-HT_1A and 5-HT₄ receptor-mediated effects on CA1 population spike amplitude were measured. While repeated corticosterone attenuated the inhibitory effect of 5-HT_1A receptor activation by 8-OH-DPAT and enhanced the excitatory effect of 5-HT₄ receptor activation by zacopride, imipramine treatment of naïve rats resulted in opposite changes. In the corticosterone plus imipramine group, the effect of 8-OH-DPAT and zacopride were not different from control, indicating that corticosterone-induced adaptive changes in the reactivity of 5-HT_1A and 5-HT₄ receptors were reversed by imipramine treatment.     

Repeated corticosterone administration increases excitatory effect of 5-HT4 receptor agonist in the rat hippocampus

The objective of the present study was to investigate whether repeated exposure of rats to high level of corticosterone affects responses of CA1 hippocampal cells to the 5-HT4 receptor agonist zacopride. To assess responsiveness of CA1 neurons to zacopride we used extracellular recording of population spikes evoked in CA1 cells by the stimulation of the Schaffer/collateral-commissural pathway in hippocampal slices. Rats were treated with corticosterone for 7 days (10 mg/kg sc, twice daily), slices were prepared two days after the last treatment. Zacopride induced an increase in the amplitude of population spike and repeated corticosterone treatment enhanced this excitatory effect. It is concluded that repeated treatment with corticosterone increases the responsiveness of hippocampal CA1 neurons to the 5-HT4 receptor activation.     

The 5-HT(1A) receptor agonist F 13640 attenuates mechanical allodynia in a rat model of trigeminal neuropathic pain

It has been reported that the treatment with a tricyclic antidepressant imipramine induces an increase in the sensitivity of 5-HT(1A) receptors and a decrease in the sensitivity of 5-HT(4) receptors in the rat hippocampus. 5-HT(1A) receptor agonists and neuroleptics also affect 5-HT(1A) receptors in different brain areas; therefore, it was of interest to compare their effects on hippocampal 5-HT receptors with the influence of the well-established antidepressant imipramine. We studied the effects of repeated treatment with imipramine, the 5-HT(1A) receptor agonists 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) and buspirone, and the neuroleptics haloperidol and clozapine on the sensitivity of rat hippocampal CA1 neurons to 5-HT(1A)- and 5-HT(4) receptor activation. Imipramine was administered for 21 days (10 mg/kg p.o., twice daily), 8-OH-DPAT for 7 days (1 mg/kg s.c., twice daily) and buspirone for 21 days (5 mg/kg s.c., twice daily). The rats received haloperidol (1 mg/kg) and clozapine (30 mg/kg) for 6 weeks in drinking water. Hippocampal slices were prepared 2 days after the last treatment with imipramine, 8-OH-DPAT or buspirone, and 5 days after the last treatment with the neuroleptics. Using an extracellular in vitro recording, we studied changes in the amplitude of stimulation-evoked population spikes, induced by 5-HT, 8-OH-DPAT and the 5-HT(4) receptor agonist zacopride. Activation of 5-HT(1A) receptors decreased, while activation of 5-HT(4) receptors increased the amplitude of population spikes. Imipramine significantly enhanced the inhibitory effects of 5-HT and 8-OH-DPAT, and attenuated the excitatory effect of zacopride. No other treatment used in the present study changed the sensitivity of hippocampal CA1 neurons to 5-HT(1A) and 5-HT(4) receptors activation. These findings indicate that adaptive changes in the sensitivity of hippocampal neurons to 5-HT(1A) and 5-HT(4) receptors agonists are specific to imipramine and may thus-at least partly-mediate its effects.     

Suppression of presynaptic responses to adenosine by activation of NMDA receptors.

The interactions between adenosine and NMDA receptors has been investigated using the paired-pulse paradigm in hippocampal slices. This technique allows the study of drug effects specifically at presynaptic terminals. The inhibitory effect of adenosine on population spikes, and the decrease of paired-pulse inhibition assessed using either population spikes or population excitatory postsynaptic potentials, were suppressed by performing the experiments in magnesium-free medium, or by superfusion of the slices with N-methyl-D-aspartate (NMDA) at a concentration (4 microM) which did not itself affect potential size. The suppressant effect of NMDA was prevented by 2-amino-5-phosphonopentanoic acid. All these interactions were still seen in the presence of bicuculline methobromide, 30 microM. Neither alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) nor kainate produced a suppression of adenosine responses. The presence of NMDA did not modify the effects of baclofen on population potentials or paired-pulse inhibition. Activating NMDA receptors by the induction of long-term potentiation or by superfusion with glycine also reduced significantly the effects of adenosine on population spikes and paired-pulse interactions. Increasing population potential size by a mechanism which did not involve the activation of NMDA receptors (increasing stimulus strength) did not change sensitivity to adenosine. When adenosine receptor-selective agonists were tested, it was found that NMDA did not modify the inhibitory effect of the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine, but did enhance the excitatory effect of the adenosine A(2A) receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680). The combined response to NMDA and CGS21680 was prevented by the adenosine A(2A) receptor selective antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385). It is concluded that NMDA receptor activation can suppress neuronal sensitivity to adenosine by acting at presynaptic sites, and that this interaction results from an increase in the excitatory action of adenosine A(2A) receptors, rather than a depression of A(1) receptor function.     

Antidepressant drugs potentiate the alpha 1-adrenoceptor effect in hippocampal slices

The effect of prolonged treatment with antidepressant drugs on the phenylephrine- and norepinephrine (NE)-evoked reaction in hippocampal slices was examined by extracellular recording of the spontaneous activity of CA1 layer neurons. The alpha 1-adrenoceptor agonists, phenylephrine and methoxamine, depressed the neuronal discharges of most of the units tested, while NE evoked both excitatory and inhibitory effects which were blocked by propranolol and phentolamine or prazosin, respectively. Imipramine, mianserin, (+)- and (-)-oxaprotiline administered subchronically (10 mg/kg p.o., twice daily for 14 days, withdrawal 48 h), potentiated the inhibitory reaction to phenylephrine. Mianserin was the only drug tested in the acute dose to effectively augment the reaction to alpha 1-adrenoceptor stimulation. Prolonged administration of mianserin and imipramine attenuated the excitatory effect to NE, which probably reflects beta-receptor down-regulation; however, only mianserin, but not imipramine, enhanced the NE-induced inhibition. The observed potentiation of the alpha 1-adrenoceptor-related inhibitory reaction to phenylephrine produced by antidepressant drugs may reflect the development of the alpha 1-adrenergic system supersensitivity in the hippocampus.     

Comparison of the effects of low and high concentrations of group I metabotropic receptor agonists on field potentials in the hippocampal CA1 region in vitro

We compared the effects of low and high concentrations of the selective group I metabotropic glutamate receptor (mGluR) agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) and the nonselective mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD) on extracellularly recorded potentials which were evoked in the rat hippocampal CA1 region by stimulation of the Schaffer collateral/commissural pathway and on intracellularly recorded electrophysiological properties of CA1 neurons, in vitro. At low concentrations (2.5 and 5 microM) DHPG and (1S,3R)-ACPD increased while at high concentrations (20 and 50 microM) they decreased population spike amplitudes. Simultaneous recordings of population spikes in the CA1 cell layer and field excitatory postsynaptic potentials (fEPSPs) in stratum radiatum of the CA1 area revealed that the enhancement of the population spike amplitude is not associated with any change in the fEPSP slope, but the decrease in population spikes is accompanied with a decrease in the fEPSP slope, suggesting that at high concentrations both agents may attenuate excitatory synaptic transmission in CA1 cells. DHPG and (1S,3R)-ACPD had a number of direct excitatory effects on CA1 pyramidal cells like a concentration-dependent depolarization and an inhibition of the slow afterhyperpolarization, which in all probability underlay the increase in the amplitude of population spikes. At high concentrations, both mGluR agonists strongly depolarized CA1 cells indicating that depolarization block of cell discharges may underlay the reduction in the population spike amplitude. Furthermore, robust cell discharges induced by the strong depolarizations, activate several secondary processes which may significantly contribute to the action of high concentrations of DHPG and (1S,3R)-ACPD. Therefore, the effects of low and high concentrations of the studied mGluR agonists may involve different mechanisms, at low concentrations the effects can be directly related to the activation of postsynaptically localized group I mGluRs while at higher concentrations the contribution of indirect effects may predominate.     

Suppression of nicotinic synaptic transmission by adenosine in myenteric ganglia of the guinea-pig gastric antrum.

Conventional intracellular recording techniques were used to investigate actions of adenosine on nicotinic cholinergic transmission in myenteric neurons of the gastric antrum. Adenosine or the more potent derivatives, 5'-N-ethylcarboxamidoadenosine (NECA), 5'-N-cyclopropylcarboxamidoadenosine, 1-deaza-2-chloro-N6-cyclopentyladenosine or N6-cyclopentyladenosine reversibly and dose dependently inhibited the fast excitatory postsynaptic potentials (fast EPSPs) in 60% of the gastric neurons. Neither adenosine nor NECA affected excitatory responses to the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium iodine. The EC50 concentration for inhibition of the fast excitatory postsynaptic potential (EPSP) by adenosine was 55 microM NECA was a more potent inhibitor than adenosine. The specific adenosine receptor antagonists 1,3-dipropyl-8-p-sulfophenyl xanthine or 1,3-dipropyl-8-(cyclopentyl) xanthine blocked the inhibitory effects of adenosine or NECA. Fast EPSPs were enhanced by superfusion of the antagonists alone, suggestive of ongoing inhibition of nicotinic transmission by endogenous adenosine. The antagonists had no effect on resting membrane properties, excitability or antidromic action potentials. In neurons with suppression of fast EPSPs, adenosine did not suppress all cholinergic inputs to the same neuron. The results suggest that adenosine inhibits nicotinic transmission by interacting with presynaptic P1 adenosine receptors located at cholinergic release sites.     

The effect of omega-conotoxin GVIA on synaptic transmission within the nucleus accumbens and hippocampus of the rat in vitro.

1. The actions of two calcium channel antagonists, the N-channel blocker omega-conotoxin GVIA (omega-CgTx) and the L-channel antagonist nisoldipine, on synaptic transmission were investigated in the hippocampus and nucleus accumbens of the rat in vitro. 2. omega-CgTx (100 nM for 10 min) produced a marked and irreversible reduction of focally evoked population spikes and intracellularly recorded excitatory postsynaptic potentials (e.p.s.ps) in the nucleus accumbens, which could not be overcome by increasing the stimulus strength. 3. Nisoldipine (10 microM for 10 min) had no effect on population spikes in the nucleus accumbens or the CA1 of the hippocampus. 4. In the hippocampus, population spikes were not irreversibly reduced by omega-CgTx (100 nM for 10 min) but rather, multiple population spikes were produced along with spontaneous synchronous discharges. This indicated that inhibitory synaptic transmission was being preferentially reduced. 5. Intracellular recordings demonstrated that omega-CgTx powerfully reduced inhibitory synaptic transmission in an irreversible manner and that excitatory transmission was also reduced but to a lesser extent. Unlike excitatory transmission in the nucleus accumbens and inhibitory transmission in the hippocampus, increasing the stimulus strength overcame the reduction of hippocampal excitatory transmission. 6. It is concluded that omega-CgTx-sensitive calcium channels are involved in the calcium entry that precedes the synaptic transmission in all these synapses. The apparent lower sensitivity of the hippocampal excitatory fibres to omega-CgTx may indicate that calcium entry that promotes transmitter release at central synapses may be mediated by pharmacologically distinct calcium channels.     

The effect of prolonged treatment with tricyclic antidepressants on the actions of 5-hydroxytryptamine in the hippocampal slice of the rat

The effect of long-term treatment with the tricyclic antidepressants imipramine (IMI) and desmethylimipramine (DMI) on neuronal responsiveness to 5-hydroxytryptamine (5-HT) was examined in the hippocampal slice preparation from the rat. Population spikes, evoked by electrical stimulation of the stratum radiatum, were recorded in the pyramidal cell layer of the CA1 region of the isolated hippocampus. When 5-HT (10(-7) to 2 X 10(-5) M) was applied there was an initial increase followed by a decrease in the amplitude of the population spike. On washout of 5-HT the amplitude increased transiently above control levels. Daily injection of 10 mg/kg of imipramine or desmethylimipramine, intraperitoneally, into rats for 4-5 weeks was found to produce a significant decrease in the inhibitory effect of 10(-5) M 5-HT, whereas there was no apparent change in the excitatory effects. The acute application of 10(-5) M imipramine or desmethylimipramine antagonized the inhibitory effect of 10(-5) M 5-HT without affecting the excitatory effects. Acute application of the 5-HT receptor antagonists cyproheptadine (10(-5) M) and ketanserin (7.5 X 10(-6) M) completely prevented the appearance of the inhibitory effect of 10(-5) M 5-HT without affecting the excitatory effects. It was concluded that the decreased inhibitory effect of 5-HT which was produced by chronic treatment with imipramine or desmethylimipramine was probably due to a reduction in the number of 5-HT receptors or a reduction in the post-receptor effector mechanisms for 5-HT.     

Caffeine-induced epileptiform field potentials in rat hippocampal slices: a pharmacological characterization

Pharmacological modulation of the epileptiform electric activity induced by caffeine, 10 mM (CAF) on rat hippocampal slices was studied upon field potential recordings in CA3 area of the slices. This concentration of CAF, reportedly releasing Ca2+ ions from the endoplasmic reticulum, led single fimbrial stimuli to evoke repetitive population spikes (PSs) and induced periodic spontaneous field bursts. Carbamazepine, 50 microM reduced (by <40%) the number of repetitive PSs and the rate of spontaneous bursting, with no significant effect on the amplitude of evoked and spontaneous bursts. Valproate, 1 mM reduced only the number (by approximately 25%), but not the amplitudes, of repetitive PSs. Clonazepam, 1 microM consistently reduced the number of repetitive PSs (by approximately 45%), their amplitudes (by 30-60%), and the amplitude of spontaneous bursts (by approximately 70%). The adenosine receptor agonists 2-chloroadenosine, 5 microM and R(-) N6-(2-phenylisopropyl)adenosine, 1 microM had only scanty anti-CAF activity. The depletor of intracellular Ca2+ stores, thapsigargin, 2 microM transiently inhibited the number of evoked PSs and spontaneous bursting. The blocker of ryanodine receptor opening, ruthenium red had an anti-CAF effect, modest at 30 microM, but very strong at 40 microM. Nifedipine, 20 microM opposed CAF-induced spontaneous bursting, but not the evoked PSs. Flunarizine, 50 microM presented only a transient tendency to delay spontaneous bursting. In conclusion, this in vitro slice model appears readily able to reveal antiepileptic properties, though it does not support unequivocal mechanistic interpretation. Nevertheless, anti-CAF activity in this model would suggest the likely involvement of the neuronal ryanodine receptor-related traffic of calcium.     

Effect of adenosine receptor agonists and other compounds on cyclic AMP accumulation in forskolin-treated hippocampal slices

The effect of adenosine analogues and some putative neurotransmitters have been studied on cyclic AMP accumulation in rat hippocampal slices treated with the adenylate cyclase activator forskolin. The effects of PGE2 and histamine were potentiated by forskolin (0.1 microM). Isoprenaline and NECA had essentially additive effects with 0.1 microM forskolin and serotonin (above 10(-4) M) inhibited forskolin-stimulated cyclic AMP accumulation. The A1-adenosine receptor selective adenosine analogue R-PIA inhibited forskolin stimulated cyclic AMP accumulation in low doses and stimulated in high. NECA, adenosine and 2-chloroadenosine uniformly stimulated cyclic AMP accumulation. 2',5'-dideoxyadenosine inhibited, but only at high concentrations. Both the stimulatory and the inhibitory effects of R-PIA were antagonized by 8-phenyltheophylline (10 microM). Enprofylline (100 microM) selectively inhibited the stimulatory effect. In the presence of enprofylline both 2-chloroadenosine showed an inhibitory effect on cyclic AMP accumulation. It is concluded that the forskolin-treated rat hippocampal slice is a useful preparation to study both stimulatory and inhibitory effects of transmitters and modulators on adenylate cyclase. The results also show that the rat hippocampus has both A1-receptors that are linked to inhibition of cyclic AMP accumulation and A2-receptors that are linked to stimulation. Furthermore, enprofylline is shown to selectively antagonize the stimulatory response, revealing inhibitory effects of compounds such as 2-chloroadenosine and adenosine.     

Cannabinoid modulation of neuronal function after oxygen/glucose deprivation in area CA1 of the rat hippocampus

Endocannabinoids released during cerebral ischemia have been implicated as neuroprotective agents. We assessed the role of cannabinoid receptors in modulating the response of neurons to oxygen/glucose deprivation (OGD), a model for in vitro ischemia, in rat hippocampal slices using extracellular recording techniques. Under control conditions, 15 min OGD resulted in only 50% recovery of CA1 field excitatory postsynaptic potentials (fEPSPs) 60 min post-insult. This post-OGD depression of function was primarily NMDA receptor-dependent as the NMDA receptor antagonist MK-801 (50 microM) promoted recovery of synaptic transmission to 76% of the baseline. Treatment with the CB1 receptor antagonist AM251 (1 microM), which prevented the depression of excitatory synaptic transmission caused by WIN55,212-2 (1 microM), also markedly enhanced recovery of function (71% of control). The enhanced recovery after OGD in the presence of AM251 was independent of both GABA(A) receptors and NMDA receptors since co-application of AM251 with either bicuculline (10 microM) or MK-801 (50 microM) did not alter recovery, or further improved recovery, respectively. These results suggest endocannabinoids released during OGD may modulate synaptic transmission and post-OGD neuronal outcome via activation of an AM251-sensitive cannabinoid receptor.     

The protein kinase C inhibitor 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) disinhibits CA1 pyramidal cells in rat hippocampal slices.

1. The effects of the protein kinase C (PKC) inhibitor 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) on evoked synaptic potentials were investigated in the CA1 region of rat hippocampal slices by use of extracellular and intracellular recording techniques. 2. Extracellular recordings showed that superfusion with H-7 (10-100 microM) increased the amplitude of the population spike and the initial slope of the dendritic field e.p.s.p. H-7 also produced the appearance of multiple population spikes in the somatic region and in the dendritic field e.p.s.p. 3. H-7 (30 microM) induced the disappearance of intracellularly recorded inhibitory potentials elicited by orthodromic stimulation of CA1 pyramidal cells. At this concentration H-7 had no effect on resting membrane potential, input membrane resistance, and spike threshold. In voltage-clamped neurones H-7 blocked the antidromically evoked inhibitory currents and the spontaneous miniature inhibitory currents. 4. The hyperpolarizing effect of bath applied gamma-aminobutyric acid (GABA, 500 microM) or isoguvacine (30 microM) was not affected by 30 microM H-7. 5. Neither the PKC activity regulator sphingosine (10-40 microM) nor the H-7 analogue N-(2-guanidinoethyl)-5-isoquinolinesulphonamide (HA-1004, 20-50 microM) which is devoid of activity on PKC at these concentrations, affected the extracellularly recorded dendritic field e.p.s.p. or population spike. 6. It is concluded that the disinhibitory effect produced by H-7 is due to the block of a H-7-sensitive PKC which is involved in the spontaneous and evoked release of GABA.     

Repeated imipramine administration enhances the effects of NMDA receptor ligands on synchronous activity in rat frontal cortex in vitro

This study assessed the effects of repeated administration (14 days) of imipramine on the function of NMDA receptors by measuring the frequency of spontaneous epileptiform discharges which develop in rat frontal cortical slices incubated in Mg2+-free conditions. Imipramine significantly enhanced both the excitatory effect of NMDA and the inhibitory effect of the competitive NMDA receptor antagonist CGP 37849 on the frequency of discharges. These results are consistent with studies indicating that chronic administration of antidepressant drugs induces adaptive changes in NMDA receptor/ channel complex in the cerebral cortex.