Thromboxane A2 agonist modulation of excitatory synaptic transmission in the rat hippocampal slice.

1. The effects of the selective thromboxane A2 (TXA2) receptor agonist I-BOP on neuronal excitability and synaptic transmission were studied in the CAl neurones of rat hippocampal slices by an intracellular recording technique. 2. Superfusion of I-BOP (0.5 microM) resulted in a biphasic change of the excitatory postsynaptic potential (e.p.s.p.), which was blocked by pretreatment with SQ 29548, a specific antagonist of TXA2 receptors. The inhibitory phase of I-BOP on the e.p.s.p. was accompanied by a decrease in neuronal membrane input resistance. 3. The sensitivity of postsynaptic neurones to glutamate receptor agonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartate (NMDA), was unchanged by I-BOP (0.5 microM) pretreatment. 4. Bath application of Ba2+ (0.5 mM) prevented both the I-BOP-induced reduction of the neuronal membrane input resistance and the blockade of e.p.s.p. induced by I-BOP. 5. Intracellular dialysis of the hippocampal CA1 neurones with GDP (10 mM) significantly attenuated the I-BOP inhibition of e.p.s.p. and membrane input resistance. Incubation of the slices with either pertussis toxin (PTX, 5 micrograms ml-1 for 12 h) or cholera toxin (CTX, 5 micrograms ml-1 for 12 h) did not affect the biphasic action of I-BOP on the e.p.s.p. or the reduction of membrane input resistance induced by I-BOP. 6. Pretreatment of the slices with the protein kinase C (PKC) inhibitor, NPC-15437 (20 microM), abolished the biphasic modulation by I-BOP (0.5 microM) of the e.p.s.p. Intracellular application of a specific PKC inhibitor, PKCI 19-36 (20 microM), completely inhibited the I-BOP reduction of e.p.s.p. The specific cyclic AMP-dependent protein kinase (PKA) inhibitor, Rp-cyclic adenosine 3',5'-monophosphate (Rp-cyclic AMPS, 25 microM), had no effect on the I-BOP action. 7. In this study we have demonstrated, for the first time, the existence of functional TXA2 receptors in the hippocampus which mediate the effects of a TXA2 agonist on neuronal excitability and synaptic transmission. Activation of the presynaptic TXA2 receptors may stimulate the release of glutamate. Conversely, activation of postsynaptic TXA2 receptors leads to inhibition of synaptic transmission resulting from a decrease in the membrane input resistance of the neurones. The pre- and postsynaptic actions of the TXA2 agonist are both mediated by PTX- and CTX-insensitive G-protein-coupled activation of PKC pathways.     

Presynaptic inhibition by neuropeptide Y in rat hippocampal slice in vitro is mediated by a Y2 receptor.

1. The action of analogues and C-terminal fragments of neuropeptide Y (NPY) was examined on excitatory synaptic transmission in area CA1 of the rat hippocampal slice in vitro, by use of intracellular and extracellular recordings, to determine by agonist profile the NPY receptor subtype mediating presynaptic inhibition. 2. Neither NPY, analogues nor fragments of NPY affected the passive or active properties of the post-synaptic CA1 pyramidal neurones, indicating their action is at a presynaptic site. 3. The full-sequence analogues, peptide YY (PYY) and human NPY (hNPY), were equipotent with NPY at the presynaptic receptor, while desamido hNPY was without activity. 4. NPY2-36 was equipotent with NPY. Fragments as short as NPY 13-36 were active, but gradually lost activity with decreasing length. NPY 16-36 had no effect on extracellular field potentials, but still significantly inhibited excitatory postsynaptic potential amplitudes. Fragments shorter than NPY 16-36 had no measurable effect on synaptic transmission. 5. The presynaptic NPY receptor in hippocampal CA1 therefore shares an identical agonist profile with the presynaptic Y2 receptor at the peripheral sympathetic neuroeffector junction.     

Triethyltin exposure suppresses synaptic transmission in area CA1 of the rat hippocampal slice

To examine the effects of TET on the electrophysiology of area CA1 of hippocampus, hippocampal slices were obtained from adult hooded rats and were maintained in vitro using standard techniques. Stimulating and recording electrodes were placed in the Schaffer collaterals and CA1 pyramidal cell body layer, respectively. Following baseline measurements, slices were exposed to either 0, 1, 3, 6, or 10 μM TET in the incubating medium. Both pyramidal cell excitability and recurrent/feedforward inhibition were suppressed in a dose-dependent manner within 3 hr postexposure. The evoked population spike and population excitatory postsynaptic potential (EPSP) were suppressed significantly by 2 hr postexposure for 1 and 3 μM TET exposures, and by 45 min postexposure for 6 and 10 μM exposures. A similar dose-dependency was observed for the suppression of recurrent/feedforward inhibition in hippocampal CA1. A second procedure tested the specificity of TET effects to axonal conduction of Schaffer collaterals. Both the stimulating and recording electrode were placed in the Schaffer collaterals so that both the Schaffer collateral population fiber volley and the CA1 pyramidal cell population EPSP could be recorded. TET exposure suppressed pyramidal cell EPSPs without significantly affecting the amplitude of Schaffer collateral fiber volleys. The results support the view that acute TET exposure suppresses synaptic transmission in area CA1 of hippocampus.     

Group III mGluR regulation of synaptic transmission at the SC-CA1 synapse is developmentally regulated

Group III metabotropic glutamate receptors (mGluRs) reduce synaptic transmission at the Schaffer collateral-CA1 (SC-CA1) synapse in rats by a presynaptic mechanism. Previous studies show that low concentrations of the group III-selective agonist, L-AP4, reduce synaptic transmission in slices from neonatal but not adult rats, whereas high micromolar concentrations reduce transmission in both age groups. L-AP4 activates mGluRs 4 and 8 at much lower concentrations than those required to activate mGluR7, suggesting that the group III mGluR subtype modulating transmission is a high affinity receptor in neonates and a low affinity receptor in adults. The previous lack of subtype selective ligands has made it difficult to test this hypothesis. We have measured fEPSPs in the presence of novel subtype selective agents to address this question. We show that the effects of L-AP4 can be blocked by LY341495 in both neonates and adults, verifying that these effects are mediated by mGluRs. In addition, the selective mGluR8 agonist, DCPG, has a significant effect in slices from neonatal rats but does not reduce synaptic transmission in adult slices. The mGluR4 selective allosteric potentiator, PHCCC, is unable to potentiate the L-AP4-induced effects at either age. Taken together, our data suggest that group III mGluRs regulate transmission at the SC-CA1 synapse throughout development but there is a developmental regulation of the subtypes involved so that both mGluR7 and mGluR8 serve this role in neonates whereas mGluR7 is involved in regulating transmission at this synapse throughout postnatal development.     

Metabotropic glutamate receptor 2 modulates excitatory synaptic transmission in the rat globus pallidus

While group II metabotropic glutamate receptors (mGluRs) are known to be expressed in the rat globus pallidus (GP), their functions remain poorly understood. We used standard patch clamping technique in GP slices to determine the effect of group II mGluR activation on excitatory transmission in this region. Activation of group II mGluRs with the group-selective agonist DCG-IV or APDC reduced the amplitude of the evoked excitatory postsynaptic currents (EPSCs) and significantly increased the paired pulse ratio suggesting a presynaptic site of action. This was further supported by double-labeling electron microscopy data showing that group II mGluRs (mGluR2 and 3) immunoreactivity is localized in glutamatergic pre-terminal axons and terminals in the GP. Furthermore, we found that LY 487379, an mGluR2-specific allosteric modulator, significantly potentiated the inhibitory effect of DCG-IV on the excitatory transmission in the GP. Co-incubation with 30 microM LY 487379 increased the potency of DCG-IV about 10-fold in the GP. We were thus able to pharmacologically isolate the mGluR2-mediated function in the rat GP using an mGluR2-specific allosteric modulator. Therefore, our findings do not only shed light on the functions of group II mGluRs in the GP, they also illustrate the therapeutic potential of mGluR-targeting allosteric modulators in neurological disorders such as Parkinson's disease.     

The pharmacology of excitatory transmission in the rat olfactory cortex slice

The pharmacology of excitatory transmission in slices of olfactory cortex of the rat, perfused with solution containing picrotoxin, has been studied by assessing the effects of cis-2,3-piperidine dicarboxylate, a nonselective antagonist of excitatory amino acid receptors, 2-amino-5-phosphonopentanoate, a selective antagonist of N-methyl-D-aspartate (NMDA) receptors and 2-amino-4-phosphonobutyrate (APB) and baclofen, which act at APB and GABAB sites, respectively, on evoked surface field potentials. Monosynaptic excitatory transmission was monitored by measuring the amplitude of the N'a'-wave, evoked on stimulation of the lateral olfactory tract, whilst di-/polysynaptic excitatory transmission was evaluated by calculating the areas of the potentials evoked on direct stimulation of the superficial and deep-lying association fibre systems. On the basis of the effects of the drugs in this and earlier studies, it is concluded that: (i) transmission at the lateral olfactory tract-pyramidal cell synapse is mediated by kainate/quisqualate but not NMDA receptors and is regulated by inhibitory APB receptors, located on the tract terminals; (ii) NMDA receptors are involved in mediating excitatory transmission at the synapses of superficial association fibres with the proximal apical dendrites of pyramidal cells with inhibitory APB receptors playing a regulatory role; (iii) transmission at synapses of association fibres with basal dendrites of pyramidal cells, is mediated in part by NMDA receptors with (presynaptic?) GABAB receptors exerting a strong inhibitory influence. These proposed roles of NMDA receptors have been confirmed in experiments in which the effects of magnesium ions on field potentials evoked in slices perfused in magnesium-free solution were monitored.     

Bicuculline-resistant paired-pulse inhibition in the rat hippocampal slice.

1. Virginiamycin, a macrolide reported to bind selectively to CCKB/gastrin receptors has been studied in a functional test, namely cholecystokinin-induced contraction of guinea-pig ileum myenteric plexus (LMMP). 2. Virginiamycin (1-10 microM) antagonized the selective CCKB agonist cholecystokinin tetrapeptide (CCK-4). The antagonism appeared not to be competitive as the highest concentration (10 microM) caused a reduction of its maximal effect. An apparent pA2 of 6.64 +/- 0.06 (s.e.) could be estimated if this depression was ignored. The selective CCKB antagonist, L-365,260 (0.01-0.3 microM) antagonized competitively the CCK-4 induced contraction and a pKB of 8.60 +/- 0.16 (s.e.) was estimated. 3. The combined dose-ratio analysis for virginiamycin, tested at 3 and 10 microM in association with 0.03 and 0.1 microM L-365,260, respectively, resulted in observed log dose-ratios of 1.39 and 1.53. That was consistent with both antagonists acting on the same receptor in LMMP. 4. These data, represent the first evidence of the antagonism of virginiamycin in a functional assay and they support the hypothesis of homogeneity between CCKB receptors in the CNS and in peripheral tissues.     

Endogenous adenosine inhibits excitatory transmission in the rat olfactory cortex slice

Excitatory neurotransmission in the rat olfactory cortex slice has been monitored by measuring the amplitude of the N-wave surface field potential evoked by stimulation of the lateral olfactory tract. Application of exogenous adenosine or aspartate depressed the N-wave amplitude and evoked synthesis of cyclic AMP. These effects were partially antagonized by theophylline and the reduction of amplitude of the N-wave was potentiated by dipyridamole. When the olfactory tract slice was stimulated, dipyridamole alone reduced the amplitude of the N-wave and increased levels of cyclic AMP, both effects being antagonized by theophylline. Exogenous adenosine significantly attenuated the K+-evoked release of [3H]D-aspartate by a mechanism insensitive to either theophylline or dipyridamole. It is concluded that synaptic activation of the olfactory cortex releases adenosine, possibly as the result of the actions of the transmitter candidate of the olfactory tract, aspartate, and that this causes sufficient adenosine to accumulate to depress excitatory transmission and elevate tissue levels of cyclic AMP although there is no positive evidence that these two effects are directly related.     

石杉碱甲增强大鼠海马脑片CA1锥体神经元的兴奋性突触传递

目的:观察石杉碱甲(Hup-A)对海马CA1锥体神经元兴奋性突触传递的影响,以探讨其增强学习记忆功能的神经细胞电生理机制。方法:应用大鼠海马脑片CA1锥体神经元细胞内记录技术,观察Hup-A对大鼠海马CA1锥体神经元膜电性质和刺激Schaffer侧支诱发的兴奋性突触后电位(EPSP)的影响。结果:(1)Hup-A(1μmol/L)灌流15min对CA1锥体神经元的膜电性质没有显著性影响。(2)Hup-A(0.3～3.0μmol/L)浓度依赖性使EPSP幅度升高、时程延长、曲线下面积增大,该作用可被阿托品(10μmol/L)预处理取消。(3)Hup-A对外源性谷氨酸诱导的去极化反应无明显影响。结论:Hup-A可增强CA1锥体神经元的兴奋性突触传递,其增强突触传递作用与M型乙酰胆碱受体激动有关。     

Pb exposure induces an imbalance of excitatory and inhibitory synaptic transmission in cultured rat hippocampal neurons

An appropriate balance of excitatory and inhibitory synapse maintains the network stability of the central nervous system. Our recent work showed lead (Pb) exposure can inhibit synaptic transmission in cultured hippocampal neurons. However, it is not clear whether Pb exposure disrupt the balance of excitatory and inhibitory synaptic transmission. Here, primary cultured hippocampal neurons from Sprague-Dawley (SD) rats were exposed to Pb (0.2 μM, 1 μM, 5 μM, respectively) from Days in Vitro (DIV) 7 to DIV 12 for 5 days and the excitatory and inhibitory synaptic transmission was examined. Patch clamp recording results showed that distinct from exposures of 0.2 μM and 5 μM, 1 μM Pb exposure significantly increased the mIPSC frequency and decreased the mEPSC frequency, leading to a uniform inhibitory outcome. Further, the number of inhibitory presynaptic puncta was significantly increased after 1 μM Pb exposure, while the number of excitatory presynaptic terminals was decreased. In addition 1 μM Pb increased the glutamic acid decarboxylase (GAD65) expression and the surface GABA_A receptor (GABA_AR) clusters. This shift might potentiate the synthesis of GABA and enhance the surface distribution of postsynaptic GABA_AR clusters in hippocampus neurons. Together, these data showed that Pb exposure disrupted the balance of excitatory and inhibitory synaptic transmission via abnormal GABAergic neurotransmission.     

Anaesthetic/amnesic agents disrupt beta frequency oscillations associated with potentiation of excitatory synaptic potentials in the rat hippocampal slice

1. Anaesthetic agents produce disruption in cognitive function typified by reductions in sensory perception and memory formation. Oscillations within the EEG gamma and beta bands have been linked to sensory perception and memory and have been shown to be modified by anaesthetic agents. 2. Synchronous gamma oscillations generated by brief tetanic stimulation in two regions of hippocampal area CA1 in slices in vitro were seen to potentiate excitatory synaptic communication between the areas. This synaptic potentiation, was seen to contribute to a transition from gamma frequency (30 - 70 Hz) to beta frequency (12 - 30 Hz) oscillations. 3. Four drugs having anaesthetic/hypnotic and amnesic properties were tested on this synchronous gamma-induced beta oscillation. Thiopental 10 - 200 microM, Diazepam 0.05 - 1.0 microM, Morphine 10 - 200 microM, and Ketamine 10 - 200 microM were all added to the bathing medium. Each drug markedly disrupted the formation of beta oscillations in a manner consistent with their primary modes of action. Thiopental and morphine disrupted synchrony of gamma oscillations and prevented potentiation of recurrent excitatory potentials measured in stratum oriens (fEPSPs). Neither diazepam, nor ketamine produced such marked changes in synchrony at gamma frequencies or reduction in potentiation of fEPSPs. However, each disrupted expression of subsequent beta oscillation via changes in the magnitude of inhibitory network gamma oscillations and the duration and magnitude of tetanus-induced depolarization respectively. 4. The degree of disruption of fEPSP potentiation correlated quantitatively with the degree of disruption in synchrony between sites during gamma oscillations. The data indicate that synchronous gamma-induced beta oscillations represent a mode of expression of excitatory synaptic potentiation in the hippocampus, and that anaesthetic/amnesic agents can disrupt this process markedly.     

Developmental regulation of hippocampal excitatory synaptic transmission by metabotropic glutamate receptors

The aims of this study were, to use agonists selective for the 3 mGlu receptor groups to identify developmental changes in their effects, and to assess the usefulness of proposed selective antagonists as pharmacological tools. Hippocampal slices (400 microm) were prepared from neonate (9 - 14 days) and young adult (5 - 7 weeks) Sprague-Dawley rats. Field excitatory postsynaptic potentials (fEPSP) were recorded from CA1. DHPG (100 microM), a group I agonist, produced a slowly developing enhancement of fEPSP slope in slices from adults. In slices from neonates, DHPG (75 microM) depressed fEPSP slope. DCG-IV (500 nM), a group II agonist, did not affect the fEPSP recorded from slices from adults whereas perfusion in neonate slices produced a sustained depression. The group III agonist L-AP4 (50 microM) was ineffective in adult slices but depressed fEPSP slope in slices prepared from neonates. DHPG-induced depression of fEPSP slope was inhibited by 4-CPG (400 microM), a group I antagonist, but was unaffected by MCCG (500 microM) and MAP4 (500 microM), group II and III receptor antagonists respectively. MCCG but not MAP4 antagonized the effects of DCG-IV with 4-CPG producing variable effects. The effect of L-AP4 was unaffected by MCCG, blocked by MAP4, and enhanced by 4-CPG. The results show that the effects of the agonists for all groups of mGlu receptors are developmentally regulated. Furthermore, MCCG and MAP4 behave as effective and selective antagonists for group II and group III mGlu receptors respectively, whereas the usefulness of 4-CPG as a group I antagonist may be limited.     

omega-Conotoxin inhibition of excitatory synaptic transmission evoked by dorsal root stimulation in rat superficial dorsal horn

A number of omega-conotoxins are potent and selective antagonists of N-type voltage-gated calcium channels (VGCCs) and are potentially effective as analgesic agents. omega-Conotoxins CVID and CVIB, venom peptides from Conus catus, inhibit N-type and N/P/Q-type VGCCs, respectively, in rat dorsal root ganglion sensory neurons. In the present study, we tested the effects of five different omega-conotoxins, CVID, CVIB, MVIIA, MVIIC and GVIA, on excitatory synaptic transmission between primary afferents and dorsal horn superficial lamina neurons of rat spinal cord. The N-type VGCC antagonists CVID (200nM) and MVIIA (500nM) completely and irreversibly inhibited excitatory postsynaptic currents (EPSCs) in the dorsal horn superficial lamina. The N- and P/Q-type VGCC antagonist CVIB (200nM) reversibly reduced evoked EPSC amplitude an average of 34+/-8%, whereas MVIIC (200nM) had no effect on excitatory synaptic transmission. In neurons receiving polysynaptic input, CVIB reduced both the EPSC amplitude and the "success rate" calculated as the relative number of primary afferent stimulations that resulted in postsynaptic responses. These results indicate that (i) the analgesic action of omega-conotoxins that antagonise N-type VGCCs may be attributed to inhibition of neurotransmission between primary afferents and superficial dorsal horn neurons, (ii) nociceptive synaptic transmission between primary afferents and superficial lamina neurons is mediated predominantly by N-type VGCCs, and (iii) in contrast to the irreversible inhibition by CVID, MVIIA and GVIA, the inhibition of excitatory monosynaptic transmission by CVIB is reversible.     

Enhanced isoflurane suppression of excitatory synaptic transmission in the aged rat hippocampus

1. The effects of the volatile anaesthetic, isoflurane, were investigated on evoked dendritic field excitatory postsynaptic potentials (f.e.p.s.p.) and antidromic and orthodromic population spikes recorded extracellularly in the CA1 cell layer region in the in vitro hippocampal slice taken from young mature (2–3 months) and old (24–27 months) Fisher 344 rats.
2. Isoflurane depressed the f.e.p.s.ps and the orthodromically-evoked population spikes in both old and young hippocampi. However, the magnitude of the anaesthetic-induced depression was greater in slices taken from old rats compared to those taken from young rats during the application of different isoflurane concentrations (0.5–5%).
3. In the presence of the GABA_A antagonist, bicuculline methiodide (15 μM), isoflurane suppressed the f.e.p.s.ps to the same extent as was observed in the absence of the GABA_A antagonist.
4. Orthodromically evoked population spikes were suppressed by isoflurane in a manner quantitatively similar to the suppression of the f.e.p.s.ps. However, antidromic population spikes and presynaptic volleys evoked in young and old slices were resistant to anaesthetic action. In addition, paired pulse facilitation ratio of the evoked dendritic f.e.p.s.ps was not affected in both young and old slices during the application of isoflurane.
5. When slices were exposed to low Ca²⁺/high Mg²⁺ solution, isoflurane (1 and 3%) depressed the f.e.p.s.ps in aged slices to the same extent as in young slices.
6. The augmented anaesthetic depression of f.e.p.s.ps in old compared to young hippocampi in the absence and presence of bicuculline, and the lack of anaesthetic effects on antidromic population spikes and presynaptic volleys in old and young slices, suggest that the increased sensitivity of anaesthetic actions in old hippocampi is due to age-induced attenuation of synaptic excitation rather than potentiation of synaptic inhibition. Furthermore, elimination of the increased sensitivity of old slices to anaesthetic actions when the slices were perfused with low Ca²⁺/high Mg²⁺ medium, which presumably would decrease intracellular [Ca²⁺], suggests that the enhanced anaesthetic effects in aged neurones might be related to increased intraneuronal [Ca²⁺] in the synaptic terminal.

     

Cannabidiol inhibits synaptic transmission in rat hippocampal cultures and slices via multiple receptor pathways

BACKGROUND AND PURPOSE

Cannabidiol (CBD) has emerged as an interesting compound with therapeutic potential in several CNS disorders. However, whether it can modulate synaptic activity in the CNS remains unclear. Here, we have investigated whether CBD modulates synaptic transmission in rat hippocampal cultures and acute slices.

EXPERIMENTAL APPROACH

The effect of CBD on synaptic transmission was examined in rat hippocampal cultures and acute slices using whole cell patch clamp and standard extracellular recordings respectively.

KEY RESULTS

Cannabidiol decreased synaptic activity in hippocampal cultures in a concentration-dependent and Pertussis toxin-sensitive manner. The effects of CBD in culture were significantly reduced in the presence of the cannabinoid receptor (CB₁) inverse agonist, {"type":"entrez-nucleotide","attrs":{"text":"LY320135","term_id":"1257555575","term_text":"LY320135"}}LY320135 but were unaffected by the 5-HT_1A receptor antagonist, WAY100135. In hippocampal slices, CBD inhibited basal synaptic transmission, an effect that was abolished by the proposed CB₁ receptor antagonist, AM251, in addition to {"type":"entrez-nucleotide","attrs":{"text":"LY320135","term_id":"1257555575","term_text":"LY320135"}}LY320135 and WAY100135.

CONCLUSIONS AND IMPLICATIONS

Cannabidiol reduces synaptic transmission in hippocampal in vitro preparations and we propose a role for both 5-HT_1A and CB₁ receptors in these CBD-mediated effects. These data offer some mechanistic insights into the effects of CBD and emphasize that further investigations into the actions of CBD in the CNS are required in order to elucidate the full therapeutic potential of CBD.     

Excitatory and inhibitory synaptic transmission is differentially influenced by two ortho-substituted polychlorinated biphenyls in the hippocampal slice preparation

Exposure to polychlorinated biphenyls impairs cognition and behavior in children. Two environmental PCBs 2,2',3,3',4,4',5-heptachlorobiphenyl (PCB170) and 2,2',3,5',6-pentachlorobiphenyl (PCB95) were examined in vitro for influences on synaptic transmission in rat hippocampal slices. Field excitatory postsynaptic potentials (fEPSPs) were recorded in the CA1 region using a multi-electrode array. Perfusion with PCB170 (10 nM) had no effect on fEPSP slope relative to baseline period, whereas (100 nM) initially enhanced then depressed fEPSP slope. Perfusion of PCB95 (10 or 100 nM) persistently enhanced fEPSP slope > 200%, an effect that could be inhibited by dantrolene, a drug that attenuates ryanodine receptor signaling. Perfusion with picrotoxin (PTX) to block GABA neurotransmission resulted in a modest increase in fEPSP slope, whereas PTX + PCB170 (1-100 nM) persistently enhanced fEPSP slope in a dose dependent manner. fEPSP slope reached > 250% of baseline period in the presence of PTX + 100 nM PCB170, conditions that evoked marked epileptiform after-potential discharges. PCB95 and PCB170 were found to differentially influence the Ca²⁺-dependence of [³H]ryanodine-binding to hippocampal ryanodine receptors. Non-coplanar PCB congeners can differentially alter neurotransmission in a manner suggesting they can elicit imbalances between inhibitory and excitatory circuits within the hippocampus. Differential sensitization of ryanodine receptors by Ca²⁺ appears to mediate, at least in part, hippocampal excitotoxicity by non-coplanar PCBs.     

Cannabinoid CB1 receptor-mediated inhibition of glutamate release from rat hippocampal synaptosomes

Cannabinoid receptors are widely expressed in the brain and have been shown to regulate synaptic transmission through a presynaptic mechanism. Using synaptosomal preparation, I show here that 2,3-dihydro-5-methyl-3-(4-morpholinyl-methyl)-pyrrolo-1,4-benzoxazin-6-yl-1-naphthalenylmethanone (WIN 55212-2) strongly depressed 4-aminopyridine-evoked glutamate release in a concentration-dependent manner, and this effect was reversed by the selective cannabinoid CB(1) receptor antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM 281). The inhibitory modulation by WIN 55212-2 was not due to a decrease in synaptosomal excitability or a direct effect on the release machinery because WIN 55212-2 did not alter 4-aminopyridine-mediated depolarization and ionomycin-induced glutamate release. In addition, the WIN 55212-2-mediated inhibition of glutamate release was blocked by the G(i)/G(o) protein inhibitor pertussis toxin, but not by the protein kinase A inhibitor 2,3,9,10,11,12-Hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo-benzodiazocine-10-carboxylic acid, hexyl ester (KT 5720). Furthermore, this inhibitory effect was associated with a decrease in 4-aminopyridine-evoked Ca(2+) influx, which could be completely prevented in synaptosomes pretreated with the N- and P/Q-type Ca(2+) channel blockers. Together, these observations indicate that activation of cannabinoid CB(1) receptors inhibit 4-aminopyridie-evoked glutamate release from hippocampal synaptosomes through a inhibitory G protein to suppress N- and P/Q-type Ca(2+) channel activity.     

Interactions of glutamate receptor agonists with long-term potentiation in the rat hippocampal slice

Previous work has described the apparent desensitisation of neuronal networks in the rat neocortex to amino acid agonists, following prior exposure several minutes earlier. Since long-term potentiation is believed to involve activation of amino acid receptors, we have now sought to determine whether long-term potentiation can modify the sensitivity of neurones to glutamate receptor agonists in rat hippocampal slices. Responses were measured as the change in population spike or postsynaptic potential (e.p.s.p.) size. Two applications of N-methyl-D-aspartate (NMDA), quinolinic acid, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or kainate, 45 min apart, did not exhibit any apparent desensitisation. However, the induction of long-term potentiation produced a marked loss of sensitivity to quinolinic acid, with smaller effects on NMDA, AMPA and kainate responses. No marked changes were obtained of e.p. s.p. size. In order to localise the cellular sites of these changes, agonists were also applied by microiontophoresis to the cell bodies or dendritic regions of CA1 neurones. Responses to quinolinic acid showed apparent desensitisation at both sites, whereas no decrease was observed in responses to NMDA or AMPA application. The induction of long-term potentiation again produced a decrease in the size of responses to NMDA and AMPA. Inhibition of nitric oxide (NO) synthase prevented the long-term potentiation-induced loss of responsiveness to NMDA, but not AMPA, implying a role for NO in the loss of NMDA sensitivity. Recordings of single cell activity during the iontophoretic application of agonists and induction of long-term potentiation showed that responses to NMDA were often suppressed to a greater extent than to quinolinic acid. The results indicate that long-term potentiation can modify the sensitivity of hippocampal neurones to glutamate receptor agonists, and that differences exist in the pharmacology of NMDA and quinolinic acid.     

Effect of phencyclidine on inhibition in the hippocampal slice

The effects of phencyclidine (PCP) on synaptic transmission were studied in the hippocampal slice. Population spikes evoked by orthodromic or antidromic stimulation were recorded from CAl pyramidal cells. Bath applied PCP (10(-4) M) reduced moderately both the orthodromic and antidromic population spikes. Lower concentrations, 5 X 10(-6) to 5 X 10(-5) M of PCP, which did not depress the population spikes, reduced inhibition of the orthodromically evoked spike in a dose dependent reversible manner. Diazepam (10(-6) to 10(-5) M) restored the inhibition despite the continued presence of PCP. It is suggested that PCP-induced seizures and other signs of hyperexcitability could be a result of reduced inhibition.     

Characterisation of the effects of ATPA,a GLU(K5) receptor selective agonist,on excitatory synaptic transmission in area CA1 of rat hippocampal slices

Kainate receptors are involved in a variety of synaptic functions in the CNS including the regulation of excitatory synaptic transmission. Previously we described the depressant action of the GLU(K5) selective agonist (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid (ATPA) on synaptic transmission in the Schaffer collateral-commissural pathway of rat hippocampal slices. In the present study we report several new features of the actions of ATPA at this synapse. Firstly, the effectiveness of ATPA is developmentally regulated. Secondly, the effects of ATPA decline during prolonged or repeated applications. Thirdly, the effects of ATPA are not mediated indirectly via activation of GABA(A), GABA(B), muscarinic or adenosine A(1) receptors. Fourthly, elevating extracellular Ca(2+) from 2 to 4 mM antagonises the effects of ATPA. Some differences between the actions of ATPA and kainate on synaptic transmission in the Schaffer collateral-commissural pathway are also noted.