Timing of the Developmental Switch in GABAA Mediated Signaling from Excitation to Inhibition in CA3 Rat Hippocampus Using Gramicidin Perforated Patch and Extracellular Recordings

Summary:  The timing of the developmental switch in the GABA_A mediated responses from excitatory to inhibitory was studied in Wistar rat CA3 hippocampal pyramidal cells using gramicidin perforated patch-clamp and extracellular recordings. Gramicidin perforated patch recordings revealed a gradual developmental shift in the reversal potential of synaptic and isoguvacine-induced GABA_A mediated responses from –55 ± 4 mV at postnatal days P0–2 to −74 ± 3 mV at P13–15 with a midpoint of disappearance of the excitatory effects of GABA at around P8. Extracellular recordings in CA3 pyramidal cell layer revealed that the effect of isoguvacine on multiple unit activity (MUA) switched from an increase to a decrease at around P10. The effect of synaptic GABA_A mediated responses on MUA switched from an increase to a decrease at around P8. It is concluded that the developmental switch in the action of GABA via GABA_A receptors from excitatory to inhibitory occurs in Wistar rat CA3 pyramidal cells at around P8–10, an age that coincides with the transition from immature to mature hippocampal rhythms. We propose that excitatory GABA contributes to enhanced excitability and ictogenesis in the neonatal rat hippocampus.     

Role of GABAA receptors in the physiology and pharmacology of sleep

Most sedative-hypnotics used in insomnia treatment target the γ-aminobutyric acid (GABA)_A receptors. A vast repertoire of GABA_A receptor subtypes has been identified and displays specific electrophysiological and functional properties. GABA_A-mediated inhibition traditionally refers to 'phasic' inhibition, arising from synaptic GABA_A receptors which transiently inhibit neurons. However, there is growing evidence that peri- or extra-synaptic GABA_A receptors are continuously activated by low GABA concentrations and mediate a 'tonic' conductance. This slower type of signaling appears to play a key role in controlling cell excitability. This review aims at summarizing recent knowledge on GABA transmission, including the emergence of tonic conductance, and highlighting the importance of GABA_A receptor heterogeneity. The mechanism of action of sedative-hypnotic drugs and their effects on sleep and the electroencephalogram will be reported. Furthermore, studies using genetically engineered mice will be emphasized, providing insights into the role of GABA_A receptors in mechanisms underlying physiological and pharmacological sleep. Finally, we will address the potential of GABA_A receptor pharmacology for the treatment of insomnia.     

Effect of a γ-Aminobutyric Acid Uptake Inhibitor, NNC-711, on Spontaneous Postsynaptic Currents in Cultured Rat Hippocampal Neurons: Implications for Antiepileptic Drug Development

Summary: The effect of a novel γ-aminobutyric acid (GABA) uptake inhibitor, NNC-711, on spontaneous postsynaptic currents was studied in cultured rat hippocampal neurons by the whole cell patch clamp method. NNC-711 decreased the amplitude of inhibitory postsynaptic currents (IPSCs) and did not prolong the decay. NNC-711 also decreased the amplitude of excitatory PSCs (EPSCs). The GABA_B receptor antagonist 2-OH saclofen abolished the effect on both IPSCs and EPSCs. NNC-711 itself induced no current and had no effect on currents induced by exogenously applied GABA. These findings suggest that duration of GABA_A-receptor mediated IPSCs is not determined by GABA uptake and that GABA uptake inhibitors may work by allowing GABA to remain in the synaptic area long enough to activate presynaptic GABA_B receptors.     

Multiple functions of GABA A and GABA B receptors during pattern processing in the zebrafish olfactory bulb

γ-Aminobutyric acid (GABA)ergic synapses are thought to play pivotal roles in the processing of activity patterns in the olfactory bulb (OB), but their functions have been difficult to study during odor responses in the intact system. We pharmacologically manipulated GABA_A and GABA_B receptors in the OB of zebrafish and analysed the effects on odor responses of the output neurons, the mitral cells (MCs), by electrophysiological recordings and temporally deconvolved two-photon Ca²⁺ imaging. The blockade of GABA_B receptors enhanced presynaptic Ca²⁺ influx into afferent axon terminals, and changed the amplitude and time course of a subset of MC responses, indicating that GABA_B receptors have a modulatory influence on OB output activity. The blockade of GABA_A receptors induced epileptiform firing, enhanced excitatory responses and abolished fast oscillations in the local field potential. Moreover, the topological reorganization and decorrelation of MC activity patterns during the initial phase of the response was perturbed. These results indicate that GABA_A receptor-containing circuits participate in the balance of excitation and inhibition, the regulation of total OB output activity, the synchronization of odor-dependent neuronal ensembles, and the reorganization of odor-encoding activity patterns. GABA_A and GABA_B receptors are therefore differentially involved in multiple functions of neuronal circuits in the OB.     

Antiepileptic Drug Mechanisms of Action

Summary: Established antiepileptic drugs (AEDs) decrease membrane excitability by interacting with neurotransmitter receptors or ion channels. AEDs developed before 1980 appear to act on sodium channels, γ-ami-nobutyric acid type A (GABA_A) receptors, or calcium channels. Benzodiazepines and barbiturates enhance GABA_A receptor-mediated inhibition. Phenytoin (PHT), carbamazepine (CBZ), and possibly valproate (VPA) decrease high-frequency repetitive firing of action potentials by enhancing sodium-channel inactivation. Ethosuximide (ESM) and VPA reduce a low threshold (T-type) calcium-channel current. The mechanisms of action of the new AEDs are not fully established. Gabapentin (GBP) binds to a high-affinity site on neuronal membranes in a restricted regional distribution of the central nervous system. This binding site may be related to a possible active transport process of GBP into neurons; however, this has not been proven, and the mechanism of action of GBP remains uncertain. Lamotrigine (LTG) decreases sustained high-frequency repetitive firing of voltage-dependent sodium action potentials that may result in a preferential decreased release of presynaptic glutamate. The mechanism of action of oxcarbazepine (OCBZ) is not known; however, its similarity in structure and clinical efficacy to CBZ suggests that its mechanism of action may involve inhibition of sustained high-frequency repetitive firing of voltage-dependent sodium action potentials. Vigabatrin (VGB) irreversibly inhibits GABA transaminase, the enzyme that degrades GABA, thereby producing greater available pools of presynaptic GABA for release in central synapses. Increased activity of GABA at postsynaptic receptors may underlie the clinical efficacy of VGB.     

Giant GABAB-mediated Synaptic Potentials Induced by Zinc in the Rat Hippocampus: Paradoxical Effects of Zinc on the GABAB Receptor

The interaction of zinc with pre- and postsynaptic GABA_B receptors was studied in adult rat hippocampal slices using intracellular recording in CA1 and CA3 pyramidal neurons. Zinc (50 – 300 μM) antagonized baclofen responses with a variable potency, whereas CGP-35348 (100 μM) or barium (300 μM) produced a more substantial and consistent inhibition. Zinc also induced giant GABA_A-mediated depolarizing potentials (GDP) in these neurons. After blocking GABA_A and excitatory synaptic transmission, monosynaptic hyperpolarizing inhibitory postsynaptic potentials (IPSP) mediated by GABA_B receptors (IPSP_B) were inhibited by CGP-35348 or barium; however, zinc increased the latency and prolonged the duration of the IPSP_B and also induced the appearance of spontaneous giant GABA_B-mediated hyperpolarizing potentials (GHP). In some cells, IPSP_Bs in zinc exhibited a multiphasic appearance. The early component was partially inhibited by 300 μM zinc and was followed by a late GHP. CGP-35348 at 100 μM inhibited the early monosynaptic IPSP_B but not the GHP; however, at 300 μM both components were blocked. Paired-pulse inhibition of the IPSP_B was used to assess the effect of zinc on presynaptic GABA_B receptors. Neither the zinc-chelating agent CP94 (400 μM) nor zinc affected this phenomenon. CGP-35348, barium and polyvalent cations, such as cadmium, copper, cobalt, manganese, iron and aluminium, failed to induce giant potentials in hippocampal neurons. It is concluded that zinc is apparently unique in synchronizing the release of GABA to produce GDPs and GHPs.     

Activation of the GABAA Receptor Inhibits the Proliferative Effects of bFGF in Cortical Progenitor Cells

Recent studies have localized γ-aminobutyric acid (GABA)-containing neurons and identified cells that express subunits of the GABA_A receptor in the proliferative zone of the developing cerebral cortex and have demonstrated a role for GABA in cortical neurogenesis. We examined here the interactions between a number of neurotrophic factors, known to be involved in cortical cell proliferation and differentiation, and the GABAergic system (GABA and GABA_A receptors) in the regulation of cell production in dissociated cortical cell cultures. We found that basic fibroblast growth factor (bFGF) increased the number of cells labelled for the α₁ subunit of the GABA_A receptor but not for the α₂, α₃ or α₅ subunits. The α₁ subunit was expressed by the majority of proliferating neuroepithelial cells as well as by differentiated neurons. We also found that activation of the GABA_A receptor by GABA or muscimol inhibited the proliferative effects of bFGF on cortical progenitors, leading to an increased number of differentiated neurons. These results suggest that bFGF stimulates cell proliferation and GABA_A receptor expression in cultured progenitor cells of the developing neocortex, and that GABA regulates cell production by providing a feedback signal that terminates cell division.     

The role of gamma-aminobutyric acid in the mechanism of action of anticonvulsant drugs

Decreased activity of gamma-aminobutyric acid, the major inhibitory neurotransmitter in CNS can be epileptogenic. Manipulation of the GABA system has been a target for development of antiepileptic drugs. The different ways for augmenting gabaergic inhibition by conventional and new AEDs are presented in this paper. Among the I generation, barbiturates and benzodiazepines are potent anticonvulsants that act as GABA modulators in postsynaptic GABA-A receptor complex but their usefulness is limited by dependence and tolerance to antiseizure activity. The II generation drugs vigabatrin and tiagabine, and to some extent gabapentin have been developed by a rationale strategy and none of them exert direct action in GABA receptors. Only two former drugs exhibit selective, strictly defined activity: vigabatrine is an irreversible inhibitor of GABA-aminotransferase and tiagabine acts as a GABA-uptake inhibitor from synaptic cleft into neurons and glia. Gabapentin binds to a novel receptors in epileptogenic areas in CNS and enhances GABA turnover. Drugs with multiple mechanisms of action, felbamate and topiramate not only potentiate gabaergic inhibition in several ways but also diminish the activity of excitatory amino acids at their NMDA or AMPA receptors; the later mechanism seems to be essential for their potential neuroprotective activity in epileptogenesis. None of gabamimetic drugs provide optimal seizure control but better tolerability of newer ones and well-established mechanisms of action provide possible harmless therapy.     

Ethanol alters synaptic activity in cultured spinal cord neurons

The acute effects of ethyl alcohol on mammalian central neurons were investigated using electrophysiological techniques and an in vitro model system, cultured fetal mouse spinal cord neurons. Intracellular recordings were made from the cultured neurons to evaluate the effect of alcohol (10-100 mM) on membrane potential, membrane permeability, amplitude of the action potential, sensitivity of the neurons to putative neurotransmitters and the process of synaptic transmission. Alcohol was applied by superfusion; putative amino acid neurotransmitters were applied by micropressure ejection. The most dramatic effect of alcohol on the spinal cord neurons was a reduction in the spontaneous activity (excitatory and inhibitory synaptic potentials and action potentials) and the glutamate evoked synaptic activity. Alcohol doses as low as 20-30 mM, concentrations which reflect blood levels during intoxication, were effective. Membrane potential, membrane permeability, and amplitude of the action potential were relatively resistant to these low doses of alcohol; at the higher alcohol doses, no effect or only modest alterations of these characteristics were observed. The responses of the neurons to the putative excitatory neuro-transmitter glutamate, and inhibitory transmitters GABA and glycine were also relatively resistant to alcohol exposure. These data indicate that acute exposure to alcohol has a predominantly inhibitory action on the activity of the cultured mammalian CNS neurons, and that this inhibition is most likely due to an alteration in the process of synaptic transmission.     

Immature Neurons and GABA Networks May Contribute to Epileptogenesis in Pediatric Cortical Dysplasia

Summary:  Cortical dysplasia (CD), a frequent pathological substrate of pediatric epilepsy surgery patients, has a number of similarities with immature cortex, such as reduced Mg²⁺ sensitivity of N-methyl-D-aspartate (NMDA) receptors and the persistence of subplate-like neurons and undifferentiated cells. Because γ-aminobutyric acid (GABA) is the main neurotransmitter in early cortical development, we hypothesized increased GABA receptor-mediated synaptic function in CD tissue. Infrared videomicroscopy and whole-cell patch clamp recordings were used to characterize the morphology and electrophysiological properties of immature and normal-appearing neurons in slices from cortical tissue samples resected for the treatment of pharmacoresistant epilepsy in children (0.2–14 years). In addition, we examined spontaneous and evoked synaptic activity, as well as responses to exogenous GABA application. We demonstrate both the presence of immature pyramidal neurons and networks in young CD tissue and the predominance of GABA synaptic activity. In addition, spontaneous GABA depolarizations frequently induced action potentials, supporting a potential excitatory role of GABA in CD. Evoked synaptic responses mediated by GABA were also prominent, and bath application of 4-aminopyridine induced rhythmic depolarizations that were blocked by bicuculline. Finally, responses to exogenous application of GABA had depolarized reversal potentials in severe compared to mild and non-CD cases. The present data support the hypothesis that CD shares features of immature cortex, with predominant and potentially excitatory GABA_A receptor-mediated neurotransmission. These results could partially explain the increased excitability of the cortical network in pediatric CD.     

Functional Heterogeneity of Hippocampal GABAA Receptors

γ-Aminobutyric acid type A (GABA_A) receptors were studied in cultured neurons taken from rat hippocampus at early postnatal stages. GABA-induced whole-cell currents showed a broad range of peak amplitudes and time-courses of desensitization. Dose – response curves of rapidly and slowly desensitizing cells revealed EC₅₀ values of 8.5 and 37.3 μM GABA, respectively, with the Hill coefficient being greater than unity. The main-state conductance of GABA_A receptor channels was 28 – 31 pS in all cells. GABA responses of low-affinity cells were more strongly affected by benzodiazepine receptor agonists (e.g. flunitrazepam, clonazepam) and inverse agonists (e.g. methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate), as compared to cells exhibiting high-affinity GABA responses. Currents were also potentiated by zolpidem, but were little affected by Ro 15-4513 and Zn²⁺. These data suggest the presence of physiologically and pharmacologically distinct GABA_A receptor isoforms in neurons of the early postnatal hippocampus, which may subserve different inhibitory control mechanisms in this brain region.     

Actions of propofol on substantia gelatinosa neurones in rat spinal cord revealed by in vitro and in vivo patch-clamp recordings

Propofol, an intravenous general anaesthetic, exerts anaesthetic actions through interaction with γ-aminobutyric acid type A (GABA_A) receptors in the supraspinal nervous system. However, whether propofol has any significant effects on synaptic transmission at the spinal level and whether it exhibits antinociceptive action is still not fully clarified. Spontaneous activity and stimulus-evoked responses of substantia gelatinosa (SG) neurones to noxious pinch stimuli were recorded using spontaneously breathing rats under propofol anaesthesia using in vivo whole-cell patch-clamp techniques. Precise actions of propofol on GABAergic and glycinergic inhibitory postsynaptic currents (IPSCs) as well as excitatory postsynaptic currents (EPSCs) in SG neurones were also analyzed in spinal cord slice preparations. At clinical doses (5 mg/kg), propofol reversibly depressed action potentials elicited by noxious mechanical stimuli applied to the skin in the majority (6/8) of SG neurons recorded under in vivo conditions. This depression may have been caused by interactions of propofol with GABA_A receptors, as decay time of GABAergic sIPSCs was prolonged after propofol injection (128 ± 11% of control, n  = 5) with minimal effect on EPSCs. Although prolongation of IPSCs in vivo was reversible, IPSCs were progressively prolonged even after washout of propofol when the effect was tested using spinal cord slices. Propofol had a mild depressant effect on Aδ- and C-afferent-mediated EPSCs. We conclude that systemic bolus injection of propofol reversibly depressed nociceptive transmission, at least in part, by enhancing postsynaptic GABA_A receptor-mediated responses in the SG.     

Despite GABAergic neurotransmission, GABAergic innervation does not compensate for the defect in glycine receptor postsynaptic aggregation in spastic mice

In the hypoglossal nucleus of wild-type mice, early mixed glycinergic-GABAergic inhibitory transmission becomes mainly glycinergic during postnatal maturation. In spastic mice (SPA), a model of human hyperekplexic syndrome, an insertion into the gene of the glycine receptor (GlyR) β subunit results in a decreased accumulation of GlyRs at postsynaptic sites and an impaired glycinergic neurotransmission. In SPA mice displaying a mild phenotype (B6C3Fe strain), a compensatory process involving an increased aggregation of GABA_A receptors (GABA_ARs) at postsynaptic sites was proposed to explain survival of mutant animals until adulthood. However, C57BL/6J strain SPA mice which express a lower amount of GlyR β subunit die 2–3 weeks after birth, suggesting that GABAergic compensation does not necessarily take place. We performed a morphofunctional study of inhibitory synapses in the developing hypoglossal nucleus of C57BL/6J SPA mice. In this mutant, the inhibitory synaptic activity was mainly GABAergic. Accordingly, we observed a developmental loss of glycinergic presynaptic terminals and an increase in the density of GABAergic presynaptic terminals during the first two postnatal weeks. In addition, while C57BL/6J SPA mice displayed a strong impairment in GlyR aggregation at postsynaptic loci, the proportion of inhibitory presynaptic terminals facing diffuse GABA_ARs significantly increased during development. Our results suggest crosstalk between postsynaptic and presynaptic elements, leading to the developmental regulation of the presynaptic terminal neurotransmitter content according to the level of postsynaptic GlyR aggregation. They also indicate that GABAergic neurotransmission does not compensate for defects in GlyR postsynaptic aggregation leading to spastic syndrome in C57BL/6J SPA mice.     

Synchronous GABAA-receptor-dependent potentials in limbic areas of the in-vitro isolated adult guinea pig brain

Epileptiform discharges are known to reflect the hypersynchronous glutamatergic activation of cortical neurons. However, experimental evidence has revealed that epileptiform synchronization is also contributed to by population events mediated by GABA_A receptors. Here, we analysed the spatial distribution of GABA_A-receptor-dependent interictal events in the hippocampal/parahippocampal region of the adult guinea pig brain isolated in vitro . We found that arterial perfusion of this preparation with 4-aminopyridine caused the appearance of glutamatergic-independent interictal potentials that were reversibly abolished by GABA_A receptor antagonism. Laminar profiles and current source density analysis performed in different limbic areas demonstrated that these GABA_A-receptor-mediated events were independently generated in different areas of the hippocampal/parahippocampal formation (most often in the medial entorhinal cortex) and propagated between interconnected limbic structures of both hemispheres. Finally, intracellular recordings from principal neurons of the medial entorhinal cortex demonstrated that the GABAergic field potential correlated to inhibitory postsynaptic potentials (membrane potential reversal, −68.12 ± 8.01 mV, n  = 5) that were interrupted by ectopic spiking. Our findings demonstrate that, in an acute seizure model developed in the adult guinea pig brain, hypersynchronous GABA_A-receptor-mediated interictal events are generated from independent sources and propagate within limbic cortices in the absence of excitatory synaptic transmission. As spared or enhanced inhibition was reported in models of epilepsy, our data may support a role of GABA-mediated signaling in ictogenesis and epileptogenesis.     

The medical management of spasticity

When spasticity produces a clinical disability by interfering with posture, motor capacity, nursing or daily living activities, medical treatment is recommended. It is mainly indicated when the muscle overactivity is diffusely distributed and should be implemented early, to prevent permanent musculoskeletal deformities or contractures. A pharmacological approach relies on the use of drugs which modulate neurotransmitters acting at the cortico-spinal level (GABA, glycine, glutamate, noradrenaline, serotonin). The aim of this treatment is to decrease spinal reflex excitability by reducing the release of excitatory neurotransmitters, or by potentiating the activity of inhibitory inputs. Evaluation of the efficacy of these drugs is determined by the therapeutic objectives which may be biomechanical, or functional. Diazepam increases presynaptic inhibition by stimulating GABA_A receptors in the brainstem and spinal cord. In double-blind studies of patients with spinal cord lesions, antispastic efficacy has been shown, but side-effects are common. Baclofen stimulates GABA_B receptors inducing a suppression of excitatory neurotransmitter release. Antispastic efficacy is sufficiently documented, but no definite effects on ambulation or activities of daily living have been proved. Tizanidine has an α₂-agonist activity (at spinal and supraspinal level) and decreases the presynaptic activity of excitatory interneurones. The main clinical effects are a reduction in tonic stretch, polysynaptic reflexes, and co-contraction, with fewer side-effects but no definite functional change. The efficacy of several other antispastic drugs is documented in a few controlled studies, but the majority of information arises from open trials or anecdotal observations.     

GABA: an excitatory transmitter in early postnatal life

In the adult mammalian CNS, GABA is the main inhibitory transmitter. It inhibits neuronal firing by increasing a Cl⁻ conductance. Bicuculline blocks this effect and induces interictal discharges. A different picture is present in neonatal hippocampal neurones, where synaptically released or exogenously applied GABA depolarizes and excites neuronal membranes - an effect that is due to a different Cl⁻ gradient. In fact, during the early neonatal period, GABA acting on GABA_A receptors provides most of the excitatory drive, whereas excitatory glutamatergic synapses are quiescent. It is suggested that during development GABA exerts mainly a trophic action through membrane depolarization and a rise in intracellular Ca²⁺.     

GABA-Mediated Synchronous Potentials and Seizure Generation

Summary: This article summarizes findings related to a synchronous, GABA-mediated potential that may contribute to the initiation and spread of epileptiform discharges within the brain. This phenomenon is observed in cortical structures such as the hippocampus, the entorhinal cortex, and the neocortex during application of low concentrations of 4-amimopyridine and is characterized at the intracellular level by a long-lasting membrane depolarization. The synchronous, GABA-mediated potential continues to occur after blockade of excitatory synaptic transmission and relays on the synchronous firing of inhibitory interneurons and consequent activation of postsynaptic (mainly type A) GABA receptors leading to a transient elevation of [K⁺]_o.
Studies performed in young rat hippocampus indicate that the synchronous, GABA-mediated potential may play a role in initiating ictal discharges under normal conditions (i.e., when excitatory amino acid receptors are operant). Moreover, a similar phenomenon may also occur in adult rat entorhinal cortex. These findings therefore indicate a novel role that is played by GABA_A receptors in limbic structures. The ability of this synchronous GABA-mediated potential to propagate in the absence of excitatory synaptic transmission may also be relevant for the propagation of synchronous activity outside conventional neuronal-synapse dependent pathways. This condition may occur in brain structures with neuronal loss and consequent disruption of normal excitatory synaptic connections such as mesial limbic structures of temporal lobe epilepsy patients with Ammon's horn sclerosis.     

Serotonin (1A) receptor ligands act on norepinephrine neuron firing through excitatory amino acid and GABA(A) receptors: a microiontophoretic study in the rat locus coeruleus.

It was previously shown that the excitatory effect of the 5-HT(1A) agonist 8-OH-DPAT on firing activity of locus coeruleus (LC) norepinephrine (NE) neurons and the inhibitory action of the 5-HT(1A) antagonist WAY 100,635 are dependent on the presence of 5-HT neurons, whereas the inhibitory action of the 5-HT(2) agonist DOI is not. Using in vivo extracellular unitary recordings performed in anesthetized rats, iontophoretic applications of the excitatory amino acid antagonist kynurenate attenuated the enhancement in firing produced by glutamate and kainate. In contrast, GABA applications decreased the firing activity of NE neurons which was attenuated by the enhancement produced by glutamate and kainate. In contrast, GABA applications decreased the firing activity of NE neurons which was attenuated by the GABA(A) receptor antagonist bicuculline. 8-OH-DPAT (10-60 microg kg(-1), i.v.) produced a dose-dependent enhancement in the firing activity of NE neurons that was abolished in the presence of kynurenate application. The selective 5-HT(1A) receptor antagonist WAY 100,635 (100 microg kg(-1), i.v.) suppressed NE firing which was reversed by the selective 5-HT(2A) antagonist MDL 100,907 (200 microg kg(-1), i.v.). In the presence of bicuculline, the inhibitory effect of WAY 100,635 was blunted. These results suggest that WAY 100,635 mainly attenuates NE neuron firing by blocking inhibitory 5-HT(1A) receptors on glutamatergic neurons, thereby enhancing glutamate release and activating excitatory amino acid receptors, possibly of the kainate subtype, on 5-HT terminals. The ensuing increased 5-HT release would then act on excitatory 5-HT(2A) receptors on GABA neurons that would ultimately mediate the inhibition of NE neurons. The prevention of the excitatory action of 8-OH-DPAT on NE neuron firing by kynurenate is also consistent with this neurocircuitry.     

Glutamate Metabotropic Receptor Agonists Depress Excitatory and Inhibitory Transmission on Rat Mesencephalic Principal Neurons

lntracellular and whole-cell patch-clamp recordings were used to evaluate the actions of different metabotropic glutamate receptor (mGluR) agonists on the synaptic inputs evoked on principal cells of the rat mesencephalon. Bath application of the group Ill mGluR agonists l-2-amino-4-phosphonobutyric acid (l-AP4) and l-serine- O -phosphonobutanoate (l-SOP) did not change the holding current of the cells held at resting potential (-60 mV) but produced a dose-dependent inhibition of the amplitude of the excitatory and inhibitory events. l-AP4 and l-SOP were more effective at inhibiting the excitatory postsynaptic currents (EPSCs) than the GABA_A and GABA_B inhibitory postsynaptic currents (IPSCs). The suppressing effects of l-AP4 and l-SOP were antagonized by ( S )-2-amino-2-methyl-4-phosphonobutanoic acid (MAP-4) but not by ±-α-methyl-4-carboxyphenylglycine (MCPG). Moreover, the group II agonist (2 S , 1' S , 2' S )-(carboxycyclopropyl)glycine (l-CCG1) and the group I agonist ( RS )-3,5-dihydrophenylglycine (3,5-DHPG) depressed in a dose-related manner the EPSC, the GABA_A IPSC and the GABA_B IPSC. The suppressing effect of the two mGluRs agonists was partially antagonized by MCPG but not by MAP-4. In addition, both l-CCG1 and 3,5-DHPG caused an inward shift of the holding current. To characterize the site of action of the metabotropic receptor agonists, experiments were performed to examine the amplitude and ratio of EPSC and GABA_A IPSC pairs. The increase of the s2/s l ratio caused by the agonists suggests that the location of the inhibitory mGluRs was presynaptic. These results indicate that the activation of presynaptic mGluRs controls the release of excitatory and inhibitory transmitters on presumed dopaminergic cells within the ventral mesencephalon.     

Lemniscal Input to Identified Neurons of the Central Nucleus of Mouse Inferior Colliculus: an Intracellular Brain Slice Study

Intracellular recordings were performed in 34 neurons in the central nucleus of the inferior colliculus in brain slice preparations of the mouse. Sixteen neurons recorded were stained intracellularly by injection of biocytin and identified as multipolar. After electrical stimulation of the lateral lemniscus, 32 of 34 neurons exhibited postsynaptic potentials (PSPs). Onset latencies of the PSPs were 5.0±2.8 ms (range 2-12 ms), presumably reflecting the lack of a significant monosynaptic input to most of the neurons recorded. An excitatory PSP (EPSP), often followed by a late inhibitory PSP (IPSP), was present in all neurons which received synaptic input. The IPSPs usually had a reversal potential positive to the cell's resting membrane potential, thus working as shunting inhibitors. Superfusion of the slice with the GABA_A antagonist bicuculline resulted in blockade of the IPSP and pronounced prolongation of the EPSP. In 50% of these cases, paroxysmal depolarizing shifts were observed in the presence of bicuculline. Blocking the non-NMDA glutamate receptors with 6,7-dinitroquinoxaline-2,3-dione resulted not only in the total disappearance of EPSPs but also of late IPSPs, indicating that the latter depend on the glutamatergic EPSPs. Furthermore, all neurons recorded must receive substantial innervation from sources within the inferior colliculus, together constituting a complex neuronal network in the inferior colliculus with an important role of the inhibitory neurotransmitter GABA in controlling network properties.