The GABAB antagonist phaclofen inhibits the late K+-dependent IPSP in cat and rat thalamic and hippocampal neurones

The impact of hydroalcoholic extract of Anacyclus pyrethrum plant on epileptic seizure induced by pentylenetetrazole in male rat

Background

The aim was to evaluate the protective effect of hydroalcoholic extract of Anacyclus pyrethrum root (APE) against pentylenetetrazole (PTZ) drug which is used for inducing epileptic seizures in animal model.

Results

50 male rats were divided: control (without any intervention), positive control 1st (received PTZ 60 mg/kg, IP), first experimental group (PTZ + Extract 500 mg/kg, gavages, 30 min before PTZ), positive control 2nd (PTZ + Phaclofen, 200 µg/µl, ICV), and second experimental group (PTZ + extract 500 mg/kg, gavage, 30 min before PTZ + Phaclofen 200 µg/µl, ICV). Several parameters were assessed during 20 min and followed up for 1.5 h. Then, the data were analyzed. APE with a dose of 500 mg/kg increased the latency time of seizures in the first experimental group, compared to the positive control 1st, also, comparison of different groups in terms of Seizure Score at the 1st time (severity of first attack) had no significant difference (P-value = 0.51, P-value = 0.34). The mean of seizure attacks (event number) was significant between the first and second positive control groups (P-value = 0.01) and also between the second positive control and the first experimental group (P-value = 0.011). Significant changes were observed in the mean score of the first and second positive control groups (P-value = 0.001) and the first experimental and second positive control groups (P-value = 0.003). In addition, the second experimental group had significant changes compared to the first positive control group (P-value = 0.014), However, no significant changes were observed between the positive control and experimental groups in terms of the severity of seizures.

Conclusion

Results have shown both blocked GABAergic receptors A and B involved in epileptic seizures. In addition, APE root increased delay time of epileptic seizures, as well as reduces epileptic seizure in dose response state.

Kinetics of GABAB autoreceptor-mediated suppression of GABA release in rat insular cortex

Release of GABA is controlled by presynaptic GABA receptor type B (GABA(B)) autoreceptors at GABAergic terminals. However, there is no direct evidence that GABA(B) autoreceptors are activated by GABA release from their own terminals, and precise profiles of GABA(B) autoreceptor-mediated suppression of GABA release remain unknown. To explore these issues, we performed multiple whole-cell, patch-clamp recordings from layer V rat insular cortex. Both unitary inhibitory and excitatory postsynaptic currents (uIPSCs and uEPSCs, respectively) were recorded by applying a five-train depolarizing pulse injection at 20 Hz. In connections from both fast-spiking (FS) and non-FS interneurons to pyramidal cells, the GABA(B) receptor antagonist CGP 52432 had little effect on the initial uIPSC amplitude. However, uIPSCs, responding to later pulses, were effectively facilitated. This CGP 52432-induced facilitation was prominent in the fourth uIPSCs, which were evoked 150 ms after the first uIPSC. The facilitation of uIPSCs was accompanied by an increase in the paired-pulse ratio. In addition, analysis of the coefficient of variation suggests the involvement of presynaptic mechanisms in CGP 52432-induced uIPSC facilitation. Paired-pulse stimulation (interstimulus interval = 150 ms) of presynaptic FS cells revealed that the second uIPSC was also facilitated by CGP 52432, which had little effect on the amplitude and interevent interval of miniature IPSCs. In contrast, uEPSCs, responding to all five stimulations of a presynaptic pyramidal cell, were less affected by CGP 52432. These results suggest that a single presynaptic action potential is sufficient to activate GABA(B) autoreceptors and to suppress GABA release in the cerebral cortex.

Mechanisms contributing to the exacerbated epileptiform activity in hippocampal slices of GABAB1 receptor subunit knockout mice

The recently developed GABAB1 receptor subunit knockout (GABAB1 -/-) mouse displays complete loss of GABAB receptor function and develops complex generalized epilepsies including absence type, audiogenic as well as spontaneous generalized seizures with electrographic spike-wave discharge signatures. To gain insight into the cellular mechanisms contributing to the generation and maintenance of this epileptic phenotype we have compared epileptiform activity induced in hippocampal slices obtained from GABAB1 -/- and wild type (GABAB1 +/+) littermates. Deletion of the GABAB1 receptor subunit had no effect on a range of passive membrane properties of CA3 pyramidale neurones, non-synaptic epileptiform field bursting and spreading depression recorded in 6mM K+/Ca2+-free medium, and inter-ictal synaptically-induced epileptiform activity induced by 100 microM 4-aminopyridine (4-AP). In contrast, synaptic epileptiform activity induced by 10 microM bicuculline, removal of extracellular Mg2+ or addition of 10 microM oxotremorine was enhanced in GABAB1 -/- slices. Acute blockade of GABAB receptors using a selective antagonist only partly mimicked these effects. It is suggested that the exaggerated in vitro epileptiform activity is caused by both acute and chronic consequences of the loss of GABAB receptor function in vivo. Specifically, enhancement of N-methyl-d-aspartate (NMDA) receptor triggered synaptic processes, arising from the loss of the GABAB receptor-mediated inhibitory postsynaptic potential (IPSP, together with a possible promotion of depolarising IPSPs due to the removal of GABAB autoreceptor function) is likely to underlie these effects.

Target-specific neuropeptide Y-ergic synaptic inhibition and its network consequences within the mammalian thalamus

Neuropeptides are commonly colocalized with classical neurotransmitters, yet there is little evidence for peptidergic neurotransmission in the mammalian CNS. We performed whole-cell patch-clamp recording from rodent thalamic brain slices and repetitively stimulated corticothalamic fibers to strongly activate NPY-containing GABAergic reticular thalamic (RT) neurons. This resulted in long-lasting (approximately 10 sec) feedforward slow IPSPs (sIPSPs) in RT cells, which were mimicked and blocked by NPY1 (Y1) receptor agonists and antagonists, respectively, and were present in wild-type mice but absent in NPY-/- mice. NPYergic sIPSPs were mediated via G-proteins and G-protein-activated, inwardly rectifying potassium channels, as evidenced by sensitivity to GDP-beta-S and 0.1 mm Ba2+. In rat RT neurons, NPYergic sIPSPs were also present but were surprisingly absent in the major synaptic targets of RT, thalamic relay neurons, where instead robust GABA(B) IPSPs occurred. In vitro oscillatory network responses in rat thalamus were suppressed and augmented by Y1 agonists and antagonists, respectively. These findings provide evidence for segregation of postsynaptic actions between two targets of RT cells and support a role for endogenously released NPY within RT in the regulation of oscillatory thalamic responses relevant to sleep and epilepsy.

Presynaptic GABAA and GABAB Receptor-mediated Phasic Modulation in Axons of Spinal Motor Interneurons

The lamprey spinal cord has been utilized to investigate the role of presynaptic inhibition in the control of the spinal motor system. Axons of the lamprey spinal cord are comparatively large because of their lack of myelination. Axons impaled with microelectrodes demonstrate depolarizing responses to the application of GABAA and GABAB receptor agonists, muscimol and baclofen. These depolarizing effects are counteracted by the specific GABAA and GABAB receptor antagonists, bicuculline and phaclofen. GABAA receptor activation leads to a gating of Cl- channels on the axons. However, the ionic mechanism leading to axonal depolarization following GABAB receptor activation is unknown. After initiation of fictive locomotion, these axons demonstrate oscillations in axonal membrane potential related to the locomotor cycle. During ficitive locomotion they depolarize in phase with the bursting of the ipsilateral ventral root of the same segment. These axonal membrane potential oscillations are due to a phasic GABAA and GABAB receptor-mediated gating of ion channels on the axonal membrane. Fictive locomotion in the lamprey spinal cord is largely unaffected by antagonism of one or other GABA receptor subtype alone, but is severely disrupted by simultaneous antagonism of both subtypes. In conclusions, we demonstrate, for the first time, an agonist-gated depolarization of a vertebrate presynaptic element measured by direct impalement of the axon under study. We also demonstrate that GABA-mediated presynaptic inhibition occurs in axons of spinal interneurons. It is not limited to the primary afferents as has previously been believed.

Recombinant GABA(B) receptors formed from GABA(B1) and GABA(B2) subunits selectively inhibit N-type Ca(2+) channels in NG108-15 cells

Efficient transfection of NG108-15 cells with GABA(B) receptor subunits was achieved using polyethylenimine. Baclofen modulated high voltage-activated Ca(2+) current in differentiated cells transfected with GABA(B1) and GABA(B2) receptor subunits or with the GABA(B2) subunit alone, but not with the GABA(B1) subunit alone. Characteristics of the current modulation were very similar for cells transfected with GABA(B1/2) and GABA(B2) subunits. Using antisense oligonucleotides against GABA(B1) subunits and also western immunoblotting, we are able to show that NG108-15 cells contain endogenous GABA(B1) subunits. Therefore, functional receptors can be formed by the combination of native GABA(B1) subunits with transfected GABA(B2) subunits, in agreement with the proposed heteromeric structure of GABA(B) receptors. Finally, we used selective channel blockers to identify the subtypes of Ca(2+) channels that are modulated by GABA(B) receptors. In fact, in differentiated NG108-15 cells, the recombinant GABA(B) receptors couple only to N-type Ca(2+) channels.

GABA(B) receptors in anterior pituitary cells. Mechanism of action coupled to endocrine effects

The activation of pituitary GABA(B) receptors by the specific agonist baclofen inhibits pituitary hormone secretion in vitro. Here we studied the mechanism of action of GABA(B) receptors in rat adenohypophysis. Anterior pituitary cells were obtained by trypsinization and were either plated for hormonal studies and cAMP determination or incubated in FURA 2AM for calcium measurements. Baclofen (BACL: 1 x 10(-5) M) significantly inhibited basal and thyrotropic releasing hormone (TRH)-stimulated (1 x 10(-7) M) PRL secretion in anterior pituitary cells from proestrous rats. In the presence of pertussis toxin (PTX: 150 ng/ml, 20 h), which leads to the uncoupling of the G(i/o)-protein from the receptor, both effects of BACL were abolished while the effect of dopamine (DA: 1 x 10(-8) M), used as an inhibitory control, was reduced from 70 to 25%. PTX also reversed BACL-induced inhibition of gonadotropin-releasing hormone (GnRH)-elicited luteinizing hormone (LH) secretion in anterior pituitary cells from 15-day-old female rats. In addition, though working in a pituitary mixed cell population, in which only some cell types possess GABA(B) receptors, BACL (1 x 10(-5) M) attenuated the forskolin-induced (0.5 microM) increase in cAMP. This effect was prevented by co-incubation with the antagonist 2 hydroxysaclofen and by preincubation with PTX. BACL (5 x 10(-5) M) and DA (5 x 10(-7) M) inhibited basal intracellular calcium concentrations ([Ca(2+)](i)) in pituitary cells and the effect of the latter was significantly stronger. The effect of BACL on [Ca(2+)](i) was abolished after preincubation with PTX. In the presence of the potassium channel blocking agents barium (200 microM and 1 mM) and tetraethylammonium (10 mM), BACL was still able to inhibit [Ca(2+)](i). Blockade of voltage-sensitive calcium channels (VSCC) with either verapamil (5 x 10(-6) M) or nifedipine (1 x 10(-6) M) completely abolished the effect of BACL on [Ca(2+)](i). In the presence of 12.5 mM potassium concentration baclofen significantly inhibited [Ca(2+)](i). In conclusion, our results describe the negative coupling of adenohypophyseal GABA(B) receptors to VSCC through PTX-sensitive G-proteins. These characteristics suggest a resemblance of these receptors to the typical presynaptic GABA(B) sites described in the central nervous system.

Comparison of antagonist potencies at pre- and post-synaptic GABA(B) receptors at inhibitory synapses in the CA1 region of the rat hippocampus

Synaptic activation of gamma-aminobutyric acid (GABA)B receptors at GABA synapses causes (a) postsynaptic hyperpolarization mediating a slow inhibitory postsynaptic potential/current (IPSP/C) and (b) presynaptic inhibition of GABA release which depresses IPSPs and leads to paired-pulse widening of excitatory postsynaptic potentials (EPSPs). To address whether these effects are mediated by pharmacologically identical receptors the effects of six GABA(B) receptor antagonists of widely ranging potencies were tested against each response. Monosynaptic IPSP(B)s were recorded in the presence of GABA(A), AMPA/kainate and NMDA receptor antagonists. All GABA(B) receptor antagonists tested depressed the IPSP(B) with an IC50 based rank order of potency of CGP55679> or =CGP56433 = CGP55845A = CGP52432>CGP51176>CGP36742. Paired-pulse EPSP widening was recorded as an index of paired-pulse depression of GABA-mediated IPSP/Cs. A similar rank order of potency of antagonism of paired-pulse widening was observed to that for IPSP(B) inhibition. Comparison of the IC50 values for IPSP(B) inhibition and paired-pulse EPSP widening revealed a close correlation between the two effects in that their IC50s lay within the 95% confidence limits of a correlation line that described IC50 values for inhibition of paired-pulse EPSP widening that were 7.3 times higher than those for IPSP(B) inhibition. Using the compounds tested here it is not possible to assign different subtypes of GABA(B) receptor to pre- and post-synaptic loci at GABAergic synapses. However, 5-10 fold higher concentrations of antagonist are required to block presynaptic as opposed to postsynaptic receptors when these are activated by synaptically released GABA.

GABA(B) receptor gene expression in monkey thalamus

Expression of gamma-amino butyric acid type B (GABA[B]) receptor gene transcripts was examined in the macaque monkey thalamus by in situ hybridization, using monkey-specific cRNA probes. GABA(B) transcript expression was widespread and of much higher density in the dorsal thalamus than in the reticular nucleus and other parts of the ventral thalamus and was highest in the epithalamus. In the dorsal thalamus, highest mRNA levels were found in the anteroventral nucleus and in the parafascicular nucleus. Sensory relay nuclei showed moderate GABA(B) mRNA levels. Neurons of all sizes were labeled, suggesting expression in relay cells and interneurons, and there was no labeling of neuroglial cells. Following 10-day periods of monocular deprivation, levels of GABA(B) mRNA were decreased in the deprived magno- and parvo-cellular laminae of the dorsal lateral geniculate nuclei, indicating activity-dependent regulation. High levels of GABA(B) receptors in the dorsal thalamus are likely to reflect the high density of synaptic inputs from the reticular nucleus while low expression in the reticular nucleus implies weak, GABA(B)-mediated intrareticular inhibition.

Changes in [K+]o evoked by baclofen in guinea pig hippocampus

K+-sensitive microelectrodes were used to record changes evoked by baclofen in extracellular potassium concentration ([K+]o) and field potentials in the stratum pyramidale (SP) and stratum radiatum (SR) in the CA1b region of guinea pig hippocampal slices in vitro. Bath applications of ( ±)-baclofen (1 µM - 3 mM for approx 5 min) evoked changes in [K+]o, which were in most cases sustained throughout agonist application and reversed during washout. The maximal (Rmax) values for curves fitted to the concentration-response data were for SP and SR, respectively, 0.59 ± 0.03 and 0.65 ± 0.03 mM, and EC50 values were 39.7 and 39.4 µM, respectively. The evoked K+ and field potential changes were significantly correlated and could be blocked by 2-OH-saclofen (50 µM) and CGP 35348 (50 µM). In <= 10% of experiments baclofen (10-50 µM) induced either a decrease or a transient increase ( <= 1 min duration) in [K+]o; in some slices with concentrations >=20 µM an initial decrease preceded a progressive increase. Pressure ejection of baclofen (100 µM for 100-900 ms) evoked increases in [K+]o and field potentials, which were larger in SR than in SP. In <= 10% of slices brief and (or) sustained application of baclofen (by either bath perfusion or pressure ejection) also evoked synchronous, repetitive interictal and ictal discharges at frequencies approx 1/s and 1/12 s, respectively, an observation that affirms a proconvulsant capacity. It is concluded that (i) although increases in [K+]o evoked by baclofen in SR compared with SP are slightly larger, they are not significantly different, (ii) GABAB receptor subtype(s) in SR and SP appear similar, as they have identical affinities, and (iii) [K+]o accumulations evoked by GABA likely include a contribution from a GABAB receptor activated K+ conductance, especially in dendritic regions.Key words: brain slices, stratum pyramidale, stratum radiatum, GABAB receptors, ion-selective microelectrodes, epileptiform activity.

Baclofen reduces GABAA receptor responses in acutely dissociated neurons of bullfrog dorsal root ganglia

The effect of baclofen on the function of the gamma-aminobutyric acidA (GABAA) receptor was examined in acutely dissociated neurons of bullfrog dorsal root ganaglia (DRG) by using the whole-cell voltage-clamp method. Baclofen (0.1-100 microM) depressed the inward currents produced by GABA (100 microM) and muscimol (100 microM). Baclofen shifted the concentration-response curve for GABA (1 microM-1 mM) downward. Baclofen decreased the maximum response (Vmax) to GABA without changing the apparent dissociation constant (Kd), suggesting a noncompetitive antagonism. The effect of baclofen on the GABA current was blocked by antagonists for the GABAB receptor; the rank order of potency was P-[3-Aminopropyl]-P-diethoxymethylphosphinic acid (CGP 55845A) > > 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P- benzyl-phosphinic acid (CGP 35348) > saclofen > > phaclofen. Baclofen produced an irreversible depression of the GABA current in neurons dialyzed with an internal solution containing guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S, 100 microM). Intracellular guanosine 5'-O-(2-thiodiphosphate) (GDP beta S, 100 microM) blocked the inhibitory effect of baclofen on the GABA current. Forskolin (10 microM) and dibutyryl N6, 2'-O-dibutyryladenosine 3':5'-cyclic monophophate (db-cyclic AMP) (200 microM) depressed the GABA current. N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9, 40 microM) and N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004, 50 microM), protein kinase A (PKA) inhibitors, reduced the depressant effect of baclofen on the GABA current. The baclofen-induced depression of the GABA current was blocked by PKI(5-24), a specific PKA inhibitor, but not by PKC(19-36), a specific protein kinase C (PKC) inhibitor. We suggest that GABAB receptors regulate the GABAA receptor function through a G-protein linked to the adenylyl cyclase-PKA pathway in bullfrog DRG neurons.

Evidence for direct and indirect mechanisms in the potent modulatory action of interleukin-2 on the release of acetylcholine in rat hippocampal slices

1. The biphasic nature of the potent modulatory action of interleukin-2 (IL-2) on hippocampal acetylcholine (ACh) release was investigated by use of brain slice superfusion. 2. Both the potentiating (10(-13) M) and inhibitory (10(-9) M) effects of IL-2 on hippocampal ACh release were stimulation-dependent and were blocked by a neutralizing IL-2 receptor antibody, suggesting the activation of typical IL-2 receptors in both cases. 3. Tetrodotoxin (TTX: 10 microM) failed to block the potentiation of ACh release induced by a very low concentration of IL-2 (10(-13) M) suggesting a direct effect on cholinergic nerve terminals. 4. In contrast, the inhibitory effect seen at a higher concentration (10(-9) M) was TTX-sensitive, and hence indicative of an indirect action. 5. To establish the nature of this intermediate mediator, blockers of nitric oxide synthesis, and of opioid and gamma-aminobutyric acid (GABA) receptors were used. Only GABAA and GABAB receptor antagonists altered the inhibitory action of IL-2, suggesting the participation of GABA as mediator. 6. Taken together, these results provide further evidence for the potent role of IL-2 in the modulation of cholinergic function in the rat hippocampus.

Synaptic and synaptically activated intrinsic conductances underlie inhibitory potentials in cat lateral amygdaloid projection neurons in vivo

The companion paper demonstrated that the responses of lateral amygdaloid (LAT) projection neurons to the stimulation of major input and output structures are dominated by monophasic hyperpolarizing potentials of large amplitude. To characterize the mechanisms underlying these inhibitory potentials, intracellular recordings of cortically evoked responses were obtained from morphologically and/or physiologically identified LAT projection neurons in barbiturate anesthetized cats. The reversal potential of the cortically evoked hyperpolarization was measured at its peak, and 115 ms later (tail), an interval corresponding to the peak latency of the gamma-aminobuturic acid-B (GABAB) response previously recorded in vitro. When recorded with K-acetate (KAc) pipettes, these reversal potentials were -86.9 +/- 1.6 mV (peak; mean +/- SE) and -90.7 +/- 1.7 mV (tail), suggesting that both Cl- and K+ conductances contribute throughout the cortically evoked hyperpolarization. The small, but consistent, difference between the two reversal potentials suggested that an additional slowly activating K(+)-mediated component contributed to the inhibitory postsynaptic potential (IPSP) tail. To determine whether Cl- conductances contributed to the evoked hyperpolarization, recordings were performed with KCl; the peak (-57.8 +/- 2.2 mV) and tail (-61.3 +/- 2.1 mV) reversal potentials were approximately 15-20 mV more depolarized than those recorded with KAc pipettes. However, the difference between the peak and tail reversals remained. In an attempt to block the Cl- conductance, recordings were obtained with pipettes filled with KAc or KCl and 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), a Cl- pump blocker that also was reported to block GABAA responses. With KAc and DIDS, the initial depolarization was prolonged and the amplitude of the hyperpolarization decreased relative to that seen with KAc alone. However, with KCl and DIDS, the reversal potential was shifted to an even greater extent than with KCl pipettes with the evoked response consisting entirely of a large depolarization, which produced a spike burst. These results suggest that LAT neurons have a Cl- pump that is blocked by DIDS, but that their Cl- channels are not blocked by DIDS. To assess the contribution of K+ conductances to cortically evoked hyperpolarizing potentials, recordings were obtained with Cs-acetate pipettes. Under these conditions, the response reversed at more depolarized potentials (peak, -71.9 +/- 1.0 mV; tail, -72.0 +/- 0.9 mV) compared with KAc recordings, with no difference between the peak and tail reversal potentials. These cells also had depolarized resting potentials (-66.2 +/- 1.8 mV) compared with those of cells recorded with KAc pipettes (-73.6 +/- 1.8 mV); however, this difference was too small to attribute the shift in reversals to a redistribution of Cl- ions across the membrane. The action potentials generated by LAT neurons under Cs+ had a shoulder that prolonged their falling phase. The increased duration of the spikes was presumably due to a dendritic Ca2+ conductance because LAT amygdaloid neurons are known to possess such conductances and Cs+ blocks the delayed rectifier and some Ca(2+)-dependent K+ currents. The dramatic reduction of this shoulder by spontaneous and evoked IPSPs suggests that the activation of dendritic conductances by back-propagating somatic action potentials is regulated tightly by synaptic events. Intracellular injection of the Ca2+ chelating agent, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (100 mM) caused a depolarization of the peak (-75.3 +/- 1.3 mV) and tail (-77.7 +/- 1.7 mV) reversal potentials during a time course of 15-45 min. Concurrently, the amplitude of the excitatory postsynaptic potential increased whereas that of the hyperpolarization decreased, suggesting that a Ca(2+)-dependent K+ conductance contributes significantly to the evoked hyperpolarization. (ABSTRACT TRUNCATED)

In vivo blockade of thalamic GABA(B) receptors increases excitatory amino-acid levels

The effect of intrathalamic application of GABA(B) receptor antagonists on the basal excitatory amino-acid levels was studied using microdialysis probes implanted in the dorsal lateral geniculate nucleus and in the ventrobasal complex. In both nuclei, continuous perfusion of the GABA(B) receptor antagonist 3-aminopropyl-(diethoxymethyl)-phosphinic acid (CGP 35348) produced an increase in the extracellular concentration of aspartate and (to a lesser extent) glutamate, but no change was observed in the level of taurine, the main amino acid involved in the regulation of brain osmolarity processes. In contrast, 3-amino-2-hydroxy-2-(4-chlorophenyl)-propanesulphonic acid (2-hydroxy-saclofen), another GABA(B) receptor antagonist, failed to affect the extracellular concentration of aspartate, glutamate and taurine. Thus, the basal level of excitatory amino acids in the thalamus in vivo is under the control of CGP 35348-sensitive GABA(B) receptors.

Tonic activation of presynaptic GABA(B) receptors on thalamic sensory afferents

The presence and role of presynaptic GABA(B) receptors in the control of excitatory amino acid-medicated transmission were investigated (using sharp electrode recordings) in the rat dorsal lateral geniculate nucleus and ventrobasal thalamus in vitro by comparing the effects of the selective GABA(B) receptor agonist, (+ or -)-baclofen, and of two antagonists, CGP 35348 and 2-hydroxy-saclofen, on the excitatory postsynaptic potentials evoked in thalamocortical neurons by stimulation of the sensory afferents. Application of CGP 35348 alone blocked the GABA(B) receptor-mediated inhibitory postsynaptic potential evoked in the dorsal lateral geniculate nucleus by stimulation of the optic tract (n = 5), but had no effect on the resting membrane potential and input resistance of thalamocortical cells (n = 6). In contrast, 2-hydroxy-saclofen caused a hyperpolarization (6.9 + or - 0.5 mV, n = 10) and a decrease in the apparent input resistance (26.3 + or - 2.6%, n = 10). This effect of 2-hydroxy-saclofen was antagonized by CGP 35348. When bicuculline was present in the perfusion medium and following intracellular injection of QX 314, GABA(A) and GABA(B) receptors in the recorded neurons were blocked. Under this condition, application of baclofen decreased the amplitude of the medial lemniscus- and optic tract-evoked excitatory postsynaptic potentials in the two thalamic nuclei investigated. This effect was fully antagonized by CGP 35348 and only partially by 2-hydroxy-saclofen. CGP 35348 alone increased (19.3 + or - 4.3%, n = 5) and 2-hydroxy-saclofen alone decreased (29.9 + or - 8.6%, n = 5) the amplitude of the excitatory postsynaptic potential. This effect of 2-hydroxy-saclofen was not blocked by CGP 35348. These results indicate that presynaptic GABA(B) receptors are present on the terminals of the sensory afferents in the rat dorsal lateral geniculate nucleus and in the ventrobasal thalamus. These receptors are tonically activated by endogenous GABA, at least in vitro, and provide a negative control mechanism by which the excitatory amino acid-mediated transmission within these nuclei can be regulated. In contrast, the endogenous GABA level is not sufficient for a tonic activation of postsynaptic GABA(B) receptors. Furthermore, these results indicate that 2-hydroxy-saclofen acts as a partial agonist on postsynaptic CGP 35348-sensitive GABA(B) receptors, and that, in addition to its antagonist action on presynaptic CGP 35348-sensitive GABA(B) receptors, it also has an effect on either presynaptic, CGP 35348-insensitive GABA(B) receptors and/or another presynaptic receptor type.

Effect of picrotoxin on antinociception in the formalin test

Subcutaneous injection of diluted formalin (0.25 microliter of 0.5%) caused a biphasic pain response in mice. The first phase of pain was observed during the first 5 min., while the second phase occurred 10-30 min. after formalin administration. With the formalin test, it was found that the antinociception produced by the GABA-A antagonist, picrotoxin, and the GABA-B antagonist, phaclofen, was abolished when employed in combination. The opioid antagonist naloxone and antimuscarinic atropine also decreased the picrotoxin response. However, sulpiride, SCH 23390, phenoxybenzamine and propranolol did not alter the picrotoxin response. Administration of naloxone, sulpiride and propranolol showed a pain response. The data indicate that dopaminergic and adrenergic mechanisms may not be involved in the picrotoxin antinociceptive effect. However, postsynaptic GABA-A and GABA-B may be involved in the drug effect, and involvement of opioid or cholinergic systems can not be excluded.

A physiological role for GABAB receptors and the effects of baclofen in the mammalian central nervous system

The inhibitory neurotransmitter GABA acts in the mammalian brain through two different receptor classes: GABAA and GABAB receptors. GABAB receptors differ fundamentally from GABAA receptors in that they require a G-protein. GABAB receptors are located pre- and/or post-synaptically, and are coupled to various K+ and Ca2+ channels presumably through both a membrane delimited pathway and a pathway involving second messengers. Baclofen, a selective GABAB receptor agonist, as well as GABA itself have pre- and post-synaptic effects. Pre-synaptic effects comprise the reduction of the release of excitatory and inhibitory transmitters. GABAergic receptors on GABAergic terminals may regulate GABA release, however, in most instances spontaneous inhibitory synaptic activity is not modulated by endogenous GABA. Post-synaptic GABAB receptor-mediated inhibition is likely to occur through a membrane delimited pathway activating K+ channels, while baclofen, in some neurons, may activate K+ channels through a second messenger pathway involving arachidonic acid. Some, but not all GABAB receptor-gated K+ channels have the typical properties of those G-protein-activated K+ channels which are also gated by other endogenous ligands of the brain. New, high affinity GABAB antagonists are now available, and some pharmacological evidence points to a receptor heterogeneity. The pharmacological distinction of receptor subtypes, however, has to await final support from a characterization of the molecular structure. The function importance of post-synaptic GABAB receptors is highlighted by a segregation of GABAA and GABAB synapses in the mammalian brain.

Effects of L-baclofen and D-baclofen on the auditory system: a study of click-evoked potentials from the inferior colliculus in the rat

The drug baclofen is a potential treatment for severe tinnitus, but its action in relieving tinnitus is not known. Baclofen is available as an approved drug only in racemic form with about equal content of the two enantiomers. In the present paper we show that L-baclofen causes a considerable (40.7%) suppression of the amplitude of the second peak in the click-evoked response from the cochlear nucleus. Bipolar recordings from the external nucleus of the inferior colliculus showed that L-baclofen caused a reduction in the amplitude of three or four distinct peaks in this response. D-Baclofen had no detectable effect on the response from the cochlear nucleus, and had only a slight effect on one component of the response from the external nucleus of the inferior colliculus. The demonstrated effect of L-baclofen on excitation in the ascending auditory pathway indicates that this drug may be a potential treatment for hyperactive auditory disorders such as tinnitus and hyperacusis.

Bicuculline enhances the late GABAB receptor-mediated paired-pulse inhibition observed in rat hippocampal slices

The inhibition of CA1 pyramidal neurones in rat hippocampal slices was studied using extracellular recordings of population spike potential responses to paired orthodromic stimulation. Variation of the interpulse interval allowed the separation of an early phase of inhibition (interpulse interval 5-20 ms), blocked by the GABAA receptor antagonist bicuculline (1 microM; n = 11), and a late phase (interpulse interval 200-400 ms) blocked by the GABAB receptor antagonist phaclofen (1 mM; n = 5) but enhanced by bicuculline (n = 11). Similar enhancement was not observed when conditioning response amplitudes were increased by increasing the stimulus strength, rather than bicuculline. Orthodromic stimulation leads to synaptic excitation of both pyramidal neurones and inhibitory interneurones, and may also lead to activation of inhibitory inputs onto interneurones. Bicuculline could prevent inhibition of the interneurones, and hence enhance the late, GABAB receptor-mediated inhibition. Conversely, the therapeutic administration of benzodiazepines would be postulated to enhance the inhibition of inhibitory interneurones, leading to an iatrogenic decrease in GABAB receptor-mediated inhibition.

A comparison of the relative activities of a number of GABAB antagonists in the isolated vas deferens of the rat

1. A series of GABAB receptor antagonists were tested against (+/-)-baclofen for activity on the presynaptic GABAB receptor in the rat vas deferens. 2. All the antagonists tested caused a rightward shift in the concentration-response curve to (+/-)-baclofen. 3. pA2 values calculated from full Schild analysis were as follows: phaclofen, pA2 = 4.3; delta-amino valeric acid, pA2 = 4.4; 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP 35348), pA2 = 5.0; 3-amino-propyl(n-hexyl)phosphinic acid (3-APHPA), pA2 = 4.5. 4. These results show that none of the above compounds possess potent antagonist activity at the GABAB receptor (i.e. pA2 > 6) in this peripheral tissue. In addition, the more recently available phosphinic acid antagonists, appear to offer no great advance over the GABAB antagonists previously available.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Status epilepticus may be caused by loss of adenosine anticonvulsant mechanisms

The inhibitory neuromodulator adenosine is an endogenous anticonvulsant that terminates brief seizures in the brain and it has been proposed that loss of adenosine or adenosine-mediating systems may play a major role in the development of status epilepticus, a seizure condition characterized by prolonged and/or recurrent seizures that last by definition, at least 20 min. In this study, the effect of specific A1-adenosine agonists and antagonists were tested for their ability to prevent and cause status epilepticus in two electrical stimulation models in rats. In a recurrent electrical stimulation model, whereas no vehicle-treated animals developed status epilepticus after 20 recurrent electrical stimulations, rats injected with 10 mg/kg of the specific A1-adenosine antagonist 8-cyclopentyl-1,3-dimethylxanthine intraperitoneally developed status epilepticus after stimulation. 8-(p-Sulphophenyl)-theophylline, which has limited penetrability into the brain when administered peripherally, did not cause status epilepticus when injected intraperitoneally. However, when 200 micrograms of 8-(p-sulphophenyl)-theophylline were administered intracerebroventricularly, status epilepticus developed in all animals, suggesting status epilepticus developed as a result of central adenosine receptor antagonism. In the second study, whereas all vehicle-treated animals developed status epilepticus after constant electrical stimulation, administration of N6-cyclohexyladenosine and N6-cyclopentyladenosine prior to stimulation suppressed the development of status epilepticus. N6-Cyclohexyladenosine was also effective in terminating status epilepticus after it had progressed for 20 min. The effects of a selective A2-agonist was also tested on both stimulation models and had no anticonvulsant effects. An electrical stimulus given to rats pretreated three days prior to stimulation with pertussis toxin, a compound which inactivates Gi-proteins, also resulted in generalized status epilepticus, suggesting that impairment of G-protein-linked receptors is involved in the development of status epilepticus. The effects of a GABAB antagonist, phaclofen, and a GABAB agonist, baclofen, were also tested in the recurrent stimulation model, as GABAB receptors are also coupled to the same subset of K+ channels as the A1-receptor. Rats given phaclofen did not develop status epilepticus after recurrent electrical stimulation, although baclofen was effective at preventing the induction of status epilepticus in the constant stimulation model. These results, together with some preliminary data obtained showing that the GABAA antagonist picrotoxin did not cause status epilepticus after recurrent stimulation, suggest that loss of GABAergic inhibition only has a minor role in status epilepticus development in our models. Brains from all animals were also assessed for brain injury.(ABSTRACT TRUNCATED AT 400 WORDS)

The physiological regulation of synaptic inhibition by GABAB autoreceptors in rat hippocampus

1. Intracellular recording techniques were used to study the effects of repetitive stimulation on monosynaptically activated inhibitory postsynaptic currents (IPSCs) in rat hippocampal slices. This was achieved by stimulation in stratum radiatum close to a recorded CA1 pyramidal neurone after pharmacological blockade of excitatory synaptic responses, using a combination of the N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonists D-2-amino-5-phosphonopentanoate (AP5; 0.04-0.1 mM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 0.02-0.04 mM), respectively. 2. Fixed-intensity stimulation at frequencies of less than 0.1 Hz evoked biphasic IPSCs of constant amplitude and waveform. In contrast, when two shocks (paired pulse) or longer trains of ten or more stimuli (i.e. tetani) were delivered at frequencies of between 0.2 and 20 Hz there was marked depression of both phases of every IPSC (by 60-100%) relative to the first or 'priming' IPSC evoked. 3. The gamma-aminobutyric acid (GABA)B receptor antagonists phaclofen (0.4-2 mM), 2-hydroxy-saclofen (0.02-0.4 mM) and 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP 35348; 0.01-1 mM) reduced or abolished, in a concentration-dependent and reversible manner, both the late phase of the IPSC (IPSCB) and paired-pulse depression of the early phase of the IPSC (IPSCA). Expressed in terms of IC50 values, all three antagonists were 5-10 times more potent at blocking IPSCB than paired-pulse depression. 4. Paired-pulse depression, at 5 and 10 Hz, has been shown to be mediated by GABA acting on presynaptic GABAB receptors (i.e. GABAB autoreceptors). We now show that GABAB receptor antagonists reverse paired-pulse depression over the entire range of frequencies (0.1-50 Hz) that it occurs. 5. GABAB receptor antagonists reversed substantially the depression of IPSCs during tetani delivered at 5 or 10 Hz. However at 20 Hz, GABAB receptor antagonists appeared to be less effective. At 100 Hz they appeared to be ineffective at reversing the depression of IPSCA; since the antagonists block IPSCB the net effect was to reduce the level of outward current. 6. At frequencies of 20 Hz or more, there was also the appearance of a slow inward current which increased in size in proportion to the frequency and number of shocks in the tetanus. This current (termed here IPSCI) was more pronounced at hyperpolarized membrane potentials and was blocked by picrotoxin (0.1 mM) or bicuculline (0.05 mM). 7. 'Priming' is considered to represent a more physiological pattern of activity than a tetanus.(ABSTRACT TRUNCATED AT 400 WORDS)

The behavior of mossy cells of the rat dentate gyrus during theta oscillations in vivo

Intracellular current clamp recordings were obtained from mossy cells (n = 6, identified by intracellular injection of biocytin) of the dorsal dentate gyrus from rats under ketamine-xylazine anesthesia. During electroencephalographic theta rhythm (4-6 Hz), recorded with a macroelectrode placed in the contralateral dorsal hippocampus near the fissure, mossy cells displayed intracellular membrane potential oscillations at low frequencies (4-6 Hz) which appeared to be phase locked to the electroencephalographic theta rhythm. The frequency of the intracellular theta rhythm was independent of the membrane potential. However, the phase difference between the intracellular and the electroencephalographic theta rhythms as well as the amplitude of the intracellular theta oscillations were voltage-dependent. These findings are consistent with the hypothesis that rhythmic GABAA receptor-mediated inhibitory postsynaptic potentials contribute to the genesis of the intracellular theta rhythm. Indeed, mossy cells displayed an early, fast inhibitory postsynaptic potential in response to electrical stimulation of the entorhinal cortex, which most likely represents a GABAA receptor-mediated event, indicating that mossy cells possess functional GABAA receptors. At the resting membrane potential, mossy cells did not fire at each cycle of the electroencephalographic theta rhythm but fired only rarely (< 1 Hz). However, when they did fire they did so preferentially in phase with the peak positivity of the electroencephalographic theta rhythm. Reconstruction of two mossy cells with axonal projections to the inner molecular layer showed that the spatial extent of the influence such weakly discharging mossy cells may have on other dentate gyrus neurons during theta oscillations can be several millimeters in the septotemporal direction. In conclusion, these findings show that mossy cells of the rat hilus during ketamine-xylazine anesthesia participate in theta oscillations of the hippocampal formation, during which their low-frequency firing may contribute to the phase-locking of a large number of spatially distributed postsynaptic neurons with postsynaptic sites in the inner molecular layer of the dentate gyrus.

CGP 55845A: a potent antagonist of GABAB receptors in the CA1 region of rat hippocampus

The new GABAB receptor antagonist CGP 55845A was tested on pre- and post-synaptic GABAB receptors in the hippocampus. CGP 55845A (1 microM) blocked (-)-baclofen (5-10 microM)-induced postsynaptic hyperpolarization and depression of evoked IPSPs and EPSPs. It also blocked three physiological consequences of GABAB receptor activation: the late IPSP, paired-pulse depression of IPSCs, and heterosynaptic depression of EPSPs. Therefore, CGP 55845A is an antagonist at pre- and post-synaptic GABAB receptors in the hippocampus and is approximately three orders of magnitude more potent than previously described GABAB receptor antagonists.

GABAB receptors modulate glycinergic inhibition and spike threshold in Xenopus embryo spinal neurones

1. The actions of GABAB receptors in the generation of the neuronal pattern underlying swimming in the Xenopus embryo have been investigated using the agonist baclofen. 2. Baclofen (10-100 microM) greatly reduced the length of swimming episodes and ventral root spike amplitude in a reversible manner. These effects were blocked by CGP 35348 (200-300 microm) and hydroxysaclofen (200-300 microM). 3. Baclofen (10-100 microM) reduced the amplitude of glycinergic IPSPs in motoneurones during fictive swimming. 4. Strychnine-sensitive spontaneous miniature inhibitory postsynaptic potentials (mIPSPs) were recorded from motoneurones. While baclofen (10-100 microM) had no effect on the amplitude of the mIPSPs it greatly decreased their frequency of occurrence. 5. GABAB receptors may therefore be present on the terminals of commissural interneurones, the only glycinergic neurones in the Xenopus embryo's nervous system, and act to reduce neurotransmitter release. 6. Baclofen reduced the reliability of action potential firing in motoneurones during fictive swimming without an apparent effect on excitation. 7. Baclofen increased the threshold to action potential firing in response to the injection of depolarizing current in motoneurones. 8. The current-voltage relationships of motoneurones were investigated. Baclofen (10-100 microM) did not change the resting membrane potential, slope conductance or the membrane rectification.

The actions of baclofen on neurones and synaptic transmission in the nucleus tractus solitarii of the rat in vitro

1. Intracellular and whole-cell patch recordings were made from sixty-seven neurones located in the nucleus tractus solitarii (NTS) in transverse slices of rat brainstem. 2. Baclofen at concentrations of 2-20 microM caused hyperpolarization from normal resting membrane potentials (Vm). This response was associated with a decrease in input resistance (Rm) tested by current pulses in discontinuous current clamp mode when membrane potential was restored to control level by current injection. In single electrode discontinuous voltage clamp mode, baclofen at these concentrations caused a small (< 50 pA) outward current associated with increased membrane conductance measured by voltage steps from holding potentials (Vh) of -50 or -60 mV. Current-voltage relations at these Vhs and the results of varying Vh between -50 and -110 mV during responses to baclofen gave a reversal potential of -73 mV. The amplitudes of baclofen responses were related to K+ concentration tested by comparing responses in media containing 1-24 mM extracellular K+, indicating that postsynaptically baclofen acts via a K+ conductance. 3. These effects were still apparent in the presence of tetrodotoxin (which did not abolish all spontaneous synaptic activity) and also in medium containing a combination of Co2+, the excitatory amino acid antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and the GABAA antagonist bicuculline which blocked synaptic activity. 4. The amplitude and frequency of spontaneous postsynaptic potentials (spPSPs) and spontaneous postsynaptic currents (spPSCs) were reduced by baclofen at concentrations (1 microM or less) which had no effect on membrane potential or holding current in current or voltage clamp recordings respectively. 5. The amplitude of evoked excitatory (evEPSPs/evEPSCs) and inhibitory (evIPSPs/evIPSCs) synaptic events elicited by electrical stimulation in the vicinity of the tractus solitarius (TS) was reduced by low concentrations of baclofen (250 nM-1 microM) which did not produce discernible postsynaptic responses. 6. In order to examine the effects of baclofen on excitatory synaptic events without contamination with inhibitory events, stimulation of the TS was carried out in the presence of bicuculline. Conversely to investigate actions on purely inhibitory synaptic responses experiments were carried out with CNQX in the bathing solution. Inhibitory synaptic responses could still be evoked, presumably by stimulation of interneurones in the vicinity of the TS. IPSPs/IPSCs were more sensitive to baclofen than EPSPs/EPSCs. 7. The effects of baclofen on membrane potential or holding current and PSP/PSCs were antagonized by 2-hydroxysaclofen (400 microM) confirming that baclofen was acting at gamma-aminobutyric acid (GABA)B receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Local and diffuse synaptic actions of GABA in the hippocampus

In the CNS, gamma-aminobutyric acid (GABA) acts as an inhibitory transmitter via ligand-gated GABAA receptor channels and G protein-coupled GABAB receptors. Both of these receptor types mediate inhibitory postsynaptic transmission in the hippocampus. In addition to these direct postsynaptic actions, GABAB receptor agonists inhibit excitatory transmission through presynaptic receptors on excitatory afferent terminals. However, a physiological role for the GABAB receptors on excitatory nerve endings has not been established. In this study, we have found a brief, heterosynaptic depression of excitatory synaptic transmission in the CA1 region of the hippocampal slice following short-lasting repetitive stimulation and determined that this inhibition is mediated by presynaptic GABAB receptors. The inhibition of GABA uptake greatly enhanced both the presynaptic action of GABA and the slow GABAB-mediated inhibitory postsynaptic current. Transmitter uptake was also found to regulate the "spill-over" of GABA at conventional GABAA synapses. These results suggest that uptake mechanisms restrict the spatial range of both point-to-point synaptic transmission mediated by GABA and its action at a distance.

Antiepileptic effects of GABAb receptor activation in area CA3 of rat hippocampus

The role of GABAb receptor activation in the expression of both interictal and ictal phenomena was investigated in slices of area CA3 of the rat hippocampal formation. Interictal-like bursts occurred following application of high frequency trains to the Schaffer collaterals. When two bursts were triggered using paired stimuli, profound depression of the second burst was seen 150-600 ms following the first burst. GABAb receptor antagonists potently reversed the paired pulse depression of the interictal-like bursts. Reversal of the paired depression was also accomplished by increasing the extracellular concentration of K+ by 2-3 mM. Additional experiments were performed in area CA3 to determine the role of GABAb receptor activation on the expression of ictal phenomena. Electrographic seizures (EGSs) were induced by application of high frequency trains. 2-Hydroxy-saclofen (200 microM) significantly decreased the duration of trains required to elicit EGSs. Taken together, these data suggest that GABAb receptor activation has potent inhibitory effects on both ictal and interictal-like events.

Intracellular electrophysiological study of suprachiasmatic nucleus neurons in rodents: inhibitory synaptic mechanisms

1. The mechanisms responsible for evoked and spontaneous fast inhibitory postsynaptic potentials (IPSPs) in the hypothalamic suprachiasmatic nucleus (SCN) were studied with intracellular recording. SCN neurons, primarily ones identified as receiving excitatory optic nerve input, were recorded in rat and guinea-pig brain slice preparations maintained in vitro. 2. In normal medium, electrical stimulation of a site dorsocaudal to the SCN evoked IPSPs from thirty-three of thirty-six neurons. The evoked IPSPs rose to the peak quickly (8.7 +/- 0.9 ms, mean +/- S.E.M.; n = 15 neurons) and decayed gradually with a time constant of 25 +/- 3 ms. Spontaneous IPSPs were present in each of the thirty-six neurons. These IPSPs had a rise-to-peak time of 7.2 +/- 1.0 ms (n = 6 neurons) and a decay time constant of 14 +/- 5 ms. 3. When recorded with potassium acetate-filled electrodes, the evoked and spontaneous IPSPs were hyperpolarizing at resting membrane potential (less negative than -70 mV) and had a reversal potential of around -75 mV. On the other hand, when recorded with potassium chloride-filled electrodes, the IPSPs were depolarizing at membrane potentials more negative than -50 mV and had an estimated reversal potential less negative than spike threshold. 4. Bath application of bicuculline (10-50 microM), a gamma-aminobutyric acidA (GABAA) receptor antagonist, resulted in a complete blockade of both the evoked (n = 16) and spontaneous (n = 13) IPSPs. The bicuculline effects were reversible, and were not associated with any significant and consistent change in baseline membrane potential or input resistance. The neurons impaled in bicuculline-containing medium (n = 11) exhibited neither spontaneous IPSPs nor evoked IPSPs. 5. In some neurons bicuculline-resistant hyperpolarizing potentials, which were similar to the fast IPSPs in time course, occurred spontaneously or were evoked by electrical stimulation of the optic nerve or the dorsocaudal site. A fast prepotential always preceded the hyperpolarizing potential, and hyperpolarizing currents blocked these events, indicating that they were not synaptic in origin. No slow IPSPs were detected. 6. The results suggest that fast IPSPs from non-retinal afferents exist in virtually all SCN neurons receiving excitatory retinal input, and that GABAA receptors associated with Cl- channels mediate the fast IPSPs.

Inhibitory responses of rat basolateral amygdaloid neurons recorded in vitro

The purpose of the present study was to characterize the ionic and pharmacological basis of the actions of synaptically released and exogenously applied GABA in basolateral amygdaloid pyramidal cells in vitro. Stimulation of forebrain afferents to pyramidal neurons in the basolateral amygdala evoked an excitatory postsynaptic potential followed by early and late inhibitory postsynaptic potentials. The early inhibitory postsynaptic potential had a reversal potential near -70 mV, was sensitive to changes in the chloride gradient across the membrane and was blocked by the GABAA antagonists picrotoxin and bicuculline methiodide but not by the GABAB antagonists phaclofen or 2-hydroxysaclofen. In contrast, the late inhibitory postsynaptic potential had a reversal potential of approximately -95 mV and was markedly reduced or abolished by GABAB antagonists. Pressure application of GABA to the surface of the slice typically elicited a triphasic response in basolateral amygdaloid pyramidal neurons consisting of a short-latency hyperpolarization that preceded or was superimposed on a membrane depolarization followed by a longer latency hyperpolarization. Each of the responses was associated with an increase in membrane conductance. Determinations of the reversal potential, ionic dependency and sensitivity to pharmacological blockade of each component of the GABA-induced response revealed that the initial hyperpolarizing (Erev approximately -70 mV) and depolarizing (Erev approximately -55 mV) responses were mediated by a GABAA-mediated increase in chloride conductance, whereas the late hyperpolarizing response (Erev approximately -82 mV) to GABA arose from a GABAB-mediated increase in potassium conductance. Experiments in which GABA was applied at various locations on the cell suggested that the short-latency hyperpolarization resulted from activation of somatic GABA receptors, whereas the depolarizing and late hyperpolarizing responses were generated primarily in the dendrites. In contrast to the complex membrane response profile elicited by GABA, pressure ejection of the GABAB agonist baclofen produced only membrane hyperpolarizations. Taken together, these results suggest that inhibitory responses that are recorded in basolateral amygdaloid pyramidal cells are mediated by activation of both GABAA and GABAB receptors. Consistent with findings elsewhere in the CNS, the early inhibitory postsynaptic potential and initial hyperpolarization and depolarizing response to local GABA application appear to involve a GABAA-mediated increase in chloride conductance, whereas the late inhibitory postsynaptic potential and the late hyperpolarizing response to GABA arise from a GABAB-mediated increase in potassium conductance.

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

Various types of inhibitory postsynaptic potentials in anterior thalamic cells are differentially altered by stimulation of laterodorsal tegmental cholinergic nucleus

The effects of stimulating the laterodorsal tegmental cholinergic nucleus upon inhibitory postsynaptic potentials recorded in relay cells of the anterior thalamic complex were studied in urethane-anesthetized cats. The inhibitory postsynaptic potentials induced in anterior thalamic relay cells by stimulating mammillary nuclei or retrosplenial cortex are generated by local-circuit inhibitory neurons since this nuclear complex is devoid of afferents from the other intrathalamic source of inhibition, the reticular thalamic nucleus. In a parallel study from this laboratory, it has been shown that cortical stimulation elicits a biphasic inhibitory postsynaptic potential consisting of two (A and B) components attributed to axonal firing of local interneurons, whereas mammillary stimulation elicits, in addition to the A-B sequence, an earlier component (a) presumably generated by presynaptic dendrites in thalamic glomeruli. In the present study, short pulse-trains applied to the laterodorsal tegmental nucleus diminished the amplitudes of A and B inhibitory components or completely suppressed them. The B component was more sensitive to the depressive effect. By contrast with the changes of the A and B components, the mammillary-evoked a inhibitory component was not reduced and, in many instances, was enhanced following laterodorsal tegmental stimulation. The effects of laterodorsal tegmental stimulation survived monoamine depletion by reserpine. We suggest that mesopontine cholinergic depressive actions on A and B inhibitory postsynaptic potentials may be due to an increased conductance in thalamocortical cells during the short-lasting nicotinic action combined with a somatic hyperpolarization of local-circuit cells, whereas the enhancement of the earliest (a) inhibitory postsynaptic potential reflects a concomitant potentiating action at the level of intraglomerular presynaptic dendrites.

The physiology of GABAB receptors in the vertebrate retina

Writing a chapter on retinal GABAB receptors is premature, as evidenced by the paucity of citations more than two years old. Despite that, this area of retinal pharmacology has made significant strides and, although it is a story without an ending, it has had an exciting beginning. To date, the experiments indicate that horizontal cell feedback to cones is mediated, at least in part, by the GABAB receptor system which probably regulates a potassium conductance. In the inner retina, GABAB receptors are found on bipolar cells, amacrines, and ganglion cells. Here, the actions are a subtle regulation of channel conductance, but the effects are a dramatic reorganization of a fundamental coding property of the retina, namely the distinction between tonic and phasic responses to light. In both the distal and proximal retina, the GABAB receptor does not appear to work alone, but instead works in concert with the GABAA receptor. The full significance of these interactions has yet to be determined. Although the discovery of the GABAB receptor has led to the resolution of several retinal mysteries, the case is far from closed. At this juncture, what can be said is that the GABAB receptor represents a unique and ubiquitous system that reveals the power of regulating calcium and potassium conductances, as opposed to the more familiar properties of the glutamate/acetylcholine regulated cationic conductances or the GABAA/glycine controlled chloride channels.

Actions of (-)-baclofen on rat dorsal horn neurons

The actions of a gamma-aminobutyric acid B (GABAB) agonist, (-)-baclofen, on the electrophysiological properties of neurons and synaptic transmission in the spinal dorsal horn (laminae I-IV) were examined by using intracellular recordings in spinal cord slice from young rats. In addition, the effects of baclofen on the dorsal root stimulation-evoked outflow of glutamate and aspartate from the spinal dorsal horn were examined by using high performance liquid chromatography (HPLC) with flourimetric detection. Superfusion of baclofen (5 nM to 10 microM) hyperpolarized, in a stereoselective and bicuculline-insensitive manner, the majority (86%) of tested neurons. The hyperpolarization was associated with a decrease in membrane resistance and persisted in a nominally zero-Ca2+, 10 mM Mg(2+)- or a TTX-containing solution. Our findings indicate that the hyperpolarizing effect of baclofen is probably due to an increase in conductance to potassium ions. Baclofen decreased the direct excitability of dorsal horn neurons, enhanced accommodation of spike discharge, and reduced the duration of Ca(2+)-dependent action potentials. Baclofen depressed, or blocked, excitatory postsynaptic potentials evoked by electrical stimulation of the dorsal roots. Spontaneously occurring synaptic potentials were also reversibly depressed by baclofen. Whereas baclofen did not produce any consistent change in the rate of the basal outflow of glutamate and aspartate, the stimulation-evoked release of the amino acids was blocked. The present results suggest that baclofen, by activating GABAB receptors, may modulate spinal afferent processing in the superficial dorsal horn by at least two mechanisms: (1) baclofen depresses excitatory synaptic transmission primarily by a presynaptic mechanism involving a decrease in the release of excitatory amino acids, and (2) at higher concentrations, the hyperpolarization and increased membrane conductance may contribute to the depressant effect of baclofen on excitatory synaptic transmission in the rat spinal dorsal horn.

The modulation of rat hippocampal synaptic conductances by baclofen and gamma-aminobutyric acid

1. We examined the effects of gamma-aminobutyric acid (GABA) and baclofen on pre- and postsynaptic membrane conductances in dissociated rat hippocampal cells. Both GABA (5 microM with 10 microM-bicuculline) and baclofen (50 microM) caused small but significant increases in membrane conductance that were blocked by 2-hydroxysaclofen (100 microM), a GABAB receptor antagonist. This increase in membrane conductance seems to be mediated by GABAB receptors. 2. At a low concentration of GABA (1 microM) which has a very small direct postsynaptic effect on GABAA receptors, no postsynaptic GABAB effect was detected. However, at this concentration, GABA near maximally attenuated both excitatory and inhibitory synaptic currents. This GABA effect on transmitter release was significantly attenuated by 2-hydroxysaclofen. 3. Baclofen was also more potent in attenuating the inhibitory synaptic conductance than increasing postsynaptic conductance. Concentrations below 1 microM diminished synaptic currents by greater than 50%. At these low baclofen concentrations 2-hydroxysaclofen significantly attenuated baclofen's reduction of synaptic currents. 4. The effects of GABA and baclofen on synaptic conductances were blocked by pretreating the cultures with pertussis toxin, suggesting that a GTP-associated protein, Gi or Go is responsible for reducing transmitter release. 5. Despite the ability of GABA to diminish inhibitory synaptic currents through GABAB receptor activation, we observed no effect of 2-hydroxysaclofen on paired-pulse depression. Therefore, these presynaptic GABAB receptors may not be true 'autoreceptors'. 6. Our findings indicate that in culture, at least, the presynaptic GABAB effect responsible for synaptic modulation has a pharmacological profile similar to the postsynaptic GABAB effect. At present, it is unnecessary to postulate two different types of GABAB receptors.

Low-frequency oscillatory activities intrinsic to rat and cat thalamocortical cells

1. Low-frequency membrane potential oscillations recorded intracellularly from thalamocortical (TC) cells of the rat and cat dorsal lateral geniculate nucleus (dLGN) and of the rat ventrobasal nucleus (VB) maintained in vitro were investigated. On the basis of their electrophysiological and pharmacological properties, four types of activity were distinguished and named: the pacemaker oscillations, the spindle-like oscillations, the 'very slow' oscillations and the 'N-methyl-D-aspartate' (NMDA) oscillations. 2. The pacemaker oscillations (95 out of 173 cells) consisted of rhythmic, large-amplitude (10-30 mV) depolarizations which occurred at a frequency of 1.8 +/- 0.3 Hz (range, 0.5-2.9 Hz) and could often give rise to single or a burst of action potentials. Pacemaker oscillations were observed when the membrane potential was moved negative to -55 and positive to -80 mV, but in a given cell the upper and lower limits of this voltage range were separated by only 13.1 +/- 0.5 mV. Above -45 mV tonic firing consisting of single action potentials was seen in the cells showing this or the other types of low-frequency oscillations. 3. The spindle-like oscillations were observed in thirty-nine (out of 173) TC cells and consisted of rhythmic (2.1 +/- 0.3 Hz), large-amplitude depolarizations (and often associated burst firing) similar to the pacemaker oscillations but occurring in discrete periods every 5-25 s and lasting for 1.5-28 s. The spindle-like oscillations were observed when the membrane potential was moved negative to -55 and positive to -80 mV and in two cells they were transformed into continuous pacemaker oscillations by depolarization of the membrane potential to -60 mV. 4. Pacemaker and spindle-like oscillations were unaffected by tetrodotoxin (TTX) or by selective blockade of NMDA, non-NMDA, GABAA, GABAB, nicotinic, muscarinic, alpha- and beta-noradrenergic receptors. 5. The 'very slow' oscillations consisted of a TTX-insensitive, slow hyperpolarization-depolarization sequence (5-15 mV in amplitude) which lasted up to 90 s and was observed in nine dLGN cells and in two VB cells. The pacemaker and the spindle-like oscillations were recorded in one cell each which also showed the 'very slow' oscillations. 6. The 'NMDA' oscillations were observed only in a 'Mg(2+)-free' medium (0 mM-Mg2+, 2-4 mM-Ca2+; 64 out of 72 cells) and consisted of large-amplitude (10-25 mV) depolarizations that did not occur at regular intervals and were intermixed with smaller depolarizations present on the baseline and on the falling phase of the larger ones.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of phaclofen and the enantiomers of baclofen on cardiovascular responses to intrathecal administration of L- and D-baclofen in the rat

In a previous study it was found that i.t. administration of L-baclofen decreased arterial pressure and heart rate while D-baclofen differentially increased arterial pressure. The objective of the present study was to determine which of these effects was blocked by prior administration of the GABAB receptor antagonist, phaclofen, and whether the effect of one enantiomer of baclofen could be blocked by prior administration of the other. The decreases in systolic and diastolic arterial pressures and in heart rate produced by i.t. administration of 70 nmol of L-baclofen were unaffected by i.t. administration of 7, 70 or 700 nmol of D-baclofen 10 min prior to administration of L-baclofen, but were blocked by administration of 5 mumol of phaclofen given 3-5 min prior to L-baclofen. On the other hand, the increases in systolic and diastolic arterial pressures induced by i.t. administration of 700 nmol of D-baclofen were blocked by 70 nmol but not by 7 nmol of L-baclofen, as well as by 2.5 mumol of phaclofen; the effect of L-baclofen cannot be attributed to a desensitization of D-baclofen-sensitive receptors as two successive doses of D-baclofen given 7 min apart had quantitatively similar effects. Phaclofen alone increased systolic and diastolic arterial pressures and heart rate. The results are interpreted as indicating that D-baclofen is not an antagonist of L-baclofen in this paradigm; rather, they suggest that L-baclofen reduces the effects of D-baclofen.(ABSTRACT TRUNCATED AT 250 WORDS)

A GABA immunocytochemical study of rat motor thalamus: light and electron microscopic observations

A light and electron microscopic study of GABA-immunoreactive neurons and profiles in the ventroanterior-ventrolateral and ventromedial nuclei of rat dorsal thalamus was conducted using antiserum raised against GABA. Less than 1% of the neurons in these motor-related nuclei exhibited GABA immunoreactivity, confirming previous reports that these nuclei are largely devoid of interneurons. Immunoreactive neurons in the ventral anterior-ventral lateral complex and ventromedial nucleus were bipolar or multipolar in shape, and tended to be smaller than non-immunoreactive neurons. GABA immunoreactivity in the neuropil consisted of labeled axon terminals and myelinated and unmyelinated axons, and was lower in the ventral anterior-ventral lateral complex and ventromedial nucleus than in neighboring thalamic nuclei. The density of neuropil immunolabeling was slightly higher in ventral anterior-ventral lateral complex than in ventromedial nucleus. GABA-immunoreactive axon terminals, collectively termed MP boutons for their medium size and pleomorphic vesicles (and corresponding to "F" profiles of some previous studies of thalamic ultrastructure), formed symmetric synapses and puncta adhaerentia contacts predominantly with large and medium-diameter (i.e. proximal) non-immunoreactive dendrites. Approximately 12 and 18% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, were GABA-immunopositive. Many of these immunoreactive profiles probably arose from GABAergic neurons in the thalamic reticular nucleus, substantia nigra pars reticulata and entopeduncular nucleus. Two types of non-immunoreactive axon terminals were distinguished based on differences in morphology and synaptic termination sites. Boutons with small ovoid profiles and round vesicles that formed prominent asymmetric synapses onto small-diameter dendrites were observed. Mitochondria were rarely observed within these boutons, which arose from thin unmyelinated axons. These boutons composed approximately 82 and 74% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, and were considered to arise predominantly from neurons in the cerebral cortex. In contrast, boutons with large terminals that contained round or plemorphic vesicles and formed multiple asymmetric synapses predominantly with large-diameter dendrites were also observed. Puncta adhaerentia contacts were also common. Mitochondria were numerous within large boutons with round vesicles, which arose from myelinated axons. Many of the large boutons were likely to have originated from neurons in the cerebellar nuclei. Approximately 6% of the boutons in the ventral anterior-ventral lateral complex and 8% in ventromedial nucleus were of the large type.(ABSTRACT TRUNCATED AT 400 WORDS)

A role for GABAB receptors in excitation and inhibition of thalamocortical cells

Gamma-aminobutyric acid (GABA) in the thalamus has mainly been associated with the inhibitory modulation of the sensory and cortical flow of information via a 'classical', chloride-dependent, GABAA receptor-mediated action. However, the discovery of a late, long-lasting potassium-dependent inhibitory postsynaptic potential (IPSP) mediated by GABAB receptors present on thalamocortical cells, has allowed new insights into our understanding of the physiological role of this neurotransmitter. In particular, work on the dorsal lateral geniculate nucleus indicates that together with a relatively weak inhibition, GABAB receptor-mediated IPSPs 'prepare' thalamocortical cells for burst firing by activating low-threshold calcium potentials. Thus, GABA in the thalamus can no longer be viewed only as a 'classical' inhibitory transmitter but also as a neuromodulator with a 'priming' role for burst firing excitation. This dual role of GABAB receptors in inhibition and excitation of thalamocortical cells might allow different interpretations of earlier findings in animals and humans, both in healthy and pathological conditions. It will also help to identify new functions for postsynaptic GABAB receptors in other parts of the central nervous system.

Electrophysiological characterization of potent agonists and antagonists at pre- and postsynaptic GABAB receptors on neurones in rat brain slices

1. Intracellular recordings were made from neurons in striatum (caudate-putamen) and substantia nigra pars compacta in rat brain slices. Three GABAB agonists, baclofen, 3-aminopropylphosphinic acid (3-APPA) and 3-aminopropyl(methyl)phosphinic acid (SK&F 97541), depressed excitatory postsynaptic potentials (e.p.s.ps) mediated by glutamate in the striatum, and hyperpolarized neurones in the substantia nigra. The ability of 3-aminopropyl(diethyoxymethyl)phosphinic acid (CGP 35348), 3-aminopropyl (hexyl)phosphinic acid (3-APHPA) and phaclofen to antagonize these responses was assessed. 2. Striatal e.p.s.ps, studied in the presence of bicuculline (30 microns), were reduced in amplitude by 92% with 6,7-dinitroquinoxaline-2,3-dione (DNQX; 30 microns). These e.p.s.ps were depressed by up to 95% by SK&F 97541 and baclofen with EC50s of 0.092 microns and 1.25 microns respectively. The maximal effect of 3-APPA was 67% with an EC50 of 0.83 microns. Agonist concentration-effect data fitted a single-site logistic model. GABAB agonists were without effect on striatal neurone membrane potential, input resistance or depolarizations induced by applied glutamate. 3. The depression of striatal e.p.s.ps by SK&F 97541 was reversibly antagonized by CGP 35348, 3-APHPA and phaclofen with estimated equilibrium dissociation constants (KB) of 11.2 +/- 1.7 microns (n = 4), 13.3 +/- 0.4 microM (n = 3) and 405 +/- 43 microM (n = 3) respectively. CGP 35348 and 3-APHPA appeared to act competitively (Schild plot slopes of 0.99 and 1.01 respectively). 4. Nigral neurones were hyperpolarized by up to 25 mV by SK&F 97541 and baclofen with EC50s of 0.15 microns and 3.6 microns respectively. The maximum hyperpolarization by 3-APPA was only 84% that of the other agonists, with an EC50 of 9.0 microM. Agonist concentration-effect data fitted a single-site logistic model. 5. The SK&F 97541-induced hyperpolarization was reversibly antagonized by CGP 35348, 3-APHPA and phaclofen with estimated KBS of 17.6 + 4.4 (n = 3), 14.0 + 1.5 (n = 4), and >400 microM (n = 1) respectively. CGP 35348 appeared competitive (Schild plot slope of 0.99). Antagonists were also tested with baclofen as agonist, yielding similar KB estimates as for SK&F 97541. 6. It is concluded that at both the presynaptic and postsynaptic sites examined, SK&F 97541 was about 10 fold more potent than baclofen or 3-APPA. The antagonists CGP 35348 and 3-APHPA (KB 1O-20 microM) were about 20 fold more potent than phaclofen. The similarities in relative agonist potency and estimated antagonist affinity between these two functionally distinct GABAB receptors renders them pharmacologically indistinguishable at present.