On the inhibitory actions of baclofen and gamma-aminobutyric acid in rat ventral midbrain culture

Presynaptic inhibition of excitatory afferents to hilar mossy cells

The hippocampus contains one very strong recurrent excitatory network formed by associational connections between CA3 pyramidal cells and another that depends largely on a disynaptic excitatory pathway between dentate granule cells. The recurrent excitatory network in CA3 has long been considered a possible location of autoassociative memory storage, whereas changes in the level and arrangement of recurrent excitation between granule cells are strongly implicated in epileptogenesis. Hilar mossy cells are likely to receive collateral input from CA3 pyramidal cells and they are key intermediaries (by mossy fiber inputs) in the recurrent excitatory network between granule cells. The current study uses minimal stimulation techniques in an in vitro preparation of the rat dentate gyrus to examine presynaptic modulation of both mossy fiber and non-mossy fiber inputs to hilar mossy cells. We report that both mossy fiber and non-mossy fiber inputs to hilar mossy cells express presynaptic gamma-aminobutyric acid type B (GABA(B)) receptors that are subject to tonic inhibition by ambient GABA. We further find that only non-mossy fiber inputs express presynaptic muscarinic acetylcholine receptors, but that bath application of cholinergic agonists produces action potential-dependent increases in ambient GABA that can indirectly inhibit mossy fiber inputs. Finally, we demonstrate that mossy cells express high-affinity postsynaptic GABA(A) receptors that are also capable of detecting changes in ambient GABA produced by cholinergic agonists. Our results are among the first to directly characterize these important collateral inputs to hilar mossy cells and may help facilitate informed comparison between primary and collateral projections in two major excitatory pathways.

Intracellular acidification in neurons induced by ammonium depends on KCC2 function

The Cl(-)-extruding neuron-specific K(+)-Cl(-) cotransporter KCC2, which establishes hyperpolarizing inhibition, can transport NH(4) (+) instead of K(+). It is, however, not clear whether KCC2 provides the only pathway for neuronal NH(4) (+) uptake. We therefore investigated NH(4) (+) uptake in cultured rat brain neurons. In neurons cultured for > 4 weeks, the response to NH(4)Cl applications (5 mM) consisted of an alkaline shift which reversed to an acid shift within seconds. Rebound acid shifts which followed brief applications of NH(4)Cl were blocked by furosemide (100 microM). They were rather insensitive to bumetanide (1 and 100 microM), in contrast to those induced in cultured glial cells. Rebound acid shifts persisted in the presence of 1 mM Ba(2+) and in Na(+)-free solution but were inhibited by extracellular K(+). In neurons with depolarizing GABA responses, indicating the absence of functional KCC2, applications of NH(4)Cl barely induced an acidosis. However, large rebound acid shifts occurred in neurons that had changed their GABA response from Ca(2+) increases to Ca(2+) decreases. Rebound acid shifts continued to increase even after the change in the GABA response had occurred and could be induced earlier in neurons transfected with KCC2 cDNA. We conclude that KCC2 provides the main pathway for fast neuronal NH(4) (+) uptake. Therefore, NH(4)Cl-induced rebound acid shifts can be used to indicate the development of KCC2 function. Further, the well known up-regulation of KCC2 function during development has the inevitable consequence of opening a major pathway for NH(4) (+) influx, which can be relevant under pathophysiological conditions.

Distinct mechanisms of presynaptic inhibition at GABAergic synapses of the rat substantia nigra pars compacta

We investigated the mechanisms of presynaptic inhibition of GABAergic neurotransmission by group III metabotropic glutamate receptors (mGluRs) and GABA(B) receptors, in dopamine (DA) neurons of the substantia nigra pars compacta (SNc). Both the group III mGluRs agonist L-(+)-2-amino-4-phosphonobutyric acid (AP4, 100 microM) and the GABA(B) receptor agonist baclofen (10 microM) reversibly depressed the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) to 48.5 +/- 2.7 and 79.3 +/- 1.6% (means +/- SE) of control, respectively. On the contrary, the frequency of action potential-independent miniature IPSCs (mIPSCs), recorded in tetrodotoxin (TTX, 1 microM) and cadmium (100 microM) were insensitive to AP4 but were reduced by baclofen to 49.7 +/- 8.6% of control. When the contribution of voltage-dependent calcium channels (VDCCs) to synaptic transmission was boosted with external barium (1 mM), AP4 became effective in reducing TTX-resistant mIPSCs to 65.4 +/- 3.9% of control, thus confirming a mechanism of presynaptic inhibition involving modulation of VDCCs. Differently from AP4, baclofen inhibited to 58.5 +/- 6.7% of control the frequency mIPSCs recorded in TTX and the calcium ionophore ionomycin (2 microM), which promotes Ca2+-dependent, but VDCC-independent, transmitter release. Moreover, in the presence of alpha-latrotoxin (0.3 nM), to promote a Ca2+-independent vesicular release of GABA, baclofen reduced mIPSC frequency to 48.1 +/- 3.2% of control, while AP4 was ineffective. These results indicate that group III mGluRs depress GABA release to DA neurons of the SNc through inhibition of presynaptic VDCCs, while presynaptic GABA(B) receptors directly impair transmitter exocytosis.

KCC2 mediates NH4+ uptake in cultured rat brain neurons

Elevated levels of NH4+ in the brain impair neuronal function. We studied the effects of NH4+ on postsynaptic inhibition of cultured rat brain neurons using whole cell recording under nominally HCO3- -free conditions. Application of NH4+ shifted the reversal potentials for spontaneous inhibitory postsynaptic currents and currents elicited by dendritic GABA applications in a positive direction because [Cl-]i increased. The positive shift of the reversal potentials of GABA-induced Cl- currents was equal on equimolar elevation of [NH4+]o or [K+]o, respectively. The NH4+-induced increase in [Cl-]i was reversed by an inhibitor of cation-anion cotransport, furosemide (0.1 mM), but not by bumetanide (0.01 mM) or by replacement of [Na+]o by Li+. We conclude that neuron-specific K-Cl cotransporter (KCC2) transports NH4+ similar to K+. Despite this fact, the small increase of [NH4+]o during metabolic encephalopathies will barely elevate [Cl-]i. However, an impairment of neuronal function may result because KCC2 provides a pathway to accumulate NH4+, and thereby, a continuous acid load to neurons.

Hyperpolarizing inhibition develops without trophic support by GABA in cultured rat midbrain neurons

During a limited period of early neuronal development, GABA is depolarizing and elevates [Ca2+]i, which mediates the trophic action of GABA in neuronal maturation. We tested the attractive hypothesis that GABA itself promotes the developmental change of its response from depolarizing to hyperpolarizing (Ganguly et al. 2001). In cultured midbrain neurons we found that the GABA response changed from depolarizing to hyperpolarizing, although GABAA receptors had been blocked throughout development. In immature neurons prolonged exposure of the cells to nanomolar concentrations of GABA or brief repetitive applications of GABA strongly diminished the elevation of [Ca+]i by GABA. As revealed by gramicidin perforated-patch recording, reduced [Ca2+]i responses were due to a diminished driving force for Cl-. This suggests that immature neurons do not have an efficient inward transport that can compensate the loss of cytosolic Cl-resulting from sustained GABAA receptor activation by ambient GABA. Transient increases in external K+, which can induce voltage-dependent Cl- entry, restored GABA-induced [Ca2+]i elevations. In mature neurons, GABA reduced [Ca2+]i provided that background [Ca2+]i was elevated by the application of an L-type Ca2+ channel agonist. This was probably due to a hyperpolarization of the membrane by Cl- currents. K(+)-Cl- cotransport maintained the gradient for hyperpolarizing Cl-currents. We conclude that in immature midbrain neurons an inward Cl- transport is not effective although the GABA response is depolarizing. Further, GABA itself is not required for the developmental switch of GABAergic responses from depolarizing to hyperpolarizing in cultured midbrain neurons.

GABA B receptor modulation of excitatory and inhibitory synaptic transmission onto rat CA3 hippocampal interneurons

Hippocampal stratum radiatum inhibitory interneurons receive glutamatergic excitatory innervation via the recurrent collateral fibers of CA3 pyramidal neurons and GABAergic inhibition from other interneurons. We examined both presynaptic- and postsynaptic-GABA(B) receptor-mediated responses at both synapse types. Postsynaptic GABA(B) receptor-mediated responses were absent in recordings from young (P16-18) but present in recordings from older animals (> or =P30) suggesting developmental regulation. In young animals, the GABA(B) receptor agonist, baclofen, inhibited the amplitude of evoked EPSCs and IPSCs, an effect blocked by prior application of the selective antagonist CGP55845. Baclofen enhanced the paired-pulse ratio and coefficient of variation of evoked EPSCs and IPSCs, consistent with a presynaptic mechanism of regulation. In addition, baclofen reduced the frequency of miniature IPSCs but not mEPSCs. However, baclofen reduced the frequency of KCl-induced mEPSCs; an effect blocked by Cd(2+), implicating presynaptic voltage-gated Ca(2+) channels as a target for baclofen modulation. In contrast, although Cd(2+) prevented the KCl-induced increase in mIPSC frequency, it failed to block baclofen's reduction of mIPSC frequency. Whereas N- and P/Q-types of Ca(2+) channels contributed equally to GABA(B) receptor-mediated inhibition of EPSCs, more P/Q-type Ca(2+) channels were involved in GABA(B) receptor-mediated inhibition of IPSCs. Finally, baclofen blocked the frequency-dependent depression of EPSCs and IPSCs, but was less effective at blocking frequency-dependent facilitation of EPSCs. Our results demonstrate that presynaptic GABA(B) receptors are expressed on the terminals of both excitatory and inhibitory synapses onto CA3 interneurons and that their activation modulates essential components of the release process underlying transmission at these two synapse types.

GABA(B) receptor-mediated inhibition of mitral/tufted cell activity in the rat olfactory bulb: a whole-cell patch-clamp study in vitro

GABA, the major inhibitory neurotransmitter involved in information processing in the olfactory bulb, is hypothesized to act through GABA(B) receptors by depressing primary neurotransmitter release at the level of olfactory nerve axon endings. The present study was designed to analyze GABA(B) receptor-mediated inhibition mechanisms by performing whole-cell patch-clamp recordings of mitral/tufted cell activity in the rat in vitro. To do so, GABA(B) receptor-mediated action was mimicked by baclofen and antagonized by saclofen. Our protocol led us to provide an original description of GABA(B) receptor-mediated inhibition exerted on mitral/tufted cells. First, their spontaneous activity was shown to be drastically abolished by baclofen. Second, their responses to olfactory nerve electrical stimulation were graded by GABA(B) receptor-mediated inhibition. Indeed, this inhibition may be described as inducing effects ranked from a slight increase in response latency to a complete response suppression.Altogether, our results corroborate the hypothesis of a presynaptic extrasynaptic GABA(B) receptor-mediated inhibition influencing mitral/tufted cell olfactory nerve responsivity. However, the involvement of postsynaptic receptors, with different properties or with different anatomical locations, cannot be ruled out, particularly in the control of spontaneous activity. In conclusion, we underline that, in the vertebrate olfactory bulb, GABA(B) receptor-mediated action appears to contribute to make mitral/tufted cell responses more salient by reducing their resting activity.

Insulin-like growth factor 1 and a cytosolic tyrosine kinase activate chloride outward transport during maturation of hippocampal neurons

The development of hyperpolarizing inhibition is an important step in the maturation of neuronal networks. Hyperpolarizing inhibition requires Cl(-) outward transport that is accomplished by KCC2, a K(+)/Cl(-) cotransporter. We show that cultured hippocampal neurons initially contain an inactive form of the KCC2 protein, which becomes activated during subsequent maturation of the neurons. We also show that this process is accelerated by transient stimulation of IGF-1 receptors. Because the transporter can be rapidly activated by coapplication of IGF-1 and an Src kinase and can be deactivated by membrane-permeable protein tyrosine kinase inhibitors, we suggest that activation of K(+)/Cl(-) cotransporter function by endogenous protein tyrosine kinases mediates the developmental switch of GABAergic responses to hyperpolarizing inhibition.

Insulin-like growth factor 1 and a cytosolic tyrosine kinase activate chloride outward transport during maturation of hippocampal neurons

The development of hyperpolarizing inhibition is an important step in the maturation of neuronal networks. Hyperpolarizing inhibition requires Cl(-) outward transport that is accomplished by KCC2, a K(+)/Cl(-) cotransporter. We show that cultured hippocampal neurons initially contain an inactive form of the KCC2 protein, which becomes activated during subsequent maturation of the neurons. We also show that this process is accelerated by transient stimulation of IGF-1 receptors. Because the transporter can be rapidly activated by coapplication of IGF-1 and an Src kinase and can be deactivated by membrane-permeable protein tyrosine kinase inhibitors, we suggest that activation of K(+)/Cl(-) cotransporter function by endogenous protein tyrosine kinases mediates the developmental switch of GABAergic responses to hyperpolarizing inhibition.

GABA(B) receptors modulate short-term potentiation of spontaneous excitatory postsynaptic currents in the rat supraoptic nucleus in vitro

High-frequency stimulation of afferents to the supraoptic nucleus (SON) results in a robust increase in the frequency and amplitude of pharmacologically isolated, tetrodotoxin-resistant, miniature excitatory postsynaptic currents (mEPSCs) lasting for 5-20 min. This increase in mEPSC frequency, termed short-term potentiation (STP), is tightly coupled to increases in action potential firing in magnocellular neurons (MCNs) suggesting a functional role for STP. gamma-Aminobutyric acid (GABA), acting selectively on GABA(B) receptors, has been shown to modulate action potential-dependent EPSCs, as well as mEPSCs in this nucleus. In this study, we examined the role of GABA in STP. Using in vitro hypothalamic slices containing the SON and the nystatin perforated-patch recording technique to record from MCNs, we tested the hypothesis that GABA modulates STP. Baclofen, a GABA(B) receptor agonist, caused a reversible decrease in the frequency of mEPSCs as well as a reduction in the magnitude and duration of STP. GABA(B) receptor antagonists blocked the baclofen-induced decrease in mEPSC frequency and reduction in STP. In addition, the antagonists by themselves increased basal mEPSC frequency while prolonging the duration of STP in most cells. By contrast, picrotoxin, a GABA(A) chloride channel blocker, had no effect on STP.These findings indicate that GABA is tonically present in the SON and its action at the GABA(B) receptor may determine the magnitude and duration of STP.

A furosemide-sensitive K+-Cl- cotransporter counteracts intracellular Cl- accumulation and depletion in cultured rat midbrain neurons

Efficacy of postsynaptic inhibition through GABAA receptors in the mammalian brain depends on the maintenance of a Cl- gradient for hyperpolarizing Cl- currents. We have taken advantage of the reduced complexity under which Cl- regulation can be investigated in cultured neurons as opposed to neurons in other in vitro preparations of the mammalian brain. Tightseal whole-cell recording of spontaneous GABAA receptor-mediated postsynaptic currents suggested that an outward Cl- transport reduced dendritic [Cl-]i if the somata of cells were loaded with Cl- via the patch pipette. We determined dendritic and somatic reversal potentials of Cl- currents induced by focally applied GABA to calculate [Cl-]i during variation of [K+]o and [Cl-] in the patch pipette. [Cl-]i and [K+]o were tightly coupled by a furosemide-sensitive K+-Cl- cotransport. Thermodynamic considerations excluded the significant contribution of a Na+-K+-Cl- cotransporter to the net Cl- transport. We conclude that under conditions of normal [K+]o the K+-Cl- cotransporter helps to maintain [Cl-]i at low levels, whereas under pathological conditions, under which [K+]o remains elevated because of neuronal hyperactivity, the cotransporter accumulates Cl- in neurons, thereby further enhancing neuronal excitability.

A furosemide-sensitive K+-Cl- cotransporter counteracts intracellular Cl- accumulation and depletion in cultured rat midbrain neurons

Efficacy of postsynaptic inhibition through GABAA receptors in the mammalian brain depends on the maintenance of a Cl- gradient for hyperpolarizing Cl- currents. We have taken advantage of the reduced complexity under which Cl- regulation can be investigated in cultured neurons as opposed to neurons in other in vitro preparations of the mammalian brain. Tightseal whole-cell recording of spontaneous GABAA receptor-mediated postsynaptic currents suggested that an outward Cl- transport reduced dendritic [Cl-]i if the somata of cells were loaded with Cl- via the patch pipette. We determined dendritic and somatic reversal potentials of Cl- currents induced by focally applied GABA to calculate [Cl-]i during variation of [K+]o and [Cl-] in the patch pipette. [Cl-]i and [K+]o were tightly coupled by a furosemide-sensitive K+-Cl- cotransport. Thermodynamic considerations excluded the significant contribution of a Na+-K+-Cl- cotransporter to the net Cl- transport. We conclude that under conditions of normal [K+]o the K+-Cl- cotransporter helps to maintain [Cl-]i at low levels, whereas under pathological conditions, under which [K+]o remains elevated because of neuronal hyperactivity, the cotransporter accumulates Cl- in neurons, thereby further enhancing neuronal excitability.

Differential expression of pre- and postsynaptic GABA(B) receptors in rat substantia nigra pars reticulata neurones

Whole-cell recordings were made from substantia nigra pars reticulata in rat midbrain slices to study the functional expression of pre- and postsynaptic GABA(B) receptors in GABA output neurones. Baclofen (up to 300 microM) dose-dependently activated a weak current which was insensitive to tetrodotoxin and Ca2+-free solution but blocked by Ba2+ and 2-OH-saclofen. The maximum current activated by baclofen (30 microM) was 43.0 +/- 4.5 pA (n = 27), representing only 23% of that in dopamine neurones. Baclofen (1-30 microM) also reduced the frequency of the GABA(A) receptor-mediated miniature inhibitory postsynaptic currents while the distribution of their amplitudes was unaffected. This presynaptic effect of baclofen, prominent at a concentration as low as 1 microM, was sensitive to 2-OH-saclofen and occluded by Cd2+, but was unaffected by Ba2+. The results suggest a predominant role of the presynaptic GABA(B) receptors in substantia nigra pars reticulata. The relative abundance of pre- and postsynaptic GABA(B) receptor subtypes in this brain region may also be important in mediating the anticonvulsant effect of baclofen in rats.

GABAB receptor-mediated inhibition of GABAA receptor calcium elevations in developing hypothalamic neurons

In the CNS, gamma-aminobutyric acid (GABA) affects neuronal activity through both the ligand-gated GABAA receptor channel and the G protein-coupled GABAB receptor. In the mature nervous system, both receptor subtypes decrease neural excitability, whereas in most neurons during development, the GABAA receptor increases neural excitability and raises cytosolic Ca2+ levels. We used Ca2+ digital imaging to test the hypothesis that GABAA receptor-mediated Ca2+ rises were regulated by GABAB receptor activation. In young, embryonic day 18, hypothalamic neurons cultured for 5 +/- 2 days in vitro, we found that cytosolic Ca2+ rises triggered by synaptically activated GABAA receptors were dramatically depressed (>80%) in a dose-dependent manner by application of the GABAB receptor agonist baclofen (100 nM-100 microM). Coadministration of the GABAB receptor antagonist 2-hydroxy-saclofen or CGP 35348 reduced the inhibitory action of baclofen. Administration of the GABAB antagonist alone elicited a reproducible Ca2+ rise in >25% of all synaptically active neurons, suggesting that synaptic GABA release exerts a tonic inhibitory tone on GABAA receptor-mediated Ca2+ rises via GABAB receptor activation. In the presence of tetrodotoxin the GABAA receptor agonist muscimol elicited robust postsynaptic Ca2+ rises that were depressed by baclofen coadministration. Baclofen-mediated depression of muscimol-evoked Ca2+ rises were observed in both the cell bodies and neurites of hypothalamic neurons taken at embryonic day 15 and cultured for three days, suggesting that GABAB receptors are functionally active at an early stage of neuronal development. Ca2+ rises elicited by electrically induced synaptic release of GABA were largely inhibited (>86%) by baclofen. These results indicate that GABAB receptor activation depresses GABAA receptor-mediated Ca2+ rises by both reducing the synaptic release of GABA and decreasing the postsynaptic Ca2+ responsiveness. Collectively, these data suggest that GABAB receptors play an important inhibitory role regulating Ca2+ rises elicited by GABAA receptor activation. Changes in cytosolic Ca2+ during early neural development would, in turn, profoundly affect a wide array of physiological processes, such as gene expression, neurite outgrowth, transmitter release, and synaptogenesis.

Enhancement of synaptic excitation by GABAA receptor antagonists in rat embryonic midbrain culture

Alterations of synaptic excitation induced by exposure to gamma-aminobutyric acid-A (GABAA) receptor antagonists were investigated employing tight-seal whole cell recording from single neurons or pairs of neurons in rat embryonic midbrain culture. Application of GABAA receptor antagonists led to sustained depolarizations followed by synchronous paroxysmal depolarization shifts (PDSs). PDSs induced a transient increase in miniature excitatory postsynaptic currents in the presence as well as in the absence of a N-methyl-aspartate receptor antagonist. The increase in glutamate release supports the excitatory drive required to reinitiate PDSs from the quiescent interburst intervals. After washout of GABAA receptor antagonists, synaptic activity remained grouped, regardless of the presence or absence of PDS blockade by tetrodotoxin (TTX). Impediment of action potential-triggered transmitter release by Cd2+ or TTX also induced grouped activity. We conclude that changes in synaptic excitation are produced by the impaired GABAA inhibition per se and by the initiation of PDSs.

Inhibition of spontaneous EPSCs and IPSCs by presynaptic GABAB receptors on rat supraoptic magnocellular neurons

1. The function of presynaptic GABA receptors in the regulation of transmitter release in supraoptic nucleus (SON) magnocellular neurons was investigated by recording spontaneous postsynaptic currents from rat magnocellular SON neurons in a slice preparation (150 microns thick, 1.8 mm in diameter) using the whole-cell patch-clamp technique. 2. Both the spontaneous EPSCs and IPSCs were TTX resistant. The EPSCs were abolished by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), whereas the IPSCs were abolished by picrotoxin, suggesting that the EPSCs and IPSCs are synaptic inputs from glutamatergic and GABAergic neurons, respectively. 3. The selective GABAB agonist, baclofen, reduced the frequency of both the EPSCs and IPSCs without affecting the amplitude. The time constant of the decay phase of both the EPSCs and IPSCs remained unchanged after baclofen application. 4. The reduction of the frequency of the synaptic currents by baclofen was dose dependent (10 nM to 100 microM) and the EC50 values were 5.8 and 8.5 microM for the EPSCs and IPSCs, respectively. 5. The effect of baclofen (10 microM) was antagonized by the selective GABAB antagonist, 2-hydroxy-saclofen (2OH-saclofen), at 300 microM. 6. When given alone, 2OH-saclofen (100 microM) increased the frequency of both the EPSCs and IPSCs without affecting their amplitude, suggesting that endogenously released GABA in the slice acts on presynaptic GABAB receptors. 7. The GABAA agonist, muscimol, reduced the frequency of EPSCs, and picrotoxin increased the frequency of the EPSCs, suggesting that GABAA receptors also participate in the presynaptic inhibition of glutamate release. 8. Taken together, these data suggest that GABAB receptors are present on the presynaptic terminals of both GABA and glutamate neurons in the SON, and that these presynaptic GABAB receptors play an important role in the regulation of the neuronal activity in SON magnocellular neurons.

GABAA receptor-mediated IPSCs in rat thalamic sensory nuclei: patterns of discharge and tonic modulation by GABAB autoreceptors

1. The patterns of discharge of spontaneous GABAA-mediated inhibitory postsynaptic currents (sIPSCs), originating from the nucleus reticularis thalami (NRT), and their modulation by GABAB autoreceptors, were studied in rat thalamocortical (TC) neurones using whole-cell voltage-clamp recordings in brain slices. 2. sIPSCs were recorded in all ventro-basal (VB) and dorsal lateral geniculate (LGN) neurones. In VB neurones, in the presence of tetraethylammonium (TEA, 5 mM), these sIPSCs can occur in bursts at frequencies of either 0.1 or 1-2 Hz. In the presence of tetrodotoxin (TTX), these bursting activities are replaced by the continuous discharge of miniature IPSCs (mIPSCs), recorded in the absence of TEA, at a frequency of 4 Hz. The kinetic properties of mIPSCs were similar in VB and LGN TC neurones. 3. In VB TC neurones the GABAB receptor agonist (+/-)-baclofen, at a concentration of 0.05 microM, decreased the mIPSC frequency by 22% without affecting their amplitude distribution. Increasing the (+/-)-baclofen concentration to 1 and 10 microM caused similar reductions (41 and 47%, respectively) in the mIPSCs frequency: these values were significantly different from the one observed with 0.05 microM (+/-)-baclofen. In LGN TC neurones, where mIPSCs originate from both NRT and local interneurone terminals, 1 microM (+/-)-baclofen produced a 66% reduction in the mIPSC frequency. 4. The GABAB receptor antagonist CGP55845A (50 nM) not only blocked the baclofen-mediated decrease in mIPSC frequency, but also produced a 52% increase in the mIPSC frequency compared with control in three out of seven neurones. Application of CGP55845A (50-500 nM) alone produced a 77% increase in the mIPSC frequency in three out of nine VB neurones, and in the LGN, CGP55845A (100 nM) produced a 53% increase in four out of nine neurones. CGP55845A (100 nM) also reversibly increased the amplitude of evoked GABAA IPSCs by 74 and 57% in three out of three VB and three out of five LGN neurones, respectively. 5. Application of GABA (1.5-5 microM) decreased the mIPSC frequency in VB TC neurones by a similar extent (48%) as 1-10 microM (+/-)-baclofen. 6. In the presence of 100 microM Cd2+, (+/-)-baclofen still decreased the mIPSC frequency by about 40%, indicating that the effect of presynaptic GABAB receptor activation on spontaneous GABA release did not occur through a reduction of voltage-dependent Ca2+ currents. 7. Cd2+ (100 microM) decreased the amplitude of both mIPSCs and isoguvacine-induced current by 30 and 19%, respectively, indicating an effect of this divalent cation on postsynaptic GABAA receptors. 8. We conclude that GABAB autoreceptors are present on the GABAergic terminals within the thalamic sensory nuclei and that these receptors can be tonically activated by the ambient GABA.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

GABAB receptor-mediated inhibition of spontaneous inhibitory synaptic currents in rat midbrain culture

1. Tight-seal, whole-cell recording was used to study GABAB receptor-mediated inhibition of spontaneous inhibitory synaptic currents in cultured rat midbrain neurones. 2. Spontaneous miniature inhibitory postsynaptic currents (mIPSCs) were recorded in tetrodotoxin (TTX), Cd2+ and Ba2+. (R)-(-)-baclofen reduced the frequency of mIPSCs through a presynaptic mechanism. The EC50 for this effect was 7 microM. It was antagonized by the GABAB receptor antagonist CGP55845A (0.5 microM). 3. In pertussis toxin (PTX)-treated cultures, some GABAB receptor-mediated reduction of the frequency of mIPSCs persisted. In contrast, PTX treatment totally abolished inhibition of miniature excitatory postsynaptic currents (mEPSCs). 4. In PTX-treated cultures, a saturating concentration of (R)-(-)-baclofen inhibited action potential-generated IPSCs but no EPSCs. 5. PTX treatment abolished the (R)-(-)-baclofen-mediated inhibition of high voltage-activated somatic Ca2+ currents and of spontaneous IPSCs depending on presynaptic Ca2+ entry. 6. We conclude that cellular mechanisms underlying GABAB receptor-mediated inhibition of mIPSCs contribute to auto-inhibition of GABA release.

GABAB receptor-mediated inhibition of tetrodotoxin-resistant GABA release in rodent hippocampal CA1 pyramidal cells

Tight-seal whole-cell recordings from CA1 pyramidal cells of rodent hippocampus were performed to study GABAB receptor-mediated inhibition of tetrodotoxin (TTX)-resistant IP-SCs. IPSCs were recorded in the presence of TTX and glutamate receptor antagonists. (R)-(-)-baclofen reduced the frequency of TTX-resistant IPSCs by a presynaptic action. The inhibition by (R)-(-)-baclofen was concentration-dependent, was not mimicked by the less effective enantiomer (S)-(+)-baclofen, and was blocked by the GABAB receptor antagonist CGP 55845A, suggesting a specific effect on GABAB receptors. The inhibition persisted in the presence of the Ca2+ channel blocker Cd2+. There was no requirement for an activation of K+ conductances by (R)-(-)-baclofen, because the inhibition of TTX-resistant IPSCs persisted in Ba2+ and Cd2+. Because the time courses of TTX-resistant IPSCs were not changed by (R)-(-)-baclofen, there was no evidence for a selective inhibition of quantal release from a subgroup of GABAergic terminals. (R)-(-)-baclofen reduced the frequency of TTX-resistant IPSCs in guinea pigs and Wistar rats, whereas the inhibition was much smaller in Sprague Dawley rats. In Cd2+ and Ba2+, beta-phorbol-12,13-dibutyrate and forskolin enhanced the frequency of TTX-resistant IPSCs. Only beta-phorbol-12, 13-dibutyrate reduced the inhibition by (R)-(-)-baclofen. We conclude that GABAB receptors inhibit TTX-resistant GABA release through a mechanism independent from the well known effects on Ca2+ or K+ channels. The inhibition of quantal GABA release can be reduced by an activator of protein kinase C.

GABAB receptor-mediated inhibition of tetrodotoxin-resistant GABA release in rodent hippocampal CA1 pyramidal cells

Tight-seal whole-cell recordings from CA1 pyramidal cells of rodent hippocampus were performed to study GABAB receptor-mediated inhibition of tetrodotoxin (TTX)-resistant IP-SCs. IPSCs were recorded in the presence of TTX and glutamate receptor antagonists. (R)-(-)-baclofen reduced the frequency of TTX-resistant IPSCs by a presynaptic action. The inhibition by (R)-(-)-baclofen was concentration-dependent, was not mimicked by the less effective enantiomer (S)-(+)-baclofen, and was blocked by the GABAB receptor antagonist CGP 55845A, suggesting a specific effect on GABAB receptors. The inhibition persisted in the presence of the Ca2+ channel blocker Cd2+. There was no requirement for an activation of K+ conductances by (R)-(-)-baclofen, because the inhibition of TTX-resistant IPSCs persisted in Ba2+ and Cd2+. Because the time courses of TTX-resistant IPSCs were not changed by (R)-(-)-baclofen, there was no evidence for a selective inhibition of quantal release from a subgroup of GABAergic terminals. (R)-(-)-baclofen reduced the frequency of TTX-resistant IPSCs in guinea pigs and Wistar rats, whereas the inhibition was much smaller in Sprague Dawley rats. In Cd2+ and Ba2+, beta-phorbol-12,13-dibutyrate and forskolin enhanced the frequency of TTX-resistant IPSCs. Only beta-phorbol-12, 13-dibutyrate reduced the inhibition by (R)-(-)-baclofen. We conclude that GABAB receptors inhibit TTX-resistant GABA release through a mechanism independent from the well known effects on Ca2+ or K+ channels. The inhibition of quantal GABA release can be reduced by an activator of protein kinase C.

5,7-Dihydroxytryptamine uptake discriminates living serotonergic cells from dopaminergic cells in rat midbrain culture

Dissociated cells from embryonic rat midbrain develop in dissociated culture into glutamatergic, GABAergic and aminergic cells. The autofluorescent serotonin analogue, 5,7-dihydroxytryptamine (5,7-DHT), is taken up by a small population of cells that is immunoreactive to 5-hydroxytryptamine. Tyrosine hydroxylase-immunoreactive cells do not accumulate 5,7-DHT. 5,7-DHT uptake, therefore, is well suited for the identification of living serotonergic cells and their discrimination from dopaminergic cells.

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.

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.

Differences in the Cs block of baclofen and 4-aminopyridine induced potassium currents of guinea pig CA3 neurons in vitro

Single-electrode current- and voltage-clamp techniques were employed to study responses elicited by (-)baclofen or gamma-aminobutyric acid (GABA) and 4-aminopyridine (4-AP) induced inhibitory postsynaptic potentials in CA3 pyramidal neurons in guinea pig hippocampal slices. All drugs were applied by the bath to submerged slices in which fast synaptic transmission was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM), bicuculline (50 microM), and picrotoxin (50 microM). (-)Baclofen (0.5 microM) and GABA (1 mM) induced equivalent-sized hyperpolarizations and input resistance decreases. The agonist induced hyperpolarization or current and 4-AP induced hyperpolarizations or currents (4-AP induced K-IPSPs or IPSCs) reversed in sign near the K-equilibrium potential (EK). The GABAB receptor antagonists, OH-saclofen (500 microM) and CGP 35348 (100 microM), reduced (-)baclofen responses, and 4-AP induced K-IPSPs, suggesting that they were mediated by GABAB receptors. Intracellular tetraethylammonium-, and extracellular barium-ions (1 mM) diminished the (-)baclofen induced current and 4-AP induced K-IPSCs. Intracellular Cs-ions blocked the (-)baclofen induced outward current at resting membrane potential but did not grossly affect the inward current recorded at membrane potentials negative to EK. 4-AP induced inwardly or outwardly directed K-IPSCs were not blocked by intracellular Cs-ions. Extracellular Cs-ions (5 mM) blocked the (-)baclofen induced inward K-current, but did not block 4-AP induced inwardly directed K-IPSCs. In conclusion, we found differences in the Cs block of activated by (-)baclofen or the endogenous transmitter GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

Baclofen as an adjuvant analgesic

Baclofen is a gamma-aminobutyric acid (GABA) agonist approved for the treatment of spasticity and commonly used in the management of many types of neuropathic pain. Controlled studies have demonstrated the efficacy of this drug in trigeminal neuralgia. Although its precise mechanism of analgesic action is unknown, it is likely that a drug-induced increase in inhibitory activity is sufficient to interrupt the cascade of neural events that culminates in aberrant activity of wide dynamic range neurons, or more rostral neurons in nociceptive pathways, that is the substrate for some types of neuropathic pain. The optimal use of baclofen as an adjuvant analgesic requires an understanding of its pharmacology, side effect spectrum, and dosing guidelines that have proven useful in clinical practice. Failure of baclofen therapy following a prolonged trial requires dose tapering prior to discontinuation due to the potential for a withdrawal syndrome.

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)

Microiontophoresis of baclofen on membrane potential and input resistance in bulbar respiratory neurons in the cat

Iontophoresis of baclofen produced hyperpolarization and a decrease in input resistance in 32 neurons and no effect in 24 neurons of the ventral respiratory group in cats. Iontophoresed phaclofen antagonized the effect of baclofen, but had negligible effects on periodic fluctuations in membrane potential and spike activity in these neurons. The hyperpolarizing effect of baclofen persisted after iontophoresis of tetrodotoxin, suggesting that baclofen acted directly at the postsynaptic gamma-aminobutyric acid (GABA)B receptors.

Characterization of synaptically elicited GABAB responses using patch-clamp recordings in rat hippocampal slices

1. Tight-seal, whole-cell voltage clamp recording techniques were used to characterize monosynaptically evoked GABAB currents in adult rat brain slices maintained at 34-35 degrees C. Responses were recorded from granule cells of the dentate gyrus following the blockade of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-, D-2-amino-5-phosphonovaleric acid (D-AP5)- and picrotoxin-sensitive fast synaptic transmission, so that the remaining synaptic currents could be studied in isolation. 2. Under these conditions, stimulation in the molecular layer elicited a slow outward current which was blocked by the selective GABAB antagonist CGP 35348 in a concentration-dependent manner (200-800 microM). This current was absent in recordings made with pipettes containing 10-15 mM of the lidocaine derivative QX-314 or when caesium was substituted for K+. 3. Increasing the [K+]o e-fold (from 2.5 to 6.8 mM) shifted the reversal potential of the GABAB current from -97.9 to -73.2 mV, as predicted by the Nernst equation. Peak conductance was constant, but in 6.8 mM [K+]o at voltages hyperpolarized to EK (equilibrium potential for potassium), a small outward rectification was evident. 4. The time course of the current could be described by fourth-power exponential activation kinetics with double exponential inactivation. At 34-35 degrees C, the average activation time constant (tau m) was 45.2 ms, while the two inactivation time constants (tau h1 and tau h2) were 110.2 and 516.2 ms, with corresponding weighting factors (wh1 and wh2) of 0.84 and 0.16, respectively. The Q10 (temperature coefficient) values for these time constants were between 1.82 and 2.31. Neither tau m, nor tau h1 and tau h2 were voltage dependent in the range from -45 to -95 mV. 5. Paired-pulse depression of the GABAB current was studied by giving identical conditioning and test stimuli over a wide range (50-5000 ms) of interstimulus intervals (ISIs). The maximal depression (48%) occurred at 200 ms ISI, and the depression lasted for over 5 s. The magnitude of paired-pulse depression was not dependent on the postsynaptic membrane potential. 6. Application of the competitive antagonist CGP 35348 such that the peak current was diminished by approximately 50% had no effect on the activation or inactivation kinetics of the current. Similarly, during paired-pulse depression the kinetics of test currents were identical to those of conditioning currents. These findings support the hypothesis that the mechanism responsible for paired-pulse depression involves a reduction in neurotransmitter release without postsynaptic alterations in K+ channel activation/inactivation kinetics.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic inhibition of miniature excitatory synaptic currents by baclofen and adenosine in the hippocampus

Presynaptic inhibition of neurotransmitter release is thought to be mediated by a reduction of axon terminal Ca2+ current. We have compared the actions of several known inhibitors of evoked glutamate release with the actions of the Ca2+ channel antagonist Cd2+ on action potential-independent synaptic currents recorded from CA3 neurons in hippocampal slice cultures. Baclofen and adenosine decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) without affecting the distribution of their amplitudes. Cd2+ blocked evoked synaptic transmission, but had no effect on the frequency or amplitude of either mEPSCs or inhibitory postsynaptic currents (IPSCs). Inhibition of presynaptic Ca2+ current therefore appears not to be required for the inhibition of glutamate release by adenosine and baclofen. Baclofen had no effect on the frequency of miniature IPSCs, indicating that gamma-aminobutyric acid B-type receptors exert distinct presynaptic actions at excitatory and inhibitory synapses.