Contribution of chloride shifts to the fade of gamma-aminobutyric acid-gated currents in frog dorsal root ganglion cells

Characterization of inhibitory GABA-A receptor activation during spreading depolarization in brain slice

Spreading depolarization (SD) is a slowly propagating wave of near complete depolarizations of neurons and glia. Previous studies have reported large GABA releases during SD, but there is limited understanding of how GABA release and receptor activation are regulated and influence the propagating SD wavefront, as well as an excitatory phase immediately following the passage of SD. The present study characterized GABA-A type receptor (GABA_AR) currents during SD generated by KCl microinjection in acute hippocampal slices from adult mice. Spontaneous GABA_AR-mediated currents (sIPSCs) were initially enhanced, and were followed by a large outward current at the wavefront. sIPSC were then transiently supressed during the late SD phase, resulting in a significant reduction of the sIPSC/sEPSC ratio. The large outward current generated during SD was eliminated by the GABA_AR antagonist gabazine, but the channel potentiator/agonist propofol failed to potentiate the current, likely because of a ceiling effect. Extracellular Cl⁻ decreases recorded during SD were reduced by the antagonist but were not increased by the potentiator. Together with effects of GABA_AR modulators on SD propagation rate, these results demonstrate a significant inhibitory role of the initial GABA_AR activation and suggest that intracellular Cl⁻ loading is insufficient to generate excitatory GABA_AR responses during SD propagation. These results provide a mechanistic explanation for facilitating effects of GABA_AR antagonists, and the lack of inhibitory effect of GABA_AR potentiators on SD propagation. In addition, selective suppression of GABA transmission in the late SD period and the lack of effect of GABA_A modulators on the duration of SD suggests that GABA modulation may not be effective approach to protect neurons during the vulnerable phase of SD.

Cl(-) concentration changes and desensitization of GABA(A) and glycine receptors

Desensitization of ligand-gated ion channels plays a critical role for the information transfer between neurons. The current view on γ-aminobutyric acid (GABA)_A and glycine receptors includes significant rapid components of desensitization as well as cross-desensitization between the two receptor types. Here, we analyze the mechanism of apparent cross-desensitization between native GABA_A and glycine receptors in rat central neurons and quantify to what extent the current decay in the presence of ligand is a result of desensitization versus changes in intracellular Cl⁻ concentration ([Cl⁻]_i). We show that apparent cross-desensitization of currents evoked by GABA and by glycine is caused by changes in [Cl⁻]_i. We also show that changes in [Cl⁻]_i are critical for the decay of current in the presence of either GABA or glycine, whereas changes in conductance often play a minor role only. Thus, the currents decayed significantly quicker than the conductances, which decayed with time constants of several seconds and in some cells did not decay below the value at peak current during 20-s agonist application. By taking the cytosolic volume into account and numerically computing the membrane currents and expected changes in [Cl⁻]_i, we provide a theoretical framework for the observed effects. Modeling diffusional exchange of Cl⁻ between cytosol and patch pipettes, we also show that considerable changes in [Cl⁻]_i may be expected and cause rapidly decaying current components in conventional whole cell or outside-out patch recordings. The findings imply that a reevaluation of the desensitization properties of GABA_A and glycine receptors is needed.

Anomalous levels of Cl- transporters cause a decrease of GABAergic inhibition in human peritumoral epileptic cortex

PURPOSE

Several factors contribute to epileptogenesis in patients with brain tumors, including reduced γ-aminobutyric acid (GABA)ergic inhibition. In particular, changes in Cl(-) homeostasis in peritumoral microenvironment, together with alterations of metabolism, are key processes leading to epileptogenesis in patients afflicted by glioma. It has been recently proposed that alterations of Cl(-) homeostasis could be involved in tumor cell migration and metastasis formation. In neurons, the regulation of intracellular Cl(-) concentration ([Cl(-) ](i) ) is mediated by NKCC1 and KCC2 transporters: NKCC1 increases while KCC2 decreases [Cl(-) ](i) . Experiments were thus designed to investigate whether, in human epileptic peritumoral cortex, alterations in the balance of NKCC1 and KCC2 activity may decrease the hyperpolarizing effects of GABA, thereby contributing to epileptogenesis in human brain tumors.

METHODS

Membranes from peritumoral cortical tissues of epileptic patients afflicted by gliomas (from II to IV WHO grade) and from cortical tissues of nonepileptic patients were injected into Xenopus oocytes leading to the incorporation of functional GABA(A) receptors. The GABA-evoked currents were recorded using standard two-microelectrode voltage-clamp technique. In addition, immunoblot analysis and immunohistochemical staining were carried out on membranes and tissues from the same patients.

KEY FINDINGS

We found that in oocytes injected with epileptic peritumoral cerebral cortex, the GABA-evoked currents had a more depolarized reversal potential (E(GABA) ) compared to those from nonepileptic healthy cortex. This difference of E(GABA) was abolished by the NKCC1 blocker bumetanide or unblocking of KCC2 with the Zn(2+) chelator TPEN. Moreover, Western blot analysis revealed an increased expression of NKCC1, and more modestly, of KCC2 transporters in epileptic peritumoral tissues compared to nonepileptic control tissues. In addition, NKCC1 immunoreactivity was strongly increased in peritumoral cortex with respect to nonepileptic cortex, with a prominent expression in neuronal cells.

SIGNIFICANCE

We report that the positive shift of E(GABA) in epileptic peritumoral human cortex is due to an altered expression of NKCC1 and KCC2, perturbing Cl(-) homeostasis, which might lead to a consequent reduction in GABAergic inhibition. These findings point to a key role of Cl(-) transporters KCC2 and NKCC1 in tumor-related epilepsy, suggesting a more specific drug therapy and surgical approaches for the epileptic patients afflicted by brain tumors.

Depolarizing shift in the GABA-induced current reversal potential by lidocaine hydrochloride

Lidocaine hydrochloride (LC-HCl) is widely used as a local anesthetic, while various adverse effects of LC-HCl, such as seizures have also been reported. Lidocaine is reported to inhibit various channels and receptors including GABA(A) receptors. Although the GABA(A) receptor-mediated response depends on Cl(-) equilibrium potential (E(Cl)), little is known about the effect of LC-HCl on E(Cl). In the present study, we investigated the effect of LC-HCl on GABA-induced currents in cultured rat hippocampal neurons with gramicidin-perforated patch-clamp recording which is known to keep the intracellular Cl(-) concentration intact. LC-HCl inhibited outward GABA-induced currents with depolarizing shift of the GABA reversal potential (E(GABA)). The LC-HCl-induced positive E(GABA) shift was not observed with conventional whole-cell patch-clamp method which cannot retain intact intracellular Cl(-) concentration. The LC-HCl action on E(GABA) was inhibited by either furosemide, a blocker of both Na(+)-K(+)-Cl(-) cotransporter (NKCC) and K(+)-Cl(-) cotransporter (KCC), or an increase in extracellular K(+) concentrations. Neither bumetanide, a specific inhibitor of NKCC, nor Na(+)-free external solution had any effect on the LC-HCl-induced E(GABA) shift. QX-314, a membrane impermeable lidocaine derivative, failed to shift E(GABA) to positive potential. Furthermore, LC-HCl caused a depolarizing shift of E(GABA) in cultured GT1-7 cells expressing KCC2 but failed to change E(GABA) in GT1-7 cells without expression of KCC2. These results suggest that the LC-HCl-induced positive E(GABA) shift is due to a blockade of KCC2. Together with the direct LC-HCl action to GABA(A) receptors, the positive E(GABA) shift induced by LC-HCl reduces the GABAergic inhibition in the central nervous system.

Activity-dependent intracellular chloride accumulation and diffusion controls GABA(A) receptor-mediated synaptic transmission

In the CNS, prolonged activation of GABA(A) receptors (GABA(A)Rs) has been shown to evoke biphasic postsynaptic responses, consisting of an initial hyperpolarization followed by a depolarization. A potential mechanism underlying the depolarization is an acute chloride (Cl(-)) accumulation resulting in a shift of the GABA(A) reversal potential (E(GABA)). The amount of GABA-evoked Cl(-) accumulation and accompanying depolarization depends on presynaptic and postsynaptic properties of GABAergic transmission, as well as on cellular morphology and regulation of Cl(-) intracellular concentration ([Cl(-)](i)). To analyze the influence of these factors on the Cl(-) and voltage behavior, we studied spatiotemporal dynamics of activity-dependent [Cl(-)](i) changes in multicompartmental models of hippocampal cells based on realistic morphological data. Simulated Cl(-) influx through GABA(A) Rs was able to exceed physiological Cl(-) extrusion rates thereby evoking HCO(3)(-) -dependent E(GABA) shift and depolarizing responses. Depolarizations were observed in spite of GABA(A) receptor desensitization. The amplitude of the depolarization was frequency-dependent and determined by intracellular Cl(-) accumulation. Changes in the dendritic diameter and in the speed of GABA clearance in the synaptic cleft were significant sources of depolarization variability. In morphologically reconstructed granule cells subjected to an intense GABAergic background activity, dendritic inhibition was more affected by accumulation of intracellular Cl(-) than somatic inhibition. Interestingly, E(GABA) changes induced by activation of a single dendritic synapse propagated beyond the site of Cl(-) influx and affected neighboring synapses. The simulations suggest that E(GABA) may differ even along a single dendrite supporting the idea that it is necessary to assign E(GABA) to a given GABAergic input and not to a given neuron.

Diazepam to Improve Acute Stroke Outcome: Results of the Early GABA-Ergic Activation Study In Stroke Trial

BACKGROUND

We tested whether diazepam, a GABA-ergic drug that also inhibits brain nitric monoxide formation, improves acute stroke prognosis.

METHODS

880 patients, randomized within 12 h of acute stroke, received diazepam 10 mg or placebo by rectiole, as soon as possible, followed by 10-mg tablets twice daily for 3 days. Primary outcome was independence (Rankin score <3) at 3 months; secondary outcome was complete recovery (Barthel index >or=95 or Rankin score <or=1).

RESULTS

Intention-to-treat analyses on all 849 patients with full follow-up (50.4% on diazepam): odds ratio (OR) 1.14, 95% CI 0.87-1.49 for primary endpoint, and an OR of 1.26 (0.90-1.76) for complete recovery, both favoring diazepam. Adjusted analyses for all stroke patients (843): OR 1.20 (0.87-1.65), and 1.25 (0.89-1.74), respectively, and for all infarct patients (748): OR 1.31 (0.93-1.85), and 1.46 (1.02-2.09; p=0.037), respectively. Analyses restricted to cardioembolic infarct patients (200) showed treatment benefit for the primary outcome: OR 2.26, 95% CI 1.07-4.76, p=0.032, and complete recovery: OR 2.65, 95% CI 1.06-6.59, p=0.037. About one third of ischemic stroke patients had 'any adverse event', without any difference between treatment groups. In 95 intracerebral hemorrhage patients, frequency of pneumonia and death were higher in the diazepam group than in the placebo group: 35 and 10%, 22 and 12%, respectively.

CONCLUSIONS

Although point estimates favored diazepam treatment in various analyses, our data did not confirm our primary hypothesis. Diazepam treatment seems beneficial in cardioembolic infarct patients, is safe in acute ischemic stroke, but may better be avoided in intracerebral hemorrhage.

Presynaptic GABAA receptors facilitate spontaneous glutamate release from presynaptic terminals on mechanically dissociated rat CA3 pyramidal neurons

Mossy fiber-derived giant spontaneous miniature excitatory postsynaptic currents have been suggested to be large enough to generate action potentials in postsynaptic CA3 pyramidal neurons. Here we report on the functional roles of presynaptic GABA(A) receptors on excitatory terminals in contributing to spontaneous glutamatergic transmission to CA3 neurons. In mechanically dissociated rat hippocampal CA3 neurons with adherent presynaptic nerve terminals, spontaneous excitatory postsynaptic currents were recorded using conventional whole-cell patch clamp recordings. In most recordings, unusually large spontaneous excitatory postsynaptic currents up to 500 pA were observed. These large spontaneous excitatory postsynaptic currents were highly sensitive to group II metabotropic glutamate receptor activation, and were still observed even after the blockade of voltage-dependent Na(+) or Ca(2+) channels. Exogenously applied muscimol (0.1-3 microM) significantly increased the frequency of spontaneous excitatory postsynaptic currents including the large ones. This facilitatory effect of muscimol was completely inhibited in the presence of 10 microM 6-imino-3-(4-methoxyphenyl)-1(6H)-pyridazinebutanoic acid HBr, a specific GABA(A) receptor antagonist. Pharmacological data suggest that activation of presynaptic GABA(A) receptors directly depolarizes glutamatergic terminals resulting in the facilitation of spontaneous glutamate release. In the current-clamp condition, a subset of large spontaneous excitatory postsynaptic potentials triggered action potentials, and muscimol greatly increased the frequency of spontaneous excitatory postsynaptic potential-triggered action potentials in postsynaptic CA3 pyramidal neurons. The results suggest that presynaptic GABA(A) receptors on glutamatergic terminals play an important role in the excitability of CA3 neurons as well as in the presynaptic modulation of glutamatergic transmission onto hippocampal CA3 neurons.

Feed-forward facilitation of glutamate release by presynaptic GABA(A) receptors

Disynaptic GABAergic inputs from Schaffer collateral (SC) afferents on to the soma of glutamatergic CA1 pyramidal neurons are involved in feed-forward inhibition in the hippocampal neural circuits. Here we report the functional roles of presynaptic GABA(A) receptors on SC afferents projecting to CA1 pyramidal neurons. Muscimol (0.5 microM), a selective GABA(A) receptor agonist, increased SC-evoked EPSC amplitude and decreased paired-pulse ratio in the slice preparation, in addition, it facilitated spontaneous glutamate release on to mechanically dissociated CA1 pyramidal neurons in an external Ca2+-dependent manner. In field recordings, muscimol at low concentrations (< or = 0.5 microM) increased not only the excitability of SC afferents but glutamate release, however, it at high concentrations (> or = 1 microM) changed bidirectionally. These results suggest that the moderate activation of presynaptic GABA(A) receptors depolarizes SC afferents and enhances SC-mediated glutamatergic transmission. When endogenous GABA was disynaptically released by brief trains of stimulation of SC afferents, the axonal excitability in addition to glutamate release was increased. The effects of endogenous GABA on the excitability of SC afferents were blocked by either SR95531 or AMPA receptor blockers, which would be expected to block disynaptic feed-forward neural circuits. The present results provide a novel form of presynaptic modulation (feed-forward facilitation) of glutamatergic transmission by presynaptic GABA(A) receptors within the intrinsic hippocampal neural circuits.

Diazepam reduces brain lesion size in a photothrombotic model of focal ischemia in rats

The neuroprotective effect of diazepam has been demonstrated in global ischemia models in vivo and in vitro [Neuroscience (2000) 471]. We studied the effect of diazepam on lesion volume in a photothrombotic model of focal brain ischemia in the rat, and the relation of such effect to time of drug administration. For this purpose we induced photochemically a focal brain lesion, and added diazepam 10 mg/kg intraperitoneally just before, at 1 and 4 h after lesion induction. After 24 h the rats were decapitated, and lesion volumes of 27 diazepam-treated rats were compared with that of 12 controls. Treated animals had a significant smaller lesion volume than controls, except those who received diazepam before induction of the lesion. We conclude that diazepam is neuroprotective in focal brain ischemia even when administered up to 4 h after ischemia onset.

Asymmetric cross-inhibition between GABAA and glycine receptors in rat spinal dorsal horn neurons

Presynaptic nerve terminals of inhibitory synapses in the dorsal horn of the spinal cord and brain stem can release both GABA and glycine, leading to coactivation of postsynaptic GABAA and glycine receptors. In the present study we have analyzed functional interactions between GABAA and glycine receptors in acutely dissociated neurons from rat sacral dorsal commissural nucleus. Although the application of GABA and glycine activates pharmacologically distinct receptors, the current induced by a simultaneous application of these two transmitters was less than the sum of currents induced by applying two transmitters separately. Sequential application of glycine and GABA revealed that the GABA-evoked current is more affected by glycine than glycine-evoked responses by GABA. Activation of glycine receptors decreased the amplitude and accelerated the rate of desensitization of GABA-induced currents. This asymmetric cross-inhibition is reversible, dependent on the agonist concentration applied, but independent of both membrane potential and intracellular calcium concentration or changes in the chloride equilibrium potential. During sequential applications, the asymmetric cross-inhibition was prevented by selective GABAA or glycine receptor antagonists, suggesting that occupation of binding sites did not suffice to induce glycine and GABAA receptors functional interaction, and receptor channel activation is required. Furthermore, inhibition of phosphatase 2B, but not phosphatase 1 or 2A, prevented GABAA receptor inhibition by glycine receptor activation, whereas inhibition of phosphorylation pathways rendered cross-talk irreversible. Taken together, our results demonstrated that there is an asymmetric cross-inhibition between glycine and GABAA receptors and that a selective modulation of the state of phosphorylation of GABAA receptor and/or mediator proteins underlies the asymmetry in the cross-inhibition.

Role of GABA A and GABA B receptors in GABA-induced inhibition of rat red nucleus neurons

We investigated GABA receptor subtypes mediating GABA-induced inhibition of red nucleus (RN) neuronal firing recorded extracellularly from anaesthetized rats. GABA response was mimicked by the GABA(A) agonists muscimol and isoguvacine in all cases and was partially blocked by the GABA(A) antagonist bicuculline. The GABA(B) agonist baclofen induced a long-lasting inhibition in 84% of cells. Neurons responding to either GABA(A) or GABA(B) agonists were equally distributed within the RN. The GABA(C) receptor agonist cis-amino-crotonic acid (CACA) did not modify RN neuronal firing; at high doses CACA occasionally induced inhibition abolished by bicuculline and thus mediated by GABA(A) receptors. We conclude that the inhibitory effects of GABA in the RN are mediated by both GABA(A) and GABA(B) receptors, whereas GABA(C) receptors are not involved.

Slow phases of GABA(A) receptor desensitization: structural determinants and possible relevance for synaptic function

GABA(A) receptor fast desensitization is thought to shape the time course of individual IPSCs. Although GABA(A) receptors also exhibit slower phases of desensitization, the possible role of slow desensitization in modifying synaptic function is poorly understood. In transiently transfected human embryonic kidney (HEK293T) cells, rat alpha1beta3delta and alpha1beta3gamma2L GABA(A) receptors showed distinct desensitization patterns during long (28 s) concentration jumps using a saturating (1 mM) GABA concentration. alpha1beta3gamma2L receptors desensitized extensively (approximately 90%), with four phases (tau(1) approximately 20 ms, tau(2) approximately 400 ms, tau(3) approximately 2 s, tau(4) approximately 10 s), while alpha1beta3delta receptors desensitized slowly and less extensively (approximately 35 %), with one or two slow phases with time constants similar to tau(3) and tau(4) of alpha1beta3gamma2L receptors. To determine the structural basis of subunit-specific desensitization, delta-gamma2L chimera subunits were expressed with alpha1 and beta3 subunits. Replacing the entire N-terminus of the gamma2L subunit with delta subunit sequence did not alter the number of phases or the extent of desensitization. Although extension of delta subunit sequence into transmembrane domain 1 (TM1) abolished the fast and intermediate components of desensitization, the two slow phases still accounted for substantial current loss (approximately 65 %). However, when delta subunit sequence was extended through TM2, the extent of desensitization was significantly decreased and indistinguishable from that of alpha1beta3delta receptors. The importance of TM2 sequence was confirmed by introducing gamma2 subunit TM2 residues into the delta subunit, which significantly increased the extent of desensitization, without introducing either the fast or intermediate desensitization phases. However, introducing delta subunit TM2 sequence into the gamma2L subunit had minimal effect on the rates or extent of desensitization. The results suggest that distinct delta subunit structures are responsible for its unique desensitization properties: lack of fast and intermediate desensitization and small contribution of the slow phases of desensitization. Finally, to investigate the possible role of slow desensitization in synaptic function, we used a pulse train protocol. We observed inhibition of peak current amplitude that depended on the frequency and duration of GABA pulses for receptors exhibiting extensive desensitization, whether fast phases were present or not. The minimally desensitizing alpha1beta3delta receptor exhibited negligible inhibition during pulse trains. Because receptors that desensitized without the fast and intermediate phases showed pulse train inhibition, we concluded that receptors can accumulate in slowly equilibrating desensitized states during repetitive receptor activation. These results may indicate a previously unrecognized role for the slow phases of desensitization for synaptic function under conditions of repeated GABA(A) receptor activation.

An electrophysiological study of muscarinic and nicotinic receptors of rat paratracheal ganglion neurons and their inhibition by Z-338

1. To study the mechanisms involved in the action of Z-338, a newly synthesized gastroprokinetic agent, experiments were performed with the paratracheal ganglion cells acutely dissociated from 2-week-old Wistar rats. The effects of Z-338 on both nicotinic and muscarinic responses of the ganglion cells were studied by nystatin perforated patch recording configuration under the current- and voltage-clamp conditions. 2. Acetylcholine (ACh) or nicotine, and muscarine or oxotremorine-M (OX-M) induced membrane depolarization with rapid and slow time courses respectively, followed by repetitive generation of action potentials in the ganglion cell. Corresponding to the membrane depolarization induced by cholinergic agents, ACh induced biphasic inward currents with rapid and slow time courses under the voltage-clamp condition. Nicotine and muscarine or OX-M evoked inward currents with rapid and slow time courses, respectively. The rapid and slow inward currents were accompanied by increase and decrease in the membrane conductance, respectively. In addition, OX-M dose-dependently suppressed the M-type K(+) current evoked in response to hyperpolarizing voltage-steps from V(H) of -25 mV to -50 mV, indicating that the activation of muscarinic acetylcholine receptors inhibits M-type K(+) current, thus inducing inward current in the ganglion cell. 3. Z-338 competitively suppressed the inward currents induced by OX-M through M(1) ACh receptor, and uncompetitively suppressed the currents induced by nicotine. 4. The inhibitory actions of Z-338 on the membrane depolarization and corresponding inward currents mediated by M(1)-muscarinic and neuronal nicotinic ACh receptors in the isolated ganglion cells were discussed in relation to the inhibitory actions on autoreceptors in the parasympathetic nerve terminals, which would explain the gastroprokinetic actions of Z-338.

GABA-induced current and circadian regulation of chloride in neurones of the rat suprachiasmatic nucleus

1. We have shown previously that GABA, the main neurotransmitter in the suprachiasmatic nucleus (SCN), has dual effects on SCN neurones, excitatory during the day and inhibitory at night. This duality has been attributed to changes in [Cl(-)](i) during the circadian cycle. To unravel the processes underlying these changes we investigated the biophysical properties of the GABAergic receptors and the regulation of [Cl(-)](i) in SCN neurones. 2. We used voltage-clamp methodology in conjunction with local application of GABA to characterise the current induced by GABA in SCN neurones within acute brain slices. This current, mediated via GABA(A) receptors, shows moderate voltage dependence, does not desensitise and can significantly alter [Cl(-)](i). 3. Loading or depletion of intracellular Cl(-) was induced by a train of GABA pulses. The recovery of intracellular Cl(-) was deduced from the change in [Cl(-)](i) calculated from the response to a test GABA pulse presented at different intervals after the conditioning train of GABA application. The time course of recovery was described by an exponential curve. Recovery following Cl(-) depletion was slower than recovery from Cl(-) loading and was further delayed during the subjective night. 4. We concluded that: (a) SCN neurones express a large number of somatic GABA(A) receptors, which give rise to a modifiable, tonic Cl(-) conductance that modulates cell excitability; (b) two Cl(-) transport mechanisms operate in SCN neurones, one that replenishes the cell with Cl(-) following Cl(-) depletion and another that removes Cl(-) after Cl(-) loading; (c) the efficiency of the replenishing mechanism is reduced during the subjective night; and (d) this reduction explains a lower [Cl(-)](i) during the night phase of the circadian cycle.

Bradykinin activates airway parasympathetic ganglion neurons by inhibiting M-currents

The action of bradykinin on neurons acutely isolated from airway parasympathetic ganglia of rats and its mechanism were investigated using the nystatin-perforated patch-clamp recording technique. Under current clamp conditions, an application of 0.1 microM bradykinin onto rat airway ganglion neurons induced a depolarization which was accompanied by the action potential firing. Bradykinin elicited inward currents with decreasing the membrane conductance when a ganglion neuron was held at a holding potential of -40 mV. The half-maximum effective concentration was 8.9 nM. The bradykinin response was mimicked by a B(2) receptor agonist, [Hyp(3)]-bradykinin, and was inhibited by HOE-140, a B(2) antagonist, suggesting the contribution of B(2) receptors. The bradykinin-induced inward current reversed at the K(+) equilibrium potential, which shifted 56.5 mV with a 10-fold change in extracellular K(+) concentration. The application of 10(-3) M Ba(2+) induced the inward current, and bradykinin failed to evoke a further inward current in the presence of Ba(2+). Bradykinin also reduced the amplitude of M-current deactivation induced by a hyperpolarizing step from a holding potential of -25 mV to -50 mV with a half-maximum effective concentration of 16 nM. Pretreatment with pertussis toxin had no effect on the bradykinin-induced inhibition of the M-current. From these results we suggest that bradykinin may be able to depolarize the airway parasympathetic ganglion neurons of rats associated with an inhibition of M-type K(+) channels through the B(2) type of bradykinin receptors.

Nitrooxy alkyl apovincaminate activates K+ currents in rat neocortical neurons

The effects of nitrooxy alkyl apovincaminate VA-045 ((+)-eburunamenine-14-carboxylic acid(2-nitroxy-ethyl ester), VA) were investigated in acutely dissociated rat neocortical neurons by using a nystatin-perforated patch recording configuration. VA activated a steady-state outward current in a concentration-dependent manner, with an EC50 of 0.65 microM. The reversal potential for the current shifted 56.5 mV with tenfold changes in the extracellular K+ concentration, suggesting that the current was carried by K+. The VA-induced current was not suppressed by apamin (1 microM), charybdotoxin (1 microM), Cs+ (3 mM), Ba2+ (3 mM), 4-aminopyridine (10 mM) or glibenclamide (10 microM), whereas tetraethylammonium suppressed the current with an IC50 of 1.4 mM. These pharmacological properties of the VA-induced current were compatible with a slowly inactivating delayed rectifier current (I(K)). It was suggested that the current activated by VA was I(K). The VA-induced current was not affected by Ca2+ depletion or by staurosporine (0.1 microM), quinacrine (10 microM), wortmanin (1 microM) or genistein (1 microM). The intracellular perfusion of GDPbetaS (0.4 mM) also had no significant effect. Thus, VA may directly activate the K+ channels.

A genetically encoded ratiometric indicator for chloride: capturing chloride transients in cultured hippocampal neurons

We constructed a novel optical indicator for chloride ions by fusing the chloride-sensitive yellow fluorescent protein with the chloride-insensitive cyan fluorescent protein. The ratio of FRET-dependent emission of these fluorophores varied in proportion to the concentration of Cl and was used to measure intracellular chloride concentration ([Cl-]i) in cultured hippocampal neurons. [Cl-]i decreased during neuronal development, consistent with the shift from excitation to inhibition during maturation of GABAergic synapses. Focal activation of GABAA receptors caused large changes in [Cl-]i that could underlie use-dependent depression of GABA-dependent synaptic transmission. GABA-induced changes in somatic [Cl-]i spread into dendrites, suggesting that [Cl-]i can signal the location of synaptic activity. This genetically encoded indicator will permit new approaches ranging from high-throughput drug screening to direct recordings of synaptic Cl- signals in vivo.

Structural domains of the human GABAA receptor 3 subunit involved in the actions of pentobarbital

This study was conducted to search for the residues of the beta3 subunit which affect pentobarbital action on the gamma-aminobutyric acid type A (GABAA) receptor. Three chimeras were constructed by joining the GABAA receptor beta3 subunit to the rho1 subunit. For each chimera, the N-terminal sequence was derived from the beta3 subunit and the C-terminal sequence from the rho1 subunit, with junctions located between the membrane-spanning regions M2 and M3, in the middle of M2, or in M1, respectively. In receptors obtained by the coexpression of alpha1 with the chimeric subunits, in contrast with those obtained by the coexpression of alpha1 and beta3, pentobarbital exhibited lower potentiation of GABA-evoked responses, and in the direct gating of Cl- currents, an increase in the EC50 together with a marked decrease in the relative maximal efficacy compared with that of GABA. Estimates of the channel opening probability through variance analysis and single-channel recordings of one chimeric subunit showed that the reduced relative efficacy for gating largely resulted from an increase in gating by GABA, with little change in efficacy of pentobarbital. A fit of the time course of the response by the predictions of a class of reaction schemes is consistent with the conclusion that the change in the concentration dependence of activation by pentobarbital is due to a change in pentobarbital affinity for the receptor. Therefore, the data suggest that residues of the beta3 subunit involved in pentobarbital binding to GABAA receptors are located downstream from the middle of the M2 region.

Contribution of changes in the chloride driving force to the fading of I(GABA) in frog melanotrophs

Chloride redistribution during type A gamma-aminobutyric acid (GABA(A)) currents (I(GABA)) has been investigated in cultured frog pituitary melanotrophs with imposed intracellular chloride concentration ([Cl(-)](i)) in the whole cell configuration or with unaltered [Cl(-)](i) using the gramicidin-perforated patch approach. Prolonged GABA exposures elicited reproducible decaying currents. The decay of I(GABA) was associated with both a transient fall of conductance (g(GABA)) and shift of current reversal potential (E(GABA)). The shift of E(GABA) appeared to be time and driving force dependent. In the gramicidin-perforated patch configuration, repeated GABA exposures induced currents that gradually vanished. The fading of I(GABA) was due to persistent shifts of E(GABA) as a result of g(GABA) recovering from one GABA application to another. In cells alternatively clamped at potentials closely flanking resting potential and submitted to a train of brief GABA pulses, a reversal of I(GABA) was observed after 150 s recording. It is demonstrated that, in intact frog melanotrophs, shifts of E(GABA) combine with genuine receptor desensitization to depress I(GABA). These findings strongly suggest that shifts of E(GABA) may act as a negative feedback, reducing the bioelectrical and secretory responses induced by an intense release of GABA in vivo.

Shift from depolarizing to hyperpolarizing glycine action in rat auditory neurones is due to age-dependent Cl- regulation

1. The inhibitory neurotransmitter glycine can elicit depolarizing responses in immature neurones. We investigated the changes in glycine responses and their ionic mechanism in developing neurones of the rat lateral superior olive (LSO), an auditory brainstem nucleus involved in sound localization. 2. Whole-cell and gramicidin perforated-patch recordings were performed from visually identified LSO neurones in brain slices and glycine was pressure applied for 3-100 ms to the soma. Glycine-evoked currents were reversibly blocked by strychnine. They were mostly monophasic, but biphasic responses occurred in approximately 30 % of P8-11 neurones in perforated-patch recordings. 3. In whole-cell recordings from P2-11 neurones, the reversal potential of glycine-evoked currents (EGly) was determined by the transmembranous Cl- gradient and corresponded closely to the Nernst potential for Cl-, regardless of age. This indicates that Cl- is the principle ion permeating glycine receptors, but is also consistent with a low relative (10-20 %) permeability for HCO3-. The Cl- gradient also determined the polarity and amplitude of glycine-evoked membrane potential changes. 4. Leaving the native intracellular [Cl-] undisturbed with gramicidin perforated-patch recordings, we found a highly significant, age-dependent change of EGly from -46.8 +/- 1.8 mV (P1-4, n = 28) to -67.6 +/- 3.3 mV (P5-8, n = 10) to -82.2 +/- 4.1 mV (P9-11, n = 18). The majority of P1-4 neurones were depolarized by glycine ( approximately 80 %) and spikes were evoked in approximately 30 %. In contrast, P9-11 neurones were hyperpolarized. 5. In perforated-patch recordings, EGly was influenced by the voltage protocol and the glycine application interval; it could be shifted in the positive and negative direction. For a given application interval, these shifts were always larger in P1-4 than in P8-11 neurones, pointing to less effective Cl- regulation mechanisms in younger neurones. 6. Furosemide (frusemide), a blocker of cation-Cl- cotransporters, reversibly shifted EGly in the negative direction in P2-4 neurones, yet in the positive direction in P8-10 neurones, suggesting the blockade of net inward and net outward Cl- transporters, respectively. 7. Taken together, age-dependent changes in active Cl- regulation are likely to cause the developmental shift from depolarizing to hyperpolarizing glycine responses. A high intracellular [Cl-] is generated in neonatal LSO neurones which decreases during maturation.

Excitatory effect of Cd2+ on cat adrenal chromaffin cells

To understand the mechanism(s) underlying the Cd2+- and Co2+-induced increases in the cytosolic free Ca2+ concentration ([Ca]i) in cat adrenal chromaffin cells, we used nystatin-perforated patch recording method and fura-2 microfluorometry. Under the current-clamp conditions, the external application of 5x10(-7) M Cd2+ slowly depolarized the cells resulting in the bursting of action potentials. Under the voltage-clamp conditions, Cd2+ evoked a slow inward current accompanied by a decrease of K+ conductance at a holding potential of -40 mV, and Co2+ mimicked Cd2+ action. In some cells (16%), Cd2+ evoked an additional rapid transient outward current associated with an increased K+ conductance and a successive slow inward current. The Cd2+-induced inward current was activated in a concentration-dependent manner with a half-maximum concentration of 9.3x10(-8) M. The Cd2+- and Co2+-induced [Ca]i increases measured with fura-2 microfluorometry were maximal at 10(-6) and 10(-5) M, respectively, and the higher concentrations of both cations caused the smaller responses. Additional transient increase in [Ca]i was often evoked upon the removal of relatively higher concentrations of these metals. It was concluded that the Cd2+-induced membrane depolarization due to the decrease in K+ conductances evoked the bursting firings resulting in the increase in [Ca]i, and consequently might stimulate the catecholamine secretion.

Multiple modulatory effects of the neuroactive steroid pregnanolone on GABAA receptor in frog pituitary melanotrophs

1. The effects of the neuroactive steroid pregnanolone (5 beta-pregnan-3 alpha-ol-20-one) on the electrical response to GABA were investigated in cultured frog pituitary melanotrophs using the patch-clamp technique. 2. Low concentrations of pregnanolone (0.01-1 microM) in the extracellular solution enhanced the current evoked by submaximal concentrations of GABAA receptor agonists and prolonged the GABA-induced inhibition of the spontaneous action potentials in a dose-dependent manner. 3. Pregnanolone augmented the opening probability of the single GABA-activated channels but did not modify the conductance levels. 4. Pregnanolone (1 microM) shifted the GABA dose-response curve towards the low GABA concentrations, reducing the EC50 from 4.2 to 1.8 microM. 5. Internal cell dialysis with pregnanolone (1 or 10 microM) did not alter the GABA-evoked current. 6. Pregnanolone accelerated the desensitization of both the current and conductance increases caused by GABA. 7. High concentrations of pregnanolone (30 microM) markedly and reversibly diminished the current evoked by 10 microM GABA. 8. At high concentrations (10-30 microM), pregnanolone induced an outward current which reversed at the chloride equilibrium potential. 9. It is concluded that, in frog pituitary melanotrophs, pregnanolone exerts a dual inverse modulation and a direct activation of the GABAA receptor-channel depending on the concentrations of both GABA and steroid. Pregnanolone acts on an extracellular site on the GABAA receptor inducing conformational changes of the receptor-channel complex, resulting in a desensitized less-conducting state.

Upregulation of GABAA current by astrocytes in cultured embryonic rat hippocampal neurons

Embryonic rat hippocampal neurons were cultured on poly-D-lysine (PDL) or a monolayer of postnatal cortical astrocytes to reveal putative changes in neuronal physiology that involve astrocyte-derived signals during the first 4 d of culture, GABA-induced Cl- current (IGABA) was quantified using outside-out and whole-cell patch-clamp recordings beginning at 30 min, when cells had become adherent. The amplitude and density (current normalized to membrane capacitance) of IGABA in neurons grown on astrocytes became statistically greater than that recorded in neurons grown on PDL after 2 hr in culture (HIC). Although the current density remained unchanged in neurons on astrocytes, that in neurons on PDL decreased and became statistically lower beginning after 2 HIC. The differences in amplitude and density of IGABA in the two groups of neurons were maintained during the 4 d experiment. The upregulation effect of astrocytes on neuronal IGABA required intimate contact between the neuronal cell body and underlying astrocytes. Suppression of spontaneous Cac2+ elevations in astrocytes by bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid that was loaded intracellularly decreased their modulatory effects on IGABA. IGABA in all cells was blocked completely by bicuculline and exhibited virtually identical affinity constants, Hill coefficients, and potentiation by diazepam in the two groups. Outside-out patch recordings revealed identical unitary properties of IGABA in the two groups. More channels per unit of membrane area could explain the astrocyte enhancement of IGABA. The results reveal that cortical astrocytes potentiate IGABA in hippocampal neurons in a contact-dependent manner via a mechanism involving astrocyte Cac2+ elevation.

Upregulation of GABAA current by astrocytes in cultured embryonic rat hippocampal neurons

Embryonic rat hippocampal neurons were cultured on poly-D-lysine (PDL) or a monolayer of postnatal cortical astrocytes to reveal putative changes in neuronal physiology that involve astrocyte-derived signals during the first 4 d of culture, GABA-induced Cl- current (IGABA) was quantified using outside-out and whole-cell patch-clamp recordings beginning at 30 min, when cells had become adherent. The amplitude and density (current normalized to membrane capacitance) of IGABA in neurons grown on astrocytes became statistically greater than that recorded in neurons grown on PDL after 2 hr in culture (HIC). Although the current density remained unchanged in neurons on astrocytes, that in neurons on PDL decreased and became statistically lower beginning after 2 HIC. The differences in amplitude and density of IGABA in the two groups of neurons were maintained during the 4 d experiment. The upregulation effect of astrocytes on neuronal IGABA required intimate contact between the neuronal cell body and underlying astrocytes. Suppression of spontaneous Cac2+ elevations in astrocytes by bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid that was loaded intracellularly decreased their modulatory effects on IGABA. IGABA in all cells was blocked completely by bicuculline and exhibited virtually identical affinity constants, Hill coefficients, and potentiation by diazepam in the two groups. Outside-out patch recordings revealed identical unitary properties of IGABA in the two groups. More channels per unit of membrane area could explain the astrocyte enhancement of IGABA. The results reveal that cortical astrocytes potentiate IGABA in hippocampal neurons in a contact-dependent manner via a mechanism involving astrocyte Cac2+ elevation.

Rapid desensitization of alpha 1 beta 1 GABA A receptors expressed in Sf9 cells under optimized conditions

alpha 1 and beta 1 subunits of human GABA A receptors were expressed in Sf9 cells using the Sf9-baculovirus system. Better expression was obtained by manipulating the system. Cell growth phase at the time of infection determined the practical range of virus titre, the period postinfection during which cells were useful for signal detection and the maximal current obtained. Cells in the early exponential phase were relatively insensitive to multiplicity of infection (MOI) whereas cells in the mid- to late-exponential phase were highly dependent on MOI and they responded with the largest Cl- current generated by GABA. Channels activated by GABA were chloride-selective. Half the maximum peak whole-cell current was obtained with 11 microM GABA. The time course of Cl- currents activated by saturating GABA concentrations in cells infected with alpha 1 beta 1-recombinant viruses was examined employing a rapid perfusion system which allowed whole-cell solution exchange in less than 1 msec. The current decay could be fitted by 3 to 4 exponentials for the first 8 sec. The initial fast current decrease had a time constant of about 23 msec. No voltage dependence of time constants was detected but the whole-cell IV relation showed outward rectification. Currents were depressed by bicuculline, penicillin and picrotoxin and potentiated by pentobarbitone.

Neonatal rat cerebellar granule and Purkinje neurons in culture express different GABAA receptors

We have established a culture system for microexplants of rat cerebellar cortical tissue in which cells develop morphologically, express type-A receptors for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and form GABAergic synaptic connections. Criteria of cell size and shape allow reliable identification of granule and Purkinje neurons, criteria confirmed by studies of the binding of antibodies to calbindin D28K and GABA. Both granule and Purkinje neurons express GABAA receptors, but granule neurons fall into two classes in terms of their sensitivity. Granule neurons which do not show spontaneous synaptic currents are relatively insensitive to GABA, while granule neurons with synaptic currents are much more sensitive. The responses of Purkinje neurons to application of 1 microM GABA are relatively insensitive to Zn2+ ion (10 microM), and are potentiated by chlordiazepoxide (100 microM) and La3+ ions (100 microM). Responses of innervated granule neurons, on the other hand, are blocked more strongly by Zn2+ ions, are less affected by chlordiazepoxide and are equally potentiated by La3+ ions. Hence these cultures provide a source of identifiable, functionally innervated cells which express distinct types of GABAA receptors.

Mutation of an arginine residue in the human glycine receptor transforms beta-alanine and taurine from agonists into competitive antagonists

Agonist binding to the inhibitory glycine receptor (GlyR) initiates the opening of a chloride-selective channel that modulates the neuronal membrane potential. Point mutations of the GlyR, substituting Arg-271 with either Leu or Gln, have been shown to underlie the inherited neurological disorder startle disease (hyperekplexia). We show that these substitutions result in the redistribution of GlyR single-channel conductances to lower conductance levels. Additionally, the binding of the glycinergic agonists beta-alanine and taurine to mutated GlyRs does not initiate a chloride current, but instead competitively antagonizes currents activated by glycine. These findings are consistent with mutations of Arg-271 resulting in the uncoupling of the agonist binding process from the channel activation mechanism of the receptor.

Regulation of K+ conductance by histamine H1 and H2 receptors in neurones dissociated from rat neostriatum

1. The effects of histamine on dissociated neostriatal neurones of the rat were investigated in the whole-cell mode using the nystatin-perforated patch recording technique. 2. Histamine evoked a net inward current accompanied by a decrease in the membrane conductance at a holding potential (Vh) of -44 mV. This response was observed in neurones considered to be interneurones based on morphology, membrane properties and the responsiveness to acetylcholine. 3. A net inward current evoked by 10(-8) to 10(-6) M histamine was inhibited in a concentration-dependent manner by the H1 receptor antagonists, pyrilamine and triprolidine. The H1 receptor agonists, 2-methylhistamine and 2-thiazolylethylamine, mimicked the histamine response, indicating that this response was mediated by the H1 receptor. 4. Histamine, at high concentrations between 10(-6) and 10(-5) M, evoked an additional net inward current with a decrease in the membrane conductance, which was inhibited by the H2 receptor antagonists, cimetidine, ranitidine and famotidine. The H2 receptor agonist, impromidine, partially mimicked the response. Thus, this additional current was considered to be mediated by the H2 receptor. 5. The reversal potentials for H1 and H2 receptor-operated currents shifted 56.9 and 59.3 mV for a 10-fold change in [K+]o, respectively, suggesting that these currents were carried by K+. 6. An analysis of change in current fluctuations mediated by H1 and H2 receptors suggested that the unitary current amplitudes of K+ channels linked to H1 and H2 receptors were 0.29 +/- 0.06 (n = 4) and 0.27 +/- 0.07 pA (n = 4), respectively. There was no significant difference between these values. The estimated mean life times (tau) for both channels were also identical (1.1 ms). 7. It was concluded that histamine reduces K+ currents in neostriatal interneurones and that both H1 and H2 receptors are involved in the response.

Sevoflurane-induced ionic current in acutely dissociated CA1 pyramidal neurons of the rat hippocampus

The electrophysiological properties of sevoflurane (Sev)-induced current (ISev) were investigated in CA1 pyramidal neurons freshly dissociated from the rat hippocampus by using the nystatin perforated patch recording configuration under voltage-clamp condition. Within the range of Sev concentrations from 3 x 10(-4) to 2 x 10(-3) M, ISev was an inward current which consisted of an initial transient peak component and a successive steady-state plateau component. The peak current component increased in a concentration-dependent manner with a conductance increase. The application of Sev over 2 x 10(-3) M, however, suppressed the peak and steady-state current components with a concomitant decrease in conductance and elicited a transient inward current ('hump' current) immediately after wash out. The current-voltage relationship for ISev showed some outward rectification suggesting a slight voltage-dependency of the ISev. The reversal potential of ISev (ESev) was close to the ECl and shifted by 52 mV for a 10-fold change in extracellular Cl- concentrations, indicating that ISev is passing through Cl- channels. The single channel conductance obtained from the analysis of the variance of ISev fluctuations was 15.3 +/- 1.3 pS.

Desensitization and noncompetitive blockade of GABAA receptors in ventral midbrain neurons by a neurosteroid dehydroepiandrosterone sulfate

Dehydroepiandrosterone sulfate (DHEAS) blocked the GABAA receptor noncompetitively in neurons grown in primary culture from the ventral midbrains of fetal rats. The apparent dissociation constant for this blockade was 4.5 microM, and one molecule of DHEAS was sufficient to block the receptor. The affinity of the blocked receptor for GABA was diminished by about one half. The findings that the DHEAS caused no rectification of chloride currents and that it did not shorten the durations of open ion channels indicated that DHEAS did not act by occluding open ion channels. Neither did it diminish their conductance. DHEAS accelerated desensitization in at least one population of receptors, diminished the amplitudes of inhibitory postsynaptic currents, and shortened their decay time constants in a concentration dependent manner.

Functional heterogeneity of hippocampal GABAA receptors

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

Regulation of GABAA receptor in cerebellar granule cells in culture: differential involvement of kinase activities

GABAA receptor function was studied in rat cerebellar granule cells in culture, by the whole-cell patch-clamp approach. The data show that GABA activates Cl- currents in these neurons which reverse at the appropriate membrane potential and are blocked by picrotoxin. The GABA-activated currents desensitize with time of application of the neurotransmitter at concentrations > or = 10(-6) M. The dose-response curve for the peak Cl- current gives a Ka value of 2.3 microM with a Hill coefficient of 1.2. The peak Cl- current elicited by GABA decreases with time of cell registration, with a time-constant of 7.3 min. Residual responsiveness though is maintained thereafter. This "run-down" phenomenon can be completely prevented by adding adenosine-5'-triphosphate + Mg2+ in the pipette solution. Treatments which directly (8-bromoadenosine-3',5'-cyclic-monophosphate; adenosine-3', 5'-cyclic-monophosphate) or indirectly (forskolin, isobutylmethylxanthine) increase the adenosine-3',5'-cyclic-monophosphate intracellular content reduce the GABA-induced Cl- current. Conversely, treatment with the protein kinase A and C inhibitor 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine potentiates the effect of GABA. On the whole, the data indicate that different protein kinase activities modulate the functional state of the GABAA receptors on granule cells from the rat cerebellum.

GABA responses and their partial occlusion by glycine in cultured rat medullary neurons

Whole-cell current responses to bath application of GABA and glycine were studied in medullary neurons cultured from embryonic rats. Two current components were seen in the responses to bath application of GABA, one component which desensitized and another which did not. These two current components have different dose-response characteristics for GABA, with the nondesensitizing component being activated more effectively and reaching its peak amplitude at lower agonist concentrations than the desensitizing one. The agonist concentrations producing half of the maximum responses are 2.8 +/- 0.3 (+/- S.E.M., n = 9) and 14.7 +/- 2.7 (n = 5) microM for the nondesensitizing and desensitizing components, respectively. The two current components for GABA are differentially affected by the antagonists, picrotoxin and bicuculline. The antagonist concentrations which block 50% of the control desensitizing and nondesensitizing responses to GABA are 33 and 320 microM for picrotoxin, and 3 and 50 microM for bicuculline, respectively. Thus, the characteristics of the GABA responses are analogous to those described previously for glycine in that there are two components which are differentially sensitive to agonist concentration [Lewis et al. (1991) J. Neurophysiol, 40, 1178-1187]. We now find there is occlusion between the responses to GABA and glycine, indicating that they share a population of receptors or channels. The occlusion was incomplete (< 80%) in half of the cells, suggesting that both agonists also activate unique receptors. Furthermore, the current responses to 35 microM GABA are blocked by the glycinergic antagonist, strychnine, with half-maximal blocking concentrations equal to 2 and 30 microM for the desensitizing and nondesensitizing components, respectively. This strychnine sensitivity is less than that for the glycine receptor. At the same time, the current responses to 100 microM glycine are sensitive to the GABAergic antagonists, picrotoxin and bicuculline. The half-maximal blocking concentrations are 36 and 120 microM picrotoxin, and 120 and 500 microM bicuculline, for the desensitizing and nondesensitizing components of the glycine response, respectively. Consequently, these results suggest that these cultured cells have at least three types of inhibitory receptors: glycine receptors, GABA receptors and GABA/glycine receptors, with all three receptors sensitive to block by strychnine, bicuculline and picrotoxin. The GABA/glycine receptor may be an immature form of the inhibitory receptor. Alternatively, some GABA and glycine receptors may have common ionophores.

Enhancement of the excitatory actions of GABA by barbiturates and benzodiazepines

Whole cell recordings from hippocampal CA1 pyramidal neurons using electrode chloride concentrations of 12-80 mM demonstrated that the effect of synaptic activation of GABAA receptors was dependent on the transmembrane chloride gradient. When the chloride reversal potential was positive to action potential threshold, GABAA receptor activation was excitatory, and anticonvulsant barbiturates and benzodiazepines enhanced this excitation. Enhancement of GABAergic excitation of interneurons may contribute to the efficacy of these drugs, while enhancement of GABAergic excitation of principal neurons may be an important mechanism of failure, such as occurs in the treatment of neonatal seizures.

Cl(-)-mediated interaction between GABA and glycine currents in cultured rat hippocampal neurons

A cross-interaction between GABA- and glycine-evoked currents was found when the two transmitters were applied in sequence to cultured embryonic rat hippocampal neurons. Whole-cell GABA-current was inhibited by a previous glycine-current flowing in the same direction (inward or outward), and potentiated by a current with opposite polarity. The same effect was caused by GABA on glycine-current. Repeated applications of GABA (glycine) elicited currents of decreasing or increasing amplitude, according to a similar pattern. Transmitter interaction was independent of external Ca2+ and of all the metabolic pathways tested, but it was blocked by specific receptor antagonists, bicuculline and strychnine. The extent of both inhibition and potentiation correlated with the amount of charge flowing through the membrane during the conditioning transmitter application, indicating that cross-modulation depends on shifts of Cl- reversal potential. This finding has both functional and methodological implications, as it suggests a new mechanism of transmitter interaction in the brain, and also that patch-clamp pipettes cannot adequately perfuse cell interior.

Neurosteroid regulation of GABAA receptor single-channel kinetic properties of mouse spinal cord neurons in culture

1. Single-channel kinetics of steroid enhancement of single gamma-aminobutyric acidA (GABA) receptor currents obtained from somata of mouse spinal cord neurones in culture were investigated using the excised outside-out patch-clamp recording technique. GABA (2 microM) and GABA (2 microM) plus androsterone (5 alpha-androstan-3 alpha-ol-17-one, AND, 10 nM-10 microM) or pregnanolone (5 beta-pregnan-3 alpha-ol-20-one, PRE, 100 nM-10 microM) applied by pressure ejection from micropipettes evoked inward currents when patches were voltage clamped at -75 mV in symmetrical chloride solutions. Averaged GABA receptor currents were increased in the presence of the steroids. 2. GABA receptor currents were recorded with at least two conductance levels, a predominant or main-conductance level of about 28 pS (which contributed 96% of the current evoked) and a minor or sub-conductance level of about 20 pS. The current amplitudes of the two conductance levels were unchanged by the steroids. The gating (opening and closing) kinetics of both of the conductance levels were analysed. Findings for the main-conductance level are summarized below. 3. Both steroids increased the average GABA receptor channel open duration. Consistent with the increased GABA receptor channel average open duration, the steroids shifted frequency histograms of GABA receptor channel open durations to longer durations. Three exponential functions were required to fit best the frequency histograms of GABA open durations, consistent with at least three kinetic open states of the main-conductance level. Time constants obtained from the GABA receptor channel open-duration frequency histograms were unchanged in the presence of the steroids. The basis for the increased average GABA receptor channel open durations by the steroids was due to an increased relative proportion of the two longer open-duration time constants. The GABA receptor channel average open durations were increased by AND and PRE in a concentration-dependent manner by shifting the proportion of openings to the longer open time constants. At a concentration of 10 microM, the prolongation of the average open duration was decreased, suggesting that the GABA receptor channel was blocked by these steroids. 4. GABA receptor channel opening frequency was increased and average channel-closed duration was decreased by AND or PRE. Consistent with this, areas of the frequency histograms of channel closed durations were shifted to shorter durations. Closed frequency distributions were fitted best with five to six exponential functions, suggesting that the channel had multiple kinetic closed states. The three briefest time constants were not greatly altered by the steroids.(ABSTRACT TRUNCATED AT 400 WORDS)

The kinetics of the response to glutamate and kainate in neurons of the avian cochlear nucleus

Neurons in the nucleus magnocellularis (nMAG) of the chicken precisely transmit auditory nerve activity via glutamatergic synapses. Using techniques for rapid application of solutions, we have explored the properties of CNQX-sensitive glutamate receptors in whole cells and outside-out patches from the nMAG. Glutamate-evoked current in patches desensitized biphasically to less than 1% of the peak current, with a fast time constant of 960 microseconds at 22 degrees C, decreasing to 570 microseconds at 33 degrees C. Dose-response studies using kainate indicated that at least two agonist molecules bind to gate the channel. We propose a kinetic model that quantitatively describes our experimental observations. The rapid kinetics of this receptor are well suited to allow phase locking of synaptic signals to auditory stimuli.

Heterogeneity in EC50 and nH of GABAA receptors on dorsal root ganglion neurons freshly isolated from adult rats

GABA activates a Cl- current through the GABAA receptor/ionophore complex that influences excitability of neurons. Studies using expression of cloned cDNAs coding for different GABAA receptor/ionophore subunits suggest that the EC50 and Hill coefficient for GABA are influenced by subunit composition. However, no direct evidence for such heterogeneity has been reported for vertebrate neurons. I have investigated the heterogeneity of EC50 and Hill coefficients (nH) of isolated dorsal root ganglion neurons using the whole-cell patch clamp technique. The EC50 for GABA varied from 26 to 107 microM among neurons. nH calculated from the logistic equation varied from 1.18 to 2.0. A negative correlation was found between the EC50 and nH (r = -0.81). Both nH and EC50 differed between some cells. However, in some instances, nH differed between cells while EC50 values were similar, and in other cells, EC50 values differed and nH was similar. In addition, when cells were categorized according to action potential shape, the EC50 and Hill coefficients differed among cell types in some instances and were similar in other instances. These findings demonstrate that different pharmacological profiles for GABA can be observed in adult mammalian neurons. Selective distribution of such pharmacological subtypes of GABAA receptors may contribute to control of neuronal excitability.

Enhancement of gamma-aminobutyric acid-activated Cl- currents in cultured rat hippocampal neurones by three volatile anaesthetics

1. The effects of the volatile anaesthetics enflurane, halothane and isoflurane on gamma-aminobutyric acid (GABA)A receptor-mediated chloride currents were studied in cultured rat hippocampal neurones. Transient current responses were obtained by brief pressure application of GABA to the cell body of neurones under voltage clamp. 2. All three anaesthetics increased the peak amplitude and duration of current 2. All three anaesthetics increased the peak amplitude and duration of current responses to brief applications of GABA. These effects were fully reversible, and did not involve alterations in the reversal potential for GABA responses. 3. The experimental concentrations of anaesthetics were measured directly using gas chromatography. The enhancement of GABA currents increased with increasing anaesthetic concentration. Clinically effective concentrations of anaesthetics (between 1 and 1.5 times MAC (minimum alveolar concentration) produced significant enhancement of GABA currents. 4. These results demonstrate that the changes in the time course of synaptic inhibition reported in the presence of the volatile anaesthetics are likely to result from modification of the function of postsynaptic GABAA receptor-channel complexes. These findings also support the hypothesis that GABAA receptor complexes serve as common molecular target sites for a variety of structurally diverse anaesthetic molecules.

Direct modulation of GABAA receptor by intracellular ATP in dissociated nucleus tractus solitarii neurones of rat

1. Effect of intracellular ATP on Cl- current (ICl) mediated by the GABA (gamma-aminobutyric acid) receptor subtype, GABAA, was studied in dissociated nucleus tractus solitarii (NTS) neurones using the whole-cell mode of patch clamp. A concentration-jump technique termed 'Y tube' was used to rapidly apply agents externally. Dissociated neurones were obtained from 1- to 3-week-old rats. 2. When the patch-pipette solution contained 2 mM-ATP, the amplitude of ICl elicited by 10(-5) M-GABA did not show any time-dependent decrease (apparent run-down), for more than 60 min after the initial recording. In the presence of ATP, the half-maximum concentration (KD) and Hill coefficient calculated from the GABA concentration-response curve were 9.12 microM and 1.47, respectively. 3. In the absence of intracellular ATP, the amplitude of GABA-induced ICl decreased with time. The relative peak amplitudes after 20 and 60 min from the initial recording were 0.40 +/- 0.09 (n = 11) and 0.16 +/- 0.05 (n = 8) with respect to the initial response. 4. Removal of Mg2+ from the internal solution induced run-down of the GABA response even in the presence of 2 mM-intracellular ATP, suggesting that both intracellular ATP (2 mM or more) and Mg2+ are necessary to prevent run-down of the GABA response. 5. Activation of dephosphorylation processes by alkaline phosphatase (100-200 microM) did not affect the GABA response in neurones perfused with internal solution containing 2 mM-ATP and 3 mM-Mg2+. Blocking the dephosphorylation process by okadaic acid, a phosphatase inhibitor, did not prevent the run-down of the GABA response. 6. Calcium influxes passing through both the voltage-dependent L-type Ca2+ channel and the glutamate receptor-operated cation channel did not affect ICl induced by GABA. 7. GABA-induced ICl was also maintained by adding 2 mM-ADP or ATP gamma S (adenosine-5'-O-3-thiotriphosphate) to the internal solution containing Mg2+. Addition of 2 mM-adenosine, AMP, cyclic AMP, AMP-PNP (adenylimido-diphosphate) or ADP beta S (adenosine-5'-O-2-thiodiphosphate) to the internal solution did not prevent the run-down of the GABA response even in the presence of 3 mM-intracellular Mg2+. Based on the chemical specificity of these ATP analogues, it is suggested that there is an ATP-sensitive binding site (ATP receptor) in the cytoplasmic side of the cell membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycine response in isolated dorsal cochlear nucleus of C57BL/6J mouse

Pharmacological properties of glycine (Gly)-induced Cl- current (ICl) in the dorsal cochlear nucleus (DCN) neurons acutely dissociated from C57BL/6J mouse were investigated in the whole-cell configuration of the patch-clamp technique. Gly-induced ICl increased in a sigmoidal manner with higher Gly concentrations. Strychnine blocked the Gly response competitively at low and non-competitively at high concentrations. Both glutamate (Glu) and N-methyl-D-aspartate (NMDA) responses were augmented by adding 10(-6) M Gly, at which concentration Gly did not induce any ICl. This facilitation was not affected by strychnine. Our results clearly show the existence of strychnine-sensitive and -insensitive glycine receptors in the DCN neurons.

Kinetics of open channel block by penicillin of single GABAA receptor channels from mouse spinal cord neurones in culture

1. Reduction by penicillin of single gamma-aminobutyric acidA (GABAA) receptor currents from somata of mouse spinal cord neurones in culture was investigated using the excised outside-out patch-clamp recording technique. 2. GABA (2 microM) alone or with penicillin (100-5000 microM) applied by pressure ejection from micropipettes evoked inward currents when patches were voltage-clamped at -75 mV in symmetrical chloride solutions. Averaged GABA receptor currents were decreased in the presence of penicillin. 3. GABA receptor currents were recorded with at least two conductance states, a more frequent or main-conductance state of about 27 pS and a less frequent sub-conductance state of about 19-20 pS. The conductances of the two states were unchanged in the presence of penicillin. The kinetic properties of the main-conductance state were analysed and are summarized below. 4. Penicillin produced a concentration-dependent reduction of GABA receptor open properties by reduction of average GABA receptor channel open duration and an increase in channel opening frequency. 5. Penicillin shifted frequency histograms of GABA receptor channel open durations to shorter durations in a concentration-dependent manner. Three exponential functions were required to fit best the frequency histograms of open durations, suggesting that the channel had at least three open states. Penicillin produced a concentration-dependent reduction in the time constants obtained from the open duration frequency histograms. 6. Frequency histograms of GABA receptor channel closed durations could be fitted with five to seven exponential functions, suggesting that the channel had multiple closed states. In the presence of increased concentration of penicillin, there was a reduction in the relative frequency of brief gaps and the appearance of new closed time constants. 7. With increased penicillin concentration, GABA receptor channel burst frequency was unchanged, burst durations were increased, the number of openings per burst was increased and the per cent time open within a burst was decreased. 8. The results suggested that penicillin produced simple open channel blockade of the GABA receptor channel. However, the experimental results also suggested that the association with and, perhaps, the dissociation of the blocker from its binding site were dependent upon the kinetic state of the open channel. Penicillin had faster association and slower dissociation rates when the channel was in unstable, brief open kinetic state than when the channel was in a more stable, longer open kinetic state. Possible models for penicillin reduction of single GABA receptor currents were simulated by computer and analysed.(ABSTRACT TRUNCATED AT 400 WORDS)

Non-competitive inhibition of GABAA responses by a new class of quinolones and non-steroidal anti-inflammatories in dissociated frog sensory neurones

1. The interaction of a new class of quinolone antimicrobials (new quinolones) and non-steroidal anti-inflammatory agents (NSAIDs) with the GABAA receptor-Cl- channel complex was investigated in frog sensory neurones by use of the internal perfusion and 'concentration clamp' techniques. 2. The new quinolones and the NSAIDs (both 10(-6)-10(-5) M) had little effect on the GABA-induced chloride current (ICI) when applied separately. At a concentration of 10(-4) M the new quinolones, and to a lesser degree the NSAIDs, produced some suppression of the GABA response. 3. The co-administration of new quinolones and some NSAIDs (10(-6)-10(-14) M) resulted in a marked suppression of the GABA response. The size of this inhibition was dependent on the concentration of either the new quinolone or the NSAID tested. The inhibitory potency of new quinolones in combination with 4-biphenylacetic acid (BPAA) was in rank order norfloxacin (NFLX) much greater than enoxacin (ENX) greater than ciprofloxancin (CPFX) much greater than ofloxacin (OFLX), and that of NSAIDs in combination with ENX was BPAA much greater than indomethacin = ketoprofen greater than naproxen greater than ibuprofen greater than pranoprofen. Diclofenac, piroxicam and acetaminophen did not affect GABA responses in the presence of ENX. 4. In the presence of ENX or BPAA, there was a small shift to the right of the concentration-response curve for GABA without any effect on the maximum response. However, the co-administration of these drugs suppressed the maximum of the GABA concentration-response curve, indicating a non-competitive inhibition, for which no voltage-dependency was observed.5. Simultaneous administration of ENX and BPAA also suppressed pentobarbitone (PB)-gated Icl. On the other hand, both PB and phenobarbitone reversed the inhibition of GABA-induced Ic, by coadministration of ENX and BPAA.6. The effect on GABAA responses of co-administration of new quinolones and NSAIDs was not via an interaction with benzodiazepine receptors coupled to the GABAA receptor, since this effect was not reversed by Rol5-1788 or diazepam.7. It is concluded that the co-administration of new quinolones and some of the NSAIDs inhibit GABAergic transmission, and could result in convulsions.

Development of GABA-mediated, chloride-dependent inhibition in CA1 pyramidal neurones of immature rat hippocampal slices

1. gamma-Aminobutyric acid (GABA)-mediated, Cl(-)-dependent inhibitory postsynaptic potentials (IPSPs) and GABA currents in immature rat hippocampal CA1 neurones were studied using the whole-cell recording technique in brain slices. 2. IPSPs evoked by electrical stimulation were observed in postnatal 2- to 5- (PN2-5), 8- to 13-(PN8-13) and 15- to 20-(PN15-20)day-old CA1 neurones. In the presence of glutamate receptor blockers 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D-2-amino-5-phosphonovaleric acid (APV), the reversal potential for the IPSP (EIPSP) was near the resting membrane potential (RMP) in the PN2-5 neurones, but 13 and 25 mV more negative than the RMP in PN8-13 and PN15-20 neurones respectively. IPSPs and GABA currents were blocked by the GABAA-receptor antagonists bicuculline or picrotoxin. 3. The reversal potential for somatic GABA currents (EGABA) was examined in the presence of tetrodotoxin (TTX). There was a strong dependence of the EGABA upon the patch pipette [Cl-] ([Cl-]p). indicating that the GABA currents were mediated by a Cl- conductance. In PN2-5 neurones, EGABA agreed with the value predicted by the Goldman-Hodgkin-Katz equation at given concentrations of internal and external anions permeable through GABA-activated Cl- channels, whereas EGABA in older neurones was 8-18 mV more negative. 4. Examination of the relations between EGABA, holding potential, [Cl-]p and resting conductance indicated that the membrane of the PN2-5 neurones was readily permeable to Cl- which followed a passive Donnan equilibrium. Passive distribution of Cl- played a decreasing role in PN8-13 neurones and in PN15-20 neurones. 5. To assess the contribution of outward Cl- co-transport, bath applications of high K+ or furosemide were performed. High K+ and furosemide caused a reversible positive shift of EGABA in PN15-20 neurones. Raising the temperature moved EGABA to a more negative potential, with a Q10 of 5 mV. A similar change of EGABA in response to high K+, but not to furosemide, was found in PN8-13 neurones. 6. The present data indicate the existence of GABAA-mediated inhibitory synaptic connections in CA1 neurones at the earliest stages of postnatal life. During the first postnatal week, Cl- ions are passively distributed and the EIPSP and EGABA are near the RMP.(ABSTRACT TRUNCATED AT 400 WORDS)

GABAA and GABAB receptors and the ionic mechanisms mediating their effects on locus coeruleus neurons

Anatomical, neurochemical, and electrophysiological studies have provided evidence that gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in the locus coeruleus (LC) nucleus. We have used intracellular recording to study the actions of GABA on putative noradrenergic neurons of the rat LC, in a brain slice preparation. GABA application in the bath, or more locally by micropressure ejection inhibited spontaneous firing and increased the conductance of LC neurons. In addition, GABA could hyperpolarize or depolarize LC neurons; the size of these responses depended on the Cl- gradient across the membrane. GABA responses were antagonized by bicuculline. These data indicate that the actions of GABA on LC neurons are primarily mediated by activation of GABAA receptors which increases the Cl- conductance. When GABA is applied to LC neurons after blockade of GABAA receptors with bicuculline, a residual action mediated by GABAB type receptors can be seen. Similar responses can be obtained with the GABAB-selective agonist baclofen. GABAB activation inhibits spontaneous firing and causes membrane hyperpolarization due to an increase in K+ conductance. Single-electrode voltage clamp experiments were used to study the voltage dependency of GABA responses in LC neurons. GABA-induced current showed outward rectification. The conductance increase caused by a given amount of GABA decreased with membrane hyperpolarization. The time constant of decay of the GABA current also decreased with membrane hyperpolarization. Due to the voltage dependency of GABA responses, GABA exerts a stronger inhibitory effect on LC neurons at depolarized potentials than at hyperpolarized potentials, which could serve as a negative feedback mechanism to control excitability of these neurons.