Aspects of the molecular pharmacology of GABAB receptors

The connection between absence-like seizures and hypothermia induced by penicillin: possible implication on other animal models of petit mal epilepsy

In this study we investigated the relationship between penicillin-induced hypothermia and petit mal epilepsy induced by this proconvulsant antibiotic. In order to find a possible dose-dependent relationship, we used two doses: 1500.000 and 1000.000 U/kg b.wt., both known as being sufficient to induce absence-like attacks with subsequent spike and wave discharges (SWD) in electrocorticogram (ECoG). Because of experimental data suggesting penicillin binding to benzodiazepine receptor recognition site, we also studied penicillin-induced changes in body temperature after diazepam pretreatment. Results of this study clearly show that penicillin in doses known to induce petit mal-like epilepsy concomitantly induces statistically significant dose-dependent decrease in body temperature. Pretreatment with diazepam completely prevents both penicillin-induced hypothermia and SWDs. On the other hand, both the diazepam and mixed diazepam + penicillin treatments did not significantly alter body temperature. These results suggest, however, that at least some of the penicillin effects described could be assigned to its binding to the benzodiazepine receptor recognition site at GABA(A) ionophore. This may have an important clinical implication because the inhibitory action of penicillin at the benzodiazepine receptor recognition site could account for the mechanism of penicillin-induced unspecific encephalopathies in humans. The relationship between petit mal epilepsy and hypothermia sheds new light on the action mechanisms of penicillin-induced absence seizures.

GABAA receptor currents recorded from Müeller glial cells of the baboon (Papio cynocephalus) retina

The effect of gamma-aminobutyric acid (GABA) application on acutely isolated, non-cultivated Muller glial cells from the baboon retina was studied using the whole-cell voltage-clamp technique. Application of GABA (0.1 mM) generated inward currents at a holding potential of -80 mV as well as an increase in current noise. The GABA-activated current had a reversal potential of 18.6 mV and was therefore supposed to be a Cl- current (ECl = 5 mV). The GABAA receptor agonist muscimol (0.1 mM) elicited an inward current and bicucullin (0.5 mM), a blocker of the GABAA receptor, diminished the GABA responses in our experiments completely. Baclofen (0.1 mM), a GABAB agonist, neither had an effect when applied under conditions where the dominant Muller cell K+ currents were unblocked, nor when the K+ currents were blocked by application of Ba2+ (1 mM). Glycine (0.1 mM) was ineffective as well. From these results we conclude that the baboon retinal Muller cells possess GABAA receptors. However, these have recently been discovered on skate Muller cells whereas GABAA receptors could not be found on Muller cells of guinea pig, pig, mouse, rat and rabbit.

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.

[D-Pen2-D-Pen5]enkephalin, a delta opioid agonist, given intracerebroventricularly in the mouse produces antinociception through medication of spinal GABA receptors

Intracerebroventricular (ICV) administration of [D-Pen2-D-Pen5]enkephalin (DPDPE), a delta opioid receptor agonist, activates a descending antinociceptive pathway that inhibits the tail-flick response in mice. Involvement of spinal GABA receptors in this response was studied by giving GABA antagonist intrathecally. First, antinociception produced by intrathecally administered isoguvacine, a GABAA agonist, was inhibited by intrathecal bicuculline (GABA receptor antagonist) or picrotoxin (chloride channel antagonist). Then, antinociception induced by ICV DPDPE was antagonized by intrathecal picrotoxin and bicuculline in a dose-and time-dependent manner. Second, intrathecal administration of 2-hydroxysaclofen, a GABAB antagonist (which inhibited antinociception induced by a GABAB agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA and B antagonists given together intrathecally produced a greater than additive antagonistic effect against ICV DPDPE-induced antinociception. Thus, the delta agonist action of DPDPE in the brain leads to activation of descending spinal pathways which involve mediation by spinal GABAA and GABAB receptors in the antinociceptive response.

Sleep disorders and anxiety: biochemical antecedents and pharmacological consequences

Evidence is presented that the most widely used and effective drugs used in the treatment of anxiety and insomnia act by indirectly activating GABA-A receptors in limbic regions of the brain. Since the discovery of the benzodiazepines, different classes of benzodiazepine receptor ligands (such as the cyclopyrroliones and imidazopyridines) have been developed which alleviate anxiety and insomnia by activating different sites on the benzodiazepine-GABA receptor complex to those activated by the 'classical' benzodiazepines as exemplified by temazepam and diazepam. There is evidence that natural ligands also exist in the mammalian brain which can modulate the benzodiazepine-GABA receptor complex. This raises the possibility that insomnia and anxiety states may arise as a consequence of a deficit in the availability of endogenous ligands that act as agonists at these sites.

The GABAergic nervous system of Caenorhabditis elegans

gamma-Aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in vertebrates and invertebrates. GABA receptors are the target of anxiolytic, antiepileptic and antispasmodic drugs, as well as of commonly used insecticides. How does a specific neurotransmitter such as GABA control animal behaviour? To answer this question, we identified all neurons that react with antisera raised against the neurotransmitter GABA in the nervous system of the nematode Caenorhabditis elegans. We determined the in vivo functions of 25 of the 26 GABAergic neurons by killing these cells with a laser microbeam in living animals and by characterizing a mutant defective in GABA expression. On the basis of the ultrastructurally defined connectivity of the C. elegans nervous system, we deduced how these GABAergic neurons act to control the body and enteric muscles necessary for different behaviours. Our findings provide evidence that GABA functions as an excitatory as well as an inhibitory neurotransmitter.

Pharmacology of GABA rho 1 and GABA alpha/beta receptors expressed in Xenopus oocytes and COS cells

1. The rho 1 protein, which we previously cloned from retina, assembles as a homooligomer that transduces the binding of gamma-aminobutyric acid (GABA) into robust chloride currents. However, its insensitivity to bicuculline, pentobarbitone and benzodiazepines, all potent agents at typical GABAA receptors, suggested that it may react atypically to other GABA agonists and antagonists. 2. cDNAs for the rho 1 and the alpha 5 beta 1 receptors for GABA were expressed as homo- and heterooligomers, respectively, in Xenopus oocytes. The selectivities of the respective receptors for various agonists were investigated using concentration-response experiments in voltage clamped cells. 3. The most potent agonists at the rho 1 receptor were trans-4-aminocrotonic acid (TACA) > GABA > muscimol; at the alpha 5 beta 1 receptor the rank order was muscimol > GABA > 4,5,6,7-tetrahydroisoxazole[4,5-c]pyridine-3-ol (THIP). The most specific agonists were cis-(2-(aminomethyl)-cyclopropyl-carboxylic acid (CAMP) and THIP for the rho 1 and the alpha 5 beta 1 receptors, respectively. 4. Comparing GABA, TACA and cis-aminocrotonic acid (CACA) at rho 1 receptors expressed in COS cells gave results almost indistinguishable from those found at oocytes; the pharmacology of rho 1 seems independent of the expression system. 5. Agonists THIP, piperidine-4-sulphonic acid (P4S), and isoguvacine, whose C-C-C-N chains are constrained by rings into a folded conformation and were potent at the alpha 5 beta 1 receptor, were among the weakest at the rho 1 receptor. However CACA and CAMP, which align better with the extended than the folded conformation, were weakest at the alpha 5 beta 1 receptor but moderately potent at the pl receptor. These findings suggest that the rho l receptor recognizes agonists in the extended conformation, in contrast to GABAA receptors, which are believed to recognize agonists in the partially folded conformation.6. In contrast to the alpha 5 beta 1 receptor, gradations in maximum responses were apparent in the rho l receptor,suggesting various degrees of partial agonism. In particular, imidazole-4-acetic acid (I4AA), whose maximum response was only 3% of GABA's maximum, had an apparent Kd for activating the rho l receptor of 16 microM; but it had an apparent Kd for competitively blocking the receptor of 0.64 microM. This difference suggests that steric constraints in the activated (open channel) receptor are tighter than in the resting receptor.7. Hill coefficients approached 2 at the rho l receptor, but were closer to unity at the alpha 5 beta 1 receptor. Thus,the rho l receptor displayed higher cooperativity.8. Unlike typical GABAA receptors, the rho l receptor was insensitive to the competitive antagonists bicuculline, SR95531, securinine, and (+)-tubocurarine.

Commentary on the mode of action of benzodiazepines

Evidence is presented showing that the benzodiazepines produce their variety of pharmacological effects by activating GABA A receptors in the mammalian brain. Different classes of benzodiazepine receptor ligands have been developed which can cause or alleviate anxiety according to the nature of their interaction with the GABA A receptor. There is now evidence that natural ligands also exist in the brain which can modulate GABA A receptor function. The changes in the responsiveness of the GABA A receptor to chronic benzodiazepine treatment is discussed with reference to the phenomenon of tolerance dependence and withdrawal.

Sensitivity of Xenopus oocytes to changes in extracellular pH: possible relevance to proposed expression of atypical mammalian GABAB receptors

Electrical recordings were made to characterize the sensitivity of Xenopus oocytes to changes in extracellular pH, and to determine whether rat cerebral cortex poly(A)+ RNA (mRNA) expressed GABAB receptors with atypical electrical properties. All oocytes showed some sensitivity to changes in pH, and those from a small fraction (< 10%) of frogs were found to be highly responsive to acidification of bathing Ringer. In these oocytes, reduction in extracellular pH elicited membrane current responses with two components: (1) Smooth, maintained currents, primarily associated with a decrease in K+ conductance. (2) Oscillatory Cl- currents, elicited through activation of the phosphoinositide/Ca2+ messenger pathway. Oocytes with highest levels of sensitivity responded to decreases as low as 0.1 pH unit (Ringer pH 7.0 lowered to pH 6.9). Rat cortex mRNA consistently showed strong expression of membrane current responses mediated by GABAA, glutamate, kainate, serotonin and acetylcholine (muscarinic) receptors, together with responses mediated by a variety of neuropeptide receptors. In these oocytes, enantiomers of the GABAB receptor agonist baclofen, at concentrations ranging between 1 microM and 10 mM (pH 7.0), activated no significant membrane current responses. However, at concentrations > 0.1 mM hydrochloride salts of baclofen caused appreciable acidification of Ringer solutions; for example, 1 mM baclofen lowered pH from 7.0 to 6.0. Thus, when assaying oocytes with high sensitivity to pH, failure to make the necessary re-adjustment could result in apparent baclofen responses that, in reality, are simply due to pH effects alone.(ABSTRACT TRUNCATED AT 250 WORDS)