Effects of dihydropyridine calcium channel ligands on rat brain gamma-aminobutyric acidB receptors

Age-related levels of GABA/benzodiazepine binding sites in cerebrum of F-344 rats: effects of exercise

We examined gamma-aminobutyric acid (GABA), benzodiazepine and convulsant sites of postsynaptic GABA/benzodiazepine receptors (GBZR) in cerebral membranes of inbred Fischer 344 male rats as a function of age. In aged rats (23 to 24 months), the benzodiazepine binding site as determined by [3H]flunitrazepam was 47% and 43% lower than corresponding values in young adult (3 to 4 months) and mature (10 to 12 months) rats, respectively. The decrease was due to the loss of binding density rather than a change in affinity. No statistically significant age-related changes in [3H]muscimol binding were observed when 5 nM or 40 nM labeled muscimol were used. GABA produced a dose-dependent stimulation of flunitrazepam binding in all age groups, but the maximum stimulation in aged animals was significantly higher (24%) than in young and mature animals. The [35S]TBPS binding site, the convulsant site of GABA/benzodiazepine receptors, was unaffected with age. We also studied the effects of exercise on GBZR binding sites of aged rats. The decline of flunitrazepam binding sites and the high sensitivity of flunitrazepam binding to regulation by GABA in aged animals were reversed by 8 to 10 weeks of endurance exercise. Endurance exercise did not have any significant effect on muscimol or TBPS binding sites. Results suggest that there are aged-related alterations of GBZR binding sites and that these modifications can be reversed by exercise.

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.

Regulation of gamma-aminobutyric acidB (GABAB) receptors in cerebral cortex during the estrous cycle

We examined binding of the GABAB receptor agonist baclofen to brain synaptic membranes as a function of the natural variations in gonadal steroids that occur during the estrous cycle of the adult rat. We found that the binding of baclofen to neocortical membranes varied systematically as a function of the estrous cycle, with the lowest binding occurring during the estrus stage. Binding to archicortical (hippocampal) and hypothalamic preparations also varied with the estrous cycle, except that the lowest level of binding in these latter cases occurred during the diestrus stage. The variation of [3H]baclofen binding during the estrous cycle was different with respect to the binding of [3H]muscimol, an agonist for GABAA receptors, and [3H]8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), an agonist for serotonin 5-HT1A receptors that shares similar G proteins and effectors with GABAB receptors. Saturation binding studies of cortical GABAB receptors showed that apparent receptor density (Bmax) rather than affinity (Kd) best accounted for the change in binding during the estrous cycle in that Bmax, like total specific binding, was at a minimum during the estrus stage. The robust regulation of GABAB receptors in neocortex was unexpected and its functional significance is at present unknown. However, the correlation of the menstrual cycle with mood and other behavioral changes, and the correlations of the estrous and menstrual cycles with seizure susceptibility, may somehow depend upon hormonal regulation of transmitter systems such as the one we have observed here.

1,4-Dihydropyridines modulate GTP hydrolysis by Go in neuronal membranes

Several lines of evidence suggest that L-type Ca2+ channels (1,4-dihydropyridine receptors) are modulated by GTP-binding proteins. We have further examined this interaction by measuring the effect of 1,4-dihydropyridines on GTPase activity in brain membranes. Dihydropyridine agonists significantly increased GTPase, reflected by an increase in the maximal rate of GTP hydrolysis, without affecting the affinity for GTP or the binding of a non-hydrolysable analogue of GTP. The stimulating effect on GTPase was abolished by antisera raised against Go alpha but not Gi alpha. L-type Ca2+ channels may act as endogenous GTPase activating proteins (GAPs) to stimulate GTP hydrolysis by Go.

Smithkline Beecham Prize for Young Psychopharmacologists: A review of the relationship between calcium channels and psychiatric disorders

The symptoms and etiology of most major psychiatric disorders probably represent an underlying disturbance of neurotransmitter function. Understanding the mechanisms which control neurotransmitter function, and in particular neurotransmitter release, is therefore of considerable importance in determining the appropriate pharmacological treatment for these disorders. Calcium entry into neurons triggers the release of a wide range of neurotransmitters and recently our understanding of the mechanisms which control neuronal calcium entry has increased considerably. Neuronal calcium entry occurs through either voltage-sensitive or receptor-operated calcium channels. This article reviews the different subtypes of calcium channel, with particular reference to their structure; drugs which act upon them; and the possible function of the subtypes identified to date. In addition, it reviews the potential role of calcium channel antagonists in the treatment of a wide range of psychiatric disorders, and concludes that these drugs may have an increasing therapeutic role particularly in the treatment of drug dependence, mood disorders and Alzheimer's disease.

Effects of calcium channel agonist and antagonists on calcium-dependent events in CA1 hippocampal neurons

The effects of a variety of calcium channel modulators on different calcium-dependent events in CA1 pyramidal hippocampal neurons were analysed using intracellular recordings in an in vitro slice preparation. The following substances were tested: the dihydropyridine calcium agonist BAY K 8644, the dihydropyridine calcium antagonist nimodipine, the phenylalkylamine verapamil and the snail toxin omega-conotoxin GVIA (omega-CgTx). BAY K 8644 increased the repolarization time of the after hyperpolarization (AHP) following a spike burst. This effect was antagonized by nimodipine. BAY K 8644 also prolonged the calcium spike and, in some cases, increased the size of the synaptic events resulting from activation of the Schaffer collateral/commissural system. Nimodipine decreased the size of the AHP in some neurons but had no consistent effect on synaptic events. Verapamil at low concentrations (1-10 microM) had no significant effects on the calcium-dependent events in the hippocampus. Increasing the concentration (up to 100 microM) led to a progressive suppression of the AHP and of the slow inhibitory postsynaptic potential (IPSP), probably via an action on potassium conductances. In addition, the baclofen-induced hyperpolarization was blocked by verapamil. Interestingly, at this higher concentration, verapamil could suppress the AHP without depressing the calcium spike. omega-CgTx selectively blocked the synaptic events (especially the IPSPs) but had no effect on non-synaptic events. This last compound exhibits a high degree of selectivity, acting on N-type calcium channels which are involved in neurotransmitter release. Our results provide evidence that different classes of agents which act on calcium channels can be used to discriminate between different calcium-dependent responses in CA1 hippocampal neurons.

Effect of 2-hydroxy-saclofen, an antagonist of GABAB action, upon the binding of baclofen and other receptor ligands in rat cerebrum

2-Hydroxysaclofen (2-OH-saclofen), a newly available compound which blocks certain physiological actions of the gamma-aminobutyric acidB (GABAB) agonist, baclofen, was found to displace [3H]baclofen at least 10-fold more potently than did phaclofen, a previously available antagonist of GABAB action. 2-OH-Saclofen reduced both the affinity and apparent density of baclofen binding sites and displaced baclofen binding at least 60-fold more potently than it displaced the binding of ligands for 3 other transmitters present in the rat cerebral cortex.