Maguire, Janet Julie; (1992) Characterization of GABA(B) receptors in the rat peripheral and central nervous system. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Recent electrophysiological studies have led to the proposal of subtypes of GABAB receptors. In the rat hippocampus the postsynaptic GABAB receptor is sensitive to the weak GABAB antagonist phaclofen and to pertussis toxin (PTX) whereas the presynaptic receptor is insensitive to both agents. To investigate the presynaptic receptor further the effect of GABAB agonists was studied on the K+-evoked release of endogenous amino acids from rat hippocampal synaptosomes. (-)Baclofen (30-100μM) produced a dose-dependent inhibition of aspartate, glutamate and GABA release evoked by 50mM K+. 3-Aminopropyl-phosphinic acid (3-APA) (30-300μM) failed to inhibit amino acid release. 3-APA is reported to inhibit [3H]-GABA binding to GABAB sites in rat whole brain membranes more potently than the prototypic GABAB ligand (-)baclofen, although in biochemical assays 3-APA is equipotent with (-)baclofen and appears to behave as a partial agonist. Thus 3-APA may distinguish between subtypes of GABAB receptors. For comparison, peripheral GABAB receptors on adrenergic nerve terminals were studied using the electrically stimulated rat anococcygeus muscle preparation. (-)Baclofen, 3-APA and its methyl derivative SKF 97541 produced a dose-dependent inhibition of the electrically-evoked release of preloaded [3H]-noradrenaline (EC50 values of 1.4μM, 0.56μM and 1.25μM respectively). CGP 35348, a selective though relatively weak GABAB antagonist, was compared with two new compounds for their ability to reverse the effect of 30μM (-)baclofen. These compounds were found to antagonize the baclofen response more potently than CGP 35348. Schild analysis of data obtained using the same preparation monitoring antagonism of baclofen-induced inhibition of transmurally-evoked contraction of the muscle indicated the presence of a single receptor type. No evidence was obtained for receptor heterogeneity on adrenergic nerve terminals. Thus, although 3-APA failed to mimic the inhibitory action of (-)baclofen on transmitter release in hippocampal synaptosomes it did produce the same response in the peripheral tissue. Experiments were performed to determine whether 3-APA exhibited a differential selectivity for CNS GABAB receptors in different regions of the rat brain. Since any lack of affinity for hippocampal receptors might go undetected in membrane binding experiments performed in whole brain preparations, studies were performed using receptor autoradiography. 3-APA inhibited [3H]-GABA binding to GABAB sites to the same extent as (-)baclofen in seventeen brain regions including the hippocampus. The lack of difference between 3-APA and (-)baclofen in the hippocampus contrasted with the findings in the release experiments and the possible reasons for this discrepancy are discussed. Following incubation of rat brain slices for 24 hours in PTX specific GABAB binding is reduced throughout the brain with the exception of the corpus striatum. This observation may reflect different post receptor coupling or an inability of the toxin to access striatal neurones. To clarify these observations the effect of PTX on GABAB binding was determined in membranes prepared from the cortices, striata, hippocampi and cerebella of adult and immature rats. Incubation for 30 min in pre-activated PTX reduced specific GABAB binding in all areas except the striatum in the adult animals (10-12 weeks), however in animals of 5-6 weeks or 7-9 weeks old the profile of the PTX effect was quite different. GABAB binding in all brain regions could be reduced by 85% by the inclusion of GTP?S in the incubating medium. Regional and age-related variations in the sensitivity of GABAB binding to PTX may therefore be due to the presence of GABAB receptors linked to different inhibitory G-proteins.

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