Quantitative evaluation of the potencies of GABA‐receptor agonists and antagonists using the rat hippocampal slice preparation

Abstract

1 CAl population spikes recorded in the rat hippocampal slice were used to assess quantitatively the potencies of GABA-receptor agonists and antagonists on mammalian CNS neurones. 2 Apart from GABA itself, GABAA-receptor agonists inhibited the CAI population spikes with potencies that correlated closely (r = 0.96) with their ability to displace [3H]-GABA from GABAAbinding sites. 3 The low potency of GABA in this preparation was attributed to the action of uptake processes as the GABA uptake inhibitor, cis-4-hydroxynipecotic acid (2 x 10-4 M), produced an approximate 6 fold increase in the potency of GABA whilst having no effect on the potency of 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol (THIP), a GABAA-receptor agonist which is not a substrate for the GABA uptake system. 4 The inhibitory effects of the selective GABAA-receptor agonists isoguvacine and muscimol were antagonized by bicuculline methochloride, which shifted the dose-response curves to the right in a parallel manner. The Schild plots for bicuculline methochloride against isoguvacine and muscimol had slopes of 1 and gave pA2 values of 6.24 and 6.10, respectively. Picrotoxin also antagonized the inhibitory effects of isoguvacine and produced parallel shifts to the right of the dose-response curve. However, the Schild plot for picrotoxin had a slope significantly less than unity (0.82) and gave a pA2 value of 6.89. 5 The novel GABAA-receptor antagonist, pitrazepin, antagonized the inhibitory effects of isoguvacine in an apparently competitive manner. The Schild plot had a slope of I and gave a pA2 of 6.69. 6 The inhibitory effects of baclofen, GABA and kojic amine were not antagonized by GABAAreceptor antagonists and were presumed to be mediated by actions at The inhibitory effects of THIP and isoguvacine were antagonized with the same potency by bicuculline methobromide. These results do not support the suggestion that THIP acts preferentially at a 'synaptic' bicuculline-sensitive, GABA receptor. 8 It is concluded that the CAI population spike in the rat hippocampal slice is a useful test system for the quantitative analysis of both GABAA-and GABAB-receptor agonists and antagonists.

Antagonistic action of pitrazepin on human and rat GABAA receptors

“…Thus, our results agree with previous observations reporting that pitrazepin antagonizes GABA-responses in rat brain hippocampal slices (GaÈ hwiler et al, 1984;Kemp et al, 1986). The IC 50 values for bicuculline and pitrazepin show that pitrazepin was ten times more potent than bicuculline in inhibiting the GABA-current; and the current-voltage relations indicate that pitrazepin does not appreciably alter the voltage-dependence and selectivity of the GABA A -gated Concentration-response data for GABA in the absence and in the presence of pitrazepin (n=4 ± 5).…”

“…channel. Furthermore, the pA 2 values for inhibition of GABAcurrents by pitrazepin determined in this study are similar to those derived using CA1 population spikes in a rat hippocampal slice preparation; which gave a slope of 1 and a pA 2 of 6.69 (Kemp et al, 1986). The similarity of pA 2 values for pitrazepin action on oocytes expressing rat cortex mRNA (therefore containing an heterogeneous population of GABA A receptors) and in oocytes expressing a 1 b 2 or a 1 b 2 g 2S human GABA A receptors, suggests strongly that the e ect of pitrazepin on GABA-currents is relatively independent of the subunit composition of GABA A receptors; at least for those studied here.…”

The affinities, potencies and efficacies of some benzodiazepine‐receptor agonists, antagonists and inverse‐agonists at rat hippocampal GABAA‐receptors

“…Hippocampal slices were prepared and maintained in a small perfusion chamber as previously described (Kemp et al, 1986). The Schaffer collateral-commissural pathway was stimulated every 30 s through metal bipolar stimulating electrodes made from two tungsten microelectrodes (TM25-5, Clark electromedical) glued together with a tip separation of 100-200 gm, and placed in the stratum radiatum.…”

“…The recorded potentials were digitised by use of a Gould OS4040 digital oscilloscope and a BBC microcomputer based system used to average and measure the peak height of the population spikes. Cumulative concentration-response curves to isoguvacine were constructed as previously described (Kemp et al, 1986). Each concentration was perfused for a 5 min period to ensure a maximum effect was achieved and the last four responses at each concentration averaged.…”

“…Furthermore, none of the studies described so far have made direct comparisons between the potencies and efficacies of benzodiazepine-receptor ligands measured electrophysiologically and their affinities and efficacies measured by binding studies. The hippocampus is amenable to both electrophysiological and binding studies and the pharmacology of GABAA-receptors on rat hipocampal neurones has recently been characterized in some detail (Kemp et al, 1986). This makes it an ideal target for the investigation of compounds which interact with GABAA-receptors in the mammalian central nervous system.…”