RESPONSES OF RAT SUBSTANTIA-NIGRA DOPAMINE-CONTAINING NEURONS TO (-)-HA-966 IN-VITRO

1 Extracellular single unit recording techniques were used to compare the effects of (-)-3-amino-1-hydroxypyrrolidin-2-one ((--)-HA-966) and (+/-)-baclofen on the activity of dopamine-containing neurones in 300 mu m slices of rat substantia nigra. Electrophysiological data were c ompared with the outcome of in vitro binding experiments designed to a ssess the affinity of (-)-HA-966 for gamma-aminobutyric acid (GABA(B)) receptors. 2 Bath application of (-)-HA-966 produced a concentration- dependent inhibition of dopaminergic neuronal firing (EC(50)=444.0 mu M; 95% confidence interval: 277.6 mu M-710.1 mu M, n = 27) which was f ully reversible upon washout from the recording chamber. Although simi lar effects were observed in response to (+/-)-baclofen, the direct-ac ting GABA(B) receptor agonist proved to be considerably more potent th an (-)-HA-966 (EC(50) = 0.54 mu M; 95% confidence interval: 0.44 mu M- 0.66 mu M, n = 29) in vitro. 3 Low concentrations of chloral hydrate ( 10 mu M) were without effect on the basal firing rate of nigral dopami nergic neurones but significantly increased the inhibitory effects pro duced by concomitant application of (-)-HA-966. 4 The inhibitory effec ts of (-)-HA-966 were completely reversed in the presence of the GABA( B) receptor antagonists, CGP-35348 (100 mu M) and 2-hydroxysaclofen (5 00 mu M). Bath application of CGP-35348 alone increased basal firing r ate. However, the magnitude of the excitation (9.2+/-0.3%) was not suf ficient to account for the ability of the antagonist to reverse fully the inhibitory effects of (-)-HA-966. 5 (-)-HA-966 (0.1-1.0 mM) produc ed a concentration-dependent displacement of [H-3]-GABA from synaptic membranes in the presence of isoguvacine (40 mu M). However, the affin ity of the drug for GABA(B) binding sites was significantly less than that of GABA (0.0005 potency ratio) and showed no apparent stereoselec tivity. 6 These results indicate that while (-)-HA-966 appears to act as a direct GABA(B) receptor agonist in vitro, its affinity for this r eceptor site is substantially less than that of GABA or baclofen and u nlikely to account for the depressant actions of this drug which occur at levels approximately ten fold lower in vivo.