Comparison between pentobarbital- and muscimol-induced feeding in satiated sheep

Abstract

Twenty sheep were used to study the mechanisms by which the intracerebral administration of pentobarbital and of muscimol induces feeding in ruminants. Injections of 1 μmol calcium induced a weak feeding response at 1 h postinjection compared with control values (108 vs. 63 g, p < 0.05). Injections of 78 μmol pentobarbital and of 100 nmol muscimol elicited strong feeding responses (p < 0.01). A preinjection of 1 μmol calcium reduced the response to pentobarbital by about 40% but did not affect the response to muscimol. Administration of 1.1 mmol sodium chloride reduced the effect to pentobarbital by about 60% but only partially decreased the effect to muscimol. Administration of picrotoxin, a GABA antagonist, slightly decreased the feeding response to pentobarbital and to muscimol. Administration of γ-vinyl GABA, an inhibitor of the enzyme GABA transaminase, did not affect feeding behavior of sheep at any of the doses tested (0–10 μmol). Injections of γ-vinyl GABA followed by equimolar injections of GABA failed to provoke any feeding response. The data suggest that pentobarbital and muscimol may induce feeding by acting on a similar hypothalamic receptor complex but by different mechanisms. The lack of effect of GABA itself remains unexplained.