Effects of the GABA agonists, baclofen and muscimol, on instrumental responding for food reward

Abstract

Intra-accumbens administration of the GABA A agonist muscimol or the GABA B agonist baclofen strongly stimulates eating behaviour in the rat. However, previous reports have suggested that neither agonist is likely to stimulate instrumental responding for food reward. In the present study, separate groups of Lister hooded rats ( N = 12, 12) were trained on a modified second-order schedule of responding, as a measure of appetitive responding for food, and then implanted with bilateral guide cannulae aimed at the nucleus accumbens shell. They were also habituated to the presentation of chow, as a measure of consummatory behaviour. They were tested on the second-order schedule following administration of vehicle, baclofen (110, 220, 660 pmol) or muscimol (220, 440, 660 pmol). Subsequently, they were given a free feeding test following an identical sequence of drug administration. Baclofen stimulated instrumental responding at an intermediate dose (220 pmol, p < 0.01), whereas muscimol had no significant effect on responding at any dose. Baclofen and muscimol stimulated free food intake at both intermediate and higher doses. These data demonstrate that the effects of intra-accumbens administration of baclofen and muscimol on instrumental responding are not equivalent and also show a clear dissociation between the dose-related effects of baclofen on appetitive and consummatory components of feeding behaviour. Baclofen, perhaps through heterosynaptic modulation of other neurotransmitter systems within the accumbens, has broader effects on motivational processing than muscimol.