THE MEDIODORSAL NUCLEUS OF THE THALAMUS IN RATS .2. BEHAVIORAL AND NEUROCHEMICAL EFFECTS OF GABA AGONISTS

The aim of this study was to determine how GABA receptors in the medio dorsal nucleus of the thalamus in rats might contribute to the regulat ion of locomotor behavior. Microinjections of the GABA(B) and GABA(A) agonists, baclofen and muscimol, into the mediodorsal nucleus produced dose-dependent increases in locomotion that were blocked by co-admini stration of the GABA(B) antagonist, 2-hydroxysaclofen. Microinjection of baclofen along the midline, lateral into the ventrolateral thalamus or into the lateral ventricles produced significantly smaller dose-de pendent increases in locomotion, indicating that the anatomical locus for baclofen-induced locomotion resides in the mediodorsal nucleus. Th e motor response elicited by microinjected baclofen was associated wit h a reduction in dopamine metabolism in the prefrontal cortex and an i ncrease in metabolism in the core of the nucleus accumbens, but not in the accumbal shell or the dorsolateral striatum. These results sugges t that GABAergic afferents to the mediodorsal nucleus may oppose a ton ic inhibitory tone on locomotor activity. The data also suggest that t he motor response produced by baclofen in the mediodorsal thalamus may arise by inhibiting the projections to the prefrontal cortex which mo dulate mesocorticolimbic dopamine transmission.