THE MEDIODORSAL NUCLEUS OF THE THALAMUS IN RATS .1. FOREBRAIN GABAERGIC INNERVATION

The aim of this study was to determine whether forebrain neurons proje cting to the mediodorsal nucleus of the thalamus in rats express gluta mate decarboxylase messenger RNA as a marker for GABAergic neurons. Fo rebrain glutamate decarboxylase messenger RNA-containing neurons that project to the mediodorsal nucleus were identified using a combination of retrograde tracing with Fluoro-Gold and in situ hybridization for the messenger RNA encoding the 67,000 molecular weight synthetic enzym e for GABA. Glutamate decarboxylase messenger RNA-containing afferents to the mediodorsal nucleus were observed in the olfactory tubercle, v ertical limb of the diagonal band of Broca, ventral pallidum, sublenti cular substantia innominata, globus pallidus, lateral preoptic area, b ed nucleus of the stria terminalis and reticular nucleus of the thalam us. The largest proportions of glutamate decarboxylase messenger RNA-c ontaining afferents to the mediodorsal nucleus were observed in the ve rtical limb of the diagonal band, ventral pallidal parts of the olfact ory tubercle and the reticular nucleus of the thalamus. Somewhat fewer glutamate decarboxylase messenger RNA-containing, retrogradely labele d neurons were observed in the subcommissural ventral pallidum and sub lenticular substantia innominata. These data suggest that a GABAergic projection from the basal forebrain to the mediodorsal nucleus of the thalamus can influence the function of this nucleus.