Ventral pallidum projections to mediodorsal nucleus of the thalamus: an anatomical and electrophysiological investigation in the rat.

Horseradish peroxidase (HRP) and single unit recording experiments were done in rats to investigate neural connections from the ventral pallidal region to the mediodorsal nucleus of the thalamus (MD). In the first series, following the diffusion or iontophoretic injection of HRP into the MD, retrogradely labeled neurons were observed throughout the rostrocaudal extent of the ipsilateral ventral pallidum. Most of the labeled neurons were found in an area between the nucleus of the diagonal band and the ventral aspect of the substantia innominata subcommissuralis. Additional labeled neurons were found in the ventral aspect of the globus pallidus and substantia innominata sublenticularis. In the second series, the region shown to contain labeled neurons was explored for single units antidromically activated by single pulse stimulation of the MD in urethane anesthetized rats. One hundred and fifty-nine single units in the subpallidal area were antidromically activated with latencies corresponding to conduction velocities of 0.2-3.9 m/s. A greater percentage of units in the subcommissural region (50.3%) were activated antidromically as compared to the sublenticular region (27.4%). In the third series, the MD was explored for single units which responded orthodromically to stimulation of the ventral pallidum. Fifty-eight percent (40/69) of MD units responded to stimulation of the subcommissural substantia innominata, whereas 90% (72/80) MD units responded to stimulation of the sublenticular substantia innominata. The most frequent type of orthodromic response observed in MD neurons was inhibition with short onset latencies (less than 10 ms). These data provide anatomical and electrophysiological evidence for the existence of direct pathways from the ventral pallidum to the MD and suggest that this projection is part of a corticosubcortical loop through which the frontal cortex with the ventral striatum and pallidum may contribute to motor function.