Pallidofugal projections to thalamus and midbrain: a quantitative antidromic activation study in monkeys and cats

The projections of monkey medial globus pallidus (and of cat entopeduncular nucleus) to thalamus and midbrain were studied with antidromic activation in order to determine the number of pallidal neurons sending axonal branches to the two sites. The animals were anesthetized with pentobarbital and several movable electrodes were used to stimulate the thalamic nuclear complex ventralis anterior - ventralis lateralis (VA-VL), the nucleus "centre médian" (CM), and the midbrain nucleus tegmenti pedunculopontinus (TPP). The responses of pallidal neurons were recorded with extracellular microelectrodes. In 3 monkeys 99% and 87% of 145 medial pallidal neurons responded antidromically to stimulation of VA-VL and TPP respectively. Reciprocal collision tests demonstrated that 86% of the 145 neurons sent axonal branches to the two sites. By comparison in 2 cats the tests demonstrated that 72% of 46 entopeduncular neurons branched to VA-VL and TPP. In 2 monkeys 68% of 53 medial pallidal neurons were shown to branch to VA-VL and CM thalamic nuclei. In the monkeys, the latencies of responses indicate that all pallidofugal fibers have the same mean conduction rate: 6 m/s. The fibers appear to branch profusely in VA-VL where less current was required to activate neurons antidromically than in TPP. The location of neurons in the medial pallidum is weakly correlated with the location of stimulation points in VA-VL activating the neurons antidromically at low threshold, suggesting some topography in the pallidothalamic projection. However there is no particular localization of medial pallidal neurons with and without branching projections. Apart from one exception, the 162 neurons recorded in the lateral pallidum failed to respond antidromically to the stimulation sites. We conclude that the great majority of medial pallidal neurons can send signals to both the thalamus and the midbrain in the cat and in the monkey.