A comparison of projections of entopeduncular neurons to the thalamus, the midbrain and the habenula in the cat.

Anatomical studies have demonstrated that the output of the striopallidal system is distributed to two areas of the thalamus: the ventrolateral-ventroanterior and the centromedian nuclei. The two areas are involved in different ways in the control of somatic motor activity. Pallidal efferents are also distributed to a still obscure tegmental area in the midbrain, the pedunculopontine nucleus, and to the lateral habenular nucleus, a structure of the limbic system. The present study compares the projections of entopeduncular neurons to the four sites in cats. The comparison is based on an estimation of the number of entopeduncular neurons sending fibers to each site and branching to more than one site. The four projection sites were stimulated electrically in anesthetized cats and the number of entopeduncular neurons excited antidromically were counted. At least 68% of entopeduncular neurons were excited antidromically by stimulation of the ventrolateral nucleus, an equal number were excited antidromically by stimulation of the nucleus centromedian and slightly fewer but still more than 50% by stimulation of the pedunculopontine nucleus. The three sites gave rise to antidromic responses of the same entopeduncular neuron in at least 33% of the cases. Only 25% of entopeduncular neurons responded antidromically to stimulation of the lateral habenular nucleus exclusively (one-third) or not (two-thirds). Some neurons recorded incidentally in the globus pallidus responded antidromically to the stimulation sites. Neurons were also recorded in the preoptico-hypothalamic area and 67% responded antidromically exclusively to the stimulation of the lateral habenular nucleus.