Identification of the pallidal and peripallidal cells projecting to the habenula in monkey.

Injections of horseradish peroxidase (HRP) into the habenula of squirrel monkeys labeled a multitude of neurons in the lateal hypothalamus and a lesser number of neurons in the internal pallidum (GPi). The distribution of labeled cells is heterogeneous and many HRP-marked pallidal neurons are intermingled with acetylcholinesterase (AChE)-rich cells in the internal medullary lamina. However, these AChE neurons do not appear to contribute significantly to the innervation of the habenula. These findings suggest species differences between rodents and primates in regard to the organization of the pallidohabenular system.