AUDITORY THALAMOCORTICAL PATHWAYS DEFINED IN MONKEYS BY CALCIUM-BINDING PROTEIN IMMUNOREACTIVITY

This study investigated differentiation of Macaca fuscata auditory tha lamus into chemically defined nuclei forming relays to auditory cortic al areas. The thalamus was stained immunocytochemically for parvalbumi n and 28 kDa calbindin in normals and in brains in which retrogradely transported tracers were injected into middle layers of auditory corti cal areas or applied to the cortical surface. Parvalbumin- and calbind in-immunoreactive cells show a complementary distribution in ventral, anterodorsal, posterodorsal, and magnocellular medial geniculate nucle i. The ventral nucleus has a high density of parvalbumin cells and few calbindin cells, and the anterodorsal nucleus has a high density of p arvalbumin cells and moderate numbers of calbindin cells, Both nuclei have a dense parvalbumin-immunoreactive neuropil formed by termination s of fibers ascending in the brachium of the inferior colliculus. The posterodorsal nucleus has approximately equal proportions of parvalbum in and calbindin cells; neuropil staining is weak but contains termina tions of calbindin-immunoreactive fibers ascending in the midbrain teg mentum. The magnocellular nucleus contains domains of parvalbumin and calbindin cells. Parvalbumin cells in the ventral nucleus project to a central core of auditory cortex with densest parvalbumin immunoreacti vity. Those in anterodorsal and posterodorsal nuclei project to surrou nding auditory fields with less dense parvalbumin immunoreactivity; th ose in the magnocellular nucleus project widely to auditory and other fields. Injections of middle cortical layers label a large majority of parvalbumin cells in the ventral, anterodorsal, or posterodorsal nucl ei and in the magnocellular nucleus. Superficial deposits label calbin din cells only, usually in more than one nucleus, implying a widesprea d projection system. (C) 1995 Wiley-Liss, Inc.