Distributions of certain neuropeptides in the primate thalamus.

The distributions of fibers and terminals immunoreactive for somatostatin (SRIF), neuropeptide Y (NPY), substance P (SP) and cholecystokinin octapeptide (CCK), were studied in the diencephalon of cynomolgus monkeys. Immunoreactivity for all 4 peptides is found in extrinsic afferent fibers innervating the dorsal thalamus, ventral thalamus and epithalamus. The distributions of such fibers are more extensive than previously described and include many relay nuclei in their zones of terminations. SP fibers are particularly concentrated in the ventral posteromedial nucleus. All peptides are especially concentrated in fibers in the intralaminar and reticular nuclei. Afferent fibers immunoreactive for each of the 4 peptides approach the thalamus by two pathways. An anterior route is formed by the classical periventricular system ascending from the hypothalamus to the epithalamus. A posterior pathway ascends in the lateral midbrain tegmentum and provides innervation to posterior, intralaminar, and many relay nuclei, plus the ventral thalamus. A basal forebrain pathway, containing SRIF and NPY immunoreactive fibers, enters the thalamus in association with the ansa lenticularis and SP fibers also ascend from the substantia nigra.