Distribution of six neuropeptides in the nucleus tractus solitarii of the rat: an immunohistochemical analysis.

Distribution of substance P-, [Leu]enkephalin-, cholecystokinin-8-, neurotensin-, avian pancreatic polypeptide- and gamma-melanocyte stimulating hormone-like immunoreactive structures were investigated in the nucleus tractus solitarii of the rat by means of the indirect immunofluorescence method. The density of the immunoreactive structures varied markedly according to neuropeptides or subnuclei, with the medial and commissural nuclei containing the highest density. This suggests that the peptides examined play a role in cardiovascular function. However, as seen in the substance P- and [Leu]enkephalin-like immunoreactive structures, these peptides were widely distributed in the nucleus tractus solitarii in addition to the commissural and medial nuclei; a high density of immunoreactive fibers in the ventral, dorsolateral and intermediate subnuclei. In addition to the immunoreactive fiber plexus, a group of immunoreactive cells was also identified in the subnuclei mentioned above. These findings strongly suggest that substance P- and [Leu]enkephalin-like immunoreactive structures are involved not only in cardiovascular function but also in other functions such as respiration, at least in the rat. Finally, the present study demonstrated that the area postrema, particularly its lateral portion, contains various neuropeptide-like structures, both neurons and fibers, substance P-, [Leu]enkephalin-, cholecystokinin-8- and neurotensin-like immunoreactive neurons and fibers, and avian pancreatic polypeptide- and gamma-melanocyte stimulating hormone-like immunoreactive fibers.