Neurotensin in the rat parabrachial region: ultrastructural localization and extrinsic sources of immunoreactivity.

We sought to determine (1) the ultrastructural localization and (2) the extrinsic sources of neurotensin-like immunoreactivity (NTLI) in the parabrachial region (PBR). The brains from untreated adult male rats and from others that received intraventricular injections of colchicine (100 micrograms/7.5 microliters saline) 24 hours prior to death were fixed by perfusion with acrolein or glutaraldehyde and paraformaldehyde. Coronal sections were immunocytochemically labeled with a polyclonal rabbit antiserum to neurotensin and the PAP method. Western dot-blots and immunocytochemical labeling with adsorbed antiserum revealed significant cross-reaction only against NT, NT8-13, and glutamine (Gln)4-NT. In the ultrastructural study, the most numerous labeled profiles were axons and axon terminals in both colchicine-treated and control animals. The terminals containing NTLI were characterized by a mixed population of small, clear and large, dense core vesicles; asymmetric junctions principally with unlabeled dendrites; and a few synaptic specializations with unlabeled axon terminals. Compared to axon terminals, relatively few perikarya or dendrites had detectable levels of NTLI in either untreated or colchicine-treated animals. The labeled perikarya measured 8-10 microns in longest cross-sectional diameter, contained NTLI throughout a narrow rim of cytoplasm, and received a few somatic synapses from unlabeled terminals. From the relative density of axon terminals and sparsity of perikarya and dendrites, we conclude that the NTLI in the PBR is principally derived from extrinsic neurons. However, the intrinsic neurons with NTLI may also contribute to the immunoreactivity in the axon terminals of the PBR. We sought to determine the precise location of the extrinsic neurons that contribute to the NTLI in axon terminals in the PBR. Following unilateral injections of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP), dual labeling was most evident in a large population of neurons located in the dorsal, medial and commissural nuclei of the solitary tracts, ipsilateral to the side of the injection. However, a few perikarya containing both the retrogradely transported WGA-HRP and immunocytochemical labels for NT were also detected in the caudal ventrolateral reticular formation, the locus coeruleus, and the paraventricular and lateral hypothalamic nuclei. We conclude that (1) NT or a closely related peptide is present in intrinsic neurons and multiple afferent pathways to the PBR; and (2) the axon terminals with NTLI have synaptic interactions with dendrites of intrinsic neurons and with axon terminals that may have either extrinsic or intrinsic origins.