TH-, NPY-, SP-, and CGRP-immunoreactive nerves in interscapular brown adipose tissue of adult rats acclimated at different temperatures: an immunohistochemical study

Interscapular brown adipose tissue (IBAT), a site of nonshivering thermogen esis in mammals, is neurally controlled. The co-existence of sympathetic an d peptidergic innervation has been demonstrated in different brown adipose depots. We studied the morphological profile of IBAT innervation and tested by immunohistochemical methods whether cold and warm stimulation are accom panied by modifications in the density of parenchymal noradrenergic nerve f ibers. We also studied the immunoreactivity of afferent fibers-which contai n calcitonin gene-related peptide (CGRP) and substance P (SP)-in different functional conditions. IBAT was obtained from adult rats (6 weeks old) accl imated at different temperatures (4 degrees, 20 degrees, and 28 degrees C). Tissue activity was evaluated by studying the immunolocalization of uncoup ling protein (UCP-1), a specific marker of brown adipose tissue. Noradrener gic and peptidergic innervation were seen to arise from morphologically dif ferent nerves. Fibers staining for tyrosine hydroxylase (TH) were thin, unm yelinated hilar nerves, and CGRP- and SP-positive fibers were in thick nerv es containing both myelinated and unmyelinated fibers. Under cold stimulati on, noradrenergic neurons produce greater amounts of TH, and their axons br anch, resulting in increased parenchymal nerve fibers density. Neuropeptide Y (NPY) probably co-localizes with TH in noradrenergic neurons, but only i n the perivascular nerve fiber network. The parenchymal distribution of NPY to interlobular arterioles and capillaries suggests that this peptide must have other functions besides that of innervating arteriovenous anastomoses , as hypothesized by other researchers. The different distribution of CGRP and SP suggests the existence of different sensory neuronal populations. Th e detection of CGRP at the parenchymal level is in line with the hypothesis of a trophic action of this peptide.