White adipose tissue sensory nerve denervation mimics lipectomy-induced compensatory increases in adiposity

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Abstract
The sensory innervation of white adipose tissue (WAT) is indicated by the labeling of sensory bipolar neurons in the dorsal root ganglion after retrograde dye placement into WAT. In addition, immunoreactivity (ir) for sensory-associated neuropeptides such as calcitonin gene-related peptide (CGRP) and substance P in WAT pads also supports the notion of WAT sensory innervation. The function of this sensory innervation is unknown but could involve conveying the degree of adiposity to the brain. In tests of total body fat regulation, partial surgical lipectomy triggers compensatory increases in the mass of nonexcised WAT, ultimately resulting in restoration of total body fat levels in Siberian hamsters and other animals. The signal that triggers this compensation is unknown but could involve disruption of WAT sensory innervation that accompanies lipectomy. Therefore, a local and selective sensory denervation was accomplished by microinjecting the sensory nerve neurotoxin capsaicin bilaterally into epididymal WAT (EWAT) of Siberian hamsters, whereas controls received vehicle injections. Additional hamsters had bilateral EWAT lipectomy (EWATx) or sham lipectomy. As seen previously, EWATx resulted in significantly increased retroperitoneal WAT (RWAT) and inguinal WAT (IWAT) masses. Capsaicin treatment significantly decreased CGRP- but not tyrosine hydroxylase-ir, attesting to the diminished and selective sensory innervation. Capsaicin-treated hamsters also had increased RWAT and, to a lesser degree, IWAT mass largely mimicking the WAT mass increases seen after lipectomy. Collectively, these data suggest the possibility that information related to peripheral lipid stores may be conveyed to the brain via the sensory innervation of WAT.