Vagaries of adipose tissue innervation.

ADIPOSE TISSUE DOES NOT SEEM to be innervated in any significant manner by the vagus nerve after all. As reported by Giordano et al. (9) in this issue, retrograde tracing with pseudorabies virus from at least one fat pad in the Siberian hamster reveals only the occasional neuron in the vagal motor nucleus and nerve fibers in adipose tissue of mouse, rat, and hamster do not exhibit any of the markers typically associated with vagal motor innervation. Adipose tissue has recently attained high prominence as a key player in obesity and its secondary health problems such as type-2 diabetes and cardiovascular disease. Originally considered as a passive storage organ, it has been found to secrete a number of important hormones and cytokines (17). It is now apparent that in obesity the expanding adipose tissue is chronically inflamed (10). The combination of increased secretion of inflammatory cytokines and reduced secretion of adiponectin leads to dyslipidemia and insulin resistance (11). Hence, there is renewed interest in the biology of adipose tissue including its innervation by the autonomic nervous system. White adipose tissue is distributed in characteristic pads in the abdominal and thoracic cavities, under the skin, and, under certain circumstances, within specific organs such as muscle and liver (7). Adipose tissue is clearly innervated by the sympathetic nervous system. Noradrenergic nerve fibers found mainly along the vasculature and occasionally contacting adipocytes are present in all fat depots of most mammals, including humans (16). They originate from postganglionic neurons located in the preand paravertebral ganglia. The postganglionic neurons are driven by cholinergic sympathetic preganglionic neurons located in the intermediolateral column of the spinal cord that receive input from a number of brain areas. The generally lipolytic action of this sympathetic innervation has been demonstrated in numerous studies using a variety of techniques. Until recently, there was little or no evidence for a similar innervation of white adipose tissue by the parasympathetic nervous system through outflow from either the vagal system or the lower spinal cord. While more or less extensive vagal efferent innervation of most thoracic and abdominal organs was demonstrated using a variety of techniques, adipose tissue together with the spleen, adrenal gland, and gonads was not among them. According to classic autonomic theory, vagal parasympathetic innervation consists of cholinergic preganglionic neurons located in the caudal brain stem and postganglionic neurons expressing either acetylcholine or other transmitters such as vasoactive intestinal peptide (VIP) and nitric oxide synthase (NOS), the so called noncholinergic, nonadrenergic nerves. Because markers for the cholinergic phenotype thus do not unequivocally point to vagal origin, retrograde (from the periphery to the brain) and anterograde (from the brain to the periphery) tracing of vagal fibers is a better approach. In contrast to the sympathetic innervation where the ganglia are located in the paravertebral or prevertebral chain (celiac ganglion, etc.), vagal ganglia are typically located within or very close to the innervated organ. The implication is that while conventional retrograde tracers injected into a given organ readily label vagal preganglionic neurons, they only label sympathetic postganglionic but not preganglionic neurons. Thus, retrograde tracers were important for providing a first qualitative assessment of vagal innervation of various organs. Initial enthusiasm for this convenient new method was, however, rapidly curtailed when serious pitfalls became apparent. It turned out that in some studies, retrograde labeling of vagal motor neurons to organs receiving relatively little innervation was vastly exaggerated by tracer leaking out of the injected target site and contaminating adjacent organs with a much heavier vagal innervation (8). Thus, unless very stringent controls are employed, false positives are often the result. Better results can be obtained with modern anterograde tracing techniques introduced and adapted to the vagal motor system in the 1980s and 1990s (1, 4, 15). Because vagal preganglionic neurons are conveniently located in two motor nuclei, a large percentage of them can be impregnated with the anterograde tracer in a single or a few injections. Also, labeling of anterogradely traced axons in the innervated organs does not compete with any other immunohistochemical marker. Therefore, anterograde tracing of vagal innervation is better suited to identify the extent of, and specific compartments/cells targeted by the innervation. This approach was instrumental in demonstrating the dense innervation by vagal preganglionic fibers of the entire alimentary canal all the way down to the colon, and the more moderate innervation of the associated pancreas, liver, and portal vein (3, 5). Double-labeling strategies also clearly demonstrated that the overwhelming majority of vagal preganglionic axons terminate on intrinsic ganglion cells located in the myenteric plexus of the gastrointestinal tract and in interlobular pancreatic ganglia (2). Most important to this discussion is that in none of these numerous studies was any significant innervation of adipose tissue noted, although white adipose tissue was not systematically searched. Transneuronal retrograde tracing with pseudorabies virus is the most recent, very powerful tool to identify neural pathways, and is ideally suited to visualize the multisynaptic autonomic outflow to peripheral organs. Giordano et al. (9) now show that virus injected into the left or right subcutaneous inguinal fat pad of the Siberian hamster infects only a handful of neurons in the area of the dorsal motor nucleus of the vagus. In addition, this scarce labeling was completely absent when the fat pad was previously sympathetically denervated by local Address for reprint requests and other correspondence: H.-R. Berthoud, Neurobiology of Nutrition Laboratory, Pennington Biomedical Research Center, Louisiana State Univ. System, 6400 Perkins Road, Baton Rouge, LA 70808 (e-mail: [email protected]). Am J Physiol Regul Integr Comp Physiol 291: R1240–R1242, 2006; doi:10.1152/ajpregu.00428.2006.