Hypothalamic neuronal origin of neuropeptide Y (NPY) or cocaine- and amphetamine-regulated transcript (CART) fibers projecting to the tuberomammillary nucleus of the rat

Abstract

Based on the importance of tuberomammillary nucleus (TMN) as a target for feeding/arousal-related functions, we aimed in the present study to investigate hypothalamic neuronal origin of neuropeptide Y (NPY) and cocaine- and amphetamine-regulated transcript (CART) fibers projecting to the histaminergic nucleus. In the first series of experiments, we examined NPY (or CART) fiber distribution within the boundary of adenosine deaminase (ADA)-immunoreactive (ir) TMN regions; extensive NPY (or CART)-ir axon terminals were observed in E4 (TMMd), E3 (TMMv), and E2 (TMVr) subdivisions. NPY varicosities co-contained vesicular GABA transporters (vGAT). CART boutons, however, contained either vGAT or vesicular glutamate transporters (vGLU), which suggested dual (or multiple) origins of CART fibers. Based on the previous observation on melanin-concentrating hormone (MCH)-ir neuronal elements in the TMN, their coexistence with CART peptide was examined in detail. In E4 subdivision, approximately 40.8% of MCH-ir somata co-contained CART, but the proportion was reduced to 24.1% in E3 region. In E2 and E1 (TMVc) regions, only MCH-ir axon terminals existed without any MCH-ir somata. In the second series of experiments, we investigated hypothalamic neuronal origin of NPY (or CART) fibers projecting to the TMN. The arcuate nucleus (Arc) was the sole source of hypothalamic NPY fibers projecting to the nucleus. In contrast, CART fibers in the TMN originated from the Arc as well as the other hypothalamic nuclei including the retrochiasmatic nucleus, paraventricular nucleus, lateral hypothalamus (LH), zona incerta (ZI), and dorsal hypothalamic area. Quantitative analysis showed that arcuate CART projection to the TMN occupied approximately 23.5% of the total hypothalamic CART input to the nucleus, while the rest originated mainly from the LH and ZI. The present observations suggested that the TMN might play a key role in energy balance and arousal, by receiving periphery-derived, first-order NPY (or CART) inputs from the Arc as well as second-order (and downstream) CART inputs from the other hypothalamic nuclei.