CRH-synthesizing neurons in the hypothalamic paraventricular nucleus (PVN) integrate neuronal and hormonal inputs and serve as a final common pathway to regulate the hypothalamic-pituitary-adrenal axis. One of the neuronal regulators of CRH neurons is neuropeptide Y (NPY) contained in axons that densely innervate CRH neurons. The three main sources of NPY innervation of the PVN are the hypothalamic arcuate nucleus and the noradrenergic and adrenergic neurons of the brainstem. To elucidate the origin of the NPY-immunoreactive (NPY-IR) innervation to hypophysiotropic CRH neurons, quadruple-labeling immunocytochemistry for CRH, NPY, dopamine-β-hydroxylase, and phenylethanolamine-N-methyltransferase was performed. Approximately 63% of NPY-IR varicosities on the surface of CRH neurons were catecholaminergic (22% noradrenergic and 41% adrenergic), and 37% of NPY-IR boutons were noncatecholaminergic. By triple-labeling immunofluorescence detection of NPY, CRH, and agouti-related protein, a marker of NPY axons projecting from the arcuate nucleus, the noncatecholaminergic, NPY-ergic axon population was shown to arise primarily from the arcuate nucleus. When NPY was administered chronically into the cerebral ventricle of fed animals, a dramatic reduction of CRH mRNA was observed in the PVN (NPY vs. control integrated density units, 23.9 ± 2.7 vs. 77.09 ± 15.9). We conclude that approximately two thirds of NPY-IR innervation to hypophysiotropic CRH neurons originates from catecholaminergic neurons of the brainstem, whereas the remaining one third arises from the arcuate nucleus. The catecholaminergic NPY innervation seems to modulate the activation of CRH neurons in association with glucoprivation and infection, whereas the NPY input from the arcuate nucleus may contribute to inhibition of CRH neurons during fasting.
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