Neuropeptide-Y Neurons Projectioning to the Medial Septum-Diagonal Band Do Not Have Access to Fenestrated Capillaries in the Rat Brain

Abstract

The aims of this study were to (1) determine the localization of neuropeptide-Y (NPY)-containing perikarya which project to the medial septum-diagonal band (MSDB), a brain region rich in gonadotropin-releasing hormone (GnRH) producing perikarya, and (2) determine if NPY neurons have access to fenestrated capillaries as well. Fluorescent retrograde tracing after microinjection of fluororuby (FR) into the MSDB, and peripheral injection of fluorogold (FG), was used in combination with immunofluorescence for NPY. Injection of FR into the MSDB resulted in retrograde labeling of neurons in the hypothalamic arcuate nucleus (ARC) and brainstem nor-adrenergic cell groups A1 and A2, as well as other regions. The largest populations of NPY neurons were located in the ventromedial ARC and several of these perikarya contained FR indicating that they project to the MSDB. Defined cell groups in the areas A1 and A2 of the brainstem also contained large numbers of NPY neuron perikarya, and several of these contained FR. In addition, we observed isolated incidences of FR-labeled NPY perikarya in the amygdala and hippocampus. A small population of NPY neurons in the ARC contained FG, indicating that they are in contact with fenestrated capillaries and hence are neuroendocrine cells; however, none of these neurons contained FR. We suggest that NPY neurons in the ARC modulate GnRH function via innervation of the MSDB and LH release via release into fenestrated capillaries of the hypophysial-portal system.