Fos Expression in Forebrain Afferents to the Hypothalamic Paraventricular Nucleus Following Swim Stress

The paraventricular nucleus of the hypothalamus (PVN) serves as the origin of the final common pathway in the secretion of glucocorticoid hormones in response to stress. Various stress-related inputs converge upon the cells of the medial parvocellular division of the PVN. These neurons, which synthesize and release corticotropin-releasing hormone, arginine vasopressin, and other secretagogues, are responsible for a cascade of events which culminates in the adrenocorticotropin-induced release of corticosteroids from the adrenal cortex. Previous data have suggested complex afferent regulation of PVN neurons, although the neuronal pathways by which the effects of stress are mediated remain to be fully disclosed. The present experiment sought to identify forebrain areas potentially involved in afferent regulation of the PVN in response to an acute stressor. Discrete injections of the retrograde tracer Fluoro-gold were delivered to the PVN, and rats were subsequently subjected to an acute swim stress. Brains were processed immunocytochemically for the simultaneous detection of the tracer and Fos, the protein product of the immediate early gene c-fos, utilized as a marker for neuronal activation. The majority of Fluoro-gold/Fos labeled neurons were detected in the parastrial nucleus, the medial preoptic area, the anterior hypothalamic area, the dorsomedial hypothalamic nucleus and adjacent posterior hypothalamic area, and, to a lesser extent, the supramammillary nucleus. These findings are discussed in relation to neural pathways mediating activation and inhibition of the hypothalamic-pituitary-adrenocortical axis.