Fig. 9.

Schematic representation of the hypothalamic neuroendocrine loop completed by AR in the ventrolateral SCN core. The SCN projects to anterior hypothalamic neurosecretory cells, which project to the anterior pituitary (63). Pituitary tropic hormones are released into the general circulation to reach the gonads. In the male, testosterone in the blood reaches the SCN, binding to AR within the SCN, serving as an endocrine afferent input. Neural inputs from the retinohypothalamic tract (RHT), intergeniculate leaflet (IGL), and raphe nucleus also reach cells of the ventrolateral core SCN. Thus, endocrine and neural signals interact within the SCN to modulate circadian timing in the system.