ESTROGEN-RECEPTIVE NEURONS IN THE ANTEROVENTRAL PERIVENTRICULAR NUCLEUS ARE SYNAPTIC TARGETS OF THE SUPRACHIASMATIC NUCLEUS AND PERI-SUPRACHIASMATIC REGION

The anteroventral periventricular nucleus (AVPv) in the rat preoptic a rea is a key site underlying control of the steroid dependent preovula tory gonadotropin surge. Estrogen and progesterone receptor-containing neurons in the preoptic/hypothalamic continuum, particularly those in the AVPv, are believed to transduce steroidal signals and, in turn co nvey this information to the LHRH system, which lacks steroid receptor s. In addition to the influence of the gonadal steroids, the precise t iming of the preovulatory gonadotropin surge is believed to be regulat ed by the hypothalamic suprachiasmatic nucleus (SCN). The SCN and peri -SCN neurons send efferent projections rostrally to the anterior preop tic area suggesting that circadian signals are communicated synaptical ly to steroid-responsive neurons in the AVPv. To test this hypothesis, ultrastructural double label immunocytochemistry was conducted to det ermine whether SCN efferents contact estrogen receptor-immunoreactive neurons in the AVPv. Brain sections with SCN injections of phaseolus v ulgaris leucoagglutinin (PHA-L) were immunostained for estrogen recept ors and PHA-L. Light and electron microscopic data show that the anter ior preoptic area received robust PHA-L-immunoreactive efferents from SCN neurons and immediately adjacent subparaventricular zone. In parti cular, the AVPv contained abundant labeled fibers and terminal boutons . Ultrastructurally, SCN- and subparaventricular zone-derived terminal s synaptically contacted the perikaryon of many estrogen receptor-immu noreactive neurons in the AVPv. The perikarya of unlabeled neurons wer e also contacted, but the majority of the labeled contacts were observ ed upon neuronal processes. These results demonstrate that estrogen re sponsive AVPv neurons are regulated by SCN efferents. Furthermore, the present data provide strong support to the idea of collective control of pituitary gonadotropin release by steroid sensitive and circadian signal neural pathways.