Corticotropin-releasing factor synapses within the paraventricular nucleus of the hypothalamus

Corticotropin-releasing factor (CRF) regulates the release of adrenocorticotropin (ACTH) from the anterior pituitary and these neurosecretory neurons reside in the paraventricular nucleus of the hypothalamus (PVN). In addition to its role as an ACTH secretogogue, exogenously administered CRF can act centrally to modify sympathetic outflow, alter various stress-induced behaviors and modulate its own secretion. Some of these effects might be mediated by CRF acting synaptically within the PVN as the nucleus is known to play a major role in integration of autonomic function. The current ultrastructural immunocytochemical study was designed to examine the range of synaptic relationships that CRF terminals make within the PVN. CRF-positive synapses were numerous, particularly in the periventricular zone. The majority of terminals formed axo-dendritic synapses and of these over 85% were the Gray's type II (symmetrical) class. Axo-somatic terminals were also encountered and both parvicellular and magnocellular neurons were innervated. Once again most of the terminals were Gray's type II. Although an innervation of unidentified structures was the most common, CRF synapses onto CRF neurons and dendrites were observed. All CRF/CRF interactions had symmetrical membrane specializations. These studies indicate that CRF could play a prominent role in the modulation of both parvicellular and magnocellular neurons within the paraventricular nucleus, including modulation of its own neurosecretory activity.