The distribution of vasopressin (VP) and oxytocin (OT) neurons in the rat supraoptic (SON), paraventricular (PVN), and accessory magnocellular (AMN) nuclei was studied by localizing both peptides on the same section with a double immunocytochemical staining procedure employing specific monoclonal antibodies (MAB). This procedure allows us to visualize the distribution of one cell type relative to the other. In the rostral SON, VP cells lie dorsal and medial to the OT cells. Near the mid-point of the nucleus along its rostral-caudal length, there is a transition zone in which the two cell types are mixed. Proceeding caudalward, the relative locations of OT and VP cells are exchanged so that most of VP cells are located in the ventral and medial sector of the nucleus, whereas the OT cells are situated dorsal and lateral. However, there is no absolute segregation of the two types of cells anywhere in the nucleus. In the anterior part of the PVN a rostral group (rPVN) of cells composed of a medial portion and a lateral wing can be recognized. Nearly all of the cells in the rPVN are oxytocin-containing. The rPVN is separated from the next group, the middle PVN (mPVN), by a cell poor zone of about 100–150 µm. The mPVN contains both OT and VP neurons. As one proceeds caudally, the OT cells extend in the rostrocaudal direction from an anterior and ventromedial location, forming a shell around a core of VP neurons. In the most caudal PVN (cPVN), a triangular cell group characterized by fusiform cells with long-beaded processes can be distinguished from the more rounded cells of the remaining PVN. Many fusiform cells in the cPVN appear to send their axons to the posterior perifornical nucleus and the nucleus of the medial forebrain bundle. Other fusiform cells of the cPVN are oriented in a rostral-caudal plane and are situated more medially in this subdivision. The dendrites of these cells project into the mPVN while their posterior processes, most of which also appear to be dendrites, project caudally along a medial route.

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.

The hypothalamo-hypophysial system of the chick embryo has been studied with a monoclonal antibody which cross-reacts with arginine vasotocin and mesotocin, using thick (100 micron) sections in conjunction with a peroxidase-conjugated rabbit anti-mouse antibody. Although weakly stained perikarya occur occasionally in the tuberal region on embryonic days 6 and 7, the most consistent immunostaining of perikarya is found in the periventricular region of the caudal midhypothalamus at the level of the optic chiasm after embryonic day 8 1/2. Synthesis of peptides, therefore, takes place while the cells are close to their site of origin. Between embryonic days 9 and 10, beaded axons run along the anterior median eminence closely apposed to the adenohypophysis, thereby forming the anlage of the zona externa. The axons of the hypothalamo-neurohypophysial tract surround the neural lobe between embryonic days 11 and 12. The caudal to rostral wave of neuronal maturation that occurs during development appears to be due to a progressive differentiation of the periventricular zone, as well as the migration of perikarya. The early periventricular perikarya at embryonic day 8 1/2 send processes rostrally in a wing-shaped formation that extends both dorso- and ventrolaterally. From embryonic days 10 to 12, perikarya can be observed in the wing-like extensions, apparently migrating to rostral levels. The dorsolateral pathway gives rise at its midportion to the lateral cell group, whereas those perikarya migrating more laterally form the anlage of the external supraoptic nucleus. The ventrolateral wing-shaped extension of perikarya appears to be directed toward the ventral group and those lateral perikarya continuous with it. The location of mature neuronal cell groups is well established by embryonic day 17.

The hypothalamo-hypophysial system of the adult chicken has been studied with a monoclonal antibody that cross-reacts with arginine vasotocin and mesotocin. We have used this antibody on thick (100 micrometers) sections in conjunction with a peroxidase-conjugated rabbit antimouse antibody that permits the visualization not only of entire perikarya, but also of long portions of their axons and dendrites. Our results confirm older concepts based on classical methods, but the more sensitive immunocytochemical method reveals that the system is more extensive than previously recognized. Immunostained neurons in the chicken are widely scattered in the hypothalamus. In the rostral preoptic region, there are three immunostained neuronal cell groups: a prominent closely packed group that extends along the ventromedial surface, a diffusely distributed lateral group, and an external group that surrounds the lateral aspect of the septomesencephalic tract. Caudally in the preoptic area and in the anterior hypothalamus, the same groups are present; but there are also conspicuous periventricular perikarya. Many of them have processes that project to the lumen of the third ventricle, as well as parallel axons that arch lateroventrally in the hypothalamus. In the midhypothalamic area, the periventricular perikarya and processes are particularly numerous at the level of the pallial commissure. The dorsal periventricular group located at the level of the dorsomedial anterior nucleus of the thalamus are the most caudal perikarya. They extend laterally in a wing-like formation. The immunostained axons from all of these perikarya form a compact hypothalamo-hypophysial tract as they run from the mid-hypothalamus to the median eminence and converge beneath the third ventricle. Axons branching from this tract innervate the zone externa of the anterior median eminence; another group of axons running in the fibrous layer of the zona interna proceeds to the neural lobe.

Light microscopic studies in our laboratory have indicated that the lateral septum, amygdala, and ventral subiculum project in a perinuclear fashion to the paraventricular (PVN), supraoptic (SON), and suprachiasmatic (SCN) nuclei (Oldfield et al., '82; Silverman and Oldfield, '84). In the present paper a combined anterograde HRP and immunocytochemical procedure has been used to determine the connectivity between these limbic efferents and peptide-containing processes emanating from the above mentioned hypothalamic nuclei. Synaptic associations were found to exist between efferents from (1) the septum and both vasopressin (VP)- and oxytocin (OX)-positive dendrites derived from cells in the PVN and SON, (2) the septum and VP dendrites dorsal to the SCN, (3) the ventral subiculum and both VP and OX dendrites arising from the PVN and SON, and (iv) the amygdala and VP dendrites from the PVN. These observations help clarify an apparent discrepancy between electrophysiological data, in which limbic efferents have been shown to influence the activity of VP and OX neurons in the PVN and SON, and anatomical evidence which indicates only a perinuclear innervation from these sites not encroaching on the hypothalamic nuclei themselves. In each case the synaptic connections are made on dendrites external to the nucleus: those lateral and ventrolateral to the PVN, dorsal to the SON, and dorsal or dorsolateral to the SCN.