Neuropeptide Cells and Fibers in the Hypothalamus and Pituitary of the Fetal Sheep: Comparison of Oxytocin and Arginine Vasopressin

Abstract

Both oxytocin (OXY) and arginine vasopressin (AVP) enhance the effects of corticotropin-releasing factor on ACTH release by the pituitary. One of these, AVP, plays a role in the control of fluid balance and responses to hypoxemic stress in the fetal sheep. To determine the possibility that OXY also participates in fetal neuroendocrine events, OXY-containing neuronal structures must first be demonstrated within the fetal endocrine hypothalamus. OXY-immunoreactive elements were examined in fetal sheep hypothalami late in gestation and compared to AVP-containing structures using immunocytochemical procedures. Six fetal sheep ranging from 126 to 144 days gestational age were delivered via cesarian section from timed pregnant Rambouillet-Columbia ewes and killed by an overdose of anesthesia. The fetal head was perfused via bilateral carotid catheters and processed for immunocytochemical localization of OXY or AVP using the avidin-biotin complex procedure. At all fetal ages examined, OXY- and AVP-containing neurons were found within the paraventricular nuclei (PVN), supraoptic nuclei (SON) and accessory magnocellular hypothalamic nuclei. OXY-containing neurons were found principally in the SON and PVN. They were generally less numerous and less intensely stained than the AVP neurons. In the SON, they concentrated along the dorsal borders of the nucleus above the AVP neurons. In PVN, clusters of OXY cells were located along the dorsal and lateral borders of the nucleus surrounding the AVP neurons; in the periventricular division, they were intermingled with the AVP neurons. Small numbers of OXYaxons were located in the external zone of the median eminence; whereas most OXY axons extended into the hypothalamo-neurohypophyseal tract and posterior lobe of the pituitary. A few of the OXY axons in the pituitary stalk were diverted to the pars intermedia. Likewise, some of the OXY fibers from the external zone of the median eminence entered the pars tuberalis but were rarely found in the distal lobe of the pituitary. In contrast, AVP axons richly innervated the external zone of the median eminence, and neural lobe. Like OXY, AVP axons from the median eminence and the pituitary stalk sent projections to the adenohypophysis. AVP fibers in the pars distalis frequently contacted corticotropes and were more numerous than OXY fibers in this region. These data provide anatomical evidence that OXY and AVP may directly regulate the fetal adenohypophysis. Of these two neuropeptides, AVP predominates anatomically.

Evolution of oxytocin pathways in the brain of vertebrates

“…However, the density of such contacts is gradually decreased in evolution. In fact, although we were able to detect a few examples of such contacts in adult rats (see Figure 4), only one paper reports on their presence in amniots: i.e., in the adenohypophysis of fetal sheep (Hoffman et al, 1989). The paracrine action of OT on pituitary cells (Hoffman et al, 1989) may occur during mammalian embryogenesis in the immature portal blood system.…”

“…In fact, although we were able to detect a few examples of such contacts in adult rats (see Figure 4), only one paper reports on their presence in amniots: i.e., in the adenohypophysis of fetal sheep (Hoffman et al, 1989). The paracrine action of OT on pituitary cells (Hoffman et al, 1989) may occur during mammalian embryogenesis in the immature portal blood system. In general, the regress of direct axo-adenar contacts during evolution may parallel the process of anatomical separation of adenohypophysis and neurohypophysis by septal connective tissue (Enemar, 1960) and coincides with the development of an effective portal blood system from reptiles onwards (Enemar, 1960).…”

Levels of pro-opiomelanocortin and prolactin mRNA in the fetal sheep pituitary following hypoxaemia and glucocorticoid treatment in late gestation

“…It is likely that the increase in CRH mRNA and the smaller increase in AVP mRNA in the parvocellular paraventricular nucleus following hypoxaemia (6 h) are associated with increased translation to CRH and AVP (Hoffman et al 1989, Curne & Brooks 1992, which are then released at the median eminence (ME) (Brooks et al 1989) into the hypophysial portal circulation. The hypophysial portal vessels are patent in late gestation (Levidiotis et al 1989) and though there is no information available on the content or ratio of CRH and AVP within the ovine fetal hypophysial circulation, stress is a powerful stimulus for CRH and AVP release at the ME in adult sheep (Engler et al 1990).…”

cFos Activity Identifies Recruitment of Luteinizing Hormone‐Releasing Hormone Neurons During the Ascending Phase of the Proestrous Luteinizing Hormone Surge

“…a markcr for catecholaniinc neurons. inncrvate both LHRH (43)(44)(45)(46) and GABA (46) ncurons in thc medial preoptic arca. although the precise identity ofthe type of catccholaminergic neuron (norcpinephrine, epinephrine or dopamine) remains unclear.…”