ONTOGENY OF ARGININE VASOPRESSIN-IMMUNOREACTIVE NEURONS IN THE HYPOTHALAMUS OF FETAL AND NEWBORN SHEEP

Arginine vasopressin (AVP) is a peptide hormone which is found in neur ons within the paraventricular (PVN) and the supraoptic (SON) nuclei o f the hypothalamus. Ln fetal sheep, this neuropeptide is involved in m aturational processes and adaptive responses to 'stress'. This study e xamined the effect of age on the total number and distribution of AVP- containing neurons in the PVN and SON of fetal sheep and newborn lambs by quantitative light-microscopic immuno-cytochemistry. Serial corona l sections of hypothalami from three groups of animals were studied: f etuses at 104-109 days of gestation (n = 6) comprising the early group , fetuses at 130-139 days of gestation (n = 5) comprising the late gro up and newborn lambs at 12-20 postnatal days (n = 5) comprising the ne onatal group. This period of development was chosen since adaptive mec hanisms to stress are operative at or near the time of birth. Hypothal amic dimensions were measured to determine if maturation had an effect on the size of the AVP-containing subregions of the hypothalamus duri ng this period of development. Dimensions included: ventricle height, optic chiasm width, distances from the dorsal margin of the ventricle to the lateral and medial margins of the optic tract, and distance bet ween the medial margins of the optic tracts. As expected, with increas e in maturational age, overall dimensions of the AVP-containing subreg ions increased significantly (p < 0.05). When early- and late-gestatio n fetuses were compared to newborn lambs, there was a significant incr ease in the total number of immunoreactive neurons in both the PVN (p < 0.01, Anova) and SON (p < 0.001, Anova) with age. With advancing age , we also observed an increase in the density of AVP neurons in the mi ddle subregion of the PVN and in the midrostral subregion of the SON. These data suggest that, during the late gestational and early postnat al period, de novo synthesis of AVP genes occurs in these hypothalamic nuclei. This study provides a baseline for further investigation to s tudy the effects of stress on these neurons in the developing ovine fe tus and newborn lamb.