Failure of bilateral paraventricular nuclear lesions to cause hypothalamic hypothyroidism in fetal sheep

Gestational hypoxia modulates expression of corticotropin-releasing hormone and arginine vasopressin in the paraventricular nucleus in the ovine fetus

Maturation of the fetal hypothalamo–pituitary–adrenocortical (HPA) axis is critical for organ maturation necessary for the fetus to transition to the ex‐utero environment. Intrauterine stressors can hasten maturation of the HPA axis leading to fetal growth restriction and in sheep, premature birth. We have previously reported that high‐altitude mediated, long‐term‐moderate gestational hypoxia (LTH) during gestation has a significant impact on the fetal HPA axis. Significant effects were observed at the level of both the anterior pituitary and adrenal cortex resulting in elevated plasma ACTH during late gestation with decreased adrenocortical expression of enzymes rate limiting for cortisol synthesis. As such, these fetuses exhibited the normal ontogenic rise in fetal plasma cortisol but an exaggerated cortisol response to acute stress. This study extended these findings to ACTH secretagogue expression in the PVN using in situ hybridization. We report that the expression of AVP but not CRH was increased in the medial parvocellular PVN (mpPVN) in the LTH fetus. This represented an increase in both AVP mRNA per neuron as well as an increase in AVP hybridizing neurons with no increase in mpPVN CRH neurons. LTH had no effect on PVN volume, area of CRH or AVP hybridization, thus LTH did not have a trophic effect on the size of the nucleus. In conclusion, there appears to be a switch from CRH to AVP as a primary ACTH secretagogue in response to LTH, supporting our previous findings of increased anterior pituitary sensitivity to AVP over CRH in the LTH fetus.

Corticotropin-releasing hormone-like axons in the adrenal glands of fetal and postnatal sheep

The presence of corticotropin-releasing hormone outside of the brain suggests that it may have functional roles besides the stimulation of pituitary adrenocorticotropin. The purpose of the present study was to determine immunohistochemically, the ontogeny of corticotropin-releasing hormone-like immunoreactivity in sheep adrenals from fetal day 100 to adult. Adrenal corticotropin-releasing hormone-like immunoreactivity: 1) was found in all animals examined, 2) existed as clusters of darkly beaded axon-like fibers over medullary cells at the medullary-cortical interface or over islands of medullary cells surrounded by cortex or rays of medullary cells extending into the cortex, 3) showed a definite increase with age, 4) was depleted in newborns < 12 h old, and 5) was found in pre- and postnatal splanchnic nerves. In conclusion, corticotropin-releasing hormone-like immunopositive fibers are present in the ovine adrenal medulla at various stages of development, supporting the concept that corticotropin-releasing hormone in pre- and postnatal sheep may play a direct local part in the regulation of adrenal function.

Distribution of calbindin-D28K in the brain of the fetal sheep in late gestation

Immunocytochemical distribution of calbindin-D28K was examined in the brains of fetal sheep at 100-105 and 120-125 days of gestation (dGA) and at term in labor (term approximately 150 dGA) and compared to adult sheep. Brains were perfused, cut frozen and immunostained as freely floating sections with a polyclonal anti-calbindin-D28K antibody. Light microscopy revealed that calbindin-D28K immunogenicity in fetal cerebellum, rhombencephalon, mesencephalon, diencephalon and telencephalon was overall very similar to that seen in the brains of adult sheep. In addition even though fetal sheep peripheral plasma cortisol concentrations increase 10-20 times over the last 3-4 weeks of gestation to peak during labor and delivery, no glucocorticoid dependent increases in calbindin-D28K in the hippocampal formation could be seen in the sheep fetus as have been demonstrated for the adult rat given exogenous glucocorticoids. It is concluded that: (1) the basic pattern of calbindin-D28K distribution seen in adult sheep brains is established by at least 100 dGA in the sheep fetus; and (2) the pattern of calbindin-D28K expression in fetal sheep hippocampal formation is not related to peripheral plasma glucocorticoid concentrations.