The regulation of ACTH secretion during stress is a multifactorial process that mainly involves two hypothalamic neurohormones: corticotrophinreleasing factor (CRF) and arginine vasopressin (AVP). In this report we measured, using semiquantitative in situ hybridization, the concentrations of CRF and AVP mRNA in hypophyseotrophic paraventricular parvocellular cell bodies of male rats after an acute (3-h) exposure to insulin-induced hypoglycaemia.
Insulin injection (2·5 IU/kg) induced a significant decrease in blood glucose levels and a strong increase in plasma ACTH concentrations. The concentration of CRF mRNA in the paraventricular nucleus (PVN) was significantly increased after insulin-induced hypoglycaemia (150% of control levels), while the number of CRF mRNA-containing cell bodies was not changed. Double-labelling experiments demonstrated that the number of CRF mRNA-containing cell bodies that also contained AVP mRNA was doubled after insulin injection. These data demonstrate that the established increased colocalization of AVP immunoreactivity in nerve terminals immunoreactive for CRF after exposure to stress follows a pretranslational activation of AVP synthesis.
Cell-by-cell analysis indicated that the mean CRF hybridization signal was increased in doublelabelled cells (about 150% of control levels), suggesting that the increase in CRF gene expression occurs equally in the AVP-synthesizing and in the AVP-deficient CRF mRNA-containing cell bodies. The mean AVP hybridization signal in the double-labelled cells was decreased, suggesting that the amount of AVP mRNA was unchanged in the cell bodies that expressed both CRF and AVP in the basal state and that AVP mRNA levels in the cell bodies recruited after insulin-induced hypoglycaemia were below control values.
Taken together, these observations suggest that the hypophyseotrophic neurones of the PVN adapt to an acute stress situation by increasing CRF gene transcription in the whole population of CRF-synthesizing cells and by increasing AVP gene transcription in a silent population of CRF-synthesizing cells. Such changes may lead to increased peptide synthesis in response to increased functional demand.