INSULIN-INDUCED HYPOGLYCEMIA INCREASES COLOCALIZATION OF CORTICOTROPIN-RELEASING FACTOR AND ARGININE-VASOPRESSIN MESSENGER-RNAS IN THE RAT HYPOTHALAMIC PARAVENTRICULAR NUCLEUS

The regulation of ACTH secretion during stress is a multifactorial pro cess that mainly involves two hypothalamic neurohormones: corticotroph in-releasing factor (CRF) and arginine vasopressin (AVP). In this repo rt we measured, using semiquantitative in situ hybridization, the conc entrations of CRF and AVP mRNA in hypophyseotrophic paraventricular pa rvocellular 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 para ventricular nucleus (PVN) was significantly increased after insulin-in duced hypoglycaemia (150% of control levels), while the number of CRF mRNA-containing cell bodies was not changed. Double-labelling experime nts demonstrated that the number of CRF mRNA-containing cell bodies th at 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 expos ure to stress follows a pretranslational activation of AVP synthesis. Cell-by-cell analysis indicated that the mean CRF hybridization signal was increased in double-labelled cells (about 150% of control levels) , suggesting that the increase in CRF gene expression occurs equally i n the AVP-synthesizing and in the AVP-deficient CRF mRNA-containing ce ll bodies. The mean AVP hybridization signal in the double-labelled ce lls was decreased, suggesting that the amount of AVP mRNA was unchange d in the cell bodies that expressed both CRF and AVP in the basal stat e 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 a dapt to an acute stress situation by increasing CRF gene transcription in the whole population of CRF-synthesizing cells and by increasing A VP gene transcription in a silent population of CRF-synthesizing cells . Such changes may lead to increased peptide synthesis in response to increased functional demand.