Inhibition of Luteinizing Hormone Secretion and Expression of c‐ fos and Corticotrophin‐Releasing Factor Genes in the Paraventricular Nucleus During Insulin‐Induced Hypoglycaemia in Sheep

Insulin can act within the brain to stimulate ovine luteinizing hormone (LH) secretion, but insulin‐induced hypoglycaemia inhibits LH via unknown brain sites, possibly involving corticotrophin‐releasing factor (CRF). Castrate male sheep, with (E+) or without (E−) subcutaneous oestradiol implants, were blood sampled every 12 min for 8 h. Insulin (0.25 or 0.5 IU/kg) was injected at 4 h via the carotid artery or jugular vein. All treatments reduced LH output with no differences between dose rate nor route of administration, but sensitivity was greater in E+ than E−sheep. There was no evidence for an effect of insulin on LH 0–1 h postinjection; however, 1–3 h after insulin, when hypoglycaemia was established, LH pulses were inhibited in both E+ and E− sheep (P<0.001). Additional intravenous (i.v.) glucose injections given 1 h (20 mmol) and 2 h (10 mmol) after insulin (0.5 IU/kg) were each followed by an LH pulse within 30 min (75% response in both E+ and E− sheep). In a separate experiment, sheep were killed 2 h after i.v. insulin (0.5 IU/kg) or saline. In‐situ hybridization revealed c‐ fos mRNA in the paraventricular nucleus (PVN), but not in any other hypothalamic nuclei nor in the hindbrain; and this was linked with increased CRF gene expression in the PVN. Similar c‐ fos and CRF gene expression was seen in insulin‐treated sheep given additional i.v. glucose (20 and 10 mmol, respectively, 40 and 20 min ante mortem ), but not in saline‐treated controls. Therefore, insulin‐induced hypoglycaemia inhibited LH secretion, with oestradiol potentiating the effect, and was associated with gonadal steroid‐independent c‐ fos gene expression and increased CRF gene expression in the PVN. The ovine PVN may be involved in mediating insulin‐induced hypoglycaemic inhibition of LH by a mechanism which might involve CRF.