Hypothalamic involvement in the activation of the pituitary-adrenocortical axis by nerve growth factor

Intravenous injection of nerve growth factor (NGF) into rats produces a dose-dependent (from 0.1 to 5 nmol/kg) increase in circulating concentrations of adrenocorticotropin (ACTH) and corticosterone. We have investigated whether this effect is produced through a direct action on a component of the hypothalamo-pituitary-adrenocortical axis. NGF (50 and 500 nM), added to the incubation medium of in vitro isolated pituitary segments or dispersed adrenal cells, did not modify either basal and stimulated release of biologically active or immunoreactive ACTH or release of corticosterone, respectively. The presence of NGF in the incubation medium of in vitro isolated hypothalami produced a dose-dependent (from 150 to 600 nM) increase of both release and content of some material with corticotropin-releasing bioactivity. The nature of this corticotropin-releasing bioactivity was determined directly by radioimmunoassays. Results have indicated that NGF induced an increase of both release and content of hypothalamic arginine-vasopressin (AVP), while no changes were observed in the release and content of hypothalamic corticotropin-releasing hormone (CRH). These results suggest that adrenocortical stimulation by NGF in vivo could be mediated by the release of hypothalamic AVP rather than CRH. The finding that in vivo NGF stimulatory effect was not abolished by the specific CRH antagonist alpha-helical CRH(9-41), while it was accompanied by an increase in circulating AVP levels, supports this interpretation. However, the fact that the hypothalamus is stimulated in vitro by NGF concentrations higher than those expected to reach this structure after systemic injection of active doses raises the possibility that other brain areas such as the hippocampus participate in NGF-induced adrenocortical activation.