Inhibition in corticotrophin and corticosterone secretion following photic stimulation in rats with 6‐hydroxydopamine injection into the medial forebrain bundle

Changes in corticotrophin (ACTH) and corticosterone (CS) serum levels, were examined in rats in which either 6‐hydroxydopamine or vehicle were injected into the medial forebrain bundle (MFB) and which were sacrificed by decapitation 14 days later. The injection of the neurotoxin, which depleted very significantly the norepinephrine (NE) content of the mediobasal hypothalamus, elevated significantly the basal levels of serum ACTH and CS, but inhibited the rise in the levels of these hormones following photic stimulation. These results would suggest that the hypothalamic NE system has normally an inhibitory effect on basal ACTH and CS secretion and that the hormonal response to photic stimulation is mediated by that system and possibly by NE MFB fibers.