ACTH and Corticosterone Secretion in Rats following Removal of the Neurointermediate Lobe of the Pituitary Gland

Surgical removal of the neurointermediate lobe of the pituitary gland (NIL-X) in the rat resulted in two abnormalities in ACTH secretion: (1) plasma ACTH and corticosterone levels were elevated in the morning and over a 24-hour period compared with levels in control (NIL-C) rats, and (2) although NIL-X and NIL-C rats had acute increases in plasma ACTH and corticosterone of equal magnitude after interoceptive stimuli (hemorrhage, surgery), NIL-X rats demonstrated markedly smaller elevations in plasma levels of these hormones after neurotropic stimuli (noise, novel environment). This subnormal adrenocortical response of NIL-X rats was not due to an impairment in perception of a neurotropic stimulus; these rats had normal latencies to paw licking and to jumping off a heated surface, yet smaller increases in plasma corticosterone after the stimulus. The impairment in ACTH response was not related to stimulus intensity, as NIL-X and NIL-C rats had equal ACTH and corticosterone secretion during both low and high levels of insulin-induced hypoglycemia. NIL-X rats demonstrated a significant elevation in daily water intake, although hematocrit, plasma Na+, K+, osmolality and protein were normal. Significant diurnal rhythms in plasma corticosterone levels and in water intake were maintained as well. The elevated morning plasma ACTH levels, the blunted hormone increases after noise, and the increase in water intake persisted in NIL-X rats 2 months after surgery. These data indicate that removal of the NIL results in (1) chronic elevations in basal ACTH and corticosterone secretion, and (2) chronic impairment in adrenocortical responses to neurotropic stimuli, but not to interoceptive stimuli. The deficit is not due to impaired perceptual capacity nor to the intensity of the stimulus.