The Effect of Plasma Cortisol Concentration on the Pituitary Adrenal Response Elicited by Electrical Stimulation of the Infundibular Area of the Awake Unrestrained Cat

A non stressful environment was established for awake unrestrained cats to test the excitability of infundibular area sites mediating ACTH release. Each subject was chronically implanted with bilateral electrodes and catheterized in the superior vena cava. Plasma corticosterone and cortisol concentration were determined by fluorometry after chromatographic separation. Samples were taken during both sham and electrical stimulation while the subjects, of their own volition, reclined in a tray. Control samples collected early in the 3-month experimental period showed cortisol values as high as the cortisol response to ACTH. Repetition of the control sampling procedure produced low cortisol levels. No discernible change in the cats’ overt behavior accompanied this decrease in cortisol. Electrical stimulation, which produced a rise in plasma cortisol, did not necessarily result in an overt behavioral reaction. When control cortisol values were high, stimulation was followed by no change or a decrease in cortisol concentration; when cortisol values were low it resulted in increased cortisol concentration. The excitatory influence of infundibular stimulation on cortisol was confirmed under barbiturate anesthesia. It is concluded that electrical stimulation of the infundibular area maximally elevates cortisol when control values are low. Repeated training lowers initially high cortisol levels.