Effects of Chronic Intracerebroventricular GABAergic Treatment on Basal and Chronic or Acute Stress-Induced Adrenocortical Activities in the Thalamic Pigeon

Abstract

Muscimol was chronically administrated to the third ventricle of thalamic pigeons by means of osmotic minipumps at the rate of 0.25 µg·h^–1 for 28 days. No abnormal behavioural sign was noted. The animals were subjected daily to chronic intermittent stress for the same 28-day period. Basal and stress-induced adrenocortical activities were evaluated by recording serial plasma corticosterone levels at the end of the experimental session. Untreated controls exhibited both components of the adaptation of the adrenocortical response to chronic stress: (1) attenuation, i.e., a decrease in magnitude and disappearance of the late rebounding phenomenon, and (2) anticipation, i.e., the occurrence of a conditioned component before stress itself. The adaptation to chronic stress was partly impaired by GABAergic treatment. The anticipatory conditioned peak subsisted but the magnitude of the post-stress peak was found not to be reduced whereas rebounding events were suppressed after chronic as well as acute stress. The basal resting levels of corticosterone were significantly lowered in muscimol-treated animals. A lesion placement in the anterior dorsomedial thalamus (ADMT) resulted in the same profile of stress-induced plasma corticosterone levels as seen after muscimol administration. Adaptation did not develop in ADMT animals and GABAergic stimulation, either acute or chronic, had no effect on their response to stress.