Does the nucleus raphes participate in the regulation of resting and stress-induced hypothalamic-pituitary-adrenocortical activity in the pigeon?

Extensive multiple electrolytic lesions were placed into the nucleus raphes of the brain stem in the pigeon. Diurnal pituitary-adrenocortical rhythmicity appeared not to be altered and basal plasma corticosterone level remained quite normal in raphe lesioned birds. Electrical stimulations through permanently implanted electrode were delivered in various central nervous structures in unanaesthetized, freely moving pigeons. Stimulations of nucleus raphes and of various parts of formatio reticularis led to a significant rise in plasma corticosterone within 16 to 19 min after the beginning of the stimulating session. Then, plasma B came again to initial level within 15 minutes. Stimulations of the corticotropic area of the hypothalamus (n. posterior medialis hypothalami) and of archistriatum dorsalis induced an early plasma corticosterone increase occurring immediately after the stimulating burst (10 min). Stimulating the n. septum medialis also had an immediate, but reverse (decrease) effect on plasma corticosterone level. Stress-induced pituitary-adrenal cortical activation exhibited a temporal pattern quite similar to that observed after brain stem (n. raphes or formatio reticularis) stimulation. It is suggested that these various limbic and brain stem areas might be involved in some "limbic system-midbrain circuit" with two components : The forebrain component might be involved in the regulation and diurnal modulation of basal hypothalamic-pituitary-adrenocortical function, the brain-stem component interferring with stress-induced responses.