Role of Brain Serotonin in the Circadian Rhythm of Corticosterone Secretion and the Corticotropic Response to Adrenalectomy in the Rat

Pharmacological and neurosurgical approaches were used to determine whether central serotoninergic systems are essential to the diurnal fluctuation of plasma corticosterone (B) and the enhanced secretivity of serotonin (A) in mice.

Abstract

Pharmacological and neurosurgical approaches were used to determine whether central serotoninergic systems are essential to the diurnal fluctuation of plasma corticosterone (B) and the enhanced secretion of ACTH induced by adrenalectomy. Normal and 3-week adrenalectomized (ADX) male rats were either acutely treated 24 h before sacrifice with parachlorophenylalanine methyl ester (PCPA, 250 mg/kg, i.p. injection), or were subjected to lesions of the nuclei centralis superior and raphe dorsalis, chief sites of serotonin(5-HT)-containing neurons. PCPA treatment produced a significant decrease in the 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels in the hypothalamus and dorsal hippocampus but did not abolish the plasma B rhythm. Lesions of the raphe nuclei did not abolish the diurnal rhythm of the steroid, either; they were followed, however, by a significant decrease in the 5-HT concentration of both brain structures, whereas no change was recorded in the noradrenergic (NE) level of either structure. Bilateral adrenalectomy induced a marked increase in the 5-HT concentration of the dorsal hippocampus. In the hypothalamus, the 5-HT level remained unaffected, whereas a slight increase in NE content was recorded. In ADX animals, raphe lesions significantly decreased the 5-HT concentration of the hypothalamus and dorsal hippocampus. No modification