An investigation of serotonergic involvement in the regulation of ACTH andcorticosterone in the olfactory bulbectomized rats

The bilateral olfactory bulbectomy resulted in significantly higher plasma concentration of corticosterone, but not of ACTH in basal conditions and mu ch higher plasma ACTH and corticosterone concentrations after 15 min of imm obilization stress than were observed in sham-operated animals. Daily treat ment with fluoxetine-a specific serotonin reuptake inhibitor-(15 mg/kg/day) had no effect on basal ACTH and corticosterone concentrations in OB rats. Fluoxetine treatment caused lower levels of ACTH, but not of corticosterone secretion, in response to immobilization stress. Bulbectomy significantly reducing 5-HT concentration in the amygdala. Stress increased serotonergic activity in the hypothalamus but not in the amygdala of OB rats. Chronic fl uoxetine treatment of both unstressed and stressed OB rats resulted in a lo wer turnover rate in the two structures. Our results suggest that the hyper corticosteronemia observed after bulbectomy in unstressed OB rats is indepe ndent of the serotonergic system in both hypothalamus and amygdala. In cont rast, they also demonstrate hypothalamic 5-HT changes in the HPA hyperactiv ity of OB rats in response to stress. Chronic fluoxetine treatment may norm alize pituitary ACTH secretion in response to stress, possibly desensitizat ion of the 5-HT receptors in the hypothalamus due to 5-HT being move availa ble at the synapses. (C) 1999 Elsevier Science Inc.