CENTRAL NERVOUS-SYSTEM MEDIATION OF PITUITARY-ADRENAL RHYTHMICITY

Evidence is presented favoring the view that there is a dissociation between the 2 aspects of pituitary adrenal operation, the stress and the rhythm modes. It is possible to abolish rhythmic function by anterior hypothalamic lesions without altering the responses to stress. During maturation of the rat CNS, the stress response appears prior to weaning while pituitary-adrenal rhythm appears after weaning. Appropriate neuropharmacologic treatment can block circadian rhythm without altering the stress response. Responsiveness to ether or immobilization may be independent of the diurnal rhythm. The stress response to a very strong stimulus, such as immobilization, habituates while the circadian rhythm does not. On the other hand there is some evidence in favor of interdependence between the 2 modes of operation: input channels for stress and rhythm may overlap since strong synchronizers of rhythm, such as food and water restriction and light can become stress stimuli. Steroid feedback can alter the timing of the circadian rhythm. Circadian variation was observed in the responses to certain stimuli. Neurotransmitters which are prominently implicated in the circadian rhythm also appear involved in the stress response.