Significance of pulsatility in the HPA axis

A stress-free automated blood sampling system has been employed to demonstrate pulsatile hypothalamo-pituitary-adrenal (HPA) activity in the rat. In females, pulses of corticosterone secretion occur approximately once/hour throughout the 24 h cycle, with variation in pulse amplitude underlying a diurnal rhythm. Males show smaller pulses of secretion which become widely spaced during the early light phase nadir. Ageing does not affect the occurrence of pulses but the diurnal variation is lost. Analysis of the relationship between the HPA response to an acute noise stress and its coincidence with the various phases of the pulse, suggests that pulsatile activity arises from alternating periods of activation and suppression. Responses to i.v. corticotropin-releasing factor are not affected by pulse phase, indicating that this relationship is not generated at the pituitary-adrenal level. This phase relationship holds for all strains of rat except the hyperresponsive Fischer-344 in which an exaggerated stress response arises from a lack of phase-dependent suppression. Patterns of pulsatile activity are also modulated by neonatal programming or chronic HPA activation arising from adjuvant-induced arthritis, with consequent impact upon the response to acute stimuli. Thus, variations in the patterns of pulsatile activity are important determinants of both basal secretion and acute responses of the HPA axis.