Endocrine and Behavioural Responses to Noise Stress:Comparison of Virgin and Lactating Female Ratsduring Non‐Disrupted Maternal Activity

Abstract

The behavioural and endocrine responses to a 10 min white noise stress have been characterized in female virgin and undisturbed lactating Sprague-Dawley rats. Animals were continuously video-taped and frequent blood samples were collected using an automated sampling system. Noise stress caused hypothalamo-pituitary-adrenal (HPA) activation, as indicated by a rapid increase in plasma corticosterone and ACTH in the virgins: corticosterone concentrations peaked 20 min after initiation of the stress before declining rapidly back to basal concentrations. In contrast, noise stress had no significant effect on either plasma corticosterone or ACTH concentrations in the lactating animals. However, 72 h after weaning the corticosterone response of the ex-lactating rats was of comparable magnitude, but longer duration to that seen in the virgins. Plasma prolactin concentrations were significantly higher in the lactating animals and declined in response to the noise whereas, a transient but reproducible increase was seen in the virgin group. In situ hybridization revealed a significantly lower basal expression of CRF mRNA in the paraventricular nucleus of lactating rats as compared to the virgins, but noise stress had no further effect. Virgin animals showed behavioural responses to the stress, including an increase in the total activity, exploratory behaviours (rearing) and displacement behaviours (grooming). Lactating animals also showed behavioural responses to the noise, but their activities were principally directed towards the pups. These data show that although lactating rats showed normal behavioural reactivity to a psychological stress they showed no statistically significant activation of the HPA axis, suggesting a dissociation of behavioural and neuroendocrine responses to this mild stress.

Physiological Roles of Prolactin in the Adrenocortical Response to Acute Restraint Stress

“…In accordance with the present results suggesting that direct adrenal effects of PRL may be important for steroidogenesis in the response to stress, there are observations from animal studies in which glucocorticoid levels can remain elevated during stress, despite de- clining CRH and/or ACTH concentrations [20,21,[40][41][42]. Previous studies have demonstrated that exercise stress induced an increase in circulating PRL in humans [43][44][45][46][47][48].…”

Oxytocin, vasopressin, and human social behavior

“…Numerous studies on the inhibitory influence of OT on stress-responsive neurohormonal systems focused on the endogenous stimulation of OT during lactation in rodents. The suckling stimulus by the newborn was found to increase OT release and decrease basal plasma levels of ACTH and cortisol [26,27,121,148,149,160].…”

Social modulation of stress responses

“…Lactation also blunts HPA axis reactivity to several stressors in rats including white noise [78], elevated plusmaze [79], forced swimming [79,80], acute cold exposure [81], and lipopolysaccharide injection [82]. Furthermore, corticosteroid response to ether stress is inversely related to litter size [83].…”