Effects of various stressors on milk release in the rat

This study investigated the effect of four stimuli on milk release (MR), namely, sound, nociception, novelty, and restraint. The role of the ensuing adrenocortical response in the suppression of MR was also evaluated. Plasma corticosterone (CORT) levels were measured at 0 (basal), 15, 30, and 60 min during the suckling sessions to determine whether elevated CORT normally associated with stress could be inhibitory to MR. Compared to nonstressed lactators, dams exposed to sound demonstrated no suppression in MR, but a significant increase in plasma CORT. Pain did not alter milk yield and elevated CORT only at the end of the first hour of exposure. During novelty, MR was suppressed and again CORT was only elevated at the end of the sampling period. Restraint decreased milk yield and increased CORT. During novelty, MR appeared to be regulated by an adrenal factor, which remains to be identified. The peripheral opiates seem to be partially involved during restraint. In conclusion, not all types of aversive stimuli interfere with MR. Of those which do, different mechanisms seem to be implicated depending upon the nature of the stressor. Furthermore, reduced MR during stress is not a direct consequence of increased CORT.