Analysis of Pituitary Prolactin and Adrenocortical Response to Ether, Formalin or Restraint in Lactating Rats: Rise in Corticosterone, but No Increase in Plasma Prolactin Levels after Exposure to Stress

Abstract

It is well established that stress causes a rise in plasma prolactin (PRL) levels of male or cycling female rats. In lactating animals, the pituitary PRL response to stress is not well understood. Therefore, the purpose of the present study was to analyze this question in lactating rats having low or elevated prestress plasma PRL levels. The animals were exposed to ether, formalin or restraint, and plasma PRL and corticosterone levels were determined. In mothers continually together with their pups, plasma PRL levels decreased significantly after exposure to ether vapor or injection of formalin under the skin. At the same time, both agents caused a significant rise in blood corticosterone concentrations. Lactating rats isolated for 4 h had very low levels of PRL before application of stress. However, neither formalin nor restraint caused any elevation in their plasma PRL levels although both interventions increased blood corticosterone concentrations. Lactating mothers receiving formalin after a 30-min suckling stimulus preceded by 4 h isolation did not show appreciable changes in pituitary PRL secretion following the administration of formalin. For information on the mechanism of the effect of stress on PRL, lactating rats were pretreated with the dopamine receptor antagonist domperidone (injecting 80 µg/kg body weight) or were adrenalectomized 7 days prior to exposure to stress. The very high levels of PRL caused by domperidone decreased markedly in animals subjected to restraint stress. Administration of formalin to adrenalectomized lactating rats continually together with their litter caused a slight immediate decrease, followed by a transitory elevation and a subsequent small second decrease in blood PRL concentration. The depression was significantly less than in nonadrenalectomized animals receiving formalin. If formalin was administered 2 days after isolation of the mothers it caused an elevation of plasma PRL concentrations indicating that the PRL stress response pattern characteristic of nonlactating animals returns within 48 h of isolation. On the basis of the present findings we conclude that in lactating animals (a) stress causes an inhibition of PRL release from the pituitary; (b) the pituitary-adrenocortical system responds to stress, (c) the inhibition of stress-induced pituitary PRL release disappears within 48 h of separation of the mother from her litter, and (d) a dopaminergic mechanism is not involved, but the adrenocortical system may participate in the stress-induced inhibition of PRL release.

Effects of Suckling on Hypothalamic-Pituitary-Adrenal Axis Responses to Psychosocial Stress in Postpartum Lactating Women

“…Interestingly, a gradual decrease in plasma PRL levels after stress has also been observed in the lactating rat (12,16) and in lactating women (17), whereas PRL levels increased in nonlactating animals and humans. In light of these findings, it may be speculated that the prolonged decrease observed in the present study might not only be due to the cessation of suckling, but could also represent a specific response to the mental stressor.…”

“…Observations in the lactating rat have shown an endocrine hyporesponsiveness to physical and psychological stressors, including attenuated secretion of ACTH (3)(4)(5)(6)(7), corticosterone (3-5, 8 -11), catecholamines (12), oxytocin (9,(13)(14)(15), and PRL (12,16).…”

Effects of Domperidone on Human Plasma Levels of Motilin, Somatostatin, Gastrin, Adrenocorticotropic Hormone and Cortisol

“…Banky et al reported that domperidone administration markedly decreased prolactin levels in animals subjected to restraint stress. 32) The plasma ACTH-IS level-time profile when blood was sampled at intervals of 120 min is shown in Fig. 3a (dotted line).…”

Prolactin Response to Formalin Is Related to the Acute Nociceptive Response and It Is Attenuated by Combined Application of Different Stressors

“…What is not well known is that under certain conditions stress can also induce a decrease in PRL secretion. The conditions whereby stress has no effect or even decreases plasma PRL are those where PRL secretion is elevated such as in lactating rats or during the proestrous afternoon surge and during the nocturnal surge of pseudopregnancy [15, 16]. …”