Central Effects of Catecholamines upon Mammary Contractility in Rats Are Neurally Mediated

Abstract

We injected, i.e., intracerebroventricularly (ICV) or systematically, small amounts of adrenaline (ADR), noradrenaline (NA), isoproterenol (ISOP) and dopamine (DA) in urethane-anesthetized lactating rats, and determined the effects on isometrically recorded intramammary pressure (IMP) responses to exogenous oxytocin (OXY). While centrally administered ADR, NA and DA provoked increased IMP responses to OXY, the β-adrenergic agonist ISOP induced the opposite effect. These effects were reversible, dose related and also occurred in hypophysectomized rats. However, when injected systematically, all adrenergic agonists but DA depressed IMP responses to OXY. Further experiments showed that central effects of catecholamines were exerted by regulating ductal tone, through the direct innervation of the mammary glands. Thus, whereas complete blockage of these effects occurred after selective denervation of the mammary glands, increased ductal tone resulted from ICV administration of ISOP. Finally, evidence was also obtained that antagonistic α- and β-adrenergic mechanisms may interact with each other to regulate milk ejection, and with afferent signals from the mammary glands. Thus, β-adrenergic inhibition upon IMP was counteracted by either NA administration or by activation of ductal mechanoreceptors. Together, these results suggest that regulation of milk ejection may involve neurally mediated influences on mammary contractility. Such actions would interact closely with afferents from the mammary gland influencing ductal tone.

Immunohistochemical Characterization of Sympathetic Chain Ganglia (SChG) Neurons Supplying the Porcine mammary Gland

“…Catecholamines circulating in the blood cause the inhibition of the oxytocin releasing from the hypothalamus (Mena et al., ; Morales et al., ,b; Wellnitz and Bruckmaier, ). Catecholamines also bind to α ‐adrenergic receptors on myoepithelial cells which surround lactiferous ducts, or occur in the nipple, and cause that relaxation (Mena et al., ; Morales et al., ,b; Wellnitz and Bruckmaier, ).…”

“…The result is a contraction of the smooth muscle cells surrounding the milk ducts or located in the nipple. The consequence of contraction of the smooth muscle cells is occlusion of milk ducts (Mena et al., ; Aceves et al., ; Morales et al., ,b; Wellnitz and Bruckmaier, ; Marina et al., ). Stimulation of the sympathetic nervous system causes constriction of the arteries of the gland.…”

Effects of Centrally Administered Prolactin upon Mammary Contractility in Anesthetized Lactating Rats

“…Together, these results indicate that central facilitatory effects of PRL upon mammary contractility in anesthetized rats were exerted through the direct innervation of the mam mary glands. Since neurally mediated, (3-adrenergic inhi bition of milk ejection through ductal constriction has previously been reported [4], perhaps inhibition of this system occurred under the influence of PRL. Alternative ly, other as yet unidentified facilitatory or inhibitory mechanisms neurally regulating the motor apparatus of the mammary gland may also be involved.…”

“…While these results may suggest a diffuse rather than a circumscribed location of the neural elements involved in the PRL effect, they also suggest that supraspinal and spinal sites of action may exist for PRLinduced facilitation of mammary contractility. On the other hand, the possibility of nonspecific depression caused by the PRL-Ab was excluded since there was no change in the capacity of centrally administered nor adrenaline [4] to increase mammary contractility.…”

“…In contrast with these effects, no effect was observed in con trol rats injected with normal rabbit serum (table 3). Also, to exclude the possibility that nonspecific depression by PRL-Ab could account for the lack of effect of centrally administered PRL, tests were performed during each experiment in which facilitation of IMP responses to OXY were induced by ICV or IT administration of nor adrenaline [4], Under such conditions, responsiveness to noradrenaline remained unaffected (table 3). These re sults therefore suggest that use of PRL-Ab was adequate to determine the specificity of the PRL effect.…”