Effect of posterior pituitary denervation (PPD) on prolactin (PRL) and alpha-melanocyte-stimulating hormone (alpha-MSH) secretion of lactating rats.

Previous data have clearly suggested that the posterior pituitary (PP), consisting of neural lobe (NL) and intermediate lobe (IL), has a role in the control of anterior pituitary PRL secretion. However, basic aspects of this regulatory mechanism like (1), the role of an intact hypothalamic innervation of the PP as well as (2) the site of production of previously found PRL releasing substance(s) have not yet been characterized. Denervation of the PP (PPD) is an effective method for having a selective lesion of the innervation of PP, indeed, PPD results in a disappearance of neurosecretory materials from NL and tyrosine hydroxylase (TH) immunoreactivity from IL, leaving blood supply of all three lobes intact. Blood samples were taken from freely moving sham an PP-denervated lactating rats before and after 4-h separation from their pups and during the suckling stimulus. PPD blocks separation-induced depletion but only attenuates suckling induced release of PRL. Furthermore, it doubles plasma level of alpha-MSH during the entire sampling period, which has been used as a marker for in vivo secretory activity of IL cells. Lack of the separation-induced depression in plasma PRL of PPD animals can be partially restored by normalizing the diabetes insipidus with treatment of a vasopressin analogue, 1-desamino-8-D-arginine-vasopressin (dDAVP). In contrast, dDAVP, neither alone nor in combination with oxytocin (OXY), can change PPD-induced elevation of plasma alpha-MSH as well as attenuation of PRL response induced by suckling. It is concluded that: (1) contribution of the THDA system parallel to the confirmed role in the regulation of alpha-MSH seems to be crucial for the depletion of plasma PRL induced by separation but not for the elevation due to suckling stimulus, (2) intact hypothalamic innervations of both NL and IL, regulating water intake and the secretion of alpha-MSH, respectively, are necessary for normal secretory responses of AL during lactation, (3) as well as for the presence of PRF activity in PP, (4) which does not solely responsible for suckling-induced PRL release. Therefore, an interplay between several substances produced by NIL of the pituitary gland must have been responsible for the intact regulation of PRL secretion during lactation.