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 robe (NL) and intermediate lobe (IL), has a rol e in the control of anterior pituitary PRL secretion. However, basic a spects of this regulatory mechanism like (1), the role of an intact hy pothalamic innervation of the PP as well as (2) the site of production of previously found PRL releasing substance(s) have not yet been char acterized. Denervation of the PP (PPD) is an effective method for havi ng a selective lesion of the innervation of PP, indeed, PPD results in a disappearance of neurosecretory materials from NL and tyrosine hydr oxylase (TH) immunoreactivity from IL, leaving blood supply of all thr ee lobes intact, Blood samples were taken from freely moving sham and PP-denervated lactating rats before and after 4-h separation from thei r pups and during the suckling stimulus, PPD blocks separation-induced depletion but only attenuates suckling induced release of PRL. Furthe rmore, it doubles plasma level of alpha-MSH during the entire sampling period, which has been used as a marker for in vivo secretory activit y 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-arg inine-vasopressin (dDAVP). In contrast, dDAVP, neither alone nor in co mbination with oxytocin (OXY), can change PPD-induced elevation of pla sma alpha-MSH as well as attenuation of PRL response induced by suckli ng. It is concluded that: (1) contribution of the THDA system parallel to the confirmed role in the regulation of alpha-MSH seems to be cruc ial for the depletion of plasma PRL induced by separation but not for the elevation due to suckling stimulus, (2) intact hypothalamic innerv ations 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 activi ty in PP, (4) which does not solely responsible for suckling-induced P RL release, Therefore, an interplay between several substances produce d by NIL of the pituitary gland must have been responsible for the int act regulation of PRL secretion during lactation. (C) 1997 Elsevier Sc ience Inc.