Prolactin (PRL), an anterior pituitary hormone, has profound effects on female fertility. In fact, hypersecretion of PRL is the major neuroendocrine-related pathology associated with female infertility. Unfortunately, the underlying defects which lead to the hypersecretion of PRL remain ill-defined. To provide a rational framework for understanding these defects and treating associated pathologies, it is essential to elucidate the cellular and molecular mechanisms of PRL secretion regulation. Normally PRL secretion is under tonic inhibition by hypothalamic dopamine (DA). Periodic, physiological surges of PRL in females require a withdrawal from this dopaminergic inhibition. Since reduced responsiveness to DA is a hallmark of hyperprolactinemic syndromes, our attention has focused on the cellular and molecular basis of DA action on the lactotrope. We have discovered and extensively characterized a DA-activated inwardly-rectifying potassium channel (KDA) in normal lactotropes. KDA activation by DA leads to hyperpolarization of the lactotrope membrane and cessation of calcium-dependent action potentials, the driving force for tonic PRL secretion. In vitro studies have demonstrated a critical role for this KDA channel in the regulation PRL release by DA. Furthermore, the functional expression of KDA is dependent upon estrogen, which may explain a long-recognized modulatory action of estrogen on this system. In these regards, the KDA channel lies at the top of a hierarchy of events in dopaminergic regulation of PRL secretion. As an initial step in elucidating the molecular basis for the physiological role of KDA in lactotropes, we have cloned three different K channel gene products from female anterior pituitary tissue. Based on functional similarities between the native channel and recombinant channels expressed in Xenopus oocytes, two of the gene products are excellent candidate subunits encoding a heteromultimeric KDA. As a logical extension of these studies, we propose to evaluate the role of this effector K channel within its physiological context- the whole animal-through transgenic technology. To this end, we will design and construct """"""""dominant-negative"""""""" mutants, capable of inhibiting wild-type KDA function in lactotropes. We will then create a transgenic mouse model in which expression of this mutant is directed to pituitary lactotropes using the PRL promoter. These studies represent a timely and important extension of the P.I.'s work and will ultimately provide insight into the basis of disorders in PRL secretion and their impact on female fertility.
