Stimulation of luteinizing hormone subunit gene expression by pulsatile intracerebroventricular microinjection of galanin in female rats.

Abstract

Although galanin, which exerts its effects both at the hypothalamic and pituitary level, has been implicated as an important neuroendocrine regulator of hypothalamic-pituitary-gonadal axis activity, there is a lack of data concerning its involvement in the regulation of gonadotropin subunit gene expression. To elucidate whether galanin can influence luteinizing hormone (LH) subunit mRNA content, as well as affect gonadotropin-releasing hormone (GnRH) receptor activity, a model based on pulsatile (one pulse per hour over 5 h) galanin (1 nM) microinjections directly into the third cerebral ventricle of ovariectomized (OVX) and/or oestrogen/progesterone-pretreated rats was used. Furthermore, to determine galanin effects on GnRH-induced LH subunit mRNA synthesis, a cocktail of 1 nM GnRH and 1 nM galanin was coadministered in a pulsatile manner to OVX/steroid primed rats. Subsequently, to obtain data concerning the role of galanin receptors in the regulation of pituitary alpha (common to LH, follicle-stimulating hormone, thyroid-stimulating hormone) and LHbeta subunit gene expression, OVX/oestrogen/progesterone rats received microinjections of 1 nM of the receptor antagonist galantide and 1 nM of galanin. In this case, both substances were administered separately, with a 30 min lag, according to which each galantide pulse always preceded a galanin pulse. Northern-blot analysis revealed that intracerebroventricular pulsatile galanin injections were effective in stimulation of both alpha and LHbeta subunit mRNA levels and that this effect was apparently steroid-dependent. Moreover, galanin also up-regulated GnRH receptor functional parameters (affinity and maximum binding capacity) but was ineffective in potentiating GnRH-induced accumulation of both subunit mRNAs. The results from the study also indicate that galanin acts through its own receptor(s) because a receptor antagonist, galantide, significantly reduced the stimulatory effect exerted by galanin on the expression of both LH subunit genes in vivo.