Characterization and Distribution of Receptors for Gonadotropin-Releasing Hormone in the Rat Hippocampus

Abstract

Distribution and properties of receptors for gonadotropin-releasing hormone (GnRH) were analyzed in the brain of adult male rats. Binding of the iodinated GnRH agonist Des-Gly¹⁰-(D-Ala⁶)-GnRH ethylamide was studied in hippocampus and anterior pituitary using three convergent approaches: quantitative autoradiography on frozen tissue, binding to fresh slices, and binding to crude membrane preparations. In all cases, binding was specific, saturable, and time, pH, and temperature dependent. Quantitative autoradiography revealed that the density of binding sites was high in the stratum oriens and stratum radiatum of the CA1-CA4 regions of Ammon’s horn. The pyramidal cell layer was faintly labelled. Binding was almost undetectable in the dentate gyrus. The highest density of sites (B_max = 11.6 + 1.0 fmol/mg protein) was observed in the stratum radiatum of the CA3 region. Under the same conditions the value obtained for pituitary tissues was 20.7 + 2.8 fmol/mg protein. Analysis of saturation curves indicated only one class of high-affinity sites for the hippocampus (CA₃; K_d = 0.28 + 0.03 nM) and for the pituitary (K_d = 0.29 ± 0.08 nM). Both native GnRH and GnRH antagonist were potent competitors of binding. Fresh slices and membrane preparations from whole hippocampus confirmed these autoradiographic data and yielded affinity constants of 0.28 + 0.01 and 0.52 + 0.08 nM, respectively. In addition, a very high binding density was present in the amygdaloid complex, while binding was barely detectable in the hypothalamus. These results demonstrate that high densities of specific GnRH receptors are present in areas concerned with the regulation of behavioral functions. GnRH may thus be involved as a neurotransmitter in the integration of endocrine and behavioral reproductive processes.

Differential Effects of an Luteinizing-Hormone-Releasing Hormone (LHRH) Antagonist Analogue on Lordosis Behavior Induced by LHRH and the LHRH Fragment Ac-LHRH^5–1⁰

“…These findings corroborate previous results from our laboratory indicating a physiologically relevant lordosis-potentiating ac tion for the LHRH decapeptide [9]. Tire rapid effect of LHRH on the electrical activity of VMH neurons [7], the identification of LHRH receptors in the VMH [4,5] and the present results demonstrating the ability of an LHRH antagonist to reduce mating behavior induced by infusion of LHRH into the VMH support the hypothesis that LHRH enhancement of mating be havior is a receptor-mediated event. The inability of the same LHRH antagonist to inhibit mating behavior induced by Ac-LHRH5-10 and a report that Ac-LHRH5-10 does not interfere with binding of an LHRH agonist to the receptor [6] suggest that the fragment does not activate behavior by interacting di rectly with the LHRH receptor.…”

“…The Ac-LHRH5-'0 fragment, however, is inactive at the pituitary level in terms of stimulating LH release [3], Although the mech anism by which the 2 peptides exert their lordosis-promoting effect is not known, the localization of LHRH receptors in the VMH [4,5] raises the possibility that the behavioral effect of the LHRH decapeptide is a receptor-mediated event. In a study of the structure-function relationships of LHRH receptors, bind ing of an LHRH agonist to the receptor was inhibited by the LHRH decapeptide, but not by the Ac-LHRH5-'0 fragment [6].…”

Prolactin Receptors in the Rat Hypothalamus: Autoradiographic Localization and Characterization

“…Recently, however, Muccioli et al [4] demonstrated the presence of specific, high affinity (0.2 x\M) PRL receptors, modulated by steroids, in male and female rat hypothala mus using an elaborated procedure for membrane pre paration. On the other hand, specific PRL binding visual ized by autoradiography after in vivo or in vitro adminis tration of the ligand appeared limited to the median eminence in both sexes [3,[5][6][7], The limited ability to de tect PRL receptors in the rat median eminence is in con trast with the numerous data demonstrating the existence of PRL-sensitive neurons in the rat hypothalamus [8,9] especially in the dopaminergic neurons of the arcuate nucleus [10][11][12], Using an autoradiographic procedure previously vali dated for GnRH receptors [13] we succeeded in localiz ing and characterizing specific PRL binding sites in sev eral hypothalamic nuclei in the adult female rat. A preliminary report of these findings has been recently presented [14],…”

Human neurons express type I GnRH receptor and respond to GnRH I by increasing luteinizing hormone expression

“…Another intriguing possibility is that aging neurons, like fetal and cancer cells, might be capable of synthesizing LH (Whitfield & Kourides 1985, Krichevsky et al 1995, Yokotani et al 1997. This latter idea is supported by the findings that (1) mRNA for LH has been localized to pyramidal neurons of the cerebral cortex and hippocampus of the aging rat brain (Lee et al 2004), (2) GnRH receptor I (GnRHR I) is localized to extrapituitary cells in the rodent brain including the hippocampus, amygdala, entorhinal cortex, and subiculum, with lower levels in the septum, frontal cortex and hypothalamus (Reubi & Maurer 1984, Badr & Pelletier 1987, Haour et al 1987, Reubi et al 1987, Jennes et al 1988, 1996, Leblanc et al 1988, Badr et al 1989, Ban et al 1990, Thompson & Moss 1992, Crumeyrolle-Arias et al 1994, Pierpaoli & Lesnikov 1997, Lu et al 1999, Granger et al 2004, and (3) the pulsatile release of GnRH I from the hypothalamus is dramatically elevated following menopause (Gill et al 2002, Gore et al 2004. The increased binding of GnRH I to gonadotrope GnRHR I results in dramatic elevations in the synthesis and secretion of gonadotropins following menopause and andropause (Gharib et al 1990, Couzinet & Schaison 1993, Woller et al 2002.…”