Septopreoptic μ Opioid Receptor Mediation of Hindbrain Glucoprivic Inhibition of Reproductive Neuroendocrine Function in the Female Rat

Abstract

Central glucostasis is a critical monitored variable in neuroendocrine regulation of pituitary LH secretion. Glucoprivic signals originating within the caudal hindbrain suppress LH. Septopreoptic mu opioid receptors (mu-R) function within neural pathways maintaining basal LH levels and mediate the effects of diverse physiological stimuli on hormone release. To identify potential sites in the septopreoptic area where ligand neuromodulatory actions may occur in response to hindbrain glucoprivic signaling, the present studies evaluated the distribution of mu-R-immunoreactive (-ir) neurons in the septopreoptic area that are genomically activated in response to caudal fourth ventricular (CV4) delivery of the glucose antimetabolite, 5-thioglucose (5TG). The effects of lateral ventricular pretreatment with the selective mu-R antagonist, d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP), on LH secretory and GnRH neuronal transcriptional responses to hindbrain glucoprivation were also evaluated. Estradiol benzoate- and progesterone-primed, ovariectomized female rats were treated by CV4 administration of 5TG or the vehicle, saline, at the onset of the afternoon LH surge. The inhibitory effects of hindbrain glucoprivation on mean plasma LH levels as well as colabeling of rostral preoptic GnRH neurons for Fos-ir were attenuated in animals pretreated by lateral ventricular delivery of CTOP. Dual immunocytochemical labeling for septopreoptic mu-R-ir and Fos-ir demonstrated a robust induction of Fos expression by receptor-positive neurons within discrete septopreoptic sites in response to CV4 5TG, a genomic response that was diminished by CTOP pretreatment. The current studies provide novel evidence for the transcriptional activation of neuroanatomically characterized, mu-R-expressing neurons by decreased hindbrain glucose utilization and show that the functional status of mu-R is critical for maximal induction of the Fos stimulus-transcription cascade in these cells by central glucoprivic signaling. The finding that receptor antagonist-mediated suppression of this genomic response is correlated with increased reproductive neuroendocrine output supports a role for these discrete mu-R-expressing neuron populations as substrates for ligand regulatory effects on the GnRH-pituitary LH axis during neuroglucopenia.

Hindbrain Neuroglucopenia Elicits Site-Specific Transcriptional Activation of Glutamate Decarboxylase-Immunopositive Neurons in the Septopreoptic Area of Female Rat Brain

“…In comparison, colocalization of GAD- and µ-R-ir was more extensive in the MEPO and MPN. We have reported that CV4 administration of 5TG elicits Fos immunoexpression by µ-R-ir neurons in the MEPO [3], and that hindbrain glucoprivation enhances GABA turnover in discrete SPO loci in a µ-R-dependent manner [11]. The current data support the likelihood that µ-R ligands may directly influence MEPO GABAergic neuronal function during hindbrain glucose imbalance.…”

“…Sections were stored at –20°C in cryoprotectant containing 30% sucrose. For each animal, two separate 1-in-8 series of sections were collected throughout the rostrocaudal extent of the SPO, and processed for sequential localization of GAD- and Fos-ir (series 1) or GAD- and µ-R-ir (series 2) by immunoperoxidase and immunofluorescence labeling, as previously described [3]. The first staining series utilized rabbit polyclonal antisera against GAD (Prod.…”

“…Evidence that pituitary gonadotropin secretion is decreased by glucose antimetabolite administration into the fourth ventricle indicates that diminished catabolic yield of specific metabolic inter-mediates and/or energy in the juxtaventricular caudal hindbrain activates neural mechanisms that inhibit the gonadotropin-releasing hormone (GnRH)-pituitary luteinizing hormone (LH) axis [1,2,3]. In the rat, hypophysiotropic GnRH neurons reside within the septopreoptic (SPO) area and project caudally to the hypothalamic median eminence, where GnRH is taken up by hypophyseal portal capillaries and conveyed to the anterior pituitary gland [4].…”

“…Evidence that hindbrain glucoprivation enhancement of GABA neurotransmission and inhibition of LH secretion are attenuated by µ opioid receptor (µ-R) antagonist pretreatment implicates these receptors in pathways mediating neuroglucopenia-sensitive GABAergic suppression of the GnRH-pituitary LH axis [3, 11]. Although µ-R immunoreactivity is demonstrable throughout the SPO [17], only µ-R-immunopositive neurons located in the LS, MS, and MEPO exhibit nuclear labeling for Fos following CV4 delivery of 5TG [3].…”

“…Although µ-R immunoreactivity is demonstrable throughout the SPO [17], only µ-R-immunopositive neurons located in the LS, MS, and MEPO exhibit nuclear labeling for Fos following CV4 delivery of 5TG [3]. These results suggest that µ-R-containing neurons in these discrete sites may function as substrates for hindbrain glucoprivic regulatory effects that suppress the GnRH-pituitary LH axis.…”