Vol 72, No 3 (2021)
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Published online: 2021-04-13

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Exogenous orexin-A downregulates luteinizing hormone secretory activity in prepubertal female rats

Lidia Martyńska1, Alina Gajewska2, Magdalena Chmielowska1, Małgorzata Kalisz1, Anna Litwiniuk1, Wojciech Bik1, Bogusława Baranowska3
Pubmed: 34010439
Endokrynol Pol 2021;72(3):238-242.

Abstract

Introduction: Orexin-A is a neuropeptide synthesized in the lateral hypothalamus. Orexin-A immunoreactive fibres overlap distribution with GnRH neurons. In adult rats, orexin A is known to affect LH secretion via GnRH release modulation. Because data concerning the impact of orexin-A on the hypothalamo-pituitary axis activity are limited, we focused on the involvement of orexin-A and receptors of NPY in the modulation of LH release and LH subunit b (Lhb) mRNA expression in prepubertal female rats.

Material and methods: Forty immature female Wistar rats were divided into 4 groups and received 2 intracerebroventricular (icv) microinjections of: 1 — artificial cerebrospinal fluid (CSF) (controls); 2 — CSF followed by orexin A; 3 — selective NPY receptor antagonist (BIBP) followed by CSF; 4 — BIBP followed by orexin A. One hour after the last microinjection, all rats were decapitated. Trunk blood was collected, and serum was stored at –20°C for the LH RIA examination. The adenohypophysis was immediately excised, flash-frozen, and kept at –80°C for RNA extraction. Real-time PCR amplification was carried out, and relative Lhb gene expression was calculated.

Results: In comparison to the CSF-treated controls with a mean LH serum concentration of 0.40 ± 0.02 ng/mL, the mean LH serum level was diminished both after orexin-A (0.27 ± 0.01 ng/mL) and after BIBP (0.30 ± 0.02 ng/mL) icv microinjections. In the presence of BIBP, orexin-A more effectively inhibited LH release (0.20 ± 0.01 ng/mL) when compared to the BIBP-treated group. Orexin-A and BIBP exerted a consistent inhibitory effect on Lhb mRNA expression levels in the anterior pituitary gland. In comparison to the CSF-treated controls, orexin-A, and BIBP-treated females responded with, respectively, 35% and 40% reduction of Lhb mRNA expression. Orexin-A and BIBP co-administration evoked a further reduction of Lhb gene transcriptional activity.

Conclusions: Orexin-A exerts a down-regulatory effect on LH synthesis and release in immature female rats. Considering that Y1R-oriented down-regulation of endogenous NPY activity did not reverse the suppressive effect of exogenous orexin-A, it might be suggested that NPY and orexin A systems can operate independently to affect gonadotropin activity in the anterior pituitary of the immature female rats. 

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