Hypothalamic norepinephrine (NE) plays an important role in the secretion of basal and postcoital gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) secretion because central infusion of exogenous NE stimulates GnRH/LH release in both sexes and coitus induces a concomitant endogenous NE/GnRH/LH release in females, but not in males.
The aims of this study were to determine whether noradrenergic neuronal synthesis and/or uptake were altered either after reduction of sex steroids by gonadectomy or after increased hypothalamohypophyseal gonadal activation by coitus. We cloned rabbit gene specific cDNA probes of tyrosine hydroxylase (TH) and NE transporter (NET), two key factors regulating NE synthesis and uptake, and established a sensitive ribonuclease protection assay to measure mRNA levels in specific noradrenergic cell groups in individual rabbits. Three weeks after gonadectomy, blood levels of LH and noradrenergic neuronal levels of TH and NET mRNAs were elevated over similar values in both intact males and females. A comparison of these same three parameters after coital activation of the brain noradrenergic-pituitary LH system revealed concomitant increases in blood LH and neuronal TH and NET mRNA levels in females between 15 and 240 min after copulation, except that TH mRNA levels declined more rapidly than NET mRNA values. In marked contrast to females, no changes in blood LH, neuronal TH or NET mRNA levels occurred after coitus in male rabbits. These observations suggest that both feedback regulation of sex steroids on basal LH secretion and coitus-induced NE/GnRH/LH release may involve transcriptional/translational processes within NE-secreting neurons. However, a clear gender difference exists in the coitally activated noradrenergic neuronal system since this neuroendocrine axis in males does not respond. Taken together, the information suggests that NE synthesis and reuptake are two key factors in mediating the regulation of GnRH/LH secretion by coitus in females, and by gonadal steroids in both male and female rabbits.
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