Gonadectomy alters tyrosine hydroxylase and norepinephrine transporter mRNA levels in the locus coeruleus in rabbits

Abstract

The central noradrenergic system has a major regulatory role on gonadotropin-releasing hormone/luteinizing hormone (GnRH/LH) secretion in rabbits. Exogenous administration of norepinephrine (NE) alters GnRH/LH release in a sex steroid-dependent manner, i.e. NE stimulates GnRH/LH release in oestrogen-primed ovariectomized (OVX) animals but not in non-primed individuals. To investigate how gonadal steroids influence noradrenergic neuronal activities in the locus coeruleus (LC), mRNA levels of tyrosine hydroxylase (TH) and NE transporter (NET), two key factors regulating NE synthesis and uptake, were compared 3 weeks after gonadectomy (GDX). Intact male (n = 5) and female (n = 6) New Zealand White rabbits were sacrificed along with castrated males (n = 4) and OVX females (n = 5). The brainstem from each individual was sectioned and the LC was punched for detection of TH and NET mRNA levels using the ribonuclease protection assay (RPA). Trunk blood was collected to determine immunoactive serum LH values. Levels of LH were elevated in both males and females after GDX. Luteinizing hormone concentrations averaged 0.10 +/- 0.05 ng/ml in intact males vs 1.64 +/- 0.31 ng/ml in castrated males (P < 0.01) and 0.30 +/- 0.08 ng/ml in intact vs 9.80 +/- 3.50 ng/ml in OVX females (P < 0.05), respectively. Removal of the gonads also increased TH mRNA levels in the LC in both males and females. In intact males, TH mRNA levels were 0.796 +/- 0.181 pg/microgram total RNA, whereas in castrates mRNA levels averaged 1.667 +/- 0.345 pg/microgram total RNA (P < 0.05). In intact females, TH mRNA levels were 0.617 +/- 0.054 pg/microgram total RNA while the OVX group averaged 1.084 +/- 0.202 pg/microgram total RNA (P < 0.05). Similar increases in NET mRNA were noted after GDX in both sexes. In males, NET mRNA levels were 1.461 +/- 0.401 pg/microgram total RNA in intacts vs 3.666 +/- 0.649 pg/microgram total RNA in castrates (P < 0.05). In females, NET mRNA levels averaged 1.336 +/- 0.212 pg/microgram total RNA and 3.297 +/- 0.835 pg/microgram total RNA in the intact and OVX groups, respectively (P < 0.05). The data indicate that GDX enhances gene expression of both TH and NET. The results support the hypothesis that the feedback regulation of sex steroids on LH secretion in rabbits of both sexes involves transcriptional/translational processes of at least TH and NET in brainstem NE cells.