The association between testicular ghrelin receptor mRNA and serum testosterone levels in immunocastrated boars

Ghrelin, the endogenous ligand for the GH secretagogue receptor (GHS-R), has various functions. The expression of ghrelin and growth hormone secretagogue receptor type 1a (GHS-R1a) has been demonstrated in rat and human testis, and ghrelin also affects testosterone (T) secretion in vitro, suggesting a role for this molecule in the direct control of testis function. However, whether this signaling system is present in pig testis, and the association with serum T remains largely unexplored. In this study we investigated the relationship between serum T levels and ghrelin and GHS-R1a gene expression in the testes of intact and immunocastrated boars. Testicular tissue and serum samples were collected from seven intact boars and seven recombinant GnRH-I immunocastrated boars. GHS-R1a gene expression in the testis was higher in immunocastrated than intact boars (2.22 ± 0.71 vs. 0.53 ± 0.23; P < 0.01). Mean serum T levels were markedly lower in immunocastrated than intact boars (0.40 ± 0.04 vs. 2.10 ± 0.94 ng/mL, P < 0.01). Ghrelin gene expression in testis did not differ between both groups of boars. GHS-R1a gene expression was positively correlated with ghrelin gene expression (r = 0.68, P < 0.001), but negatively correlated with serum T concentrations (r = −0.83, P < 0.001). Overall, the existence of ghrelin and GHS-R1a gene expression in pig testis, and the increase in GHS-R1a gene expression in the testes and the inverse correlation with serum T in immunocastrated boars are highly suggestive of a role for ghrelin in the regulation of mammalian testicular function.