Hormonal control of prostatic thyrotropin-releasing hormone (TRH) testosterone modulates prostatic TRH concentrations

The presence of extremely high concentrations of authentic TRH in the rat prostate prompted us to examine whether prostatic TRH concentrations were under hormonal control. Both the immunoreactive TRH content per prostate and TRH immunoreactivity expressed per 100 mg prostatic protein were lower in animals 2 weeks after hypophysectomy than in sham-operated and calorically restricted weight-matched controls. Since many of the prostatic functions are testosterone dependent, we assessed the possibility that testosterone modulated prostatic TRH concentrations. We measured prostatic TRH concentrations in the following four groups of sexually mature male Sprague-Dawley rats: group I, sham operated; group II, castrated; group III, castrated animals with 5-mm Silastic testosterone implants; and group IV, castrated animals with 20-mm testosterone implants. Prostatic TRH concentrations in these four groups 2 weeks after surgery were 600.5 +/- 33.3, 65.1 +/- 22.6, 169.4 +/- 55.3, and 609.5 +/- 144.3 (+/-SE) ng/100 mg protein. There was good linear correlation between prostatic TRH concentrations and serum testosterone concentrations (r = 0.65; P less than 0.01). By subjecting the pooled prostatic extracts from each group to ion exchange chromatography on a SP-Sephadex C-25 column and measuring the proportion of immunoreactivity coeluting with authentic TRH, it was shown that the fall in prostatic TRH immunoreactivity after castration and hypophysectomy was indeed due to a loss of authentic TRH. We conclude that the prostatic TRH concentrations are under hormonal control and appear to be modulated by serum testosterone concentrations. This is the first demonstration of hormonal regulation of a neuropeptide in a mammalian extrahypothalamic site and suggests a physiological role for this neuropeptide at this site.