Intrathecal methiothepin and thyrotropin-releasing hormone effects on penile erection

Intrathecal thyrotropin-releasing hormone (TRH) potently inhibits penile er ection at all doses (100, 500, 1000 or 5000 pmol) tested so far. Since the serotonin receptor antagonist methiothepin (MT) inhibits TRH responses in o ther systems, this study tested the hypothesis that MT-sensitive receptors mediate the effect of TRH on penile erection in rats. When compared to cont rols, the highest doses of IT TRH (0, 10 or 500 pmol) or MT (5 or 50 nmol) significantly altered penile reflex latency. When coadministered (50 nmol M T/500 pmol TRH), the effect of TRH was reversed, suggesting that the high d ose of MT antagonized the inhibitory actions of TRH. The low dose of MT (5 nmol) did not block the 500 pmol TRH inhibition of reflex latency. These da ta further suggest that MT sensitive receptors are important in (1) mediati ng normal penile reflexes and (2) mediating the inhibitory response to TRH. (C) 2001 Elsevier Science Inc. All rights reserved.