Evidence for the Modulation of Sexual Behavior by α-Adrenoceptors in Male Rats

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Abstract
Clonidine, a commonly used antihypertensive agent believed to act by stimulation of central α- adrenoceptors, produced a dose-related suppression of ejaculatory behavior in sexually vigorous male rats throughout the range of treatment (0.0125–0.5 mg/kg). Treatment with 0.25 mg/kg virtually eliminated ejaculatory behavior, without altering the number of animals mounting and intromitting. These effects were maintained for at least 4 h after treatment. Prazosin, another antihypertensive (but acting by blockade of α1-adrenoceptors), increased latencies to initiation of copulation, to ejaculation, and to reinstatement of copulation following ejaculation. Additionally, prazosin (1 mg/kg) pretreatment failed to attenuate or prevent the clonidine-induced suppression of ejaculation. In contrast, yohimbine, a drug which preferentially blocks α2-adrenoceptors (2 mg/kg, 20 min prior to mating tests), caused a facilitation of copulatory behavior as evidenced by drastic decreases in ejaculation latency and intercopulatory and postejaculatory intervals. Pretreatment with yohimbine completely prevented the clonidine-induced suppression of ejaculation, while clonidine attenuated the facilitatory effects of yohimbine, suggesting a competitive interaction. These data lead to the suggestion that increased excitatory adrenergic activity results in increased sexual arousal either by blockade of α2- or stimulation of α1-adrenocep-tors. Alternatively, stimulation of α2- or blockade of α1-adrenoceptors results in diminished sexual motivation. While the precise implications of this animal research for the clinic are as yet unclear, further research could lead to important developments in the pharmacological treatment of sexual dysfunctions.