Serotonin 5-HT and 5-HT receptors co-mediate the RU 24969-induced locomotor activity of male and female preweanling rats.

The serotonin (5-HT) agonist RU 24969 robustly increases the locomotor activity of adult male rats and mice; however, studies using selective antagonists alternately report that 5-HT, 5-HT, or both receptor types mediate RU 24969's locomotor activating effects. The purpose of the present study was to extend these past findings by administering a selective 5-HT agonist and/or antagonists to male and female preweanling rats. This age group was tested because younger rats often exhibit psychopharmacological responses that are quantitatively or qualitatively different from adult rats. In a series of experiments, male and female preweanling rats were pretreated with vehicle, the 5-HT antagonist WAY 100635 (0.5, 1, 5, or 10 mg/kg), or the 5-HT antagonists NAS-181 (5 or 10 mg/kg) or SB 216641 (5 or 10 mg/kg) 30 min before assessment of locomotor activity. Rats were injected with saline or RU 24969 immediately prior to testing. Results showed that RU 24969 (0.625, 1.25, 2.5, or 5 mg/kg) significantly increased the locomotor activity of both male and female preweanling rats (no sex differences were apparent). Antagonism of either the 5-HT or the 5-HT receptor was sufficient to significantly reduce the locomotor activity of RU 24969-treated preweanling rats. Unexpectedly, NAS-181 did not act as a silent receptor antagonist, as both doses of NAS-181 significantly increased the locomotor activity of saline-treated preweanling rats. In sum, the present results show that both the 5-HT and 5-HT receptor systems mediate locomotion during the late preweanling period, and this mediation does not vary according to sex.