Effects of local application of 5-HT and 8-OH-DPAT into the dorsal and median raphe nuclei on motor activity in the rat

The spontaneous motor activity of male Wistar rats was recorded after the local application of 5-HT (10 or 40 micrograms) or 8-OH-DPAT (1 or 5 micrograms) into the dorsal raphe (DR) or median raphe (MR) nuclei. Motor activity was monitored for 30 min, beginning 10 min after injections. The injection volume was 0.5 microliters and injections were made by means of 31-ga needles at a rate of 0.33 microliters min-1. The raphe nuclei were approached at 30 degrees in order to avoid penetration of the cerebral aqueduct and also to avoid passage of the DR with injections aimed at the MR. The application of 5-HT into the DR produced a marked suppression of locomotor activity and rearing, and no or a slight increase in these variables were noted after MR injections. 8-OH-DPAT injections into the DR resulted in a decrease in locomotor activity and rearing after the 5- but not the 1-microgram dose, whereas a marked locomotor stimulation was found after injections into the MR. In the latter case 1 microgram of 8-OH-DPAT was more effective than 5 micrograms, possibly due to overstimulation by the higher dose. The results demonstrate different roles for serotonergic mechanisms in the DR and MR in the mediation of spontaneous motor activity in the rat, and also a possible specific role for 5-HT1 receptors. The involvement of 5-HT1 receptors was further supported by the ability of (-)pindolol, 2 mg kg-1 SC, to antagonise 8-OH-DPAT-induced hyperactivity after application into the MR.

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