Evidence that central 5-hydroxytryptaminergic neurones are involved in the anxiolytic activity of buspirone.

1. A two-compartment exploratory test was used to assess the role of central 5-hydroxytryptaminergic neurones in the anxiolytic activity of buspirone in rats. 2. Buspirone 0.1 mg kg-1, administered subcutaneously 15 min before testing, significantly increased black-white transitions (BWT) in control rats but had no effect in animals injected intracerebroventricularly one week before with 150 micrograms 5,7-dihydroxytryptamine (in 20 microliters). 3. Infusion of buspirone in the median raphe (but not in the dorsal raphe) significantly enhanced BWT, at doses from 1 micrograms to 10 micrograms (in 0.5 microliters). Buspirone 5 and 10 micrograms, but not 1 microgram, administered in the median raphe, significantly enhanced motor activity of rats during the first 10 min of testing in the activity cages. 4. The effect on BWT of 5 micrograms buspirone in the median raphe was completely antagonized in animals which had received either 5,7-dihydroxytryptamine intraventricularly, 150 micrograms (in 20 microliters), one week before or an infusion of 0.1 microgram (in 0.5 microliter) (-)-propranolol in the same area 5 min before. (-)-Propranolol infused in the median raphe did not modify the effect of buspirone on locomotion. 5. Infusion of 5 micrograms buspirone (in 0.5 microliter) in the median raphe significantly enhanced punished responses in a conflict test with no effect on unpunished responding. Buspirone infused in the dorsal raphe had no effect on punished or unpunished responding over a wide dose range. 6. The results indicate that at the relatively low dose used in the present study buspirone produces an anxiolytic effect by acting on central 5-hydroxytryptaminergic neurones. It is likely that activation of 5-hydroxytryptamine1A-receptors in the median raphe is involved.