Rationale: Previous studies have shown that buspirone, a partial 5-HT1A receptor agonist, produces antinociceptive effects in rats and mice; Ca2+ plays a critical role as a second messenger in mediating nociceptive transmission. 5-HT1A receptors have been proven to be coupled functionally with various types of Ca2+ channels in neurons, including N-, P/Q-, T-, or L-type. It was of interest to investigate the involvement of extracellular/intracellular Ca2+ in buspirone-induced antinociception. Objectives: To determine whether central serotonergic pathways participate in the antinociceptive processes of buspirone, and investigate the involvement of Ca2+ mechanisms, particularly L-voltage-gated Ca2+ channels and Ca2+/caffeine-sensitive pools, in buspironeinduced antinociception. Methods: Antinociception was assessed using the hot-plate test (55degreesC, hind-paw licking latency) in mice treated with either buspirone (1.25-20 mg/kg i.p.) alone or the combination of buspirone and fluoxetine (2.5-10 mg/kg i.p.), 5-HTP (25 mg/kg i.p.), nimodipine (2.5-10 mg/kg i.p.), nifedipine (2.5-10 mg/kg i.p.), CaCl2 (25-200 nmol per mouse i.c.v.), EGTA (530 nmol per mouse i.c.v.), or ryanodine (0.25-2 nmol per mouse i.c.v.). Results: Buspirone dose dependently increased the licking latency in the hot-plate test in mice. This effect of buspirone was enhanced by fluoxetine, 5-HTP, nimodipine, and nifedipine. Interestingly, central administration of Ca2+ reversed the antinociceptive effects of buspirone. In contrast to these, ryanodine or EGTA administered centrally potentiated buspirone-induced antinociception. Conclusions Decreasing neuronal Ca2+ levels potentiated buspirone-induced antinociception; conversely, increasing intracellular Ca2+ abolished the antinociceptive effects of buspirone. These results suggest that Ca2+ influx from extracellular fluid and release of Ca2+ from Ca2+/caffeine- sensitive microsomal pools may be involved in buspirone-induced antinociception.