Serotonin plays an important role in locomotor pattern generation during early development. In the isolated in vitro rat and mouse spinal cord, bath applied 5-HT can evoke long-lasting rhythmic burst discharges from ventral roots. The 5-HT receptor subtypes that contribute to 5-HT evoked locomotion are considered to be 5-HT 1,2,7. However, a recent SFN abstract (Aduonum et al, 2001) showed that in some species of the neonatal rat 5-HT evoked locomotion persisted when all known 5-HT receptors were blocked. We tested the hypothesis that D1 and D2 receptors partly contribute to 5-HT (30 μM) induced locomotion using the isolated mouse (Swiss Webster) in vitro spinal cord as a model. Blockade of either D1 receptors (2 μM SCH-23390) or D2 receptors (10 μM Amisulpiride) did not completely block the locomotor-like discharge recorded from the L2 and L5 ventral roots (3/3 animals). However, bath application of both D1 and D2 antagonists did block the locomotor rhythm (4/4 animals). Blockade of 5-HT2 (100 nM ketanser...