Effects of intra-VL PAG microinjections of vehicle (0.05% DMSO in ACSF), PGE2 (0.01, 0.1 and 1 pg) alone, or PGE2 (1 pg) in combination with L355677 (0.1 pg) on the spontaneous firing of RVM ON (A) or OFF (B) cells. Vehicle or drugs were administered at time 0, whereas L 335677 (0.1 pg) was administered 10 min beforehand. Each point represents the mean ± S.E.M of 6-7 neurons of different treated groups of rats. * indicates a statistically significant difference vs vehicle and° versus PGE2 (1 pg). P values < 0.05 were considered statistically significant.