Influence of serotonergic drugs on restraint stress induced analgesia

The effects of drugs selectively effecting central serotonergic systems on immobilization-induced analgesia (SIA) were tested in the rat. The drugs were used in dose ranges previously shown to effect emotional processes. SIA was tested using the tail withdrawal method. It was found that pretreatment of rats with para-chlorophenylalanine (p-CPA), an inhibitor of serotonin synthesis, significantly attenuated SIA, measured immediately after stress session. Ritanserin, a 5-HT2A/2C receptor antagonist, ondansetron, a 5-HT3 receptor antagonist and citalopram, a selective serotonin re-uptake blocker increased the baseline pain threshold, whereas 8-OH-DPAT, a full 5-HT1A receptor agonist and buspirone, a partial 5-HT1A receptor agonist expressing also high affinity towards dopaminergic D2 receptors, were without effect on pain perception and stress induced analgesia. It has been concluded, that modification of SIA by serotonergic drugs probably merely reflects changes in the activity of the 5-HT system on the spinal cord level, with minor, if any, contribution of supraspinal emotional centers.