Smooth muscle stimulation by an immunomodulatory compound muramyl dipeptide: does it involve serotoninergic system?

Contractions evoked by muramyl dipeptide (MDP), a synthetic compound possessing immunostimulatory properties, were studied in several isolated nerve-smooth muscle preparations. The contractions by micromolar concentrations of MDP were evoked either by direct interaction with smooth muscle (rat stomach strip) or at least partly indirectly via neurogenic stimulation (guinea pig ileum); the effect was stereospecific since the MDP-D was not active. The insensitivity of the preparations to serotonin (5-HT), either inherent (vas deferens) or after 5-HT antagonists or after desensitization to 5-HT, prevented or markedly reduced the contractile activity by MDP. On the other hand, a nanomolar concentration of MDP enhanced the sensitivity of the rat stomach strip to 5-HT.