Role of serotonergic and noradrenergic systems in a model of visceral pain

The effects of selective manipulations of activity of the serotonergic and noradrenergic systems were examined in the rat model of visceral pain. It was found that neither p-chlorophenylalanine(p-CPA)- nor N-chloro-ethyl-2,2--bromo-benzylamine(DSP-4)-induced strong and selective depletion of the brain and spinal cord serotonin and noradrenaline, respectively, changed in a significant way rat visceral pain perception. On the other hand, 8-OH-DPAT, a full selective 5-HT1A receptor agonist, prazosin, an alpha1-adrenoceptor antagonist, clonidine, an alpha2-adrenoceptor agonist, and two beta-adrenoceptor antagonists: propranolol and metoprolol, dose-dependently reduced the number of body writhes induced by intraperitoneally administered 2% solution of acetic acid (the writhing test). The results obtained with selective receptor ligands, DSP-4 and p-CPA, indicate that the noradrenergic and serotonergic innervation of the central nervous system contribute in a complex way to the animal behavior in the writhing test. The 5-HT1A receptors and alpha2-adrenoceptors play an inhibitory role in the expression of rat behavior in this model of visceral pain. On the other hand, adrenergic alpha1 and beta1 receptors facilitate the behavioral effects of the irritant agent.