| Abstract: | The study aims to establish the nature of the chemical mediator which produces the IP (presynaptic inhibition) of the mechanoreceptive afferents reaching the NTS (nucleus tractus solitarius) of the frog. To this end we have examined the effects of the administration of SP (substance P) and of one of its antagonists in the IV ventricle, in both normal and unilaterally axotomized preparations at the level of the glossopharyngeal nerve. In particular we have examined the size of the afferent discharge of the glossopharyngeal-hypoglossus reflex arc and the PAD (primary afferent depolarization) phenomena recorded from the dorsal root of the XII. While in normal preparations the SP reduces the size of the reflex discharge, on the contrary the antagonist increases it; the electrical activity of PAD appears to be enhanced by SP and reduced by the antagonist. Lastly SP normalises the enhanced response produced by axotomy. All the observed effects favour the hypothesis that the IP, which appears in the NTS with the activation of the mechanoreceptive afferents, is brought about by the release of S.P. from their central endings. |
