Effect of midbrain raphe nucleus stimulation on somatosensory evoked potential in cat

Experiments were carried out to clarify the effect of serotonin [5-hydroxytryptamine (5-HT)] upon the somatosensory evoked potential (SEP) elicited by superficial radial nerve stimulation in cat, and the following results were obtained: (i) in untreated (control) animals, a significant reduction occurred in amplitude of the SEP in response to conditioning stimulation of the dorsal raphe nucleus in the midbrain, within 100 ms of the interval between conditioning and test stimuli; (ii) in 5-HT depleted animals prepared by intraperitoneal injection of p-chlorophenylalanine (PCPA), the amplitude of SEP significantly increased in comparison with that of nontreated animals. Conditioning stimulation of the raphe nucleus in 5-HT depleted animals failed to cause any remarkable change in the SEP; (iii) the diminuation of the SEP by raphe nucleus conditioning reappeared following intraventricular administration of a large dose of 5-HT (200 micrograms) in the PCPA-treated animals. These results have demonstrated that the serotonergic system with its cell bodies located in the raphe nucleus of the midbrain has the function of reducing the amplitude of the SEP, thus suggesting that it acts as a suppressive control on the excitability of the cortex to sensory input.