RAPHE INDUCED INHIBITION OF INTRALAMINAR THALAMIC UNITARY ACTIVITIES AND ITS BLOCKADE BY PARA-CHLOROPHENYLALANINE IN CATS

Extracellular recordings were made of intralaminar thalamic neurons responding to stimulation of the radial nerves. The effects on this response of a conditioning stimulation of the median raphe nucleus were studied in both untreated and p-CPA [chlorophenylalanine] pretreated cats. Of thalamic responses to somatic stimulation in untreated cats, 92% was inhibited by conditioning stimulation of the raphe nucleus. The intensities of conditioning stimulation required for the inhibition were significantly higher in cats pretreated with p-CPA than in cats not treated with the drug. Raphe projection to intralaminar thalamus exerts an inhibitory influence on thalamic activities, and that the inhibition is mediated by a putative neurotransmitter, serotonin. The possible involvement of the median raphe nucleus in pain processing was discussed.