Raphe induced inhibition of intralaminar thalamic unitary activities and its blockade by para-chlorophenylalanine in cats.

Extracellular recordings were made of intralaminar thalamic neurones 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 pretreated cats. Ninety-two percent of thalamic responses to somatic stimulation in untreated cats 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. It is suggested that raphe projection to intralaminar thalamus excerts 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 is discussed.