Is substance P a primary afferent neurotransmitter for nociceptive input? II. Spinalization does not reduce and intrathecal morphine potentiates behavioral responses to substance P.

Scratching elicited by the intrathecal (i.t.) administration of substance P (SP) into the lumbosacral spinal cord of rats was not reduced by spinalization or i.t. pretreatment with the analgesic morphine. Spinalization also did not affect scratching elicited by i.t. kainic acid and potentiated scratching elicited by i.t. strychnine, picrotoxin, and L-glutamic acid. Intrathecal morphine did, however, reduce scratching elicited by i.t. strychnine and kainic acid. These findings demonstrate that the scratching elicited by i.t. SP and other neuroexcitatory agents is a spinally mediated response. That this response, when elicited by SP, is not inhibited by the analgesic morphine strongly suggests that SP does not elicit scratching by action at the primary afferent synapse and that the response is not indicative of pain.