Stimulation of 5-HT1 receptors in the spinal cord changes substance P-induced behaviour

This study examined whether intrathecal (i.th.) injection of different 5-hydroxytryptamine (5-HT) receptor agonists modulated the behavioural response to substance P. Given intrathecally, substance P produces a behavioural syndrome consisting of biting of the lower parts of the abdomen and reciprocal hindlimb scratching, which may be indicative of nociceptive stimulation. The number of substance P-induced bites was reduced when counted 5 min after intrathecal injection of 5-HT, p-chloroamphetamine (PCA) which causes release of 5-HT from neuronal terminals, the non-selective 5-HT receptor agonist quipazine, the selective 5-HT1 receptor agonists (+)-8-hydroxy-2-(di-n-propylamino)tetralin [(+)-8-OH-DPAT], 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole (RU 24969) and 1(m-chlorophenyl)piperazine (mCPP), but was unchanged by treatment with the 5-HT2/5-HT1C receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). The number of scratches was significantly increased 5 min after injection of 5-HT and RU 24969. The results showed that intrathecal injection of 5-HT agonists, with a high affinity for the 5-HT1 receptor subtypes, reduced the total number of responses induced by intrathecal injection of substance P, whereas a 5-HT2/5-HT1C receptor agonist did not affect the behavioural response to the intrathecal injection of substance P.