SPINAL ANTINOCICEPTIVE EFFECT OF SUBSTANCE-P ON THE RESPONSES INDUCEDBY INTRATHECALLY INJECTED NMDA IN MICE

Substance P (SP) has previously been shown to be involved in the trans mission of nociceptive information in the spinal dorsal hem. Ln this s tudy we investigated whether a functional interaction exists between S P and excitatory amino acids in the spinal cord of mice. Behavioral re sponses were observed after intrathecal co-administration of SP and N- methyl-D-aspartate (NMDA). The high dose (12 pmol) of SP potentiated N MDA (0.25 nmol)-induced behavior consisted of caudally directed lickin g and biting, while the low dose (1 pmol) of SP significantly reduced this behavior. This inhibitory effect of low dose of SP was blocked by intrathecal co-administration of opioid receptor antagonist naloxone (4 nmol). These results suggest that SP is also involved in the antino ciception which is dependent on opioid mechanisms at the spinal level.