Effects of lysergic acid diethylamide (LSD) and adjuvant-induced inflammation on desensitization to and metabolism of substance P in the mouse spinal cord

We have previously shown that the caudally directed biting and scratching response to repeated intrathecal (i.t.) injections of substance P (SP) is decreased by the third injection of SP and that this apparent desensitization to SP is less pronounced in mice pretreated with Freund's adjuvant. This study was designed to study the mechanism of this desensitization to SP and to examine the effect of lysergic acid diethylamide tartrate (LSD) on desensitization. Our results indicate that while 25 μg of LSD/kg body weight i.p. in naive mice had no effect on the response to a single injection of SP, LSD decreased the development of desensitization to SP-induced behaviors. In contrast, identical injections of LSD in adjuvant-pretreated mice not only failed to prevent desensitization but enhanced the degree of apparent desensitization to SP. Tolerance developed to the effects of LSD on desensitization to SP-induced behaviors in both adjuvant- and saline-pretreated mice. When injected i.t. with SP, LSD failed to alter the degree of desensitization to SP-induced behaviors, suggesting that the effect of LSD is not produced at the spinal cord level. Separation and quantification of SP and its metabolites in the spinal cord using high performance liquid chromatography (HPLC) techniques indicated that either a single injection of LSD or pretreatment with Freund's adjuvant produced similar patterns of changes in the concentrations of SP-related peptides in mouse spinal cord. LSD treatment in mice presumed to be in chronic pain produced no further change in the concentrations of SP-related peptides, together suggesting that there may be a common mechanism in the effects of LSD and of chronic pain on the development of desensitization to SP-induced behaviors and on SP metabolism.