Regulation of sigma activity by the amino-terminus of substance P in the mouse spinal cord: involvement of phencyclidine (PCP) sites not linked to N-methyl-D-aspartate (NMDA) activity.

Behavioral responses to kainic acid (KA) injected intrathecally in mice are enhanced by N-but not C-terminal fragments of substance P (SP). Repeated injections of KA result in sensitization to KA-induced activity, an effect that appears to be mediated by SP N-terminal activity and inhibited by PCP ligands. The present study was initiated to determine whether the ability of SP N-terminal fragments to enhance KA activity is also sensitive to PCP ligands. We compared the effect of a PCP ligand, dizocilpine (MK-801), to that of haloperidol, a sigma ligand and dopamine antagonist. MK-801 (1 nmol) failed to alter the enhancement of behavioral responses to KA (25 pmol) produced by SP(1-7) (22.5 pmol, 30 min). However, pretreatment with 1 nmol of either haloperidol or the N-terminal SP antagonist, [D-Pro2-D-Phe7]SP(1-7) [D-SP(1-7)], prevented potentiation of KA by SP(1-7). Like SP(1-7), 5 nmol of the sigma ligand 1,3-di(2-tolyl)guanidine (DTG) also enhanced behaviors elicited by KA, and this effect was also blocked by haloperidol or D-SP(1-7), but not spiperone (2.5 nmol), a dopamine antagonist. Together these data suggest that sigma receptors are involved in the potentiation of KA. A large dose of SP(1-7) (10 nmol) or DTG (20 nmol) did not alter the response to KA 24 hr later, yet further potentiated responses to KA 30 min after SP(1-7) (22.5 pmol) or DTG (5 nmol), suggesting sensitization to the effects of these compounds.(ABSTRACT TRUNCATED AT 250 WORDS)