The role of spinal voltage-sensitive calcium channels (VSCC) in a behavioral model of prolonged nociception was examined in rats. Blockade of VSCC by the trivalent cations neodymium (NdCl3) and lanthanum (LaCl3) resulted in a dose-dependent suppression of both phases of the response to formalin. omega-Conopeptides, which selectively block N-type VSCC, also produced a dose-dependent inhibition of both the initial behavior [phase 1; ED50 (nmol): SNX-111 (0.003) > SNX-185 (0.010) > SNX-239 (0.16) >> SNX-159 (> 0.26); SNX-199 (> 0.30)] and the facilitated response [phase 2; ED50 (nmol): SNX-111 (0.003) > SNX-185 (0.009) > SNX-239 (0.020) > SNX-159 (0.120) = SNX-199 (0.230)]. In contrast, SNX-231 (0.24 nmol), which is selective for a non-L/non-N site and also the L-type VSCC blockers nifedipine (24 nmol), nimodipine (29 nmol), verapamil (200 nmol), and diltiazem (220 nmol), had minimal effects on either phase of the formalin test at the highest dose examined. The P-type channel blocker omega-agatoxin IVA produced a 40% inhibition of phase 1 at the highest dose and phase 2 was suppressed in a dose-dependent fashion (ED50, 0.001 nmol). The response latency to a high-threshold thermal stimulus (the 52.5 degrees C hot plate) was moderately (20%) increased by NdCl3 (0.30 nmol) and SNX-111 (0.008 nmol), but not verapamil (200 nmol) and omega-agatoxin IVA (0.006 nmol). High doses of the N-type VSCC produced characteristic shaking behavior, serpentine-like tail movements, and impaired coordination. However, at antinociceptive doses there was no significant motor effect, though three of the N-type antagonists produced some tail movements. These studies demonstrate that VSCC of the N- and P-type, but not L-type, are involved in facilitated nociceptive processing at the spinal level.

Injection of formalin into the hind paw evokes a biphasic flinching of the injured paw. Pharmacological characterization of this behavior has implicated the spinal release of excitatory amino acids (EAAs) and cyclooxygenase (COX) products. To address this hypothesis, we examined the effect of paw formalin injection on release of EAAs and prostaglandin E2-like immunoreactivity (PGE2-LI) from the spinal cord in unanesthetized rats using a dialysis probe placed in the lumbar subarachnoid space. To assess the contribution of spinal COX products, the effects of S(+)- and R(-)-ibuprofen (active and inactive COX inhibitors) were examined. Paw formalin injection evoked a biphasic spinal release of PGE2-LI with an increase above resting concentrations of 110% in the 0-10 min sample, and of 83% in the 20-30 min sample. Significantly increased release of glutamate (Glu; 110%) and aspartate (Asp; 112%) was only observed in the 0-10 min sample. Saline injection into the paw had no effect on behavior, PGE2-LI, or EAA release. Intraperitoneal administration of 10 mg/kg, but not 1 mg/kg, S(+)-ibuprofen reduced paw flinching, blocked the elevated levels of PGE2-LI, and suppressed Glu and Asp release to 50% of control. Intrathecal delivery of 10 micrograms, but not 1 microgram, S(+)-ibuprofen also suppressed formalin-induced behavior, PGE2-LI, Glu, and Asp release. R(-)-ibuprofen showed no effect on formalin-induced behaviors or spinal release. These data demonstrate that paw formalin injection produces spinal release of PGE2-LI corresponding to the biphasic behavioral response and that the evoked release is blocked by antinociceptive doses of COX inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)