Spinal Cord Differentially Modulates lntracerebroventricularly Administered Morphine- and -Endorphin-lnduced Antinociception in the Mouse1

Our study was designed to study the modulatory role of nitric oxide/cyclic guanosine monophosphate in the spinal cord on the antinociception induced by morphine and f3-endorphin given supraspinally. The antinociception was assessed by the tail-flick test in male ICR mice. The antinociception induced by intracerebroventricularly (i.c.v.) administered morphine was po-tentiated by intrathecal (i.t.) injection of N#{176}-nitro-L-arginine (2 /.Lg), hemoglobin (120.tg) or methylene blue (5 j.g), but was attenuated by i.t. administered L-arginine (20 Ag) or 3-morpho-lino-sydnonimine (SIN-i, 5 pg). In contrast, the antinociception induced by i.c.v. administered fJ-endorphin was attenuated by i.t. pretreatment with N’-nitro-L-arginine (2.tg) and the atten-uation of -endorphin-induced antinociception by N”-nitro-L-arginine was reversed by i.t. administered L-arginine (20 p.g). The antinociception induced by i.c.v. administered (3-endorphi