Antinociception induced by intracerebroventricular or intrathecal administration of gentamicin in rats.

1. The intrathecal (i.t.) administration of gentamicin (AD50 = 3.98 micrograms) produced marked and dose-dependent antinociception in rats, as assessed by the tail-flick test. A weaker effect was obtained following intracerebroventricular drug administration (AD50 = 50.12 micrograms). 2. A transitory but dose-dependent antagonism of the effect of i.t. gentamicin was obtained following i.t. administration of calcium chloride (4 and 8 micrograms). 3. The magnitude and duration of morphine antinociception (5 micrograms, i.t.) were enhanced by the previous (intraperitoneal) i.p. or i.t. administration of gentamicin. 4. These results indicate that gentamicin-induced antinociception may depend on its calcium-antagonist property and involves drug diffusion to a spinal site of action.