[The effects of intrathecally administered baclofen on somato-sympathetic reflex potentials]

We investigated the antinociceptive effect of intrathecally administered S(-)-baclofen and R (+)-baclofen in cats using somato-sympathetic reflex potentials derived from lumbar sympathetic ganglion by stimulation of the femoral nerve. S(-)-baclofen 10 mg maximally reduced both A reflex potential and C reflex potential to 65.3% and 83.7% of control values, respectively, 1 minute after the administration, but these changes were not significant. The inhibition of A reflex potential at this dosage was greater than that of blood pressure and heart rate induced by the same dosage of S(-)-baclofen. While, R(+)-baclofen 1 mg maximally inhibited A reflex potential to 48.6% of control value 20 minutes after the administration, and this change was significant (P < 0.05). The inhibition of A reflex potential was greater than that of blood pressure and heart rate induced by the same dosage of R(+)-baclofen. This dosage of baclofen reduced C reflex potential to 66.4% of control value 10 minutes after the administration. The degree of reduction of A and C reflex potentials induced by 1 mg of R(+)-baclofen was higher in comparison with that induced by 10 mg of S(-)-baclofen. These reduction of A and C reflex potentials were reversed by 1 to 1.5 mg of intrathecally administered saclofen. These results indicate that R(+)-baclofen has higher potency than S(-)-baclofen and suggest that S(-)-baclofen and R(+)-baclofen show antinociceptive effect by intrathecal administration. On the other hand, it is possible that these two drugs exert antinociceptive effect via A delta fiber.