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

Antinociceptive effect of intrathecal neostigmine has been investigated using withdrawal responses in laboratory experiments. However, neostigmine administered in this route can induce muscle weakness or sedation. As a result, these effect can modify withdrawal responses. In this study we investigated the antinociceptive effect of intrathecally administered neostigmine bromide (NB) in cats objectively and quantitatively using somato-sympathetic reflex potentials.

Anesthetized cats were inserted with a spinal catheter via atlanto-occipital membrane and 10 micrograms (n = 5), 50 micrograms (n = 5), 250 micrograms (n = 8) or 2 mg (n = 5) of NB was administered through this route. Somato-sympathetic reflex potentials (A and C reflex) derived from lumbar sympathetic ganglion by stimulation of the femoral nerve and mean arterial blood pressure (MAP) and heart rate (HR) in each dose were monitored and recorded.

Reflex potential was reduced in a dose dependent fashion by NB but these changes were not significant. On the other hand, 250 micrograms and 2 mg of NB maximally reduced C reflex potential to 55.2% and 24.0% of control values respectively 20 minutes after the administration (P < 0.01), and this reduction was reversed by 250 to 2 mg of intrathecal administration of atropine sulfate to 89.4%, 68.6% of control values respectively. MAP and HR decreased in dose dependent fashion by NB to 57.6%, 65.2% of control values after the administration of 2 mg of NB and these cardiovascular changes recovered by intrathecally administered atropine sulfate.

These results objectively indicate that intrathecally administered neostigmine bromide shows antinociceptive effect and this inhibition is partially mediated by cholinergic mechanism.