Figure 4. (A) Lidocaine 500 micro gram was administered intrathecally to six neuropathic rats (10 micro liter of 5% lidocaine in saline). This dose was chosen to exceed hypothetical cerebrospinal fluid concentrations of lidocaine achieved with intravenous infusion. All rats developed complete, transient lower body flaccidity (effective spinal anesthesia). After resolution of motor weakness, paw withdrawal thresholds were compared to predrug values. The significant albeit small effect at 45 min (P = 0.02, repeated measures analysis of variance with means comparisons contrast tests) was not present at 30 min and was absent by 60 min. (B) Catheters were implanted at the site of nerve ligation with orifices directed centrally (6 rats). After full recovery and documentation of withdrawal thresholds to von Frey hairs, catheters were injected with 50 micro liter of 2% lidocaine (1 mg = approximately 4 mg/kg in a 250 g rat). Transient analgesia to paw pinch and partial paw weakness were seen in all six rats (data not shown), demonstrating satisfactory delivery to the ligation site. By 15 min after treatment, von Frey hair thresholds were again indistinguishable from pretreatment (P = 0.97, repeated measures analysis of variance, means comparisons contrasts). X-axes: time; y-axes: paw withdrawal thresholds expressed as percent of maximum possible effect.