Antinociceptive effects of phenobarbital in "tail-flick" test and deafferentation pain

Deafferentation pain has been related to abnormal electrical hyperactivity in the neurons of the sensory relays in the central nervous system. This electrical activity resembles the epileptoid pattern observed in experimental epileptoid foci. With the aim of preventing this hyperactivity, rats were given long-term treatment with phenobarbital after sciatic transection and dorsal cervical rhizotomy. Daily intramuscular injections of saline solution or 5 and 10 mg/kg of phenobarbital were administered for 20 days, starting 10 days before surgery. Larger doses of phenobarbital delayed the onset and reduced the severity of autotomy. In a test of acute pain, the effect of intraperitoneal (1-16 mg) and intrathecal (100-500 micrograms) phenobarbital was studied by measuring the "tail-flick" response latency. Intraperitoneal phenobarbital did not modify acute pain, but 500 micrograms of intrathecal phenobarbital increased the threshold of pain. These results indicate that (a) phenobarbital, a drug with anticonvulsant activity, reduces deafferentation behavior in rats, and (b) intrathecal phenobarbital has an antinociceptive action in acute experimental pain.