The analgesic action of baclofen [beta-(4-chlorophenyl)-gamma-aminobutyric acid].

When administered to mice, baclofen induced dose-related antinocisponsive activity in the stretch, hot plate and tail-flick tests. Although high doses also impaired motor coordination, as measured by the rotating rod test, the following observations suggest that the pronounced antinocisponsive effect of higher doses did not reflect a reduced capacity to respond to perceived pain: motor impairment, having shorter onset of action, was fully developed before the appearance of any analgesic action; mice made completely tolerant to the motor effect were still analgesic; and other agents which severely impaired rotating rod performance did not necessarily show antinocisponsive activity in the hot plate test. The mechanism of action of baclofen is not similar to that of morphine since naloxone did not antagonize the analgesic effect of baclofen, and cross-tolerance between morphine- and baclofen-induced analgesia could not be demonstrated.