Alternative approaches to analgesia: baclofen as a model compound.

1 It is suggested that analgesia could be produced by drug action at the spinal level through (a) interference with neurotransmission at primary afferent terminals; (b) enhancement of the ;gate control' of the sensory input to the spinal cord mediated through descending spinal tracts; or (c) increased presynaptic inhibition of primary afferents by a direct action.2 Baclofen (9.4-70.3 mumol/kg, i.p.), which may mimic spinal presynaptic inhibition, produced a dose-dependent increase in the response times of mice in a hot-plate test, but high doses also impaired motor function.3 Morphine hydrochloride (5.3-40 mumol/kg, i.p.) increased the response time of mice in the hot-plate test and had little effect on motor function.4 Combination of baclofen (9.4 or 23.4 mumol/kg) with morphine (13.3 mumol/kg) produced greater increases in response time than either drug administered alone but with little concurrent effect on motor function.5 The possibility that baclofen may have some analgesic action and a potentiating effect on other analgesics is discussed.