THE LACK OF EFFECT OF BACLOFEN ON ACTION-POTENTIALS OF SPINAL MOTOR AXON COLLATERAL TERMINATIONS IN-VIVO

In the lumbar ventral horn of pentobarbitone-anaesthetised cats, (-)-b aclofen reduces both the synaptic release of excitatory transmitter fr om muscle group Ia afferent terminations and tile duration of tile pre synaptic action potentials of these terminations, presumably by interf ering with the influx of calcium ions through voltage-activated channe ls. Baclofen, however, has little or no effect on cholinergic excitati on at motor axon collateral synapses on spinal Renshaw cells and, in t he present study, was found not to reduce the duration of the action p otential of axon collateral terminations located in the vicinity of Re nshaw cells in pentobarbitone-anaesthetised cats. Furthermore, in cont rast to group Ia terminations, a 4-aminopyridine-sensitive potassium c onductance could not be detected as contributing to axon collateral te rmination action potentials. These results suggest that there may be d ifferences in presynaptic ion fluxes associated with transmitter relea se at the intraspinal terminations of group Ia afferent fibres and mot or axon collaterals in the cat spinal cord.