EFFECTS OF BACLOFEN ON JUNCTIONAL TRANSMISSION IN RAT PHRENIC DIAPHRAGM PREPARATION

The pre- and postsynaptic effects of the antispastic drug β(p-chlorophenyl)-γ-aminobutyric acid (baclofen, Lioresal) on the rat phrenic diaphragm neuromuscular junction were studied. At very high concentrations (2.10-4 M) baclofen decreases the efficiency of synaptic transmission, as indicated by a decrease in the size of endplate potentials). More detailed electrophysiological examination indicated that this depression of transmission is caused by both a presynaptic change (the quantal content decreases) and postsynaptic changes (the input resistance of the muscle fibres is decreased and the decay phase of the end plate current is shortened). At somewhat lower concentrations of baclofen (2.10-5 M) the depression of transmission is less pronounced. At the lowest concentrations used in this study (2.10-6 M) baclofen increases the efficiency of synaptic transmission (the endplate potentials increase in size). This seems to be caused by an increase in quantal content with no significant postsynaptic action. It is concluded that at concentrations likély to be reached after therapeutic administration, the action of baclofen on the neuromuscular junction does not explain its antispastic action and probably does not contribute significantly to it. © 1979.