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Abstract

A survey of effects of 4-aminopyridine (4-AP) on synaptic transmission in the cat spinal cord (Jankowska, Lundberg, Rudomin and Syková, 1977) was made with a twofold aim: (i) to compare effects of 4-AP on excitatory and inhibitory synapses in various spinal pathways and (ii) to test these effects under conditions closest to clinical conditions, allowing comparison of effects of systemic and of local or in vitro applications. 4-AP has been found to facilitate all tested excitatory as well as inhibitory responses. It enhanced monosynaptic and polysynaptic excitation and di synaptic and polysynaptic inhibition of motoneurones, excitation of ascending tract cells and presynaptic depolarization of primary afferents; these synaptic actions were evoked by different types of afferents and various descending systems and were mediated via different interneuronal systems. In the case of the disynaptic inhibitory reflexes the evidence has been obtained that 4-AP may enhance both the excitation of the interposed interneurones and the inhibitory effects exerted by these interneurones on the motoneurones.The described effects were evoked by 4-AP in doses up to 1 mg/kg. They appeared within the first minute of the injection, reached maximum after about 10–15 min and remained stable during at least several hours.In an analysis of action of 4-AP on primary afferents and motoneurones no indications have been found of its direct effects on excitability of fibres or cells to electrical stimuli. No systematic changes in membrane potential of motoneurones, nor in duration of action potentials of these neurones or of primary afferents were seen. The study leads to the conclusion that small doses of 4-AP enhance various spinal reflexes primarily due to facilitation of synaptic transmission.