Evoked Activity in Nerve Trunks of the Rat: 4-Aminopyridine-Induced Modifications

In experiments on rats, we studied 4-aminopyridine (4-AP)-induced modifications of the excitability of peripheral nerve fibers in an efferent trunk, the ventral root (VR), and in a mixed trunk including both afferent and efferent fibers, the sciatic nerve (SN). For this purpose, we examined how 4-AP influenced the parameters of integral action potentials recorded from the VR and SN in three experimental modes. These were: (i) stimulation of the SN and recording of antidromic action potentials from the VR in vivo after systemic injections of 4-AP into the animal, (ii) stimulation of a preparation of the SN dissected from the animal after systemic injection of 4-AP and recording of action potentials from another segment of the same preparation in vitro, and (iii) stimulation of an SN preparation and recording of action potentials from another region of this preparation in vitro, but after direct application of the solution of 4-AP to this preparation. It was found that 4-AP significantly increased the threshold for generation of action potentials and enhanced their amplitude, decreased the duration of action potentials recorded from the VR, and shortened the refractory period following these responses. The drug also significantly increased the amplitude and decreased the duration of action potentials recorded from the SN in vitro after systemic injections of the agent, but the threshold for response generation in this preparation noticeably dropped; the post-response refractory period in this case showed no changes. Modifications of action potentials recorded from the SN in vitro after direct applications of 4-AP were in general similar to the described above. Other examined parameters of action potentials (chronaxia and dynamics of an increase in the amplitude related to intensification of stimulation) showed no significant changes under the influence of 4-AP. We conclude that 4-AP increases the excitability of nerve fibers in the nerve trunks under study, but not to the point where the electrical interaction between excited and nonexcited fibers in the fiber conductors under study (VR and SN) overcomes the threshold.