Effects of 4-aminopyridine on sympathetic preganglionic neuron activity.

Sympathetic preganglionic neuron (SPN) activity (mass or single unit discharge) was recorded extracellularly from axons of the cervical sympathetic trunk (CST) in unanaesthetized, midcollicular decerebrate or C1 spinal anemically decerebrate cats which were paralyzed and artificially ventilated. Phrenic motoneuron activity was also recorded in the midcollicular preparation. The drug 4-aminopyridine (4-AP) was administered intravenously. Doses of 0.5 to 1.0 mg/kg increased the background firing rate of SPNs in a dose-dependent manner. These doses increased the repetition rate of phrenic nerve bursts and, in a parallel fashion, that of the inspiration-synchronous activity of the CST. In contrast, the action of 4-AP on activity evoked by stimulation of high threshold myelinated afferents in the radial, femoral or pelvic nerve, or in the L7 dorsal root, was very variable. The evoked responses could be increased, decreased or not affected by the drug. Thus, treatment with 4-AP produces a graded increase in background firing of SPNs, but the drug is ineffective at reliably enhancing somato- or viscero-sympathetic reflexes.