Axon-to-axon transmission in tullidora (buckthorn) neuropathy

Oral administration of ether extracts of the tullidora ( Karwinskia humboldtiana ) fruit, which contains an identified neurotoxin, produced flaccid hind limb paralysis in cats after a latency of 4 to 7 weeks. Acute experiments were conducted after the paralysis was evident. Spinal roots of lumbar and sacral segments were transected as close as possible to the spinal cord and divided into several filaments. Stimulation of some filaments distal to the transection evoked action potentials in other filaments (axon-to-axon transmission or cross talk) after a latency of at least 8 ms. Cross-talk responses frequently consisted of multiple discharges. Axon-to-axon transmission was seen only between motor axons and disappeared when hind limb nerves were transected 10 to 15 cm from the spinal cord. Twin pulses were applied to a filament at various intervals; the pulse intensity was adjusted so that the conditioning pulse was subthreshold to elicit cross talk, but the test pulse frequently elicited it (temporal facilitation). In three fully studied fibers the facilitation was prolonged to 50 to 80 ms. In some cases, no cross talk was evoked in a given filament by individual stimulation of two other filaments, but simultaneous stimulation of the same filaments did evoke cross talk (spatial facilitation). Series of periodic bursts of activity spontaneously occurred in those axons responding with multiple discharges to single stimulation of other axons. At low temperatures (about 30 degrees C) the stimulus could trigger essentially similar series of bursts. Single motoneurons were intracellularly stimulated by brief depolarizing pulses. The action potential elicited by the stimulus was followed after several msec by a secondary train of discharges generated at the periphery ("back firing").