Enhancement of resting activity in postganglionic vasoconstrictor neurones following short-lasting repetitive activation of preganglionic axons.

Resting activity in postganglionic neurones supplying the cat hindlimb was enhanced after activation of preganglionic axons in the lumbar sympathetic trunk with short trains of stimuli (50 stimuli at 25 Hz). The characteristics of this enhancement were as follows: 1) It has values of 120 to 600% and lasts for 4 to more than 40 min. 2) It can be elicited heterosynaptically by repetitive stimulation of the peripheral end of a cut white ramus. 3) It is dependent on the activation of thin, probably unmyelinated, preganglionic axons. 4) It is probably produced by non-nicotinic (muscarinic and non-cholinergic) synaptic mechanisms in the sympathetic chain ganglia. 5) Vasoconstrictor neurones can be activated in this way, but not sudomotor neurones. The results argue that nicotinic transmission of activity from pre- to postganglionic vasoconstrictor neurones can be modulated heterosynaptically by slow non-nicotinic synaptic processes.