Synaptic and somatic effects of axotomy in the intact, innervated rat sympathetic neuron

A biophysical description of the axotomized rat sympathetic neuron is reported, obtained by the two-electrode voltage-clamp technique in mature, intact superior cervical ganglia in vitro. Multiple aspects of neuron functioning were tested. Synaptic conductance activated by the whole presynaptic input decreased to 29% of the control value (0.92 muS per neuron) 1 day after axotomy and to 18% after 3 days. Despite the decrease in amplitude of the macroscopic current, miniature excitatory postsynaptic current (mEPSC) mean conductance, acetylcholine (ACh) equilibrium potential, and EPSC decay time constant were unaffected. Synaptic efficacy was tested during paired-pulse or maintained stimulation (5, 10, and 15 Hz, 10-s duration). Quantal release in axotomized neurons was preserved during the tetanus despite the reduction of the initial EPSC amplitude, suggesting that ACh secretion depended on the number of surviving synapses; each of them exhibited dynamic behavior during trains similar to that of normal synapses. Facilitation of EPSC amplitude was noted in 2-day axotomized neurons during the first few impulses in the train. Voltage-dependent potassium currents (the delayed I(KD) and the transient I(A)) exhibited an early drastic decrease in peak amplitude; these effects persisted 7 days after axotomy. Marked changes in I(A) kinetics occurred after injury: the steady-state inactivation curve shifted by up to +17 mV toward positive potentials and the voltage sensitivity of inactivation removal became steeper. I(A) impairment was reflected in a reduced inward threshold charge for discharge and reduced spike repolarization rate. Synaptic and somatic data were applied in a mathematical model to describe the progressive decrease in the safety factor, and the eventual failure of ganglionic transmission after axotomy.