FAST NA-PIG SUBSTANTIA-NIGRA PARS COMPACTA NEURONS( SPIKE GENERATION IN DENDRITES OF GUINEA)

Electric fields were applied to study the regenerative properties of s ubstantia nigra pars compacta neurons in guinea-pig brain slices. Two types of spikes, of high or low amplitude, were generated in both the soma-hyperpolarizing and the soma-depolarizing directions of the field . The different sensitivity of the spikes to somatic polarization sugg ested that the high-amplitude spikes were generated near the cell body , whereas the low-amplitude spikes were generated at a distance from t he soma. Application of tetrodotoxin or intracellular injection of QX 314 abolished both types of spike. The spikes were not inhibited in th e presence of glutamate receptor antagonists or during Ca2+ channel bl ockade. Blockers of gap junctional conductance (sodium propionate, oct anol and halothane) did not affect the field-induced spikes. The spike generation was highly sensitive to changes in membrane conductance in duced by current injection in the soma or by external field applicatio n. The ability of a conditioning field stimulation to affect the spike generation in different neuronal compartments suggested that a transi ent outward current was generated in the dendrites. The field-induced spikes were facilitated by synaptic stimulation and, in some neurons, low-amplitude spikes were generated by synaptic potentials in the abse nce of field application. These results suggest that channels responsi ble for Na+ spike generation reside in the dendrites, and are influenc ed by spatially distributed voltage-dependent K+ currents and by synap tic input. Copyright (C) 1996 IBRO. Published by Elsevier Science Ltd.