AUTOACTIVITY OF A5 NEURONS - ROLE OF SUBTHRESHOLD OSCILLATIONS AND PERSISTENT NA+ CURRENT

A5 noradrenergic neurons play a key role in autonomic regulation, noci ception, and respiration. The purpose of the present experiments was t o characterize some of the intrinsic properties of A5 cells in vitro. Whole cell recordings were obtained from 85 spinally projecting neuron s of the ventrolateral pens of neonate rats. Immunohistochemistry show ed that 60% of the ventrolateral pontine cells were noradrenergic. Eig hty percent of A5 neurons were spontaneously active (0.1-5.5 spikes/s) . Their discharge rate was unchanged by a mixture of synaptic blockers that eliminated postsynaptic potentials (PSPs). The nonnoradrenergic cells could not be distinguished from A5 cells on the basis of dischar ge rate, action potential duration, inward rectification, input resist ance, or accommodation. A5 cells displayed subthreshold irregular osci llations of the membrane potential (main frequency component 0.5-2 Hz) . These oscillations were unchanged in the presence of low external Ca 2+-high Mg2+ and were very reduced by hyperpolarizing the cells below -65 mV. The oscillations were partially attenuated by 1 mu M tetrodoto xin (TTX) and were eliminated by reducing external Na+ (27 mM). Steppi ng the membrane potential from -65 to -50 mV for 200 ms revealed the p resence of a transient and a persistent inward current that were both blocked by 0.1 mu M TTX Or by extracellular Na+ reduction. In conclusi on, most A5 neurons are spontaneously active in vitro. They display ir regular subthreshold membrane potential oscillations generated by volt age-activated conductances that include a persistent TTX-sensitive Na current. Most of the activity of A5 cells appears due to intrinsic pr operties rather than to synaptic inputs.