NA-DEPENDENT NEURITIC SPIKES INITIATE CA2+-DEPENDENT SOMATIC PLATEAU ACTION-POTENTIALS IN INSECT DORSAL PAIRED MEDIAN NEURONS()

The origin of plateau action potentials was studied in short-term cult ures of dorsal paired median (DPM) neurons dissociated from the termin al abdominal ganglion of the cockroach, Periplaneta americana. Spontan eous plateau action potentials were recorded by intracellular microele ctrodes in cell bodies that had neurite stumps. These action potential s featured a fast initial depolarization followed by a plateau. Howeve r, only fast spikes of short duration were observed when the cell was hyperpolarized from the resting membrane potential. These two differen t components of the action potentials could be separated by applying d epolarizing current pulses from a hyperpolarized holding potential. Ap plication of 200 nM tetrodotoxin (TTX) abolished both fast and slow ph ases, but depolarization to the original resting potential by steady c urrent injection triggered slow monophasic action potentials that coul d be blocked by 3 mM CoCl2. In contrast, DPM neurons without neurites were not spontaneously active. In these cells, calcium-dependent slow monophasic action potentials were only recorded immediately after impa lement or with current pulse stimulation. Immunocytochemical observati ons showed that dorsal unpaired median (DUM) neuron cell bodies, which are known to exhibit spontaneous sodium-dependent action potentials, reacted with an antibody directed against a synthetic peptide correspo nding to the SP19 segment of voltage-activated sodium channels. In con trast, the antibody did not stain DPM neuron cell bodies but gave inte nse, patchy staining only in the neurite. Whole cell patch-clamp exper iments performed on isolated DPM neuron cell bodies without a neurite revealed the presence of an inward current that did not inactivate com pletly within the duration of the test pulse. This current was insensi tive to both 100 nM TTX and sodium-free saline. It was defined as a hi gh-voltage-activated calcium current according to its high threshold o f activation (-30 mV) and its sensitivity to 1 mM CdCl2 and 100 nM ome ga-conotoxin GVIA. Our findings demonstrate that spontaneous sodium-de pendent spikes arising from the neurite are required to initiate slow somatic calcium-dependent action potentials in DPM neurons.