Ionic Species Involved in the Electrical Activity of Single Adult Aminergic Neurones Isolated from the Sixth Abdominal Ganglion of the Cockroach Periplaneta Americana

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Abstract
Adult neurones were obtained by dissociation of the dorsal area of the sixth abdominal (A6) ganglion of the cockroach, and electrical properties were studied with the patch-clamp technique. The neurones showed spontaneous fast action potentials, similar to those recorded with microelectrodes in neurones in situ along the dorsal median line of the A6 ganglion. Synthetic saxitoxin (sSTX) at concentrations of 10 × 10−8 to 1.0×10−7mol l−1 suppressed the action potential (AP) and induced a dose-dependent hyperpolarization of the resting potential, suggesting that two types of sSTX-sensitive Na+ channels are present. The resting potential was dependent on the external concentration of both Na+ and K+, with a similar sensitivity to each, yielding a slope of about 43 mV per 10-fold change in concentration. The delayed outward rectification present under control conditions was reduced by tetraethylammonium chloride (TEA-Cl, 1.0×10−2mol l−1). TEA-Cl or Ca2+-free saline abolished the afterhyperpolarization and increased the overshoot and duration of triggered APs, indicating that a calcium-activated potassium conductance contributes to the falling phase of the AP. At 3.0×10−3mol l−1, the Ca2+ channel blockers MnCl2, CoCl2 and NiCl2 lengthened the AP. A blocker-dependent increase in the overshoot and threshold of the AP and reduction of the afterhyperpolarization were observed, probably reflecting the relative potencies of these ions in blocking Ca2+ channels and thus the Ca2+- activated K+ conductance. Increasing MgCl2 concentration by 3.0 × 10−3mol l−1 had no effect on the AP, indicating that the positive shift of the threshold is due to the blockade of Ca2+ channels present at this potential. The results suggest that these isolated neurones are dorsal unpaired median neurones previously studied in a number of insect species.