Ionic currents in neurones cultured from embryonic cockroach (Periplaneta americana) brains

Neurones isolated from embryonic cockroach brains were maintained in culture for up to 8 weeks. A single patch electrode was used to record voltage changes in response to injected current, membrane ionic currents under whole-cell voltage-clamp conditions or single-channel currents from isolated membrane patches. The voltage changes in response to injected current that depolarized the cell indicated increases in membrane permeability to calcium and potassium. These observations were confirmed using a voltage clamp. The potassium current observed in the youngest cultures turned on with a delay and was blocked by tetraethylammonium (TEA) and 4-aminopyridine (4-AP). Two kinds of decrease in the outward potassium current were observed. One may be associated with extracellular potassium accumulation, inactivation of the potassium channel or inactivation of a calcium channel. The other appears to be a voltage-dependent inactivation. The magnitude of the calcium permeability appeared to increase as the cultures developed, being most prominent in cultures more than 2 weeks old. Single-channel conductance measured from an analysis of records from six isolated membrane patches ranged from 15 to 110 pS. Except for one channel, the probability of the channels being open did not change appreciably with membrane potential. Our results suggest that much of the increase in potassium permeability may be due an increase in intracellular calcium level.