A novel, calcium-induced triphasic current response in silent, non-bursting Aplysia neurons

The cyclic GMP-induced inward current in neuron R14 of Aplysia californica: similarity to a FMRFamide-induced inward current

Injecting cGMP into Aplysia neuron R14 induced an inward current similar to one elicited by application of FMRFamide to the outside of that cell. In contrast, injection of cAMP into R14 caused a long-lasting outward current and conductance increase. Phosphodiesterase inhibitors increased the cGMP and FMRFamide-induced inward currents in R14. The cGMP-induced inward current is voltage dependent and is largely carried by Na+. It is also strongly and inversely dependent on both external [Ca2+] and [Cl-], although these ions are not significant current carriers. Changing external [K+] had no effect. Voltage and ion dependencies of the cGMP-induced inward current are similar to those of an inward current induced by FMRFamide. Thus cGMP may be a second messenger to FMRFamide in producing a slow inward current in R14. cGMP does not appear to be a second messenger to FMRFamide in most Aplysia neurons.

A calcium-activated, nonselective cationic conductance in Aplysia silent neurons

We have previously reported the activation of a triphasic current response by calcium injection in voltage-clamped, nonbursting neurons of Aplysia californica. Present evidence indicates that the second phase, a delayed inward current that peaks 10-20 seconds after the end of the injection, is a calcium-activated, nonselective cationic conductance. It can be carried by both sodium and calcium, is not sensitive to chloride concentration changes, but is voltage sensitive, decreasing in amplitude with hyperpolarization.

A long-lasting inward current induced by intracellular injection of calcium ions in identified neurons of Helix pomatia

Intracellular Ca2+ is known to activate an early unspecific inward and a potassium outward current in various vertebrate and invertebrate preparations. In the identified neurons B1, B2 and B3 of the buccal ganglia of Helix pomatia an intracellular application of Ca2+ induced a late long-lasting inward current besides the currents mentioned. Ca2+ was injected by ionophoresis or pressure pulses. The long-lasting inward current was reversed at membrane potentials of between -30 and -40 mV, whereas the equilibrium potential of the unspecific inward and potassium outward current was in the range of -20 to +10 mV and -50 to -70 mV, respectively. The late inward current was accompanied by an increase of the membrane conductance and suppressed by the Cl- channel blocker 9-anthracencarbonic acid. The equilibrium potential shifted with changes in the extracellular Cl- concentration. The long-lasting inward current is suggested to be activated by intracellular Ca2+ and to be carried by Cl-.