Potassium currents in submucous neurones of guinea-pig caecum and their synaptic modification

1. Intracellular recordings were made from submucous neurones of the guinea-pig caecum. In most experiments, membrane currents were measured using a single-electrode voltage clamp. 2. A potassium current dependent on calcium influx occurred at rest (approximately equal to 200 pA at -60 mV). The amplitude of the current was increased up to 1 nA at -35 mV and decreased to zero at -100 mV; when fully activated the current did not show any inactivation. An inward calcium current, of 15-25 pA in amplitude near -60 mV and insensitive to omega-conotoxin (0.5 microM), probably activated the potassium current. 3. Step depolarizations from potentials negative to -80 mV evoked a transient (less than or equal to 200 ms at -40 mV) potassium current which was blocked by 4-aminopyridine (1-3 mM). Hyperpolarizing commands to potentials negative to -87 mV evoked an inwardly rectifying potassium current which was selectively blocked by caesium (1-2 mM). The residual cell current between -100 and -40 mV in calcium-free solution containing tetraethylammonium (20 mM), caesium (2 mM) and 4-amino-pyridine (3 mM) conformed to constant field assumptions. This current was called a background potassium current. 4. Decrease in membrane conductance during the slow excitatory postsynaptic current (EPSC) was due predominantly (greater than or equal to 90%) to a reduction in the calcium-activated potassium current at -35 mV, but due almost exclusively to a reduction in the background potassium current at potentials more negative than -100 mV. The relative contribution of the two currents to the slow EPSC was entirely dependent on the relative contribution of the currents to the membrane conductance at given potentials. 5. The transient potassium current was unaffected or slightly enhanced during the slow EPSC. The inwardly rectifying potassium current was unaffected during the slow EPSC. 6. Three tachykinins (substance P, substance K and neurokinin B; 3-800 nM), forskolin (1-30 microM), 8-bromoadenosine 3':5'-cyclic monophosphate (8-bromo cyclic AMP; 1-3 mM), 3-isobutyl-1-methylxanthine (0.3-1 mM) mimicked the conductance changes during the slow EPSC in a concentration-dependent manner. 7. It is concluded that the slow excitatory synaptic potential in the submucous plexus, presumably mediated by peptidergic transmitters, results from an inactivation of two distinct potassium currents, at least one of which is controlled by intracellular calcium ions.