DIFFERENT CALCIUM CHANNELS ARE COUPLED TO POTASSIUM CHANNELS WITH DISTINCT PHYSIOLOGICAL ROLES IN VAGAL NEURONS

Whole-cell and sharp microelectrode recordings were obtained from neur ons of rat dorsal motor nucleus of the vagus (DMV) in transverse slice s of the rat medulla maintained in vitro. Calcium currents were studie d with sodium currents blocked with tetrodotoxin, potassium currents b locked by perfusing the cell with caesium as the main cation and using barium as the charge carrier. From a holding potential of -60 mV, inw ard currents activated at potentials positive of -50 mV and peaked aro und 0 mV. Voltage clamping the neuron at more hyperpolarised potential s did not reveal any low-threshold inward current. The inward current was effectively blocked by cadmium (100 mu M) and nicked (1 mM), sugge sting that it is carried by voltage-dependent calcium channels. The in ward current could be separated into three pharmacologically distinct components: 40% of the whole cell current was omega-conotoxin sensitiv e; 20% of the current was nifedipine sensitive; and the rest was block ed by high concentrations of cadmium and nickel. This remaining curren t cannot be due to P-type calcium channels as omega-agatoxin had no ef fect on the inward current. Nifedipine had no significant effect on th e action potential. Application of omega-conotoxin reduced the calcium component of the action potential and significantly reduced the potas sium current underlying the afterhyperpolarization. Application of cha rybdotoxin slowed action potential repolarization. When N-type calcium channels were blocked with omega-conotoxin, charybdotoxin was still e ffective in slowing repolarization. In contrast, charybdotoxin was ine ffective when calcium influx was blocked with the nonspecific calcium channel blocker cadmium. These results show that, during the action po tential, both omega-conotoxin-sensitive and omega-conotoxin-insensitiv e calcium channels contribute to calcium influx. Calcium influx via N- type channels is responsible for activating potassium channels underly ing the afterhyperpolarization. In contrast, influx via omega-CgTx and nifedipine-insensitive channels is responsible for activating potassi um channels contributing to action potential repolarization. Calcium-a ctivated potassium channels are thought to be in close proximity to vo ltage-dependent calcium channels. These results therefore suggest that in vagal neurons different calcium channels are colocalized with diff erent calcium-activated potassium channels that subserve distinct func tional roles.