SODIUM AND CALCIUM CURRENTS IN ACUTELY DISSOCIATED NEURONS FROM RAT SUPRACHIASMATIC NUCLEUS

1. Neurons were acutely dissociated from the suprachiasmatic nucleus (SCN) of adult rats and studied with whole-cell and perforated-patch recordings at room temperature. 2. Acutely dissociated SCN neurons had spherical cell bodies of 12 mum in average diameter. The recorded cells were randomly selected and had either no process (38%), one (41%), two (19%), or three processes (2%). They had a resting potential of about -60 mV, an input resistance of approximately 5 GOMEGA, and a cell capacitance of approximately 7 pF. 3. The dissociated neurons had variable spontaneous firing rates, typically (76%) < 1 Hz. 4. Under current clamp, continuous current injection elicited repetitive action potentials. 1 muM tetrodotoxin (TTX) reduced the amplitudes of the action potentials as well as the firing rate, whereas 200 muM Cd2+ stopped repetitive firing altogether. Action potentials were completely eliminated with Cd2+ and TTX present. These results suggest that both Na+ and Ca2+ contribute to the action potential in these cells. 5. With 200 muM Cd2+ present to block calcium currents, a train of brief depolarizing pulses could still elicit repetitive sodium action potentials, but these became attenuated at stimulating frequencies as low as 1 Hz. 6. Under voltage clamp, the sodium current was activated at about -40 mV and peaked at about -10 mV. It inactivated with a time constant of approximately 0.5 ms at 0 mV, and in steady state the current was half-inactivated at about -60 mV. Recovery of the current from inactivation showed two very different phases with time constants of approximately 30 and 600 ms at -60 mV. The slow phase was probably responsible for the very low firing rate of the sodium action potential. 7. In the absence of external sodium, depolarization-activated calcium action potentials were preferentially blocked by 20 muM Cd2+, whereas a posthyperpolarizing depolarization (or anode break) was preferentially reduced by 100 muM Ni2+. These differential effects hinted at the presence of both low-threshold and high-threshold calcium currents in these cells. 8. Voltage-clamp experiments confirmed the presence of a low-threshold, transient calcium current that was activated by depolarizations above -70 mV. It inactivated with a time constant of approximately 25 ms between -50 and -30 mV. Steady-state inactivation was half-complete at about -90 mV and complete at about -70 mV. This current was only weakly inhibited by 20 muM Cd2+ but almost completely blocked by 100 muM Ni2+. It was insensitive to nifedipine and omega-conotoxin GVIA. 9. High-threshold, slowly inactivating calcium currents were also observed, being activated by depolarizations positive to -30 mV. The currents were almost completely blocked by 20 muM Cd2+, whereas 100 muM Ni2+ was only partially effective. 10 muM nifedipine blocked 18% of the currents, 0.3 muM omega-conotoxin blocked 39% of the currents, and a combination of these two drugs blocked 50% of the currents. 10. In summary, I have examined the excitability of isolated SCN neurons and the underlying voltage-gated sodium and calcium currents. The sodium current has a slow phase of recovery from inactivation, which may impose a limit on the firing rate of these neurons. High-threshold calcium currents also appear to be important for maintaining repetitive firing of these cells owing to their weak inactivation. The findings reported here provide an initial basis for studying the circadian rhythm in excitability of these neurons.