Ionic currents in cultured rat suprachiasmatic neurons.
Whole-cell voltage-clamp recordings were made from cultured neurons obtained by dissociation of the suprachiasmatic area of rat fetuses. Neurons were held for seven to 14 days in culture. These neurons possessed several voltage-dependent ionic currents. A transient inward Na+ current was present, which could be completely blocked by tetrodotoxin. No inward Ca2+ currents were detected. Three types of outward K+ currents were recorded, which could be separated to a reasonable extent by their differences in voltage sensitivity and pharmacology. These K+ currents corresponded to the transient current IA, the delayed rectifier current IKo and a calcium-dependent current IK(Ca) as described in other neurons. The A current activated at -50 mV, reached half-maximal conductance at about -30 mV and maximum conductance between 0 and 30 mV. During depolarizing steps it inactivated completely within 100 ms and steady-state inactivation was half-maximal at -66 mV. The outward rectifier activated at -30 mV, reached half-maximal conductance close to 0 mV and maximum conductance at about 70 mV. Slow inactivation of IKo occurred with 50% reduction in amplitude at the end of 2 s depolarizations above 0 mV. The K+ channel blocker 4-amino-pyridine (4 mM) reduced the amplitude of IA by 21% and of IKo by 32%, whereas tetraethylammonium (10 mM) decreased IA by 27% and IKo by 83%. The calcium-dependent K+ component was also voltage dependent and was present at voltages more positive than 0 mV. No inward rectifying K+ current was present. Considering its voltage dependence, IA must play a role in determining the excitability of these neurons, through its probable influence on the action potential threshold and interspike interval. Both IA and IKo should take part in membrane repolarization following an action potential. The Ca(2+)-dependent current should also contribute to repolarization following any event which gives rise to an increase in intracellular Ca2+. Apart from IA, which may make a slight contribution, none of these currents appear to be involved in determining the resting membrane potential. All three outward current components will act together in suprachiasmatic neurons to control their spontaneous firing frequency, which is the major feature of the output of these neurons in vivo. Variations in properties of these conductances could contribute to the circadian rhythm in firing frequency described in suprachiasmatic hypothalamic neurons.
Walsh
I B
IB
Laboratory of Physiology, Leiden University, The Netherlands.
van den Berg
R J
RJ
Rietveld
W J
WJ
D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000502:physiology; D002478:Cells, Cultured; D004553:Electric Conductivity; D007477:Ions; D008959:Models, Neurological; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000502:physiology*; D051381:Rats; D012964:Sodium / Q000502:physiology*; D013493:Suprachiasmatic Nucleus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D007477:Ions; D013757:Tetraethylammonium Compounds; D019789:Tetraethylammonium; D012964:Sodium; D015761:4-Aminopyridine; D011188:Potassium; D002118:Calcium