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Section of Electrophysiology, National Institute on Alcohol Abuse and Alcoholism, Rockville, MD 20852.
1. The voltage-activated outward currents in diencephalon-derived neuroendocrine pineal cells, dissociated from rats aged 1 day to 3 weeks post-natal, were studied with the whole-cell variation of the patch-clamp technique and compared with those of adult rats (1-3 months post-natal). 2. Thirty-five per cent of the 1-week-old cells displayed a single slowly inactivating outward current that had properties which distinguished it from the classical IA and IK currents. This current, named IK(d) for developmental, activated at potentials near -35 mV. Its time to half-maximal activation (t 1/2) ranged from 16 ms at -30 mV to 4 ms at + 15 mV. No other membrane currents were apparent with depolarizing steps up to +80 mV. 3. IK(d) displayed slow inactivation at depolarized potentials. The time constant for this inactivation was on the order of several hundred milliseconds. The curve for steady-state inactivation disclosed that the current was 50% inactivated near -90 mV. This current was not found in cells dissociated from animals 4 or more weeks of age. 4. The reversal potential determined from the amplitude of the tail current at various repolarizing voltages was -76 mV. Tetraethylammonium and 4-aminopyridine reduced the amplitude of the current. The amplitude and time course of this current was not affected by the removal of external Ca2+. Similarly, removal of Cl- did not affect the current characteristics. 5. Sixty-five per cent of the 1-week-old cells displayed IA and IK. IK rose slowly with time and displayed a threshold of activation near -20 mV. No current decay was observed during a 160 ms pulse. IA activated with step potentials positive to -50 mV. This current rose faster than IK(d) and IK, and it had a significant decay over a 160 ms pulse. 6. IA and IK were observed as early as 1 day after birth. Comparison of the time course of activation of IA and IK from young and adult animals showed a small increase (2-3 ms at 0 mV) in the time to peak and half-maximal current, respectively. With a step potential to -20 mV, the time constant of decay of IA increased from 34.6 ms in 2-day-old animals to 42.9 ms in adult animals. 7. The results indicate that unlike adult pineal cells, some cells from young animals express a kinetically distinct outward current (IK(d)) which was observed in the absence of IA and IK.(ABSTRACT TRUNCATED AT 400 WORDS)
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