Frequency-dependent potentiation of voltage-activated responses only in the intact neurohypophysis of the rat.

The loose-patch-clamp technique was used with multiple-pulse protocols to study the frequency dependence of currents from the surface of the intact rat neurohypophysis (NH) and hypothalamus. In the NH, but not in the corresponding supraoptic nucleus of the hypothalamus, an initial, single pulse of 3-8 ms duration (long pulse) potentiated a secondary pulse response starting 20-50 ms after the initial pulse. Potentiation was abolished by 4-aminopyridine (4-AP), but not by tetraethylammonium (TEA) chloride or tetrandrine, indicating the participation of A-type potassium currents. Potentiation was also abolished by CdCl2, CoCl2 or 1 microM nicardipine, indicating the participation of calcium currents. The potentiation was reduced significantly in the presence of 4-6 mM extracellular CaCl2, indicating that the potentiation is not due to calcium influx. An initial train with as few as two pulses, each of 0.3-0.7 ms duration (short pulses) at 64-1,100 Hz also potentiated the secondary short pulse response significantly. We conclude that voltage-gated channels underlie this potentiation, which is due to interstitial calcium and potassium homeostasis changes induced by action potential activity and occurs only in the intact NH. A model is proposed for the participation of calcium and potassium channels in the burst patterning that is optimal for secretion from the NH.