Differential distribution of a potassium current in immunocytochemically identified supraoptic magnocellular neurones of the rat.

Abstract

Magnocellular hypothalamo-neurohypophysial neurones display characteristic firing patterns, related to the hormone they release. To identify the membrane currents that may underlie these firing patterns, we performed whole-cell recording of freshly dissociated magnocellular neurones from the supraoptic nucleus. After recording, cells were immunocytochemically identified by using highly selective monoclonal antibodies raised respectively against vasopressin (AVP) and oxytocin (OT) neurophysin. In 64 out of 131 neurones (48.8%), we detected the presence of a transient potassium current whose kinetic properties were characteristic of an A-current. The A-current was activated by depolarisation over -40 mV, and inactivated rapidly with a monoexponential decay (tau = 28 +/- 2.7 ms; n = 33 at 0 mV). Using conditioning prepulses of 50 ms, the voltage dependence of the inactivation was determined, and the data were adequately fit with a Boltzman equation (half-maximal inactivation: -42.5 mV). The steady-state time-dependent inactivation curve was determined using a prepulse potential at -40 mV, and data were best described with a mono-exponential equation (tau = 89.7 ms). The sensitivity to 1 mM 4-amino-pyridine (63 +/- 9% inhibition, n = 6), and a reversal potential close to the theoretical Nernst equilibrium for potassium (-56.3 +/- 1 mV, n = 6, vs. -58 mV) confirmed that the transient current studied was indeed an A-type potassium current. Immunocytochemical identification revealed that the A-current was selectively expressed in OT-neurophysin-positive cells. As previous work in hypothalamic slice preparations suggests that the A-current is expressed by both AVP cells and OT cells, the present data suggest that whereas the A-current is expressed in the soma of OT cells, it may be expressed only on the dendritic tree of AVP cells, which is truncated in the dispersed cell preparation used here. This distribution may play a role in the specific firing characteristics of magnocellular neurones.