DIFFERENTIAL DISTRIBUTION OF A POTASSIUM CURRENT IN IMMUNOCYTOCHEMICALLY IDENTIFIED SUPRAOPTIC MAGNOCELLULAR NEURONS OF THE RAT

Magnocellular hypothalamo-neurohypophysial neurones display characteri stic firing patterns, related to the hormone they release. To identify the membrane currents that may underlie these firing patterns, we per formed whole-cell recording of freshly dissociated magnocellular neuro nes from the supraoptic nucleus. After recording, cells were immunocyt ochemically identified by using highly selective monoclonal antibodies raised respectively against vasopressin (AVP) and oxytocin (OT) neuro physin. In 64 out of 131 neurones (48.8%), we detected the presence of a transient potassium current whose kinetic properties were character istic of an A-current. The A-current was activated by depolarisation o ver -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 v oltage dependence of the inactivation was determined, and the data wer e adequately fit with a Boltzman equation (half-maximal inactivation: -42.5 mV). The steady-state time-dependent inactivation curve was dete rmined using a prepulse potential at -40 mV, and data were best descri bed 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 poten tial 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 identificat ion revealed that the A-current was selectively expressed in OT-neurop hysin-positive cells. As previous work in hypothalamic slice preparati ons suggests that the A-current is expressed by both AVP cells and OT cells, the present data suggest that whereas the A-current is expresse d in the soma of OT cells, it may be expressed only on the dendritic t ree of AVP cells, which is truncated in the dispersed cell preparation used here. This distribution may play a role in the specific firing c haracteristics of magnocellular neurones.