Electrical membrane properties of trapezoid body neurons in the rat auditory brain stem are preserved in organotypic slice cultures.

The medial nucleus of the trapezoid body (MNTB) is a conspicuous structure in the mammalian auditory brain stem. It is a major component of the superior olivary complex and is involved in sound localization. Recently, organotypic slice culture preparations of the superior olivary complex were introduced to investigate the development of inhibitory and excitatory projections (Sanes and Hafidi, 1996; Lohmann et al., 1998). In the present article, we further assessed the organotypicity of our culture system (Lohmann et al., 1998) and examined electrical membrane properties of MNTB neurons expressed under culture conditions. To do so, MNTB neurons from early postnatal rats (P3-5) were studied after 3-6 days in vitro (DIV) by whole-cell patch-clamp recordings. Their mean resting potential was -59 mV, the input resistance averaged 171 Momega, and the average time constant was 3 ms. Four types of voltage-activated conductances were observed in voltage-clamp recordings. All cells expressed a tetrodotoxin (TTX)-sensitive sodium current. Two types of potassium currents could be characterized: a tetraethylammonium (TEA) -sensitive and a 4-aminopyridine (4-AP)-sensitive conductance, both of which are composed of a transient and a sustained component. Finally, an inwardly rectifying current, activated by hyperpolarizing voltage steps, was found. In current-clamp recordings, depolarizing current pulses typically elicited a single action potential. In the presence of 4-AP, however, these current pulses induced a train of action potentials. The duration of action potentials was increased by 4-AP and the afterhyperpolarization was reduced. Hyperpolarizing current injections induced a "sag" in the membrane potential, indicating the influence of an inwardly rectifying current. Our results demonstrate that MNTB neurons in slice cultures have electrical membrane properties comparable to those of their counterparts in acute slices.