Systematic variation of potassium current amplitudes across the tonotopic axis of the rat medial nucleus of the trapezoid body.

Many central auditory nuclei preserve the tonotopic organization of their afferent inputs, generating a frequency "map" across the nucleus. In the medial nucleus of the trapezoid body (MNTB) the most medial neurons receive inputs corresponding to the highest frequency sounds and the most lateral neurons have the lowest characteristic frequencies. Whole-cell patch recording from MNTB principal neurons in rat brainstem slices demonstrates a corresponding tonotopic organization of voltage-gated outward potassium currents. Medial MNTB neurons had larger total outward K+ current amplitudes than lateral neurons and similar medial to-lateral gradients were observed for two K+ current subtypes distinguished by their low and high voltage activation thresholds. In contrast, a third K+ conductance with an intermediate voltage threshold and slower kinetics showed an inverse gradient (being smallest in medial MNTB). The orthogonal axes of MNTB did not exhibit potassium current gradients (dorsal-to-ventral, or rostral-to-caudal). The input resistance was unchanged across the MNTB, but a slow capacitative component was enhanced in lateral neurons. These data demonstrate that the intrinsic properties of rat MNTB neurons are tuned across the tonotopic axis so as to promote shorter action potentials, faster firing and therefore greater accuracy in transmission of auditory information in the high characteristic frequency regions.