There is an orderly topographic arrangement of neurones within auditory brainstem nuclei based on sound frequency. Previous immunolabelling studies in the medial nucleus of the trapezoid body (MNTB) have suggested that there may be gradients of voltage-gated currents underlying this tonotopic arrangement. Here, our electrophysiological and immunolabelling results demonstrate that underlying the tonotopic organization of the MNTB is a combination of medio-lateral gradients of low-and high-threshold potassium currents and hyperpolarization-activated cation currents. Our results also show that the intrinsic membrane properties of MNTB neurones produce a topographic gradient of time delays, which may be relevant to sound localization, following previous demonstrations of the importance of the timing of inhibitory input from the MNTB to the medial superior olive (MSO). Most importantly, we demonstrate that, in the MNTB of congenitally deaf mice, which exhibit no spontaneous auditory nerve activity, the normal tonotopic gradients of neuronal properties are absent. Our results suggest an underlying mechanism for the observed topographic gradient of neuronal firing properties in the MNTB, show that an intrinsic neuronal mechanism is responsible for generating a topographic gradient of time-delays, and provide direct evidence that these gradients rely on spontaneous auditory nerve activity during development.
We have investigated the membrane properties of brainstem auditory neurons in a mouse model of congenital deafness (dn ⁄ dn). Whole-cell recordings were made from visualized neurons in slices of the medial nucleus of the trapezoid body (MNTB) and anteroventral cochlear nucleus (AVCN). We have recently demonstrated that MNTB neurons in deaf mice are more excitable than in normal mice, due in part to a reduced expression of low-threshold potassium currents. In this study, we have examined the contribution of hyperpolarization-activated (I h ) channels to the membrane properties of MNTB and AVCN neurons. Our results show that I h is larger in MNTB neurons from deaf mice than in normal mice. In contrast, no significant differences were found in I h or excitability between AVCN bushy cells from dn ⁄ dn and normal mice. Experimental evidence and neuronal modelling suggests that, in the MNTB of normal mice, a small contribution of I h helps to reduce temporal summation of synaptic potentials. A larger I h in neurons from deaf mice has a much greater effect in reducing temporal summation of synaptic potentials, counteracting to some extent the greater excitability of these cells. Our results provide further insight into the role of activity during development in regulating the membrane and firing properties of central neurons.
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