Brain size-dependent scaling of dendritic and somatic functions in an evolutionary conserved neuronal circuit in mammals.

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In mammals, brain and neuron size enlarges with increasing head size. Inevitably, an increase in neuron size results in an increased membrane capacitance. To achieve size-independent synaptic integration and, thus, the function, neuronal properties like input resistance, dendritic morphology, synaptic conductance as well as the number and location of ion channels and synapses must scale proportionally. A prerequisite to investigate the cellular mechanisms of such a scaling is an evolutionary conserved neuronal population that fulfills the same circuit function independent of head and brain size. The neurons of the medial nucleus of the trapezoid body (MNTB) fulfill these criteria and are involved in binaural processing and spectro-temporal integration in the ascending auditory pathway. Quantification of the biophysical and morphological cellular parameters of MNTB neurons and their synaptic input size as well as channel and synapse location in differently sized mammalian species, i.e. Etruscan shrew, gerbil and rat, allows us to grasp the consequences of brain size-dependent neuron scaling. Our electrophysiological and immunofluorescence findings will culminate in a computational model to understand the functional significances of individual neuronal elements. Moreover, we can utilize this comparative approach to investigate the functional role of MNTB dendrites, which remains largely unknown. Specifically, by quantitatively determining the influence of dendritic synaptic inputs on synaptic latency and the success of action potential generation, we can capture their potential role in the generation of high frequency outputs.