Dendritic speeding of synaptic potentials in an auditory brainstem principal neuron

Principal cells of the medial nucleus of the trapezoid body (MNTB) in the mammalian auditory brainstem receive most of their strong synaptic inputs directly on the cell soma. However, these neurons also grow extensive dendrites during the first four postnatal weeks. What are the functional roles of these dendrites? We studied the morphology and growth of the dendrites in the mouse MNTB using both electron microscopy and confocal fluorescence imaging from postnatal day 9 (P9; pre-hearing) to P30. The soma of principal cells sprouted 1 to 3 thin dendrites (diameter ~ 1.5 microns) by P21 to P30. Each dendrite bifurcated into 2-3 branches and spanned an overall distance of about 80 to 200 microns. By contrast, at P9-11 the soma had 1 to 2 dendrites that extended for only 25 microns on average. Patch clamp experiments revealed that the growth of dendrites during development correlates with a progressive decrease in the input resistance, whereas acute removal of dendrites during brain slicing leads to higher input resistances. Accordingly, recordings of excitatory postsynaptic potentials (EPSPs) evoked by afferent fiber stimulation show that EPSP decay is faster in P21-24 neurons with intact dendrites than in neurons without dendrites. This dendritic speeding of the EPSP reduces the decay time constant 5-fold, which will impact significantly synaptic current summation and the ability to fire high-frequency spike trains. These data suggest a novel role for dendrites in auditory brainstem neurons: the speeding of EPSPs for faster and more precise output signal transfer.\n\nSignificance StatementAuditory circuits that compute sound localization express different types of specialized synapses. Some are capable of fast, precise and sustained synaptic transmission. As the paradigm example, principal cells of the MNTB receive a single calyx-type nerve terminal on their soma and this large excitatory synapse produces fast and brief supra-threshold EPSPs that can trigger trains of high frequency spikes. However, these neurons also extend thin and long dendrites with unknown function. We examined the relationship between dendritic morphology, passive electrical properties and EPSP waveform. We found that more mature neurons with intact dendrites have lower input resistances and short EPSP waveforms, ideally suited for conveying precise timing information, whereas immature neurons with shorter dendrites and higher input resistance have longer lasting EPSPs.