Required input number for action potential generation in auditory Brainstem nuclei

The number of input fibers required to generate suprathreshold excitation varies between auditory brainstem nuclei. Furthermore, during developmental circuit refinement the input size of a single fiber is changed. The number of individual fibers that generate suprathreshold excitation is therefore highly specific for each nucleus. In relay nuclei, like the medial nucleus of the trapezoid body (MNTB) or the ventral nucleus of the lateral lemniscus (VNLL), between one and two inputs are present and generate supra-threshold excitation even during ongoing activity with only a limited number of failures. In structures that perform integration of spatial cues, like the medial superior olive (MSO) or the dorsal nucleus of the lateral lemniscus (DNLL), a larger convergence is observed. The number of inputs that are minimally required to drive these neurons suprathreshold during ongoing activity is not fully clarified. To estimate the number of required inputs that evoke supra-threshold excitation we recorded in current-, voltage- and dynamic-clamp in acute brain slices from neurons of the MNTB, VNLL, MSO and DNLL from juvenile (postnatal day 9/10) and mature (postnatal day 26-90) Mongolian gerbils. Towards our aim we first determined strength-duration relationships to estimate the stimulation time dependence of action potential current thresholds. Next, we measured unitary EPSC IV-relationships to quantify the time course and size of the AMPAR and the voltage-dependent NMDAR components. Using the EPSC time course we estimated from the strength-duration relationship the fiber numbers required for supra-threshold excitation. As synaptic transmission is history dependent, we determined the synaptic depression in 1.2 mM extracellular [Ca2+]. From the amount of depression, we inferred the required input number for ongoing supra-threshold activity. We found that in juvenile VNLL neurons two to three and in adult one input were sufficient to generate ongoing supra-threshold excitation. For MNTB neurons our estimate yielded no failures in juvenile and the emerging of failures in adult neurons with a single input. MSO neurons required between 10 and 25 and between 5 and 10 inputs in juvenile and adult animals respectively. DNLL neurons were surprisingly excitable, with only four to eight and two to four inputs required to sustain ongoing activity in juvenile and adult animals respectively. In the next step we aim to verify the estimated number of inputs by using dynamic-clamp and to further untangle the contribution of NMDAR and AMPAR currents and post-synaptic adaptations on ongoing supra-threshold excitation.