Functional and structural properties of strong inhibitory synapses in a binaural auditory circuit

Neuronal circuits are defined in their function and computational outcome by the synaptic interplay of Excitation and Inhibition (E/I). One step further in understanding the role of these opposite interactors in neuronal computations consists in determining their relative contribution, in terms of input strength and convergence. In the auditory brainstem, neurons in the Lateral Superior Olive (LSO) rely upon binaural E/I integration for computing sound-source localization. We made use of this circuit as a model for understanding the role of synaptic connectivity in information processing. We characterized the functional connectivity of the inhibitory and excitatory inputs onto a given LSO neuron. We found that LSO neurons receive few but large inhibitory inputs with a unitary conductance of 9 nS (under physiological intracellular Cl- concentration), although the unitary strength of inputs scattered over a 10-fold range. We estimated that unitary inhibitory postsynaptic currents (IPSCs) were mediated by a large number of synchronously released glycine vesicles (on average: 48 ± 39, range 9 - 115). The large quantal content suggests that these IPSCs are generated by a high number of functional release sites. Electron-microscopy based reconstructions revealed the structural specialization of the inhibitory axons which contacted the soma and proximal dendrites of a LSO neuron multiple times via large varicosities (diameter range 1 ¿ 11 µm), each containing on average three active zones. In contrast to the strong unitary IPSCs, excitatory inputs were numerous but had a small unitary conductance (~ 0.7 nS). A computational model of E/I integration showed that the strength and convergence of excitatory inputs determines how spontaneous firing is transmitted from upstream neurons, whereas the strength and convergence of inhibitory inputs determines the dynamic range of the tuning curve. We then aimed to study mechanisms underlying the developmental acquisition of strong inhibitory inputs on LSO neurons. Input-output curves were gradual at post-natal days (P) 5 and 6 but showed, in most cases, clear steps from P7 onwards, indicative of a reduction in the number of synaptic inputs and a growth in the strength of the individual inputs. In addition, we wished to investigate whether Synaptotagmin1 (Syt1) represents the Calcium sensor that guarantees the co-release of glutamate which has been known to occur at the immature inhibitory MNTB - LSO synapses. Genetic deletion of Syt1 caused asynchronous glutamate release only during the first 2 ¿ 3 post-natal days, suggesting that Syt1 might gradually become redundant with another Calcium sensor after ~ P5. In conclusion, this study highlights the functional and structural specializations of inhibitory synapses optimized for fulfilling the computational properties of binaural neurons devoted to sound localization. It also suggests that the basic synaptic connectivity at this connection is laid down in the first week of postnatal development.