Morphology and Dendrite-Specific Synaptic Properties of Midbrain Neurons Shape Multimodal Integration

Multimodal integration facilitates object recognition and response to sensory cues. This depends on spatiotemporal coincidence of sensory information, recruitment of NMDA-type glutamate receptors and inhibitory feedback. Shepherd’s crook neurons (SCNs) in the avian optic tectum (TeO) are an ideal model for studying cellular mechanism of multimodal integration. They receive different sensory modalities through spatially segregated dendrites, are important for stimulus selection and have an axon-carrying dendrite (AcD). We performed whole-cell patch-clamp experiments in chicken midbrain slices of both sexes. We emulated visual and auditory input in vitro by stimulating presynaptic afferents electrically. Simultaneous stimulation enhanced responses inversely depending on stimulation amplitude demonstrating the principle of inverse effectiveness. Contribution of NMDA-type glutamate receptors prolonged postsynaptic events for visual inputs only, causing a strong modality-specific difference in synaptic efficacy. We designed a multicompartment model to study the effect of morphological and physiological parameters on multimodal integration by varying the distance between soma and axonal origin and the amount of NMDA receptor (NMDAR) contribution. These parameters changed the preference of the model for one input channel and adjusted the range of input rates at which multimodal enhancement occurred on naturalistic stimulation. Thus, the unique morphology and synaptic features of SCNs shape the integration of input at different dendrites and generates an enhanced multimodal response.