Dendritic Morphology of an Inhibitory Retinal Interneuron Enables Simultaneous Local and Global Synaptic Integration

Amacrine cells, inhibitory interneurons of the retina, feature synaptic inputs and outputs in close proximity throughout their dendritic trees, making them notable exceptions to prototypical somato-dendritic integration with output transmitted via axonal action potentials. The extent of dendritic compartmentalization in amacrine cells with widely differing dendritic tree morphology, however, is largely unexplored. Combining compartmental modeling, dendritic Ca²⁺ imaging, targeted microiontophoresis and multielectrode patch-clamp recording (voltage and current clamp, capacitance measurement of exocytosis), we investigated integration in the AII amacrine cell, a narrow-field electrically coupled interneuron that participates in multiple, distinct microcircuits. Physiological experiments were performed with in vitro slices prepared from retinas of both male and female rats. We found that the morphology of the AII enables simultaneous local and global integration of inputs targeted to different dendritic regions. Local integration occurs within spatially restricted dendritic subunits and narrow time windows and is largely unaffected by the strength of electrical coupling. In contrast, global integration across the dendritic tree occurs over longer time periods and is markedly influenced by the strength of electrical coupling. These integrative properties enable AII amacrines to combine local control of synaptic plasticity with location-independent global integration. Dynamic inhibitory control of dendritic subunits is likely to be of general importance for amacrine cells, including cells with small dendritic trees, as well as for inhibitory interneurons in other regions of the CNS.SIGNIFICANCE STATEMENT Our understanding of synaptic integration is based on the prototypical morphology of a neuron with multiple dendrites and a single axon at opposing ends of a cell body. Many neurons, notably retinal amacrine cells, are exceptions to this arrangement, and display input and output synapses interspersed along their dendritic branches. In the large dendritic trees of some amacrine cells, such arrangements can give rise to multiple computational subunits. Other amacrine cells, with small dendritic trees, have been assumed to operate as single computational units. Here, we report the surprising result that despite a small dendritic tree, the AII amacrine cell simultaneously performs local integration of synaptic inputs (over smaller dendritic subregions) and global integration across the entire cell.