A Central Role for Mixed Acetylcholine/GABA Transmission in Direction Coding in the Retina

Abstract

A surprisingly large number of neurons throughout the brain are endowed with the ability to co-release both a fast excitatory and inhibitory transmitter. The computational benefits of dual transmitter release, however, remain poorly understood. Here, we address the role of co-transmission of acetylcholine (ACh) and GABA from starburst amacrine cells (SACs) to direction-selective ganglion cells (DSGCs). Using a combination of pharmacology, optogenetics, and linear regression methods, we estimated the spatiotemporal profiles of GABA, ACh, and glutamate receptor-mediated synaptic activity in DSGCs evoked by motion. We found that ACh initiates responses to motion in natural scenes or under low-contrast conditions. In contrast, classical glutamatergic pathways play a secondary role, amplifying cholinergic responses via NMDA receptor activation. Furthermore, under these conditions, the network of SACs differentially transmits ACh and GABA to DSGCs in a directional manner. Thus, mixed transmission plays a central role in shaping directional responses of DSGCs.

“Silent” NMDA Synapses Enhance Motion Sensitivity in a Mature Retinal Circuit

“…Previous studies provided mixed views on whether presynaptic pathways to DSGCs, including those mediated by AMPA and NMDA receptors, scale proportionately as a function of contrast (Lipin et al, 2015; Poleg-Polsky and Diamond, 2016b; Sethuramanujam et al, 2016; Stafford et al, 2014). Here, the paired recordings directly demonstrate that high-sensitive inputs to starbursts scale with NMDA receptor mediated inputs, but not with GABA/AMPA/ACh inputs to DSGCs (Figure 2).…”

“…As these glutamate signals are non-directional (Figure S2) (Park et al, 2014; Yonehara et al, 2013), they would likely produce non-directional spiking responses in DSGCs if conventional AMPA synapses were utilized. By using silent synapses, DSGC responses become reliant on coincident starburst cholinergic inputs for their activation (Brombas et al, 2017; Sethuramanujam et al, 2016), and thus circumvent the potential deleterious effects of non-linear processing by starbursts.…”

“…Second, in contrast to earlier reports (Lee et al, 2010), we found GABA and ACh inputs to be activated at similar contrast thresholds (Figure 2C). The precise activation of cholinergic inputs could be inferred from measurements of excitation made at -60 mV, because they are known to dominate the non-NMDA excitatory inputs to DSGCs at low stimulus contrasts (Sethuramanujam et al, 2016). Although cholinergic signals are non-DS, the combination of E/I signals generated by the starburst network contain accurate direction information (Sethuramanujam et al, 2016).…”

“…Interestingly, in the direction-selective (DS) retinal circuit that we examine here, we recently observed that under cholinergic receptor blockade, bipolar cell glutamate inputs to ON-OFF DS ganglion cells (DSGCs; output neurons that relay directional information from retina to higher visual centers) evoked by low-contrast stimuli were mediated by silent NMDA synapses (Sethuramanujam et al, 2016). The AMPA receptor-mediated component of the DSGC's synaptic response only appeared at higher stimulus contrasts.…”

“…In this model, unbalanced glutamate signals are rendered silent at DSGCs because they are processed predominantly by NMDA receptors (Figure 1D & E), which alone do not drive spiking (Sethuramanujam et al, 2016). NMDA receptors serve to amplify coincident ACh/GABA signals from starbursts that contain accurate directional information (Lee et al, 2010; Sethuramanujam et al, 2016). Thus, the requirement for high/low sensitivity bipolar cells to drive balanced E-I is obviated.…”