Cholinergic transmission in direction selective ganglion cells: Synaptic or Paracrine?

Abstract

Purpose : In the retina, cholinergic transmission is largely paracrine. Acetylcholine (ACh) is released from dendrites of starburst amacrine cells (SACs) residing in discrete layers of the inner-plexifom layer, while ACh receptors are expressed diffusely amongst a variety of amacrine, bipolar and ganglion cells. However, in the case of DSGCs, whose dendrites co-stratify with those of SACs, the nature of cholinergic transmission is currently being debated. Functional studies indicate that DSGCs receive a relatively fast ‘synaptic’ ACh excitation mediated by SACs, with somas located on the preferred- and null-sides of the DSGC’s receptive field. On the other hand, electron microscopy studies indicate that well-defined SAC-DSGC synaptic connections are predominantly made by null-side SACs. While, this asymmetric SAC connectivity forms the substrate for the null-direction GABAergic inhibition that is important for the generation of DS in ganglion cells, the tacit assumption is that cholinergic transmission is paracrine (Briggman et al., 2011). To resolve the discrepancy between functional and anatomical studies, here we sought to characterize the kinetic properties of cholinergic transmission at DSGCs using electrophysiological techniques.

Methods : Cholinergic transmission was studied using SAC-DSGC paired voltage clamp recordings, performed in mouse lines where these cells were genetically labelled. SACs were briefly depolarized from -60mV to 0 mV, evoking robust ACh currents in DSGCs.

Results : Electrophysiological recordings revealed fast miniature events mediated by nAChRs in DSGCs (rise time: 1±0.1 ms; t_decay: 4±0.2 ms), indicating a synaptic component for cholinergic transmission. However, brief depolarizations of the preferred-SACs through a patch electrode produced ACh currents in DSGCs (125±22 pA at -60mV; E_Cl; n = 6) that were significantly slower to rise (6±1 ms) compared to miniature synaptic events. Evoked ACh currents remained slow even when SAC were minimally stimulated (20% failure rate) producing an average peak current of 34±3 pA in DSGCs. The slow kinetics of the post-synaptic response was consistent with paracrine modes of transmission.

Conclusions : Our results demonstrate for the first time the ‘quantal’ nature of cholinergic signalling in the retina. Together, with previous findings, we suggest that cholinergic synapses mediate both paracrine and synaptic forms of transmission in the DS circuit.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.