Figure 7: Cholinergic signalling controls light-evoked dendritic spike initiation.

(a) Reconstruction of an ON-DSGC showing the placement of recording electrodes and the preferred direction movement of a light bar; the blue coloured section of the dendritic tree feeds to the dendritic recording site. (b) The generation of dendritic spikes (dendritic recording (blue traces), positions 1 and 2 in a), and back-propagating action potentials (positions 3 and 4 in a) were attenuated by antagonism of nAChRs (hexamethonium (Hex); 100 μM) when a preferred direction light bar was swept across the receptive field. Traces are aligned to the peak of somatically recorded action potentials (APs). (c) Summary of the reduction of AP output and dendritic spike generation by hexamethonium. (d) Spatial pattern of somato-dendritic spike delay under control (black symbols) and following the blockade of nAChRs (Hex; red symbols). (e) Reconstruction of simultaneously recorded ON-SAC and ON-DSGCs, showing the placement of recording electrodes. (f) ON-DSGC somatic (black traces) and dendritic (blue traces) recordings show that the presynaptic SAC drives dendritic spike generation (reconstruction in a; recordings were made in GABAzine (10 μM); five consecutive SAC-evoked supra-threshold excitatory responses are shown). The lower overlain traces show the dendro-somatic attenuation of a sub-threshold voltage response evoked by SAC activation (green trace). (g) Quantification of the sites of SAC-DSGC close apposition in recorded DSGC dendritic subtrees (left graph, vertical line represents average dendritic recording site; 145±5 μm from the soma; n=5). Note that all detected appositions were distal to dendritic recording sites. The right graph shows that dendritic spikes preceded action potentials in all supra-threshold trials.