Inhibitory responses driven by SCN electrical stimulation arise primarily via non-VIP expressing neurons. a Left shows a schematic of the recording configuration for experiments using SCN electrical and optogenetic stimulation in VIP+/cre; Ai32+/− slices, with 465 nm blue light flashes illuminating a region > 500 μm diameter centred on the SCN (see the “Methods” section). Right plot illustrates proportions of responding cells and their overlap following optical and electrical stimulation (bubble size proportional to population sizes). b Top panel shows peri-stimulus raster for a representative cell exhibiting inhibitory responses to optogenetic stimulation under baseline conditions and following GABAA receptor blockade (BIC; 20 μm (+)-bicuculline) in the absence and presence of ionotropic glutamate receptor antagonists (iGlu-X; 50 μm D-AP5 and 20 μm CNQX). Lower panel shows mean ± SEM change in firing across the population of responding cells (n = 8) under the various conditions. c Mean ± SEM responses of neurons exhibiting inhibitory responses to optogenetic stimulation before and after ionotropic GABA/glutamate receptor blockade. Data analysed by mixed-effects linear model (F2,9.9 = 6.1; P = 0.02) with Sidak’s post-tests. * = P < 0.05. Raw data values used for statistical analysis can be found in Additional file 8.