The large majority of ON bipolar cells were unaffected by this form of cross-modal regulation. Having observed an action of olfactory stimulation on the visual signal as it is transmitted by bipolar cells, we investigated how far the responses of postsynaptic ganglion cells were also affected. To monitor signals

across large populations of neurons in vivo, we made a line of zebrafish expressing selleck compound the calcium reporter GCaMP3.5 under the eno2 promoter, which drives expression in RGCs (Bai et al., 2007; Figure 3A). Responses were then quantified in RGC dendrites through different strata of the IPL. Step changes in luminance were a relatively ineffective stimulus for RGCs, so we examined the effects of an olfactory stimulus on full-field stimuli modulated

at 5 Hz. The advantage of this in vivo imaging approach over electrophysiology is that it allows stimulation of the olfactory system while observing activity across a large population of RGCs. Responses from RGC dendrites were classified as OFF (Figures 3B and 3C), ON (Figures 3D and GSK1120212 datasheet 3E), or ON-OFF (Figures 3F and 3G), according to the responses to steps of light (Figure S3). A total of 334 responses from n = 5 eno2::GCaMP3.5 fish were collected. Methionine induced a reduction in gain of OFF and ON-OFF RGCs at contrasts of 50% and above, without an appreciable effect on ON RGCs. These results are consistent with the reduced gain of responses to contrast observed in OFF bipolar cell terminals, but not ON, following application of methionine (Figure 2). They also confirm that the actions of the ORC are evident in the retinal output, as previously demonstrated by Maaswinkel and Li (2003) and Huang et al. (2005). How does an olfactory stimulus modulate synaptic transmission through bipolar cells? Existing evidence suggests that a key signal is dopamine released by IPCs (Umino and Dowling,

1991 and Huang et al., 2005). To investigate how dopaminergic signaling might be involved in modulating synaptic activity of bipolar cells, we injected agonists or antagonists of dopamine receptors into the anterior chamber of one eye of a fish, with a parallel sham injection into the other eye acting as a control. The first manipulation first was to activate dopamine receptors by injecting the agonist [3H] 2-amino-6,7-dihydroxy 1,2,3,4-tetrahydronapthalene (ADTN) at an estimated concentration of 0.2 μM (see Experimental Procedures). In OFF terminals, ADTN increased the amplitude of SyGCaMP2 responses to all but the brightest lights and luminance sensitivity (I1/2) increased by a factor of ∼420 ( Figures 4A and 4B; n = 92 terminals). In ON terminals, ADTN increased the amplitude of the SyGCaMP2 response to bright lights by 108% and increased luminance sensitivity by a factor of 15 ( Figures 4C and 4D). Strong activation of dopamine receptors therefore potentiated presynaptic calcium signals in both ON and OFF bipolar cells, an effect opposite to an olfactory stimulus.