Distribution of interneurons expressing EGFP from the 5HT3 receptor promoter (Tg(Htr3a-EGFP)DH30Gsat, www.gensat.org) colabelled in red for calretinin (Swant, 1:2000) and counterstained with DAPI (blue) to show the cell layers. EGFP expression was amplified with chicken anti-GFP (Abcam, 1:2000). 5HT3-EGFP expressing cells and fibers are present throughout all layers of the hippocampus but enriched in the hilus and stratum lacunosum moleculare (see associated images). Calretinin immunoreactivity is prominent in fibers within the inner molecular layer of the dentate gyrus and is also present in small EGFP positive bipolar neuronal cell bodies within the hilus and granule cell layer. Calretinin immunoreactive cells belong to an interneuron class that signal to other interneurons. See Freund and Buzaki (Pubmed ID 8915675) for more information on hippocampal interneurons. Section (40 um) was mounted with Fluoromount G and imaged with a 32X, 0.4 NA LD objective on a Zeiss Axiovert microscope. AF568 was excited at 562/40 nm and emission collected at 624/40 nm with Semrock filters and a single band dichroic (530-585 nm reflected, 601-800 nm transmitted). AF488 was excited at 482/35 nm and emission collected at 536/40 nm with Semrock filters and a single band dichroic (446-499 nm reflected, 513-735 nm transmitted). DAPI was excited at 350/50 nm and emission collected at 460/50 nm with a CLP400 dichroic (Chroma). Single images were captured for each channel using a Zeiss Axiocam MR CCD and pseudo-colored using Axiovision. Linear background subtraction was used on the presented image. See Davis and Puhl 2011 (Pubmed ID: 21305052) for a detailed staining protocol. This image is part of a series characterizing EGFP expression from the 5HT3 promoter throughout the brain.