GABAergic cell types in the superficial layers of the mouse superior colliculus

To begin to unravel the complexities of GABAergic circuits in the superior colliculus (SC), we utilized mouse lines that express green fluorescent protein (GFP) in cells that contain the 67 kDa isoform of glutamic acid decarboxylase (GAD67‐GFP), or Cre‐recombinase in cells that contain glutamic acid decarboxylase (GAD; GAD2‐cre). We used Cre‐dependent virus injections in GAD2‐Cre mice and tracer injections in GAD67‐GFP mice, as well as immunocytochemical staining for gamma amino butyric acid (GABA) and parvalbumin (PV) to characterize GABAergic cells that project to the pretectum (PT), ventral lateral geniculate nucleus (vLGN) or parabigeminal nucleus (PBG), and interneurons in the stratum griseum superficiale (SGS) that do not project outside the SC. We found that approximately 30% of SGS neurons in the mouse are GABAergic. Of these GABAergic neurons, we identified three categories of potential interneurons in the GAD67‐GFP line (GABA+GFP ~45%, GABA+GFP + PV ~15%, and GABA+PV ~10%). GABAergic cells that did not contain GFP or PV were identified as potential projection neurons (GABA only ~30%). We found that GABAergic neurons that project to the PBG are primarily located in the SGS and exhibit narrow field vertical, stellate, and horizontal dendritic morphologies, while GABAergic neurons that project to the PT and vLGN are primarily located in layers ventral to the SGS. In addition, we examined GABA and GAD67‐containing elements of the mouse SGS using electron microscopy to further delineate the relationship between GABAergic circuits and retinotectal input. Approximately 30% of retinotectal synaptic targets are the presynaptic dendrites of GABAergic interneurons, and GAD67‐GFP interneurons are a source of these presynaptic dendrites. We used Cre‐dependent virus injections in GAD2‐Cre mice and tracer injections in GAD67‐ green fluorescent protein (GFP) mice, as well as immunocytochemical staining for gamma amino butyric acid (GABA) and parvalbumin (PV) to characterize GABAergic cells of the stratum griseum superficiale (SGS) via confocal and electron microscopy. The Venn diagram illustrates the approximate proportions of GABAergic SGS neurons listed to the right. GABAergic neurons that contain GFP in the GAD67‐GFP line and/or are labeled with a PV antibody are identified as interneurons. The remaining cells are potential projection neurons that include narrow field vertical, stellate, and horizontal cells that project to the parabigeminal nucleus. The GAD6