Green fluorescent protein expression and colocalization with calretinin, parvalbumin, and somatostatin in the GAD67‐GFP knock‐in mouse

γ‐aminobutyric acid (GABA)ergic neurons in the central nervous system regulate the activity of other neurons and play a crucial role in information processing. To assist an advance in the research of GABAergic neurons, here we produced two lines of glutamic acid decarboxylase–green fluorescence protein (GAD67‐GFP) knock‐in mouse. The distribution pattern of GFP‐positive somata was the same as that of the GAD67 in situ hybridization signal in the central nervous system. We encountered neither any apparent ectopic GFP expression in GAD67‐negative cells nor any apparent lack of GFP expression in GAD67‐positive neurons in the two GAD67‐GFP knock‐in mouse lines. The timing of GFP expression also paralleled that of GAD67 expression. Hence, we constructed a map of GFP distribution in the knock‐in mouse brain. Moreover, we used the knock‐in mice to investigate the colocalization of GFP with NeuN, calretinin (CR), parvalbumin (PV), and somatostatin (SS) in the frontal motor cortex. The proportion of GFP‐positive cells among NeuN‐positive cells (neocortical neurons) was approximately 19.5%. All the CR‐, PV‐, and SS‐positive cells appeared positive for GFP. The CR‐, PV, and SS‐positive cells emitted GFP fluorescence at various intensities characteristics to them. The proportions of CR‐, PV‐, and SS‐positive cells among GFP‐positive cells were 13.9%, 40.1%, and 23.4%, respectively. Thus, the three subtypes of GABAergic neurons accounted for 77.4% of the GFP‐positive cells. They accounted for 6.5% in layer I. In accord with unidentified GFP‐positive cells, many medium‐sized spherical somata emitting intense GFP fluorescence were observed in layer I. J. Comp. Neurol. 467:60–79, 2003. © 2003 Wiley‐Liss, Inc.