Expression of the Genes GAD67 and Distal-less-4 in the Forebrain of Xenopus laevis Confirms a Common Pattern in Tetrapods AURORA BROX, LUIS PUELLES, BEATRIZ FERREIRO, AND LORETA MEDINA * Department of Human Anatomy, Faculty of Medicine, University of Murcia, 30100 Murcia, Spain ABSTRACT We investigated whether -amino butyric acidergic (GABAergic) cell populations correlate positionally with specific Dlx-expressing histogenetic territories in an anamniote tetrapod, the frog Xenopus laevis. To that end, we cloned a fragment of Xenopus GAD67 gene (xGAD67, expressed in GABAergic neurons) and compared its expression with that of Distal-less-4 gene (xDll-4, ortholog of mouse Dlx2) in the forebrain at late larval and adult stages. In Xenopus, GABAergic neurons were densely concentrated in xDll-4–positive territories, such as the telen- cephalic subpallium, part of the hypothalamus, and ventral thalamus, where nearly all neurons expressed both genes. In contrast, the pallium of Xenopus generally contained dispersed neurons expressing xGAD67 or xDll-4, which may represent local circuit neurons. As in amniotes, these pallial interneurons may have been produced in the subpallium and migrated tangentially into the pallium during development. In Xenopus, the ventral division of the classic lateral pallium contained extremely few GABAergic cells and showed only low signal of the pallial gene Emx1, suggesting that it may represent the amphibian ventral pallium, homologous to that of amniotes. At caudal forebrain levels, a number of GABAergic neurons was observed in several areas (dorsal thalamus, pretectum), but no correlation to xDll-4 was observed there. The location of GABAergic neurons in the forebrain and their relation to the developmental regulatory genes Dll and Dlx were very similar in Xenopus and in amniotes. The close correlation in the expression of both genes in rostral forebrain regions supported the notion that Dll/Dlx are among the genes involved in the acquisition of the GABAergic phenotype. J. Comp. Neurol. 461:370 –393, 2003. © 2003 Wiley-Liss, Inc. Indexing terms: subpallium; ventral pallium; basal ganglia; ventral thalamus; tangential migrations In mammals, birds, and reptiles, -amino butyric aci- dergic (GABAergic) neurons are densely concentrated in the diencephalic ventral thalamus, pretectum, and the telencephalic basal ganglia (Reiner et al., 1984, 1998; Me- dina and Reiner, 1995; Marı ´n et al., 1998b; Guirado et al., 1999; Katarova et al., 2000). In contrast, adjacent territo- ries such as the diencephalic dorsal thalamus or the tel- encephalic pallial regions contain mostly glutamatergic and aspartergic neurons and only a dispersed population of GABAergic neurons (Reiner et al., 1984, 1998; Reiner, 1991, 1993; Medina and Reiner, 1995, 2000; Guirado et al., 2000; Stu ¨ hmer et al., 2002). Recent data have indi- cated that the production of GABAergic neurons in the rostral forebrain is largely restricted to specific histoge- netic territories that express developmental regulatory genes of the Dlx family (Price et al., 1991; Bulfone et al., 1993; Marı ´n and Rubenstein, 2001). In amniotes, the ven- tral thalamus in the diencephalon and the basal ganglia in the telencephalic subpallium, both rich in GABAergic neu- rons, are characterized by expression of Dlx genes during Grant sponsor: Spanish Ministry of Science and Technology; Grant num- ber: DGICYT PB98-0397; Grant number: BFI2000-1359-C02-02; Grant sponsor: Se ´neca Foundation; Grant number: PB14/FS/97; Grant number: PI-64/00862/FS/01; Grant number: PB/25/FS/99; Grant sponsor: Spanish Ministry of Health; Grant number: FIS 01/0057-02. *Correspondence to: Loreta Medina, Department of Human Anatomy, Faculty of Medicine, University of Murcia, 30100 Murcia, Spain. E-mail: lmedina@um.es Received 16 August 2002; Revised 10 December 2002; Accepted 24 Feb- ruary 2003 DOI 10.1002/cne.10688 Published online the week of May 12, 2003 in Wiley InterScience (www. interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 461:370 –393 (2003) © 2003 WILEY-LISS, INC.