434 Purinergic Signalling (2010) 6:429–445 Fig. 2 Cells in the periventricular zone of the lateral ventricle actively proliferate. a Schematic representation of the anterior telencephalon of larval X. laevis. b Overview over a horizontal section of the anterior telencephalon (b1) in which all cell nuclei were stained with propidium iodide (red fluorescence) and proliferating cells were visualized with a BrdU incorporation assay (green fluorescence). Proliferating cells appear yellow. Higher magnification of the periventricular region of different animals (b2-1–b2-3 and b3). Abbre- viations: ON olfactory nerve, MOB main olfactory bulb, AOB accessory olfactory bulb, PVZ periventricular zone, gl glomerular layer/glomeruli, mcl mitral cell layer, gcl granule cell layer, lv lateral ventricle, pc periglomerular cells [Ca2+]i increases were reduced in presence of suramin (200 μM; Fig. 4a and b) and PPADS (200 μM; Fig. 4c and d). Nevertheless, the observed effect of suramin on 2MeSATP-induced responses in both brain regions and on ATP-induced responses in the PVZ was not statistically significant. It has been shown that glutamate mediates most or all excitatory synaptic interactions between OB cells via ionotropic receptors [1, 25], and extracellular nucleotides acting on purinergic receptors have been shown to be able to induce glutamate release from neuronal [26, 27] and glial cells [26, 28, 29]. To exclude a contribution of ionotropic glutamate receptors in the [Ca2+]i increases recorded in cells of the anterior telencephalon, we tested the effect of the AMPA and NMDA receptor antagonist CNQX and APV. Figure 4e and f show that the ATP-induced peak responses remained virtually unchanged in both regions when the ionotropic glutamate receptors were blocked. To test whether metabotropic and/or ionotropic purinergic receptors are expressed in the OB and the PVZ, we performed experiments with Ca2+-free extracellular bath solution. Al- though in both brain regions, 2MeSATP- and ATP-induced [Ca2+]i increases were significantly reduced in Ca2+-free bath solution, the reduction was consistently greater in the OB (Fig. 5). PVZ cells of all slices tested clearly retained some residual responses. After returning to standard bath solution, the responses recovered almost completely. This set of data unambiguously shows that OB cells are endowed almost exclusively with ionotropic P2X receptor(s), while PVZ cells in addition to an important fraction of ionotropic P2X receptors most likely express also some metabotropic purinergic receptors. To substantiate the above results and to obtain an indication of which purinergic receptor subtypes are expressed in the OB and the PVZ, we examined the potency of nine purinergic agonists known to differentially activate purinergic receptor subtypes, and compared their efficacy to elicit [Ca2+]i increases in cells of these brain regions. The obtained results are shown in Fig. 6. Thereby, the most evident differences were observed with adenosine and ADP. While these two agonists were almost ineffective in the OB, they induced clear [Ca2+]i increases in cells of the PVZ. Also ATP induced significantly stronger responses in cells of the PVZ, but in contrast to adenosine and ADP, it induced clear responses also in the OB. 2MeSATP and BzATP elicited strong [Ca2+]i increases in cells of both regions. While 2MeSATP was almost equipotent in the OB and the PVZ, BzATP was more effective in the OB. α,β-MeATP, β,γ-MeATP, uridine and UTP were inactive or very weakly active in both regions. Immunohistochemical localization of purinergic receptors To gain additional information about the purinergic receptor subtypes expressed in the anterior telencephalon, we