442 ADP [12, 51]. The faint UTP-mediated [Ca2+]i increases in PVZ cells are also in line with the expression of P2Y subtypes [12, 51]. BzATP also induced clear responses in the PVZ. This could suggest that also PVZ cells express P2X7 subtypes [52, 53], but also other P2X subtypes have been shown to be sensitive to BzATP [10, 12, 52, 53]. Immunohistochemical localization of purinergic receptors We obtained positive P2X7- and P2X4-like stainings in the anterior telencephalon of larval X. laevis. No clear stainings were achieved with antibodies against P2X1, P2X2, P2Y2, and P2Y4 receptor subtypes. The antibody against the P2X7 subtype gave a clear somatic staining pattern in both OB and PVZ cells. This result is in line with our calcium imaging data that strongly suggested the presence of the P2X7 subtype. The successful P2X4-like staining pattern suggests an expression of also this subtype. Thereby, the observed P2X4-positive tubular structures starting from the ependymal layer of the lateral ventricle and extending through the PVZ into the OB could represent appendages of radial glia-like cells, or other proliferating precursor cells [6, 7, 55, 56]. These cells have been shown to possess a morphology resembling the observed structures [7, 55, 56]. This fits also very well to our observations of the ultrastructure of the PVZ, which suggested the presence of radial glia-like cells. As the used antibodies have been raised against rat P2 receptors, especially the negative results should be interpreted with caution. In the future, antibodies specifically raised against X. laevis purinergic receptors, in situ hybridization, or single-cell PCR experi- ments will hopefully provide a conclusive insight about the individual purinergic receptors subtypes expressed in the OB and the neurogenic PVZ. Together, our pharmacological and immunohistochem- ical data clearly show that cells of the OB and the PVZ of larval X. laevis express purinergic receptors. We got strong evidence that OB cells almost exclusively express ionotropic P2X subtypes, and that cells of the PVZ most likely express a combination of ionotropic P2X and metabotropic P1 and P2Y subtypes. The exact identifica- tion of specific purinergic subtypes is inherently difficult, particularly in lower vertebrates such as amphibians. Nevertheless, the collective evidence of the present study permits a relatively clear survey of which purinergic receptor families are expressed in the anterior telenceph- alon, and more importantly revealed significant differ- ences in the expression profile of purinergic receptors and the nucleotide-induced response behavior of cells of the OB and the PVZ. This knowledge will certainly aid future work dealing with questions regarding the physiological implication of the purinergic system in the anterior telencephalon of larval X. laevis. Purinergic Signalling (2010) 6:429–445 On the physiological functions of the purinergic system in the anterior telencephalon Olfactory bulb Long-term application of purinergic agonists and antagonists did not apparently modulate odorant-induced responses of OB cells. This strongly suggests that unlike other signaling systems, such as the noradrenergic or the GABAer- gic system (for a review see [25]), the purinergic system is most probably not involved in the immediate processing of olfactory information in the OB of larval X. laevis. Of course, more focused studies will be necessary to completely rule out an involvement of the purinergic system in the fine-tuning of olfactory information, but the results of the present study gave no indication in this direction. The presence of the P2X7 subtype rather indicates that nucleotide-induced [Ca2+]i signaling is involved in the sorting of bulbar interneurons. In the neuronal network of the OB a subset of interneurons are constantly renewed throughout the animals’ life [1, 57], and cell death through apoptosis has been shown to play a crucial role in the lifelong cellular turnover [1, 58, 59]. On the other hand, perturbations of [Ca2+]i have been shown to be involved in the initiation of cell death through apoptosis [60, 61], and the activation of the P2X7 receptor has been brought into relation with the initiation of apoptosis in a variety of cells types [62, 63]. Among all purinergic receptors, the P2X7 subtype has an interesting and unique characteristic. If activated briefly, it behaves like a non-selective cation channel, but a prolonged activation leads to the formation of a large non-selective pore permeable to molecules of up to 900 Da in size, including calcium [64]. It is therefore plausible that P2X7 signaling is involved in the induction of apoptosis in OB cells that need to be sorted. To substantiate this assertion, future detailed studies focusing specifically on the involvement of this receptor in mechanisms regulating the cell turnover in the OB will be necessary. Periventricular zone of the lateral ventricles Many mole- cules and their receptors have been shown to be involved in the regulation of neurogenesis [65, 66]. There is some evidence that also purinergic receptors might be involved (for a review, see [67]), but nucleotides to date are certainly among the least investigated groups of regulatory molecules. It is known that ependymal cells lining the murine ventricular system express purinergic receptors [14, 41, 42] and nucleotide-degrading enzymes [15]. It has also been shown that neurospheres derived from the adult mouse subventricular zone express metabotropic P2Y receptors and ectonucleotidases [68]. Application of nucleotides evoked fast [Ca2+]i transients, and augmented cell proliferation in co-operation with growth factors [68]. Also neuronal progenitors of the subgranular zone are known to respond