Purinergic Signalling (2010) 6:429–445 shows that the proliferation rate of neural stem/progenitor cells in larval X. laevis is extremely high. Responses to nucleotides and functional investigation of the involved purinergic receptors Application of 2MeSATP and ATP, two nucleotides known to activate many purinergic receptor subtypes [9], induced strong [Ca2+]i increases in both, OB and PVZ cells. The fact that suramin and PPADS, two frequently used P2 receptor antagonists [9], attenuated these responses in both cell pools confirmed the involvement of purinergic receptors. In the OB, glutamate mediates most or all excitatory synaptic interactions between the different cells populating this area of the brain via ionotropic receptors [1, 25]. Also, it has been reported that activation of purinergic receptors can induce neurotransmitter release, including glutamate, from neuronal [26, 27] and glial cells [26, 28, 29]. Therefore, we thought it would be necessary to exclude a possible contribution of ionotropic glutamate receptor activation, especially in the [Ca2+]i increases recorded in cells of the OB. To do so, we compared ATP-induced [Ca2+]i transients of cells of the anterior telencephalon, with and without the AMPA and NMDA receptor antagonist CNQX and APV in the bath solution. Blocking the ionotropic glutamate recep- tors did not significantly change the peak responses of cells of the anterior telencephalon. To get a first indication of which receptor subtypes are involved, we investigated the effect of the omission of calcium from the extracellular solution. Although in Ca2+-free bath solution the 2MeSATP- and ATP-induced [Ca2+]i increases were significantly reduced in both brain regions, in all of the slices tested the reduction was consistently greater in the OB. While PVZ cells of all slices tested clearly retained some residual responses, in the vast majority of OB cells the responses were virtually abolished. This faint but clear difference between the two cell pools strongly suggested that the nucleotide-induced [Ca2+]i increases were almost entirely due to activation of ionotropic P2X receptors in OB cells, while in the PVZ evidently a combination of ionotropic (P2X) and metabotropic purinergic receptors (P1 and/or P2Y receptors) contributed to the responses. To verify this supposition and in the attempt to find clear differences in the purinergic receptor expression in the OB and the PVZ, we compared the efficacy of nine known purinergic agonists (see “Materials and methods” section). Significant differences could be observed with adenosine, ADP and ATP. Thereby, adenosine and ADP are particularly interesting. Both agonists were virtually inactive in the OB, while they induced considerable [Ca2+]i increases in the PVZ. Although also ATP induced significantly stronger responses in cells of the PVZ, it is less adequate to differentiate between the two regions, as it induced strong 441 responses also in the OB. α,β-MeATP and UTP were only very weakly active in both brain regions. β,γ-MeATP and uridine were completely inactive. 2MeSATP elicited strong but not significantly different [Ca2+]i increases in cells of the OB and the PVZ. Also, BzATP elicited clear but not significantly different [Ca2+]i increases in both brain regions, but it was apparently more effective in the OB. The obtained agonist data does not allow to exactly identify the purinergic receptor subtypes involved in the Ca2+ signaling mechanisms in the anterior telencephalon of larval X. laevis, but it certainly allows a gross classification of the subtypes that are present in the different brain zones. The virtually complete absence of 2MeSATP- and ATP- induced responses in Ca2+-free extracellular solution together with the failure to elicit responses with adenosine, a selective P1 receptor agonist, strongly suggests that OB cells express almost exclusively ionotropic P2X receptors [9]. The very faint UTP- and ADP-mediated [Ca2+]i increases could suggest a very weak expression of P2Y receptor subtypes [12, 51]. The presence of very few P2Y receptor subtypes on some cells of the OB could also explain the very faint residual nucleotide-induced responses in Ca2+-free extracel- lular solution. The agonist responses of OB cells are not consistent with that reported for any of the seven known P2X subtypes [9, 10], suggesting that OB cells express multiple P2X subtypes. ATP, known to activate all known P2X receptor assemblies [10, 52, 53], was less potent than 2MeSATP. Already at concentrations of 5 to 10 μM 2MeSATP induced stronger responses than 50 μM ATP (Fig. 3b). This is an interesting result and apparently rather specific for the OB of larval X. laevis, as at most P2X subtypes in many species ATP is at least equipotent to 2MeSATP [9, 10, 52, 53]. BzATP, in turn, induced stronger responses than ATP. As BzATP has a higher potency than ATP only at the P2X7 subtype [10, 52, 53], this result strongly suggest the presence of the P2X7 subtype in the OB. In contrast to the OB cells, cells of the PVZ consistently retained some residual 2MeSATP- and ATP-induced [Ca2+]i transients in Ca2+-free extracellular solution. This is a strong indication that in addition to ionotropic P2X subtypes, PVZ cells also express metabotropic purinergic receptors, known to be coupled to intracellular cascades linked to calcium release from internal stores [9, 12, 51]. The fact that also in the PVZ the omission of extracellular calcium in the bath solution significantly reduced the nucleotide-induced responses shows that the major fraction of purinergic receptors are P2X subtypes. On the other hand, the responses to adenosine, a selective agonist for P1 receptors [9, 54], nicely show that PVZ cells express also functional P1 receptors. 2MeSATP was the most potent agonist also in cells of the PVZ. The fact that responses to ADP were in the range of those to ATP suggests that PVZ cells express at least one P2Y subtype highly sensitive to