Bardy and Pallotto What role for neurons generated in adults? Table 1 | Comparison of recent studies blocking adult neurogenesis and examining the consequences of the blockade for olfactory behavior. Experimental model Methods for blocking adult neurogenesis NEW NEURONS REMAINING* Olfactory bulb Dentate gyrus OLFACTORY TESTS Spontaneous/Habituation Detection threshold Discrimination Short-term memory Reward-motivated Detection threshold Discrimination Long-term associative memory to discriminate odors Fear-conditioned Odor cued Non odor cued (tone or contextual) Breton-Provencher et al. (2009) Adult mice C57BL/6J Pharmaco: AraC in LV ∼0% ∼50% ↓ Perf. No effect ≠odors No effect 30 min ↓ Perf. 60–120 min – – No effect 24 h 1 week – – Moreno et al. (2009) Adult mice C57BL/6J Pharmaco: AraC in SVZ ∼0% ∼35% – ↓ Perf. ≈odors + Enrichment – – – – – – Lazarini et al. (2009) Adult mice C57BL/6J Irradiation of SVZ ∼30% ∼100% – No effect ≠odors No effect 30 min No effect No effect ↓ Perf. 1 month – – Valley et al. (2009) Adult mice C57BL/6J Irradiation of SVZ + RMS ∼4% ∼100% – – – – – – Less freezing No effect Imayoshi et al. (2008) Adult mice C57BL/6J Genetic: NSE-DTA mice ∼5% ∼5% – No effect – – – No effect 24 h 1 week 1 month – Less freezing *Proportion of newly generated neurons (identiﬁed by DCX or BrdU) arriving in the olfactory bulb (OB) or in the dentate gyrus (DG) after the blockade of neurogenesis. ↓ Perf., decrease in performance. –, not reported. could still remember and discriminate the reward-associated odor. Four other independent research groups have also recently studied the effect of blocking adult neurogenesis on olfactory behavior (Imayoshi et al., 2008; Lazarini et al., 2009; Moreno et al., 2009; Valley et al., 2009). However, no clear consen- sus emerges from their results. Different methods to block adult neurogenesis and to evaluate olfactory behavior have been used, making it difﬁcult to compare and to inter- pret the various results obtained. We have tried to summarize the results obtained in these studies in Table 1. Overall, these independent studies seem to show, con- sistently, that blocking adult neurogenesis in the OB does not affect spontaneous discrimination between different odors (Imayoshi et al., 2008; Breton-Provencher et al., 2009; Lazarini et al., 2009). However, another recent study showed that blocking adult neurogenesis resulted in a dramatic decrease in performance in a task involv- ing discrimination between two perceptu- ally similar odorants after passive exposure (Moreno et al., 2009). Unlike Breton- Provencher et al., Lazarini et al. reported that the blockade of adult neurogenesis had no effect on odor detection thresholds. This difference in the results obtained by these two studies may be accounted for by differ- ences in the nature of the tests, which were reward-motivated in Lazarini’s study but not in Breton-Provencher’s study. However, spontaneous olfactory tasks do not seem to be the only tasks requiring adult neurogen- esis. The blockade of adult neurogenesis in the OB by focused irradiation, results in a decrease in freezing behavior conditioned by odor cues (Valley et al., 2009), and an impairment of the long-term memory (1 month) of reward-associated odor (Lazarini et al., 2009; but see Imayoshi et al., 2008). Interestingly, although mice with impaired neurogenesis may perform well in learning tests, the amount of time for which the learned information is stored in their memory seems to depend on adult neurogenesis. Adult neurogenesis seems to play a key role in various types of speciﬁc olfactory behavior and does not appear to be restricted to spontaneous, reward- motivated or fear-conditioned learning (see Table 1). EFFECT OF BLOCKING ADULT NEUROGENESIS ON OLFACTORY BULB CIRCUIT ACTIVITY Breton-Provencher et al. tried to associ- ate the results obtained in their behavior experiments with putative changes in the Frontiers in Neuroscience | Neurogenesis March 2010 | Volume 4 | Article 24 | 2