Michele A. Noonan

Understanding the fate of adult-generated neurons and the mechanisms that influence them requires consistent labeling and tracking of large numbers of stem cells. We generated a nestin-CreER(T2)/R26R-yellow fluorescent protein (YFP) mouse to inducibly label nestin-expressing stem cells and their progeny in the adult subventricular zone (SVZ) and subgranular zone (SGZ). Several findings show that the estrogen ligand tamoxifen (TAM) specifically induced recombination in stem cells and their progeny in nestin-CreER(T2)/R26R-YFP mice: 97% of SGZ stem-like cells (GFAP/Sox2 with radial glial morphology) expressed YFP; YFP+ neurospheres could be generated in vitro after recombination in vivo, and maturing YFP+ progeny were increasingly evident in the olfactory bulb (OB) and dentate gyrus (DG) granule cell layer. Revealing an unexpected regional dissimilarity in adult neurogenesis, YFP+ cells accumulated up to 100 d after TAM in the OB, but in the SGZ, YFP+ cells reached a plateau 30 d after TAM. In addition, most SVZ and SGZ YFP+ cells became neurons, underscoring a link between nestin and neuronal fate. Finally, quantification of YFP+ cells in nestin-CreER(T2)/R26R-YFP mice allowed us to estimate, for example, that stem cells and their progeny contribute to no more than 1% of the adult DG granule cell layer. In addition to revealing the dynamic contribution of nestin-expressing stem cells to adult neurogenesis, this work highlights the utility of the nestin-CreER(T2)/R26R-YFP mouse for inducible gene ablation in stem cells and their progeny in vivo in the two major regions of adult neurogenesis.

Drugs of abuse dynamically regulate adult neurogenesis, which appears important for some types of learning and memory. Interestingly, a major site of adult neurogenesis, the hippocampus, is important in the formation of drug-context associations and in the mediation of drug-taking and drug-seeking behaviors in animal models of addiction. Correlative evidence suggests an inverse relationship between hippocampal neurogenesis and drug-taking or drug-seeking behaviors, but the lack of a causative link has made the relationship between adult-generated neurons and addiction unclear. We used rat intravenous cocaine self-administration in rodents, a clinically relevant animal model of addiction, to test the hypothesis that suppression of adult hippocampal neurogenesis enhances vulnerability to addiction and relapse. Suppression of adult hippocampal neurogenesis via cranial irradiation before drug-taking significantly increased cocaine self-administration on both fixed-ratio and progressive-ratio schedules, as well as induced a vertical shift in the dose-response curve. This was not a general enhancement of learning, motivation, or locomotion, because sucrose self-administration and locomotor activity were unchanged in irradiated rats. Suppression of adult hippocampal neurogenesis after drug-taking significantly enhanced resistance to extinction of drug-seeking behavior. These studies identify reduced adult hippocampal neurogenesis as a novel risk factor for addiction-related behaviors in an animal model of cocaine addiction. Furthermore, they suggest that therapeutics to specifically increase or stabilize adult hippocampal neurogenesis could aid in preventing initial addiction as well as future relapse.

Relapse, a major problem in the treatment of cocaine addiction, is proposed to result in part from neuroadaptations in the hippocampus. We examined how a mediator of hippocampal neuroplasticity, adult neurogenesis in the subgranular zone (SGZ), was regulated by cocaine self-administration (CSA), and whether these changes were reversed by 4 weeks of withdrawal (CSA-WD) versus continued cocaine self-administration (CSA-CONT). Rats self-administered intravenous cocaine or saline for 3 weeks and were killed 2 h (CSA) or 4 weeks (CSA-WD, CSA-CONT) after injection with the S-phase marker bromodeoxyuridine (BrdU). Cells in several stages of adult neurogenesis were quantified: proliferating cells labeled by BrdU (2 h) or Ki-67; immature neurons labeled by doublecortin; and adult-generated neurons labeled with BrdU (4 weeks) and the mature neuronal marker NeuN. CSA decreased proliferation in both the SGZ and the subventricular zone (SVZ), a source of adult-generated olfactory neurons, changes reversed by CSA-WD. Unexpectedly, CSA-WD and CSA-CONT resulted in more immature doublecortin-immunopositive (+) neurons in the posterior SGZ and a normal number of adult-generated BrdU+ neurons in the SGZ, suggesting an enduring impact of CSA regardless of whether cocaine intake was stopped or continued. However, only CSA-WD rats had more adult-generated neurons with punctate BrdU staining, an indicator of enhanced maturity. These data suggest a mechanism for the cognitive and olfactory deficits seen in cocaine addicts, and further suggest that adult-generated neurons should be considered for their potential role in cocaine addiction and hippocampal-mediated relapse after cocaine withdrawal.