Impact of cocaine on adult hippocampal neurogenesis in an animal model of differential propensity to drug abuse

Hippocampal plasticity (e.g. neurogenesis) likely plays an important role in maintaining addictive behavior and/or relapse. This study assessed whether rats with differential propensity to drug-seeking behavior, bred Low-Responders (bLR) and bred High-Responders (bHR) to novelty, show differential neurogenesis regulation after cocaine exposure. Using specific immunological markers, we labeled distinct populations of adult stem cells in the dentate gyrus at different time-points of the cocaine sensitization process; Ki-67 for newly born cells, NeuroD for cells born partway, and 5-bromo-2'-deoxyuridine for older cells born prior to sensitization. Results show that: (i) bHRs exhibited greater psychomotor response to cocaine than bLRs; (ii) acute cocaine did not alter cell proliferation in bLR/bHR rats; (iii) chronic cocaine decreased cell proliferation in bLRs only, which became amplified through the course of abstinence; (iv) neither chronic cocaine nor cocaine abstinence affected the survival of immature neurons in either phenotype; (v) cocaine abstinence decreased survival of mature neurons in bHRs only, an effect that paralleled the greater psychomotor response to cocaine; and (vi) cocaine treatment did not affect the ratio of neurons to glia in bLR/bHR rats as most cells differentiated into neurons in both lines. Thus, cocaine exerts distinct effects on neurogenesis in bLR vs. bHR rats, with a decrease in the birth of new progenitor cells in bLRs and a suppression of the survival of new neurons in bHRs, which likely leads to an earlier decrease in formation of new connections. This latter effect in bHRs could contribute to their enhanced degree of cocaine-induced psychomotor behavioral sensitization.