Neurons are born throughout adulthood in the hippocampus and show enhanced plasticity compared with mature neurons. However, there are conflicting reports on whether or not young neurons contribute to performance in behavioral tasks, and there is no clear relationship between the timing of maturation of young neurons and the duration of neurogenesis reduction in studies showing behavioral deficits. We asked whether these discrepancies could reflect differences in the properties of young neurons in mice and rats. We report that young neurons in adult rats show a mature neuronal marker profile and activity-induced immediate early gene expression 1-2 weeks earlier than those in mice. They are also twice as likely to escape cell death, and are 10 times more likely to be recruited into learning circuits. This comparison holds true in two different strains of mice, both of which show high rates of neurogenesis relative to other background strains. Differences in adult neurogenesis are not limited to the hippocampus, as the density of new neocortical neurons was 5 times greater in rats than in mice. Finally, in a test of function, we find that the contribution of young neurons to fear memory is much greater in rats than in mice. These results reveal substantial differences in new neuron plasticity and function between these two commonly studied rodent species.

Stress strongly inhibits proliferation of granule cell precursors in the dentate gyrus, while voluntary running has the opposite effect. Few studies, however, have examined the possible effects of these environmental manipulations on the maturation and survival of young granule cells. We examined number of surviving granule cells and the proportion of young neurons that were functionally mature, as defined by seizure-induced immediate-early gene expression, in 14 and 21 day-old granule cells in mice that were given access to a running wheel, restrained daily for 2 hours, or given no treatment during this period. Importantly, treatments began two days after BrdU injection, to isolate effects on survival from those on cell proliferation. We found a large increase in granule cell survival in running mice compared with controls at both time points. In addition, running increased the proportion of granule cells expressing the immediate-early gene Arc in response to seizures, suggesting that it speeds incorporation into circuits, i.e., functional maturation. Stressed mice showed no change in Arc expression, compared to control animals, but, surprisingly, showed a transient increase in survival of 14-day-old granule cells, which was gone 7 days later. Examination of cell proliferation, using the endogenous mitotic marker proliferating cell nuclear antigen (PCNA) showed an increase in cell proliferation after 12 days of running but not after 19 days of running. The number of proliferating cells was unchanged 24 hours after the 12 th or 19 th episode of daily restraint stress. These findings demonstrate that running has strong effects on survival and maturation of young granule cells as well as their birth and that stress can have positive but short-lived effects on granule cell survival.

The ecological dominance of ants has to a great extent been achieved through their collective action and complex social organization. Ants provide diverse model systems to examine the neural underpinnings of individual behavior and group action that contribute to their evolutionary success. Core elements of ant colony structure such as reproductive and ergonomic division of labor, task specialization, and social integration are beginning to be understood in terms of cellular neuroanatomy and neurochemistry. In this review we discuss the neuroethology of colony organization by focusing on the role of biogenic amines in the control of social behavior in ants. We examine the role of neuromodulation in significant sociobiological characteristics of ants, including reproductive hierarchies, colony foundation, social food flow, nestmate recognition, territoriality, and size- and age-related sensory perception and task performance as well as the involvement of monoamines in collective intelligence, the ultimate key to the global dominance of these remarkable superorganisms. We conclude by suggesting future directions for the analysis of the aminergic regulation of behavior and social complexity in ants.

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