The adrenal hormone corticosterone (CORT) acts on brain to mediate physiology and behavior. In songbirds, behavioral effects of CORT vary across species, environmental conditions, and life history stage, with several mechanisms proposed to account for these divergent results. Although blood CORT levels are well characterized, few studies measure CORT within the brain itself. Here we used in vivo microdialysis to measure CORT in two regions of the zebra finch brain, the hippocampus (HP) and caudal nidopallium (cNp). Our results show that we can successfully measure physiological levels of CORT in brain within 15- to 30-minute intervals of dialysate collection. Moreover, we found that levels in the cNp were generally lower than levels in the HP. Surprisingly, whereas plasma CORT levels increased in response to a standard stressor, no stress-induced surge was detected in the HP or cNp. In addition, although a diel CORT rhythm was observed in plasma, the rhythm in brain was attenuated and only observed when levels were integrated over a 4-hour time period. Regional differences in brain CORT levels were reflected in local mRNA expression levels of the CORT-inactivating enzyme 11β-hydroxysteroid dehydrogenase type 2 with levels elevated in the cNp relative to the HP. Region-specific CORT metabolism may therefore play a role in buffering the brain from CORT fluctuations.

Age-related decrements in cognitive ability have been proposed to stem from deteriorating function of the hippocampus. Many birds are long lived, especially for their relatively small body mass and elevated metabolism, making them a unique model of resilience to aging. Nevertheless, little is known about avian age-related changes in cognition and hippocampal physiology. We studied spatial cognition and hippocampal expression of the age-related gene, Apolipoprotein D (ApoD), and the immediate early gene Egr-1 in zebra finches at various developmental time-points. In a first experiment, middle-aged adult males outperformed middle-aged females in learning correct food locations in a 4-arm maze, but all birds remembered the task equally well after a 5- or 10-day delay. In a second experiment comparing young and old birds, aged birds showed minimal evidence for deterioration in spatial cognition or motivation relative to young birds, except that aged females showed less rapid gains in accuracy during spatial learning than young females. These findings indicate that sex differences in hippocampus-dependent spatial learning and decline with age are phylogenetically conserved. With respect to hippocampal gene expression, adult females expressed Egr-1 at significantly greater levels than males after memory retrieval, perhaps reflecting a neurobiological compensation. Contrary to mammals, ApoD expression was elevated in young zebra finches compared to aged birds. This may explain the near absence of decrements in spatial memory due to age, possibly indicating an alternative mechanism of managing oxidative stress in aged birds.

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