Bimanual motor deficits in older adults predicted by diffusion tensor imaging metrics of corpus callosum subregions.

Age-related changes in the microstructural organization of the corpus callosum (CC) may explain declines in bimanual motor performance associated with normal aging. We used diffusion tensor imaging in young (n = 33) and older (n = 33) adults to investigate the microstructural organization of seven specific CC subregions (prefrontal, premotor, primary motor, primary sensory, parietal, temporal and occipital). A set of bimanual tasks was used to assess various aspects of bimanual motor functioning: the Purdue Pegboard test, simultaneous and alternating finger tapping, a choice reaction time test and a complex visuomotor tracking task. The older adults showed age-related deficits on all measures of bimanual motor performance. Correlation analyses within the older group showed that white matter fractional anisotropy of the CC occipital region was associated with bimanual fine manipulation skills (Purdue Pegboard test), whereas better performance on the other bimanual tasks was related to higher fractional anisotropy in the more anterior premotor, primary motor and primary sensory CC subregions. Such associations were less prominent in the younger group. Our findings suggest that structural alterations of subregional callosal fibers may account for bimanual motor declines in normal aging.

Chronic behavioral stress induces apical dendritic reorganization in pyramidal neurons of the medial prefrontal cortex.

Both the hippocampus and the medial prefrontal cortex (mPFC) play an important role in the negative feedback regulation of hypothalamic-pituitary-adrenal (HPA) activity during physiologic and behavioral stress. Moreover, chronic behavioral stress is known to affect the morphology of CA3c pyramidal neurons in the rat, by reducing total branch number and length of apical dendrites. In the present study, we investigated the effects of behavioral stress on the mPFC, using the repeated restraint stress paradigm. Animals were perfused after 21 days of daily restraint, and intracellular iontophoretic injections of Lucifer Yellow were carried out in pyramidal neurons of layer II/III of the anterior cingulate cortex and prelimbic area. Cellular reconstructions were performed on apical and basal dendrites of pyramidal neurons in layer II/III of the anterior cingulate and prelimbic cortices. We observed a significant reduction on the total length (20%) and branch numbers (17%) of apical dendrites, and no significant reduction in basal dendrites. These cellular changes may impair the capacity of the mPFC to suppress the response of the HPA axis to stress, and offer an experimental model of stress-induced neocortical reorganization that may provide a structural basis for the cognitive impairments observed in post-traumatic stress disorder.

Naloxone suppression and morphine enhancement of voluntary wheel-running activity in rats.

This study examines the effects of the opioid receptor antagonist, naloxone, and the agonist, morphine, on voluntary wheel-running activity (WR) in rats. Male Sprague-Dawley rats were given 1-h access to a running wheel under non-deprived conditions. Naloxone injections (1.0, 0.5, or 0.25 mg/kg, ip), administered immediately before access to running wheels, dose-dependently suppressed WR. In another experiment, subjects were given 6-h access to running wheels under nondeprived conditions for 5 consecutive days. Morphine injections (2.0 mg/kg, sc) were found to increase WR after an initial suppression. These data demonstrate that naloxone inhibits WR, while morphine both suppresses and enhances WR depending on time and dose. These are in agreement with data on other behaviors that indicate that endogenous opioid systems play a major role in the mediation of motivational behaviors.