Brenda J. Anderson

Exercise and the brain: angiogenesis in the adult rat cerebellum after vigorous physical activity and motor skill learning.

This study compared the morphology of cerebellar cortex in adult female rats exposed for 1 month to repetitive exercise, motor learning, or an inactive condition. In the exercise conditions, rats that were run on a treadmill or housed with access to a running wheel had a shorter diffusion distance from blood vessels in the molecular layer of the paramedian lobule when compared to rats housed individually or rats that participated in a motor skill learning task. Rats taught complex motor skills substantially increased the volume of the molecular layer per Purkinje neuron and increased blood vessel number sufficiently to maintain the diffusion distance. These results dissociate angiogenesis associated with increased neuropil volume (as seen in the motor learning group) from angiogenesis associated with increased metabolic demands (as seen in the exercise groups). While the volume fraction of mitochondria did not differ among groups, the mitochondrial volume fraction per Purkinje cell was significantly increased in the motor skill rats. This appears to parallel the previously reported increase in synapses and associated neuropil volume change.

Chronic administration of corticosterone impairs spatial reference memory before spatial working memory in rats

Corticosterone (CORT), the predominant glucocorticoid in rodents, elevated for 21 days damages hippocampal subregion CA3. We tested the hypothesis that CORT would impair spatial memory, a hippocampal function. In each of the three experiments, rats received daily, subcutaneous injections of either CORT (26.8 mg/kg body weight in sesame oil) or sesame oil vehicle alone (VEH). CORT given for 21 or 56 days effectively attenuated body weight gain and reduced selective organ and muscle weights. All behavioral testing was done on tasks that are minimally stressful and avoid deprivation. For each experiment, testing commenced 24h after the last injection. CORT given for 21 days did not impair spatial working memory in the Y-maze (Experiments 1 and 2). After 56-day administration of CORT, spatial working memory was impaired in the Y-maze (Experiment 2). CORT given for 21 days also failed to impair spatial working memory in the Barnes maze (Experiment 3). However, in trials that depended solely on reference memory, the VEH group improved in performance, whereas the CORT group did not. In conclusion, CORT elevated over a period of 21 days did not impair spatial working memory, but impaired the formation of a longer-term form of memory, most likely reference memory. Impairments in spatial working memory are seen only after longer durations of CORT administration.

The effects of chronic glucocorticoid exposure on dendritic length, synapse numbers and glial volume in animal models: Implications for hippocampal volume reductions in depression

Glucocorticoids (GCs) are hormones secreted by the adrenal glands as an endocrine response to stress. Although the main purpose of GCs is to restore homeostasis when acutely elevated, animal studies indicate that chronic exposure to these hormones can cause damage to the hippocampus. This is indicated by reductions in hippocampal volume, and changes in neuronal morphology (i.e., decreases in dendritic length and number of dendritic branch points) and ultrastructure (e.g., smaller synapse number). Smaller hippocampal volume has been also reported in humans diagnosed with major depressive disorder or Cushings disorder, conditions in which GCs are endogenously and chronically elevated. Although a number of studies considered neuron loss as the major factor contributing to the volume reduction, recent findings indicated that this is not the case. Instead, alterations in dendritic, synaptic and glial processes have been reported. The focus of this paper is to review the GC effects on the cell number, dendritic morphology and synapses in an effort to better understand how these changes may contribute to reductions in hippocampal volume. Taken together, the data from animal models suggest that hippocampal volumetric reductions represent volume loss in the neuropil, which, in turn, under-represent much larger losses of dendrites and synapses.

Synapse loss from chronically elevated glucocorticoids: Relationship to neuropil volume and cell number in hippocampal area CA3

Individuals with clinical disorders associated with elevated plasma glucocorticoids, such as major depressive disorder and Cushings syndrome, are reported to have smaller hippocampal volume. To understand how the hippocampus responds at the cellular and subcellular levels to glucocorticoids and how such changes are related to volume measures, we have undertaken a comprehensive study of glucocorticoid effects on hippocampal CA3 volume and identified elements in the neuropil including astrocytic volume and cell and synapse number and size. Male Sprague‐Dawley rats were injected with corticosterone (40 mg/kg), the primary glucocorticoid in rodents, or vehicle for 60 days. The CA3 was further subdivided so that the two‐thirds of CA3 (nearest the dentate gyrus) previously shown to be vulnerable to corticosterone could be analyzed as two separate subfields. Corticosterone had no effect on neuropil volume or glial volume in the proximal subfield but caused a strong tendency for astrocytic processes to make up a larger proportion of the tissue and for volume of tissue made of constituents other than glial cells (primarily neuronal processes) to be smaller in the middle subfield. Within the neuropil, there were no cellular or subcellular profiles that indicated degeneration, suggesting that corticosterone does not cause prolonged damage. Corticosterone did not reduce cell number or cell or nonperforated synapse size but did cause a pronounced loss of synapses. This loss occurred in both subfields and, therefore, was independent of volume loss. Together, the findings suggest that volume measures can underestimate corticosterone effects on neural structure. J. Comp. Neurol. 498:363–374, 2006.

Exercise increases metabolic capacity in the motor cortex and striatum, but not in the hippocampus

Acute bouts of exercise have been shown to produce transient increases in regional cerebral glucose utilization, oxygen uptake, and cerebral blood flow in motor cortex, striatum, and hippocampus. The purpose of this study was to determine whether or not chronic exercise will cause long-term metabolic plasticity in brain structures activated during physical activity. The activity of cytochrome oxidase (COX), is coupled to the production of ATP, and reflects long-term plasticity in metabolic capacity. The present study examined whether or not 6 months of voluntary exercise would increase COX activity in the striatum, sensorimotor cortex, and three hippocampal subfields. Five-month-old, female Long-Evans hooded rats were randomly assigned to a control or exercise condition. Exercising rats had running wheels attached to their home cages. After the training period, fresh brains were rapidly frozen and sectioned with a cryostat. COX activity was measured using COX histochemical methods and optical densitometry. Rats in the exercise condition had significantly higher optical density in the hindlimb and forelimb motor cortices (18%, P<0.01) and dorsolateral caudate putamen (17%, P<0.01), but not in the ventrolateral caudate putamen or any subfield of the hippocampus. Although exercise is believed to increase neuronal activity in the hippocampus, motor cortex and striatum, only limb representations in the motor cortex and striatum increase bioenergetic capacity after regular exercise.

Neurochemical organization of the macaque striate cortex: Correlation of cytochrome oxidase with Na+K+ATPase, NADPH-diaphorase, nitric oxide synthase, and N-methyl-d-aspartate receptor subunit 1

Previously, we found that cytochrome oxidase-rich zones in the supragranular layers of the macaque striate cortex had more asymmetric, glutamate-immunoreactive synapses than the surrounding, cytochrome oxidase-poor regions. A major glutamate receptor family is N-methyl-D-aspartate, which is implicated in the stimulation of nitric oxide synthase and in the production of nitric oxide, a gaseous intra- and inter-cellular messenger. To determine if energy-generating and energy-utilizing enzymes bore any spatial relationship with neurochemicals associated with glutamatergic neurotransmission in the monkey visual cortex, serial cortical sections were processed histochemically for cytochrome oxidase and NADPH-diaphorase, and immunohistochemically for sodium/potassium-ATPase, nitric oxide synthase, and N-methyl-D-aspartate receptor subunit 1 protein, respectively. The general patterns were similar among the five neurochemicals, with layers 4C, 6 and supragranular puffs being labelled, although the intensity of labelling differed among them. Monocular impulse blockade with tetrodotoxin for two to four weeks induced a down-regulation of all five neurochemicals not only in deprived layer 4C ocular dominance columns, but also in deprived rows of puffs. Thus, the regulation of all five neurochemicals in the mature visual cortex is activity-dependent. Combined cytochrome oxidase histochemistry and nitric oxide synthase immunohistochemistry in the same sections revealed that double-labelled cells were primarily medium-sized non-pyramidals in various cortical layers. Likewise, those that were double-labelled by N-methyl-D-aspartate receptor subunit 1 immunohistochemistry and nitric oxide synthase immunogold silver staining in the same sections were of the medium-sized non-pyramidal neurons. At the ultrastructural level, combined cytochrome oxidase cytochemistry and postembedding immunogold labelling for nitric oxide synthase showed that immunogold particles for nitric oxide synthase were more heavily concentrated in cytochrome oxidase-rich type C cells. These medium-sized non-pyramidal cells were previously found to be gamma aminobutyric acid-immunoreactive and received both gamma aminobutyric acid- and glutamate-immunoreactive axosomatic synapses. Thus, our results are consistent with an enrichment of excitatory synaptic interactions in metabolically active regions of the primate visual cortex that involves glutamate-related neurochemicals, such as N-methyl-D-aspartate receptors and nitric oxide synthase. These interactions impose a higher energy demand under normal conditions and are down-regulated by retinal impulse blockade.

A new method for the investigation of capillary structure.

Numerous physiological conditions as well as behavioral conditions have been shown to influence central nervous system vascular structure. Many of the methods used to investigate these structural alterations take advantage of the visibility of viscous substances (e.g. India ink in gelatin) perfused into the vasculature. The high viscosity of the solution, however, can cause incomplete vessel perfusion. The aim of the present study was to test whether or not capillaries seen in tissue perfused with fixative, embedded in celloidin and stained with Methylene Blue-Azure II (n=6) could be a useful alternative for the investigation of brain vascular structure. The method was compared to tissue from six rats perfused with India ink in gelatin and stained with cresyl violet. Qualitatively, vessels in the standard perfused tissue embedded in celloidin yielded clear vessels with stained pericytes. The two methods did not differ in branch point to cell ratio, length of individual capillaries, vessel length per mm(3), and capillary tortuosity. The capillary diameter was greater in the celloidin embedded tissue than in the India ink perfused tissue. Measuring the diameter between vessel walls appears to provide a more accurate measure than the widest distance between India ink pigments. Quantitative comparisons suggest that perfusion with standard fixative followed by embedding in celloidin provides vascular quantification comparable to that from India ink perfused tissue. The present method has several advantages, which include visualization of pericytes, increased probability of complete perfusion, clear view of cells that might otherwise be obscured by opaque vessels, and the possibility of using the alternate cerebral hemisphere for investigation of vascular ultrastructure.

Plasticity of gray matter volume: The cellular and synaptic plasticity that underlies volumetric change

Fifty years ago, Mark Rosenzweig and coworkers described environmental effects on brain chemistry and gross brain weight. William Greenough then used stereological tools, electron microscopy, and the Golgi stain to demonstrate that enrichment led to dendritic growth and synapse addition. Together these forms of plasticity accounted for cortical expansion and a reduction in cell density. In parallel with other investigators, Greenough demonstrated that these effects were not limited to the rodent, the cortex, or development, but instead generalize to many species, brain regions, and life stages. Studies of the anatomical effects of enrichment foreshadowed the recent empirical evidence for cortical volumetric increases after environmental experience and training in humans. Since research in humans is limited to regional effects, the analysis of the cellular and synaptic effects of enrichment, and their contribution to volumetric increases can inform us of the potential cellular and subcellular plasticity the leads to volume change in humans.

Cerebellar and brainstem circuits involved in classical eyeblink conditioning.

Model systems are one useful strategy for the investigation of the mechanisms of learning. Whereas mammalian model systems generally do not offer the ease of identifying circuitry and exploring cellular mechanisms of learning that is realized with invertebrate preparations /37,97/, research involving the rabbit classical eyeblink conditioning paradigm has now reached the state at which much of the basic conditioning neural circuit appears to have been identified /9,65,66,85,89,91/. Despite a dispute as to precisely where in the circuitry convergence of the associated stimuli may occur, there is substantial evidence identifying the stimulus input pathways and motor output pathway. The present summary of this research details these paths. In addition, the proposed sites of convergence of the conditioning stimuli are discussed. Finally, a hypothesized neural circuit responsible for classical eyeblink conditioning is presented along with some suggestions for future research directions.

Chronic corticosterone affects brain weight, and mitochondrial, but not glial volume fraction in hippocampal area CA3.

Corticosterone (CORT), the predominant glucocorticoid in rodents, is known to damage hippocampal area CA3. Here we investigate how that damage is represented at the cellular and ultrastructural level of analyses. Rats were injected with CORT (26.8 mg/kg, s.c.) or vehicle for 56 days. Cell counts were estimated with the physical disector method. Glial and mitochondrial volume fractions were obtained from electron micrographs. The effectiveness of the CORT dose used was demonstrated in two ways. First, CORT significantly inhibited body weight gain relative to vehicles. Second, CORT significantly reduced adrenal gland, heart and gastrocnemius muscle weight. Both the adrenal and gastrocnemius muscle weight to body weight ratios were also significantly reduced. Although absolute brain weight was reduced, the brain to body weight ratio was higher in the CORT group relative to vehicles, suggesting that the brain is more resistant to the effects of CORT than many peripheral organs and muscles. Consistent with that interpretation, CORT did not alter CA3 cell density, cell layer volume, or apical dendritic neuropil volume. Likewise, CORT did not significantly alter glial volume fraction, but did reduce mitochondrial volume fraction. These findings highlight the need for ultrastructural analyses in addition to cellular level analyses before conclusions can be drawn about the damaging effects of prolonged CORT elevations. The relative reduction in mitochondria may indicate a reduction in bioenergetic capacity that, in turn, could render CA3 vulnerable to metabolic challenges.