The impact of the diabetic condition on the CNS, specifically on limbic structures, has been extensively studied by several groups including ours. These contributions suggested that hippocampal alterations, including impairments in adult neurogenesis, could be potentially associated with disturbances in cognitive functions and an increased risk of depression and dementia 
On the other hand, environmental complexity induces multiple changes in the brain, promoting reorganization of sensorial cortices (i.e. visual, olfactory and tactile cortices develop in response to the exposure to environmental stimuli), enhancing synaptic plasticity (by increasing LTP), dendritic spine density and expression of synaptic proteins, receptors and neurotrophins 
. The beneficial effects of environmental enrichment on the hippocampus have been described in animal models of physiological and pathological conditions 
, counting aging 
, stress 
, Alzheimer's disease 
and traumatic brain injury 
among other situations, but, to our knowledge, this is the first report describing an environmental effect on the hippocampus in an animal model of diabetes.
Here, we present evidence of the positive effect that the exposure to a complex environment has on the hippocampus of diabetic mice. Cell proliferation, differentiation and survival of newborn neurons, vascularization of the dentate gyrus and dendritic complexity of mature hippocampal neurons were improved after a short exposure of streptozotocin-induced diabetic mice to an environment that incorporated social, sensorial and cognitive stimuli.
The evaluation of the cell proliferation, assessed by Ki67 immunodetection in the dentate gyrus, showed a tendency for diminishing in diabetic animals in standard caging and a significant increase with EE. A high proliferation rate has been associated with enrichment paradigms that specifically included physical exercise adding a wheel in the cage for voluntary running 
. This component of the EE was purportedly excluded from our experiments to minimize effects of mixed stimuli. We found an increased proliferation after EE with limited physical exercise only in diabetic animals suggesting that environmental effects on this parameter become evident specifically under the pathological context. Survival of newly generated cells, studied by BrdU administration before starting the exposure to enriched or standard conditions, was markedly reduced in diabetic mice housed in normal cages, in agreement with previous results 
. Surprisingly, a short period –ten days- of EE was able to increase cell survival in diabetic animals. Moreover, also as we have previously published, hippocampal volume is not affected by diabetes at 20 days of evolution 
Differentiation of newborn cells into a neuronal phenotype, evaluated as the percentage of BrdU-positive cells that showed TuJ1 co-staining, was diminished in diabetic mice. Potential mechanisms underlying this phenomenon could be an increased differentiation towards a glial phenotype, a delay in newborn cell differentiation and an accelerated maturation of TuJ1+ cells leading to the loss of the marker. A further thorough analysis of the cell differentiation dynamic should be done to elucidate this point. It is interesting to note that diabetic animals housed under enriched conditions showed neuronal differentiation levels similar to non-diabetic animals, demonstrating the capacity of sensorial and social stimuli to reverse disease-associated neuronal changes. The total number of granular neurons in the DG did not change between experimental groups Indicating that, at least in the studied temporal frame, diabetes or the differential housing conditions are not able to affect this factor.
Within the EE- induced changes in the dentate gyrus, our results showed an increase in the arborization of newly generated neurons. The dendritic length of DCX+ more mature neurons showed a decline in diabetic mice and a significant increase after exposure to the EE. Interestingly, cells expressing DCX, that will generate mature granule neurons, were described as specially responsive to certain enrichment paradigms 
and, in contrast, particularly vulnerable in conditions such as diabetes, aging and chronic stress 
Using a modified version of the Golgi silver impregnation technique we studied the dendritic ramification, dendritic length and spine density of hippocampal pyramidal neurons and these changes were prevented by EE. These parameters were clearly reduced in CA1 neurons of diabetic animals. Sholl analysis allowed us to conclude that the reduction in the dendritic ramification and length was more evident near the neuronal soma. There is probably a regional-dependent differential modulation of the dendritic tree by the diabetic condition. Brain derived neurotrophic factor (BDNF) is abundant in the hippocampus, among other cerebral structures, and its constitutive secretion is intimately involved with proximal processes growth 
. Nitta et al have described a reduction in the brain content of BDNF protein and mRNA levels in experimental diabetes 
and, in accordance, we found decreased BDNF mRNA levels in the hippocampus of streptozotocin-treated mice using in situ
hybridization (unpublished results). Based on the data obtained in the present study, environmental enrichment seems to positively regulate the dendritic length of mature pyramidal neurons, being this effect more evident at zones that are proximal to the neuronal soma. Modulation of the dendritic complexity takes place at different levels and could be associated to changes in the synaptic function and also in animal behavior. Experimental diabetes is linked to cognitive impairment and behavioral changes together with hypothalamic-pituitary-adrenal axis dysregulation, including high plasma glucocorticoid levels, that could be coupled with the mentioned hippocampal disturbances 
. Along this line, a possible modulating effect of EE on the hypothalamic-pituitary-adrenal axis and related hormones in diabetic mice should not be discarded.
Another important finding of the present work is related to the brain vasculature in diabetic animals and the strong effect of EE. The lectin-positive area in the DG, specifically corresponding to blood vessel area, was lower in streptozotocin-treated mice housed in standard cages compared with controls. Concordantly, we have previously found a diminished vascular area in the granular cell layer of type 2 diabetic Goto-Kakisaki rats using von Willebrand factor immunohistochemistry 
. This result constitutes a novel fact matching with other hippocampal disturbances associated with this disease. In this line, vessel abnormalities like vessel regression, hypoperfusion, endothelial degeneration, abnormal vessel growth, size and shape and microangiopathy are found in neurological disorders and are often associated with neuronal loss 
. Neurogenesis in the adult dentate gyrus occurs in a neurovascular niche, where endothelial cells play a crucial role. Furthermore, a poor neurogenic potential has been linked to reduced vascular density in aging 
. Improvement of cerebral blood flow could be a probable mechanism underlying the benefits of EE on the CNS, as the exposure to complex environments stimulates capillary branching and surface area 
and promotes endothelial cell proliferation 
in the brain. In addition, VEGF, a neurotrophic and pro-angiogenic factor, is upregulated after EE 
, possibly playing a role in the regulation of adult neurogenesis. A short term exposure -ten days- to an enriched environment was able to significantly increase the vessel area in the DG from diabetic mice. This last result, together with the effect on neuronal survival and dendritic development in new neurons, arborization and spine density in mature neurons constitute a clear evidence of the strong influence that experience exerts on hippocampal plasticity. Diabetic animals exposed to the enriched environment showed a slight but significant decrease in the glycemia. Though we can not discard a relationship of this effect with the multiple changes that environmental enrichment exerted in the hippocampus, it is important to consider that glycemia was over 28 mM in both diabetic groups, denoting a severe diabetic status. As reported by Stranahan et al 
in streptozotocin-induced diabetic rats, increased glucose circulating levels do not correlate with changes in glucose or insulin levels in the hippocampus, indicating an indirect mechanism of damage. Possible intermediate pathways could be an increased activity of the HPA axis 
, upregulation of the receptor for advanced glycation end products (RAGE) expression in the brain 
, brain vascular inflammation 
and increased oxidative stress 
. In a previous study done in spontaneously type 1 diabetic mice (NOD mice), we found that hippocampal abnormalities are evident before the onset of hyperglycemia, indicating that other pathological, glycemia-independent pathways are activated 
Notably, in our experiments, control animals exposed to EE did not show significant differences in the studied parameters compared to standard-housed mice, with the exception of an increase in the dendritic length of DCX+ cells. Doublecortin+ cells in the dentate gyrus seem to be particularly sensitive to changes in the environment even in normal animals. Possibly, a prolonged exposure time may be required for changes to become evident in normal, young animals.
Remarkably, in the literature there is no complete consensus about the protocols of EE used by different research groups. Differences can be found regarding objects included, duration of EE, cage size, type of enrichment and the combination or not of EE with physical exercise 
. Running can promote neurogenesis, synaptic plasticity and learning 
. In streptozotocin-induced diabetic rats, treadmill running was shown to improve synaptic plasticity in the DG 
. However, physical exercise alone may not be able to promote morphological changes in the dendritic trees of hippocampal neurons 
. Several similarities in the effects of both aging and diabetes can be found in the hippocampus 
and an interaction between diabetes and aging was described 
. As in aging, increased oxidative stress is present in the diabetic brain 
and constitutes another factor leading to the diabetes-induced behavioral deficits 
. In aged rats, EE was shown to potentially protect against oxidative stress and to improve cognitive performance 
. As mentioned, the duration of EE is a key factor to be considered when brain plasticity is studied. Bindu and colleagues demonstrated that a short-term exposure to an EE was able to enhance dendritic branching in ventral subicular-lesioned rats while few changes were found on control animals 
. In agreement to that, our results suggest that in a pathological situation with several hippocampal disturbances, like type 1 diabetes, a short term exposure to EE could be enough to promote a potent brain response involving an enhancement of adult neurogenesis, dendritic growth and spine density as well as an increase of the hippocampal vascular area.
Our data could represent a new approach to the prevention of some of the central nervous system complications correlating with type 1 diabetes. Even though learning and memory testing is still needed to link these results with behavioral changes in streptozotocin-treated mice, the diabetic brain reveals that it is still able to react in response to environmental challenges.