Elsevier

Brain Research

Volume 1033, Issue 1, 1 February 2005, Pages 51-57
Brain Research

Research report
Changes in number of synapses and mitochondria in presynaptic terminals in the dentate gyrus following cerebral ischemia and rehabilitation training

https://doi.org/10.1016/j.brainres.2004.11.017Get rights and content

Abstract

Damage to the adult brain can result in adaptive plasticity in regions adjacent to the site of the principal insult and that the plastic changes may be modulated by post-injury rehabilitation training. In this study, we examined the effects of rehabilitation training on synaptic morphology in the dentate gyrus following transient global cerebral ischemia and the metabolic correlates of the ultrastructural changes. Forty adult male Wistar rats were included in the study and assigned to either ischemia or sham group. Following ischemic or sham surgery, rats were randomized to either complex environment housing (EC), exercise (EX), or social condition (SC, paired housing) group. Electron microscopy and unbiased stereological methods were used to evaluate synaptic plasticity and the number and size of mitochondria in synaptic axon terminals. Increased number of granule neurons was seen in all ischemic groups and in the sham EC rats. Changes in the number of synapses per neuron in the outer and inner molecular layers of the dentate gyrus parallel those seen in granule neurons. Similarly, ischemia and behavioral experience in EC independently increased the number of synaptic mitochondria in presynaptic terminals in both the outer and inner molecular layers; however, no significant changes were seen in mitochondrial size. These data suggest a link between behavioral training and synaptic plasticity in the region adjacent to the injury and that the likely metabolic correlate of this synaptic plasticity is increased number of mitochondria at synaptic axon terminals.

Introduction

Evidence exists that damage to the adult brain results in morphological changes in regions adjacent to the site of the principal insult [12], [24], [25], [36], [41]. These changes include post-injury sequelae of neurotoxic and degenerative events as well as an apparently adaptive neural plasticity. For example, cortical injury has been found to result in increased dendritic arbors and synaptogenesis [6], [25] in the non-damaged cortical regions. In addition, reorganization of cortical representations in the remaining regions of the cortex was seen after cortical damage [11], [31]. In cerebral ischemia, changes seen in neuronal and non-neuronal cells in regions adjacent to the injury include dendritic restructuring [3], [10], [23], reactive changes in glia [28], [43], reactive synaptogenesis [21], [26], and enhanced neurogenesis [27], [35], [49]. Ischemic insult has also been shown to activate a variety of potential growth-promoting processes including increased expression of neurotrophic factors, specifically nerve growth factor and brain-derived neurotrophic factor [33], [45], and cell adhesion molecules [14].

Reports have shown that some of the injury-induced morphological changes seen in the adult brain may be modulated by post-insult rehabilitation training such as experience in a complex environment. Housing in an enriched environment before or after focal ischemic injury resulted in increased dendritic spine density as well as improved behavioral outcome on several sensorimotor tasks [22], [23], [40]. Behavioral experience in a complex environment following cerebral ischemia also resulted in increased number of synapses with perforated postsynaptic densities and terminals that form multiple synapses [4]. Furthermore, enriched environment housing after ischemic insult has been shown to alter expression of several neurotrophic factors, such as nerve growth factors and basic fibroblast growth factor [15], [44]. Other rehabilitation training paradigms, such as motor-skill training after cortical injury, or constraint-induced therapy after clinical stroke were able to promote reorganization of cortical maps in tissues adjacent to the injury and were associated with motor recovery [31], [32], [34], [39]. These reports provide converging evidence that rehabilitation therapies instituted after brain injury may remodel neuronal circuitry in the tissues surrounding the injury and that such reorganization may possibly contribute to functional recovery.

In the previous electron microscopic study, we demonstrated decreased neuron density in the anterior and medial CA1 in ischemic animals and greater number of synapses per neuron was also evident in this group. Although the ischemia-induced synaptic structural changes were evident in our previous study, the exact mechanism of why these changes occur is not fully understood. Furthermore, the metabolic correlates of the injury- and behaviorally-induced synaptic changes reported in our previous study and those of others are not known. By using tissues generated in the previous electron microscopic study [4], stereological procedures were used to examine the changes in synapse number in the dentate gyrus (because of its proximity to the hippocampus proper) after cerebral ischemia and behavioral training. In addition, the possible metabolic correlates of these synaptic changes were examined by quantifying the number and size of mitochondria in synaptic axon terminals. The mitochondria are important determinants of cerebral metabolism [37], [48] since oxidative phosphorylation is the major energy-synthesizing pathway used by the central nervous system [16].

Section snippets

Methods

The quantitative analyses in the present study used tissues generated from a previous study which examined the effects of rehabilitation training after cerebral ischemia on the ratio of synapses-to-neuron and changes in synaptic configuration in hippocampal CA1 [4]. Briefly, the four-vessel occlusion method was used to induce transient global cerebral ischemia as previously described. Adult male Wistar rats 3–4 months of age (body weight of 350–375 g at the time of surgery) were used in the

Granule cell neurons

Fig. 2 shows the total number of granule neurons in the dentate gyrus. A significant main effect of ischemia (F(5/34) = 6.82, P < 0.05) as well as behavioral training (F(5/34) = 6.99, P < 0.05) was seen but there was no GROUP × BEHAVIORAL TRAINING interaction (F(5/34) = 4.19, P > 0.05). Subsequent planned comparisons showed significantly increased number of granule neurons in the dentate gyrus in the ischemia EX and ischemia SC animals as compared to the sham EX and sham SC groups (F(1/34) =

Discussion

The main findings in this study are that: (1) either cerebral ischemia or behavioral experience in EC was able to independently increase the total number of granule cell neurons in the dentate gyrus, (2) increased ratio of synapses-to-neuron in both the OML and IML of the dentate gyrus was seen following behavioral training and cerebral ischemia, and (3) behavioral training and ischemic injury resulted in changes in the number of mitochondria at the presynaptic terminals per neuron in both OML

Acknowledgments

This work was supported by the National Institutes of Health grant #RO1 NR05260. We are grateful for the use of facilities at the University of Illinois Research Resources Center, Electron Microscopy Services, as well as the assistance of Mai Nguyen in developing the electron micrograph pictures.

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