Research reportChanges in number of synapses and mitochondria in presynaptic terminals in the dentate gyrus following cerebral ischemia and rehabilitation training
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.
References (49)
- et al.
Behaviorally-induced ultrastructural plasticity in the hippocampal region after cerebral ischemia
Brain Res.
(2004) - et al.
Adult neurogenesis is regulated by adrenal steroids in the dentate gyrus
Neuroscience
(1994) - et al.
Functional recovery of forelimb response capacity after forelimb primary motor cortex damage in the rat is due to the reorganization of adjacent areas of cortex
Neuroscience
(1995) - et al.
Experience-dependent structural plasticity in cortex heterotropic to focal sensorimotor cortical damage
Exp. Neurol.
(2000) - et al.
Environmental enrichment alters nerve growth factor-induced gene A and glucocorticoid receptor messenger RNA expression after middle cerebral artery occlusion in rats
Neuroscience
(1999) - et al.
Stress and hippocampal neurogenesis
Psychiatry
(1999) - et al.
Synaptogenesis and dendritic growth in the cortex opposite unilateral sensorimotor cortex damage in adult rats: a quantitative electrom microscopic examination
Brain Res.
(1996) - et al.
Exercise induces angiogenesis but does not alter movement representations within rat motor cortex
Brain Res.
(2002) - et al.
Temporal profile of nerve growth factor-like immunoreactivity after transient focal cerebral ischemia in rats
Brain Res.
(1996) - et al.
Comparative analyses of synaptic densities during reactive synaptogenesis in the rat dentate gyrus
Brain Res.
(2004)