Neuroprotective effects of enriched environment housing after transient global cerebral ischaemia are associated with the upregulation of insulin-like growth factor-1 signalling.

AIMS: Use of enriched environment (EE) housing has been shown to promote recovery from cerebral ischaemic injury but the underlying mechanisms of their beneficial effects remains unclear. Here we examined whether the beneficial effects of EE housing on ischaemia-induced neurodegeneration and cognitive impairment are associated with increased insulin-like growth factor-1 (IGF-1) signalling in the hippocampus. METHODS: Forty-two adult male Wistar rats were included in the study and received either ischaemia or sham surgery. Rats in each group were further randomized to either: EE or standard laboratory cage housing (control). Rats were placed in their assigned housing condition immediately after recovery from anaesthesia. Behavioural testing in the cued learning and discrimination learning tasks were conducted 2 weeks after ischaemia. Rats were euthanized after behavioural testing and the hippocampus was analysed for IGF-1 level, IGF-1 receptor (IGF-1R) activation, protein kinase B (Akt) pathway activation, neurone loss and caspase 3 expression. RESULTS: Our data showed that EE housing: (1) mitigated ischaemia-induced neuronal loss; (2) attenuated ischaemia-induced increase in caspase 3 immunoreactivity in the hippocampus; (3) ameliorated ischaemia-induced cognitive impairments; and (4) increased IGF-1R activation and signalling through the Akt pathway after ischaemic injury. CONCLUSION: Ultimately, these findings suggest the possibility that IGF-1 signalling may be one of the underlying mechanisms involved in the beneficial effects of EE in optimizing recovery following cerebral ischaemic injury.

Retracted: Involvement of insulin-like growth factor-1 in chemotherapy-related cognitive impairment.

Here we examined the involvement of insulin-like growth factor 1 (IGF-1) on chemotherapy-induced cognitive impairment. Sixty-four ovariectomized female Sprague-Dawley rats were included in the study and given cyclophosphamide, methothrexate, and 5-fluorouracil (CMF) drug combination or saline (control). CMF was given once a week for 4 weeks. In one experiment, behavioral testing using the cued learning and spontaneous object recognition tasks were performed either: at the end of treatment or 4 weeks after treatment. In another experiment, rats from the chemotherapy and saline groups received either: continuous insulin-like growth factor 1 (IGF-1) or vehicle delivered subcutaneously via osmotic pump for 21 days (started the week after completion of therapy). Bromodeoxyuridine injections were given for 3 consecutive days starting at 2 weeks after completion of chemotherapy to assess the survival of proliferating cells. Increased levels of IGF-1 and activation of its receptor as well as increased activation of Akt and Erk1/2, its downstream signaling pathways was seen immediately after completion of chemotherapy but decreased 4 weeks after treatment. Behavioral testing showed CMF-induced cognitive impairment after completion of therapy and persisted for 4 weeks. We also found that giving IGF-1 significantly increased activation of its receptor, and the Akt and Erk1/2 pathways, and most importantly attenuated chemotherapy-induced cognitive impairment. CMF-induced neuronal apoptosis was also seen and the ratio of surviving cells that proliferate was higher compared to the number of apoptotic cells in the CMF rats given IGF-1. These results suggest that IGF-1 is involved in CMF-induced cognitive impairment by modulating cell death and cell proliferation.

Chronic neuroinflammation and cognitive impairment following transient global cerebral ischemia: role of fractalkine/CX3CR1 signaling.

Although neuroinflammation has been studied extensively in animal models of cerebral ischemia, their contrasting functions are still not completely understood. A major participant in neuroinflammation is microglia and microglial activation usually regulated by the chemokine CX3CL1 (fractalkine) and its receptor, CX3CR1. Here, we examined the involvement of CX3CR1 on ischemia-induced chronic neuroinflammation and cognitive function using small interfering RNA (siRNA). Forty adult male Wistar rats were included in the study and received either ischemia or sham surgery then were randomized to receive either CX3CR1 siRNA or scrambled RNA as control starting at 7 days after reperfusion. Behavioral testing commenced 28 days after siRNA delivery and all rats were euthanized after behavioral testing. Our data showed that: (i) transient global cerebral ischemia significantly decreased fractalkine/CX3CR1 signaling in the hippocampus; (ii) inhibition of CX3CR1 function exacerbated the ischemia-induced chronic increase in microglial activation and pro-inflammatory cytokine levels; (iii) inhibition of CX3CR1 function worsened ischemia-induced chronic cognitive impairment; (iv) inhibition of CX3CR1 function in sham rats resulted in increased IL-1ß expression and impaired behavioral performance. However, no significant effect of CX3CR1 on ischemia-induced neurodegeneration was seen. The present study provides important insight to understanding the involvement of CX3CR1 in chronic neuroinflammation and cognitive impairment.

Astrocytic changes in the hippocampus and functional recovery after cerebral ischemia are facilitated by rehabilitation training.

In this study we examined whether astrocytic and basic fibroblast growth factor changes after cerebral ischemia can be influenced by rehabilitation training and if these changes are associated with functional improvement. After receiving either ischemia or sham surgery, male adult Wistar rats were assigned to one of two rehabilitation training group: complex environment housing (EC) or paired housing as controls (CON). Rats were tested in the water maze after 14 days of rehabilitation training. Results showed increased expression of reactive astrocytes (GFAP) in all ischemic animals and in the sham EC rats with a significant overall increased seen in the ischemia EC housed animals. The pattern of basic fibroblast growth factor (FGF-2) expression seen was somewhat similar to that of GFAP. Behavioral data showed that even though all animals learned to perform the water maze task over time, the ischemia CON rats took longer to learn the task while all the ischemia EC animals performed as well as the sham groups. Regression analysis showed that increased GFAP was able to explain some of the variances in the behavioral parameters in the water maze of the ischemia EC rats suggesting that the activation of astrocytes in this group probably mediated enhanced functional recovery. Lastly, it is possible that the favorable effect of astrocyte activation after cerebral ischemia was mediated by FGF-2.

Amelioration of cognitive impairment and changes in microtubule-associated protein 2 after transient global cerebral ischemia are influenced by complex environment experience.

In this study we examined whether expression of microtubule-associated protein 2 (MAP2) after transient global cerebral ischemia can be influenced by behavioral experience and if the changes are associated with functional improvement. Rats received either ischemia or sham surgery then assigned to: complex environment housing (EC) or social housing (SC) as controls for 14 days followed by water maze testing. Upregulation of MAP2 was seen in all ischemic animals with a significant overall increase evident in the EC housed rats. Behaviorally, all animals learned to perform the water maze task over time but the ischemia SC rats had the worst performance overall while all the EC housed animals demonstrated the best performance in general. Regression analysis showed that increase MAP2 expression was able to explain some of the variance in the behavioral parameters in the water maze suggesting that this cytoskeletal protein probably played a role in mediating enhanced functional outcomes.

Dentate gyrus neurogenesis after cerebral ischemia and behavioral training.

Neurogenesis in the mammalian brain continues throughout adulthood. Several factors have been shown to influence neurogenesis, including experience in a complex environment (EC), exercise (EX), and ischemic insult. The authors investigated the effects of behavioral rehabilitation training following transient global cerebral ischemia on the number of new cells in the dentate gyrus that incorporated bromodeoxyuridine (BrdU), a thymidine analog that labels cells undergoing DNA replication. Seventy-two animals were included in the study, and 4-vessel occlusion was used to induce cerebral ischemia while control animals were subjected to anesthesia and sham surgery alone. Within 3 days of surgery, rats were randomly assigned to either EC, EX, or control (paired housing in standard laboratory conditions) groups. All animals were sacrificed 2 weeks after behavioral training. Immunohistochemistry results showed an increased number of BrdU-labeled cells in the subgranular zone of the dentate gyrus in all ischemic groups and in the EC and EX sham groups, although no significant group differences were seen. Examination of cell phenotype showed that almost all BrdU-positive cells colabeled with TuJ1, an immature neuron marker, in all animals whereas only a few BrdU-positive cells colabeled with NeuN, a mature neuron marker. BrdU/NeuN-labeled cells were seen only in the sham and ischemia EC groups. No new cells showed glial fibrillary acidic protein, astrocyte marker, colabeling. These results suggest that the adult brain has an inherent regenerative capacity after insult and that behavioral training following injury does not have an additive effect on neurogenesis. Finally, the enhanced maturation of BrdU-positive cells seen in the EC rats is probably modulated by environmental cues.