17Beta-estradiol blocks NMDA-induced increases in regional cerebral O(2) consumption

Targeting Estrogen Signaling in the Radiation-induced Neurodegeneration: A Possible Role of Phytoestrogens

Radiation for medical use is a well-established therapeutic method with an excellent prognosis rate for various cancer treatments. Unfortunately, a high dose of radiation therapy comes with its own share of side effects, causing radiation-induced non-specific cellular toxicity; consequently, a large percentage of treated patients suffer from chronic effects during the treatment and even after the post-treatment. Accumulating data evidenced that radiation exposure to the brain can alter the diverse cognitive-related signaling and cause progressive neurodegeneration in patients because of elevated oxidative stress, neuroinflammation, and loss of neurogenesis. Epidemiological studies suggested the beneficial effect of hormonal therapy using estrogen in slowing down the progression of various neuropathologies. Despite its primary function as a sex hormone, estrogen is also renowned for its neuroprotective activity and could manage radiation-induced side effects as it regulates many hallmarks of neurodegenerations. Thus, treatment with estrogen and estrogen-like molecules or modulators, including phytoestrogens, might be a potential approach capable of neuroprotection in radiation-induced brain degeneration. This review summarized the molecular mechanisms of radiation effects and estrogen signaling in the manifestation of neurodegeneration and highlighted the current evidence on the phytoestrogen mediated protective effect against radiationinduced brain injury. This existing knowledge points towards a new area to expand to identify the possible alternative therapy that can be taken with radiation therapy as adjuvants to improve patients' quality of life with compromised cognitive function.

Sex Differences and Role of Gonadal Hormones on Glutamate LevelAfter Spinal Cord Injury in Rats: A Microdialysis Study

Introduction:

Sex differences in outcomes of Spinal Cord Injury (SCI) suggest a sex-hormone-mediated effect on post-SCI pathological events, including glutamate excitotoxicity. This study aimed to investigate the importance of gonadal hormones on glutamate release subsequent to SCI in rats.

Methods:

After laminectomy at T8–T9, an electrolytic lesion was applied to the spinothalamic tracts of male and female rats. Using spinal microdialysis, we assessed glutamate levels at the site of lesion in both intact and gonadectomized rats for 4 hours. In this way, we examined the sex differences in the glutamate concentrations.

Results:

The peak retention time of glutamate level was 10.6 min and spinal glutamate concentration reached a maximum level 40 min following SCI. In male SCI rats, gonadectomy caused a significant elevation of glutamate level (P<0.001) following injury which was maximum 40 min post-SCI as well. However, no significant alterations were seen in gonadectomized female rats.

Conclusion:

The significant differences in glutamate levels between both intact and gonadectomized SCI male and female rats show the sex-hormone-related mechanisms underlying the molecular events in the second phase of SCI. It seems that the role of male gonadal hormones to prevent glutamate excitotoxicity is more prominent. The exact mechanisms of these hormones on the functional recovery after SCI should be clarified in further studies.

Estradiol attenuates spinal cord injury-related central pain by decreasing glutamate levels in thalamic VPL nucleus in male rats

Central neuropathic pain (CNP) is a complicated medical problem that involves both the spinal and supraspinal regions of the central nervous system. Estrogen, a neuroprotective agent, has been considered a possible candidate for CNP treatment. In this study, we examined the effects of a single dose of 17β-estradiol on glutamate levels in the ventral posterolateral (VPL) nucleus of the rat thalamus. Furthermore, we determined whether there was a correlation between glutamate levels and neuropathic pain induced by unilateral electrolytic spinothalamic tract (STT) lesion. STT lesioning was performed in male Wistar rats at the T8-T9 vertebrae; rats were then administered 17β-estradiol (4 mg/kg, i.p.) 30 min after injury. Glutamate samples were collected using a microdialysis probe and quantified by high performance liquid chromatography. Mechanical allodynia (MA) and thermal hyperalgesia (TH) thresholds were measured pre-injury and 7, 14, and 28 days post-injury. We found that STT lesion significantly increased glutamate levels in the ipsilateral VPL nucleus 14 and 28 days post-injury; this was accompanied by allodynia and hyperalgesia in the hind paws of the rats. Administering 17β-estradiol to the rats decreased glutamate levels in the ipsilateral VPL nucleus and significantly increased MA and TH thresholds. These results suggest that glutamate in the VPL nucleus of the thalamus is involved in the pathology of neuropathic pain after STT injury; furthermore, 17β-estradiol may attenuate this neuropathic pain by decreasing glutamate levels.

Reduced effect of stimulation of AMPA receptors on cerebral O₂ consumption in a rat model of autism

Previous work demonstrated that basal alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor activity did not contribute to the elevated regional cerebral O₂ consumption in the brains of Eker rat (an autism-tuberous sclerosis model). We tested the hypothesis that increased stimulation of AMPA receptors also would not augment cerebral O₂ consumption in the Eker rat. Three cortical sites were prepared for administration of saline, 10⁻⁴ and 10⁻³ M AMPA in young (4 weeks) male control Long Evans and Eker rats (70-100 g). Cerebral blood flow (¹⁴C-iodoantipyrine) and O₂ consumption (cryomicrospectrophotometry) were determined in isoflurane anesthetized rats. Receptor levels were studied through Western analysis of the GLuR1 subunit of the AMPA receptor. We found significantly increased cortical O₂ consumption (+33%) after 10⁻⁴ M AMPA in control rats. The higher dose of AMPA did not further increase consumption. In the Eker rats, neither dose led to a significant increase in cortical O₂ consumption. Regional blood flow followed a similar pattern to oxygen consumption but cortical O₂ extraction did not differ. Cortical AMPA receptor protein levels were significantly reduced (-21%) in the Eker compared to control rats. Both O₂ consumption and blood flow were significantly elevated in the pons of the Eker rats compared to control. These data demonstrate a reduced importance of AMPA receptors in the control of cortical metabolism, related to reduced AMPA receptor protein, in the Eker rat. This suggests that increasing AMPA receptor activity may not be an effective treatment for children with autism spectrum disorders as they also have reduced AMPA receptor number.

Effects of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor blockade on increased cerebral O(2) consumption in Eker rats

Previous work had demonstrated that there was elevated regional cerebral O2 consumption in the brains of a tuberous sclerosis model (Eker rat). We tested the hypothesis that the increased cerebral O2 consumption was related to an increased contribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors to the control of cerebral metabolism. Young (4weeks) male control Long Evans (N=14) and Eker (N=14) rats (70-100g) were divided into control and NBQX (50mg/kg, AMPA antagonist) treated animals. Cerebral regional blood flow (14C-iodoantipyrine) and O2 consumption (cryomicrospectrophotometry) were determined in isoflurane anesthetized rats. We found significantly increased basal O2 consumption (+85%) in the cortex, hippocampus, cerebellum and pons of the Eker rats. Regional cerebral blood flow was also elevated in Eker rats at baseline (+53%) as was cerebral O2 extraction (+22%). NBQX significantly lowered O2 consumption in all regions of the Long Evans rats (-41%). Cerebral O2 consumption was lowered in the cortex to a lesser extent (-24%) and there were no changes in other regions of the Eker rats after NBQX. Cerebral blood flow was reduced in the Long Evans, but not the Eker rats, after NBQX. In conclusion, Eker rats had significantly elevated cerebral O2 consumption, O2 extraction and blood flow, but this was not related to AMPA receptor activation. In fact, the importance of AMPA receptors in the control of basal cerebral O2 consumption was reduced.

Cerebral O(2) consumption in young Eker rats, effects of GABA blockade: implications for autism

Since there is a strong correlation between tuberous sclerosis and autism, we used a tuberous sclerosis model (Eker rat) to test the hypothesis that the increased regional cerebral O(2) consumption in the Eker rat might be associated with autism. We also examined whether this increased cerebral O(2) consumption was related to changes in the activity of the gamma-aminobutyric acid (GABA) inhibitory system. Young (4 weeks) male control Long Evans (n=14) and Eker (n=14) rats (70-100g) were divided into control and bicuculline (1mg/kg/min for 2 min then 0.1mg/kg/min for 13 min, GABA(A) receptor antagonist) treated animals. Cerebral regional blood flow ((14)C-iodoantipyrine) and O(2) consumption (cryomicrospectrophotometry) were determined in isoflurane anesthetized rats. We found significantly increased basal O(2) consumption in the cortex (6.3+/-0.7 ml O(2)/min/100g Eker vs. 5.1+/-0.2 ml O(2)/min/100g control), hippocampus and cerebellum, but not the pons. Regional cerebral blood flow was also elevated in the cortex and hippocampus in Eker rats at baseline, but cerebral O(2) extractions were similar. Bicuculline significantly increased O(2) consumption in the cortex (6.5+/-0.3) and all other regions of the control rats, but had no effect on cortex (5.9+/-1.5) or other regions of the Eker rats. Cerebral blood flow followed a similar pattern. In conclusion, Eker rats had significantly elevated cerebral O(2) consumption and blood flow, but this was not affected by GABA receptor blockade. This suggested a reduced activity of the GABA(A) receptor in the brains of Eker rats. This may have important implications in the treatment of autism.

Mechanisms involved in the cerebrovascular dilator effects of N-methyl-d-aspartate in cerebral cortex

Glutamate and its synthetic analogues N-methyl-d-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) are potent dilator agents in the cerebral circulation. The close linkage between neural activity-based release and actions of glutamate on neurons and the related decrease in cerebral vascular resistance is a classic example in support of the concept of tight coupling between increased neural activity and cerebral blood flow. However, mechanisms involved in promoting cerebral vasodilator responses to glutamatergic agents are controversial. Here we review the development and current status of this important field of research especially in respect to cerebrovascular responses to NMDA receptor activation.

Increased cerebral oxygen consumption in Eker rats and effects of N-methyl-D-aspartate blockade: Implications for autism

Because there is a strong correlation between tuberous sclerosis and autism, we used a tuberous sclerosis model (Eker rat) to test the hypothesis that these animals would have an altered regional cerebral O2 consumption that might be associated with autism. We also examined whether the altered cerebral O2 consumption was related to changes in the importance of N-methyl-D-aspartate (NMDA) receptors. Young (4 weeks) male control Long Evans (N = 14) and Eker (N = 14) rats (70-100 g) were divided into control and CGS-19755 (10 mg/kg, competitive NMDA antagonist)-treated animals. Cerebral regional blood flow (14C-iodoantipyrine) and O2 consumption (cryomicrospectrophotometry) were determined in isoflurane-anesthetized rats. NMDA receptor protein levels were determined by Western immunoblotting. We found significantly increased basal O2 consumption in the cortex (6.2 +/- 0.6 ml O2/min/100 g Eker vs. 4.7 +/- 0.4 Long Evans), hippocampus, cerebellum, and pons. Regional cerebral blood flow was also elevated in Eker rats at baseline, but cerebral O2 extraction was similar. CGS-19755 significantly lowered O2 consumption in the cortex (2.8 +/- 0.3), hippocampus, and pons of the Long Evans rats but had no effect on cortex (5.8 +/- 0.8) or other regions of the Eker rats. Cerebral blood flow followed a similar pattern. NMDA receptor protein levels (NR1 subunit) were similar between groups. In conclusion, Eker rats had significantly elevated cerebral O2 consumption and blood flow, but this was not related to NMDA receptor activation. In fact, the importance of NMDA receptors in the control of basal cerebral O2 consumption was reduced. This might have important implications in the treatment of autism.