Brain-derived estrogen exerts anti-inflammatory and neuroprotective actions in the rat hippocampus
Introduction
17β-Estradiol (E2, estrogen) is a steroid hormone that has been implicated to be neuroprotective against a variety of neurodegenerative disorders, including stroke, Alzheimer’s disease (AD) and Parkinson’s disease, although controversy exists (Brann et al., 2007, Yao and Brinton, 2012, Simpkins et al., 2012, Bourque et al., 2012). With respect to stroke, studies in rats, mice and gerbils found a sex difference in brain injury following cerebral ischemia, with young adult female animals having smaller infarct volume as compared to young adult males (Brann et al., 2007, Alkayed et al., 1998, Park et al., 2006). Similarly, a number of studies have documented sex differences in stroke risk and outcome in humans, with women generally protected against stroke, at least until menopause (Murphy et al., 2004, Di Carlo et al., 2003). Many groups, including our own, have shown that exogenous administration of E2 dramatically reduces infarct volume in cortex and hippocampus following focal or global cerebral ischemia (GCI) in ovariectomized female mice, rats and gerbils, and in male rats and gerbils (Brann et al., 2007, Zhang et al., 2008, Simpkins et al., 1997, Dubal et al., 1998, Shughrue and Merchenthaler, 2003, Zhang et al., 1998).
It has been generally assumed that the neuroprotective effects of E2 are primarily due to ovarian-derived E2. However, work by a number of laboratories has shown that certain areas of the brain exhibit high expression of the E2 generating enzyme, aromatase, which has raised the possibility that brain-derived E2 may have important roles in the CNS. For instance, work within the last decade in rodents, birds, monkeys, and humans has shown that forebrain structures, in particular the hippocampus CA1–CA3 regions, exhibits high expression of aromatase as indicated by in situ hybridization, RT-PCR and immunohistochemical analysis, and can produce significant levels of E2 levels that are equivalent to or even higher than that observed in the circulation (Veiga et al., 2005, Hojo et al., 2004, Azcoitia et al., 2011, Higaki et al., 2012, Yague et al., 2008, Fester et al., 2011, Garcia-Segura, 2008, Mukai et al., 2010, Shen et al., 1994). It should be noted that the cerebral cortex has also been reported to express aromatase (Azcoitia et al., 2011, Stoffel-Wagner et al., 1999, Srivastava et al., 2010), and thus brain-derived E2 may also regulate cortical functions. In support of this possibility, global aromatase knockout mice have been reported to have greater cortical damage following focal cerebral ischemia than wild type ovariectomized mice, suggesting that brain-derived E2 may have neuroprotective actions in the cerebral cortex (McCullough et al., 2003).
With respect to the hippocampus, treatment of cultured mouse hippocampal neurons with an aromatase inhibitor has been reported to result in a significant decrease in axon outgrowth and dendritic spines in the CA1 region (Fester et al., 2011, Mukai et al., 2010, Kretz et al., 2004, Rune and Frotscher, 2005, von Schassen et al., 2006), as well as a significant decrease of long-term potentiation (LTP) amplitude, dendritic spines and synapses in hippocampal slices in vitro (Grassi et al., 2011, Vierk et al., 2012). These results suggest that local E2 in the hippocampus may modulate synaptic function. Interestingly, studies in songbirds have also shown that inhibiting aromatase by intracerebral administration of aromatase inhibitors results in increased damage and apoptosis in the brain after a penetrating injury (Wynne and Saldanha, 2004, Wynne et al., 2008). Aromatase inhibition has also been reported to result in increased hippocampal damage in male rats following excitotoxic injury (Azcoitia et al., 2001).
It is well known that the hippocampal CA1 region is highly vulnerable to GCI, which can occur after cardiac arrest, asphyxiation, and hypotensive shock (Neumann et al., 2013, Harukuni and Bhardwaj, 2006), and can lead to significant neuronal damage, cognitive defect and mortality. It is currently unknown whether brain-derived E2 in the hippocampal CA1 region has a neuroprotective role against GCI, and whether it can modulate neuroinflammation that occurs after GCI. To address these deficits in our knowledge, the goals of the current study were: (1) to access whether aromatase and local E2 levels change in the hippocampus following GCI, (2) to determine the cell types containing aromatase and local E2 expression in ischemic and non-ischemic animals, and (3) to assess whether antisense oligonucleotide knockdown of aromatase and local E2 levels in the hippocampus affects GCI-induced neurodegeneration and inflammation in ovariectomized rats.
Section snippets
Animal model of global cerebral ischemia
All procedures were approved by the Georgia Regents University Institutional Animal Care and Use Committee and were conducted in accordance with the National Institutes of Health guidelines for animal research. Adult Sprague Dawley female SD rats were obtained from Harlan Inc., and studied at 3 months of age. All the rats were bilaterally ovariectomized one week before the induction of GCI. GCI was performed by four-vessel occlusion (4-VO) as described previously (Pulsinelli and Brierley, 1979,
Results
We first examined aromatase protein expression in the hippocampus and cortex of adult ovariectomized rats using immunohistochemistry. As shown in Fig. 1A, the hippocampal regions CA1, CA3 and dentate gyrus (DG) showed high levels of immunoreactive aromatase staining, while the cortex displayed only moderate staining. We next examined the cell type of expression of aromatase in the hippocampal CA1 region and its temporal pattern following GCI. As shown in Fig. 1B, triple immunohistochemical
Discussion
It has been known for some time that gonadal-derived and exogenous E2 can protect the vulnerable hippocampal CA1 region from GCI-induced neuronal damage and cell death (Brann et al., 2007, Zhang et al., 2009, Jover et al., 2002, Merchenthaler et al., 2003). The results of the current study add to our understanding by revealing for the first time an important neuroprotective and anti-inflammatory role of local brain-derived E2 in the rat hippocampus CA1 region following GCI. Clinically, GCI can
Acknowledgement
This research was supported by Research Grant (NS050730) from the National Institutes of Neurological Disorders and Stroke, National Institutes of Health.
References (44)
- et al.
Estradiol synthesis within the human brain
Neuroscience
(2011) - et al.
Signaling pathways mediating the neuroprotective effects of sex steroids and SERMs in Parkinson’s disease
Front Neuroendocrinol.
(2012) - et al.
Neurotrophic and neuroprotective actions of estrogen: basic mechanisms and clinical implications
Steroids
(2007) - et al.
Neural 17beta-estradiol facilitates long-term potentiation in the hippocampal CA1 region
Neuroscience
(2011) - et al.
Mechanisms of brain injury after global cerebral ischemia
Neurol. Clin.
(2006) - et al.
Response of ERbeta and aromatase expression in the monkey hippocampal formation to ovariectomy and menopause
Neurosci. Res.
(2012) - et al.
Modulation of synaptic plasticity by brain estrogen in the hippocampus
Biochimica et biophysica acta
(2010) - et al.
Neurosteroid synthesis in the hippocampus: role in synaptic plasticity
Neuroscience
(2005) - et al.
Isolation and characterization of a zebra finch aromatase cDNA: in situ hybridization reveals high aromatase expression in brain
Brain Res. Mol. Brain Res.
(1994) - et al.
Estrogen prevents the loss of CA1 hippocampal neurons in gerbils after ischemic injury
Neuroscience
(2003)
Expression of CYP19 (aromatase) mRNA in different areas of the human brain
J. Steroid. Biochem. Mol. Biol.
Aromatase distribution in the monkey temporal neocortex and hippocampus
Brain Res.
Aromatase expression in the normal and epileptic human hippocampus
Brain Res.
Estrogen regulation of mitochondrial bioenergetics: implications for prevention of Alzheimer’s disease
Adv. Pharmacol.
Effects of gender and estradiol treatment on focal brain ischemia
Brain Res.
Gender-linked brain injury in experimental stroke
Stroke
Brain aromatase is neuroprotective
J. Neurobiol.
A priming role of local estrogen on exogenous estrogen-mediated synaptic plasticity and neuroprotection
Exp. Mol. Med.
Chaihu-shugan-san administration ameliorates perimenopausal anxiety and depression in rats
PLoS One
Morphine protects against intracellular amyloid toxicity by inducing estradiol release and upregulation of Hsp70
J. Neurosci.
Sex differences in the clinical presentation, resource use, and 3-month outcome of acute stroke in Europe: data from a multicenter multinational hospital-based registry
Stroke
Estradiol protects against ischemic injury
J. Cereb. Blood Flow Metab.
Cited by (103)
Chlorine disinfection byproduct of diazepam affects nervous system function and possesses gender-related difference in zebrafish
2022, Ecotoxicology and Environmental SafetySynthesis and impact of neuroestradiol on hippocampal neuronal networks
2022, Current Opinion in Endocrine and Metabolic ResearchDoes obesity-associated insulin resistance affect brain structure and function of adolescents differentially by sex?
2022, Psychiatry Research - Neuroimaging