Research reportRaloxifene and estradiol benzoate both fully restore hippocampal choline acetyltransferase activity in ovariectomized rats
Introduction
Choline acetyltransferase (ChAT) is the rate-limiting enzyme in the synthesis of acetylcholine (ACh), a neurotransmitter critically involved in cognition. ACh-synthesizing neurons are selectively lost in Alzheimer's disease (AD), a disease characterized by progressive deterioration of cognitive function [7]. In AD patients, ACh and ChAT activity are reduced in the cerebral cortex, hippocampus and nucleus basalis of Meynert and those reductions correlate with cognitive impairment 3, 21. Estrogen replacement in post-menopausal women has been reported to improve cognitive function and may decrease the risk of developing AD 28, 33. Conversely, estrogen deprivation decreases ChAT activity in the hippocampus and impairs performance in cognitive tests in rats [34], and these deficits can be reversed by estrogen replacement 9, 10, 17, 34. Further, in rats, lesions that disrupt cholinergic neurotransmission impair learning and memory while treatments that increase cholinergic activity facilitate performance in learning and memory tasks 25, 34. The mechanism underlying estrogen's effects on cholinergic function has not been elucidated, but may be related to its ability to increase expression of brain-derived neurotrophic factor (BDNF) 11, 35which enhances ChAT activity by itself [24]and (or) to its ability to promote neuronal outgrowth and survival of cells in vitro [23].
Raloxifene is a prototypical selective estrogen receptor modulator (SERM), that exhibits tissue-specific estrogen agonist and antagonist properties [20]. Raloxifene functions as an estrogen receptor (ER) agonist to preserve bone and decrease serum cholesterol levels in both post-menopausal women [8]as well as estrogen deficient animals [2]. In other tissues, however, raloxifene functions as an ER antagonist in that it does not stimulate proliferation of the uterine endometrium or stimulate breast tissue and can block estrogen's stimulatory effects in these tissues 8, 37, 38. The effects of raloxifene in the central nervous system (CNS) have not been well characterized. The present study was conducted to compare the effects of raloxifene and estrogen on measures of cholinergic activity in ovariectomized (OVX) rats. The results of these studies indicate that raloxifene and estrogen produce similar increases in ChAT activity in the hippocampus. Further, the combination of estrogen and raloxifene increased brain ChAT activity and simultaneously blocked the estrogen-induced increase in uterine weight. This profile suggests that benzothiophene SERMs like raloxifene may be useful in the treatment of age-related cognitive decline and Alzheimer's related dementias that involve decreased cholinergic neurotransmission in brain. A preliminary report of some of these data has recently been presented [41].
Section snippets
Animals
In all experiments, 6-month-old OVX or sham-operated rats (Harlan Sprague–Dawley, Indianapolis, IN, USA) were assigned to various treatment groups 3 weeks post-surgery. Animals were group-housed at room temperature held constant at 22°C and with ad libitum access to food and water. Rats were injected subcutaneously once daily, for 3 or 10 days with estradiol benzoate (EB, 0.03 or 0.3 mg/kg), raloxifene HCl (3.0 mg/kg) or vehicle (20% CDX) and ChAT activity was measured in various brain regions
Results
As has been previously reported in rats [34], ovariectomy reduced hippocampal ChAT activity by approximately 30% when compared to intact SHAM-controls across experiments. There was some variability from experiment to experiment in both the absolute ChAT activity of a given brain region and in the magnitude of the reduction resulting from ovariectomy. However, a reduction in hippocampal ChAT activity in OVX rats compared to sham-operated controls was observed in every experiment. As can be seen
Discussion
SERMs have ER agonist and/or antagonist properties, depending on the tissue, in various species, including man [20]. Raloxifene is a prototypical SERM which has ER agonist activity in bone but is an ER antagonist in the breast and uterus [2]. Given the presence and regional distribution of ER in brain 5, 18, we have initiated studies to characterize various SERMs as to their ER agonist or antagonist actions within the CNS. ChAT is an ER-responsive gene product in the hippocampus. ChAT activity
Acknowledgements
The contribution of David Cox to the manuscript preparation is appreciated. The authors thank the Lilly Center for Women's Health for generous support.
References (41)
- et al.
Behavioral and neurochemical effects following neurotoxic lesions of a major cholinergic input to the cerebral cortex in the rat
Pharmacol. Biochem. Behav.
(1983) - et al.
Effects of estrogen replacement on the relative levels of choline acetyltransferase, trkA, and nerve growth factor messenger RNAs in the basal forebrain and hippocampal formation of adult rats
Exp. Neurol.
(1994) Effects of estrogen on basal forebrain cholinergic neurons vary as a function of dose and duration of treatment
Brain Res.
(1997)Estradiol increases choline acetyltransferase activity in specific basal forebrain nuclei and projection areas of female rats
Exp. Neurol.
(1985)- et al.
Estradiol and progesterone regulate neuronal structure and synaptic connectivity in adult as well as developing brain
Exp. Gerontol.
(1994) Acetylcholine, aging, and Alzheimer's disease
Pharmacol. Biochem. Behav.
(1997)- et al.
SDZ ENA 713 facilitates central cholinergic function and ameliorates spatial memory impairment in rats
Behav. Brain Res.
(1997) - et al.
Ovarian steroid deprivation results in a reversible learning impairment and compromised cholinergic function in female Sprague–Dawley rats
Brain Res.
(1994) - et al.
Effect of estrogen during menopause on risk and age at onset of Alzheimer's disease
Lancet
(1996) - et al.
Non-steroidal antioestrogens-receptor binding and biological response in rat uterus, rat mammary carcinoma and human breast cancer cells
J. Steroid Biochem.
(1984)
Differential response of estrogen receptor alpha and estrogen receptor beta to partial estrogen agonists/antagonists
Mol. Pharmacol.
Raloxifene (LY139481 HCI) prevents bone loss and reduces serum cholesterol without causing uterine hypertrophy in ovariectomized rats
J. Clin. Invest.
Chemical pathology of this organic dementias: II. Quantitative estimation of cellular changes in post-mortem brains
Brain
Selective estrogen receptor modulators: an alternative to hormone replacement therapy
Proc. Soc. Exp. Biol. Med.
Estrogen receptors in human nontarget tissues: biological and clinical implications
Endocr. Rev.
Time-dependent effect of tamoxifen therapy on endometrial pathology in asymptomatic postmenopausal breast cancer patients
Int. J. Gynecol. Pathol.
The role of interactions between the cholinergic system and other neuromodulatory systems in learning and memory
Synapse
Effects of raloxifene on bone mineral density, serum cholesterol concentrations, and uterine endometrium in postmenopausal women
N. Engl. J. Med.
Charaterization of raloxifene binding and transactivation properties of the estrogen receptor-beta (Erbeta) — ASBMR 19th Annual Metting
J. Bone Miner. Res.
Senile dementia-Alzheimer's type and estrogen
Horm. Metab. Res.
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