Elsevier

Brain Research

Volume 847, Issue 1, 13 November 1999, Pages 98-104
Brain Research

Research report
Raloxifene and estradiol benzoate both fully restore hippocampal choline acetyltransferase activity in ovariectomized rats

https://doi.org/10.1016/S0006-8993(99)02062-4Get rights and content

Abstract

Selective estrogen receptor modulators (SERMs) demonstrate tissue-specific estrogen receptor (ER) agonist or antagonist properties. Raloxifene, a prototypical SERM, has ER agonist properties in bone and on cholesterol metabolism but full antagonist properties in the uterus and breast. To characterize the ER agonist/antagonist profile of raloxifene in the brain, we have examined its effect on the activity of a known estrogen-responsive gene product, choline acetyltransferase (ChAT), in the hippocampus and other brain regions of 6-month-old ovariectomized (OVX) Sprague–Dawley rats. Three weeks post-ovariectomy, animals received estradiol benzoate (EB, 0.03 mg or 0.3 mg kg-1 day-1 for 3 or 10 days); raloxifene HCl (3.0 mg kg-1 day-1 for 3 or 10 days), tamoxifen (3.0 mg kg-1 day-1 for 10 days) or vehicle (20% CDX). As previously reported, ChAT activity decreased by approximately 20%–50% in the hippocampus of OVX compared with SHAM-operated control rats with no change in ChAT activity observed in the hypothalamus. Raloxifene or EB reversed the OVX-induced decrease in ChAT activity in the hippocampus but did not change ChAT activity in the hypothalamus. Animals that received combined EB (0.03 mg/kg) plus raloxifene (1 mg/kg) or tamoxifen alone (3.0 or 10 mg/kg) also showed increased hippocampal ChAT activity. Raloxifene failed to increase uterine weight and blocked the estrogen-induced increase in uterine weight, while another SERM, tamoxifen, increased uterine weight. These data demonstrate that raloxifene has estrogen-like properties on hippocampal ChAT activity in vivo, and suggest that benzothiophene SERMs may exert estrogen-like beneficial effects on cholinergic neurotransmission in brain without producing peripheral stimulation of breast or uterine tissue.

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.

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