Levels of trkA and BDNF mRNA, but not NGF mRNA, fluctuate across the estrous cycle and increase in response to acute hormone replacement

Abstract

Recent studies suggest that hormone replacement therapy can help to reduce the risk and severity of Alzheimer's-related dementia in postmenopausal women. We have hypothesized that these effects are due, in part, to the ability for estrogen and progesterone to enhance hippocampal function, as well as the functional status of cholinergic projections to the hippocampus and cortex, by influencing the expression of specific neurotrophins and neurotrophin receptors. In the present study, quantitative in situ hybridization techniques were used to determine whether the levels of trkA mRNA in the basal forebrain, and nerve growth factor (NGF) mRNA and brain-derived neurotrophic factor (BDNF) mRNA in the hippocampus, are significantly affected by physiological changes in circulating gonadal steroids. Gonadally intact animals were sacrificed at different stages of the estrous cycle and ovariectomized animals were sacrificed at different times following the administration of either estrogen or estrogen plus progesterone. In gonadally intact animals, significant fluctuations in the levels of trkA mRNA in the medial septum (MS), and BDNF mRNA in regions CA1 and CA3/4 of the hippocampus, were detected across the estrous cycle. In animals that received hormone replacement, a significant increase (30.4%) in trkA mRNA was detected in the MS of animals sacrificed 24 h following estrogen administration. Levels of trkA mRNA in the MS declined to control levels over the next 48 h; however, a single injection of progesterone administered 48 h after estradiol appeared to prevent any further decline in trkA mRNA over the next 24 h. In addition, significant increases in BDNF mRNA were detected in the dentate granule cell layer (73.4%), region CA1 (28.1%), and region CA3/4 (76.9%) of animals sacrificed 53 h after receiving estrogen and 5 h after receiving progesterone. No significant changes in trkA mRNA were detected in the nucleus basalis magnocellularis, and no significant changes in NGF mRNA were detected in the hippocampus. These data demonstrate that levels of trkA mRNA in the MS, and BDNF mRNA in the hippocampus, are affected by physiological changes in the levels of circulating gonadal steroids and are elevated in response to acute hormone replacement. The relevance of these effects to the ability for estrogen replacement to enhance cholinergic activity and hippocampal function, and thereby reduce the risk and severity of Alzheimer's-related dementia in postmenopausal women, is discussed.

Introduction

Alzheimer's disease is an age-related neurodegenerative disease characterized by a progressive loss of cognitive functions with corresponding memory loss and dementia. Studies have shown that after the age of 65, the prevalence of dementia and Alzheimer's disease doubles every five years [19]and that women are at greater risk than men (see Ref. [14]for review). However, recent clinical studies suggest that estrogen replacement may help to reduce the risk and severity of Alzheimer's-related dementia in postmenopausal women 15, 16, 34, 33, 36, 43.

Recently, we and others have shown that, in rats, cholinergic neurons located within specific regions of the basal forebrain are significantly affected by changes in circulating estrogens. These neurons are the major source of cholinergic innervation to the hippocampal formation and cortex and have been shown by numerous studies to play an important role in learning and memory processes 5, 27, 35. Loss of these neurons is also thought to contribute significantly to cognitive decline associated with aging and Alzheimer's disease 5, 44. We have recently shown that, in rats, long-term loss of ovarian function results in decreases in specific measures of basal forebrain cholinergic function over and above those associated with normal aging [6], suggesting that the long-term loss of circulating estrogens in postmenopausal women may contribute to an age-related decline in basal forebrain cholinergic function. This is consistent with other recent studies showing that estrogen replacement in young adult rats produces increases in basal forebrain cholinergic function 7, 10, 13, 24, 40, and suggests that one mechanism by which estrogen replacement may help to reduce the risk and severity of Alzheimer's-related dementia in women is by enhancing the functional status of cholinergic projections emanating from the MS and NBM.

Cholinergic neurons in the MS and NBM are significantly affected by nerve growth factor (NGF), a polypeptide growth factor produced in the hippocampus and cortex [23]. Effects of NGF are mediated by binding with trkA, a membrane-bound, tyrosine kinase receptor [1], and studies suggest that over 90% of the trkA-expressing cells detected in the MS and NBM are cholinergic neurons projecting to the hippocampus and cortex 4, 12. Brain-derived neurotrophic factor (BDNF) is an NGF-related polypeptide which, like NGF, is expressed in the hippocampal formation and cortex and is thought to play an important role in learning and memory processes [23]. Like NGF, BDNF can enhance the functional status of cholinergic neurons in the basal forebrain 31, 32. BDNF has also been shown to play an important role in the formation of long-term potentiation in the hippocampus 21, 37, the promotion of activity-dependent dendritic growth [26], and protection from apoptotic cell death [3], all of which are relevant to age-related changes in learning and memory processes.

Decreases in both trkA and BDNF mRNA 30, 39, as well as TrkA protein [29]have been detected in association with Alzheimer's disease, suggesting that decreases in the expression of, and/or responsiveness to, specific neurotrophins may contribute to the functional decline associated with the disease. Significant decreases in trkA expression in the MS have also been observed as a function of aging and ovariectomy in female rats 2, 6, suggesting that the combination of aging and long-term hormone deprivation may contribute to decreases in basal forebrain cholinergic survival and function by decreasing the responsiveness of the cholinergic neurons to endogenous NGF.

The purpose of the present study was to determine whether the expression of trkA mRNA in the basal forebrain, as well as NGF mRNA and BDNF mRNA in the hippocampus, are significantly affected by physiological changes in circulating gonadal steroids as assessed by fluctuations across the estrous cycle, as well as by the response to acute replacement with physiological levels of estrogen or estrogen plus progesterone.