Expression of estrogen receptor (ER) α and β in mouse cerebral cortex: Effect of age, sex and gonadal steroids
Introduction
Estrogen receptor (ER) is a member of the nuclear receptor superfamily of transcription factors, which also includes receptors for sex steroids, thyroid hormone and retinoids as well as many orphan receptors for which no ligand has been identified [19]. There are two types of ERα and ERβ coded by two separate genes located on different chromosomes. Upon binding to its ligand, both receptors undergo conformational changes leading to the formation of either ERα/ERβ homodimer or heterodimer [12], [18]. ERα and ERβ are implicated in several brain functions such as migration of neurons during embryonic development [55], [56], protection against brain injury [14], [60], [61] and improvement in memory and learning [17], [30], [44], increase in choline acetyl transferase activity [34], growth of synapses [7], [46] and decrease of β amyloid level [32].
The brain exhibits sexual differences in its structure and function throughout the animal kingdom [36]. The wide range of effects associated with brain sexual differentiation represents the result of a complex response cascade, initiated by genes and prolonged by the effects of gonadal hormones. Early exposure to a masculine hormonal milieu results in the development of male specific brain while the absence of such a hormonal milieu shifts the development to a female specific brain. Apart from other changes in the brain, variation in the endogenous hormone level was shown to be responsible for differential distribution and density of androgen receptor (AR) and ER [57] in different regions of the brain during early development [36]. Gonadal steroids regulate ER level in the rat brain around 7–10 days after birth [20], [27], thereafter ER concentration declines rapidly and attains the adult pattern. However, in adult female rat brain, the level of ERα mRNA changes during different estrous phases [49]; thus, suggesting the involvement of sex steroids in the control of ER level beyond the early developmental stages. In contrast to adult animals, circulating gonadal hormone level changes with advancing age [28], [50], [51]. While these changes are abrupt in female, they are slow but steady in male [15]. Such sex-related decline in the level of circulating hormones may modulate ERα and ERβ level differentially in male and female during aging [48], [52].
The in situ and immunohistochemical studies conducted in female rat brain revealed that the number of cells expressing ERα and ERβ mRNA and protein show region specific changes during aging [2], [9], [10], [59]. In the present study, Western blot analysis showed differential effect of age, sex and gonadal steroid hormones on the expression of full length ERα and ERβ protein in the cerebral cortex of AKR mice.
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Animals and hormone treatment
AKR mice were maintained in a colony at 25 ± 2 °C with free access to standard mice feed and drinking water. They were exposed to 12-h light:12-h dark schedule. The average life span of AKR mice in our colony is 70 ± 5 weeks. We used AKR mice for our study because of their short life span. Experimental protocols were approved by the animal ethical committee of Banaras Hindu University, Varanasi, India. For each experiment, adult (25 ± 2 weeks) and old (65 ± 5 weeks) male and female mice were divided
Age dependent differences in ERα and ERβ level
To know the effect of age, the levels of ERα and ERβ in adults were compared with those in old (Fig. 1, Fig. 2). The adult level was considered as 100%. ERα level did not change in old. In contrast to ERα, ERβ level decreased significantly in both old male (40%, P < 0.05) and female (75%, P < 0.05).
Sex dependent differences in ERα and ERβ level
To know the effect of sex, the level of ERα and ERβ in adult male was considered as 100%. Western blot analysis (Fig. 1) revealed higher level of ERα protein (162%, P < 0.05) in adult female than adult
Discussion
In the present study, we have examined the effect of age, sex and gonadal sex steroid hormones on the levels of ERα and ERβ in the mice cerebral cortex. The cerebral cortex is a recognized target of gonadal sex steroids. In addition to developmental influences [37], behavioral [8] and biochemical [16] studies suggest that estrogen also stimulates the mature cerebrum [35]. Estrogen exerts potent modulatory effects on multiple neuronal networks. Although it is clear that many of the effects of
Acknowledgements
This work was supported by grants from the Department of Biotechnology (BT/PRO831/HRD/15/87/97 and BT/PR3593/Med/14/468/2004) and Department of Science and Technology, Government of India (SP/SO/B05/99) to M.K.T. P.K.S. is a recipient of Junior Research Fellowship from the University Grants Commission, India.
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