Research report
Administration of raloxifene reduces sensorimotor and working memory deficits following traumatic brain injury

https://doi.org/10.1016/j.bbr.2006.02.026Get rights and content

Abstract

Hormonal differences between males and females have surfaced as a crucial component in the search for effective treatments after experimental models of traumatic brain injury (TBI). Recent findings have shown that selective estrogen receptor modulators (SERMs) may have therapeutic benefit. The present study examined the effects of raloxifene, a SERM, on functional recovery after bilateral cortical contusion injury (bCCI) or sham procedure. Male rats received injections of raloxifene (3.0 mg/kg, i.p.) or vehicle (1.0 ml/kg, i.p.) 15 min, 24, 48, 72, and 96 h after bCCI or sham procedure. Rats were tested on both sensorimotor (bilateral tactile removal and locomotor placing tests) and cognitive tests (reference and working memory in the Morris water maze). Raloxifene-treated animals showed a significant reduction in the initial magnitude of the deficit and facilitated the rate of recovery for the bilateral tactile removal test, compared to vehicle-treated animals. The raloxifene-treated animals also showed a significant improvement in the acquisition of working memory compared to vehicle-treated animals. However, raloxifene did not significantly improve the acquisition of reference memory or locomotor placing ability. Raloxifene treatment also did not result in a significant reduction in the size of the lesion cavity. Thus, the task-dependent improvements seen following raloxifene treatment do not appear to be the result of cortical neuroprotection. However, these results suggest that raloxifene improves functional outcome following bCCI and may present an interesting avenue for future research.

Introduction

The Center for Disease Control and Prevention estimates 1.5 million people per year endure some form of traumatic brain injury (TBI) in the United States [40]. Each year approximately 50,000 people die from TBI and another 80,000–90,000 people become permanently disabled as a result of TBI [41], [45]. Depending upon the location of the specific injury, TBI may cause cognitive, movement, sensation, and/or emotional impairments [45]. Some of the leading causes of TBI are vehicle crashes, firearms, and falls [45]. Developing effective treatments for TBI victims has been challenging. Most treatments have failed to move beyond pre-clinical trials, have not been successful with humans or have shown undesired side-effects. Currently there are no effective pharmacological treatments for brain injury [44].

It has become apparent that sex differences exist between males and females when responding to stroke or trauma [15]. A majority of reports suggests that females may be better protected because of high levels of estrogen [35]. Presently there is an abundance of research on estrogen and its possible neuroprotective characteristics and effects on functional recovery after brain injury. Estrogen may work in several ways. It may bind directly to and activate the alpha or beta receptor, it may act indirectly by engaging in cross-talk with other signaling processes, or it may act receptor-independently [3]. Recent research has confirmed that estrogens influence cognition and memory, fine motor skills, balance, and movement [4], [16], [23], [30], [36], [38], [47], [49].

The antioxidant effects of estrogen may also play an important role in recovery of function after TBI. Estrogen may be able to lessen the secondary injury cascade associated with TBI by limiting lipid peroxidation. Research has shown that male rats tend to use more of their Vitamin E reserve (an endogenous antioxidant) after severe hemispheric ischemia compared to female rats, suggesting that more antioxidant action is present in female rats compared to males [15]. It has also been shown that male rats experience a greater loss of ascorbate, which is the main water-soluble antioxidant in the central nervous system, during ischemia compared to female rats [10], thus supporting the same conclusions from the Vitamin E study [15]. These findings are most likely due to the protective antioxidant elements (e.g., estrogen) present in the female brain that is lacking or absent in the male brain.

Several experimental stroke studies have shown that ovariectomy, which removes the principle estrogen reserve in female rats, results in neurological damage similar to that in males [48], [1]. These findings suggest that the depletion of the fundamental source of estrogen in female rats may decrease neuroprotection. Furthermore, administration of 17β-estradiol in male rats has been shown to reduce infarct size after stroke in a manner similar to that seen in female rats [46], [17]. A recent review by Stein [43] highlighted important research projects that have examined the effects of estrogen and progesterone treatment on traumatic injury. One study in particular found that estrogen treatment after fluid percussion injury improved outcome in male rats while having a detrimental effect in female rats [9]. The detrimental effects of estrogen in females may have resulted from changes in energy metabolism that was receptor-mediated, while the beneficial effects of estrogen in males may have resulted from receptor-independent actions [9]. Estrogen has the ability to act directly on the glutamate NMDA receptor, which increases response to excitatory amino acids [39]. These excitatory effects would be more observable in females since they have much more estrogen binding capacity compared to males [5].

However, more recent studies have found a detrimental effect of 17β-estradiol on male subjects. For example, 17β-estradiol increased hippocampal damage in male rats during induced status epilepticus [11]. It has also been shown that treatment with estrogen in gonadectomized male rats resulted in more dopamine loss in the striatum than in female rats [29]. This loss was comparable to that of intact male rats. In addition, medroxyprogesterone acetate, a common element of hormone replacement medications, might actually block the neuroprotective effects of 17β-estradiol [31]. This suggests some women who are taking hormone replacement may not be able to benefit from the protective effects of 17β-estradiol. Given some of these potential problems with direct administration effects of estrogen its possible usefulness may be limited. Thus, compounds with similar mechanisms of action but without the side-effects may prove more beneficial.

Selective estrogen receptor modulators (SERMs) may have the ability to mimic the protective characteristics of estrogens and may lack the complications. The Food and Drug Administration approved the use of raloxifene in 1997. This drug has several promising features. Post-menopausal women have most commonly used it as a form of hormone replacement therapy. SERMs have become a positive alternative for women who cannot take estrogen supplements [37]. They may also be another treatment option for men and women who cannot benefit from estrogen treatment after brain injury.

SERMs have a tissue-selective pharmacology that allows them to act as either an estrogen agonist or antagonist [24]. Raloxifene does not promote the growth of tumors in the breast or uterus, and it has the ability to increase bone density and lower serum cholesterol levels [32], [34]. Raloxifene can also act as a partial estrogen agonist and antagonist [32]. Research has revealed that when estrogen was absent, raloxifene exhibited agonistic effects. When estrogen was present, raloxifene acted like an agonist but at the same time antagonized some effects of estrogen. Raloxifene has also been shown to be neuroprotective against the β-amyloid peptide25–35, H2O2 (hydrogen peroxide), and excitotoxic glutamate [32].

Inconsistent results from various research projects have forced researchers to recognize that estrogen may not be beneficial across genders. In fact, many studies have found estrogen to have a detrimental effect on males and their recovery of function after TBI. SERMs have the ability to mimic the actions of estrogen and promise to be an effective treatment option for men and women who are unresponsive to estrogen. It is possible then, that SERMs may also have the ability to protect the brain after injury and improve functional recovery. The purpose of this research was to examine the effects of raloxifene after bilateral cortical contusion injury in the male rat. If raloxifene can mimic the actions of estrogen, perhaps it can also improve outcome after TBI.

Section snippets

Subjects

Twenty-four male Sprague-Dawley rats (∼3 months old; 325–372 g) were used for this study. All experimental procedures were reviewed and approved by the Institutional Animal Care and Use Committee and the study was conducted in a facility certified by the American Association for the Accreditation of Laboratory Animal Care. Rats were individually housed in standard cages and had free access to food and water. Animals were housed under a 12-h light/dark cycle. Rats were handled daily for

Cognitive assessment: reference memory

Latency to reach the submerged platform was analyzed in a 3 × 4 mixed ANOVA. The factors included were group (raloxifene, vehicle, sham) and day (1–4) as the repeated measure. Rats became more successful at locating the hidden platform on successive trials; the main effect of day was statistically significant [F(3, 63) = 7.16, P = 0.001]. Unilateral contusion injury produced a significant difference in the severity of the reference memory deficit; the main effect of group was statistically

Discussion

The present study has demonstrated that raloxifene treatment following bCCI improved functional recovery in a task-dependent manner. Administration of raloxifene facilitated the acquisition of working memory in the MWM and caused a significant reduction in the initial magnitude of the deficit and facilitated the rate of recovery in the tactile removal test. However, raloxifene treatment did not facilitate the acquisition of reference memory in the MWM or reduce the magnitude of the impairment

Acknowledgement

Research supported by grant NS045647 from NINDS.

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    1

    Present address: Department of Psychology, Virginia Commonwealth University, Richmond, VA 23284, USA. Tel.: +1 804 828 1193.

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