Tamoxifen disrupts consolidation and retrieval of morphine-associated contextual memory in male mice: interaction with estradiol

Hippocampal Estrogen Signaling Mediates Sex Differences in Retroactive Interference

Despite being a crucial physiological function of the brain, the mechanisms underlying forgetting are still poorly understood. Estrogens play a critical role in different brain functions, including memory. However, the effects of sex hormones on forgetting vulnerabilitymediated by retroactive interference (RI), a phenomenon in which newly acquired information interferes with the retrieval of already stored information, are still poorly understood. The aim of our study was to characterize the sex differences in interference-mediated forgetting and identify the underlying molecular mechanisms. We found that adult male C57bl/6 mice showed a higher susceptibility to RI-dependent memory loss than females. The preference index (PI) in the NOR paradigm was 52.7 ± 5.9% in males and 62.3 ± 13.0% in females. The resistance to RI in female mice was mediated by estrogen signaling involving estrogen receptor α activation in the dorsal hippocampus. Accordingly, following RI, females showed higher phosphorylation levels (+30%) of extracellular signal-regulated kinase1/2 (ERK1/2) in the hippocampus. Pharmacological inhibition of ERK1/2 made female mice prone to RI. The PI was 70.6 ± 11.0% in vehicle-injected mice and 47.4 ± 10.8% following PD98059 administration. Collectively, our data suggest that hippocampal estrogen α receptor-ERK1/2 signaling is critically involved in a pattern separation mechanism that inhibits object-related RI in female mice.

Tamoxifen Blocks the Development of Motivational Features of an Addiction-Like Phenotype in Female Rats

Women become addicted sooner after initiating cocaine use as compared to men. Preclinical studies reveal a similar vulnerability in females, with findings from ovariectomized rats suggesting that estradiol mediates the enhanced vulnerability. However, since ovariectomy depletes not only estradiol, but all ovarian hormones, its role in a physiological context is not clear. Thus, the goal of this study was to determine the role of estradiol in the development of an addiction-like phenotype in ovary-intact females treated chronically with the selective estrogen receptor (ER) modulator tamoxifen. We hypothesized that tamoxifen, by antagonizing ERs, would block the development of an addiction-like phenotype as defined by an enhanced motivation for cocaine (assessed under a progressive-ratio schedule), and a heightened vulnerability to relapse (assessed under an extinction/cue-induced reinstatement procedure). Effects were examined following extended access cocaine self-administration (24-h/day; 4-discrete trials/h; 1.5 mg/kg/infusion) and 14-days of abstinence, conditions optimized for inducing an addiction-like phenotype. As predicted, motivation for cocaine was increased following extended-access self-administration and protracted abstinence in the vehicle (sesame oil) and no-injection control groups, but not in the tamoxifen group indicating that ER signaling is critical for the development of this feature of an addiction-like phenotype. Surprisingly, the increase in motivation for cocaine following abstinence was also attenuated in the vehicle group as compared to no-injection controls suggesting that oil/injections also affected its development. Contrary to our hypothesis, tamoxifen did not decrease vulnerability to relapse as this group responded at similar levels during initial extinction sessions and cue-induced reinstatement testing as compared to controls. Tamoxifen did, however, impair extinction learning as this group took longer to extinguish as compared to controls. Taken together, these findings indicate that estradiol is critical for the extinction of drug-associated cues and the development of motivational features of addiction.

Sex differences in nicotine preference

Smoking is the major cause of preventable deaths worldwide, and although there is a decline in overall smoking prevalence in developed countries, the decline in women is less pronounced than in men. Women become dependent faster and experience greater difficulties in quitting. Similar trends have been observed in animal models of nicotine/tobacco addiction. Individual differences in vulnerability to drug abuse are also observed in nicotine/tobacco addiction and point to the importance of sex differences. This Review, summarizes findings from three experimental approaches used to depict nicotine preference in animal models, intravenous and oral nicotine self-administration and nicotine-induced conditioned place preference. Nicotine preference is considered to be reflected in the animal's motivation to administer the drug (intravenously or orally) or to prefer an environment paired with the presence of the drug (conditioned place preference). These approaches all point to the importance of sex and age of the subjects; the preference of females and adolescents appear to be more pronounced than that of males and adults, respectively. A closer look at these factors will help us understand the mechanisms that underlie nicotine addiction and develop strategies to cope. Ignoring sex differences and reaching conclusions based only on studies using male subjects has resulted in erroneous generalizations in the past. Sex differences in nicotine preference have been clearly documented, and awareness on this aspect of nicotine dependence will significantly impact our success in translational research. © 2016 Wiley Periodicals, Inc.

The Working Memory and Dorsolateral Prefrontal-Hippocampal Functional Connectivity Changes in Long-Term Survival Breast Cancer Patients Treated with Tamoxifen

BACKGROUND

Tamoxifen is the most widely used drug for treating patients with estrogen receptor-sensitive breast cancer. There is evidence that breast cancer patients treated with tamoxifen exhibit cognitive dysfunction. However, the underlying neural mechanism remains unclear. The present study aimed to investigate the neural mechanisms underlying working memory deficits in combination with functional connectivity changes in premenopausal women with breast cancer who received long-term tamoxifen treatment.

METHODS

A total of 31 premenopausal women with breast cancer who received tamoxifen and 32 matched healthy control participants were included. The participants completed n-back tasks and underwent resting-state functional magnetic resonance imaging, which measure working memory performance and brain functional connectivity, respectively. A seed-based functional connectivity analysis within the whole brain was conducted, for which the dorsolateral prefrontal cortex was chosen as the seed region.

RESULTS

Our results indicated that the tamoxifen group had significant deficits in working memory and general executive function performance and significantly lower functional connectivity of the right dorsolateral prefrontal cortex with the right hippocampus compared with the healthy controls. There were no significant changes in functional connectivity in the left dorsolateral prefrontal cortex within the whole brain between the tamoxifen group and healthy controls. Moreover, significant correlations were found in the tamoxifen group between the functional connectivity strength of the dorsolateral prefrontal cortex with the right hippocampus and decreased working memory performance.

CONCLUSION

This study demonstrates that the prefrontal cortex and hippocampus may be affected by tamoxifen treatment, supporting an antagonistic role of tamoxifen in the long-term treatment of breast cancer patients.

Tamoxifen improves cholinergically modulated cognitive performance in postmenopausal women

Tamoxifen (TMX) is a selective estrogen receptor modulator that is used as an estrogen receptor antagonist for the treatment and prevention of breast cancer. Whether TMX has antagonist activities in the human brain is less clear and its effects on cognitive function have not been experimentally explored. This study examined how TMX affected cognitive performance in older women using a model of anticholinergic drug-induced cognitive dysfunction. Twenty-one postmenopausal women were administered 20 mg of oral TMX or placebo for 3 months. Participants then took part in five drug challenges using the anticholinergic antinicotinic agent mecamylamine (MECA) and antimuscarinic agent scopolamine (SCOP) and were tested on a comprehensive battery including tasks of attention and psychomotor function, verbal episodic memory, and spatial navigation. After a 3-month placebo washout, participants were then crossed over to the alternate treatment and repeated the drug challenges after 3 months. Compared with placebo treatment, TMX significantly attenuated the impairment from cholinergic blockade on tasks of verbal episodic memory and spatial navigation, but effects on attentional/psychomotor tasks were more variable. Analysis by APOE genotype showed that APO ɛ4+ women showed a greater beneficial effect of TMX on reversing the cholinergic impairment than APO ɛ4- women on most tasks. This study provides evidence that TMX may act as an estrogen-like agonist to enhance cholinergic system activity and hippocampally mediated learning.

Is basic research providing answers if adjuvant anti-estrogen treatment of breast cancer can induce cognitive impairment?

Adjuvant treatment of cancer by chemotherapy is associated with cognitive impairment in some cancer survivors. Breast cancer patients are frequently also receiving endocrine therapy with selective estrogen receptor modulators (SERMs) and/or aromatase inhibitors (AIs) to suppress the growth of estradiol sensitive breast tumors. Estrogens are well-known, however, to target brain areas involved in the regulation of cognitive behavior. In this review clinical and basic preclinical research is reviewed on the actions of estradiol, SERMs and AIs on brain and cognitive functioning to see if endocrine therapy potentially induces cognitive impairment and in that respect may contribute to the detrimental effects of chemotherapy on cognitive performance in breast cancer patients. Although many clinical studies may be underpowered to detect changes in cognitive function, current basic and clinical reports suggest that there is little evidence that AIs may have a lasting detrimental effect on cognitive performance in breast cancer patients. The clinical data on SERMs are not conclusive, but some studies do suggest that tamoxifen administration may form a risk for cognitive functioning particularly in older women. An explanation may come from basic preclinical research which indicates that tamoxifen often acts agonistic in the absence of estradiol but antagonistic in the presence of endogenous estradiol. It could be hypothesized that the negative effects of tamoxifen in older women is related to the so-called window of opportunity for estrogen. Administration of SERMs beyond this so-called window of opportunity may not be effective or might even have detrimental effects similar to estradiol.

The effects of lesion of the olfactory epithelium on morphine-induced sensitization and conditioned place preference in mice

Animals attain information about their environment through different sense organs. For example, the dominant external resource about the environment for rodents is obtained through olfaction. Many environmental conditions (stress or enriched environment) are known to affect an animal's susceptibility to drug addiction. However, it is not known how external information is integrated and paired with drug stimuli to develop into addictive behavior. Here, we investigated the effects of olfactory epithelium lesions induced with ZnSO4 effusion (ZnE) on morphine-induced sensitization and conditioned place preference in mice. We found that the lesion of the olfactory epithelium attenuated the repeated morphine (40 mg/kg)-induced behavioral sensitization and morphine-induced conditioned place preference (CPP) behaviors, such as hyper-locomotion during morphine (40 mg/kg) conditioned training. Additionally, the expression of FosB-like proteins, transcription factors associated with behavioral alterations, in the nucleus accumbens of the brain was attenuated in morphine administered mice treated by ZnE. Taken together, these results indicated that lesion of the olfactory epithelium lead to a decrease in morphine sensitization and CPP behavior in mice as well as modulate specific molecular markers of neuroadaption to drugs of abuse. These findings also suggest that olfaction plays an important role in the development of addictive behaviors that can be modulated by external actions.

Effects of repeated administration of chemotherapeutic agents tamoxifen, methotrexate, and 5-fluorouracil on the acquisition and retention of a learned response in mice

RATIONALE

A number of cancer chemotherapeutic agents have been associated with a loss of memory in breast cancer patients although little is known of the causality of this effect.

OBJECTIVES

To assess the potential cognitive effects of repeated exposure to chemotherapeutic agents, we administered the selective estrogen receptor modulator tamoxifen or the antimetabolite chemotherapy, methotrexate, and 5-fluorouracil, alone and in combination to mice and tested them in a learning and memory assay.

METHODS

Swiss-Webster male mice were injected with saline, 32 mg/kg tamoxifen, 3.2 or 32 mg/kg methotrexate, 75 mg/kg 5-fluorouracil, 3.2 or 32 mg/kg methotrexate in combination with 75 mg/kg 5-fluorouracil once per week for 3 weeks. On days 23 and 24, mice were tested for acquisition and retention of a nose-poke response in a learning procedure called autoshaping. In addition, the acute effects of tamoxifen were assessed in additional mice in a similar procedure.

RESULTS

The chemotherapeutic agents alone and in combination reduced body weight relative to saline treatment over the course of 4 weeks. Repeated treatment with tamoxifen produced both acquisition and retention effects relative to the saline-treated group although acute tamoxifen was without effect except at a behaviorally toxic dose. Repeated treatment with methotrexate in combination with 5-fluorouracil produced effects on retention, but the magnitude of these changes depended on the methotrexate dose.

CONCLUSIONS

These data demonstrate that repeated administration of tamoxifen or certain combination of methotrexate and 5-fluorouracil may produce deficits in the acquisition or retention of learned responses which suggest potential strategies for prevention or remediation might be considered in vulnerable patient populations.

Lithium chloride disrupts consolidation of morphine-induced conditioned place preference in male mice: the nitric oxide/cyclic GMP signaling pathway

Lithium effects on brain functions such as cognition, attention, learning and memory are well-established for ages; however, the way it affects these functions and its precise mechanism of action remains unknown. The purpose of this study was to determine the effects of lithium on the consolidation of morphine-associated conditioned place preference and the possible involvement of the NO/cGMP pathway. Using an unbiased conditioned place preference (CPP) model, the effects of lithium (1-100 mg/kg, i.p.), nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) (5-100 mg/kg, i.p.), nitric oxide precursor L-arginine (50-150 mg/kg, i.p.) and phosphodiesterase inhibitor sildenafil (5-40 mg/kg, i.p.) on the consolidation of morphine-induced CPP were assessed. In addition, the possible interaction between lithium, L-arginine and sildenafil or subeffective doses of lithium and L-NAME on the consolidation of morphine-induced contextual memory was evaluated. NMRI mice were used in all studies. Lithium (5-30 mg/kg, i.p.), immediately after conditioning trials, significantly reduced the time spent by mice in the reward-paired compartment. Although post-training administration of L-arginine, sildenafil or L-NAME had no significant effect on the consolidation of CPP, concomitant administration of L-arginine (50-150 mg/kg) and sildenafil (5-10 mg/kg) with lithium (30 mg/kg) prevented the impairing effect of lithium. Also, co-administration of sub-effective doses of lithium (1 mg/kg) and L-NAME (5 mg/kg) disrupted consolidation of CPP. However, delayed administration of effective doses of lithium, which shows specific effect on memory consolidation, did not affect morphine-induced CPP. Lithium seems to inhibit consolidation of morphine-induced CPP and this impairing effect might be via nitric oxide/cyclic GMP pathway.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Implication of dopaminergic projection from the ventral tegmental area to the anterior cingulate cortex in μ-opioid-induced place preference

Despite the importance of prefrontal cortical dopamine in modulating reward, little is known about the implication of the specific subregion of prefrontal cortex in opioid reward. We investigated the role of neurons projecting from the ventral tegmental area (VTA) to the anterior cingulate cortex (ACG) in opioid reward. Microinjection of the retrograde tracer fluorogold (FG) into the ACG revealed several retrogradely labelled cells in the VTA. The FG-positive reactions were noted in both tyrosine hydroxylase (TH)-positive and -negative VTA neurons. The released levels of dopamine and its major metabolites in the ACG were increased by either the electrical stimulation of VTA neurons or microinjection of a selective μ-opioid receptor (MOR) agonist, (D-Ala²,N-MePhe⁴,Gly-ol⁵) enkephalin (DAMGO), into the VTA. MOR-like immunoreactivity was seen in both TH-positive and -negative VTA neurons projecting to the ACG. The conditioned place preference induced by intra-VTA injection of DAMGO was significantly attenuated by chemical lesion of dopaminergic terminals in the ACG. The depletion of dopamine in the ACG induced early extinction of μ-opioid-induced place preference. The levels of phosphorylated DARPP32 (Thr34) and phosphorylated CREB (Ser133) were increased in the ACG of rats that had maintained the morphine-induced place preference, whereas the increases of these levels induced by morphine were blocked by pre-treatment of a selective dopamine D1 receptor antagonist SCH23390. These findings suggest that VTA-ACG transmission may play a crucial role in the acquisition and maintenance of μ-opioid-induced place preference. The activation of DARPP32 and CREB through dopamine D1 receptors in the ACG could be implicated in the maintenance of μ-opioid-induced place preference.