Sex Differences in the Ventral Tegmental Area and Nucleus Accumbens Proteome at Baseline and Following Nicotine Exposure

Temporal sex specific brain gene expression pattern during early rat embryonic development

Background: The classical concept of brain sex differentiation suggests that steroid hormones released from the gonads program male and female brains differently. However, several studies indicate that steroid hormones are not the only determinant of brain sex differentiation and that genetic differences could also be involved. Methods: In this study, we have performed RNA sequencing of rat brains at embryonic days 12 (E12), E13, and E14. The aim was to identify differentially expressed genes between male and female rat brains during early development. Results: Analysis of genes expressed with the highest sex differences showed that Xist was highly expressed in females having XX genotype with an increasing expression over time. Analysis of genes expressed with the highest male expression identified three early genes, Sry2, Eif2s3y, and Ddx3y. Discussion: The observed sex-specific expression of genes at early development confirms that the rat brain is sexually dimorphic prior to gonadal action on the brain and identifies Sry2 and Eif2s3y as early genes contributing to male brain development.

Nicotine addiction: More than just dopamine

Despite decades of research and anti-tobacco messaging, nicotine addiction remains an important public health problem leading to hundreds of thousands of deaths each year. While fundamental studies have identified molecular, circuit-level and behavioral mechanisms important for nicotine reinforcement and withdrawal, recent studies have identified additional pathways that are important for both nicotine seeking and aversion. In particular, although dopaminergic mechanisms are necessary for nicotine-dependent reward and drug-seeking, novel glutamate and GABA signaling mechanisms in the mesolimbic system have been identified for their contributions to reward-related behaviors. An additional area of active investigation for nicotine addiction focuses on molecular mechanisms in the habenula-interpeduncular pathway driving nicotine aversion and withdrawal. Across all these domains, sex differences in the molecular basis of nicotine-induced behaviors have emerged that identify important new directions for future research. Recent studies reviewed here highlight additional pathways that could provide therapeutic targets for smoking cessation and problematic nicotine vaping.

Poor inhibitory control predicts sex-specific vulnerability to nicotine rewarding properties in mice

RATIONALE

The risk of becoming addicted to tobacco varies greatly from individual to individual, raising the possibility of behavioural biomarkers capable of predicting sensitivity to nicotine reward, a crucial step in the development of nicotine addiction. Amongst all of nicotine's pharmacological properties, one of central importance is the enhancement of cognitive performances, which depend on the balance between attentional processes and inhibitory control. However, whether the cognitive enhancement effects of nicotine are predictive of sensitivity to its rewarding properties is still unknown.

OBJECTIVE

Using male and female mice, we investigated whether the effects of nicotine on cognitive performances are predictive of sensitivity to the rewarding properties of nicotine and, if so, whether this relationship is sex dependent.

METHODS

Naïve male and female mice were first assessed for their performances in both baseline conditions and following nicotine injection (0.15 and 0.30 mg/kg) in a cued-Fixed Consecutive Number task (FCNcue) measuring both optimal (attention) and premature (inhibitory control) responding. Next, all mice underwent nicotine-induced conditioned place preference (CPP) in order to evaluate inter-individual differences in response to nicotine reward (0.30 mg/kg).

RESULTS

Results showed that males and females benefited from the effect of nicotine as a cognitive enhancer in the FCNcue task. However, only those males displaying poor inhibitory control, namely high-impulsive animals, subsequently displayed sensitivity to nicotine reward. In females, sensitivity to nicotine reward was independent of FCNcue performances, in both basal and nicotine conditions.

CONCLUSION

Thus, our study suggests that poor inhibitory control and its modulation by nicotine may be a behavioural biomarker for sensitivity to nicotine reward and consequent vulnerability to nicotine addiction in males but not females.

The choroid plexus links innate immunity to CSF dysregulation in hydrocephalus

The choroid plexus (ChP) is the blood-cerebrospinal fluid (CSF) barrier and the primary source of CSF. Acquired hydrocephalus, caused by brain infection or hemorrhage, lacks drug treatments due to obscure pathobiology. Our integrated, multi-omic investigation of post-infectious hydrocephalus (PIH) and post-hemorrhagic hydrocephalus (PHH) models revealed that lipopolysaccharide and blood breakdown products trigger highly similar TLR4-dependent immune responses at the ChP-CSF interface. The resulting CSF "cytokine storm", elicited from peripherally derived and border-associated ChP macrophages, causes increased CSF production from ChP epithelial cells via phospho-activation of the TNF-receptor-associated kinase SPAK, which serves as a regulatory scaffold of a multi-ion transporter protein complex. Genetic or pharmacological immunomodulation prevents PIH and PHH by antagonizing SPAK-dependent CSF hypersecretion. These results reveal the ChP as a dynamic, cellularly heterogeneous tissue with highly regulated immune-secretory capacity, expand our understanding of ChP immune-epithelial cell cross talk, and reframe PIH and PHH as related neuroimmune disorders vulnerable to small molecule pharmacotherapy.

ERK signaling is required for nicotine-induced conditional place preference by regulating neuroplasticity genes expression in male mice

Nicotine is an addictive compound that interacts with nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA), inducing a release of dopamine in the nucleus accumbens (NAc). When neurons undergo repeated exposure to nicotine, several adaptive changes in neuroplasticity occur. Activation of nAChRs involves numerous intracellular signaling cascades that likely contribute to neuroplasticity and ultimately the establishment of nicotine addiction. Nevertheless, the molecular mechanisms underlying this adaptation remain unclear. To explore the effects of nicotine on neuroplasticity, a stable nicotine-induced conditioned place preference (CPP) model was constructed by intravenous injection in mice. Using a PCR array, we observed significant changes in the expression of synaptic plasticity-related genes in the VTA (16 mRNAs) and NAc (40 mRNAs). When mice were pre-treated with PD98059, an extracellular signal-regulated kinase (ERK) inhibitor, more gene expression changes in the VTA (53 mRNAs) and NAc (60 mRNAs) were found. Moreover, PD98059 pre-treatment blocked the increased p-ERK/ERK and p-CREB/CREB ratios and decreased the expression of synaptic plasticity-related proteins such as SAP102, PSD95, synaptophysin, and BDNF, these changes might contribute to preventing the establishment of nicotine-induced CPP. Furthermore, neurons from the VTA and NAc of nicotine CPP mice had an increased dendritic spine density and complexity of dendritic morphology by Golgi staining. PD98059 also blocked this dynamic. These results demonstrate that repeated exposure to nicotine may remold the expression of neuroplasticity-related genes by activating the ERK signaling pathway in the VTA and NAc, and is related to the establishment of nicotine-induced CPP.

Relapse-like behavior and nAChR sensitization following intermittent access nicotine self-administration

Many tobacco smokers consume nicotine intermittently, but the underlying mechanisms and neurobiological changes associated with intermittent nicotine intake are unclear. Understanding intermittent nicotine intake is a high priority, as it could promote therapeutic strategies to attenuate tobacco consumption. We examined nicotine intake behavior and neurobiological changes in male rats that were trained to self-administer nicotine during brief (5 min) trials interspersed with longer (15 min) drug-free periods. Rats readily adapted to intermittent access (IntA) SA following acquisition on a continuous access (ContA) schedule. Probabilistic analysis of IntA nicotine SA suggested reduced nicotine loading behavior compared to ContA, and nicotine pharmacokinetic modeling revealed that rats taking nicotine intermittently may have increased intake to maintain blood levels of nicotine that are comparable to ContA SA. After IntA nicotine SA, rats exhibited an increase in unreinforced responses for nicotine-associated cues (incubation of craving) and specific alterations in the striatal proteome after 7 days without nicotine. IntA nicotine SA also induced nAChR functional upregulation in the interpeduncular nucleus (IPN), and it enhanced nicotine binding in the brain as determined via [¹¹C]nicotine positron emission tomography. Reducing the saliency of the cue conditions during the 5 min access periods attenuated nicotine intake, but incubation of craving was preserved. Together, these results indicate that IntA conditions promote nicotine SA and nicotine seeking after a nicotine-free period.

Toxic effects of smokeless tobacco on female reproductive health: A review

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Smokeless tobacco use can lead to impairments of ovarian function, morphology, oocyte quality and hormonal regulation.

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Use of smokeless tobacco during pregnancy has adverse health effects on both the mother and fetus.

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Exposure to smokeless tobacco in utero has long term health consequences on offspring.

The habitual consumption of tobacco in its various form is widespread and a serious public health issue globally. In particular, the use of smokeless tobacco has increased substantially due to its easy availability and misconception that it is relatively harmless compared to smoking. Tobacco use has been well established from numerous studies as a causative agent of devastating illnesses such as cancer, insulin resistance, hypertension, acute respiratory disease, osteoporosis, etc. Limited but growing evidence have also suggested its role in adversely affecting reproductive capabilities and outcomes in women of reproductive age and during pregnancy. This paper provides an updated review on available literature regarding the negative effects of smokeless tobacco use on female reproductive health, during pregnancy and its adverse consequences on the offspring. Existing data suggests the association between chronic smokeless tobacco use and impairment of ovarian morphology and function, oocyte quality, hormonal perturbations, fetal development and long-term health effects on the fetus. Improved understanding of these issues can contribute to better awareness of the dangers of smokeless tobacco products.

Specificity of Varenicline in Blocking Mesolimbic Circuit Activation to Natural and Drug Rewards

The mesolimbic dopamine (DA) system reinforces behaviors that are critical for survival. However, drug dependence can occur when drugs of abuse, such as nicotine, highjack this reinforcement system. Pharmacologically targeting the DA system to selectively block drug reinforcement requires a detailed understanding of the neural circuits and molecular pathways that lead to the reward-based activation of mesolimbic circuits. Varenicline is an approved smoking cessation drug that has been shown to block nicotine-evoked DA increases in the nucleus accumbens (NAc) through action on nicotinic acetylcholine receptors. Because these receptors have been implicated in the reinforcement of other addictive substances, we explored the possibility that varenicline could broadly affect reward processing. We used in vivo fiber photometry to monitor midbrain DA neuron activity and striatal DA levels following either natural or drug rewards in mice treated with varenicline. We demonstrate that varenicline pretreatment enhances the suppression of nicotine-evoked DA release by attenuating DA neuron activity in the VTA. Varenicline's ability to attenuate DA release is highly specific to nicotine, and varenicline slightly elevates DA release when co-administered with morphine or ethanol. Furthermore, varenicline has no effect on DA release in response to naturally rewarding behavior such as food intake or exercise. These results demonstrate the exquisite specificity with which varenicline blocks nicotine reward and highlight the complexity with which different rewards activate the mesolimbic DA system.

Administration of brain-derived neurotrophic factor in the ventral tegmental area produces a switch from a nicotine nondependent D1R-mediated motivational state to a nicotine dependent-like D2R-mediated motivational state

Brain-derived neurotrophic factor (BDNF) has been implicated in the transition from a nondependent motivational state to a drug-dependent and -withdrawn motivational state. Chronic nicotine can increase BDNF in the rodent brain and is associated with smoking severity in humans; however, it is unknown whether this increased BDNF is linked functionally to the switch from a nicotine-nondependent to a nicotine-dependent state. We used a place conditioning paradigm to measure the conditioned responses to nicotine, showing that a dose of acute nicotine that nondependent male mice find aversive is found rewarding in chronic nicotine-treated mice experiencing withdrawal. A single BDNF injection in the ventral tegmental area (in the absence of chronic nicotine treatment) caused mice to behave as if they were nicotine-dependent and in withdrawal, switching the neurobiological substrate mediating the conditioned motivational effects from dopamine D1 receptors to D2 receptors. Quantification of gene expression of BDNF and its receptor, tropomyosin-receptor-kinase B (TrkB), revealed an increase in TrkB mRNA but not BDNF mRNA in the VTA in nicotine-dependent and -withdrawn mice. These results suggest that BDNF signaling in the VTA is a critical neurobiological substrate for the transition to nicotine dependence. The modulation of BDNF signaling may be a promising new pharmacological avenue for the treatment of addictive behavior.

Estrogen receptors and the aging brain

The female sex hormone estrogen has been ascribed potent neuroprotective properties. It signals by binding and activating estrogen receptors that, depending on receptor subtype and upstream or downstream effectors, can mediate gene transcription and rapid non-genomic actions. In this way, estrogen receptors in the brain participate in modulating neural differentiation, proliferation, neuroinflammation, cholesterol metabolism, synaptic plasticity, and behavior. Circulating sex hormones decrease in the course of aging, more rapidly at menopause in women, and slower in men. This review will discuss what this drop entails in terms of modulating neuroprotection and resilience in the aging brain downstream of spatiotemporal estrogen receptor alpha (ERα) and beta (ERβ) signaling, as well as in terms of the sex differences observed in Alzheimer's disease (AD) and Parkinson's disease (PD). In addition, controversies related to ER expression in the brain will be discussed. Understanding the spatiotemporal signaling of sex hormones in the brain can lead to more personalized prevention strategies or therapies combating neurodegenerative diseases.

Mitochondria-Related Nuclear Gene Expression in the Nucleus Accumbens and Blood Mitochondrial Copy Number After Developmental Fentanyl Exposure in Adolescent Male and Female C57BL/6 Mice

The potency of the synthetic opioid fentanyl and its increased clinical availability has led to the rapid escalation of use in the general population, increased recreational exposure, and subsequently opioid-related overdoses. The wide-spread use of fentanyl has, consequently, increased the incidence of in utero exposure to the drug, but the long-term effects of this type of developmental exposure are not yet understood. Opioid use has also been linked to reduced mitochondrial copy number in blood in clinical populations, but the link between this peripheral biomarker and genetic or functional changes in reward-related brain circuitry is still unclear. Additionally, mitochondrial-related gene expression in reward-related brain regions has not been examined in the context of fentanyl exposure, despite the growing literature demonstrating drugs of abuse impact mitochondrial function, which subsequently impacts neuronal signaling. The current study uses exposure to fentanyl via dam access to fentanyl drinking water during gestation and lactation as a model for developmental drug exposure. This perinatal drug-exposure is sufficient to impact mitochondrial copy number in circulating blood leukocytes, as well as mitochondrial-related gene expression in the nucleus accumbens (NAc), a reward-related brain structure, in a sex-dependent manner in adolescent offspring. Specific NAc gene expression is correlated with both blood mitochondrial copy number and with anxiety related behaviors dependent on developmental exposure to fentanyl and sex. These data indicate that developmental fentanyl exposure impacts mitochondrial function in both the brain and body in ways that can impact neuronal signaling and may prime the brain for altered reward-related behavior in adolescence and later into adulthood.