Sex-dependent diversity in ventral tegmental dopaminergic neurons and developmental programing: A molecular, cellular and behavioral analysis

Residential exposure to urban traffic is associated with the poorer neurobehavioral health of Ecuadorian schoolchildren

PURPOSE

We investigated whether chronic traffic-generated air pollution containing fine and ultrafine particulate matter is associated with reduced neurobehavioral performance and behavioral dysfunction in urban Ecuadorian schoolchildren. Also, we examined the effect of child hemoglobin and sociodemographic risk factors on these neurocognitive outcomes.

METHODS

A convenience sample of healthy children aged 8-14 years attending public schools were recruited in Quito, Ecuador. Child residential proximity to the nearest heavily trafficked road was used as a proxy for traffic-related pollutant exposure. These included high exposure (<100 m), medium exposure (100-199 m) and low exposure (≥ 200 m) from the nearest heavily trafficked road. The Behavioral Assessment and Research System (BARS), a computerized test battery assessing attention, memory, learning and motor function was used to evaluate child neurobehavioral performance. The Child Behavior Checklist (CBCL/6-18) was used to assess child behavioral dysfunction as reported by mothers. The data were analyzed using multiple linear regression.

RESULTS

Children with the highest residential exposure to traffic pollutants (< 100 m) had significantly longer latencies as measured by match to sample (b = 410.27; p = 0.01) and continuous performance (b = 37.90; p = 0.02) compared to those living ≥ 200 m away. A similar but non-significant association was observed for reaction time latency. Children living within 100 m of heavy traffic also demonstrated higher scores across all CBCL subscales although only the relationship with thought problems (p = 0.05) was statistically significant in the adjusted model.

CONCLUSION

The study findings suggest that children living within 100 m of heavy traffic appear to experience subtle neurobehavioral deficits that may result from fine and ultrafine particulate matter exposure.

Sex-dependent regulation of social reward by oxytocin: an inverted U hypothesis

The rewarding properties of social interactions are essential for the expression of social behavior and the development of adaptive social relationships. Here, we review sex differences in social reward, and more specifically, how oxytocin (OT) acts in the mesolimbic dopamine system (MDS) to mediate the rewarding properties of social interactions in a sex-dependent manner. Evidence from rodents and humans suggests that same-sex social interactions may be more rewarding in females than in males. We propose that there is an inverted U relationship between OT dose, social reward, and neural activity within structures of the MDS in both males and females, and that this dose-response relationship is initiated at lower doses in females than males. As a result, depending on the dose of OT administered, OT could reduce social reward in females, while enhancing it in males. Sex differences in the neural mechanisms regulating social reward may contribute to sex differences in the incidence of a large number of psychiatric and neurodevelopmental disorders. This review addresses the potential significance of a sex-dependent inverted U dose-response function for OT's effects on social reward and in the development of gender-specific therapies for these disorders.

Sex Differences in the Rat Hippocampal Opioid System After Oxycodone Conditioned Place Preference

Although opioid addiction has risen dramatically, the role of gender in addiction has been difficult to elucidate. We previously found sex-dependent differences in the hippocampal opioid system of Sprague-Dawley rats that may promote associative learning relevant to drug abuse. The present studies show that although female and male rats acquired conditioned place preference (CPP) to the mu-opioid receptor (MOR) agonist oxycodone (3 mg/kg, I.P.), hippocampal opioid circuits were differentially altered. In CA3, Leu-Enkephalin-containing mossy fibers had elevated levels in oxycodone CPP (Oxy) males comparable to those in females and sprouted in Oxy-females, suggesting different mechanisms for enhancing opioid sensitivity. Electron microscopy revealed that in Oxy-males delta opioid receptors (DORs) redistributed to mossy fiber-CA3 synapses in a manner resembling females that we previously showed is important for opioid-mediated long-term potentiation. Moreover, in Oxy-females DORs redistributed to CA3 pyramidal cell spines, suggesting the potential for enhanced plasticity processes. In Saline-injected (Sal) females, dentate hilar parvalbumin-containing basket interneuron dendrites had fewer MORs, however plasmalemmal and total MORs increased in Oxy-females. In dentate hilar GABAergic dendrites that contain neuropeptide Y, Sal-females compared to Sal-males had higher plasmalemmal DORs, and near-plasmalemmal DORs increased in Oxy-females. This redistribution of MORs and DORs within hilar interneurons in Oxy-females would potentially enhance disinhibition of granule cells via two different circuits. Together, these results indicate that oxycodone CPP induces sex-dependent redistributions of opioid receptors in hippocampal circuits in a manner facilitating opioid-associative learning processes and may help explain the increased susceptibility of females to opioid addiction acquisition and relapse.

The Sex Chromosome Hypothesis of Schizophrenia: Alive, Dead, or Forgotten? A Commentary and Review

The X chromosome has long been an intriguing site for harboring genes that have importance in brain development and function. It has received the most attention for having specific genes underlying the X-linked inherited intellectual disabilities, but has also been associated with schizophrenia in a number of early studies. An X chromosome hypothesis for a genetic predisposition for schizophrenia initially came from the X chromosome anomaly population data showing an excess of schizophrenia in Klinefelter's (XXY) males and triple X (XXX) females. Crow and colleagues later expanded the X chromosome hypothesis to include the possibility of a locus on the Y chromosome and, specifically, genes on X that escaped inactivation and are X-Y homologous loci. Some new information about possible risk loci on these chromosomes has come from the current large genetic consortia genome-wide association studies, suggesting that perhaps this hypothesis needs to be revisited for some schizophrenias. The following commentary reviews the early and more recent literature supporting or refuting this dormant hypothesis and emphasizes the possible candidate genes still of interest that could be explored in further studies.

Ethanol actions on the ventral tegmental area: novel potential targets on reward pathway neurons

The ventral tegmental area (VTA) evaluates salience of environmental stimuli and provides dopaminergic innervation to many brain areas affected by acute and chronic ethanol exposure. While primarily associated with rewarding and reinforcing stimuli, recent evidence indicates a role for the VTA in aversion as well. Ethanol actions in the VTA may trigger neuroadaptation resulting in reduction of the aversive responses to alcohol and a relative increase in the rewarding responses. In searching for effective pharmacotherapies for the treatment of alcohol abuse and alcoholism, recognition of this imbalance may reveal novel strategies. In addition to conventional receptor/ion channel pharmacotherapies, epigenetic factors that control neuroadaptation to chronic ethanol treatment can be targeted as an avenue for development of therapeutic approaches to restore the balance. Furthermore, when exploring therapies to address reward/aversion imbalance in the action of alcohol in the VTA, sex differences have to be taken into account to ensure effective treatment for both men and women. These principles apply to a VTA-centric approach to therapies, but should hold true when thinking about the overall approach in the development of neuroactive drugs to treat alcohol use disorders. Although the functions of the VTA itself are complex, it is a useful model system to evaluate the reward/aversion imbalance that occurs with ethanol exposure and could be used to provide new leads in the efforts to develop novel drugs to treat alcoholism.

Sex-dependent impact of early-life stress and adult immobilization in the attribution of incentive salience in rats

Early life stress (ELS) induces long-term effects in later functioning and interacts with further exposure to other stressors in adulthood to shape our responsiveness to reward-related cues. The attribution of incentive salience to food-related cues may be modulated by previous and current exposures to stressors in a sex-dependent manner. We hypothesized from human data that exposure to a traumatic (severe) adult stressor will decrease the attribution of incentive salience to reward-associated cues, especially in females, because these effects are modulated by previous ELS. To study these factors in Long-Evans rats, we used as an ELS model of restriction of nesting material and concurrently evaluated maternal care. In adulthood, the offspring of both sexes were exposed to acute immobilization (IMO), and several days after, a Pavlovian conditioning procedure was used to assess the incentive salience of food-related cues. Some rats developed more attraction to the cue predictive of reward (sign-tracking) and others were attracted to the location of the reward itself, the food-magazine (goal-tracking). Several dopaminergic markers were evaluated by in situ hybridization. The results showed that ELS increased maternal care and decreased body weight gain (only in females). Regarding incentive salience, in absolute control animals, females presented slightly greater sign-tracking behavior than males. Non-ELS male rats exposed to IMO showed a bias towards goal-tracking, whereas in females, IMO produced a bias towards sign-tracking. Animals of both sexes not exposed to IMO displayed an intermediate phenotype. ELS in IMO-treated females was able to reduce sign-tracking and decrease tyrosine hydroxylase expression in the ventral tegmental area and dopamine D1 receptor expression in the accumbens shell. Although the predicted greater decrease in females in sign-tracking after IMO exposure was not corroborated by the data, the results highlight the idea that sex is an important factor in the study of the long-term impact of early and adult stressors.

Common Biological Mechanisms of Alcohol Use Disorder and Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are highly comorbid. Although recent clinical studies provide some understanding of biological and subsequent behavioral changes that define each of these disorders, the neurobiological basis of interactions between PTSD and AUD has not been well-understood. In this review, we summarize the relevant animal models that parallel the human conditions, as well as the clinical findings in these disorders, to delineate key gaps in our knowledge and to provide potential clinical strategies for alleviating the comorbid conditions.

Manganese in teeth and neurobehavior: Sex-specific windows of susceptibility

BACKGROUND

Manganese (Mn) is an essential element required for growth and development, but higher body burdens have been associated with neurobehavioral decrements in children.

OBJECTIVES

We examined whether prenatal or postnatal Mn measured in deciduous teeth was associated with scores on a test of visuospatial learning and memory.

METHODS

Deciduous teeth were collected from 142 participants (ages 10-14years) residing near varied ferro‑manganese industry in Italy. Mn concentrations were measured in prenatal and postnatal tooth regions by laser ablation inductively coupled plasma mass spectrometry (ICP-MS). The Virtual Radial Arm Maze (VRAM), an animal-human analogue task, was used to assess visuospatial learning and memory. We used generalized additive, linear and zero-inflated Poisson mixed regression models to estimate associations between prenatal or postnatal Mn concentrations and repeated measures of all four VRAM outcomes: time, distance, working and reference memory errors. Effect measure modification by sex was examined in stratified models.

RESULTS

U-shaped associations between prenatal Mn and VRAM outcomes were observed among girls only (pGAMM=0.001 to 0.02 in stratified models). Compared to the mid-tertile of prenatal Mn, girls in the highest tertile took 7.7s [95% CI: -6.1, 21.5] longer to complete the task, traveled 2.3 maze units [0.1, 4.4] farther, and committed more working and reference memory errors (β for count ratio=1.33 [1.01, 1.83]; 1.10 [0.98, 1.24], respectively). This association was not observed among boys. In contrast, for postnatal Mn, no significant associations were found, and patterns were similar for boys and girls.

CONCLUSIONS

The prenatal period may be a critical window for the impact of environmental Mn on visuospatial ability and executive function, especially for females.

Sex differences in addiction

Women exhibit more rapid escalation from casual drug taking to addiction, exhibit a greater withdrawal response with abstinence, and tend to exhibit greater vulnerability than men in terms of treatment outcome. In rodents, short-term estradiol intake in female rats enhances acquisition and escalation of drug taking, motivation for drugs of abuse, and relapse-like behaviors. There is also a sex difference in the dopamine response in the nucleus accumbens. Ovariectomized female rats exhibit a smaller initial dopamine increase after cocaine treatment than castrated males. Estradiol treatment of ovariectomized female rats enhances stimulated dopamine release in the dorsolateral striatum, but not in the nucleus accumbens, resulting in a sex difference in the balance between these two dopaminergic projections. In the situation where drug-taking behavior becomes habitual, dopamine release has been reported to be enhanced in the dorsolateral striatum and attenuated in the nucleus accumbens. The sex difference in the balance between these neural systems is proposed to underlie sex differences in addiction.

Low Retinal Dehydrogenase 1 (RALDH1) Level in Prepubertal Boys with Autism Spectrum Disorder: A Possible Link to Dopamine Dysfunction?

Objective

Retinal dehydrogenase 1 (RALDH1) is a cytosolic enzyme which acts both as a source of retinoic acid (RA) and as a detoxification enzyme. RALDH1 has key functions in the midbrain dopaminergic system, which influences motivation, cognition, and social behavior. Since dopamine has been increasingly linked to autism spectrum disorder (ASD), we asked whether RALDH1 could contribute to the autistic phenotype. Therefore, we investigated for the first time the levels of RALDH1 in autistic patients. To further assess the detoxification function of RALDH1, we also explored 4-hydroxynonenal protein adducts (4-HNE PAs) and reduced glutathione (GSH) levels. Moreover, considering the effect of testosterone on RALDH1 expression, we measured the second to fourth digit ratio (2D:4D ratio) for both hands, which reflects exposure to prenatal testosterone.

Methods

Male patients with ASD (n=18; age, 62.9±4.3 months) and healthy controls (n=13; age, 78.1±4.9 months) were examined. Erythrocyte RALDH1, serum 4-HNE PAs and erythrocyte GSH levels were measured using colorimetric assays, and digit lengths were measured using digital calipers.

Results

We found significantly lower (−42.9%) RALDH1 levels in autistic patients as compared to controls (p=0.032). However, there was no difference in 4-HNE PAs levels (p=0.368), GSH levels (p=0.586), or 2D:4D ratios (p=0.246 in the left hand, p=0.584 in the right hand) between healthy controls and autistic subjects.

Conclusion

We concluded that a subset of autistic patients had a low RALDH1 level. These results suggest that low RALDH1 levels could contribute to the autistic phenotype by reflecting a dopaminergic dysfunction.

Sex-Dependent Effects of Stress on Immobility Behavior and VTA Dopamine Neuron Activity: Modulation by Ketamine

BACKGROUND

Stress constitutes a risk factor across several psychiatric disorders. Moreover, females are more susceptible to stress-related disorders, such as depression, than males. Although dopamine system underactivation is implicated in the pathophysiology of depression, little is known about the female dopamine system at baseline and post-stress.

METHODS

The effects of chronic mild stress were examined on ventral tegmental area dopamine neuron activity and forced swim test immobility by comparing male and female rats. The impact of a single dose of the rapid antidepressant ketamine (10 mg/kg, i.p.) on forced swim test immobility and ventral tegmental area function was then tested.

RESULTS

Baseline ventral tegmental area dopamine activity was comparable in both sexes. At baseline, females exhibited roughly double the forced swim test immobility duration than males, which corresponded to ~50% decrease in ventral tegmental area dopamine population activity compared with similarly treated (i.e., post-forced swim test) males. Following chronic mild stress, there was greater immobility duration in both sexes and reduced ventral tegmental area dopamine neuron activity by approximately 50% in males and nearly 75% in females. Ketamine restored behavior and post-forced swim test ventral tegmental area dopamine activity for up to 7 days in females as well as in both male and female chronic mild stress-exposed rats.

CONCLUSIONS

These data suggest increased female susceptibility to depression-like phenotypes (i.e., greater immobility, ventral tegmental area hypofunction) is associated with higher dopamine system sensitivity to both acute and repeated stress relative to males. Understanding the neural underpinnings of sex differences in stress vulnerability will provide insight into mechanisms of disease and optimizing therapeutic approaches in both sexes.

Innervation of the medial prefrontal cortex by tyrosine hydroxylase immunoreactive fibers during adolescence in male and female rats

Adolescence is associated with continued maturation of the cerebral cortex, particularly the medial prefrontal cortex (mPFC). We have previously documented pruning in the number of neurons, dendrites, and synapses in the rat mPFC from preadolescence to adulthood, with the period of pubertal onset being particularly important. We hypothesized that dopaminergic innervation of this region, critical for executive functions, would also be influenced by pubertal onset. Here, we measured changes in the volume of tyrosine hydroxylase (TH) immunoreactive axons in all layers of the male and female mPFC from preadolescence to adulthood (postnatal Day (P) 25, 35, 45, 60, and 90) as a marker of dopaminergic innervation. Assessing both total fiber volume and length, TH fibers were quantified by multiplying the mPFC volume by fiber density. While there were subtle layer-specific changes, TH fiber volume and length increased between P25 and P90 in both males and females. Contrary to our hypothesis, a role for pubertal onset in TH innervation of this region was not discernable. In summary, axons immunoreactive for TH increase with similar trajectories in the mPFC of male and female rats from pre-puberty to young adulthood.

A systematic review of sex and age factors in neuroleptic malignant syndrome diagnosis frequency

OBJECTIVE

To examine sex and age distributions in neuroleptic malignant syndrome (NMS) patients based on a systematic literature review.

METHOD

EMBASE and PubMed databases were searched to identify any observation of NMS published from January 1, 1998 through November 1, 2014 that was accessible and interpretable (using language translation software). Redundant and equivocal reports were excluded. Sex ratio and age distributions were examined using standard graphical techniques and measures of association.

RESULTS

Twenty-eight independent sex ratio estimates were included. Males predominated in most (75%) estimates with an overall median sex ratio of 1.47 (95% CI, 1.20-1.80). NMS incidence peaked at age 20-25 years and declined steadily thereafter, with males consistently outnumbering females at all but the oldest age intervals.

CONCLUSION

NMS patients are 50% more likely to be males, and NMS is most likely to occur in young adulthood.

Cellular basis of brain maturation and acquisition of complex behaviors in salamanders

The overall bauplan of the tetrapod brain is highly conserved, yet significant variations exist among species in terms of brain size, structural composition and cellular diversity. Understanding processes underlying neural and behavioral development in a wide range of species is important both from an evolutionary developmental perspective as well as for the identification of cell sources with post-developmental neurogenic potential. Here we characterize germinal processes in the brain of Notophthalmus viridescens and Pleurodeles waltl during both development and adulthood. Using a combination of cell tracking tools, including clonal analyses in new transgenic salamander lines we examine the origin of neural stem and progenitor cells found in the adult brain, determine regional variability in cell cycle length of progenitor cells, and show spatio-temporally orchestrated neurogenesis. We analyze how maturation of different brain regions and neuronal subpopulations are linked to the acquisition of complex behaviors, and how these behaviors are altered upon chemical ablation of dopamine neurons. Our data analyzed from an evolutionary perspective reveal both common and species-specific processes in tetrapod brain formation and function.

Gender Differences in Risk Factors for Adolescent Binge Drinking and Implications for Intervention and Prevention

Alcohol use, particularly binge drinking (BD), is a major public health concern among adolescents. Recent national data show that the gender gap in alcohol use is lessening, and BD among girls is rising. Considering the increase in BD among adolescent girls, as well as females' increased risk of experiencing more severe biopsychosocial negative effects and consequences from BD, the current review sought to examine gender differences in risk factors for BD. The review highlights gender differences in (1) developmental-related neurobiological vulnerability to BD, (2) psychiatric comorbidity and risk phenotypes for BD, and (3) social-related risk factors for BD among adolescents, as well as considerations for BD prevention and intervention. Most of the information gleaned thus far has come from preclinical research. However, it is expected that, with recent advances in clinical imaging technology, neurobiological effects observed in lower mammals will be confirmed in humans and vice versa. A synthesis of the literature highlights that males and females experience unique neurobiological paths of development, and although there is debate regarding the specific nature of these differences, literature suggests that these differences in turn influence gender differences in psychiatric comorbidity and risk for BD. For one, girls are more susceptible to stress, depression, and other internalizing behaviors and, in turn, these symptoms contribute to their risk for BD. On the other hand, males, given gender differences across the lifespan as well as gender differences in development, are driven by an externalizing phenotype for risk of BD, in part, due to unique paths of neurobiological development that occur across adolescence. With respect to social domains, although social and peer influences are important for both adolescent males and females, there are gender differences. For example, girls may be more sensitive to pressure from peers to fit in and impress others, while male gender role stereotypes regarding BD may be more of a risk factor for boys. Given these unique differences in male and female risk for BD, further research exploring risk factors, as well as tailoring intervention and prevention, is necessary. Although recent research has tailored substance use intervention to target males and females, more literature on gender considerations in treatment for prevention and intervention of BD in particular is warranted.

Enduring, Sexually Dimorphic Impact of In Utero Exposure to Elevated Levels of Glucocorticoids on Midbrain Dopaminergic Populations

Glucocorticoid hormones (GCs) released from the fetal/maternal glands during late gestation are required for normal development of mammalian organs and tissues. Accordingly, synthetic glucocorticoids have proven to be invaluable in perinatal medicine where they are widely used to accelerate fetal lung maturation when there is risk of pre-term birth and to promote infant survival. However, clinical and pre-clinical studies have demonstrated that inappropriate exposure of the developing brain to elevated levels of GCs, either as a result of clinical over-use or after stress-induced activation of the fetal/maternal adrenal cortex, is linked with significant effects on brain structure, neurological function and behaviour in later life. In order to understand the underlying neural processes, particular interest has focused on the midbrain dopaminergic systems, which are critical regulators of normal adaptive behaviours, cognitive and sensorimotor functions. Specifically, using a rodent model of GC exposure in late gestation (approximating human brain development at late second/early third trimester), we demonstrated enduring effects on the shape and volume of the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) (origins of the mesocorticolimbic and nigrostriatal dopaminergic pathways) on the topographical organisation and size of the dopaminergic neuronal populations and astrocytes within these nuclei and on target innervation density and neurochemical markers of dopaminergic transmission (receptors, transporters, basal and amphetamine-stimulated dopamine release at striatal and prefrontal cortical sites) that impact on the adult brain. The effects of antenatal GC treatment (AGT) were both profound and sexually-dimorphic, not only in terms of quantitative change but also qualitatively, with several parameters affected in the opposite direction in males and females. Although such substantial neurobiological changes might presage marked behavioural effects, in utero GC exposure had only a modest or no effect, depending on sex, on a range of conditioned and unconditioned behaviours known to depend on midbrain dopaminergic transmission. Collectively, these findings suggest that apparent behavioural normality in certain tests, but not others, arises from AGT-induced adaptations or compensatory mechanisms within the midbrain dopaminergic systems, which preserve some, but not all functions. Furthermore, the capacities for molecular adaptations to early environmental challenge are different, even opponent, in males and females, which may account for their differential resilience or failure to perform adequately in behavioural tests. Behavioural "normality" is thus achieved by the midbrain dopaminergic network operating outside its normal limits (in a state of allostasis), rendering it at greater risk to malfunction when challenged in later life. Sex-specific neurobiological programming of midbrain dopaminergic systems may, therefore, have psychopathological relevance for the sex bias commonly found in brain disorders associated with these systems, and which have a neurodevelopmental component, including schizophrenia, ADHD (attention/deficit hyperactivity disorders), autism, depression and substance abuse.

From molecules to behavior: An integrative theory of autism spectrum disorder

Autism spectrum disorder (ASD) comprises a group of neurodevelopmental disorders for which various theories have been proposed. Each theory brings valuable insights and has experimental evidence backing it, yet none provides an overarching explanation for each of the pathological aspects involved in ASD. Here we present an integrative theory of ASD, centered on a sequence of events spanning from the molecular to the behavioral level. We propose that an abnormality in the interplay between retinoic acid and sex hormones predisposes an individual to specific molecular malfunctions. In turn, this molecular syndrome generates an altered brain connectivity between the cerebellum, the midbrain dopaminergic areas, and the prefrontal cortex. Lastly, this disconnection would generate specific behavioral traits traditionally involved in ASD. Therefore, this paper represents a step forward in unifying different levels of pathological features into novel integrated testable hypotheses.

Sex Differences in Animal Models: Focus on Addiction

The purpose of this review is to discuss ways to think about and study sex differences in preclinical animal models. We use the framework of addiction, in which animal models have excellent face and construct validity, to illustrate the importance of considering sex differences. There are four types of sex differences: qualitative, quantitative, population, and mechanistic. A better understanding of the ways males and females can differ will help scientists design experiments to characterize better the presence or absence of sex differences in new phenomena that they are investigating. We have outlined major quantitative, population, and mechanistic sex differences in the addiction domain using a heuristic framework of the three established stages of the addiction cycle: binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation. Female rats, in general, acquire the self-administration of drugs and alcohol more rapidly, escalate their drug taking with extended access more rapidly, show more motivational withdrawal, and (where tested in animal models of "craving") show greater reinstatement. The one exception is that female rats show less motivational withdrawal to alcohol. The bases for these quantitative sex differences appear to be both organizational, in that estradiol-treated neonatal animals show the male phenotype, and activational, in that the female phenotype depends on the effects of gonadal hormones. In animals, differences within the estrous cycle can be observed but are relatively minor. Such hormonal effects seem to be most prevalent during the acquisition of drug taking and less influential once compulsive drug taking is established and are linked largely to progesterone and estradiol. This review emphasizes not only significant differences in the phenotypes of females and males in the domain of addiction but emphasizes the paucity of data to date in our understanding of those differences.

Gonadectomy but not biological sex affects burst-firing in dopamine neurons of the ventral tegmental area and in prefrontal cortical neurons projecting to the ventral tegmentum in adult rats

The mesocortical and mesolimbic dopamine systems regulate cognitive and motivational processes and are strongly implicated in neuropsychiatric disorders in which these processes are disturbed. Sex differences and sex hormone modulation are also known for these dopamine-sensitive behaviours in health and disease. One relevant mechanism of hormone impact appears to be regulation of cortical and subcortical dopamine levels. This study asked whether this regulation of dopamine tone is a consequence of sex or sex hormone impact on the firing modes of ventral midbrain dopamine neurons. To address this, single unit extracellular recordings made in the ventral tegmental area and substantia nigra were compared among urethane-anaesthetized adult male, female, gonadectomized male rats. These comparisons showed that gonadectomy had no effect on nigral cells and no effects on pacemaker, bursty, single-spiking or random modes of dopamine activity in the ventral tegmental area. However, it did significantly and selectively increase burst firing in these cells in a testosterone-sensitive, estradiol-insensitive manner. Given the roles of prefrontal cortex (PFC) in modulating midbrain dopamine cell firing, we next asked whether gonadectomy's effects on dopamine cell bursting had correlated effects on the activity of ventral tegmentally projecting prefrontal cortical neurons. We found that gonadectomy indeed significantly and selectively increased burst firing in ventral tegmentally projecting but not neighbouring prefrontal cells. These effects were also androgen-sensitive. Together, these findings suggest a working model wherein androgen influence over the activity of PFC neurons regulates its top-down modulation of mesocortical and mesolimbic dopamine systems and related dopamine-sensitive behaviours.

Making Sense of: Sensitization in Schizophrenia

Sensitization is defined as a process whereby repeated intermittent exposure to a given stimulus results in an enhanced response at subsequent exposures. Next to robust findings of an increased dopamine synthesis capacity in schizophrenia, empirical data and neuroimaging studies support the notion that the mesolimbic dopamine system of patients with schizophrenia is more reactive compared with healthy controls. These studies led to the conceptualization of schizophrenia as a state of endogenous sensitization, as stronger behavioral response and increased dopamine release after amphetamine administration or exposure to stress have been observed in patients with schizophrenia. These findings have also been integrated into the neurodevelopmental model of the disorder, which assumes that vulnerable neuronal circuits undergo progressive changes during puberty and young adulthood that lead to manifest psychosis. Rodent and human studies have made an attempt to identify the exact mechanisms of sensitization of the dopaminergic system and its association with psychosis. Doing so, several epigenetic and molecular alterations associated with dopamine release, neuroplasticity, and cellular energy metabolism have been discovered. Future research aims at targeting these key proteins associated with sensitization in schizophrenia to enhance the knowledge of the pathophysiology of the illness and pave the way for an improved treatment or even prevention of this severe psychiatric disorder.

Toll-like Receptors in the Vascular System: Sensing the Dangers Within

Toll-like receptors (TLRs) are components of the innate immune system that respond to exogenous infectious ligands (pathogen-associated molecular patterns, PAMPs) and endogenous molecules that are released during host tissue injury/death (damage-associated molecular patterns, DAMPs). Interaction of TLRs with their ligands leads to activation of downstream signaling pathways that induce an immune response by producing inflammatory cytokines, type I interferons (IFN), and other inflammatory mediators. TLR activation affects vascular function and remodeling, and these molecular events prime antigen-specific adaptive immune responses. Despite the presence of TLRs in vascular cells, the exact mechanisms whereby TLR signaling affects the function of vascular tissues are largely unknown. Cardiovascular diseases are considered chronic inflammatory conditions, and accumulating data show that TLRs and the innate immune system play a determinant role in the initiation and development of cardiovascular diseases. This evidence unfolds a possibility that targeting TLRs and the innate immune system may be a novel therapeutic goal for these conditions. TLR inhibitors and agonists are already in clinical trials for inflammatory conditions such as asthma, cancer, and autoimmune diseases, but their study in the context of cardiovascular diseases is in its infancy. In this article, we review the current knowledge of TLR signaling in the cardiovascular system with an emphasis on atherosclerosis, hypertension, and cerebrovascular injury. Furthermore, we address the therapeutic potential of TLR as pharmacological targets in cardiovascular disease and consider intriguing research questions for future study.

Prenatal particulate air pollution and neurodevelopment in urban children: Examining sensitive windows and sex-specific associations

BACKGROUND

Brain growth and structural organization occurs in stages beginning prenatally. Toxicants may impact neurodevelopment differently dependent upon exposure timing and fetal sex.

OBJECTIVES

We implemented innovative methodology to identify sensitive windows for the associations between prenatal particulate matter with diameter ≤ 2.5 μm (PM2.5) and children's neurodevelopment.

METHODS

We assessed 267 full-term urban children's prenatal daily PM2.5 exposure using a validated satellite-based spatio-temporally resolved prediction model. Outcomes included IQ (WISC-IV), attention (omission errors [OEs], commission errors [CEs], hit reaction time [HRT], and HRT standard error [HRT-SE] on the Conners' CPT-II), and memory (general memory [GM] index and its components - verbal [VEM] and visual [VIM] memory, and attention-concentration [AC] indices on the WRAML-2) assessed at age 6.5±0.98 years. To identify the role of exposure timing, we used distributed lag models to examine associations between weekly prenatal PM2.5 exposure and neurodevelopment. Sex-specific associations were also examined.

RESULTS

Mothers were primarily minorities (60% Hispanic, 25% black); 69% had ≤12 years of education. Adjusting for maternal age, education, race, and smoking, we found associations between higher PM2.5 levels at 31-38 weeks with lower IQ, at 20-26 weeks gestation with increased OEs, at 32-36 weeks with slower HRT, and at 22-40 weeks with increased HRT-SE among boys, while significant associations were found in memory domains in girls (higher PM2.5 exposure at 18-26 weeks with reduced VIM, at 12-20 weeks with reduced GM).

CONCLUSIONS

Increased PM2.5 exposure in specific prenatal windows may be associated with poorer function across memory and attention domains with variable associations based on sex. Refined determination of time window- and sex-specific associations may enhance insight into underlying mechanisms and identification of vulnerable subgroups.

Counteractive effects of antenatal glucocorticoid treatment on D1 receptor modulation of spatial working memory

RATIONALE

Antenatal exposure to the glucocorticoid dexamethasone dramatically increases the number of mesencephalic dopaminergic neurons in rat offspring. However, the consequences of this expansion in midbrain dopamine (DA) neurons for behavioural processes in adulthood are poorly understood, including working memory that depends on DA transmission in the prefrontal cortex (PFC).

OBJECTIVES

We therefore investigated the influence of antenatal glucocorticoid treatment (AGT) on the modulation of spatial working memory by a D₁ receptor agonist and on D1 receptor binding and DA content in the PFC and striatum.

METHODS

Pregnant rats received AGT on gestational days 16-19 by adding dexamethasone to their drinking water. Male offspring reared to adulthood were trained on a delayed alternation spatial working memory task and administered the partial D₁ agonist SKF38393 (0.3-3 mg/kg) by systemic injection. In separate groups of control and AGT animals, D₁ receptor binding and DA content were measured post-mortem in the PFC and striatum.

RESULTS

SKF38393 impaired spatial working memory performance in control rats but had no effect in AGT rats. D₁ binding was significantly reduced in the anterior cingulate cortex, prelimbic cortex, dorsal striatum and ventral pallidum of AGT rats compared with control animals. However, AGT had no significant effect on brain monoamine levels.

CONCLUSIONS

These findings demonstrate that D₁ receptors in corticostriatal circuitry down-regulate in response to AGT. This compensatory effect in D₁ receptors may result from increased DA-ergic tone in AGT rats and underlie the resilience of these animals to the disruptive effects of D₁ receptor activation on spatial working memory.

Striatal NOS1 has dimorphic expression and activity under stress and nicotine sensitization

Nicotine exerts its addictive influence through the meso-cortico-limbic reward system, where the striatum is essential. Nicotine addiction involves different neurotransmitters, nitric oxide (NO) being especially important, since it triggers the release of the others by positive feedback. In the nervous system, NO is mainly produced by nitric oxide synthase 1 (NOS1). However, other subtypes of synthases can also synthesize NO, and little is known about the specific role of each isoform in the process of addiction. In parallel, NOS activity and nicotine addiction are also affected by stress and sexual dimorphism. To determine the specific role of this enzyme, we analyzed both NOS expression and NO synthesis in the striatum of wild-type and NOS1-knocked out (KO) mice of both sexes in situations of nicotine sensitization and stress. Our results demonstrated differences between the caudate-putamen (CP) and nucleus accumbens (NA). With respect to NOS1 expression, the CP is a dimorphic region (27.5% lower cell density in males), but with a stable production of NO, exclusively due to this isoform. Thus, the nitrergic system of CP may not be involved in stress or nicotine addiction. Conversely, the NA is much more variable and strongly involved in both situations: its NO synthesis displays dimorphic variations at both basal (68.5% reduction in females) and stress levels (65.9% reduction in males), which disappear when nicotine is infused. Thus, the KO animals showed an increase in NO production (21.7%) in the NA, probably by NOS3, in an attempt to compensate the lack of NOS1.

Biological factors underlying sex differences in neurological disorders

The prevalence, age of onset, pathophysiology, and symptomatology of many neurological and neuropsychiatric conditions differ significantly between males and females. Females suffer more from mood disorders such as depression and anxiety, whereas males are more susceptible to deficits in the dopamine system including Parkinson's disease (PD), attention-deficit hyperactivity disorder (ADHD), schizophrenia, and autism spectrum disorders (ASD). Until recently, these sex differences have been explained solely by the neuroprotective actions of sex hormones in females. Emerging evidence however indicates that the sex chromosome genes (i.e. X- and Y-linked genes) also contribute to brain sex differences. In particular, the Y-chromosome gene, SRY (Sex-determining Region on the Y chromosome) is an interesting candidate as it is expressed in dopamine-abundant brain regions, where it regulates dopamine biosynthesis and dopamine-mediated functions such as voluntary movement in males. Furthermore, SRY expression is dysregulated in a toxin-induced model of PD, suggesting a role for SRY in the pathogenesis of dopamine cells. Taken together, these studies highlight the importance of understanding the interplay between sex-specific hormones and sex-specific genes in healthy and diseased brain. In particular, better understanding of regulation and function of SRY in the male brain could provide entirely novel and important insights into genetic factors involved in the susceptibility of men to neurological disorders, as well as development of novel sex-specific therapies.

Reinforcement learning in depression: A review of computational research

Despite being considered primarily a mood disorder, major depressive disorder (MDD) is characterized by cognitive and decision making deficits. Recent research has employed computational models of reinforcement learning (RL) to address these deficits. The computational approach has the advantage in making explicit predictions about learning and behavior, specifying the process parameters of RL, differentiating between model-free and model-based RL, and the computational model-based functional magnetic resonance imaging and electroencephalography. With these merits there has been an emerging field of computational psychiatry and here we review specific studies that focused on MDD. Considerable evidence suggests that MDD is associated with impaired brain signals of reward prediction error and expected value ('wanting'), decreased reward sensitivity ('liking') and/or learning (be it model-free or model-based), etc., although the causality remains unclear. These parameters may serve as valuable intermediate phenotypes of MDD, linking general clinical symptoms to underlying molecular dysfunctions. We believe future computational research at clinical, systems, and cellular/molecular/genetic levels will propel us toward a better understanding of the disease.

Sex-specific disruption of murine midbrain astrocytic and dopaminergic developmental trajectories following antenatal GC treatment

The mammalian midbrain dopaminergic systems arising in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) are critical for coping behaviours and are implicated in neuropsychiatric disorders where early life challenges comprise significant risk factors. Here, we aimed to advance our hypothesis that glucocorticoids (GCs), recognised key players in neurobiological programming, target development within these systems, with a novel focus on the astrocytic population. Mice received antenatal GC treatment (AGT) by including the synthetic GC, dexamethasone, in the mothers' drinking water on gestational days 16-19; controls received normal drinking water. Analyses of regional shapes and volumes of the adult SNc and VTA demonstrated that AGT induced long-term, dose-dependent, structural changes that were accompanied by profound effects on astrocytes (doubling/tripling of numbers and/or density). Additionally, AGT induced long-term changes in the population size and distribution of SNc/VTA dopaminergic neurons, confirming and extending our previous observations made in rats. Furthermore, glial/neuronal structural remodelling was sexually dimorphic and depended on the AGT dose and sub-region of the SNc/VTA. Investigations within the neonatal brain revealed that these long-term organisational effects of AGT depend, at least in part, on targeting perinatal processes that determine astrocyte density and programmed cell death in dopaminergic neurons. Collectively, our characterisation of enduring, AGT-induced, sex-specific cytoarchitectural disturbances suggests novel mechanistic links for the strong association between early environmental challenge (inappropriate exposure to excess GCs) and vulnerability to developing aberrant behaviours in later life, with translational implications for dopamine-associated disorders (such as schizophrenia, ADHD, autism, depression), which typically show a sex bias.

Sex Differences and the Impact of Chronic Stress and Recovery on Instrumental Learning

We have previously shown that 21-day chronic restraint stress impacts instrumental learning, but overall few studies have examined sex differences on the impact of stress on learning. We further examined sex differences in response to extended 42-day chronic stress on instrumental learning, as well as recovery from chronic stress. Rats were tested in aversive training tasks with or without prior appetitive experience, and daily body weight data was collected as an index of stress. Relative to control animals, reduced body weight was maintained from day 22 through day 42 across the stress period for males, but not for females. Stressed males had increased response speed and lower learning efficiency during appetitive acquisition and aversive learning. Males overall showed slower escape shaping times and more shock exposure. In contrast, stressed females showed slower appetitive response speeds and higher appetitive and aversive efficiency but overall reduced avoidance rates during acquisition and maintenance for transfer animals and during maintenance for aversive-only animals. These tasks reveal important nuances on the effect of stress on goal-directed behavior and further highlight sexually divergent effects on appetitive versus aversive motivation. Furthermore, these data underscore that systems are temporally impacted by chronic stress in a sexually divergent pattern.

Sex differences in PTSD resilience and susceptibility: Challenges for animal models of fear learning

PTSD occurs in only a small fraction of trauma-exposed individuals, but risk is twice as high in women as in men. The neurobiological basis for this discrepancy is not known, but the identification of biological determinants of resilience and susceptibility in each sex could lead to more targeted preventions and treatments. Animal models are a useful tool for dissecting the circuits and mechanisms that underlie the brain's response to stress, but the vast majority of this work has been developed and conducted in males. The limited work that does incorporate female animals is often inconsistent across labs and does not broadly reflect human populations in terms of female susceptibility to PTSD-like behaviors. In this review, we suggest that interpreting male vs. female comparisons in these models be approached carefully, since common behavioral outcome measures may in fact reflect distinct neural processes. Moreover, since the factors that determine resilience and susceptibility are likely at least in part distinct in men and women, models that take a within-sex approach to response variability may be more useful in identifying critical mechanisms for manipulation.

Embryological exposure to valproic acid disrupts morphology of the deep cerebellar nuclei in a sexually dimorphic way

Autism spectrum disorders (ASD) is diagnosed in males at a much higher rate than females. For this reason, the majority of autism research has used male subjects exclusively. However; more recent studies using genetic sex as a factor find that the development of the male and female brain is differentially affected by ASD. That is, the natural sex-specific differences that exist between male and female brains lead to sexually dimorphic expressions of autism. Here we investigate the putative sexual dimorphism that exists in the deep cerebellar nuclei of male and female rats exposed to valproic acid (VPA) on embryological day 12.5. We find natural sex-specific differences in adult nucleus area, length, and estimated cell populations. Therefore VPA exposure during embryology creates some sex-specific deficits such as higher cell counts in the VPA males and lower cell counts in the VPA females. At the same time, some effects of VPA exposure occur regardless of sex. That is, smaller nucleus area and length lead to truncated nuclei in both VPA males and females. These deficits are more pronounced in the VPA males suggesting that genetic sex could play a role in teratogenic susceptibility to VPA. Taken together our results suggests that VPA exposure induces sexually dimorphic aberrations in morphological development along a mediolateral gradient at a discrete region of the hindbrain approximate to rhombomere (R) 1 and 2. Sex-specific disruption of the local and long-range projections emanating from this locus of susceptibility could offer a parsimonious explanation for the brain-wide neuroanatomical variance reported in males and females with ASD.

The ventral tegmentum and dopamine: A new wave of diversity

Projection systems arising from the ventral tegmental area (VTA) and the substantia nigra (SN) have a critical role in a broad range of functions, as well as in the etiology, symptoms and treatment of neurological and psychiatric diseases. Mostly studied for its dopamine neurons, the ventral tegmentum is in fact heterogeneous at cellular and functional levels. This special issue of Neuroscience gathered some experts in the field to review the connectivity of the ventral mesencephalic dopaminergic complex, its cellular heterogeneity with attention given to glutamate neurons, the D2 autoreceptor and the cholinergic controls of dopamine activity, the influence of neurotrophins, the controls of bursting activity and the heterogeneity of neuronal activity across traits and states, the pedunculopontine tegmental and the sensory controls of dopamine activity, the sex-dependent diversity, the links between circadian and dopamine systems, the functional antero-posterior heterogeneity of the VTA and the role of its GABA tail (tVTA/rostromedial tegmental nucleus (RMTg)), the functional heterogeneity of the VTA outputs, the place of dopamine in cortico-basal ganglia circuitry, the different roles of the D1 and D2 striatal pathways and the role of dopamine in associative learning and memory. Recent progress also highlights the need for molecular markers of functional subpopulations within the ventral tegmentum, for deeper developmental knowledge of this region, and for a single cell level of connectomic. It also raises the question of inter-individual, sex, strain and species heterogeneity, and conversely the question of data generalization in a context of human pathology models, which warrant comparative studies and translational effort.

The antero-posterior heterogeneity of the ventral tegmental area

The ventral tegmental area (VTA) is a brain region processing salient sensory and emotional information, controlling motivated behaviors, natural or drug-related reward, reward-related learning, mood, and participating in their associated psychopathologies. Mostly studied for its dopamine neurons, the VTA also includes functionally important GABA and glutamate cell populations. Behavioral evidence supports the presence of functional differences between the anterior VTA (aVTA) and the posterior VTA (pVTA), which is the topic of this review. This antero-posterior heterogeneity concerns locomotor activity, conditioned place preference and intracranial self-administration, and can be seen in response to ethanol, acetaldehyde, salsolinol, opioids including morphine, cholinergic agonists including nicotine, cocaine, cannabinoids and after local manipulation of GABA and serotonin receptors. It has also been observed after viral-mediated manipulation of GluR1, phospholipase Cγ (PLCγ) and cAMP response element binding protein (CREB) expression, with impact on reward and aversion-related responses, on anxiety and depression-related behaviors and on pain sensitivity. In this review, the substrates potentially underlying these aVTA/pVTA differences are discussed, including the VTA sub-nuclei and the heterogeneity in connectivity, cell types and molecular characteristics. We also review the role of the tail of the VTA (tVTA), or rostromedial tegmental nucleus (RMTg), which may also participate to the observed antero-posterior heterogeneity of the VTA. This region, partly located within the pVTA, is an inhibitory control center for dopamine activity. It controls VTA and substantia nigra dopamine cells, thus exerting a major influence on basal ganglia functions. This review highlights the need for a more comprehensive analysis of VTA heterogeneity.