Cocaine Reduces the Neuronal Population While Upregulating Dopamine D2-Receptor-Expressing Neurons in Brain Reward Regions: Sex-Effects

Brain dysfunctions and neurotoxicity induced by psychostimulants in experimental models and humans: an overview of recent findings

Preclinical and clinical studies indicate that psychostimulants, in addition to having abuse potential, may elicit brain dysfunctions and/or neurotoxic effects. Central toxicity induced by psychostimulants may pose serious health risks since the recreational use of these substances is on the rise among young people and adults. The present review provides an overview of recent research, conducted between 2018 and 2023, focusing on brain dysfunctions and neurotoxic effects elicited in experimental models and humans by amphetamine, cocaine, methamphetamine, 3,4-methylenedioxymethamphetamine, methylphenidate, caffeine, and nicotine. Detailed elucidation of factors and mechanisms that underlie psychostimulant-induced brain dysfunction and neurotoxicity is crucial for understanding the acute and enduring noxious brain effects that may occur in individuals who use psychostimulants for recreational and/or therapeutic purposes.

Psychostimulants and social behaviors

Mounting evidence from animal models and human studies indicates that psychostimulants can significantly affect social behaviors. This is not surprising considering that the neural circuits underlying the regulation and expression of social behaviors are highly overlapped with those targeted by psychostimulants, which in most cases have strong rewarding and, consequently, addictive properties. In the present work, we provide an overview regarding the effects of illicit and prescription psychostimulants, such as cocaine, amphetamine-type stimulants, methylphenidate or modafinil, upon social behaviors such as social play, maternal behavior, aggression, pair bonding and social cognition and how psychostimulants in both animals and humans alter them. Finally, we discuss why these effects can vary depending on numerous variables such as the type of drug considered, acute versus long-term use, clinical versus recreational consumption, or the presence or absence of concomitant risk factors.

Decoupling Dopamine Synthesis from Impulsive Action, Risk-Related Decision-Making, and Propensity to Cocaine Intake: A Longitudinal [18F]-FDOPA PET Study in Roman High- and Low-Avoidance Rats

Impulsive action and risk-related decision-making (RDM) are two facets of impulsivity linked to a hyperdopaminergic release in the striatum and an increased propensity to cocaine intake. We previously showed that with repeated cocaine exposure, this initial hyperdopaminergic release is blunted in impulsive animals, potentially signaling drug-induced tolerance. Whether such dopaminergic dynamics involve changes in dopamine (DA) synthesis as a function of impulsivity is currently unknown. Here, we investigated the predictive value of DA synthesis for impulsive action, RDM, and the propensity to take cocaine in a rat model of vulnerability to cocaine abuse. Additionally, we assessed the effects of cocaine intake on these variables. Rats were tested sequentially in the rat Gambling Task (rGT) and were scanned with positron emission tomography and [18F]-FDOPA to respectively assess both impulsivity facets and striatal DA synthesis before and after cocaine self-administration (SA). Our results revealed that baseline striatal levels of DA synthesis did not significantly predict impulsive action, RDM, or a greater propensity to cocaine SA in impulsive animals. Besides, we showed that impulsive action, but not RDM, predicted higher rates of cocaine taking. However, chronic cocaine exposure had no impact on DA synthesis, nor affected impulsive action and RDM. These findings indicate that the hyper-responsive DA system associated with impulsivity and a propensity for cocaine consumption, along with the reduction in this hyper-responsive DA state in impulsive animals with a history of cocaine use, might not be mediated by dynamic changes in DA synthesis.

Neurogenetics and Epigenetics of Loneliness

Loneliness, an established risk factor for both, mental and physical morbidity, is a mounting public health concern. However, the neurobiological mechanisms underlying loneliness-related morbidity are not yet well defined. Here we examined the role of genes and associated DNA risk polymorphic variants that are implicated in loneliness via genetic and epigenetic mechanisms and may thus point to specific therapeutic targets. Searches were conducted on PubMed, Medline, and EMBASE databases using specific Medical Subject Headings terms such as loneliness and genes, neuro- and epigenetics, addiction, affective disorders, alcohol, anti-reward, anxiety, depression, dopamine, cancer, cardiovascular, cognitive, hypodopaminergia, medical, motivation, (neuro)psychopathology, social isolation, and reward deficiency. The narrative literature review yielded recursive collections of scientific and clinical evidence, which were subsequently condensed and summarized in the following key areas: (1) Genetic Antecedents: Exploration of multiple genes mediating reward, stress, immunity and other important vital functions; (2) Genes and Mental Health: Examination of genes linked to personality traits and mental illnesses providing insights into the intricate network of interaction converging on the experience of loneliness; (3) Epigenetic Effects: Inquiry into instances of loneliness and social isolation that are driven by epigenetic methylations associated with negative childhood experiences; and (4) Neural Correlates: Analysis of loneliness-related affective states and cognitions with a focus on hypodopaminergic reward deficiency arising in the context of early life stress, eg, maternal separation, underscoring the importance of parental support early in life. Identification of the individual contributions by various (epi)genetic factors presents opportunities for the creation of innovative preventive, diagnostic, and therapeutic approaches for individuals who cope with persistent feelings of loneliness. The clinical facets and therapeutic prospects associated with the current understanding of loneliness, are discussed emphasizing the relevance of genes and DNA risk polymorphic variants in the context of loneliness-related morbidity.

The gut-brain axis mediates bacterial driven modulation of reward signaling

OBJECTIVE

Our goal is to investigate if microbiota composition modulates reward signaling and assess the role of the vagus in mediating microbiota to brain communication.

METHODS

Male germ-free Fisher rats were colonized with gastrointestinal contents from chow (low fat (LF) ConvLF) or HF (ConvHF) fed rats.

RESULTS

Following colonization, ConvHF rats consumed significantly more food than ConvLF animals. ConvHF rats displayed lower feeding-induced extracellular DOPAC levels (a metabolite of dopamine) in the Nucleus Accumbens (NAc) as well as reduced motivation for HF foods compared to ConvLF rats. Dopamine receptor 2 (DDR2) expression levels in the NAc were also significantly lower in ConvHF animals. Similar deficits were observed in conventionally raised HF fed rats, showing that diet-driven alteration in reward can be initiated via microbiota. Selective gut to brain deafferentation restored DOPAC levels, DRD2 expression, and motivational drive in ConvHF rats.

CONCLUSIONS

We concluded from these data that a HF-type microbiota is sufficient to alter appetitive feeding behavior and that bacteria to reward communication is mediated by the vagus nerve.

Are There Prevalent Sex Differences in Psychostimulant Use Disorder? A Focus on the Potential Therapeutic Efficacy of Atypical Dopamine Uptake Inhibitors

Psychostimulant use disorders (PSUD) affect a growing number of men and women and exert sizable public health and economic burdens on our global society. Notably, there are some sex differences in the onset of dependence, relapse rates, and treatment success with PSUD observed in preclinical and clinical studies. The subtle sex differences observed in the behavioral aspects of PSUD may be associated with differences in the neurochemistry of the dopaminergic system between sexes. Preclinically, psychostimulants have been shown to increase synaptic dopamine (DA) levels and may downregulate the dopamine transporter (DAT). This effect is greatest in females during the high estradiol phase of the estrous cycle. Interestingly, women have been shown to be more likely to begin drug use at younger ages and report higher levels of desire to use cocaine than males. Even though there is currently no FDA-approved medication, modafinil, a DAT inhibitor approved for use in the treatment of narcolepsy and sleep disorders, has shown promise in the treatment of PSUD among specific populations of affected individuals. In this review, we highlight the therapeutic potential of modafinil and other atypical DAT inhibitors focusing on the lack of sex differences in the actions of these agents.

THC exposure during adolescence increases impulsivity-like behavior in adulthood in a WIN 55,212-2 self-administration mouse model

Background

Cannabis addiction is a chronically relapsing disorder lacking effective treatment. Regular cannabis consumption typically begins during adolescence, and this early cannabinoid exposure may increase the risk for drug addiction in adulthood.

Objective

This study investigates the development of cannabis addiction-like behavior in adult mice after adolescent exposure to the main psychoactive component of cannabis, Δ⁹-tetrahydrocannabinol (THC).

Methods

Adolescent male mice were exposed to 5 mg/kg of THC from postnatal days 37 to 57. Operant self-administration sessions of WIN 55,212-2 (12.5 μg/kg/infusion) were conducted for 10 days. Mice were tested for three addiction-like criteria (persistence of response, motivation, and compulsivity), two parameters related to craving (resistance to extinction and drug-seeking behavior), and two phenotypic vulnerability traits related to substance use disorders (impulsivity and reward sensitivity). Additionally, qPCR assays were performed to detect differentially expressed genes in medial prefrontal cortex (mPFC), nucleus accumbens (NAc), dorsal striatum, and hippocampus (HPC) of "addicted" and "non-addicted" mice.

Results

Adolescent THC exposure did not modify WIN 55,212-2 reinforcement nor the development of cannabis addiction-like behavior. Inversely, THC pre-exposed mice displayed impulsive-like behavior in adulthood, which was more pronounced in mice that developed the addiction-like criteria. Moreover, downregulated drd2 and adora2a gene expression in NAc and HPC was revealed in THC pre-exposed mice, as well as a downregulation of drd2 expression in mPFC of vehicle pre-treated mice that developed addiction-like behaviors.

Discussion

These findings suggest that adolescent THC exposure may promote impulsivity-like behavior in adulthood, associated with downregulated drd2 and adora2a expression in NAc and HPC.

Unraveling Psychiatric Disorders through Neural Single-Cell Transcriptomics Approaches

The development of single-cell and single-nucleus transcriptome technologies is enabling the unraveling of the molecular and cellular heterogeneity of psychiatric disorders. The complexity of the brain and the relationships between different brain regions can be better understood through the classification of individual cell populations based on their molecular markers and transcriptomic features. Analysis of these unique cell types can explain their involvement in the pathology of psychiatric disorders. Recent studies in both human and animal models have emphasized the importance of transcriptome analysis of neuronal cells in psychiatric disorders but also revealed critical roles for non-neuronal cells, such as oligodendrocytes and microglia. In this review, we update current findings on the brain transcriptome and explore molecular studies addressing transcriptomic alterations identified in human and animal models in depression and stress, neurodegenerative disorders (Parkinson's and Alzheimer's disease), schizophrenia, opioid use disorder, and alcohol and psychostimulant abuse. We also comment on potential future directions in single-cell and single-nucleus studies.

Cocaine's effects on the reactivity of the medial prefrontal cortex to ventral tegmental area stimulation: optical imaging study in mice

BACKGROUND AND AIMS

The prefrontal cortex (PFC) is modulated by dopaminergic and glutamatergic neurons that project from the ventral tegmental area (VTA) and disruption of this modulation might facilitate impulsive behaviors during cocaine intoxication. Here, we assessed the effects of acute cocaine (30 mg/kg, i.p.) on the reactivity of the PFC to VTA stimulation.

METHODS

Using a genetically encoded calcium indicator (GCaMP6f), we optically imaged the neuronal Ca2+ reactance in medial PFC (mPFC) in response to 'tonic-like' (5 Hz) and 'phasic-like' (50 Hz) electrical VTA stimulation. The high temporal and spatial resolutions of our optical system allowed us to capture single Ca2+ neuronal transients from individual stimuli with 'tonic-like' stimulation and to visualize single neuronal activation evoked by 'phasic-like' VTA stimulation.

RESULTS

'Tonic-like' VTA stimulation induced a rapid increase in mean neuronal Ca2+ in mPFC followed by a plateau and recovery upon termination of stimulation. After cocaine, the mPFC sensitivity to 'tonic-like' VTA stimulation was attenuated, with a 50.4% reduction (P = 0.03) in the number of Ca2+ transients corresponding to single electrical stimuli but the recovery time was lengthened (4.30 ± 0.25 sec to 5.41 ± 0.24 sec, P = 0.03). 'Phasic-like' stimulation evoked a rapid Ca2+ fluorescence increase in mPFC with an immediate decay process, and while cocaine did not affect the peak response (7.17 ± 1.07% versus 7.13 ± 0.96%, P = 0.98) it shortened the recovery time to baseline (3.27 ± 0.11 sec versus 2.38 ± 0.23 sec, P = 0.005).

CONCLUSIONS

Acute cocaine impairs reactivity of medial prefrontal cortex (mPFC) to ventral tegmental area stimulation, decreasing its sensitivity to 'tonic-like' stimulation and lengthening the recovery time to return to baseline while shortening it for phasic stimulation. These changes in mPFC might contribute to cocaine binging during intoxication.

Memantine Attenuates Cocaine and neuroHIV Neurotoxicity in the Medial Prefrontal Cortex

Individuals with substance use disorder are at a higher risk of contracting HIV and progress more rapidly to AIDS as drugs of abuse, such as cocaine, potentiate the neurotoxic effects of HIV-associated proteins including, but not limited to, HIV-1 trans-activator of transcription (Tat) and the envelope protein Gp120. Neurotoxicity and neurodegeneration are hallmarks of HIV-1-associated neurocognitive disorders (HANDs), which are hypothesized to occur secondary to excitotoxicity from NMDA-induced neuronal calcium dysregulation, which could be targeted with NMDA antagonist drugs. Multiple studies have examined how Gp120 affects calcium influx and how cocaine potentiates this influx; however, they mostly focused on single cells and did not analyze effects in neuronal and vascular brain networks. Here, we utilize a custom multi-wavelength imaging platform to simultaneously study the neuronal activity (detected using genetically encoded Ca²⁺ indicator, GcaMP6f, expressed in neurons) and hemodynamic changes (measured by total hemoglobin and oxygenated hemoglobin within the tissue) in the prefrontal cortex (PFC) of HIV-1 Tg rats in response to cocaine and evaluate the effects of the selective NMDA antagonist drug memantine on cocaine and HIV neurotoxicity compared to those of non-HIV-1 Tg animals (controls). Our results show that memantine improved cocaine-induced deficit in cerebral blood volume while also attenuating an abnormal increase of the neuronal calcium influx and influx duration in both control rats and HIV-1 Tg rats. Cocaine-induced neuronal and hemodynamic dysregulations were significantly greater in HIV-1 Tg rats than in control rats. With memantine pretreatment, HIV-1 Tg rats showed attenuated cocaine’s effects on neuronal and hemodynamic responses, with responses similar to those observed in control rats. These imaging results document an enhancement of neuronal Ca²⁺ influx, hypoxemia, and ischemia with cocaine in the PFC of HIV-1 Tg rats that were attenuated by memantine pretreatment. Thus, the potential utility of memantine in the treatment of HAND and of cocaine-induced neurotoxicity deserves further investigation.

Psychomotor Symptoms in Chronic Cocaine Users: An Interpretative Model

According to the latest estimates, there are around 24.6 million cocaine users worldwide, and it is estimated that around a quarter of the population worldwide has used cocaine at some point in their lifetime. It follows that such widespread consumption represents a major risk for public health. Long-term use of cocaine, in addition to being related to many cerebral and cardiovascular diseases, is increasingly associated with a higher incidence of psychomotor symptoms and neurodegenerative disorders. In recent years, numerous studies have shown an increased risk of antipsychotic-induced extrapyramidal symptoms (EPSs) in patients with psychotic spectrum disorders comorbid with psychostimulant misuse, particularly of cocaine. In the present paper, we describe the case of a young patient on his first entry into a psychiatric setting with previous cocaine misuse who rapidly presented psychomotor symptoms and was poorly responsive to symptomatic therapy consisting of benzodiazepines and anticholinergics, in relation to the introduction of various antipsychotics (first, second, and third generation). Furthermore, we propose neurobiological mechanisms underlying the hypothesized increased vulnerability to psychomotor symptoms in chronic cocaine abusers. Specifically, we supposed that the chronic administration of cocaine produces important neurobiological changes, causing a complex dysregulation of various neurotransmitter systems, mainly affecting subcortical structures and the dopaminergic and glutamatergic pathways. We believe that a better understanding of these neurochemical and neurobiological processes could have useful clinical and therapeutic implications by providing important indications to increase the risk–benefit ratio in pharmacological choice in patients with psychotic spectrum disorders comorbid with a substance use disorder.

An Increase in Peripheral Temperature following Cocaine Administration Is Mediated through Activation of Dopamine D2 Receptor in Rats

Drug addiction has become a worldwide problem, affecting millions of people across the globe. While the majority of mechanistic studies on drug addiction have been focused on the central nervous system, including the mesolimbic dopamine system, the peripheral actions of drugs of abuse remain largely unknown. Our preliminary study found that the systemic injection of cocaine increased peripheral skin temperature. This led us to our present study, which investigated the mechanisms underlying the increase in peripheral temperature following cocaine injection. Male Sprague Dawley rats were anesthetized with pentobarbital sodium, and peripheral skin temperature measurements were taken using a thermocouple needle microprobe and an infrared thermal camera. Cocaine injection caused an acute rise in peripheral body temperature, but not core body temperature, about 10 min after injection, and the temperature increases were occluded by systemic injection of dopamine D2 receptor antagonist L741,626, but not D1 receptor antagonist SCH23390. In addition, systemic administration of bromocriptine, a dopamine D2 receptor agonist, significantly increased peripheral temperature. Infrared thermal imaging showed that the thermal increases following cocaine injection were predominantly in the distal areas of the forelimbs and hindlimbs, relative to core body temperature. Treatment with cocaine or bromocriptine decreased the size of skin blood vessels without affecting the expression of dopamine D2 receptors. These results suggest that increased peripheral temperature in skin following cocaine injection is associated with the activation of the dopamine D2 receptor.

Sex Differences in Dopamine Receptors and Relevance to Neuropsychiatric Disorders

Dopamine is an important neurotransmitter that plays a key role in neuropsychiatric illness. Sex differences in dopaminergic signaling have been acknowledged for decades and have been linked to sex-specific heterogeneity in both dopamine-related behaviours as well as in various neuropsychiatric disorders. However, the overall number of studies that have evaluated sex differences in dopamine signaling, both in health and in these disorders, is low. This review will bring together what is known regarding sex differences in innate dopamine receptor expression and function, as well as highlight the known sex-specific roles of dopamine in addiction, depression, anxiety, schizophrenia, and attention deficit hyperactivity disorder. Due to differences in prognosis, diagnosis, and symptomatology between male and female subjects in disorders that involve dopamine signaling, or in responses that utilize pharmacological interventions that target dopamine receptors, understanding the fundamental sex differences in dopamine receptors is of vital importance for the personalization of therapeutic treatment strategies.

Ca2+ channel blockade reduces cocaine's vasoconstriction and neurotoxicity in the prefrontal cortex

Cocaine profoundly affects both cerebral blood vessels and neuronal activity in the brain. The vasoconstrictive effects of cocaine, concurrently with its effects on neuronal [Ca²⁺]_i accumulation are likely to jeopardize neuronal tissue that in the prefrontal cortex (PFC) could contribute to impaired self-regulation and compulsive cocaine consumption. Here we used optical imaging to study the cerebrovascular and neuronal effects of acute cocaine (1 mg/kg i.v.) and to examine whether selective blockade of L-type Ca²⁺ channels by Nifedipine (NIF) (0.5 mg/kg i.v.) would alleviate cocaine's effects on hemodynamics (measured with cerebral blood volume, HbT), oxygenation (measured with oxygenated hemoglobin, HbO₂) and neuronal [Ca²⁺]_i, which were concomitantly measured in the PFC of naive rats. Our results show that in the PFC acute cocaine significantly reduced flow delivery (HbT), increased neuronal [Ca²⁺]_i accumulation and profoundly reduced tissue oxygenation (HbO₂) and these effects were significantly attenuated by NIF pretreatment. They also show that cocaine-induced vasoconstriction is distinct from its increase of neuronal [Ca²⁺]_i accumulation though both of them contribute to hypoxemia and both effects were attenuated by NIF. These results provide evidence that blockade of voltage-gated L-type Ca²⁺ channels might be beneficial in preventing vasoconstriction and neurotoxic effects of cocaine and give support for further clinical investigations to determine their value in reducing cocaine's neurotoxicity in cocaine use disorders.