Teratogenic effects of maternal drug abuse on developing brain and underlying neurotransmitter mechanisms

Abstract

The aim of this review is to highlight our knowledge of the various drugs of abuse that can prove potential teratogens affecting the brain and cognitive development in an individual exposed to maternal consumption of such agents. Among several drugs of abuse in women, we specifically highlighted the commonly used alcohol, nicotine, opioids, cannabis, cocaine and marijuana. These drugs can affect the fetal development and slow the cognitive maturation apart from physical disabilities. However, no known therapy exists to counter the toxic potential of these drugs. Several researchers used animal models of drug abuse to understand the underlying mechanisms affecting brain development and the relevant neurotransmitter system. Identifying such targets can potentially help in drug discovery research. We reported in depth analysis of such mechanisms and discussed the potential targets for drug development research.

Introduction

Teratogens are agents that affect the normal fetal development and subsequently cause cognitive deficits in an individual.They can be classified into six different groups, namely: ionizing radiation, chemical substances, infectious agents, nutritional or metabolic factors, autoimmune reactions, and factors linked to the increase in maternal age (Cerrizuela et al., 2020). During pregnancy many women take medications for health reasons and their effect on fetal development may still be unknown. Often many drugs are abused during pregnancy which are considered as potential teratogens. The extent of damage depends on the maternal metabolism, pharmacodynamics of the drug, innate resistance of the fetus to the agent and the time of exposure or the dose of the agent (Polifka and Friedman, 1991).

Among the many drugs of abuse that are prevalent in western world, we focused our review on the teratogenic potential of alcohol, nicotine, opioids, cocaine, and marijuana. Data collected from a national survey conducted in 2012 showed that in the United States, a total of 5.9 % of women who are pregnant use illicit drugs, exposing over 380,000 infants to illicit substances. Of these pregnant women, 15.9 % smoke cigarettes and 8.5 % drink alcohol, causing over one million infants to be exposed to tobacco and over 550,000 infants to be exposed to alcohol in utero (United States Department Of Health And Human Services. Substance Abuse And Mental Health Services Administration. Center For Behavioral Health Statistics And Quality, 2014).Their use among pregnant women show a slow decline over the past few years (Table 1). Many individuals conceive under the influence of drug abuse such as “unintended alcohol exposed pregnancy or AEP ” and they sometimes lack the knowledge of its effect on the fetal development (Cannon et al., 2015). The report also suggested around 1 in 30 of all non-pregnant women are at risk of AEP. Consumption of such agents irrespective of the stage of pregnancy may be harmful for fetal development. Rapid brain development occurs during fetal and early neonatal life during which the brain volume increases and maturation of cognitive abilities take place (Song et al., 2017). Prenatal exposure of teratogens may impede these developmental timeframes and dramatically impair the cognitive development in the child. Each brain region has its own timeline and rate of development in a normal environment (Arias et al., 2015). With exposure to potential teratogens, the rate of neuronal maturation and functions potentially gets affected leading to cognitive slowdown or decline (Weiss et al., 2007). There is no effective treatment to date to prevent the teratogenic effect of these agents on the developing fetus. Understanding the underlying mechanism of neuronal plasticity impairment from fetal toxicity by these agents would help us focus on treatment strategies and identify target molecules for effective drug development improving learning and memory in these patients. Indeed, studies have indicated critical periods during brain development that are usually affected by acute exposure of the teratogen (Georgieff et al., 2018). Long term effects can be the sharp decline in quality of life, financial hardships and increased medical costs and social burden associated with care of these individuals (Shankaran et al., 2007). Recent surge in substance use have emphasized the need to study the complex brain developmental pathways that are potentially targeted by substance use.

We know these drugs affect the fetal brain development and impair cognitive maturity in the prenatally exposed individual. We also know that limited studies are available that focus on the mechanisms of these drugs in the fetal environment using animal models (Ross et al., 2015). In this review, we will broadly focus on potential prenatal mechanisms associated with drugs of abuse influencing brain development and cognitive functions in the offspring exposed during the fetal stage. This review will also benefit future studies identifying specific drug targets and improving treatment outcomes.

Alcohol use in pregnancy has been correlated with an increased risk of miscarriage, stillbirth and infant mortality, various congenital anomalies, and low birthweight. The effect of alcohol on fetal brain development has been a topic of research for decades. Drinking during pregnancy leads to a range of mental and physical defects in the child which is known under an umbrella term called Fetal Alcohol Spectrum Disorder (FASD) (Becker et al., 1990). FASD encompasses Fetal Alcohol Syndrome, partial Fetal Alcohol Syndrome, and brain developmental disorders and birth defects related to alcohol consumption during pregnancy. According to the Centers for Disease Control and Prevention (CDC), FASD is very prevalent in the US to a maximum of 1.5–2.0 cases per 1000 births. The National Survey on Drug Use and Health (NSDUH) predicted from their survey (2011–2012) that around 8.5 % of pregnant women in the US (aged between 15 and 44) consumed alcohol during pregnancy. Drinking any form of alcohol is not safe at any stage of pregnancy. There are various long-term deficits associated with prenatal drinking, some of which are dampened speech and language outcomes, executive functioning deficits, and various cognitive and behavioral challenges throughout adolescence and adulthood (Forray, 2016). Still, the major cause of FASD remains unknown (U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES). Alcohol is therefore a known teratogen whose use in pregnancy impacts the normal development of human fetuses. This promotes serious developmental alterations and generates a wide range of physical, behavioral, cognitive, and neurological impairments (Almeida et al., 2020).

Alcohol has been found to adversely impact brain development by reducing the size of the brain, particularly the hippocampus, and decreasing the number of neuronal cells (Golub et al., 2015). Quantitative structural magnetic resource imaging (MRI) studies of patients with prenatal exposure to alcohol, often with FASD, have consistently reported reductions of total brain, white matter, and gray matter with larger proportional reductions in specific deep gray matter structures such as the caudate and putamen (Little and Beaulieu, 2020). Studies show alcohol may also decrease the total number of neurons and glial cells in barrel cortex brain development (Sabzalizadeh et al., 2020). Alcohol also causes a significant reduction in the number of dorsal hippocampal neurons and impairs long term potentiation (LTP) in the fetal alcohol rodent model (Gil-Mohapel et al., 2010; Shimono et al., 2002). This is also reflected in behavioral assessment with spatial memory tasks in the offspring (Toosi et al., 2019). Similarly, in humans, fetal alcohol exposure yields reduced intelligence and cognitive abilities in the individual. Patients, as they age, have shown shrinkage in corpus callosum, caudate, and cerebellar volumes compared to age matched controls (Inkelis et al., 2020). The effect of prenatal alcohol exposure on structural shrinkage of brain areas may indicate perturbation of certain epigenetic factor regulation. It is not very clear if certain regions are resistant to the teratogenic effect of alcohol or if any of the affected regions have the capacity to recover as age progresses (Lebel et al., 2012). This study also provides evidence that timely cognitive interventions may potentially improve the cognitive ability in these patients. The amount and time of exposure may dictate the severity of symptoms seen in these individuals. However, whether the structural shrinkage can be prevented is still a matter of debate and research.

In a clinical setting, cognition is measured in terms of intelligence quotient (IQ). IQ less than 70 is indicative of impaired cognitive ability. In FASD children, general intellectual ability is decreased even when they have a good average IQ (Astley et al., 2009; Reid et al., 2017). FASD children with normal IQ are often seen to be deficient in executive functioning, a set of mental skills that include working memory, flexible thinking, and self-control (Davis et al., 2013; Greenspan et al., 2016). The hippocampus plays an important role in spatial learning and contextual learning abilities (Hamilton et al., 2003). One study observed spatial learning deficits among FASD children. They found FASD children performed poorly as compared to the controls suggesting hippocampal dependent deficits. All these studies aptly provide evidence of the extent of impairment in brain functioning in FASD children. Proper social and emotional assessments are necessary considering the prenatal alcohol related developmental deficits seen in children (Crawford et al., 2020). An important treatment goal is providing psychological counselling and the development of support groups for these individuals.

Animal models of prenatal alcohol exposure were similarly used to understand the cognitive and memory deficit effects of alcohol. Morris water maze is used to map the spatial locations where the animal has to escape to a hidden platform submerged under opaque water (Vorhees and Williams, 2006). Prenatal alcohol damage to the hippocampus considerably damages the capacity to learn spatial cues. Prenatal alcohol exposed rats take a longer time to reach the escape platform due to hippocampal dysfunction (Winocur et al., 2005). In summary, the spatial deficits observed in rodent models show variable age dependency of prenatal alcohol damage. A more significant effect is seen in younger animals with delayed testing after the training session. The dose of alcohol and severity in spatial deficits as observed with various maze tests strongly support the hypothesis that prenatal alcohol affects the developing hippocampus.

Oxidative stress and subsequent damage to the neuronal cells may be linked to learning and memory deficits seen in prenatal alcohol models (Motaghinejad et al., 2021). The neuronal signaling pathway impairments may lead to sustained loss of neuronal functions. Brain Derived Neurotrophic Factor (BDNF), the most abundant neurotrophin in the brain, has been shown to counter the damage caused by oxidative stress. We and others have reported deficits in BDNF expression in the brain in prenatal alcohol exposed models. Reduced BDNF function can also further damage the neuronal plasticity and hence cause overall impairment of learning and memory in the offspring (Bhattacharya et al., 2015; Feng et al., 2005). Consequently, enhancement of BDNF activity has shown to improve learning and memory impairment in the prenatal models. Enhanced BDNF expression from an enriched environment has shown to reverse spatial and contextual memory deficits in the prenatal alcohol exposed animals (Tipyasang et al., 2014). Changes in expression of glutamatergic receptors, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-Methyl-D-aspartic acid or N-Methyl-D-aspartate (NMDA) may explain the observed deficits in learning and memory in prenatal alcohol models. Indeed, studies have shown reduced AMPA receptors in the neocortex in alcohol exposed models but no changes in NMDA receptors (Bellinger et al., 2002). In the hippocampus, we observed enhanced calcium impermeable GluR2 subtype of AMPA receptors (Bhattacharya et al., 2015). In another model of moderate alcohol consumption, there was a reported decrease in GluN2B subtype of NMDA receptor and increased expression of GluN1 containing NMDAR subunits in the dentate gyrus of the hippocampus (Brady et al., 2013). Overall, these studies indicate that alcohol modulates glutamatergic synapses and the effect is region specific and model dependent. The amount of alcohol and time of exposure may change the synaptic protein expression of NMDA and AMPA receptors in the developing brain, affecting synaptic plasticity and resulting in learning and memory deficits. The alteration of γ-Aminobutyric acid (GABA) synapses can also play a role in the deficits seen in these models. Reduced parvalbumin basket cells were reported in a moderate drinking prenatal alcohol model emphasizing the long-term impairment of brain development and learning and memory deficits (Madden et al., 2020). Alterations of such excitation/inhibition in the developing brain may potentially harm the neuronal synaptic efficacy. BDNF can effectively rescue such balance in the brain and may prove an important therapeutic target in FASD related disorders (Eckert et al., 2021). BDNF can enhance Integrin Linked Kinase (ILK), a serine threonine kinase that regulates various downstream signalling processes in different brain regions (Xu et al., 2015). Enhancement of BDNF expression increases ILK activity in the brain and potentially modulates synaptic neurogenesis and dendritic morphogenesis. Infact, ILK has been found to interact with scaffolding proteins such as Postsynaptic density protein-95 (PSD95) in the synapse and AMPA receptors (Chen et al., 2010). Our study also reported decreased ILK activity in the hippocampus of the prenatal alcohol model (Bhattacharya et al., 2015). Altogether, enhancement of BDNF expression or enhancement of ILK activity may attenuate the impairment of synaptic plasticity and deficits of learning and memory in FASD.

While a fall in traditional nicotine use through cigarettes has been seen for > a decade, since 2016 > 7.2 % of women still report to have smoked during pregnancy (Drake et al., 2018). Recent models have also shown possible vascular changes in fetal brain development (Raghunathan et al., 2020). Nicotine is the main psychoactive agent in tobacco smoking that can cross the placental barrier (Vaglenova et al., 2008). In addition to the original combustible administration of tobacco, there has been a significant increase in non-combustible nicotine products in recent years. However, products such as electronic cigarettes (e-cigarettes), smokeless tobacco, and nicotine-replacement therapies (NRTs) still make nicotine a prevalent drug (McGrath-Morrow et al., 2020). Some agencies in Europe have even claimed e-cigarette use as a safe alternative to standard cigarettes during pregnancy, but the CDC and US Preventive Services Task Force both disagree with this notion, stating that the fetus’ lungs and brain may still be at risk (van der Eijk et al., 2017). Regardless of the vehicle in which nicotine is consumed, its teratogenic effects are undeniable and well-researched. As are its possibly life-long deficits such as behavioural problems, attention deficit disorders, anxiety, depression, conduct disorders, and learning disabilities (England et al., 2017).

Nicotine is a teratogen that impacts the fetal brain through various effects on dopaminergic innervation of the prefrontal cortex, decreased striatal volume and cerebellum volume, and reduced cortical thickness and cell packing density and altered neuronal morphology (Roy and Sabherwal, 1994). Nicotine also has the effect of decreasing striatal volume and the number of striatal neuronal numbers (Levin et al., 2015). Hippocampal neuronal loss is also evident in prenatal nicotine exposed rat models (Chen et al., 2006). This is important since the striatum and hippocampus play a critical role in learning and memory. Also, reduced cerebellar growth, and smaller width of the lateral ventricles are observed in offspring exposed to maternal nicotine consumption (Roza et al., 2007). Overall, there is a reduction in cerebellar grey matter and total parenchymal volume indicative of atrophy in children as measured with MRI (Rivkin et al., 2008). Cognitive impairment in prenatal nicotine exposure is likely associated with this observation. Long-lasting impact on brain size and volume due to gestational exposure is reflected in behavioral deficits seen in these individuals (Fryer et al., 2012).

Smoking contains a multitude of chemicals that may interfere with prenatal development. Nicotine is one of these chemicals which is a known potential teratogen. The serotonin transporter function in the presynaptic neuron terminal is compromised with nicotine exposure leading to decreased serotonin turnover (Slotkin et al., 2006). Prenatal exposure of nicotine can reduce the threshold of developing depressive disorders and indeed prevalence of these disorders increases with maternal smoking (Menezes et al., 2013). Incidence of Attention Deficit Hyperactivity Disorder (ADHD) is dependent on norepinephrine and dopamine neurotransmitter expression in the brain centers such as the frontal cortex. A non-ADHD brain can adequately balance the responses to stimulation without the interference of emotions or sensations (Ptacek et al., 2019). The deficiency of the neurotransmitters, however, compromises the brain’s ability to suppress such stimuli resulting in ADHD symptoms. This has been shown in several animal studies (Alkam et al., 2017; Thomas et al., 2000). Nicotine exposure during gestation can modify expression of several genes in the developing brain including dopamine related genes (Eicher et al., 2013). Indeed, prenatal nicotine exposure reduces the release of dopamine in the prenatal brain (Alkam et al., 2017). Maternal smoking is associated with variations in dopaminergic receptor genes and ADHD symptoms in the exposed children (Neuman et al., 2007). Choline supplementation reversed the epigenetic modifications seen in chromatin remodeling genes, Smarca2 and Bahccl induced by nicotine (Gitik et al., 2018). It is yet to be investigated if choline supplementation in children exposed to prenatal nicotine can augment the cognitive outcome. Similar to prenatal alcohol, glutamatergic modulations are also seen in prenatal nicotine-exposed animals in brain regions associated with addiction and memory (McNair and Kohlmeier, 2015; Wang and Gondré-Lewis, 2013). Early activation of nicotinic receptors may alter the synaptic expression and trafficking of the glutamate receptors associated with hippocampal memory deficits with gestational nicotine exposure (Wang et al., 2011). Alteration of cortical inhibition-excitation balance is also seen in the brain with maternal nicotine exposure decreasing GABA to non-GABA neurons (Martin et al., 2020). Altogether, more research to understand choline supplementation during adolescence, reducing premature nicotinic receptor activation with nicotine exposure or promoting trkB medicated glutamatergic trafficking are necessary to get a deeper understanding of neuroprotective mechanisms against nicotine.

Prenatal cannabis use has been correlated with adverse outcomes regarding the proper growth and maturation of fetal brains (Jaques et al., 2014). Additionally, cannabis use in pregnancy has also been seen to cause diminished executive functioning skills and attention, increased behavioral issues, and poorer academic achievement, all of which can be severely life altering consequences (Warner et al., 2014).

There have been recent increases in the decriminalization and legalization of cannabis worldwide which further emphasizes the importance of better understanding the medical implications of cannabis to the developing brain (Hurd et al., 2019). According to a recent study, cannabis use within pregnant women was seen to more than double in the United States from 2010 to 2017 (Volkow et al., 2019). In 2017, 4.7 % of women reported cannabis use during their pregnancy in order to alleviate pain, anxiety, and nausea (Ko et al., 2020) Due to its widespread acceptance as a harmless drug, pregnant women frequently use cannabis, especially to mitigate morning sickness (Dickson et al., 2018). Prenatal cannabis exposure has been seen to affect the endogenous cannabinoid system which is crucial for the normal development of the brain (Hurd et al., 2019). The cannabis exposed infant is predisposed to various psychiatric disorders and long-term neurocognitive effects which are developmental in etiology (Hurd et al., 2019).

Cannabinoids are molecules such as marijuana, tetrahydrocannabinol (THC), and cannabidiol (CBD) that stimulate cannabinoid receptors. Prenatal exposure to cannabinoids could negatively impact specific cognitive skills, especially attention and memory (Grant et al., 2020). These deficits in memory and attention could potentially affect long-term success in school and the workplace. Various longitudinal studies have demonstrated that individuals exposed prenatally to cannabis develop executive function deficits as adults, many of which are associated with changes in cortical excitatory neurotransmission (Hurd et al., 2019). Gross effects of cannabinoids on the central nervous system include a lack of iris or coloboma and presence of the brain outside the cranium or exencephaly (Grant et al., 2020).

Cannabis use during pregnancy has pronounced effects on the mesocorticolimbic system, which can then impact the offspring’s future psychiatric health (Hurd et al., 2019). The neurotransmitter involved in this system is dopamine, which has been seen to not only play a role in many psychiatric disorders, but also influence forebrain circuit formation and neuronal development (Hurd et al., 2019). In utero cannabis exposure has demonstrated changes in the dopamine D2 receptor (DRD2) gene expression within the mesocorticolimbic system (Hurd et al., 2019). One study found that D2 receptor expression was reduced in nucleus accumbens, an essential brain reward region after maternal cannabis exposure (DiNieri et al., 2011). Because the mesocorticolimbic system is involved in various processes such as emotional recognition, motivation, and cognition, alterations to the fetal dopaminergic system due to maternal cannabis use can influence the development of various psychiatric disorders.

The endocannabinoid system not only plays a role in various developmental processes such as cell proliferation and migration, but also is involved in controlling movement, memory, and emotions (Trezza et al., 2008). Any alterations in the activity of the endocannabinoid system during periods of high neuronal plasticity, such as the prenatal period, can therefore lead to long-term neurobehavioral deficits. THC, the active ingredient in marijuana, causes a sedative-like effect by binding to the cannabinoid-1 (CB-1) receptor (Trezza et al., 2008). One study examined the impact of THC exposure on cortical projection neuron development by administering THC to pregnant mice (de Salas-Quiroga et al., 2015). Results from this study demonstrated that prenatal THC exposure interfered with subcerebral projection neuron generation, in turn producing changes in fine motor functioning in the adult offspring by altering corticospinal connectivity (de Salas-Quiroga et al., 2015). Furthermore, prenatal THC administration also demonstrated an increase in seizure susceptibility due to its interference with CB-1 dependent regulation of GABAergic and glutamatergic neuron development (de Salas-Quiroga et al., 2015). These findings indicate that prenatal exposure to THC can lead to long-term damaging consequences in the offspring due to the disruption of the CB-1 signaling pathway.

Opioids produce sedative, analgesic and euphoric effects when bound to specific receptors in the brain. Classification of opioids is based on their derivation, which include synthetic such as methadone, natural such as morphine, and semi-synthetic which includes buprenorphine and oxycodone (Blandthorn et al., 2018). Over the past ten years there has been an increase in the use of opioids by pregnant women. A recent increase in opioid use in pregnancy has been noted, likely due to the epidemic of opioid prescription misuse. Specifically, there has been a five-fold increase in opiate use in pregnancy between 2000 and 2009 in the United States (Hayes and Brown, 2012). Two of the most common opioids being abused are methadone and heroin. Every year there are 7000 births in which the baby was exposed to opioids in utero. Maternal opioid use while pregnant is associated with an augmented risk of neonatal abstinence syndrome, in which exposure to opiates while in utero leads to a postnatal withdrawal syndrome in the neonate (Patrick et al., 2012). Furthermore, postnatal growth deficiency, sudden infant death syndrome, neurobehavioral issues, and microcephaly are all also associated with opioid exposure during pregnancy (Minozzi et al., 2013). Indications of neonatal abstinence syndrome are tremors, irritability, inability to regulate temperature, seizures, and inadequate suckling. Overall, these lead to failure to thrive and even death (Bhuvaneswar et al., 2008). The child is assessed in three major categories which include gastrointestinal, autonomic nervous system and central nervous system using the modified Finnegan Scoring tool. The score determines the level of treatment from supportive to pharmacologic, usually involving administering oral morphine (Blandthorn et al., 2018).

Methadone has been shown to result in higher rates of delay and impairment using neurodevelopmental outcomes across all domains (Chang, 2020). Women who are addicted to heroin are often prescribed methadone during their pregnancy. This aids in the prevention of using heroin during the pregnancy, and decreases the incidence of preterm labor and intrauterine death (Monnelly et al., 2018). Neonates exposed to methadone in utero were found to have a lower fractional anisotropy in the external and internal capsule, and in the inferior longitudinal fasciculus. This indicates that these white matter tracts have decreased axonal diameter and fiber density (Monnelly et al., 2018). Opioids have a direct impact on the myelination process that occurs early on in development (Vestal-Laborde et al., 2014). Resting state functional magnetic resonance imaging (fMRI) performed on infants who were exposed to opioids in utero showed abnormal connectivity between cortical regions and the amygdala (Radhakrishnan et al., 2021).

Methadone enters the mothers blood stream, crosses the placenta, and acts as an opioid agonist (Monnelly et al., 2018). Opioid exposure shows an increase in myelin basic protein, myelin oligodendrocyte myoprotein, and myelin proteolipid protein. These elevated protein levels lead to compacted myelin sheaths surrounding an abnormally large number of axons in the corpus callosum. The increased maturation of pre oligodendrocytes could cause a disruption in the connectivity process (Vestal-Laborde et al., 2014). Two receptors play a key role in the effects of opioids on the cerebrum during development. The NMDAR and u-opioid receptor (MOR) have a downstream signaling pathway that when bound activates extracellular signal related kinase (ERK) and Ca/calmodulin protein kinase II (CaMKII). For up to two weeks post birth, reduced binding of the MOR was present in pups who were administered methadone. The downstream signaling involving CaMKII and ERK during development resulted in decreased activation after the administration of methadone in utero. Overall, this resulted in a decrease in the transcription of proteins that are imperative for brain development when methadone is administered (Kongstorp et al., 2020).

Cocaine, a drug of the Erythroxylum coca plant, has for the most part seen a decline in use over the last few decades but has seen a recent re-spike in use since 2013 due to increased cultivation (Oliveira and Dinis-Oliveira, 2018). Cocaine itself in any form passes the placenta through simple diffusion and is known to cause vasoconstriction of the placenta, frequent eclampsia, and constriction of the uterus, leading to fetal hypoxia, miscarriage, abortion, and premature birth with small head circumference and low birth weight (Souza-Silva et al., 2020). No concrete data exists to get an exact estimation of cocaine use during pregnancy. One report suggested around 75,000 women abused cocaine during pregnancy (Cain et al., 2013). After prenatal cocaine exposure, studies have found that children may show subtle but significant behavioral and cognitive deficits, information processing, and performing critical cognitive tasks as they grow (Delaney-Black et al., 2000), (Li et al., 2011).

When a fetus is exposed to cocaine, the brain retains lasting structural and functional deficits. A study imaged the caudate nucleus of adolescents that had been exposed to high amounts of cocaine in utero. This study found the caudate to be significantly decreased in size compared to those not exposed to cocaine in utero (Avants et al., 2007). Another study administered female rats with cocaine and saline during pregnancy and lactation. Upon birth they studied the locomotor sensitization and activity of the male offspring, and concluded there was an age and dose dependent enhancement of both the sensitization and activity of locomotion (Barbosa-Méndez and Salazar-Juárez, 2020). Another group of researchers subjected pregnant rats to cocaine smoke upon which both mother and pup were studied. In the pup, the fontanelle experienced delayed closure, and using a brain mass and skull cap ratio, the brain mass was noted to be significantly decreased (Souza-Silva et al., 2020). It was previously known that arousal dysregulation was associated with prenatal cocaine exposure. In 2019 researchers conducted longitudinal studies which examined a link between age and exposure. Focusing on intrinsic functional connectivity, they used fMRI to examine patients who were exposed to cocaine prenatally at 14 then 16 years old and control groups of the same age. Upon examination, from ages 14–16 there was an increase in functional connectivity in the amygdala (Li et al., 2019).

The mechanism by which cocaine affects the brain is through the prevention of monoamine receptor signaling by impairing the transporters. A neurotransmitter that is targeted by cocaine is dopamine. One study showed that there was a decrease in the functioning of the dopamine D1 receptor and a significant increase in the function of D2 after the administration of cocaine from embryonic day 8 through day 14. In the striatum and cortex, high cyclic adenosine monophosphate (cAMP) expression was observed post exposure, but cAMP returned to physiologic levels upon the blockage of the adenosine A2a receptor (Kubrusly and Bhide, 2010). Another study found a connection between cocaine dependence and the C-C chemokine receptor (CCR5). They found an increase in the expression of the CCR5 gene upon the continuous exposure of cocaine in a Sprague-Dawley rat. This upregulation was localized to the ventral trigeminal area and the nucleus accumbens (Nayak et al., 2020). Previous research has found that the Methyl-CpG Binding Protein 2 (MeCP2) negatively regulates the expression of brain derived neurotrophic factor BDNF though binding to its promoter region. This study focused on the level of microglial activation upon intravenous cocaine use and its effects on MeCP2 and BDNF. Researchers found that upon cocaine administration, in the microglia, MeCP2 was phosphorylated, preventing it from translocating and binding to BDNF, and therefore BDNF underwent transcriptional activation. Overall this could indicate a link between cocaine usage and decreased synaptic plasticity (Cotto et al., 2018).