An examination of sex differences in the effects of early-life opiate and alcohol exposure

Abstract

Early-life exposure to drugs and alcohol is one of the most preventable causes of developmental, behavioural and learning disorders in children. Thus a significant amount of basic, animal and human research has focused on understanding the behavioural consequences and the associated neural effects of exposure to drugs and alcohol during early brain development. Despite this, much of the previous research that has been done on this topic has used predominantly male subjects or rodents. While many of the findings from these male-specific studies may ultimately apply to females, the purpose of this review is to highlight the research that has also examined sex as a factor and found striking differences between the sexes in their response to early-life opiate and alcohol exposure. Finally, we will also provide a framework for scientists interested in examining sex as a factor in future experiments that specifically examine the consequences of early-life drug and alcohol exposure.

1. Introduction

Research in the field of developmental toxicology has demonstrated that there are a number of ways by which early-life exposure to drugs or alcohol may affect the development of the nervous system. There are a number of important ongoing processes in the developing brain that are likely to be disrupted by early-life drug and alcohol exposure, including cell differentiation, cell determination, cell migration, synaptic formation, synaptic pruning and naturally occurring cell death or apoptosis. These processes occur at much higher rates in the developing brain than in the adult brain and are critical for the proper establishment of neural circuits. Thus, early-life drug and alcohol exposure can disrupt any one or more of these critical cellular processes in the developing brain, with significant consequences for neural function and behaviour later in life. These effects of early-life drug and alcohol exposure can lead to alterations in the ability of the exposed individual to adapt, respond or learn from their environment, and in turn these deficits in behaviour can subsequently lead to further alterations in neural function and behaviour.

The nervous system is also highly adaptive and intrinsically self-regulating, constantly functioning to maintain a critical homeostasis. Disturbing nervous system function with drug and alcohol exposure can also lead to compensatory adaptations, though the nature of these compensatory processes may differ depending on the age and we would also argue the sex of the exposed individual. As a classic example, chronic blockade of dopamine (DA) receptors in the adult male brain leads to an upregulation of DA receptor binding [1]. However, chronic blockade of these same receptors in the developing male brain results in decreased DA receptor binding [2]. Simply stated, pharmacologically blocking the DA receptors in the adult brain induces receptor upregulation necessary to maintain homeostatic equilibrium in DA neurotransmission in the adult brain; however, the same drug produced a new homeostasis in the developing brain, wherein fewer DA receptors are needed due to the chronic decrease in DA receptor activation [3]. As a result of important findings such as this, neuroscientists have come to an understanding that the developing brain is quite distinct from the adult brain, particularly in its adaptive potential and vulnerability to events such as drug and alcohol exposure.

The purpose of this review is to highlight that sex, being male or female, is also an important factor that can influence both the vulnerability and the adaptive response to early-life drug or alcohol exposure. Figure 1 provides a simple diagram of the ways by which sex can interact with the pre- and postnatal environment to modulate the effects of drug and alcohol exposure on the developing brain. There are two notable differences between the prenatal and postnatal windows of development, particularly with regard to drug and alcohol exposure in males and females. The first notable difference is the interaction of the mother and developing fetus that occurs during the prenatal period but does not occur during the postnatal period. During the prenatal period, the mother is the primary consumer of the drug or alcohol. This is perhaps the most common manner by which children are initially exposed to drugs or alcohol. The mother's metabolism of the drug or alcohol can significantly impact the amount of drug or drug metabolites that are passed on to the developing fetus [4–8] (figure 1a). Continuous drug and alcohol exposure can also produce a stress or immune response in the mother that can impact the developing fetus [9–11] (figure 1b). Additionally, the placenta has the capacity to respond to the drug itself and in doing so indirectly affects the developing fetus through changes in its function [12,13]. The placenta can also be impacted by the sex of the fetus (figure 1c). There are known sex differences in the human placental transcriptome [14] and in the mouse placental transcriptome [15]; and these sex differences in gene expression are often exaggerated following insults such as maternal stress [16], poor maternal diet [14] and decreased maternal oxygen levels [17]. Thus future research should examine whether sex differences exist in the transfer of maternal drug or alcohol concentrations across the placenta to the fetal bloodstream or brain, and whether prenatal drug exposure may also affect placental gene transcription in a sex-specific manner. Postnatal drug exposure predominantly involves drugs being administered directly to the neonate, not through the mother. This is perhaps less common with illicit drugs, but still of note and important as a topic in basic research because neonatal infants and young children can receive therapeutic drugs such as opiates, as needed. In addition, because rodents are born prematurely during what is considered the third trimester equivalent for humans [18], many developmental toxicology studies administer drugs and alcohol to the neonatal animal to mimic third trimester-equivalent drug exposure. Given the discussion above regarding pre- versus postnatal differences in drug and alcohol exposure, there are obvious caveats with the administration of drugs and alcohol to a neonatal animal as a model of prenatal exposure. Despite this, it is a common model for developmental toxicologists and therefore, understanding how this period of development is impacted by sex (figure 1) can inform these researchers as to how sex may impact postnatal drug and alcohol exposure.

Figure 1.

Sex can impact early-life drug and alcohol exposure via a number of sex-specific mechanisms. Prenatal drug (left panel) exposure can be quite distinct from postnatal drug exposure (right panel). Prenatal exposure to drugs or alcohol is the result of maternal drug or alcohol consumption. (a) The mother metabolizes the drug/alcohol, (b) which can also result in the activation of the maternal immune system. Immune molecules produced by the mother and metabolites from the drug/alcohol can pass from the mother to the fetus. These are not factors that impact postnatal exposure to drugs or alcohol. (c) There are known sex differences in placental gene transcription, which can be exacerbated by early-life events such as stress or potentially drugs/alcohol. (d) Males begin to secrete testosterone prenatally, which differentiates the body and the brain. These circulating hormones can also impact how drugs or alcohol can affect the male body or the brain. (e) Finally, sex differences in gene expression, the result of XX- or XY-specific genes, can also produce a sex-specific response to drug or alcohol exposure. Postnatal exposure to drugs and alcohol can also result in sex-specific effects, via different mechanisms. (f) First sexual differentiation that occurred prenatally, via testosterone, has resulted in sex differences in the body and brain that can impact how the male responds to postnatal drug or alcohol exposure. (g) Secondly, sex differences in metabolism may emerge now that the neonates are solely responsible for metabolism of the drug, not the mother. (h) Just as it was before birth, sex differences in gene expression, the result of XX- or XY-specific genes, are still a factor that can impact the response to postnatal drug and alcohol.

The second reason that the prenatal period is quite distinct from the postnatal period with regard to the effects of drug and alcohol exposure has to do with the sex of the baby and potential sex differences. Males begin to secrete testosterone prenatally, during mid- to late gestation (figure 1d). This occurs in a number of mammalian species (including humans and rodents) and is a critical developmental process that results in the sexual differentiation of the brain and ultimately behaviour in males [19–21]. Testosterone levels are high in males during prenatal development and then decrease precipitously after birth such that both males and females have no circulating sex hormones postnatally. As a result, the sex of the fetus can profoundly influence the effects of drug and alcohol exposure during late prenatal development. However, once the body and brain have been sexually differentiated by prenatal testosterone exposure, the male brain is, from that point on, distinct from the female. Thus, sex is a critical factor to consider when examining the potential effects of postnatal drug and alcohol exposure because of the previous differentiation of the male body and brain by testosterone (figure 1f). In addition, sex differences may also exist in the metabolism of the drugs by the neonate during the postnatal period, which may impact the subsequent effects of the drugs on postnatal brain development (figure 1g). Finally, one should also consider the potential genomic effects of sex on the effects of early-life drug or alcohol exposure (figure 1h). For example, kidney cells from female mice on embryonic day 10.5, prior to the secretion of prenatal testosterone, are more vulnerable to the effects of alcohol treatment than male cells [22]. These data indicate that sex differences exist, even in the absence of circulating sex hormones [23,24], and thus sex should always be considered even at the earliest ages of prenatal drug and alcohol exposure.

After decades of research, it is clear that there are a number of differences between males and females, other than their apparent physiological features. For example, males and females can respond to various stressors, infections, medications and certain environmental factors in drastically different ways, which points to a need for both sexes to be used in all studies including those that examine the effects of early-life drug and alcohol exposure. There is mounting evidence that sex plays an important role in the risk of drug abuse and addiction in adults [25,26]. Aspects such as the prevalence of drug use and physiological reactions to various substances are significantly different between males and females [27,28]. For example, although fewer females use drugs than males, they are more susceptible to a faster progression from drug use to dependence [29]. In addition, males experience greater analgesia compared to females when given the same dose of morphine [30,31]. Males and females process and respond to drugs and alcohol differently, and the purpose of this review is to highlight some of the known differences in developing males and females.

2. Opiates

Opiates are a class of drugs derived from opium that are primarily used to manage pain. Some common opiates include morphine and codeine, as well as synthetic drugs like oxycodone, or illicit drugs such as heroin. While opiates have been used for centuries and are highly valued in the field of medicine for their analgesic properties, they are also frequently misused and abused. Abuse of opioids such as prescription pain killers and heroin affects an estimated 2.5 million Americans of all socio-economic classes, races, genders and backgrounds [32]. This dangerous problem has become a growing concern as emergency room visits [33] and fatal overdoses due to opium-derived drugs have almost quadrupled since 2000 [34]. Millions of dollars are spent each year on substance abuse research in hopes of discovering potential side effects, therapies and new treatment plans for users and addicts. As the prevalence of opiate use in the United States is on the rise, it is becoming more important that researchers investigate how prenatal exposure to opiate drugs affects children immediately and long-term, and more specifically, how sex differences may play a role in these effects.

(a). Prenatal exposure to opiates

Many research studies examining the effects of opiate abuse during pregnancy focus on the use of morphine or opioid agonists, like methadone, which are commonly used as therapeutic treatment for heroin addiction. Not only does the drug itself affect the fetus, but oftentimes the opiate-addicted mothers do not receive adequate prenatal care, including wellness checks, proper nutrients and vitamins, and may be exposed to other environmental toxins such as second-hand smoke. This, in addition to the socio-economic status and conditions that the child may grow up in, may have confounding effects on the results of human studies, making it difficult to definitively draw conclusions about the singular effects of prenatal exposure to opiates in humans as you will see in the discussion below.

In general, opiate exposure during gestation has been linked to growth retardation and increased mortality risk in both animal and human studies [35]. Pups of methadone-treated rat dams weigh less than unexposed pups [36], although interestingly once born, exposed males recover to their normal sex-matched weight sooner than female pups [37]. The pups exposed to methadone also have an increased risk of death [38], though this is not sex-dependent. Offspring exposed to opiates prenatally also have differences in adrenal weight and spleen weight into adulthood that are sex-dependent, indicating that the effects of prenatal opiate exposure impact many aspects of physiology that can in turn affect the brain and behaviour in a sex-specific manner [39]. In humans, reports have shown that young children born to heroin-addicted mothers have significantly lower weight and head circumference [40]. These data suggest that although exposure to opiates may not cause severe malformations or birth defects like those seen in fetal alcohol syndrome (FAS) [41], the effects of prenatal opiate exposure can still be detrimental to the child's overall health and development in the first years of life.

In addition to stunted growth, there are notable changes in cognition and behaviour in offspring prenatally exposed to opiates. Male mice exposed to heroin in utero showed impairments in short-term memory using an object recognition task [42]. Another study used a symmetrical maze and found that males were more hyperactive if they were prenatally exposed to morphine, though there was no evidence of cognitive differences compared to control animals. Morphine-exposed females from this same study showed significant impairments in learning but only if they were ovariectomized. These cognitive effects in females were subsequently reversed with the injection of the sex hormones oestradiol and progesterone, indicating that the absence or a decrease in circulating gonadal hormones in females may precipitate the negative cognitive effects of prenatal morphine exposure [43]. Male rats exposed to prenatal morphine also show memory deficits in a passive avoidance retention task earlier in development, as juveniles; however, these cognitive deficits are reversed post-puberty [44]. In contrast, females exposed to morphine prenatally still show deficits even at the later ages. Findings such as these suggest that females may be more at risk to the cognitive deficits associated with prenatal opiate exposure. In contrast, males may show deficits early in development that are later reversed or obscured later on by some inherent developmental process that only occurs during puberty in males or with the increase in testosterone levels during puberty. That said, others have found that male offspring exposed to opiates prenatally are continuously at risk of increased anxiety-like and depressive-like behaviours later in life [39,45], indicating that the sex of the fetus may influence the timing of and the specific neural circuits or systems that are affected by prenatal opiate exposure.

In humans, the rate of attention hyperactivity deficit disorder (ADHD) was found to be significantly higher in children born to mothers who used heroin during gestation [46], though sex was controlled for in this particular study. In addition, a small study found that methadone-exposed children had trending lower IQ scores and greater reports of anxiety, aggression and rejection compared with control children [47]. There have been other studies, however, that show little to no significant cognitive differences between babies exposed to opiates and control subjects later in life [48,49], which suggests that other factors (not controlled for in these studies) likely influence the risk of behavioural or cognitive deficits associated with prenatal opiate exposure. Interestingly, children exposed to heroin prenatally that were then adopted into a controlled household at a young age experienced behavioural deficits associated with ADHD at much lower rates [46]. While all of these studies test different types of learning and behaviour, including hyperactivity, anxiety and aggression—most suggest that opiate-exposed offspring have a higher if not significant chance of developing cognitive and behavioural problems. The confounding effects of socio-economic status and parental guidance are interesting and noted in the work of Ornoy et al. [46] and reviewed extensively in Fodor et al. [35], indicating that while the absence of opiates during pregnancy is important, so too is the environment in which the child is raised. In addition, though sex differences have not been widely studied in human studies or seen in many results, the interaction of prenatal opiate exposure, poor environmental conditions or stress, and the sex of the individual are likely important to consider when examining the immediate and long-term consequences of prenatal opiate exposure on cognitive outcomes. This is particularly important given that there is substantial evidence that males may be more vulnerable to the impact of early-life stressors such as maternal separation [50–53]. In addition, the observations of Slamberova et al. [43] and Nasiraei-Moghadam et al. [44] in rodents suggest that sex hormones during puberty may play a role in the sex-dependent risk of cognitive and behavioural deficits caused by prenatal opiate exposure.

In addition to cognitive deficits, prenatal opiate exposure also impacts later-life nociception and the reaction to subsequent drug exposure well into later stages of life. Enters et al. [37] found neonatal males exposed prenatally to methadone had an enhanced analgesic response to morphine compared to control male pups, while females treated with methadone prenatally did not show the same effect. Interestingly, on postnatal day 21, both males and females had a diminished analgesic response to subsequent morphine exposure [37]. Other findings report that prenatal morphine exposure results in greater sensitivity to the analgesic effects of morphine in adult male rats than same-sex controls; however, the same effect of enhanced analgesia is not seen in females [54]. Taken together, these results suggest that prenatal opiate exposure can influence subsequent physiological responses to pain or analgesia, particularly in males, following opiate re-exposure and that these effects are age- and sex-dependent.

(b). Postnatal exposure to opiates

Babies born to opiate-using mothers often undergo extensive observation for opiate dependency and indications of withdrawal known as neonatal abstinence syndrome, or NAS. This diagnosis of NAS is used to describe the symptoms and behaviours that newborns experience when their exposure to a drug is stopped abruptly following birth. Similar to withdrawal in adults, babies who suffer from NAS show signs of irritability, tremors, excessive/inconsolable crying and hypertonia [55,56]. To alleviate the suffering of these children, doctors initiate medical interventions on 30% [57] to 91% [58] of neonates to help wean them off of their drug dependence. To date, there have been no observed differences in the number of males and females that develop NAS [59]. Similarly, Holbrook & Kaltenbach [60] also discovered that the rate of occurrence, length of treatment and maximum dose required to alleviate NAS is not sex-dependent either. Because of the limited number of studies on the effects of pre- and postnatal opiate exposure, it is not well known what causes an individual to be predisposed to suffer from NAS following prenatal drug exposure [61]. Though these studies suggest that sex may not be a driving factor, the full impact of sex on the prevalence, severity and treatment of NAS should be investigated more closely.

The most common manner in which neonates or young children are likely exposed to opiates postnatally is via post-operative analgesia. Repetitive untreated pain and distress can impair the developing brain and have both short- and long-term negative consequences [62,63]. For example, neonatal inflammatory pain decreases locomotor activity in adult rats [64] and decreases pain sensitivity later in life, though more so in females than in males [65]. Neonatal pain also rapidly alters markers of pain and stress in the pup [66,67], and produces lifelong deficits in the response to stress in both male and female rats [68]. Thus, effective pain management in neonates is recognized as necessary and effective; and interestingly opiate administration at the time of inflammatory pain actually alleviates many of these negative effects described above. Morphine administration at the time of neonatal injury can reverse decreased locomotor activity and alterations in pain sensitivity later in life [64,66]. In addition, analgesia for early-life pain prevents deficits in anxiety and stress in both male and female rats [66]. Thus postnatal opiate administration is necessary and beneficial when administered either for NAS, surgery or severe pain. However, future studies are still needed to explore potential sex differences in the effects or interactions of postnatal opiate exposure with environmental conditions (maternal care) or concomitant physiological conditions (pain or withdrawal) as these experiments will better inform our understanding of specific clinical populations and potential sex differences in these populations.

3. Alcohol

Alcohol consumption during pregnancy is the most common cause of preventable mental retardation in the world, estimated to affect 1–5% of live births each year [69,70]. Alcohol consumed by the mother can easily pass through the placenta, directly affecting the developing fetus. In addition, alcohol can also indirectly affect the fetus by impacting the mother's critical hormonal balance and placental function [11,12,71]. The developing brain is particularly vulnerable to the toxicological effects of alcohol. Alcohol exposure during fetal development can lead to a number of complications, including overt brain damage [72]. The range of cumulative effects and symptoms of the brain damage caused by alcohol exposure are referred to as fetal alcohol spectrum disorders (FASD) [73]. FASD is associated with deficits in general intelligence, memory, language, attention, learning, executive functioning, motor skills and adaptive functioning [74–76]. The effects of FASD also increase the susceptibility of the exposed offspring to develop depression, stress and anxiety disorders later on in life [77].

One of the most well-known FASD is FAS, though it sometimes goes undiagnosed if certain characteristics are not evident. Microcephaly, a reduced cranium size, and microencephaly, small brain size, along with evidence of central nervous system (CNS) dysfunction, are common characteristics that lead to a diagnosis of FAS. When children lack microcephaly and other facial malformations, it is difficult to diagnose them with overt FAS [78]. International FAS birth rates have increased from 0.2 to 8.2% in just the past 10 years [79]. The rising FAS rates may be due to more accurate diagnosis and awareness; however, this remains to be confirmed. Research shows that FASD is more prevalent in young boys than in young girls (on average 12.9 out of male 1000 births compared to 10.4 out of 1000 female births); however, interestingly, there is no sex difference in the rate of FASD diagnosis when the children are diagnosed later in life [80]. These findings would suggest that the diagnosis of FASD is sex- and age-dependent, with males being more likely to be diagnosed with FASD at earlier ages, perhaps because females only meet certain diagnostic criteria at later ages or because the characteristics of FASD are more pronounced in males than in females. Future research should consider sex differences in the developmental aetiology or presentation of FASD symptoms as these findings could provide sex-specific criteria for diagnosing FASD as early as possible in both males and females.

Fetal alcohol exposure leads to a number of neural and behavioural disorders that have been extensively documented in the human literature and studied using rodent models. There is a direct correlation between the amount of alcohol exposure and its negative effects on the brain [81]. In humans, fetal alcohol exposure is known to lower IQ scores along the entire exposure spectrum, such that on average, children who are diagnosed with FAS have the lowest IQ. Along with a lower IQ, fetal alcohol exposure also increases the likelihood of learning deficits in these children [74]. Rodent studies have also demonstrated a strong inverse relationship between blood alcohol concentration (BAC) of the rat during exposure and weight of the brain at birth [82]. These effects are likely the direct result of cell death in the brain. Several studies have used various alcohol exposure paradigms in rodents to find that ethanol causes robust cell death in multiple brain regions during the first week of postnatal life [82–86]. Notably, none of these seminal studies examined alcohol-induced cell death in females. One study has examined both males and females in a model of third trimester binge drinking but found no effect of the alcohol on the number of neurons or glia in the neocortex at this age [87]. In another study, an acute third trimester binge dose of alcohol produced decreased brain volumes in both male and female adult rats, with significant decreases in both parvalbumin interneurons and layer II pyramidal neurons [88]. Interestingly, this same study found that only males exposed to this binge dose of alcohol during the third trimester equivalent showed an increase in neurogenesis in the dentate gyrus as adults [88], which suggests that there may be compensatory mechanisms following early-life alcohol exposure that are sex-specific.

Before neuroimaging technology existed, the only way to obtain physiological evidence regarding the effects of FASD on the developing brain in humans was through autopsies. Through these examinations, researchers found widespread damage throughout the brain in infants that had been exposed to high levels of prenatal alcohol. Many of these children fail to develop a corpus callosum, anterior commissure, and have significant damage within the brainstem and cerebellum [89]. Follow-up studies reported additional damage around the ventricles, basal ganglia, hippocampus, pituitary gland and optic nerve, ultimately suggesting severe widespread damage caused by fetal alcohol exposure [78,90]. Using MRI technology, others have reported overall volume reductions in the cranial, cerebral and cerebellar brain areas in individuals with FASD [41,72]. These structural studies examined both males and females diagnosed with FAS or FASD, and reported no significant sex differences in the effect of the fetal alcohol exposure, but that does not discount potential sex differences in the function of many of these brain regions that are adversely impacted by fetal alcohol exposure. More recently sex has been examined in models of fetal alcohol exposure and these experiments reveal interesting differences between males and females, discussed below, in the effects of fetal alcohol exposure at the cellular and molecular level of analysis that significantly impact behaviour.

The hypothalamus and the hypothalamic–pituitary–adrenal axis (HPA) are particularly affected by fetal alcohol exposure and have been a primary target of investigation. Fetal alcohol exposure affects HPA axis activity through changes in the hypothalamus and brain areas that are functionally connected to the hypothalamus [91], with the result that HPA tone is increased throughout the lifespan [71], though to a greater extent in males than in females. A number of laboratories have found no difference between males and females exposed to fetal alcohol in the stimulated release of adrenocorticotropin-releasing hormone [92–94], which would suggest that the hypersensitivity to stress that the animals express is the result of altered negative feedback in the brain. In fact, fetal alcohol exposure dysregulates the ‘stress axis' by altering gene expression including pituitary pro-opiomelanocortin gene transcripts and beta-endorphin expression in the brain [77,95,96], both of which are changed to a greater extent in males than in females. These effects are similar in the human literature as well. Female infants (age 5–7 months) that had been exposed to fetal alcohol had greater change in heart rate during stress and negative affect while males of the same age previously exposed to fetal alcohol had greater increases in cortisol as a result of stress [97]. The mechanism by which the HPA axis is programmed by fetal alcohol exposure is not fully understood, but it is known that these effects of fetal alcohol on the stress axis can be transmitted through the male germline to subsequent generations such that only male offspring exhibit increased stress responses into the next generation [96]. Data such as these reveal that sex can significantly impact the early-life programming of specific neural systems by fetal alcohol exposure and that these effects can transfer to the offspring thereby influencing the susceptibility of disease in subsequent generations in a sex-dependent manner.

We also have preliminary data indicating that low doses of fetal alcohol exposure can produce changes in immune function that can subsequently impact behaviour (LS Terasaki, J Gomez, JM Schwarz 2015, unpublished data). A very low dose of ethanol (BAC = 0.08%) produced sex-specific inflammation in the fetal brain. Notably, this same dose of alcohol also produced inflammation in the placenta that was not sex-specific. The immune system may be one physiological system that is particularly sensitive to even low levels of teratogens such as ethanol, even in the absence of overt neural cell death. In particular, others have also noted that microglia, the immune cells in the brain, are quick to respond to alcohol-induced neuronal cell death and require a few days to ‘deactivate’ following the initial alcohol exposure [98]. This same study found no sex differences in the level of microglial activation caused by binge levels of alcohol exposure, contrary to what we have seen following low levels of fetal alcohol exposure. Our findings and the findings of others, such as Wierzba-Bobrowicz et al. [99], suggest that microglia activation and immune activation could be early markers for alcohol-induced damage, and thus further studies should examine sex differences in this particular response to fetal alcohol exposure.

Sex differences have also been identified in social recognition and memory tasks after early-life alcohol exposure. Fetal alcohol significantly impaired social recognition memory in males in all variations of the test, while fetal alcohol exposure impaired social recognition memory in females only when the task was more challenging [100]. Males exposed to fetal alcohol also exhibit decreases in social investigation, contact behaviour and play fighting, indicating that alcohol has striking effects on typical social behaviour in males [101]. While males have greater levels of oxytocin receptor binding than females in the amygdala overall and this is thought to play a role in social behaviour, fetal alcohol exposure decreased oxytocin receptor binding similarly in both males and females [100], which suggests that similar neural consequences of fetal alcohol exposure in males and females may not produce similar effects on behaviour. Fetal alcohol exposure, particularly during the third trimester equivalent, results in deficits in the ability of hippocampal synapses to express long-term potentiation in adolescent and adult males; however, the same cannot be said for females [102]. Similarly, male rats exposed to fetal alcohol show deficits in reversal learning at PN28, though not PN63 [103].

Alcohol has many implications on the developing brain and body. In both animal and human literature, it is known to cause devastating effects that are irreversible including cranial and facial malformations, learning disabilities, social and memory disabilities, and altered brain structure and function. These effects also differ by sex such that males appear to be more vulnerable to these effects of fetal alcohol exposure. A growing amount of research has been done on the sexual dimorphism of fetal alcohol exposure on the developing brain in animal models; however, human literature is not yet as substantial. It is important that researchers continue to examine both sexes when looking into FAS and FASD in humans as it is clear that males and females respond differently to this devastating CNS insult.

4. Discussion

The research discussed in §§2 and 3 highlights known sex differences in the immediate and long-term consequences of early-life drug and alcohol exposure. Taken together, these data reveal interesting trends and some additional caveats that should be considered for future experiments. Given the number of sex differences noted here in the effects of early-life drug and alcohol exposure, it is important that researchers continue to examine males and females in both basic biomedical research and epidemiological research. The effects of early-life drug exposure impact males and females differently in an age-dependent manner that also depends upon the dose administered and the endpoint of analysis. In addition, this discussion does not highlight articles that may have used equal numbers of males and females in their experimental design yet either see no significant differences between the sexes in their analysis or do not identify whether sex was appropriately analysed as a factor. Most researchers consider sex to only be an important variable in adult animals when circulating sex hormones are high; however, as this review highlights, sex impacts biology from the moment of conception. Thus, studies examining prenatal drug and alcohol exposure should also use equal numbers of males and females in their experiments. Ultimately, early-life drug and alcohol exposure has significant effects on the brain and behaviour in both males and females; however, a careful analysis of future basic animal research and clinical data may reveal important and significant sex differences in these effects that can inform diagnostic criteria and therapeutic or behavioural interventions targeting affected children.

Authors' contributions

L.T., J.G. and J.S. contributed equally to the manuscript concept and design, drafting and revision of subsequent versions, and completion of the final version.

Competing interests

We have no competing interests.

Funding

The authors acknowledge the funding sources for ongoing work in the laboratory including R21MH101663 to J.M.S. and P20GM103653 to J.M.S.