The placenta as a target of opioid drugs

Abstract

Opioid drugs are analgesics increasingly being prescribed to control pain associated with a wide range of causes. Usage of pregnant women has dramatically increased in the past decades. Neonates born to these women are at risk for neonatal abstinence syndrome (also referred to as neonatal opioid withdrawal syndrome). Negative birth outcomes linked with maternal opioid use disorder include compromised fetal growth, premature birth, reduced birthweight, and congenital defects. Such infants require lengthier hospital stays necessitating rising health care costs, and they are at greater risk for neurobehavioral and other diseases. Thus, it is essential to understand the genesis of such disorders. As the primary communication organ between mother and conceptus, the placenta itself is susceptible to opioid effects but may be key to understanding how these drugs affect long-term offspring health and potential avenue to prevent later diseases. In this review, we will consider the evidence that placental responses are regulated through an endogenous opioid system. However, maternal consumption of opioid drugs can also bind and act through opioid receptors express by trophoblast cells of the placenta. Thus, we will also discuss the current human and rodent studies that have examined the effects of opioids on the placenta. These drugs might affect placental hormones associated with maternal recognition of pregnancy, including placental lactogens and human chorionic gonadotropin in rodents and humans, respectively. A further understanding of how such drugs affect the placenta may open up new avenues for early diagnostic and remediation approaches.

Opioid usage among pregnant women have risen dramatically, and such drugs can affect placental function by affecting endogenous opioid receptors that regulate production of human chorionic gonadotropin (hCG), placental lactogens, and other genes.

Graphical Abstract

Graphical Abstract.

Introduction

Opioids were first identified in the opium poppy plant, but many of the current ones are synthetic forms. They are primarily used as analgesic agents to control pain associated with a range of causes. Agonistic and partial agonist opioids act by binding to opioid receptors that are stimulated by endogenous endorphins, the body’s natural substances to reduce pain and produce feelings of well-being. The effectiveness of such drugs in mitigating pain has resulted in prescribed and nonprescribed abusage of opioids, in particular by pregnant women. Opioid abuse has risen to be one of the leading noninfectious disease public health concerns and economic challenges facing the United States [1]. Opioid use disorder (OUD) is a particular health concern in women of childbearing age [2] with OUD during pregnancy estimated to affect 5.6 per 1000 live birth infants [3]. Neonates exposed during gestation to opioids are at risk for neonatal abstinence syndrome (NAS) [4]. Maternal OUD has been associated with poor fetal growth, increased risk for premature births, low birthweight offspring, and congenital defects [5, 6]. Adult-onset diseases due to developmental origin of health and disease (DOHaD) effects of these drugs are also possible [7, 8]. For all these reasons, it is essential that we understand how opioids alter the trajectory of fetal organ development and predispose to later disorders that might arise later in life.

Opioids circulating in the maternal blood can easily cross the placenta. In humans, nonhuman primates, and rodents, the trophoblast (TB) cells of the placenta are directly bathed in maternal blood thereby ensuring continual exposure. In this review, we will consider the evidence to date that opioids affect placental structure and function. Surprisingly, the TB cells express opioid receptors, and evidence suggests that endogenous opioids, such as endorphins, may even govern essential physiological processes in these cells. Consequently, maternal consumption of opioids that transit through the placenta may tamper the vital regulatory processes. Much though remains to be determined how opioids target the placenta. Thus, open-ended questions and future directions will be explored. By understanding how this class of drugs act on the placenta, we may be able to establish early diagnostic and remediation strategies to prevent later diseases with a DOHaD origin.

General mechanisms of opioid signaling and actions

Endogenous and pharmaceutical opioids act by binding to opioid receptors that are classically present in the central nervous system, peripheral nervous system, and gastrointestinal system [9–11], although as detailed below such receptors have been identified in the placenta. Based on their activity to bind and activate such receptors, opioids are classified as agonists, partial agonists, or antagonists. Examples of agonists include morphine, methadone, oxycodone (OXY), and butorphanol. Buprenorphine is considered a partial agonist as it shows reduced binding and activation of opioid receptors. Naltrexone and naloxone act as competitive opioid receptor antagonists and are used to reverse in cases of opioid overdose or in the cases of OUD [12, 13]. The structures for one of the main endogenous opioids, β-endorphin, and example agonists and antagonists are shown in Figure 1. Their binding affinity to the different opioid receptors is depicted in Table 1.

Figure 1.

Structures of an β-endorphin, an endogenous opioid, and select pharmaceutical opioid agonists and antagonists are shown. The structure of β-endorphin is from PubChem (https://pubchem.ncbi.nlm.nih.gov/compound/beta-Endorphin-_1-9#section=2D-Structure). All the other structures are derived from http://www.chemspider.com/.

Table 1.

Select opioid drugs and their binding affinity to different types of opioid receptors

	Receptor type
Opioid	MOP	KOP	DOP	NOP
β-endorphin	+++	+++	+++	−
Morphine	+++	+	+	−
Oxycodone	+++	+	+	−
Hydromorphone	+++	+	+	−
Butorphanol	−	++	+	−
Methadone	+	+++	+++−	−
Fentanyl	+++	−	−	−
Codeine	++	+	−	−
Buprenorphine	++	−

MOP = μ opioid receptor; KOP = κ opioid receptor; DOP = δ opioid receptor; NOP = nociceptin opioid peptide receptor. – = No affinity; + = low affinity; ++ = moderate affinity; +++ = high affinity.

The main receptor forms include μ, κ, and δ opioid receptors [14], but up to 17 forms have been reported [14]. There is also a lesser characterized subtype termed opioid receptor-like (ORL1), also called opioid related nociceptin receptor 1 (OPRL1), nociceptin opioid peptide receptor (NOP), nociceptin/orphanin FQ (N/OFQ) receptor, or kappa-type 3 opioid receptor.

Opioid receptors are seven-transmembrane domain receptors that couple to intracellular signaling molecules by activating heterotrimeric G proteins [14]. Binding of partial agonist/agonist results in dissociation of Gα and Gβ_γ subunits from each other. They are then free to stimulate intracellular signaling pathways. Coupling of the four main opioid receptors (μ, κ, δ, and OPR1) to G proteins suppresses cAMP formation. The primary effect of such receptors though is to mediate calcium and potassium ion channels. Potassium channel deactivation that results from this signaling cascade leads to cellular hyperpolarization and reduction in tonic neural activity. All four receptors result in suppression of Ca⁺² currents in the cell, and some opioid agonists may act acutely to reduce calcium content in the synaptic vesicles. By inhibiting adenyl cyclase activity, cAMP-dependent Ca⁺² influx is also suppressed. Opioid receptor activation can also affect mitogen-activated protein kinases (MAPK) signaling [14]. These pathways are illustrated in Figure 2 from [14].

Figure 2.

Diagram of opioid receptor signaling and recycling. The primary four opioid receptor subtypes, μ, δ, κ, and ORL1, act via similar pathways. Opioid receptors are seven-transmembrane domain receptors that couple to G proteins. Binding of partial agonist/agonist results in dissociation of Gα and Gβ_γ subunits from each other. Coupling of the four main opioid receptors (μ, κ, δ, and OPR1) to G proteins suppresses cAMP formation. All four opioid receptors result in suppression of Ca⁺² currents in the cell. By inhibiting adenyl cyclase activity, cAMP-dependent Ca⁺² influx is also suppressed. In contrast, K⁺ channels are stimulated that results hyperpolarization of the cell. Opioid receptor activation can also affect MAPK signaling. Abbreviations: α G-protein alpha subunit, arrestin phosphorylation-dependent GPCR scaffold; βγ, G-protein beta-gamma subunit; cAMP, cyclic adenosine monophosphate; ERK ½, extracellular signal-regulated kinase; JNK, c-Jun N-terminal kinase; p38, p38 MAPK; P, phosphorylation. This figures has been reproduced from [14] with permissions obtained through Copyright Clearance Center’s RightsLink service (order number 5203420264867).

ORL-1 (OPRL1 or NOP) is also a GPCR with 65% structure homology to the other members of the opioid family, but its endogenous 17 amino acid neuropeptide ligand known as nociceptin (N/OFQ) or peptide nociceptin (PNOC) may induce nociceptive and antinociceptive actions, which could be due to signaling pathways shown in Figure 2 and other novel mechanisms [15]. Endogenous opioid peptides and morphine-like drugs have little binding affinity. NOP signaling is important in regulating instinctive and emotional behaviors. Antagonists to this receptor are currently being considered for treating depression, Parkinson’s disease, and as nonaddictive painkillers. Such drugs might be especially useful in treating pregnant women with depression or pain as this receptor exhibits minimal expression in the placenta (https://www.ncbi.nlm.nih.gov/gene?Db=gene&Cmd=DetailsSearch&Term=4987). OPRl1 likely activates similar pathway as other opioid receptors, as shown in Figure 2, but some of its effects may also involve other mechanisms that remain to be determined. For instance, nociceptin inhibits uterine myometrial contractions, but this physiological response is only partly mediated via heterotrimeric G proteins coupled to OPRL1 receptors leading to increased intracellular cAMP levels and through activation of Ca⁺²-dependent K⁺ channels [16]. A comprehensive map of all known opioid receptor signaling pathways is available at https://www.wikipathways.org/index.php/Pathway:WP5093 [17].

The primary medical usage of opioids is to treat pain arising from a variety of causes, including those acute and chronic in origin [18, 19]. Other reasons such drugs might be prescribed are to treat diarrhea as it inhibits gastrointestinal peristalsis allowing more time for water reabsorption by the large intestine [20, 21]. If used chronically though to treat diarrhea, such drugs can then lead to constipation that necessitates treatment with an opioid receptor antagonist, such as naloxegol. Additionally, the opioid drugs, such as codeine, hydrocodone, and butorphanol, have been used with varying success as cough suppressants or antitussive agents [22–26]. Opioid drugs may also be used in case of shortness of breath or dyspnea due to chronic obstructive pulmonary disease, cancer, and, more recently, COVID-19 [27–31]. Side effects of such drugs when used for any of the above conditions relate to their central and peripheral effects and include sedation, euphoria, nausea, respiratory depression, bronchospasm, hypotension, rash, itchiness, urinary retention, and constipation. Long-term usage of opioids has been linked to hypogonadism, endocrinopathies, including hypoandrogenism, and male and female infertility disorders (Figure 3) [32–38].

Figure 3.

Proposed model of how endogenous opioids and pharmaceutical opioids act within the placenta.

Opioid tolerance occurs when there is need to increase the dosage to obtain the same desired analgesia [19, 39–41]. However, increasing dosages can more often than not lead to one or more of the undesirable side effects, resulting in patients discontinuing the prescribed opioid drug or treating themselves with increasing dosages to maintain the same or even greater analgesics effects compared to when the drug was initially prescribed [19, 39–41]. All of the current opioid drugs demonstrate analgesic tolerance and the potential for addiction that can result in OUD. Opioid addiction and OUD remain at epidemic levels in the United States and globally and will likely continue to escalate in coming decades [40]. Opioid use disorder is estimated to affect presently over 16 million individuals globally and 2.1 million in the United States, and it contributes to over 120 000 deaths annually [40].

Those who attempt to discontinue opioid drugs may experience severe withdrawal symptoms and increased pain responses, further accounting for why many individuals continue to use and even abuse these drugs. The mechanisms underlying opioid tolerance are likely multifaceted but appear to primarily involve regulation of opioid receptors. Such responses involve desensitization of opioid receptors with chronic treatment, phosphorylation changes, β-arrestin binding, endocytosis, resensitization, and potential recycling of these receptors [41]. Chronic opioid treatment may lead to a reduction in opioid receptors both due to decreased synthesis and cellular internalization and subsequent degradation of these receptors [42–44]. Both mechanisms leave fewer opioid receptors to respond ingested opioids and correspondingly contribute to opioid tolerance. The demands of pregnancy, especially coupled to underlying diseases, such as gestational diabetes, obesity, and other disorders, result in many pregnant women experiencing acute and chronic pain. Between 2008 and 2012, 2.5% of pregnant women were prescribed greater than a 1-month supply of a chronic opioid drug [45]. The current rate of OUD during pregnancy is approximately 5.6 per 1000 live births [3]. One report suggested that more than 85% of pregnancies in women plagued with OUD were left untreated [46]. Pregnant women with OUD had delayed and reduced rates of prenatal care compared to women without substance use disorders [47]. Opioid use is associated with reduced fecundity in women trying to conceive [48]. We are at the nascence of understanding the ramifications for the mother and her infant on opioid usage while pregnant. It is increasingly becoming apparent that such drugs can target the placenta, which will be considered in the following sections.

Reproductive and pregnancy complications associated with maternal opioid usage

In this section, we will consider female reproductive deficiencies and obstetric complications associated with maternal opioid treatment in rats and humans. Female rats treated with morphine sulfate from periconception through lactation weigh less during gestation but more during lactation than saline control counterparts [49]. Morphine-treated rats produced reduced litter sizes, increased number of stillborn, had increased infant mortality, and growth retardation of their offspring was evident [49]. In humans, opioid use is associated with reduced fecundity in women trying to conceive and potential pregnancy loss [48]. Birth outcomes associated with maternal OUD include poor fetal growth, potential premature birth, low birthweight, and possible congenital defects [5, 6, 48]. Female rats treated with OXY have prolonged estrous cycles and reduced number of corpora lutea [50]. In short, rodent model experiments and human epidemiological studies indicate that opioid drugs can compromise female reproductive function and lead to pregnancy complications that might affect fetal and infant survival. Those offspring who do survive may exhibit neurobehavioral disorders, as detailed in the next section.

Neurobehavioral disorders in offspring exposed prenatally to opioids

Extensive evidence in rodent models and humans links prenatal exposure to opioid drugs and neurobehavioral disorders in infants and later in life. Such effects of opioids might be due to direct effects on fetal brain development, but others might originate due to opioid-induced placental dysfunction with secondary disturbances on the placenta–brain axis, as detailed below. We will first consider effects seen in rodent model studies.

We found that developmental exposure of mice to OXY is linked with adult metabolic and behavioral disruptions, including cognition and exploratory behaviors [51]. In offspring born to Sprague–Dawley dams that were permitted to self-administer OXY, they elicit fewer calls at PND 3, but this pattern shifts at PND 9 with higher maternal OXY intake linked with greater number of calls by their pups [52]. In utero and postnatal exposure of Sprague–Dawley rats to OXY enhances anxiety-like behaviors and leads to social deficits that can be transmitted to subsequent generations [53]. Prenatal exposure of Sprague–Dawley rats to other opioid drugs, methadone or buprenorphine, results in later social deficits and cognitive deficits [54]. Other reports also suggest early exposure to OXY and likely other opioid drugs affects later cognitive responses. Male offspring born to female Sprague–Dawley rats treated with OXY throughout breeding and gestation have impairments in spatial learning and memory [55]. Spatial memory of male but not female Wistar rats and grand offspring is compromised in those derived from dams treated with morphine [53]. Rat offspring derived from female and male parents treated with morphine have memory deficiency that is presumably due to suppression of long-term potentiation in the hippocampus [56]. In rodents, early exposure to opioid drugs might also increase later anxiogenic behaviors. For instance, a previous study with rats found that prenatal exposure to morphine increases anxiety-like behaviors [57].

How maternal exposure to opioids and NAS relate to later behavioral outcomes in children has been a topic of active concern. In this section, we will consider select papers that establishes clear linkages between developmental exposure to opioid drugs and later deficits in one or more neurobehavioral domains. At birth, prenatal exposed infants have significant reductions in several brain regions, including smaller relative voles of deep gray matter, thalamic and sub-thalamic components, brainstem, and they have reduced amount of cerebrospinal fluid [58]. One cohort study found that the degree of NAS strongly correlates with poor neurodevelopmental outcomes in cognition, language acquisition, and motor skills that collectively resulted in more overall behavioral problems at 18 months of age [59]. Another report that tested at 24 months of age infants exposed during gestation to methadone versus those who were not found exposed to this opioid exhibit significant reductions in attention, regulation, and motor skills but are more excitable and easily aroused [60]. Furthermore, these infants demonstrate nonoptimal reflexes, hypertonicity, and evidence of stress abstinence. Similar outcomes were obtained in another report with 2-year children that found methadone exposed individuals have lower motor, cognitive, and language scores and compromised self, emotional, feeding behaviors, and sensory processing than unexposed children [61]. A prospective study reported that children who were prenatally exposed to opioids and diagnosed with NAS within the first 3 years of life are more likely to exhibit poorer growth and development, mental health, musculoskeletal formation, but show increased risk for infection, neonatal, sensory, and social deficits [62]. Another prospective study that followed children from 2 to 9 years of age found that those exposed early on to opioid drugs demonstrate emotional and behavioral problems that progressed with age [63]. Taken together, the rodent model and human prospective studies strongly suggest that maternal OUD and NAS are associated with offspring neurobehavioral disorders that appear to affect brain development and subsequent cognition, emotional, motor, sensory, anxiogenic, and communication behaviors. It is uncertain whether these changes are due to direct effects on the brain versus those mediated through the placenta that is increasingly being recognized to be a target of opioid drugs, as detailed in the next sections.

Endogenous opioids and signaling in the placenta

Dating back to the 1980s, there has been an interest in characterizing endogenous opioids present within the placenta and specific opioid receptors [64–67]. The naturally occurring opioids identified in the placenta include β-endorphin, enkephalin, leucine enkephalin, and dynorphins 1–8 and 1–13 with dynorphin 1–8 being the major form in the human placental villi [68]. In the rat placenta, dopamine and apomorphine stimulate the secretion of β-endorphin, and these responses occur in a dose-dependent manner [69]. Kappa opioid receptors are the primary ones expressed by human placental cells [70–73]. In the mouse placenta, the delta opioid receptor and opioid precursor, proopiomelanocortins are co-expressed in TB giant cells [74]. Mu opioid receptor forms, including μ4 and μ1 opioid receptor, have also been identified in human placental cells [75]. Binding of morphine to these placental receptors results in a rapid release of nitric oxide that is inhibited by naloxone [75].

Several other actions have been ascribed to opioid signaling in the placenta, but it is not clear if different endogenous opioids induce varying affects and whether the physiological responses change throughout gestation. The data below represent what is currently known about the effects of these endogenous compounds on the placenta, but much remains to be determined. Addition of β-endorphin to cultured human choriodecidual cells suppressed interleukin-8 (IL-8) production [76]. This cytokine has been implicated in triggering parturition, and thus, opioid inhibition of IL-8 may help prevent preterm labor. Kappa opioid receptors in the placenta appear to be coupled to L-type calcium channels that may stimulate transduction signaling pathways and conductance across the placental villi [77]. Opioid binding to kappa receptors results in increase human chorionic gonadotropin (hCG) release by human TB tissue, and this response may also be mediated by locally produced gonadotropin-releasing hormone (GnRH) [78]. Another study by this same research group suggested that other opioid receptors, including mu and delta forms, may also promote opioid-induced release of hCG by TB cells [79, 80]. Another group suggested that the effects of endogenous opioids and hCG secretion are time dependent with β-endorphin inhibiting hCG at 7–9 weeks of gestation but stimulating this hormone at 11 weeks of gestation [81]. Kappa opioid receptors also appear to stimulate release of human placental lactogen from TB cells, which may also involve extracellular calcium [82]. Kappa opioid receptors in human placental villi suppress acetylcholine release [83]. The collective studies to date suggest that endogenous opioids increase hCG and placental lactogen production by the placenta, and some of these effects may involve Ca⁺² signaling pathways.

Intriguingly, the effects of opioid activity in the placenta persist beyond birth. Placentophagy or consumption of the placenta after birth is practiced by some cultures. This behavior has been documented in almost all female terrestrial eutherian mammals totaling over 4000 species [84–86] but is largely absent in aquatic (such as cetaceans) and semi-aquatic (pinniped) mammals [86]. While placentophagy may have evolved as a predator avoidance response, it has also been proposed to induce beneficial responses for the mother, including nutrient reclamation, suppressing postpartum depression, and analgesic effects [16]. To reduce the cannibalistic concern of eating raw flesh, several methods now exist to cook the placenta or prepare encapsulated pelleted forms of it. Select animal model studies have been performed to determine whether this practice yields any health benefits for new mothers. One study found that placental ingestion by rats enhanced δ- and κ-opioid antinociceptive responses but blunted μ-opioid antinociception [87]. The findings suggest that placentophagy might alleviate some pain responses. Consumption of the placenta by Long-Evans rats reversed centrally administered morphine inhibition of gut peristalsis but had no effect when this drug was systemically administered [88]. Thus, besides exhibiting analgesic properties, placentophagy may improve gastrointestinal function in new mothers. In rats, endogenous opiates stimulate placentophagy and pup grooming during and immediately after birth, as these behaviors are suppressed by an opioid antagonist [89]. The analgesic properties resulting from ingestion of the placenta and amniotic fluid might be attributed to placental opioid-enhancing factor [90]. Even though consumption of the placenta might lead to these beneficial analgesic responses, many physicians and organizations, such as the Society of Obstetricians and Gynaecologists of Canada (SOGC) and Center for Disease Control, advise against this practice, as can also lead to disease transmission and increase risk of exposure to heavy metals and other environmental toxicants [16, 91, 92].

Effects of opioid pharmaceutical agents on the placenta

Opioid drugs appear to lead to a range of pathological and molecular change in human and mouse placental cells. Pregnant women prescribed either methadone or buprenorphine strongly correlated with placental abnormalities that included delayed villous maturation, placentomegaly, and corresponding decreased fetal-to-placental ratio relative to women not prescribed these mediations while pregnant [93]. Analogous findings of delayed villous maturation in pregnant women exposed to opioid therapy were reported in a cohort study that spanned from 2010 to 2012 and included more than 1000 women between the exposed and nonexposed groups [94]. Morphine administration to pregnant mothers significantly correlates with vasoconstriction of placental vasculature that may compromise nutrient and oxygen delivery to the placenta and thereby the fetus [95].

Chronic consumption of the opioid drug, methadone, during pregnancy is associated with downregulation or desensitization of opioid receptors in human placental villous tissue [96]. Such treatment is also associated with opioid tolerance by human placental tissue, presumably due to suppression of kappa opioid receptors, structural confirmation changes to such receptors that alter the binding to endogenous opioids, and possibly downstream signaling pathways [97]. In human placental TB cell lines, JEG3 and BeWo, methadone, buprenorphine, and norbuprenorphine activate aryl hydrocarbon receptor that stimulates an increase in breast cancer resistant protein (BCRP) transcript in these cells [98]. Treatment of human placental microsomes with methadone or the partial opioid agonist, buprenorphine, inhibits aromatase conversion of testosterone to estrogen [99].

Infants with severe NOWS show reductions in placental aromatase mRNA expression [100]. A follow-up study by this same group revealed that syncytiotrophoblast aromatase immunostaining is reduced in opioid exposed cases compared to nonexposed infants [101]. Infants with severe NOWS have lower placental aromatase immunoreactivity in these cells than those without severe NOWS. Examination of the transcriptome profile in the placenta of infant with NOWS revealed significant gene expression changes relative to infants without this condition [102]. The top upregulated genes were cytochrome P450 Family 1 Subfamily A Member 1 (CYP1A1), FP671120.3, Checkpoint DNA Exonuclease (RAD1), and RN7SL856P, and the most significantly downregulated genes included RNA5SP364, Glutamate Ionotropic Receptor NMDA Type Subunit 2A (GRIN2A), Unc-5 Netrin Receptor D (UNC5D), DMBT1P1, MIR3976HG, LINC02199, LINC02822, POU3F3 Adjacent Non-Coding Transcript 1 (PANTR1), AC012178.1, and Catenin Alpha 2 (CTNNA2). One pilot study considered whether the placenta of infants with NOWS demonstrates epigenetic changes in the form of altered methylation of the OPRM1 gene [103], but this limited study did not find any such associations. However, global DNA methylation and other epigenetic changes, in particular miRNA and other small RNA profiles, should be examined in the placentas of these neonates.

We sought to determine whether maternal Oxycontin (OXY) exposure affects the morphology and transcriptome profile as determined by RNAseq in E 12.5 mice placenta [104]. Maternal OXY treatment diminished the parietal TB giant cell area and maternal blood vessel area within the labyrinth region. OXY exposure transformed placental gene expression profiles in a sexually dimorphic manner with female placenta having upregulation of several placental enriched genes, including Carcinoembryonic antigen-related cell adhesion molecule 11 (Ceacam11), Carcinoembryonic antigen-related cell adhesion molecule 14 (Ceacam14), Carcinoembryonic antigen-related cell adhesion molecule 12 (Ceacam12), Carcinoembryonic antigen-related cell adhesion molecule 13 (Ceacam13), prolactin family 7, subfamily b, member 1 (Prl7b1), Prolactin family 2, subfamily b, member 1 (Prl2b1), cathepsin Q (Ctsq), and Trophoblast-specific protein alpha (Tpbpa). Placenta of OXY exposed males exhibited expression changes in several ribosomal proteins. Weighted correlation network analysis revealed that in OXY females versus control females, select modules correlated with placental histological changes induced by OXY. Such associations were absent in the male OXY versus control male comparison. Pathways potentially affected in OXY females based on gene sets in these modules include extracellular matrix reorganization, vascular endothelial growth factor (VEGF) signaling, and regulation of actin bioskeleton, collagen biosynthesis, peptide hormone signaling, interferon signaling, interferon gamma signaling, and triglyceride metabolism and catabolism.

The current studies indicate that opioid drugs can affect placental structure, transcriptome profile, and select proteins, such as aromatase. At least one study has linked such changes in the placenta to the degree of infant NOWS severity. However, it is not clear from the current work if there are developmental windows where exposure to opioid drugs can lead to more pronounced effects. While there are selective placental histoarchitecture changes, opiates do not appear in general to prevent placentation, although as noted above some studies indicate that mothers with OUD might experience fetal loss, which could be due improper placentation.

Placental transfer and metabolism of opioid drugs

Many opioid drugs might be transferred across the placenta where they can collect and cause harmful changes, but they also be transferred across the placenta to the fetus, where such compounds might directly influence fetal development [105]. The placenta possesses some ability to metabolize certain opioids, but the full range of such metabolite derivatives on the fetus requires further exploration. In this section, we will consider some of the studies that have examined the disposition and metabolism of opioid drugs in the placenta. By using an ex vivo transfer approach with preterm and term placenta, it was determined that preterm transfer and clearance index of methadone are reduced in preterm compared to term placenta [106], suggesting that placental transfer and metabolism of at least this opioid drug might fluctuate throughout pregnancy. Other work suggests that levels of placental methadone and its metabolite, [2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), may predict NAS severity [107]. Other data suggest that placental transfer of three opioid drugs, fentanyl, alfentanil, and sufentanil, escalates with increased maternal blood flow [108]. Positron emission tomography and 11C-labeled morphine administration to pregnant Rhesus monkeys have revealed that this drug is readily transferred across the placenta with a rapid appearance of conjugated morphine metabolites [109]. In pregnant baboons, morphine-3-beta-glucuronide (M3G), an active metabolite of morphine, is actively transferred across the placenta to the fetus [110]. In contrast, DALDA (Tyr-D-Arg-Phe-Lys-NH2), a potent and highly selective μ agonist, shows limited transfer across the sheep placenta to the fetus [111]. While the investigators conclude that these results suggest this compound might be a promising analgesic in pregnant women, the sheep placenta is less invasive than that of humans and nonhuman primates. Thus, the findings might not be fully translateable. Most results indicate that parental opioid drugs and their metabolites are readily transferred across the placenta to the fetus.

Conclusions and future directions

With the increasing number of pregnant women afflicted with OUD, it is essential to ascertain the full range of effects such drugs have on the conceptus, especially at the time when new organ systems are being launched. The proximity of the placenta to maternal blood renders it vulnerable to opioid and other drugs circulating in the maternal bloodstream. Figure 3 provides a model of how endogenous and pharmaceutical opioids might act in the placenta. Cell culture approaches, in particular placental organoids, TB stem cells, and differentiated placental lines, will likely be useful in examining how endogenous and pharmaceutical opioids affect different TB cell lineages.

While we are in the infancy of understanding how opioids target the placenta, the downstream effects, especially on the brain, should also be in our line of sight. This close association between the two organs has led to coinage of the term, placental–brain axis [112, 113]. How opioids influence this axis remains uncertain. It is clear that children and rodent models developmentally exposed to opioids are at risk for neurobehavioral disorders [51, 59, 60, 114–117]. The degree to which these stems from direct effects of the fetal brain or through the placenta–brain axis remains to be determined. In defining the contribution of the placenta to such later diseases, conditional transgenic models might be employed. For instance, through usage of the Cre-Lox system, select opioid receptors could be abolished in the placenta as a whole or individual TB cells. Examples of such placental specific Cre-models include Cre-Cyp19 that encodes placental-specific aromatase cytochrome P450 to ablate a given gene in the whole placenta and Cre-Prl2c2 (proliferin) to delete it selectively in parietal trophoblast giant cells (pTGC) and spiral artery GC of the placenta [118, 119]. Other such transgenic models exist and are being developed.

Extracellular vesicles (EV) might be important in carrying the cargo load from the placenta to the fetal brain and other organs [120]. Based on their architecture and contents, EVs are categorized into varying groups of microvesicles, exosomes, and apoptotic bodies. Such structures act like a shell to encase and protect their contents from GI destruction and metabolism and in the process shuttle these structures to remote target organs. Exosomes or EVs contain proteins (receptors, transcription factors, extracellular matrix proteins, and enzymes), lipids, DNA, mRNA, miRNA, and other metabolites [120]. We recently discovered that exposure to the endocrine disruptor, bisphenol A, affects miRNAs in the placenta that act upon mRNA associated with fetal brain development [121]. No studies to date have explored whether opioids affect placental miRNA and EV contents.

In conclusion, OUD in pregnant women can lead to pernicious effects on the placenta. The complete extent to which opioids affect individual TB lineages, including their morphology and molecular machinery, awaits future studies. Future experiments also need to consider the extent to which opioid treatments shape the placenta–brain axis whether this be through hormones, neurotransmitters, EV contents, including miRNA, or other factors. All of these studies are imperative in developing improved mitigation strategies to combat opioid-induced pathologies in the placenta and preventing DOHaD effects.

Author contributions

CSR researched, wrote, and edited the manuscript.

Data availability

The current article describes previously published work. RNA sequence data reported from my laboratory have been deposited in the Gene Expression Omnibus (GEO) database https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE152172.

Conflict of interest

The authors have declared that no conflict of interest exists.