Introduction

Proper development and functioning of the neocortex critically depends on the coordinated production and migration of excitatory and inhibitory neurons [1–3]. Representing 10–25% of total number of cortical neurons, GABAergic interneurons expressing parvalbumin (PV+) and somatostatin (SOM+) are the main source of inhibition. GABAergic interneurons control the activity of pyramidal neurons [4, 5] and play a crucial role in shaping cortical maturation at various stages of development [4]. Disruptions of cortical GABAergic circuitry at several stages of development contribute to various neurodevelopmental disorders [6] including autism [7, 8], epilepsy [1, 9, 10] and schizophrenia [8, 11].

During in utero development, GABAergic interneuronal system is vulnerable to several agents including prenatal ethanol exposure [12–14]. Prenatal ethanol exposure can alter the migration of GABAergic interneurons [14–16] which might represent a potential mechanism by which prenatal ethanol exposure can lead to the postnatal behavioral and cognitive dysfunctions encountered with Fetal Alcohol Spectrum Disorders (FASDs). The exact molecular mechanisms by which prenatal ethanol exposure can affect the GABAergic interneurons during their cortical migration are still unclear. This article discusses different molecular mechanisms by which prenatal ethanol exposure can alter GABAergic interneuronal cortical migration.

Embryology of GABAergic Interneurons

There are three distinct classes of GABAergic interneurons in the rodent neocortex according to the molecular markers they express: PV+, somatostatin (SOM+), and 5-hydroxytryptamine 3A receptor positive (5-HT3AR+) cells [17, 18]. The vast majority of murine GABAergic interneurons in the neocortex are derived from the subpallium [19], in which PV+ and SOM+ cells originate from medial ganglionic eminence (MGE) [19] whereas most 5-HT3AR+ cells originate from the caudal ganglionic eminence (CGE) [18] and preoptic area (POA) [20, 21].

MGE is the primary source of GABAergic cortical interneurons, accounting for 50–60% of cortical GABAergic interneurons [22, 23]. Following a tangential migration pattern, inhibitory GABAergic interneurons migrate throughout the developing telencephalon to reach the cortex [24, 25]. Migratory pathways followed by CGE- and POA-derived interneurons to reach their cortical destination are largely distinct from those used by MGE-derived interneurons. Whereas MGE-derived cells migrate toward the rostrocaudal regions of the neocortex [26], CGE-derived and POA-derived cells primarily migrate toward the caudal pole [27, 28], and rostral region of the neocortex [20] respectively. This suggests that interneurons born in the CGE and POA respond, at least in part, to a different set of guidance cues [29].The migration of GABAergic interneurons generated from MGE commences around embryonic day 9.5 (E9.5) with a peak at E13.5 -E16.5 [30] followed by the migration of neurons generated in other areas. GABAergic interneurons start reaching the cortex by E14. Upon reaching the cortex, GABAergic interneurons integrate themselves in an “inside-out” pattern where earlier coming older interneurons lie deeper to the newly coming younger ones [31–33]. MGE-derived interneurons are expected to reach their adult brain-like maturation by postnatal days 16–21 [31]. In humans, neuronal migration takes place predominantly between 12 and 20 weeks of gestation [34].

The migration of neurons from the subpallial origins to the cerebral cortex is a complicated process involving the activity of various motogens, chemotactic factors, transcription factors, as well as neurotransmitters [35, 36]. Chemical cues such as semaphorines and ephrins are expressed as gradients in the brain and serve as attracting or repelling signals for migrating cells [37, 38]. Several prenatal factors including stress [39] and pharmacological agents [40] can disrupt the GABAergic interneuronal system. This review will only focus on the effect of prenatal ethanol exposure on GABAergic interneuronal migration.

Fetal Alcohol Spectrum Disorders (FASDs)

Prenatal ethanol exposure results in neurodevelopmental deficits and lifelong disability in offspring [41]. FASDs, which encompasses fetal alcohol syndrome, alcohol-related birth defects, and alcohol-related neurodevelopmental disorder, are estimated to affect at least 1% of all births in the United States [42]. In pregnant women, the prevalence of any alcohol use and binge drinking in the past 30 days was 10.2 and 3.1%, respectively [43]. Animal studies showed that binge-like drinking patterns in pregnant females are particularly dangerous to fetal brain development, even if the total amount of ethanol consumed is less than that consumed in a more continuous drinking pattern [44]. Children with FASDs are often described as hyperactive, distractible, impulsive, with short attention spans, impaired cognitive functions and learning difficulties [45, 46]. Epidemiological studies have suggested that alcohol use problem of parents might be related to the hyperactivity or inattention symptoms in their children [47]. Prenatal ethanol exposure in experimental animals results in hyperactivity [48] and impaired executive functions [49]. The mechanism by which prenatal ethanol exposure contribexposure contributes to such myriad of symptoms is not fully understood.

Prenatal Ethanol Exposure Impacts GABAergic Interneuron Cortical Migration

Prenatal ethanol exposure results in profound effects on cortical neuronal migration [50, 51]. Several studies have suggested that abnormal migration of GABAergic cortical interneurons is involved in FASDs (Table 1). Monkeys exposed prenatally to ethanol once per week for 4 or 24 weeks starting from the first week of gestation showed reduced GABA expressing neurons in somatosensory cortex [52]. In guinea pigs, ethanol administered through almost the whole pregnancy (from day 2 until the day before delivery) was associated with reduced expression of glutamic acid decarboxylase (GAD), the protein marker for GABAergic neurons, in layers II–III of somatosensory cortex [53]. The involvement of superficial cortical layers (layers II–III) suggested that chronic prenatal ethanol treatment seems to affect mostly the late-generated GABAergic cells [16]. In rats, the offspring of pregnant females exposed to ethanol over the whole gestation showed 45% fewer PV+ neurons in the anterior cingulate cortex [54]. Over all, it has been suggested that chronic prenatal ethanol consumption may result in reduced GABAergic cell density in specific cortical regions.

Table 1 Effect of prenatal ethanol exposure on the migration of GABAergic interneuron migration in experimental animals
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A different set of experiments came up with different conclusions. Prenatal ethanol exposure for 14 days (E0.5–E14.5) in mice resulted in increased density of GABAergic interneurons in all cortical layers of medial prefrontal cortex (mPFC) at E14.5 which was proposed to be due to increased tangential migration of GABAergic interneurons [14]. Sukorput et al. showed that gestational ethanol exposure during the peak of tangential migration of GABAergic interneurons (E13.5–E16.5 in mice corresponding to mid-first trimester in humans) resulted in increased density of GABAergic interneurons in layer V of mPFC. This was associated with significant electrophysiological and behavioral consequences. Mice exposed prenatally to ethanol during such critical period showed dysregulated neuronal excitability in the form of increased GABA-mediated inhibitory drive over pyramidal neurons. Behavioral deficits in the form of hyperactivity and long-term PFC-dependent executive dysfunction were also observed in these mice [15]. In another study done in mice, ethanol exposure between E11 and E14 produced mosaic abnormal distribution of GABA expressing cells in the somatosensory cortex at P10 and a tendency, though non-significant, for increased GABA expressing cells at P180 [16] which may suggest that alcohol may also alter cortical organization of already migrated GABAergic interneurons.

In summary, it seems that the type of exposure (chronic versus acute) could account for the differences observed in various models [16]. However, the mechanisms by which prenatal ethanol exposure can influence the programmed GABAergic migratory process is still poorly understood. This review proposes that ethanol can alter one or more of the factors regulating GABAergic interneuronal migration process which include neurotransmitters such GABA and dopamine, epigenetic and trophic factors.

GABAergic Pathway of Ethanol-Induced Abnormal GABAergic Interneuron Migration

There are two main categories of GABA receptors, the ionotropic GABA receptors, GABAA and GABAc, and the metabotropic GABAB receptors. Activation of GABAA receptors causes the opening of a channel formed at the center of the receptor complex, allowing the diffusion of chloride ions across the cell membrane [55]. GABAB receptors are coupled to K+ and/or Ca2+ channels via a G-protein mediated pathway or in a membrane delimited manner [56, 57]. There is clear evidence for the involvement of GABAA and GABAB receptors in neurodevelopmental disorders [57, 58]. Migration of GABAergic interneurons are largely controlled by GABAergic signaling [59, 60]. Reducing ambient GABA activity results in improper migration process with the accumulation of interneurons at the corticostriatal junction [61]. Migrating interneurons express GABAA and GABAB receptors, and their GABA responsiveness increases with the progression of the migration process [62]. In line with this, diazepam, a benzodiazepine which augments the activation of GABAA receptors, substantially increased the motility rate of migrating GABAergic interneurons [63]. The tangentially migrating GABAergic interneurons in the marginal zone of neonatal mice are also impaired after inhibition of GABAA receptors in vivo [63] demonstrating the important influence of endogenous GABA on the tangential migration.

It was suggested that tangentially migrating GABAergic neurons are themselves a source for GABA [59] which might constitute a promigratory signal autoactivating GABA receptors [64]. It has been shown that GABA can affect the migration process in a dose and receptor dependent manner and the differential role of GABA receptors on neuronal migration might differ according to type of targeted neuron [64–68]. Acting on GABAergic interneurons, GABA can have a selective action at individual layers, being able to generate action potentials in layers V–VI but not in layers II–III [69]. Overall, GABA seems to affect the migration and function of GABAergic interneurons.

GABAergic signaling is a well-known target of alcohol. Alcohol potentiates GABA-mediated signaling through different mechanisms including increasing GABA release and increasing GABAA receptor activity [70–72]. It has been proposed that alcohol can increase the tangential migration by augmenting GABA signaling. In utero ethanol exposure elevated the ambient level of GABA and increased the sensitivity of MGE-derived cells to GABA promoting premature migration [14]. As different neurons respond differently to GABA, ethanol mimicking GABA action through the activation of GABAA could target preferentially the neurons that are more sensitive to GABA mediated depolarization i.e. layer V which might explain why prenatal ethanol exposure increased the migration to this particular cortical layer [15]. However, neurons less sensitive to GABAA-mediated depolarization i.e. layers II–III might be modulated by alcohol through metabotropic GABA receptors producing an opposite effect on migration [53].

GABAB receptors have been identified in the developing cerebral cortex [73] with particularly high density in tangentially orientated neurons in the lower intermediate zone (LIZ) of the cortex [74]. Several lines of evidence suggest that GABAB receptors activation might play an important role in cortical development. Pharmacological studies indicate that GABAB receptors activation stimulates migration of neurons in immature cortical regions [75]. Blockade of GABAB receptors with a specific antagonist, CGP52432, resulted in a concentration-dependent accumulation of these tangentially migrating neurons in the ventricular/sub-ventricular zones of the cortex and fewer cells were observed in the cortical plate/marginal zone and LIZ indicating an important modulatory role of GABAB receptors in the migration of cortical interneurons [76]. The role of GABAB receptors in mediating effect of prenatal ethanol exposure on GABAergic interneuronal migration is to be identified.

Dopaminergic Pathway of Alcohol-Induced Abnormal GABAergic Interneuron Migration

Another possible mechanism by which ethanol can influence the migration of GABAergic interneuron is modulation of dopaminergic transmission. Dopaminergic signaling has been shown to modulate the migration of GABAergic interneurons which express D1 and D2 receptors. D1 receptor normally functions to promote cortical interneuron migration while D2 receptor knockouts possess increased migratory capability [77]. The dopaminergic system is known to be significantly influenced by ethanol [78, 79] including prenatal ethanol exposure [80]. Ethanol enhanced dopaminergic signaling mediated through D1 [81]. Prenatal ethanol exposure led to the persistent abnormal synaptic plasticity via disturbing the balance between D1 and D2 mediated signaling, allowing more D1 over D2 activity [82]. Theoretically, altered dopaminergic signaling may provide a potential mechanism by which ethanol can influence the GABAergic interneurons migration. As dopamine receptors are widely distributed in the central nervous system, it might be difficult to use pharmacological agents to understand their contribution to prenatal ethanol-induced abnormal interneuronal migration. Selective knock down of dopamine receptors in migrating GABAergic neurons might be a more useful strategy in this context.

Prenatal Alcohol Exposure and Epigenetic Factors

Epigenetic regulation of gene expression has been shown to play a pivotal role in developmental processes. Recent studies have demonstrated epigenetic alterations in the etiology of FASDs [83]. Hyperactivity has been shown in offsprings of male mice exposed to preconception ethanol which may refer to the involvement of epigenetic factors [84]. Prenatal alcohol exposure caused altered DNA methylation pattern [84–86] and changed the expression of key epigenomic regulators e.g. DNA methyltransferase 1 (DNMT1), DNMT3a, and methyl CpG binding protein 2 (MeCP2) [87]. MeCP2 is a transcriptional regulator that binds to methylated DNA. MeCP2 regulates the expression of Brain-Derived Neurotrophic Factor (BDNF). BDNF is known to influence GABAergic interneuronal migration (see below), thus MeCP2 might be involved in the regulation of GABAergic interneuron maturation indirectly by regulating BDNF [88, 89]. MeCP2 regulates the transcription of DLX5, a transcription factor critical for the migration and maturation of PV+ interneurons [90]. Thus, it is possible that ethanol influences the GABAergic interneuron migration through an altered DNA methylation-MeCP2-BDNF/DLX5 pathway.

MicoRNAs (miRNAs) are small noncoding RNAs that regulate gene expression by binding to the 3′ untranslated region (3′UTR) of mRNA leading to its breakdown or translational repression. It has been shown that dysregulated miRNA mediated activity significantly influence GABAergic neuronal development and migration [91]. In vivo and in vitro studies showed that prenatal alcohol exposure resulted in several miRNA alterations both in the mother and the developing fetus [92, 93]. Alterations in miRNA expression were also found in primary neuronal cultures from the cortex of mice (E15) following chronic intermittent ethanol exposure and withdrawal [94]. Rats exposed to prenatal ethanol show altered miRNA expression in amygdala and striatum [95]. miRNAs have been implicated in the development of brain damage in response to prenatal ethanol exposure [96]. Experimental evidence indicates that the expression of miRNAs is altered following exposure to alcohol during development, and this may be one of the mechanisms underlying alcohol-related teratogenesis [97, 98]. miRNAs disrupted by ethanol exposure during critical periods, can affect neuronal migration by regulating several biological pathways regulating interneuronal migration such as BDNF [92].

Prenatal Alcohol Exposure and Neurotrophic Factors

The guidance of GABAergic interneurons from the subpallium to the developing cortex relies on multiple factors [99, 100]. Several neuronal growth factors such as Hepatocyte Growth Factor (HGF) [101], BDNF [102] and glial cell line-derived neurotrophic factor (GDNF) [103, 104] regulate the migration of cortical GABAergic interneurons. By stimulating TrkB-mediated pathways, BDNF strongly influences tangential interneuronal migration in the developing nervous system [102]. GDNF, through its receptor, GFRalpha, mediated signaling has been implicated in the development of GABAergic interneurons. GFRalpha signaling guides the development of a subset of PV+ expressing GABAergic interneurons in specific cortical regions [103]. HGF serves as an important molecular cue for the dispersion of ganglionic eminence-generated interneurons to their appropriate locations in the dorsal telencephalon. HGF receptor, MET, expression was evident in a polarized pattern on migrating cells from GE explants and its exogenous ligand disrupts normal cell migration [101].

Prenatal alcohol exposure has been found to influence the expression or the levels of these factors [105, 106] which may impact GABAergic interneuron migration. However, the effect of ethanol on the expression of these growth factors and their receptors is rather complex and there is clear deficit in our knowledge regarding their role in mediating the effects of prenatal alcohol exposure. Although adult mice showed reduced BDNF in response to prenatal ethanol exposure, neonatal rats that were exposed to ethanol throughout gestation [105] did not show change in BDNF levels in their cortex [107]. Other studies showed also that ethanol may reduce TrkB expression [108] thus indirectly influencing BDNF-mediated effects. The effect of alcohol on GDNF is also poorly understood. Alcohol increased expression of GDNF in ventral tegmental area [109] and cortex [105]. It reduced the expression of GDNF mRNA expression in neuronal progenitor cells generated from telencephalic tissue derived from E15–17 [106].

Currently, we cannot conclusively establish which of the different factors mentioned above (neurotransmitters, epigenetic regulators or trophic factors) and possibly others, are the major players linking prenatal alcohol exposure to abnormal GABAergic interneuronal migration (Fig. 1). The most probable is a complex interplay, with some of these factors being regulated in a maladaptive manner resulting in FASDs [16]. Despite the accumulating evidence about the involvement of GABAergic interneurons in the FASDs, it has been shown that prenatal exposure to alcohol during early pregnancy (E0–E8) i.e. before the critical period of tangential migration can still produce persistent behavioral dysfunction [110], which suggests the multifactorial nature of FASDs. In addition, the prolonged inhibitory effect related to ethanol can develop due to enhanced postsynaptic GABA signaling [111], which renders the functional and electrophysiological impact of ethanol on GABAergic signaling more complex.

Fig. 1
figure 1

Factors that might be involved in mediating the effect of prenatal ethanol exposure on GABAergic interneuronal migration

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Conclusion and Future Directions

Prenatal ethanol exposure affects the cortical migration of GABAergic interneurons which impacts cortical function and development. Understanding the mechanisms by which prenatal alcohol exposure can influence GABAergic interneuronal migration process is not an easy task. First, the process of GABAergic interneuronal migration is a complex phenomenon controlled by precisely orchestrated pathways and several important aspects of which are not fully understand. Second, alcohol can affect several molecular pathways and neurotransmitter systems on almost every neuronal cell-type. Although alcohol-free pregnancy is the best choice for the mother and the newborn, it might be difficult to achieve, this necessitates clear understanding of the mechanism by which alcohol can cause a permanent neuroanatomical and functional impairment for the possible generation of a safe “antidote” that might be successful in preventing or at least minimizing ethanol’s teratogenic effects.