Prenatal exposure to cocaine reduces the number and enhances reactivity of A10 dopaminergic neurons to environmental stress

Modulation by cocaine of dopamine receptors through miRNA-133b in zebrafish embryos

The use of cocaine during pregnancy can affect the mother and indirectly might alter the development of the embryo/foetus. Accordingly, in the present work our aim was to study in vivo (in zebrafish embryos) the effects of cocaine on the expression of dopamine receptors and on miR-133b. These embryos were exposed to cocaine hydrochloride (HCl) at 5 hours post-fertilization (hpf) and were then collected at 8, 16, 24, 48 and 72 hpf to study the expression of dopamine receptors, drd1, drd2a, drd2b and drd3, by quantitative real time PCR (qPCR) and in situ hybridization (ISH, only at 24 hpf). Our results indicate that cocaine alters the expression of the genes studied, depending on the stage of the developing embryo and the type of dopamine receptor. We found that cocaine reduced the expression of miR-133b at 24 and 48 hpf in the central nervous system (CNS) and at the periphery by qPCR and also that the spatial distribution of miR-133b was mainly seen in somites, a finding that suggests the involvement of miR-133b in the development of the skeletal muscle. In contrast, at the level of the CNS miR-133b had a weak and moderate expression at 24 and 48 hpf. We also analysed the interaction of miR-133b with the Pitx3 and Pitx3 target genes drd2a and drd2b, tyrosine hydroxylase (th) and dopamine transporter (dat) by microinjection of the Pitx3-3'UTR sequence. Microinjection of Pitx3-3'UTR affected the expression of pitx3, drd2a, drd2b, th and dat. In conclusion, in the present work we describe a possible mechanism to account for cocaine activity by controlling miR-133b transcription in zebrafish. Via miR-133b cocaine would modulate the expression of pitx3 and subsequently of dopamine receptors, dat and th. These results indicate that miRNAs can play an important role during embryogenesis and in drug addiction.

Prenatal cocaine exposure enhances long-term potentiation induction in rat medial prefrontal cortex

Prenatal exposure to cocaine has been reported to produce long-lasting cognitive deficits, but the underlying mechanisms remain largely unknown. Here, we report that the induction of long-term potentiation (LTP) at excitatory synapses onto layer V pyramidal neurons in the medial prefrontal cortex (mPFC) is facilitated in rats exposed to cocaine in utero (3 mg/kg, intravenous twice daily during embryonic days 10-20). This facilitated LTP is caused by a reduction of A-type γ-aminobutyric acid (GABA(A)) receptor-mediated inhibition of mPFC pyramidal neurons. Biochemical experiments revealed a significant decrease in the surface expression of GABA(A) receptor α₁ subunits and total protein levels of γ₂ and δ subunits in mPFC slices from rats exposed to cocaine in utero. Prenatal cocaine exposure also leads to enhanced mPFC pyramidal neuronal excitability. However, the development of behavioural sensitization to repeated cocaine administration was impaired in rats that were exposed to cocaine in utero. These results suggest that prenatal cocaine exposure causes a long-lasting reduction of GABAergic inhibition in mPFC layer V pyramidal neurons, leading to an increased susceptibility of excitatory synapses to LTP induction during the postnatal period.

The canary in the coalmine: the sensitivity of mesolimbic dopamine to environmental adversity during development

The hypothalamic-pituitary-adrenal axis has been the focus of extensive research with regard to the phenotypic plasticity this system shows in response to environmental influences on mammalian development. This review proposes that the mesolimbic dopamine system is similarly reactive to indicators of environmental adversity during development. Physical, physiological, and toxicological stressors encountered during perinatal development have been routinely demonstrated to affect dopamine neurophysiology, most likely through consequent exposure to maternal glucocorticoids or a reduction in oxygen supply. However, findings remain inconsistent with regard to the nature of impact these events have on the dopamine system. Both hyper- and hypo-dopaminergic changes have been noted. This review argues that the directionality of change is a function of chronicity and severity of the insult, and that both resultant phenotypes are adaptive developmental responses, despite their potential for conferring vulnerability for psychopathology in humans.

A meta-analysis of animal studies on disruption of spatial navigation by prenatal cocaine exposure

Water-maze testing has been used to assess prenatal cocaine (PCOC)-induced deficits in behavioral studies of spatial navigation and memory abilities. Effects of PCOC in acquisition or in probe trials over water-maze testing days were rarely detected. Despite an absence of effects of PCOC when data were collapsed over multiple days, there was a potential difference when examined during the first day of acquisition training, characterized by a PCOC-associated decrease in learning efficiency but not capacity. Here, we review studies of PCOC-related changes in day-1 water-maze acquisition training and examine the relationship between experimental methodologies and PCOC-treatment procedures and the variability in effect size estimates across studies. The results revealed a significant increase in latencies to goal platform on acquisition training day-1 in PCOC-exposed offspring vs. controls (effect size: r=0.44). Significant effects attributable to variations in the PCOC-treatment procedures across studies were also identified. The moderating variable of PCOC "dose" was significant as lower doses of PCOC exposure yielded larger treatment effects. "Duration" of PCOC exposure was not significant, although a trend for greater effects was observed in studies that employed longer daily treatment schedules or schedules administered in later gestational periods. This analysis identified a consistent difference in acquisition training day-1 of water-maze testing in PCOC-exposed offspring indicating a PCOC-induced deficiency in spatial learning. These findings of impaired spatial learning efficiency are of particular interest given clinical scenarios involving acutely impaired spatial memory and related learning in PCOC-exposed children that highlight the potential consequences in classroom learning.

Prenatal cocaine exposure enhances responsivity of locus coeruleus norepinephrine neurons: role of autoreceptors

Children exposed to cocaine during gestation have a higher incidence of neurobehavioral deficits. The neurochemical bases of these deficits have not been determined, but the pharmacology of cocaine and the nature of the abnormalities suggest that disruptions in catecholaminergic systems may be involved. In the current study, we used a rat model of prenatal cocaine exposure to examine the impact that this exposure has on the locus coeruleus (LC) noradrenergic system in offspring. Pregnant rats received twice-daily i.v. injections of cocaine (3 mg/kg) or saline between gestational days 10 and 20, and progeny were tested as juveniles. Exposure to a mild stressor elevated an index of norepinephrine turnover in the prefrontal cortex and also increased Fos expression in tyrosine hydroxylase-positive LC neurons in rats exposed to prenatal cocaine but not in rats exposed to prenatal saline. No change in the number of tyrosine hydroxylase-positive neurons in the LC was observed between the two prenatal treatment groups. Specific binding of [125I]-para-iodoclonidine, a radioligand with specificity for high affinity alpha2A-adrenergic receptors, was decreased in the LC of rats exposed to prenatal cocaine compared with prenatal saline controls. As alpha2-adrenergic receptors on LC norepinephrine neurons function as autoreceptors, their down-regulation by prenatal cocaine exposure provides a plausible mechanism for the observed heightened reactivity of norepinephrine neurons in these animals. These data indicate that prenatal cocaine exposure results in lasting changes to the regulation and responsivity of rat LC norepinephrine neurons. A similar dysregulation of LC norepinephrine neurons may occur in children exposed to cocaine during gestation, and this may explain, at least partly, the increased incidence of cognitive deficits that have been observed in these subjects.

Prenatal exposure to cocaine is associated with increased number of spine synapses in rat prelimbic cortex

Prenatal exposure to cocaine has been associated with cognitive deficits in children and in animal models. An excess activation of pyramidal neurons in the prefrontal cortex has been proposed as a potential cause for these deficits based on previous studies. The goal of this study was to determine if prenatal exposure to cocaine was associated with an increase in the number of excitatory synapses on dendritic spines in layer II/III of the prelimbic cortex. Frontal cortex of young adult male and female rats, exposed to either saline or cocaine (3 mg/kg i.e., twice a day, embryonic day 10-20), were examined using electron microscopy and the number of asymmetric spines synapses were estimated using the physical disector method. Both male and female rats prenatally exposed to cocaine had about twice as many synapses on dendritic spines as the prenatal saline controls. The increase in number of excitatory synaptic inputs associated with prenatal cocaine exposure could contribute to the increased neuronal activation and cognitive deficits noted.

Specificity of prenatal cocaine on inhibition of locus coeruleus neurite outgrowth

Prenatal cocaine exposure induces alterations in attentional function that presumably involve locus coeruleus noradrenergic neurons and their projections. Previous reports indicate that embryonic rat locus coeruleus neurons exposed to cocaine, both in vitro and in vivo, showed in decreased cell survival and inhibition of neurite outgrowth, and that the effects were most deleterious during early gestation. The present study performed in vitro addressed the specificity of the inhibitory effects of cocaine by comparing locus coeruleus neurite formation and extension to that of dopaminergic substantia nigra neurons following exposure to a physiologically-relevant dose of cocaine (500 ng/ml, two times a day, for four days) during peak neuritogenesis. Following cocaine treatment, immunocytochemistry (anti-norepinephrine antibody to locus coeruleus; anti-tyrosine hydroxylase antibody to substantia nigra) and image analysis were performed to measure a variety of neurite outgrowth parameters. For locus coeruleus neurons, cocaine treatment decreased the 1) number of cells initiating neurites [P<0.001], 2) mean number [P<0.05] and length of neurites [P<0.0001], 3) mean number [P<0.0016] and length of branched neurites [P<0.0006], and 4) mean length of the longest neurites [P<0.0001]. In comparison, substantia nigra neurons were not significantly affected by cocaine for any of the parameters examined. More importantly, a significant interaction between cocaine treatment and brain region was observed [P<0.0002] indicating greater vulnerability of locus coeruleus, relative to substantia nigra neurons, to cocaine exposure. These data support our hypothesis that cocaine targets the noradrenergic system by negatively regulating locus coeruleus neuronal outgrowth, which likely affects pathfinding, synaptic connectivity, and ultimately attentional behavior in cocaine-exposed offspring.

Prenatal cocaine exposure specifically alters spontaneous alternation behavior

Our laboratory has previously characterized a rabbit model of gestational cocaine exposure in which permanent alterations in neuronal morphology, cell signaling and psychostimulant-induced behavior are observed. The cellular and molecular neuroadaptations produced by prenatal cocaine occur in brain regions involved in executive function and attention, such as the anterior cingulate and medial prefrontal cortices. Therefore, in the present study, we have measured the effects of prenatal cocaine exposure on specific behavioral tasks in adult offspring whose mothers were treated with cocaine (3mg/kg, twice a day, E16-E25). We assessed non-spatial, short-term memory in a two-object recognition task and found no deficits in memory or exploratory behaviors in cocaine-exposed offspring in this paradigm. We also evaluated a different memory task with a more robust attentional component, using spontaneous alternation in a Y maze. In this task, young adult rabbits exposed to cocaine prenatally exhibited a significant deficit in performance. Deficits in spontaneous alternation can be induced by a wide variety of behavioral and cognitive dysfunctions, but taken together with previous findings in this and other animal models, we hypothesize that prenatal exposure to cocaine alters highly specific aspects of cognitive and emotional development.

Prenatal cocaine exposure: effects on locomotor activity in rat offspring

This study examines the developmental effects of prenatal exposure to cocaine in the rat, evaluated during the first month of life through open-field behavior. The offspring of Wistar dams that received 60mg/kg of cocaine, from gestational day 8 to 22, were examined in the open-field during the second, third and fourth weeks of postnatal life in three consecutive 15-min daily sessions, starting on postnatal day (PND) 14, (PND 14-16), PND 21 (PND 21-23) and PND 28 (PND 28-30). Results show that prenatal exposure to cocaine increased total activity and rearing behavior on PND 22 and PND 29. Also, on PND 14, cocaine-exposed animals reared significantly more than control rats. There were no significant differences in the frequency of center and peripheral ambulation, nor in the defecation rate. The present results evidence alterations in the emotional behavior of rats prenatally exposed to cocaine. The delayed onset of exploration in the open-field observed in cocaine-exposed animals suggests that they take more time to become habituated to a novel and open environment.

Brief exposure to predator odor and resultant anxiety enhances mesocorticolimbic activity and enkephalin expression in CD-1 mice

The present study assessed alterations in mesolimbic enkephalin (ENK) mRNA levels after predator [2,5-dihydro-2,4,5-trimethylethiazoline (TMT)] and non-predator (butyric acid) odor encounter and/or light-dark (LD) testing in CD-1 mice immediately, 24, 48 and 168 h after the initial odor encounter and/or LD testing. The nucleus accumbens, ventral tegmental area, basolateral (BLA), central (CEA) and medial amygdaloid nuclei, prelimbic and infralimbic cortex were assessed for fos-related antigen (FRA) and/or ENK mRNA as well as neuronal activation of ENK neurons (FRA/ENK). Mice exposed to TMT displayed enhanced freezing and spent less time in the light of the immediate LD test relative to saline- or butyric acid-treated mice. Among mice exposed to TMT, LD anxiety-like behavior was associated with increased FRA in the prelimbic cortex and accumbal shell and decreased ENK-positive neurons in the accumbal core. Mice displaying high TMT-induced LD anxiety exhibited increased ENK-positive neurons in the BLA, CEA and medial amygdaloid nuclei relative to mice that displayed low anxiety-like behavior in the LD test after TMT exposure. In the BLA and CEA, 'high-anxiety' mice also displayed increased FRA/ENK after TMT exposure and LD testing. In contrast to neural cell counts, the level of ENK transcript was decreased in the BLA and CEA of 'high-anxiety' mice after TMT exposure and LD testing. These data suggest that increased FRA may regulate stressor-responsive genes and mediate long-term behavioral changes. Indeed, increased ENK availability in mesolimbic sites may promote behavioral responses that detract from the aversiveness of the stressor experience.

Prenatal exposure to cocaine selectively disrupts the development of parvalbumin containing local circuit neurons in the medial prefrontal cortex of the rat

Exposure to cocaine in utero can result in cognitive deficits potentially through a disruption in the inhibitory processes of the frontal cortex. One potential mechanism is through alterations in the inhibitory local circuit neurons containing the calcium-binding protein, parvalbumin. Parvalbumin-immunostaining primarily identifies 2 types of local circuit neurons: larger, rounder, axo-somal basket cells and smaller, more-spindle shaped, axo-axonic chandelier cells. Both are thought to have critical impact on the excitatory/inhibitory balance due to the proximal site of projection on pyramidal neurons. Calretinin, another calcium-binding protein, identifies a distinct sub-population of inhibitory local circuits that impinges more distally on the dendritic arbor and serves as a control population for this study. Here, we examine local circuit neurons containing either parvalbumin or calretinin in adolescent male rats (approximately 45 days old) exposed to saline or cocaine (3 mg/kg, intravenous twice a day during embryonic days 10 to 20). Prenatal cocaine exposure caused select changes in the parvalbumin, but not calretinin, containing cells in the frontal cortex. Specifically, prenatal cocaine exposure is associated with a 50% reduction in spindle-shaped parvalbumin-immunoreactive cells potentially indicating a select loss of chandelier cells or a shift to a rounder shape. Additionally, a reduction in the number of dendrites of parvalbumin-immunoreactive cells in rats exposed to cocaine in utero was noted. Other measures of both parvalbumin- and calretinin-immunoreactive cells were unchanged, including total number of cells, distribution by depth, and sizes of cells. These changes to the excitatory/inhibitory balance in the frontal cortex may contribute to the cognitive deficits associated with prenatal cocaine exposure.

Beneficial effects of the sigma1 receptor agonists igmesine and dehydroepiandrosterone against learning impairments in rats prenatally exposed to cocaine

In utero cocaine (IUC) exposure results in offspring rats in complex neurochemical and behavioral alterations, particularly affecting learning and memory processes. We examined here the impact of IUC exposure on memory functions in male and female offspring rats and report that selective sigma(1) (sigma(1)) receptor agonists are effective in reversing the deficits. Dams received a daily cocaine, 20 mg/kg ip, injection between gestational days E17 to E20. Learning was examined in offspring between day P30 and P41 using delayed alternation in the T-maze, water-maze learning and passive avoidance. Both male and female rats prenatally exposed to cocaine showed delayed alternation deficits and impairments of acquisition of a fixed platform position in the water maze, as shown by higher acquisition latencies and diminutions of time spent in the training quadrant during the probe test. The acquisition of a daily changing platform position also demonstrated impaired working memory. Finally, passive avoidance deficits were observed. Pretreatment with the synthetic sigma(1) agonist igmesine (0.1-1 mg/kg ip) or the neuroactive steroid dehydroepiandrosterone (DHEA 10-40 mg/kg ip) reversed the prenatal cocaine-induced learning deficits in offspring rats for each test. The sigma(1) antagonist BD1063 (1 mg/kg ip) failed to affect performances alone but blocked the igmesine and DHEA effects, confirming the involvement of the sigma(1) receptor. IUC exposure thus results in marked memory deficits, affecting spatial and nonspatial short- and long-term memories in juvenile male and female offspring rats. The activation of the sigma(1) neuromodulatory receptor allows a complete behavioral recovery of the memory functions in prenatally cocaine-exposed rats.

Prenatal cocaine exposure alters potassium-evoked dopamine release dynamics in rat striatum

The emerging profile for the effects of prenatal cocaine exposure presents two prominent features in the exposed offspring: cognitive/attention deficits and an age-associated trend toward motor/tone abnormalities up to 2 years of age. One candidate mechanism underlying these clinical features is long-lasting alterations to dopamine (DA) neuron function. However, the impact of prenatal cocaine exposure on DA release in dopaminergic terminal fields in vivo in mature offspring is poorly understood. Long-Evans female rats were implanted with an i.v. access port, bred, and given saline or cocaine-HCl (3 mg/kg/ml) for gestational days (GD) 8-14 (1x/day), GD 15-21 (2x/day), or GD 8-21 (1x/day-GD 8-14, 2x/day-GD 15-21). Using in vivo high-speed chronoamperometric recordings, potassium-stimulated DA release was measured in striatum of anesthetized male offspring 90-150 days after birth. There was a trend toward increased potassium-evoked DA signal amplitudes in offspring exposed to cocaine at any time period examined. In offspring exposed to cocaine during GD 8-21 and GD 15-21, but not at GD 8-14, there were significant decreases in the clearance capacity of the potassium-evoked DA signal compared with control offspring. The time required to clear 80% of the evoked DA signal (T(80)) in striatum for DA was significantly prolonged (approximately 150% of control) and this effect was further increased in the mean-evoked DA concentration range for these two groups. We also measured total dopamine transporter (DAT) and tyrosine hydroxylase protein levels in these offspring by blot immunolabeling and found a small, but significant, decrease in DAT protein in striatum from offspring exposed at GD 8-21 and GD 15-21. Collectively, these data demonstrate that prenatal cocaine exposure during dopamine neuron neurogenesis has long-lasting effects on DA neuron function lasting into early adulthood which may be related in part to steady state DAT protein levels. These molecular events may be associated with established cognitive deficits and perhaps the trends seen in altered motor behavior.

Cocaine effects on the developing brain: current status

The present paper reports on the results obtained in a rabbit model of prenatal cocaine exposure that mimics the pharmacokinetics of crack cocaine in humans, and relates these findings to studies in other species including humans. A general finding is that prenatal exposure to cocaine during neurogenesis produces dysfunctions in signal transduction via the dopamine D(1) receptor and alterations in cortical neuronal development leading to permanent morphological abnormalities in frontocingulate cortex and other brain structures. Differences in the precise effects obtained appear to be due to the dose, route and time of cocaine administration. Related to these effects of in utero cocaine exposure, animals demonstrate permanent deficits in cognitive processes related to attentional focus that have been correlated with impairment of stimulus processing in the anterior cingulate cortex. The long-term cognitive deficits observed in various species are in agreement with recent reports indicating that persistent attentional and other cognitive deficits are evident in cocaine-exposed children as they grow older and are challenged to master more complex cognitive tasks.

Impaired sustained attention and altered reactivity to errors in an animal model of prenatal cocaine exposure

Although correlations have been reported between maternal cocaine use and impaired attention in exposed children, interpretation of these findings is complicated by the many risk factors that differentiate cocaine-exposed children from SES-matched controls. For this reason, the present dose-response study (0, 0.5, 1.0, or 3.0 mg/kg cocaine HCl) was designed to explore the effect of prenatal cocaine exposure on visual attention in a rodent model, using an intravenous injection protocol that closely mimics the pharmacokinetic profile and physiological effects of human recreational cocaine use. In adulthood, animals were tested on an attention task in which the duration, location, and onset time of a brief visual cue varied randomly between trials. The 3.0 mg/kg exposed males committed significantly more omission errors than control males during the final 1/3 of each testing session, specifically on trials that followed an error, which implicates impaired sustained attention and increased reactivity to committing an error. During the final 1/3 of each testing session, the 0.5 and 1.0 mg/kg exposed females took longer to enter the testing alcove at trial onset, and failed to enter the alcove more frequently than control females. Because these effects were not seen in other tasks of similar duration and reinforcement density, these findings suggest an impairment of sustained attention. This inference is supported by the finding that the increase in omission errors in the final block of trials in each daily session (relative to earlier in the session) was significantly greater for the 1.0 mg/kg females than for controls, a trend also seen for the 0.5 mg/kg group. Unlike the cocaine-exposed males, who remain engaged in the task when attention is waning, the cocaine-exposed females appear to opt for another strategy; namely, refusing to participate when their ability to sustain attention is surpassed.

Axo-axonic structures in the medial prefrontal cortex of the rat: reduction by prenatal exposure to cocaine

The cognitive deficits associated with prenatal exposure to cocaine have been hypothesized to be the results of changes in the anatomy and function of the frontal cortex. In this study, pregnant dams were treated with cocaine (3 mg/kg i.v. twice a day) and the resulting adolescent (postnatal day, approximately 45) male offspring were killed for immunocytochemical determination of the total linear measure, number, location, and lengths of inhibitory GABA transporter-1 immunoreactive axo-axonic structures commonly called "candles" or "cartridges" in the medial prefrontal cortex. These inhibitory structures are the axon terminals of GABAergic cells that impinge on the initial axon segments of excitatory pyramidal neurons. We report that prenatal cocaine exposure decreased the number of these inhibitory candles. The greatest reduction of candles was observed in the ventral prelimbic cortex. Additionally, there was a subtle difference in the pattern of distribution of candles, namely the depth of the initial candle in the ventral portions of the prefrontal cortex was greater in rats exposed to prenatal cocaine. However, there was no overt change in the number of cells that were immunoreactive for the calcium-binding protein parvalbumin, an indicator of a subset of GABAergic interneurons that includes axo-axonic chandelier cells. We conclude that exposure to cocaine in utero disrupts the development of the axo-axonic cells in the prefrontal cortex and this disruption could contribute to the cognitive deficits reported with prenatal cocaine exposure.

Enduring effects of prenatal cocaine exposure on attention and reaction to errors

Rats exposed to cocaine prenatally were administered a series of 3-choice visual attention tasks, with the most pronounced deficits seen in a task in which the onset time, location, and duration of a visual cue varied unpredictably between trials. The cocaine-exposed rats were less accurate than controls but did not differ in the rate of premature responses or omission errors. The pattern of errors, coupled with response latency data, implicated deficits in the ability to rapidly engage attention and maintain a high level of alertness to the task. The cocaine-exposed rats also exhibited a blunted reaction to an error on the previous trial, possibly reflecting an alteration in emotional regulation and/or error monitoring. Implications for underlying neuropathology are discussed.

Prenatal cocaine exposure alters sensitivity to the effects of idazoxan in a distraction task

The present study was designed to test whether prenatal cocaine (COC) exposure alters sensitivity to the attentional effects of idazoxan (IDZ), an alpha-2 adrenergic antagonist that increases coeruleocortical NE activity. The task assessed subjects' ability to selectively attend to an unpredictable light cue and disregard olfactory distractors. IDZ increased commission errors specifically under conditions of distraction, an effect that was similar in the COC and control groups. In contrast, COC animals were significantly more sensitive than controls to the effects of IDZ on omission errors and nontrials. The pattern of effects suggests that the differential treatment response to IDZ on these latter measures resulted from an alteration in norepinephrine (NE)-modulated dopamine release in the COC animals, reflecting lasting changes in dopaminergic and/or noradrenergic systems as a result of the early cocaine exposure. Based on the behavioral measures that showed a differential response to IDZ in the COC animals, it seems likely that these changes may contribute to the alterations in sustained attention and arousal regulation that have been reported in both animals and humans exposed to cocaine in utero.