Exposure to prenatal nicotine transiently increases neuronal nicotinic receptor subunit alpha7, alpha4 and beta2 messenger RNAs in the postnatal rat brain

Impact of Prenatal Nicotine Exposure on Placental Function and Respiratory Neural Network Development

Smoking during pregnancy is associated with multiple undesirable outcomes in infants, such as low birth weight, increased neonatal morbidity and mortality, and catastrophic conditions like sudden infant death syndrome (SIDS). Nicotine, the most addictive and teratogenic substance in tobacco smoke, reaches and crosses the placenta and can be accumulated in the amniotic fluid and distributed by fetal circulation, altering the cholinergic transmission by acting on the nicotinic acetylcholine receptors (nAChRs) expressed from very early gestational stages in the placenta and fetal tissue. Because nAChRs influence the establishment of feto-maternal circulation and the emergence of neuronal networks, prenatal nicotine exposure can lead to multiple alterations in newborns. In this mini-review, we discuss the undeniable effects of nicotine in the placenta and the respiratory neural network as examples of how prenatal nicotine and smoking exposition can affect brain development because dysfunction in this network is involved in SIDS etiology.

Long-term effects of early postnatal nicotine exposure on cholinergic function in the mouse hippocampal CA1 region

In rodent models of smoking during pregnancy, early postnatal nicotine exposure results in impaired hippocampus-dependent memory, but the underlying mechanism remains elusive. Given that hippocampal cholinergic systems modulate memory and rapid development of hippocampal cholinergic systems occurs during nicotine exposure, here we investigated its impacts on cholinergic function. Both nicotinic and muscarinic activation produce transient or long-lasting depression of excitatory synaptic transmission in the hippocampal CA1 region. We found that postnatal nicotine exposure impairs both the induction and nicotinic modulation of NMDAR-dependent long-term depression (LTD). Activation of muscarinic receptors decreases excitatory synaptic transmission and CA1 network activity in both wild-type and α2 knockout mice. These muscarinic effects are still observed in nicotine-exposed mice. M1 muscarinic receptor activity is required for mGluR-dependent LTD. Early postnatal nicotine exposure has no effect on mGluR-dependent LTD induction, suggesting that it has no effect on the function of m1 muscarinic receptors involved in this form of LTD. Our results demonstrate that early postnatal nicotine exposure has more pronounced effects on nicotinic function than muscarinic function in the hippocampal CA1 region. Thus, impaired hippocampus-dependent memory may arise from the developmental disruption of nicotinic cholinergic systems in the hippocampal CA1 region.

Differential Effects of Nicotine Exposure on the Hippocampus Across Lifespan

BACKGROUND

Nicotine exposure affects the hippocampus through activation of hippocampal nicotinic acetylcholine receptors (nAChRs), which are present throughout excitatory and inhibitory hippocampal circuitry. The role of cholinergic functioning in the hippocampus varies across developmental stages so that nicotine exposure differentially affects this region depending upon timing of exposure, producing developmentally distinct changes in structure, function, and behavior.

METHODS

We synthesize findings across literature in this area to comprehensively review current understanding of the unique effects of nicotine exposure on the hippocampus throughout the lifespan with a focus on hippocampal morphology, cholinergic functioning, and hippocampusdependent learning and memory.

CONCLUSIONS

Chronic and acute nicotine exposure differentially affect hippocampus structure, functioning, and related learning and memory in the perinatal period, adolescence, and aging. Age-related differences in sensitivity to nicotine exposure should be considered in the research of nicotine addiction and the development of nicotine addiction treatments.

Nicotinic acetylcholine receptors: upregulation, age-related effects and associations with drug use

Nicotinic acetylcholine receptors are ligand-gated ion channels that exogenously bind nicotine. Nicotine produces rewarding effects by interacting with these receptors in the brain's reward system. Unlike other receptors, chronic stimulation by an agonist induces an upregulation of receptor number that is not due to increased gene expression in adults; while upregulation also occurs during development and adolescence there have been some opposing findings regarding a change in corresponding gene expression. These receptors have also been well studied with regard to human genetic associations and, based on evidence suggesting shared genetic liabilities between substance use disorders, numerous studies have pointed to a role for this system in comorbid drug use. This review will focus on upregulation of these receptors in adulthood, adolescence and development, as well as the findings from human genetic association studies which point to different roles for these receptors in risk for initiation and continuation of drug use.

Early postnatal nicotine exposure causes hippocampus-dependent memory impairments in adolescent mice: Association with altered nicotinic cholinergic modulation of LTP, but not impaired LTP

Fetal nicotine exposure from smoking during pregnancy causes long-lasting cognitive impairments in offspring, yet little is known about the mechanisms that underlie this effect. Here we demonstrate that early postnatal exposure of mouse pups to nicotine via maternal milk impairs long-term, but not short-term, hippocampus-dependent memory during adolescence. At the Schaffer collateral (SC) pathway, the most widely studied synapses for a cellular correlate of hippocampus-dependent memory, the induction of N-methyl-D-aspartate receptor-dependent transient long-term potentiation (LTP) and protein synthesis-dependent long-lasting LTP are not diminished by nicotine exposure, but rather unexpectedly the threshold for LTP induction becomes lower after nicotine treatment. Using voltage sensitive dye to visualize hippocampal activity, we found that early postnatal nicotine exposure also results in enhanced CA1 depolarization and hyperpolarization after SC stimulation. Furthermore, we show that postnatal nicotine exposure induces pervasive changes to the nicotinic modulation of CA1 activity: activation of nicotinic receptors no longer increases CA1 network depolarization, acute nicotine inhibits rather than facilitates the induction of LTP at the SC pathway by recruiting an additional nicotinic receptor subtype, and acute nicotine no longer blocks LTP induction at the temporoammonic pathway. These findings reflect the pervasive impact of nicotine exposure during hippocampal development, and demonstrate an association of hippocampal memory impairments with altered nicotinic cholinergic modulation of LTP, but not impaired LTP. The implication of our results is that nicotinic cholinergic-dependent plasticity is required for long-term memory formation and that postnatal nicotine exposure disrupts this form of plasticity.

Nicotine during pregnancy: changes induced in neurotransmission, which could heighten proclivity to addict and induce maladaptive control of attention

Prenatal exposure to nicotine, occurring either via maternal smoking or via use of transdermal nicotine patches to facilitate cigarette abstinence by pregnant women, is associated with ∼ 13% of pregnancies worldwide. Nicotine exposure during gestation has been correlated with several negative physiological and psychosocial outcomes, including heightened risk for aberrant behaviors involving alterations in processing of attention as well as an enhanced liability for development of drug dependency. Nicotine is a terotogen, altering neuronal development of various neurotransmitter systems, and it is likely these alterations participate in postnatal deficits in attention control and facilitate development of drug addiction. This review discusses the alterations in neuronal development within the brain's major neurotransmitter systems, with special emphasis placed on alterations within the laterodorsal tegmental nucleus, in light of the role this cholinergic nucleus plays in attention and addiction. Changes induced within this nucleus by gestational exposure to nicotine, in combination with changes induced in other brain regions, are likely to contribute to the transgenerational burden imposed by nicotine. Although neuroplastic changes induced by nicotine are not likely to act in isolation, and are expected to interact with epigenetic changes induced by preconception exposure to drugs of abuse, unraveling these changes within the developing brain will facilitate eventual development of targeted treatments for the unique vulnerability for arousal disorders and development of addiction within the population of individuals who have been prenatally exposed to nicotine.

Electrophysiological changes in laterodorsal tegmental neurons associated with prenatal nicotine exposure: implications for heightened susceptibility to addict to drugs of abuse

Prenatal nicotine exposure (PNE) is a risk factor for developing an addiction to nicotine at a later stage in life. Understanding the neurobiological changes in reward related circuitry induced by exposure to nicotine prenatally is vital if we are to combat the heightened addiction liability in these vulnerable individuals. The laterodorsal tegmental nucleus (LDT), which is comprised of cholinergic, GABAergic and glutamatergic neurons, is importantly involved in reward mediation via demonstrated excitatory projections to dopamine-containing ventral tegmental neurons. PNE could lead to alterations in LDT neurons that would be expected to alter responses to later-life nicotine exposure. To examine this issue, we monitored nicotine-induced responses of LDT neurons in brain slices of PNE and drug naive mice using calcium imaging and whole-cell patch clamping. Nicotine was found to induce rises in calcium in a smaller proportion of LDT cells in PNE mice aged 7-15 days and smaller rises in calcium in PNE animals from postnatal ages 11-21 days when compared with age-matched control animals. While inward currents induced by nicotine were not found to be different, nicotine did induce larger amplitude excitatory postsynaptic currents in PNE animals in the oldest age group when compared with amplitudes induced in similar-aged control animals. Immunohistochemically identified cholinergic LDT cells from PNE animals exhibited slower spike rise and decay slopes, which likely contributed to the wider action potential observed. Further, PNE was associated with a more negative action potential afterhyperpolarization in cholinergic cells. Interestingly, the changes found in these parameters in animals exposed prenatally to nicotine were age related, in that they were not apparent in animals from the oldest age group examined. Taken together, our data suggest that PNE induces changes in cholinergic LDT cells that would be expected to alter cellular excitability. As the changes are age related, these PNE-associated alterations could contribute differentially across ontogeny to nicotine-mediated reward and may contribute to the particular susceptibility of in utero nicotine exposed individuals to addict to nicotine upon nicotine exposure in the juvenile period.

Prenatal nicotine exposure selectively affects nicotinic receptor expression in primary and associative visual cortices of the fetal baboon

Exposure to nicotine during pregnancy via maternal cigarette smoking is associated with visual deficits in children. This is possibly due to the activation of nicotinic acetylcholine receptors (nAChRs) in the occipital cortex, which are important in the development of visual mapping. Using a baboon model, we explored the effects of prenatal nicotine on parameters in the primary and associated visual cortices. Pregnant baboons were infused with nicotine (0.5 mg/h, intravenous) or saline from 86 days gestation. At 161 days gestation, fetal brains were collected (n = 5 per group) and the occipital lobe assessed for nAChRs and markers of the serotonergic and catecholaminergic systems using tissue autoradiography and/or high-performance liquid chromatography. Neuronal nAChRs and serotonergic markers were expressed in a region- and subunit-dependent manner. Prenatal nicotine exposure was associated with increased binding for (3) H-epibatidine sensitive nAChRs in the primary visual cortex [Brodmann areas (BA) 17] and BA 18, but not BA 19, of the associative visual cortex (P < 0.05). Markers of the serotonergic or catecholaminergic systems were not significantly altered. Thus, prenatal nicotine exposure is associated with alterations in the cholinergic system in the occipital lobe, which may aid in the explanation of the appearance of visual deficits in children from mothers who smoke during pregnancy.

Possible role of the α7 nicotinic receptors in mediating nicotine's effect on developing lung - implications in unexplained human perinatal death

Background

It is well known that maternal smoking during pregnancy is very harmful to the fetus. Prenatal nicotine absorption, in particular, is associated with alterations in lung development and functions at birth and with respiratory disorders in infancy. Many of the pulmonary disorders are mediated by the interaction of nicotine with the nicotinic receptors (nAChRs), above all with the α7 nAChR subunits that are widely expressed in the developing lung. To determine whether the lung hypoplasia frequently observed in victims of sudden fetal and neonatal death with a smoker mother may result from nicotine interacting with lung nicotinic receptors, we investigated by immunohistochemistry the possible presence of the α7 nAChR subunit overexpression in these pathologies.

Methods

In lung histological sections from 45 subjects who died of sudden intrauterine unexplained death syndrome (SIUDS) and 15 subjects who died of sudden infant death syndrome (SIDS), we applied the radial alveolar count (RAC) to evaluate the degree of lung maturation, and the immunohistochemical technique for nAChRs, in particular for the α7 nAChR subunit identification. In the same cases, an in-depth study of the autonomic nervous system was performed to highlight possible developmental alterations of the main vital centers located in the brainstem.

Results

We diagnosed a “lung hypoplasia”, on the basis of RAC values lower than the normal reference values, in 63% of SIUDS/SIDS cases and 8% of controls. In addition, we observed a significantly higher incidence of strong α7 nAChR immunostaining in lung epithelial cells and lung vessel walls in sudden fetal and infant death cases with a smoker mother than in age-matched controls. Hypoplasia of the raphe, the parafacial, the Kölliker-Fuse, the arcuate and the pre-Bötzinger nuclei was at the same time present in the brainstem of these victims.

Conclusions

These findings demonstrate that when crossing the placenta, nicotine can interact with nicotinic receptors of both neuronal and non-neuronal cells, leading to lung and nervous system defective development, respectively. This work stresses the importance of implementing preventable measures to decrease the noxious potential of nicotine in pregnancy.

Stimulation of nicotine reward and central cholinergic activity in Sprague-Dawley rats exposed perinatally to a fat-rich diet

RATIONALE

While clinical studies show maternal consumption of palatable fat-rich diets during pregnancy to negatively impact the children's behaviors and increase their vulnerability to drug abuse, the precise behavioral and neurochemical mechanisms mediating these phenomena have yet to be examined.

OBJECTIVE

The study examined in rats whether gestational exposure to a high-fat diet (HFD) can increase the offspring's propensity to use nicotine and whether disturbances in central nicotinic cholinergic signaling accompany this behavioral effect.

METHODS

Rat offspring exposed perinatally to a HFD or chow diet were characterized in terms of their nicotine self-administration behavior in a series of operant response experiments and the activity of acetylcholinesterase (AChE) and density of nicotinic ACh receptors (nAChRs) in different brain areas.

RESULT

Perinatal HFD compared to chow exposure increased nicotine-self administration behavior during fixed ratio and dose-response testing and caused an increase in breakpoint using progressive ratio testing, while nicotine seeking in response to nicotine prime-induced reinstatement was reduced. This behavioral change induced by the HFD was associated with a significant reduction in activity of AChE in the midbrain, hypothalamus, and striatum and increased density of β2-nAChRs in the ventral tegmental area and substantia nigra and of α7-nAChRs in the lateral and ventromedial hypothalamus.

CONCLUSIONS

Perinatal exposure to a HFD increases the vulnerability of the offspring to excessive nicotine use by enhancing its reward potential, and these behavioral changes are accompanied by a stimulation of nicotinic cholinergic signaling in mesostriatal and hypothalamic brain areas important for reinforcement and consummatory behavior.

Gestational nicotine treatment modulates cell death/survival-related pathways in the brains of adolescent female rats

Gestational exposure to nicotine affects brain development, leading to numerous behavioural and physiological deficits in the offspring during adolescence. To analyse the molecular mechanisms underlying these effects, a pathway-focused oligonucleotide microarray was used to determine gene expression profiles in five brain regions (i.e. amygdala, prefrontal cortex, nucleus accumbens, periventricular nucleus of the hypothalamus, and caudate putamen CPu) of adolescent rats that received nicotine or saline during gestation. Following appropriate statistical and Gene Set Enrichment Analyses, 24 cell death/survival-related pathways were found to be significantly modulated by gestational nicotine. On the basis of their biological functions, these pathways can be classified into three categories: growth factor, death receptor, and kinase cascade. We employed a quantitative real-time PCR array to verify the findings by measuring the expression of 29 genes involved in cell death/survival-related pathways. Together, our findings indicate that gestational nicotine exposure has significant effects on gene expression in cell death/survival-related pathways in the brains of adolescent offspring. Such effects appear to be brain region-specific and are realized through regulation of the expression of growth factors and receptors, caspases, kinases, and transcription factors. On the basis of these findings, we offer a hypothetical model to explain how gestational nicotine exposure may affect cell death and survival in the brains of adolescent offspring by regulating the balance between growth-factor and death-receptor pathways.

Early exposure to nicotine during critical periods of brain development: Mechanisms and consequences

Tobacco use during pregnancy continues to be a major problem with more than 16% of pregnant women in the United States continuing to smoke during pregnancy. Tobacco smoke is known to contain more than 4,000 different chemicals, and while many of these compounds have the potential to interfere with proper neurodevelopment, there is direct evidence that nicotine, the major psychoactive substance present in tobacco, acts as a neuroteratogen. Nicotine activates, and subsequently desensitizes, neuronal nicotinic acetylcholine receptor subtypes (AChRs), which are expressed in the developing central nervous system (CNS) prior to the in-growth of cholinergic neurons. Nicotinic AChRs are present by the first trimester of development in both humans and rodents, and activation of these receptors by acetylcholine is thought to play a critical role in CNS development. The purpose of the current review is to provide an overview of the role that nicotinic AChRs play in the developing CNS and to describe the effects of nicotine exposure during early development on neuronal cell biology, nicotinic AChR expression and neurotransmitter system (e.g., dopamine, norepinephrine, serotonin) function. In particular, differences that occur as a result of the timing and duration of nicotine exposure will be discussed. Emphasis will be placed on preclinical studies examining particular periods of time which correspond to periods of prenatal development in humans (i.e., first, second and third trimesters). Finally, the effects of early nicotine exposure on neurobehavioral development as it pertains to specific disorders, i.e., attention deficit hyperactivity disorder (ADHD), depression and addiction, will be discussed.

Respiratory dysfunctions induced by prenatal nicotine exposure

1. Maternal tobacco smoking is the principal risk factor associated with sudden infant death syndrome (SIDS), a leading cause of death of infants under 1 year of age. Victims of SIDS show a higher incidence of respiratory control abnormalities, including central apnoeas, delayed arousal responses and diminished ventilatory chemoreflexes. 2. Nicotine is likely the link between maternal tobacco smoking and SIDS. Prenatal nicotine exposure can alter the breathing pattern and can reduce hypoxia- and hypercarbia-induced ventilatory chemoreflexes. In vitro approaches have revealed that prenatal nicotine exposure impairs central chemosensitivity, switching the cholinergic contribution from a muscarinic to a nicotinic receptor-based drive. In addition, serotonergic, noradrenergic, GABAergic, glycinergic and glutamatergic, among others, are affected by prenatal nicotine. 3. Here we propose that prenatal nicotine affects the respiratory network through two main processes: (i) reorganization of neurotransmitter systems; and (ii) remodelling of neural circuits. These changes make breathing more vulnerable to fail in early postnatal life, which could be related to the pathogenesis of SIDS.

Separate or sequential exposure to nicotine prenatally and in adulthood: Persistent effects on acetylcholine systems in rat brain regions

Nicotine is a developmental neurotoxicant but the proposed "sensitization-homeostasis" model postulates that even in adulthood nicotine permanently reprograms synaptic function. We administered nicotine to rats throughout gestation or in adulthood (postnatal days PN90-107), simulating plasma levels in smokers, with evaluations on PN105, PN110, PN120, PN130 and PN180. We assessed nicotinic acetylcholine receptor (nAChR) binding, choline acetyltransferase activity, a marker for acetylcholine (ACh) terminals, and hemicholinium-3 (HC3) binding to the choline transporter, an index of ACh presynaptic activity. Prenatal nicotine exposure elicited persistent deficits in HC3 binding in male cerebral cortex and female striatum, but little change in other parameters. Nicotine given in adulthood produced profound nAChR upregulation lasting 2 weeks after discontinuing treatment. Decrements in cerebrocortical and striatal HC3 binding emerged during withdrawal and persisted through PN180, indicative of reduced ACh synaptic activity. Prenatal nicotine did not evoke any major alterations in the response to nicotine given in adulthood. The effects seen here are substantially different from those found previously for nicotine given to adolescent rats, which showed more prolonged nAChR upregulation and profound, widespread and persistent deficits in markers of ACh synaptic function; for adolescents, prenatal nicotine exposure desensitized nAChR responses, exacerbated withdrawal-induced ACh functional deficits, and worsened the long-term outcome. Our results indicate that the effects of nicotine during prenatal or adolescent stages are indeed distinct from the effects in adults, but that even adults show persistent changes after nicotine exposure, commensurate with the sensitization-homeostasis model. These effects may contribute to lifelong vulnerability to readdiction.

Combined exposure to nicotine and ethanol throughout full gestation results in enhanced acquisition of nicotine self-administration in young adult rat offspring

RATIONALE

Epidemiological evidence shows positive correlation between either maternal cigarette smoking or alcohol consumption on subsequent drug-taking behavior in offspring. However, the consequences of full gestational exposure to both drugs have not been studied experimentally despite concurrent use frequently reported among women of childbearing age. Such comorbid gestational drug exposure may increase susceptibility to acquiring cigarette smoking (i.e., nicotine self-administration), a major gateway drug.

OBJECTIVES

We developed a noninvasive rat model for exposure to both nicotine (2-6 mg kg(-1) day(-1)) and EtOH (4 g/kg gavage) that continued throughout pregnancy and postnatal (P) days 2-12, the rodent equivalent of the human third trimester, a critical brain developmental period. Offspring with this full gestational exposure to both drugs (Nic+EtOH) were compared to controls: nicotine alone, EtOH alone, pair-fed (comparable nutrition and handling), and ad libitum chow-fed. At P60-90, offspring had unlimited chronic access to acquire i.v. nicotine self-administration.

RESULTS

There were no differences in gender ratio, stillbirths, birth weights, righting reflex, eye opening age, or weight gain. However, Nic+EtOH offspring of both genders acquired nicotine self-administration (15 or 30 microg kg(-1) injection(-1)) more rapidly, at a higher percentage, and at a higher level than offspring in the other cohorts.

CONCLUSION

Full gestational Nic+EtOH exposure produced no overt alterations in standard postnatal measures but resulted in an enhanced acquisition of nicotine self-administration in young adult offspring.

Long-term behavioral and developmental consequences of pre- and perinatal nicotine

Research has shown that cigarette use during pregnancy can result in increased fetal mortality, sudden infant death syndrome, and behavioral and attentional disorders during childhood. Neurochemical and behavioral consequences of prenatal nicotine exposure have been well documented although few studies have examined long-term behavioral consequences that persist into adulthood. In this study, fifty-eight male and female Long-Evans rats were exposed to chronic nicotine prenatally and postnatally via subcutaneous infusions (0.96 mg/kg/day) in the dam. Nicotine exposure continued in the pups via maternal milk until the dams' osmotic mini-pumps became exhausted at approximately postnatal day (P) 11. At weaning, animals were group housed until behavioral testing at P60 to assess spatial learning and memory in the Morris water maze (MWM). Mild deficits in spatial learning were observed in nicotine-exposed females. These behavioral differences were accompanied by significant reduction in weight gain of nicotine-exposed females beginning at puberty, suggesting a hormonal interaction. Long-term effects of nicotine exposure were less striking in males. Nicotine-exposed males had significantly slower swim speeds than controls, but latency to reach the hidden platform was equal between groups by the conclusion of testing. Weight gain in males did not differ between groups as a result of prenatal nicotine exposure.

Chronic neonatal nicotine upregulates heteromeric nicotinic acetylcholine receptor binding without change in subunit mRNA expression

Smoking during pregnancy chronically exposes the fetus to nicotine resulting in long-term behavioral and cognitive deficits. Nicotine binds to neuronal nicotinic acetylcholine receptors (nAChRs), pentameric ligand-gated ion channels widely expressed in the nervous system. Chronic nicotine upregulates high-affinity nAChRs in animals and smokers. Here we determined if chronic nicotine treatment during a developmental period corresponding to the human third trimester regulates nAChR expression. Rat pups were intubated orally three times per day with or without nicotine (6 mg/kg/day) from postnatal day 1 to 8. Subunit mRNA expression was assessed by in situ hybridization. Expression of heteromeric and homomeric nAChR receptor was evaluated by autoradiography using (125)I-epibatidine and (125)I-alphabungarotoxin, respectively. nAChR expression was analyzed in cortex, hippocampus, thalamus and medial habenula from autoradiograms using computer assisted image analysis. Nicotine induced significant upregulation of heteromeric but not homomeric nAChRs in hippocampus, cortex and thalamus without changes in subunit mRNA expression. No effect of chronic nicotine on receptor expression was detected in the medial habenula, suggesting that nicotine's effect was mainly on alpha4beta2-type heteromeric nAChRs. The nicotine-induced upregulation was reversed after nicotine withdrawal. Receptor blockade by DHbetaE, an antagonist for heteromeric alpha4/beta2 nAChRs, did not prevent upregulation but increased expression to a similar degree as nicotine. Combination of both drugs had a cumulative effect. Thus, although transient, intermittent nicotine exposure as seen in smoking mothers is sufficient to upregulate heteromeric nAChRs during a critical period of brain development and could contribute to the behavioral deficits found in children whose mother smoked.

Gestational nicotine-induced changes in adolescent neuronal activity

Smoking during pregnancy is associated with numerous physiological and neurobehavioral deficits in infants, which persist into adolescence. To better understand the underlying mechanisms, we have treated pregnant rats with nicotine and have evaluated expression of the immediate early gene c-fos, as a measure of neuronal activity, in the brains of adolescent male offspring. Pregnant dams were infused with nicotine (3 mg/kg/day) or saline from gestational day (G) 4 until G18. After birth on G22, litters were cross fostered and weaned at postnatal day (P) 21. Brain sections from adolescent offspring, aged P38-40, were analyzed by in situ hybridization for regional c-fos mRNA expression in response to acute injection of saline or nicotine (0.03, 0.1, 0.3 mg/kg). Acute nicotine challenge increased c-fos expression within nucleus accumbens shell, lateral bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus, dorsal lateral geniculate, and superior colliculus, whereas c-fos expression was decreased in prelimbic cortex. There was no effect of gestational nicotine treatment on acute nicotine-induced alterations in c-fos mRNA levels. However, basal c-fos mRNA expression within infralimbic cortex and nucleus accumbens core was increased by gestational nicotine treatment. These data indicate that gestational nicotine does not produce global changes in nicotine-induced c-fos expression in adolescent brain. However, gestational drug exposure changes basal neuronal activity within mesocorticolimbic structures that are critical for motivated behavior. Such changes may underlie some of the behavioral deficits in attention, cognition, and impulse control that have been reported in the offspring of smoking mothers.

Gestational nicotine exposure reduces nicotinic cholinergic receptor (nAChR) expression in dopaminergic brain regions of adolescent rats

Children of women who smoked during pregnancy are at increased risk of dependence when smoking is initiated during adolescence. We previously reported that gestational nicotine exposure attenuated dopamine release induced by nicotine delivered during adolescence. In this study, we determined the effects of gestational nicotine exposure on nicotinic cholinergic receptor (nAChR) expression. Timed pregnant rats received nicotine (2 mg/kg/day) or vehicle via mini-osmotic pumps during gestation. Treatments continued in pups via maternal nursing during postnatal days (PN) 2-14 (equivalent to the human in utero third trimester). On PN35, 125I-epibatidine binding to nAChR was measured. The Bmax values (fmol/mg) in prefrontal cortex (PFC), nucleus accumbens (NAcc), substantia nigra (SN) and ventral tegmental area (VTA) were reduced by 26.6% (P<0.05), 32.6% (P<0.01), 23.0% (P<0.01) and 27.6% (P<0.05), respectively. In addition, gender differences were found in vehicle-treated groups; in SN and VTA, females were 79.3% (P<0.005) and 82.9% (P=0.08) of males, respectively. The expression of nAChR subunit mRNAs was measured using real-time RT-PCR on laser-capture microdissected tissues. In adolescent VTA, gestational nicotine exposure reduced (P<0.05) nAChR subunit mRNAs encoding alpha3 (53.0%), alpha4 (23.9%), alpha5 (46.7%) and beta4 (61.4%). In NAcc core, the treatment increased alpha3 mRNA (75.8%). In addition, the number of neurons in VTA was reduced by 15.0% (P<0.001). These studies indicate that gestational exposure to nicotine induces long-lasting changes in nAChR expression that may underlie the vulnerability of adolescents to dependence on nicotine.

Smoking during early pregnancy affects the expression pattern of both nicotinic and muscarinic acetylcholine receptors in human first trimester brainstem and cerebellum

Prenatal nicotine exposure is associated with an increased risk of complications during pregnancy and childhood. In this study the expression of nicotinic and muscarinic acetylcholine receptors in first trimester pons, medulla oblongata and cerebellum from abortus (5-12 weeks of gestation) of smoking and nonsmoking women was compared. A significant age-related increase in binding of nicotinic receptor subtype alpha4 was found in both pons and cerebellum only in fetal tissue from non-smoking women, while a similar increase was observed in medulla oblongata from fetuses exposed to smoking. A significant age-related increase in binding of muscarinic receptor subtype m2 was observed in pons from abortus of smoking compared with non-smoking women. The gene expression pattern of both alpha4 and alpha7 nicotinic receptor subunits was changed after smoking in all three regions investigated. Smoking also changed the expression of m1 and 2 muscarinic receptor mRNA in pons, m1 mRNA in cerebellum and the m3 mRNA in medulla oblongata. The findings indicate that early prenatal nicotine exposure affects the normal developmental pattern of the cholinergic system in human fetal brain.

Prenatal nicotine exposure alters the response to nicotine administration in adolescence: effects on cholinergic systems during exposure and withdrawal

Maternal smoking during pregnancy increases the likelihood that the offspring will become smokers in adolescence. In the current study, we evaluated effects of prenatal and adolescent nicotine exposure in rats to assess whether there is a biological basis for this relationship. Pregnant rats were given nicotine or vehicle throughout pregnancy and the offspring then again received nicotine or vehicle during adolescence (postnatal days PN30-47.5), using a regimen (6 mg/kg/day by subcutaneous infusion) that produces plasma nicotine levels similar to those in smokers. Evaluations were made in the cerebral cortex and midbrain during adolescent nicotine administration (PN45) and for up to 1 month after the end of treatment. We assessed the magnitude and persistence of nicotinic acetylcholine receptor (nAChR) upregulation; in addition, we evaluated cholinergic synaptic activity by comparing the effects on choline acetyltransferase (ChAT), a constitutive marker for cholinergic nerve terminals, with those on hemicholinium-3 (HC-3) binding to the presynaptic choline transporter, which is regulated by nerve impulse activity. Prenatal nicotine exposure had only minor effects on nAChRs but produced persistent cholinergic hypoactivity (reduced HC-3 binding relative to ChAT) throughout adolescence and into adulthood (PN75). Adolescent nicotine exposure evoked robust nAChR upregulation and also suppressed cholinergic activity. Prenatal nicotine exposure reduced the upregulation of nAChRs evoked by adolescent nicotine but worsened the cholinergic hypoactivity during withdrawal. Our results indicate that prenatal nicotine exposure alters the subsequent response to nicotine in adolescence, effects that may contribute to the association between maternal smoking during pregnancy and subsequent adolescent smoking in the offspring.

Functional nicotinic acetylcholine receptors on subplate neurons in neonatal rat somatosensory cortex

The establishment of cortical synaptic circuits during early development requires the presence of subplate neurons (SPn's), a heterogeneous population of neurons capable to integrate and process synaptic information from the thalamus, cortical plate, and neighboring SPn's. An accumulation of cholinergic afferents and nicotinic acetylcholine receptors (nAChRs) has been documentated in the subplate around birth. To assess the developmental role of the cholinergic innervation onto SPn's, we used whole cell patch-clamp recordings of visually identified and biocytin-labeled SPn's in neonatal rat somatosensory cortex. Functional nAChRs were present in 92% of the investigated SPn's. Activation of postsynaptic nAChRs by local application of agonists elicited a brief membrane depolarization associated with a barrage of action potentials and large inward currents reversing around 0 mV. According to our pharmacological data, excitation of SPn's is mediated by alpha4beta2 receptors. In contrast, functional alpha7 nAChRs could not be identified on SPn's. Activation of nAChRs affected neither the spontaneous synaptic activity of SPn's nor the synaptic connections between thalamus and SPn's and within subplate. Nicotine, at concentrations reaching the developing brain by maternal smoking, induced a severe desensitization of nAChRs and an increase in the baseline noise. These results indicate that nAChR-mediated excitation of SPn's may stabilize the developing synaptic circuits and suggest the involvement of nAChRs located on SPn's in the fetal tobacco syndrome.

Higher expression of alpha7 nicotinic acetylcholine receptors in human fetal compared to adult brain

Neuronal nicotinic acetylcholine receptors are thought to be involved in regulation of several processes during neurogenesis of the brain. In this study the expression of the alpha7 nicotinic acetylcholine receptor subtype was investigated in human fetal (9-11 weeks of gestation), middle-aged (28-51 years) and aged (68-94 years) medulla oblongata, pons, frontal cortex, and cerebellum. The specific binding of the alpha7 receptor antagonist [(125)I]alpha-bungarotoxin was significantly higher in fetal than in both middle-aged and aged medulla oblongata and aged pons. No significant decrease in [(125)I]alpha-bungarotoxin binding sites was observed from fetal to adult cortex and cerebellum. The alpha7 mRNA expression was significantly higher in all fetal brain regions investigated, except for aged cortex, than in corresponding middle-aged and aged tissue. The high expression of alpha7 nicotinic acetylcholine receptors in fetal compared to adult brain supports the view that these receptors play an important role during brain development.

Neurotransmitter regulation of neural development: acetylcholine and nicotinic receptors

Several neurotransmitter systems have been related to developmental processes during the past decade. In this review, we discuss the evidence that the nicotinic acetylcholine receptors could have an additional function during development that may be unrelated to their role in cholinergic neurotransmission in the vertebrate brain. Both temporal expression data and in vitro and in vivo studies with nicotinic agonists and antagonists have provided direct support for a role of nicotinic receptors in neural developmental processes such as neurite outgrowth and differentiation. A similar picture has emerged for other neurotransmitter and receptor systems as well, which generates a new view of neural processes during both development and mature life.

Nicotine-induced alterations in the expression of nicotinic receptors in primary cultures from human prenatal brain

The nicotinic receptor proteins and gene transcripts for the different nicotinic receptor subunits exist in human prenatal brain already at 4-5 weeks of gestation. The early presence of nicotinic receptors suggests an important role for these receptors in modulating dendritic outgrowth, establishment of neuronal connections and synaptogenesis during development. When measurements of nicotinic receptors using [(3)H]epibatidine (labelling both the alpha3 and alpha4 subtype) and [(3)H]cytisine (labelling the alpha4 subtype) were performed in intact cells from the cortex, subcortical forebrain and mesencephalon (7.5-11 weeks of gestation), the highest specific binding for both ligands was detected in cells from mesencephalon, followed by subcortical forebrain and cortex. The effects of nicotine exposure were studied in primary cultures of prenatal brain (7.5-11 weeks of gestation). Treatment with nicotine (1-100 microM) for 3 days significantly increased the specific binding of [(3)H]epibatidine and [(3)H]cytisine in cortical cells but not in cells from subcortical forebrain and mesencephalon brain regions, indicating region-specific differences in the sensitivity to nicotine exposure. Relative quantification of mRNA showed that the expression of the nicotinic receptor subunits alpha3 and alpha7, but not alpha4, was increased in cortical cells after nicotine treatment. These findings support the assumption of a potential risk of disturbance in the functional role of nicotinic receptors during brain development as a consequence of maternal smoking during pregnancy.

Risks and benefits of nicotine to aid smoking cessation in pregnancy

Cigarette smoking during pregnancy is the single largest modifiable risk for pregnancy-related morbidity and mortality in the US. Addiction to nicotine prevents many pregnant women who wish to quit smoking from doing so. The safety and efficacy of nicotine replacement therapy (NRT) for smoking cessation during pregnancy have not been well studied. Nicotine is classified by the US Food and Drug Administration as a Pregnancy Category D drug. Animal studies indicate that nicotine adversely affects the developing fetal CNS, and nicotine effects on the brain may be involved in the pathophysiology of sudden infant death syndrome (SIDS). It has been assumed that the cardiovascular effects of nicotine resulting in reduced blood flow to the placenta (uteroplacental insufficiency) is the predominant mechanism of the reproductive toxicity of cigarette smoking during pregnancy. Short term high doses of nicotine in pregnant animals do adversely affect the maternal and fetal cardiovascular systems. However, studies of the acute effects of NRT in pregnant humans indicate that nicotine alone has minimal effects upon the maternal and fetal cardiovascular systems. Cigarette smoking delivers thousands of chemicals, some of which are well documented reproductive toxins (e.g. carbon monoxide and lead). A myriad of cellular and molecular biological abnormalities have been documented in placentas, fetuses, and newborns of pregnant women who smoke. The cumulative abnormalities produced by the various toxins in cigarette smoke are probably responsible for the numerous adverse reproductive outcomes associated with smoking. It is doubtful that the reproductive toxicity of cigarette smoking is primarily related to nicotine. We recommend the following. Efficacy trials of NRT as adjunctive therapy for smoking cessation during pregnancy should be conducted. The initial dose of nicotine in NRT should be similar to the dose of nicotine that the pregnant woman received from smoking. Intermittent-use formulations of NRT (gum, spray, inhaler) are preferred because the total dose of nicotine delivered to the fetus will be less than with continuous-use formulations (transdermal patch). A national registry for NRT use during pregnancy should be created to prospectively collect obstetrical outcome data from NRT efficacy trials and from individual use. The goal of this registry would be to determine the safety of NRT use during pregnancy, especially with respect to uncommon outcomes such as placental abruption. Finally, our review of the data indicate that minimal amounts of nicotine are excreted into breast milk and that NRT can be safely used by breast-feeding mothers.

Prenatal nicotine alters vigilance states and AchR gene expression in the neonatal rat: implications for SIDS

Maternal smoking is a major risk factor for sudden infant death syndrome (SIDS). The mechanisms by which cigarette smoke predisposes infants to SIDS are not known. We examined the effects of prenatal nicotine exposure on sleep/wake ontogenesis and central cholinergic receptor gene expression in the neonatal rat. Prenatal nicotine exposure transiently increased sleep continuity and accelerated sleep/wake ontogeny in the neonatal rat. Prenatal nicotine also upregulated nicotinic and muscarinic cholinergic receptor mRNAs in brain regions involved in regulating vigilance states. These findings suggest that the nicotine contained in cigarette smoke may predispose human infants to SIDS by interfering with the normal maturation of sleep and wake.

Differential alterations in nicotinic receptor alpha6 and beta3 subunit messenger RNAs in monkey substantia nigra after nigrostriatal degeneration

Our previous studies showed that alpha4, alpha6, alpha7, beta2, beta3 and beta4 nicotinic acetylcholine receptor messenger RNAs are present in monkey substantia nigra, with a particularly intense and localized labelling of the alpha6 and beta3 subunit messenger RNAs to this brain region. Because loss of nigrostriatal neurons is a central feature of Parkinson's disease and evidence suggests that nicotinic agonists potentiate antiparkinsonian effects of L-dopa, experiments were done to determine whether nicotinic receptor subunit messenger RNAs and binding sites were altered in the basal ganglia after nigrostriatal degeneration. Squirrel monkeys (Saimiri sciureus) were rendered parkinsonian by systemic injection of the selective dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine. Behavioral studies showed that this treatment decreased baseline motor activity to 36+/-11% of control. One month after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration, caudate and putamen dopamine levels were reduced to 51+/-15% and 43+/-6% of control, respectively, while the number of tyrosine hydroxylase-positive neurons in the substantia nigra was 75+/-6% of control. Despite the reduction in nigral cell number after nigrostriatal degeneration, there were no changes in alpha4, alpha7, beta2 and beta4 messenger RNA levels in the substantia nigra. In contrast, alpha6 mRNA levels were significantly increased (143+/-10%) and the beta3 transcript decreased (62+/-6%) in the substantia nigra after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Declines were also observed in [125I]epibatidine binding in both the caudate-putamen and substantia nigra, with no change in alpha7 receptor binding. These results may suggest a dissociation in the regulation of receptor messenger RNA and binding sites, and/or that there are differential alterations in the different receptor subtypes measured using [125I]epibatidine. The changes in the two nicotinic receptor subunit messenger RNAs, alpha6 and beta3, which exhibit a selective localization to the substantia nigra, may indicate that nicotinic receptors containing these subunits are altered after nigrostriatal degeneration.

The developing cholinergic system as target for environmental toxicants, nicotine and polychlorinated biphenyls (PCBs): implications for neurotoxicological processes in mice

During neonatal life, offspring can be affected by toxic agents either by transfer via mother's milk or by direct exposure. In many mammalian species the perinatal period is characterized by a rapid development of the brain - "the brain growth spurt" (BGS). This period in the development of the mammalian brain is associated with numerous biochemical changes that transform the feto-neonatal brain into that of the mature adult. In rodents, the cholinergic transmitter system undergoes a rapid development during the neonatal period, a time when spontaneous motor behaviour also reaches peak activity. We have observed that low-dose exposure to environmental toxicants such as nicotine, polychlorinated biphenyls (PCBs) and polybrominated diphenylethers (PBDE, flame retardants) during the "BGS" can lead to irreversible changes in adult brain function in the mouse. The induction of persistent effects on behaviour and cholinergic nicotinic receptors in the adult animal appears to be limited to a short period during neonatal development. Furthermore, the neurotoxic effects were shown to develop over time, indicating a time-response/time-dependent effect. This indicates that environmental toxicants, such as nicotine, PCBs and probably PBDEs, might be involved in the slow, implacable induction of neurodegenerative disorders and/or interfere with normal aging processes.

Nicotine exposure during the neonatal brain growth spurt produces hyperactivity in preweanling rats

Despite warning labels and increases in evidence of the adverse effects of tobacco use, women continue to use tobacco products during pregnancy. Cigarette smoking has been linked to increased prenatal mortality, increased incidence of SIDS, reductions in birth weight, and disruptions in CNS and behavioral development. Animal model systems have critically established the causal relationship between nicotine and adverse developmental outcome. The present study examines the behavioral effects of nicotine exposure in the rat during the third trimester equivalent of the human brain growth spurt, a period of rapid development of the cholinergic systems and a period during which the CNS is particularly vulnerable to a number of insults. Sprague-Dawley rat pups were exposed to nicotine (6.0 mg/kg/day) from postnatal days (PD) 4-9 via an artificial rearing procedure. This procedure ensures that observed effects are not due to nutritional deficits. Two control groups were employed, an artificially reared control group and a normally reared control group. Activity level was measured on PD 18-19. Nicotine-exposed subjects were significantly overactive compared to both control groups, which did not differ significantly from one another. This behavioral alteration was observed in the absence of nicotine-induced body weight deficits. These results suggest that women who use tobacco products during late gestation may place their fetuses at risk for hyperactivity later in life, particularly during early adolescence.

Neonatal nicotine exposure alters hippocampal EEG and event-related potentials (ERPs) in rats

A consensus is forming that nicotine can damage the developing rat central nervous system. However, few studies have assessed the electrophysiological effects of neonatal nicotine exposure in rodents in brain regions known to be sensitive to the teratogenic properties of nicotine. In a previous study it was reported that 1.0 and 4.0 mg/kg/day nicotine exposure from postnatal days 4-9, a developmental period corresponding to human third-trimester exposure, significantly altered hippocampal event-related potentials (ERPs) but did not effect cortical ERPs, cortical EEG, or hippocampal EEG. Because alterations in behavior and cortical/hippocampal neurochemistry and morphology have been reported following nicotine exposure, the present study used a higher dose of nicotine during the postnatal period (6.0 mg/kg/day) determine if functional changes in the EEG of these regions might contribute to behavioral changes that have been observed. Male Sprague-Dawley rats were exposed to 6. 0 mg/kg/day nicotine via gastric infusion using an artificial rearing, "pup-in-the-cup," technique for 6 consecutive days (postnatal days 4-9). At adulthood, EEG and auditory ERPs were recorded from the cortex and hippocampus. There were no significant differences in EEG or ERPs recorded from the cortex between nicotine-treated and control subjects. Examination of the hippocampal EEG revealed significantly decreased power in the 1-2-Hz frequency band of nicotine-treated rats. In addition, there was a significantly attenuated P300 ERP response to a noise tone in the nicotine-treated rats compared to controls. These data indicate that neonatal nicotine exposure alters functional activity in the hippocampus of adult rats. These effects are likely to be the result of synaptic disorganization in the hippocampus, and indicate that neonatal nicotine exposure exerts teratogenic effects on the developing central nervous system, particularly the hippocampus, which persist into adulthood.

Exposure to nicotine during a defined period in neonatal life induces permanent changes in brain nicotinic receptors and in behaviour of adult mice

Neonatal exposure to low doses of nicotine has been shown to prevent the development of low-affinity nicotine-binding sites, and to elicit a different behaviour response to nicotine in the mice as adults. This study has identified a defined period during the development of neonatal mouse brain for the induction of these permanent changes. Neonatal mice, aged either 3, 10, or 19 days were exposed to nicotine, 66 micrograms nicotine-base/kg b.wt., s.c. twice daily, on 5 consecutive days. In the cerebral cortex, high- and low-affinity (HA and LA) nicotine-binding sites were assayed (3H-nicotine/nicotine) in neonatal male mice aged 8, 15, and 24 days and in adult mice aged 4 months. Spontaneous behaviour and nicotine-induced behaviour were observed in 4-month-old male mice. The spontaneous behaviour test did not indicate any difference between saline- and nicotine-treated mice, whereas the nicotine-induced behaviour test revealed a hypoactive response to nicotine, though only in mice given nicotine on days 10-14. The response of controls and the other age categories to nicotine was an increased activity. At no time during the neonatal period could LA nicotine-binding sites be found following nicotine treatment, but the persistence of this effect was evident only in adult mice exposed on days 10-14.

Effect of pre- and postnatal nicotine exposure on vasopressinergic system in rats

Timed pregnant Sprague-Dawley rats were infused subcutaneously either with nicotine (NIC, 6 mg kg-1 day-1; n=17) or saline (control, n=15) on the 3rd day of gestation, via Alzet osmotic pumps, for 28 days. After the parturition, the pups of both, control and NIC infused dams, were each randomly divided into 2 groups and placed to be nursed as following: (1) control dams nursing pups born to control mother (control group); (2) control dams nursing pups born to NIC-infused mother (prenatal NIC group); (3) NIC-infused dams nursing pups born to control mother (postnatal NIC group); (4) NIC-infused dams nursing pups born to NIC-infused mother (pre- and postnatal NIC group). Vasopressin (VP) was measured by RIA in plasma, neurointermediate lobe (NIL) and hypothalamus (HT) in the pups of both sexes, at the following age: 0 (within 24 h after birth); 1, 2, 3, 4 and 6 weeks. At the age of 3, 4 and 6 weeks, the isolated NILs were individually superfused and VP was measured as a basal release and the response to a 10-min 56 mM potassium stimulation. A marked suppression in the activity of VP-ergic system was observed in both sexes of offspring exposed to NIC prenatally, being first detectable at the age of 3 weeks, when the HT-NIL system becomes fully developed. However, the significant changes were observed at the age of 6 weeks: decreased serum VP concentration, lower VP contents in the HT and NIL, and suppressed VP release, basal and stimulated, from the isolated NIL. Postnatal exposure to nicotine was ineffective.