Developmental nicotine exposure alters AMPA neurotransmission in the hypoglossal motor nucleus and pre-Botzinger complex of neonatal rats

Chronic, Episodic Nicotine Alters Hypoglossal Motor Neuron Function at a Critical Developmental Time Point in Neonatal Rats

Developmental nicotine exposure (DNE), alters brainstem neurons that control breathing, including hypoglossal motor neurons (XIIMNs), which innervate the tongue. Here, we tested the hypothesis that chronic, episodic DNE (eDNE), which mimics nicotine replacement therapies such as e-cigarettes or nicotine gum, alters the function of nicotinic acetylcholine receptors (nAChRs), XIIMN intrinsic properties, and tongue muscle function in vivo similar to what we have observed with a chronic, sustained exposure model. We delivered nicotine to pregnant Sprague Dawley rats through drinking water and studied pups of either sex in two age groups: postnatal day (P)1–P5 and P10–P12, which encompasses a critical period in brain development. At P1–P5, eDNE was associated with delayed recovery of nAChRs from desensitization; however, there were no changes in the magnitude of desensitization, XIIMN intrinsic properties, or tongue muscle function in vivo. By P10–P12, eDNE XIIMNs had lower peak firing frequencies in response to depolarizing current injection, larger delayed rectifier potassium currents, and continued to exhibit delayed nAChR recovery. Moreover, this age group exhibited a blunted and delayed tongue muscle response to nasal occlusion in vivo, indicating that changes to XIIMN intrinsic properties is an important mechanism behind this effect, as it is not produced by altered nAChR function alone. Together, these results show that eDNE alters XIIMNs and tongue muscle function during a critical period in brain development and that the specific effects of chronic nicotine exposure may be pattern dependent.

The Association Between E-Cigarette Use Behaviors and Saliva Cotinine Concentration Among Healthy E-Cigarette Users in Malaysia

Electronic cigarettes (e-cigarettes) have rapidly increased in popularity within the last 2 years in Malaysia. The study aims to understand the association between e-cigarette use behaviors and salivary cotinine (a CYP2AA metabolite of nicotine) concentration to inform the development of future e-cigarette control policies. A convenience sample of saliva from 144 e-cigarette users was obtained between November and December 2015. The study participants used refill liquid containing between 0 and 12 mg/ml of nicotine. The overall median cotinine concentration of the study participants was 81.1 ng/ml (interquartile range = 8.5-195.8). Among the zero-nicotine and single e-cigarette users, the median cotinine level was 51.1 (interquartile range = 8.20-125.35) ng/ml. Factors significantly associated with a higher salivary cotinine concentration were dual use of e-cigarettes and tobacco cigarettes, regular and daily e-cigarette use, a longer duration of e-cigarette use, using a higher amount of e-liquid, and a shorter duration to finish a refill. Multivariate analysis revealed that e-cigarette use of 1-6 and 6-12 months (but not 1 month and below) was significantly associated with a higher cotinine concentration. Cotinine found in zero-nicotine e-liquids implies the importance of stringent regulatory governance for the consistency of labeled nicotine content of e-cigarette liquid in the market. Zero-nicotine e-cigarette users should also be informed of the likelihood of environmental exposure to tobacco smoke. Future studies conducted on larger samples are warranted to validate the association between duration of e-cigarette use and salivary cotinine concentration as well as to investigate underlying mechanisms.

Effects of inflammation on the developing respiratory system: Focus on hypoglossal (XII) neuron morphology, brainstem neurochemistry, and control of breathing

Breathing is fundamental to life and any adverse change in respiratory function can endanger the health of an organism or even be fatal. Perinatal inflammation is known to adversely affect breathing in preterm babies, but lung infection/inflammation impacts all stages of life from birth to death. Little is known about the role of inflammation in respiratory control, neuronal morphology, or neural function during development. Animal models of inflammation can provide understanding and insight into respiratory development and how inflammatory processes alter developmental phenotype in addition to providing insight into new treatment modalities. In this review, we focus on recent work concerning the development of neurons, models of perinatal inflammation with an emphasis on two common LPS-based models, inflammation and its impact on development, and current and potential treatments for inflammation within the respiratory control circuitry of the mammalian brainstem. We have also discussed models of inflammation in adults and have specifically focused on hypoglossal motoneurons (XII) and neurons of the nucleus tractus solitarii (nTS) as these nuclei have been studied more extensively than other brainstem nuclei participating in breathing and airway control. Understanding the impact of inflammation on the developmental aspects of respiratory control and breathing pattern is critical to addressing problems of cardiorespiratory dysregulation in disease and this overview points out many gaps in our current knowledge.

Maternal cigarette smoke exposure disturbs glutamate/GABA balance in pFRG of neonatal rats

We previously found that maternal cigarette smoke (CS) exposure resulted in impairment of central chemoreception and oxidative stress and mitochondrial dysfunction of parafacial respiratory group (pFRG, a critical site for mammalian central chemoreception) in neonatal rats. The present work was carried out to identify if maternal CS exposure could disturb the glutamate (GLU)-ergic and γ-aminobutyric acid (GABA)-ergic balance in pFRG of neonatal rats. We found that maternal CS exposure induced a decrease in GLU content and consequently in GLU/GABA ratio in pFRG of neonatal rats. Maternal CS exposure also decreased glutamine content and glutaminase and glutamine synthetase activity in offspring pFRG. In addition, expression of vesicular glutamate transporter 2 was depressed, and those of glutamate transporter 1 and GABA transporter 3 were elevated by maternal CS exposure. These results indicate that maternal CS exposure leads to a disturbance of GLU/GABA balance in pFRG of the neonatal rats, which might contribute to the suppression of central chemoreception in maternal CS-exposed offspring.

Cellular and Molecular Changes in Hippocampal Glutamate Signaling and Alterations in Learning, Attention, and Impulsivity Following Prenatal Nicotine Exposure

Over 70 million European pregnant women are smokers during their child-bearing years. Consumption of tobacco-containing products during pregnancy is associated with several negative behavioral outcomes for the offspring, including a higher susceptibility for the development of attention-deficit/hyperactive disorder (ADHD). In efforts to minimize fetal exposure to tobacco smoke, many women around the world switch to nicotine replacement therapies (NRTs) during the gestational period; however, prenatal nicotine exposure (PNE) in any form has been associated with alterations in cognitive processes, including learning, memory, and attention. These processes are controlled by glutamatergic signaling of hippocampal pyramidal neurons within the CA1 region, suggesting actions of nicotine on glutamatergic transmission in this region if present prenatally. Accordingly, we aimed to investigate hippocampal glutamatergic function following PNE treatment in NMRI mice employing molecular, cellular electrophysiology, and pharmacological approaches, as well as to evaluate cognition in the rodent continuous performance task (rCPT), a recently developed mouse task allowing assessment of learning, attention, and impulsivity. PNE induced increases in the expression levels of mRNA coding for different glutamate receptors and subunits within the hippocampus. Functional alterations in AMPA and NMDA receptors on CA1 pyramidal neurons of PNE mice were suggestive of higher GluA2-lacking and lower GluN2A-containing receptors, respectively. Finally, PNE was associated with reduced learning, attention, and enhanced impulsivity in the rCPT. Alterations in glutamatergic functioning in CA1 neurons parallel changes seen in the spontaneously hypertensive rat ADHD model and likely contribute to the lower cognitive performance in the rCPT.

Impact of inflammation on developing respiratory control networks: rhythm generation, chemoreception and plasticity

The respiratory control network in the central nervous system undergoes critical developmental events early in life to ensure adequate breathing at birth. There are at least three "critical windows" in development of respiratory control networks: 1) in utero, 2) newborn (postnatal day 0-4 in rodents), and 3) neonatal (P10-13 in rodents, 2-4 months in humans). During these critical windows, developmental processes required for normal maturation of the respiratory control network occur, thereby increasing vulnerability of the network to insults, such as inflammation. Early life inflammation (induced by LPS, chronic intermittent hypoxia, sustained hypoxia, or neonatal maternal separation) acutely impairs respiratory rhythm generation, chemoreception and increases neonatal risk of mortality. These early life impairments are also greater in young males, suggesting sex-specific impairments in respiratory control. Further, neonatal inflammation has a lasting impact on respiratory control by impairing adult respiratory plasticity. This review focuses on how inflammation alters respiratory rhythm generation, chemoreception and plasticity during each of the three critical windows. We also highlight the need for additional mechanistic studies and increased investigation into how glia (such as microglia and astrocytes) play a role in impaired respiratory control after inflammation. Understanding how inflammation during critical windows of development disrupt respiratory control networks is essential for developing better treatments for vulnerable neonates and preventing adult ventilatory control disorders.

Developmental Nicotine Exposure Alters Synaptic Input to Hypoglossal Motoneurons and Is Associated with Altered Function of Upper Airway Muscles

Nicotine exposure during the fetal and neonatal periods [developmental nicotine exposure (DNE)] is associated with ineffective upper airway protective reflexes in infants. This could be explained by desensitized chemoreceptors and/or mechanoreceptors, diminished neuromuscular transmission or altered synaptic transmission among central neurons, as each of these systems depend in part on cholinergic signaling through nicotinic AChRs (nAChRs). Here, we showed that DNE blunts the response of the genioglossus (GG) muscle to nasal airway occlusion in lightly anesthetized rat pups. The GG muscle helps keep the upper airway open and is innervated by hypoglossal motoneurons (XIIMNs). Experiments using the phrenic nerve-diaphragm preparation showed that DNE does not alter transmission across the neuromuscular junction. Accordingly, we used whole cell recordings from XIIMNs in brainstem slices to examine the influence of DNE on glutamatergic synaptic transmission under baseline conditions and in response to an acute nicotine challenge. DNE did not alter excitatory transmission under baseline conditions. Analysis of cumulative probability distributions revealed that acute nicotine challenge of P1–P2 preparations resulted in an increase in the frequency of nicotine-induced glutamatergic inputs to XIIMNs in both control and DNE. By contrast, P3–P5 DNE pups showed a decrease, rather than an increase in frequency. We suggest that this, together with previous studies showing that DNE is associated with a compensatory increase in inhibitory synaptic input to XIIMNs, leads to an excitatory-inhibitory imbalance. This imbalance may contribute to the blunting of airway protective reflexes observed in nicotine exposed animals and human infants.

Premature birth, homeostatic plasticity and respiratory consequences of inflammation

Infants who are born premature can have persistent apnea beyond term gestation, reemergence of apnea associated with inflammation during infancy, increased risk of sudden unexplained death, and sleep disorder breathing during infancy and childhood. The autonomic nervous system, particularly the central neural networks that control breathing and peripheral and central chemoreceptors and mechanoreceptors that modulate the activity of the central respiratory network, are rapidly developing during the last trimester (22-37 weeks gestation) of fetal life. With advances in neonatology, in well-resourced, developed countries, infants born as young as 23 weeks gestation can survive. Thus, a substantial part of maturation of central and peripheral systems that control breathing occurs ex-utero in infants born at the limit of viability. The balance of excitatory and inhibitory influences dictates the ultimate output from the central respiratory network. We propose in this review that simply being born early in the last trimester can trigger homeostatic plasticity within the respiratory network tipping the balance toward inhibition that persists in infancy. We discuss the intersection of premature birth, homeostatic plasticity and biological mechanisms leading to respiratory depression during inflammation in former premature infants.

Diverse physiological properties of hypoglossal motoneurons innervating intrinsic and extrinsic tongue muscles

The mammalian tongue contains eight muscles that collaborate to ensure that suckling, swallowing, and other critical functions are robust and reliable. Seven of the eight tongue muscles are innervated by hypoglossal motoneurons (XIIMNs). A somatotopic organization of the XII motor nucleus, defined in part by the mechanical action of a neuron's target muscle, has been described, but whether or not XIIMNs within a compartment are functionally specialized is unsettled. We hypothesize that developing XIIMNs are assigned unique functional properties that reflect the challenges that their target muscle faces upon the transition from in utero to terrestrial life. To address this, we studied XIIMNs that innervate intrinsic and extrinsic tongue muscles, because intrinsic muscles play a more prominent role in suckling than the extrinsic muscles. We injected dextran-rhodamine into the intrinsic longitudinal muscles (IL) and the extrinsic genioglossus, and physiologically characterized the labeled XIIMNs. Consistent with earlier work, IL XIIMNs (n = 150) were located more dorsally within the nucleus, and GG XIIMNs (n = 55) more ventrally. Whole cell recordings showed that resting membrane potential was, on average, 9 mV more depolarized in IL than in GG XIIMNs (P = 0.0019), and the firing threshold in response to current injection was lower in IL (-31 ± 23 pA) than in GG XIIMNs (225 ± 39 pA; P < 0.0001). We also found that the appearance of net outward currents in GG XIIMNs occurred at more hyperpolarized membrane potentials than IL XIIMNs, consistent with lower excitability in GG XIIMNs. These observations document muscle-specific functional specializations among XIIMNs.NEW & NOTEWORTHY The hypoglossal motor nucleus contains motoneurons responsible for innervating one of seven different muscles with notably different biomechanics and patterns of use. Whether or not motoneurons innervating the different muscles also have unique functional properties (e.g., spiking behavior, synaptic physiology) is poorly understood. In this work we show that neonatal hypoglossal motoneurons innervating muscles that shape the tongue (intrinsic longitudinal muscles) have different electrical properties than those innervating the genioglossus, which controls tongue position.

Prenatal Nicotine Exposure in Rodents: Why Are There So Many Variations in Behavioral Outcomes?

Introduction

The World Health Organization (WHO) reported that smoking cessation rates among women have stagnated in the past decade and estimates that hundreds of millions of women will be smokers in the next decade. Social, environmental, and biological conditions render women more susceptible to nicotine addiction, imposing additional challenges to quit smoking during gestation, which is likely why more than 8% of pregnancies in Europe are associated with smoking. In epidemiological investigations, individuals born from gestational exposure to smoking exhibit a higher risk of development of attention-deficit/hyperactive disorder (ADHD) and liability to drug dependence. Among other teratogenic compounds present in tobacco smoke, nicotine actions during neuronal development could contribute to the observed outcomes as nicotine misleads signaling among progenitor cells during brain development. Several experimental approaches have been developed to address the consequences of prenatal nicotine exposure (PNE) to the brain and behavior but, after four decades of studies, inconsistent data have been reported and the lack of consensus in the field has compromised the hypothesis that gestational nicotine exposure participates in cognitive and emotional behavioral deficits.

Aims

In this review, we discuss the most commonly used PNE models with focus on their advantages and disadvantages, their relative validity, and how the different technical approaches could play a role in the disparate outcomes.

Results

We propose methodological considerations, which could improve the translational significance of the PNE models.

Conclusions

Such alterations might be helpful in reconciling experimental findings, as well as leading to development of treatment targets for maladaptive behaviors in those prenatally exposed.

Implications

In this article, we have reviewed the advantages and disadvantages of different variables of the commonly used experimental models of PNE. We discuss how variations in the nicotine administration methods, the timing of nicotine exposure, nicotine doses, and species employed could contribute to the disparate findings in outcomes for PNE offspring, both in behavior and neuronal changes. In addition, recent findings suggest consideration of epigenetic effects extending across generations. Finally, we have suggested improvements in the available PNE models that could contribute to the enhancement of their validity, which could assist in the reconciliation of experimental findings.

Acute lung injury in neonatal rats causes postsynaptic depression in nucleus tractus solitarii second-order neurons

Acute Lung Injury (ALI) alters pulmonary reflex responses, in part due to changes in modulation within the lung and airway neuronal control networks. We hypothesized that synaptic efficacy of nucleus tractus solitarii (nTS) neurons, receiving input from lung, airway, and other viscerosensory afferent fibers, would decrease following ALI. Sprague Dawley neonatal rats (postnatal days 9-11) were given intratracheal installations of saline or bleomycin (a well-characterized model that reproduces the pattern of ALI) and then, one week later, in vitro slices were prepared for whole-cell and perforated whole-cell patch-clamp experiments (postnatal days 16-21). In preparations from ALI rats, 2nd-order nTS neurons had significantly decreased amplitudes of both spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs), compared to saline controls. Rise and decay times of sEPSCs were slower in whole-cell recordings from ALI animals. Similarly, the amplitude of tractus solitarii evoked EPSCs (TS-eEPSCs) were significantly lower in 2^nd-order nTS neurons from ALI rats. Overall these results suggest the presence of postsynaptic depression at TS-nTS synapses receiving lung, airway, and other viscerosensory afferent tractus solitarii input after bleomycin-induced ALI.

Calcium rises induced by AMPA and nicotine receptors in the ventral tegmental area show differences in mouse brain slices prenatally exposed to nicotine

Nicotine exposure during gestation is associated with a higher risk of adverse behavioral outcomes including a heightened liability for dependency to drugs of abuse, which can exhibit drug-specificity influenced by gender. This enhanced liability suggests that nicotine use during pregnancy alters neural development in circuits involved in motivation and reward-based learning. The ventral tegmental area (VTA) is critical in motivated behaviors and we hypothesized that gestational exposure to nicotine alters the development of excitatory circuits in this nucleus. Accordingly, in VTA brain slices from male and female mice exposed to nicotine during the prenatal period (PNE) and controls, we compared cellular rises in calcium induced by AMPA receptor and nicotinic acetylcholine receptor (nAChR) stimulation by use of the ratiometric calcium binding dye, Fura-2AM. We found that AMPA induced smaller amplitude calcium rises in the PNE VTA, which was an effect only detected in males. Further, while the amplitude did not vary between treatment and control in females, a greater number of cells responded with rises in calcium in the PNE. Conversely, the proportions of cells responding with calcium rises induced by nAChR stimulation did not change in either gender according to treatment. However, larger rises in calcium in PNE females were detected. When taken together our data show that excitatory signaling in the VTA is altered in a gender-specific manner by PNE and suggest that alterations in signaling could play a role in drug-specific differences in maladaptive, motivated behaviors exhibited by males and females born to mothers exposed to nicotine during pregnancy. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 2018.

Developmental nicotine exposure alters glycinergic neurotransmission to hypoglossal motoneurons in neonatal rats

We tested the hypothesis that nicotine exposure in utero and after birth [developmental nicotine exposure (DNE)] disrupts development of glycinergic synaptic transmission to hypoglossal motoneurons (XIIMNs). Glycinergic spontaneous and miniature inhibitory postsynaptic currents (sIPSC/mIPSC) were recorded from XIIMNs in brain stem slices from 1- to 5-day-old rat pups of either sex, under baseline conditions and following stimulation of nicotinic acetylcholine (ACh) receptors with nicotine (i.e., an acute nicotine challenge). Under baseline conditions, there were no significant effects of DNE on the amplitude or frequency of either sIPSCs or mIPSCs. In addition, DNE did not alter the magnitude of the whole cell current evoked by bath application of glycine, consistent with an absence of change in postsynaptic glycine-mediated conductance. An acute nicotine challenge (bath application of 0.5 μM nicotine) increased sIPSC frequency in the DNE cells, but not control cells. In contrast, nicotine challenge did not change mIPSC frequency in either control or DNE cells. In addition, there were no significant changes in the amplitude of either sIPSCs or mIPSCs in response to nicotine challenge. The increased frequency of sIPSCs in response to an acute nicotine challenge in DNE cells reflects an enhancement of action potential-mediated input from glycinergic interneurons to hypoglossal motoneurons. This could lead to more intense inhibition of hypoglossal motoneurons in response to exogenous nicotine or endogenous ACh. The former would occur with smoking or e-cigarette use while the latter occurs with changes in sleep state and with hypercapnia. NEW & NOTEWORTHY Here we show that perinatal nicotine exposure does not impact baseline glycinergic neurotransmission to hypoglossal motoneurons but enhances glycinergic inputs to hypoglossal motoneurons in response to activation of nicotinic acetylcholine (ACh) receptors with acute nicotine. Given that ACh is the endogenous ligand for nicotinic ACh receptors, the latter reveals a potential mechanism whereby perinatal nicotine exposure alters motor function under conditions where ACh release increases, such as the transition from non-rapid-eye movement to rapid-eye movement sleep, and during hypercapnia.

Prenatal nicotine exposure alters postsynaptic AMPA receptors and glutamate neurotransmission within the laterodorsal tegmentum (LDT) of juvenile mice

Despite dissemination of information regarding the harm on fetal development of smoking while pregnant, the number of pregnancies associated with nicotine exposure appears to have stagnated. Presence of nicotine during neural formulation is associated with a higher susceptibility of drug dependence, suggesting an altered development of neurons in circuits involved in saliency and motivation. The laterodorsal tegmental nucleus (LDT) plays a role in coding stimuli valence via afferents to mesolimbic nuclei. Accordingly, alterations in development of neural mechanisms in the LDT could be involved in vulnerability to drug dependency. Therefore, we examined the effect of prenatal nicotine exposure (PNE) on glutamatergic functioning of LDT neurons in mouse brain slices using whole-cell, patch clamp concurrent with fluorescence-based calcium imaging. PNE was associated with larger amplitudes of AMPA-induced currents, and greater AMPA-mediated rises in intracellular calcium. AMPA/NMDA ratios and the AMPA-current rectification index were lower and higher, respectively, consistent with changes in the functionality of AMPA receptors in the PNE, which was substantiated by a greater inhibition of evoked and spontaneous glutamatergic synaptic events by a selective inhibitor of GluA2-lacking AMPA receptors. Paired pulse ratios showed a decreased probability of glutamate release from presynaptic inputs, and fluorescent imaging indicated a decreased action potential-dependent calcium increase associated with PNE. When taken together, our data suggest that PNE alters LDT glutamatergic functioning, which could alter output to mesolimbic targets. Such an alteration could play a role in altered coding of relevancy of drug stimuli that could enhance risk for development of drug dependency.

Developmental nicotine exposure alters potassium currents in hypoglossal motoneurons of neonatal rat

We previously showed that nicotine exposure in utero and after birth via breast milk [developmental nicotine exposure (DNE)] is associated with many changes in the structure and function of hypoglossal motoneurons (XIIMNs), including a reduction in the size of the dendritic arbor and an increase in cell excitability. Interestingly, the elevated excitability was associated with a reduction in the expression of glutamate receptors on the cell body. Together, these observations are consistent with a homeostatic compensation aimed at restoring cell excitability. Compensation for increased cell excitability could also occur by changing potassium conductance, which plays a critical role in regulating resting potential, spike threshold, and repetitive spiking behavior. Here we test the hypothesis that the previously observed increase in the excitability of XIIMNs from DNE animals is associated with an increase in whole cell potassium currents. Potassium currents were measured in XIIMNs in brain stem slices derived from DNE and control rat pups ranging in age from 0 to 4 days by whole cell patch-clamp electrophysiology. All currents were measured after blockade of action potential-dependent synaptic transmission with tetrodotoxin. Compared with control cells, XIIMNs from DNE animals showed significantly larger transient and sustained potassium currents, but this was observed only under conditions of increased cell and network excitability, which we evoked by raising extracellular potassium from 3 to 9 mM. These observations suggest that the larger potassium currents in nicotine-exposed neurons are an important homeostatic compensation that prevents "runaway" excitability under stressful conditions, when neurons are receiving elevated excitatory synaptic input.NEW & NOTEWORTHY Developmental nicotine exposure is associated with increased cell excitability, which is often accompanied by compensatory changes aimed at normalizing excitability. Here we show that whole cell potassium currents are also increased in hypoglossal motoneurons from nicotine-exposed neonatal rats under conditions of increased cell and network excitability. This is consistent with a compensatory response aimed at preventing instability under conditions in which excitatory synaptic input is high and is compatible with the concept of homeostatic plasticity.

Evaluating the control: minipump implantation and breathing behavior in the neonatal rat

We evaluated genioglossus (GG) gross motoneuron morphology, electromyographic (EMG) activities, and respiratory patterning in rat pups allowed to develop without interference (unexposed) and pups born to dams subjected to osmotic minipump implantation in utero (saline-exposed). In experiment 1, 48 Sprague-Dawley rat pups (Charles-River Laboratories), ages postnatal day 7 (P7) through postnatal day 10 (P10), were drawn from two experimental groups, saline-exposed (n = 24) and unexposed (n = 24), and studied on P7, P8, P9, or P10. Pups in both groups were sedated (Inactin hydrate, 70 mg/kg), and fine-wire electrodes were inserted into the GG muscle of the tongue and intercostal muscles to record EMG activities during breathing in air and at three levels of normoxic hypercapnia [inspired CO2 fraction (FiCO2 ): 0.03, 0.06, and 0.09]. Using this approach, we assessed breathing frequency, heart rate, apnea type, respiratory event types, and respiratory stability. In experiment 2, 16 rat pups were drawn from the same experimental groups, saline-exposed (n = 9) and unexposed (n = 7), and used in motoneuron-labeling studies. In these pups a retrograde dye was injected into the GG muscle, and the brain stems were subsequently harvested and sliced. Labeled GG motoneurons were identified with microscopy, impaled, and filled with Lucifer yellow. Double-labeled motoneurons were reconstructed, and the number of primary projections and soma volumes were calculated. Whereas pups in each group exhibited the same number (P = 0.226) and duration (P = 0.093) of respiratory event types and comparable motoneuron morphologies, pups in the implant group exhibited more central apneas and respiratory instability relative to pups allowed to develop without interference.

Influence of developmental nicotine exposure on glutamatergic neurotransmission in rhythmically active hypoglossal motoneurons

Developmental nicotine exposure (DNE) is associated with increased risk of cardiorespiratory, intellectual, and behavioral abnormalities in neonates, and is a risk factor for apnea of prematurity, altered arousal responses and Sudden Infant Death Syndrome. Alterations in nicotinic acetylcholine receptor signaling (nAChRs) after DNE lead to changes in excitatory neurotransmission in neural networks that control breathing, including a heightened excitatory response to AMPA microinjection into the hypoglossal motor nucleus. Here, we report on experiments designed to probe possible postsynaptic and presynaptic mechanisms that may underlie this plasticity. Pregnant dams were exposed to nicotine or saline via an osmotic mini-pump implanted on the 5th day of gestation. We used whole-cell patch clamp electrophysiology to record from hypoglossal motoneurons (XIIMNs) in thick medullary slices from neonatal rat pups (N=26 control and 24 DNE cells). To enable the translation of our findings to breathing-related consequences of DNE, we only studied XIIMNs that were receiving rhythmic excitatory drive from the respiratory central pattern generator. Tetrodotoxin was used to isolate XIIMNs from presynaptic input, and their postsynaptic responses to bath application of l-glutamic acid (glutamate) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were studied under voltage clamp. DNE had no influence on inward current magnitude evoked by either glutamate or AMPA. However, in cells from DNE animals, bath application of AMPA was associated with a right shift in the amplitude distribution (P=0.0004), but no change in the inter-event interval distribution of miniature excitatory postsynaptic currents (mEPSCs). DNE had no influence on mEPSC amplitude or frequency evoked by glutamate application, or under (unstimulated) baseline conditions. Thus, in the presence of AMPA, DNE is associated with a small but significant increase in quantal size, but no change in the probability of glutamate release.

Plasticity in respiratory motor neurons in response to reduced synaptic inputs: A form of homeostatic plasticity in respiratory control?

For most individuals, the respiratory control system produces a remarkably stable and coordinated motor output-recognizable as a breath-from birth until death. Very little is understood regarding the processes by which the respiratory control system maintains network stability in the presence of changing physiological demands and network properties that occur throughout life. An emerging principle of neuroscience is that neural activity is sensed and adjusted locally to assure that neurons continue to operate in an optimal range, yet to date, it is unknown whether such homeostatic plasticity is a feature of the neurons controlling breathing. Here, we review the evidence that local mechanisms sense and respond to perturbations in respiratory neural activity, with a focus on plasticity in respiratory motor neurons. We discuss whether these forms of plasticity represent homeostatic plasticity in respiratory control. We present new analyses demonstrating that reductions in synaptic inputs to phrenic motor neurons elicit a compensatory enhancement of phrenic inspiratory motor output, a form of plasticity termed inactivity-induced phrenic motor facilitation (iPMF), that is proportional to the magnitude of activity deprivation. Although the physiological role of iPMF is not understood, we hypothesize that it has an important role in protecting the drive to breathe during conditions of prolonged or intermittent reductions in respiratory neural activity, such as following spinal cord injury or during central sleep apnea.

Developmental nicotine exposure alters cholinergic control of respiratory frequency in neonatal rats

Prenatal nicotine exposure with continued exposure through breast milk over the first week of life (developmental nicotine exposure, DNE) alters the development of brainstem circuits that control breathing. Here, we test the hypothesis that DNE alters the respiratory motor response to endogenous and exogenous acetylcholine (ACh) in neonatal rats. We used the brainstem-spinal cord preparation in the split-bath configuration, and applied drugs to the brainstem compartment while measuring the burst frequency and amplitude of the fourth cervical ventral nerve roots (C4VR), which contain the axons of phrenic motoneurons. We applied ACh alone; the nicotinic acetylcholine receptor (nAChR) antagonist curare, either alone or in the presence of ACh; and the muscarinic acetylcholine receptor (mAChR) antagonist atropine, either alone or in the presence of ACh. The main findings include: (1) atropine reduced frequency similarly in controls and DNE animals, while curare caused modest slowing in controls but no consistent change in DNE animals; (2) DNE greatly attenuated the increase in C4VR frequency mediated by exogenous ACh; (3) stimulation of nAChRs with ACh in the presence of atropine increased frequency markedly in controls, but not DNE animals; (4) stimulation of mAChRs with ACh in the presence of curare caused a modest increase in frequency, with no treatment group differences. DNE blunts the response of the respiratory central pattern generator to exogenous ACh, consistent with reduced availability of functionally competent nAChRs; DNE did not alter the muscarinic control of respiratory motor output. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1138-1149, 2016.

Developmental nicotine exposure disrupts dendritic arborization patterns of hypoglossal motoneurons in the neonatal rat

Maternal smoking or use of other products containing nicotine during pregnancy can have significant adverse consequences for respiratory function in neonates. We have shown, in previous studies, that developmental nicotine exposure (DNE) in a model system compromises the normal function of respiratory circuits within the brainstem. The effects of DNE include alterations in the excitability and synaptic interactions of the hypoglossal motoneurons, which innervate muscles of the tongue. This study was undertaken to test the hypothesis that these functional consequences of DNE are accompanied by changes in the dendritic morphology of hypoglossal motoneurons. Hypoglossal motoneurons in brain stem slices were filled with neurobiotin during whole-cell patch clamp recordings and subjected to histological processing to reveal dendrites. Morphometric analysis, including the Sholl method, revealed significant effects of DNE on dendritic branching patterns. In particular, whereas within the first five postnatal days there was significant growth of the higher-order dendritic branches of motoneurons from control animals, the growth was compromised in motoneurons from neonates that were subjected to DNE. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1125-1137, 2016.

Influence of developmental nicotine exposure on the ventilatory and metabolic response to hyperthermia

To determine whether developmental nicotine exposure (DNE) alters the ventilatory and metabolic response to hyperthermia in neonatal rats (postnatal age 2-4 days), pregnant dams were exposed to nicotine (6 mg kg(-1) of nicotine tartrate daily) or saline with an osmotic mini-pump implanted subdermally on day 5 of gestation. Rat pups (a total of 72 controls and 72 DNE pups) were studied under thermoneutral conditions (chamber temperature 33°C) and during moderate thermal stress (37.5°C). In all pups, core temperature was similar to chamber temperature, with no treatment effects. The rates of pulmonary ventilation (V̇(I)), O2 consumption (V̇(O2)) and CO2 production (V̇(CO2)) did not change with hyperthermia in either control or DNE pups. However, V̇(I) was lower in DNE pups at both chamber temperatures, whereas the duration of spontaneous apnoeas was longer in DNE pups than in controls at 33°C. The V̇(I)/V̇(O2) ratio increased at 37.5°C in control pups, although it did not change in DNE pups. To simulate severe thermal stress, additional pups were studied at 33°C and 43°C. V̇(I) increased with heating in control pups but not in DNE pups. As heat stress continued, gasping was evoked in both groups, with no effect of DNE on the gasping pattern. Over a 20 min recovery period at 33°C, V̇(I) returned to baseline in control pups but remained depressed in DNE pups. In addition to altering baseline V̇(I) and apnoea duration, DNE is associated with subtle but significant alterations in the ventilatory response to hyperthermia in neonatal rats.

Developmental nicotine exposure enhances inhibitory synaptic transmission in motor neurons and interneurons critical for normal breathing

Nicotine exposure in utero negatively affects neuronal growth, differentiation, and synaptogenesis. We used rhythmic brainstems slices and immunohistochemistry to determine how developmental nicotine exposure (DNE) alters inhibitory neurotransmission in two regions essential to normal breathing, the hypoglossal motor nucleus (XIIn), and preBötzinger complex (preBötC). We microinjected glycine or muscimol (GABAA agonist) into the XIIn or preBötC of rhythmic brainstem slices from neonatal rats while recording from XII nerve roots to obtain XII motoneuron population activity. Injection of glycine or muscimol into the XIIn reduced XII nerve burst amplitude, while injection into the preBötC altered nerve burst frequency. These responses were exaggerated in preparations from DNE animals. Quantitative immunohistochemistry revealed a significantly higher GABAA receptor density on XII motoneurons from DNE pups. There were no differences in GABAA receptor density in the preBötC, and there were no differences in glycine receptor expression in either region. Nicotine, in the absence of other chemicals in tobacco smoke, alters normal development of brainstem circuits that are critical for normal breathing.

Harmful effects of nicotine

With the advent of nicotine replacement therapy, the consumption of the nicotine is on the rise. Nicotine is considered to be a safer alternative of tobacco. The IARC monograph has not included nicotine as a carcinogen. However there are various studies which show otherwise. We undertook this review to specifically evaluate the effects of nicotine on the various organ systems. A computer aided search of the Medline and PubMed database was done using a combination of the keywords. All the animal and human studies investigating only the role of nicotine were included. Nicotine poses several health hazards. There is an increased risk of cardiovascular, respiratory, gastrointestinal disorders. There is decreased immune response and it also poses ill impacts on the reproductive health. It affects the cell proliferation, oxidative stress, apoptosis, DNA mutation by various mechanisms which leads to cancer. It also affects the tumor proliferation and metastasis and causes resistance to chemo and radio therapeutic agents. The use of nicotine needs regulation. The sale of nicotine should be under supervision of trained medical personnel.

Developmental nicotine exposure adversely effects respiratory patterning in the barbiturate anesthetized neonatal rat

Neonates at risk for sudden infant death syndrome (SIDS) are hospitalized for cardiorespiratory monitoring however, monitoring is costly and generates large quantities of averaged data that serve as poor predictors of infant risk. In this study we used a traditional autocorrelation function (ACF) testing its suitability as a tool to detect subtle alterations in respiratory patterning in vivo. We applied the ACF to chest wall motion tracings obtained from rat pups in the period corresponding to the mid-to-end of the third trimester of human pregnancy. Pups were drawn from two groups: nicotine-exposed and saline-exposed at each age (i.e., P7, P8, P9, and P10). Respiratory-related motions of the chest wall were recorded in room air and in response to an arousal stimulus (FIO2 14%). The autocorrelation function was used to determine measures of breathing rate and respiratory patterning. Unlike alternative tools such as Poincare plots that depict an averaged difference in a measure breath to breath, the ACF when applied to a digitized chest wall trace yields an instantaneous sample of data points that can be used to compare (data) points at the same time in the next breath or in any subsequent number of breaths. The moment-to-moment evaluation of chest wall motion detected subtle differences in respiratory pattern in rat pups exposed to nicotine in utero and aged matched saline-exposed peers. The ACF can be applied online as well as to existing data sets and requires comparatively short sampling windows (∼2 min). As shown here, the ACF could be used to identify factors that precipitate or minimize instability and thus, offers a quantitative measure of risk in vulnerable populations.

Influence of developmental nicotine exposure on spike-timing precision and reliability in hypoglossal motoneurons

Smoothly graded muscle contractions depend in part on the precision and reliability of motoneuron action potential generation. Whether or not a motoneuron generates spikes precisely and reliably depends on both its intrinsic membrane properties and the nature of the synaptic input that it receives. Factors that perturb neuronal intrinsic properties and/or synaptic drive may compromise the temporal precision and the reliability of action potential generation. We have previously shown that developmental nicotine exposure (DNE) alters intrinsic properties and synaptic transmission in hypoglossal motoneurons (XIIMNs). Here we show that the effects of DNE also include alterations in spike-timing precision and reliability, and spike-frequency adaptation, in response to sinusoidal current injection. Current-clamp experiments in brainstem slices from neonatal rats show that DNE lowers the threshold for spike generation but increases the variability of spike-timing mechanisms. DNE is also associated with an increase in spike-frequency adaptation and reductions in both peak and steady-state firing rate in response to brief, square wave current injections. Taken together, our data indicate that DNE causes significant alterations in the input-output efficiency of XIIMNs. These alterations may play a role in the increased frequency of obstructive apneas and altered suckling strength and coordination observed in nicotine-exposed neonatal humans.

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.

Prenatal nicotine is associated with reduced AMPA and NMDA receptor-mediated rises in calcium within the laterodorsal tegmentum: a pontine nucleus involved in addiction processes

Despite huge efforts from public sectors to educate society as to the deleterious physiological consequences of smoking while pregnant, 12–25% of all babies worldwide are born to mothers who smoked during their pregnancies. Chief among the negative legacies bestowed to the exposed individual is an enhanced proclivity postnatally to addict to drugs of abuse, which suggests that the drug exposure during gestation changed the developing brain in such a way that biased it towards addiction. Glutamate signalling has been shown to be altered by prenatal nicotine exposure (PNE) and glutamate is the major excitatory neurotransmitter within the laterodorsal tegmental nucleus (LDT), which is a brainstem region importantly involved in responding to motivational stimuli and critical in development of drug addiction-associated behaviours, however, it is unknown whether PNE alters glutamate signalling within this nucleus. Accordingly, we used calcium imaging, to evaluate AMPA and NMDA receptor-mediated calcium responses in LDT brain slices from control and PNE mice. We also investigated whether the positive AMPA receptor modulator cyclothiazide (CYZ) had differential actions on calcium in the LDT following PNE. Our data indicated that PNE significantly decreased AMPA receptor-mediated calcium responses, and altered the neuronal calcium response to consecutive NMDA applications within the LDT. Furthermore, CYZ strongly potentiated AMPA-induced responses, however, this action was significantly reduced in the LDT of PNE mice when compared with enhancements in responses in control LDT cells. Immunohistochemical processing confirmed that calcium imaging recordings were obtained from the LDT nucleus as determined by presence of cholinergic neurons. Our results contribute to the body of evidence suggesting that neurobiological changes are induced if gestation is accompanied by nicotine exposure. We conclude that in light of the role played by the LDT in motivated behaviour, the cellular changes in the LDT induced by exposures to nicotine prenatally, when combined with alterations in other reward-related regions, could contribute to the increased susceptibility to smoking observed in the offspring.

Decreased spinal synaptic inputs to phrenic motor neurons elicit localized inactivity-induced phrenic motor facilitation

Phrenic motor neurons receive rhythmic synaptic inputs throughout life. Since even brief disruption in phrenic neural activity is detrimental to life, on-going neural activity may play a key role in shaping phrenic motor output. To test the hypothesis that spinal mechanisms sense and respond to reduced phrenic activity, anesthetized, ventilated rats received micro-injections of procaine in the C2 ventrolateral funiculus (VLF) to transiently (~30min) block axon conduction in bulbospinal axons from medullary respiratory neurons that innervate one phrenic motor pool; during procaine injections, contralateral phrenic neural activity was maintained. Once axon conduction resumed, a prolonged increase in phrenic burst amplitude was observed in the ipsilateral phrenic nerve, demonstrating inactivity-induced phrenic motor facilitation (iPMF). Inhibition of tumor necrosis factor alpha (TNFα) and atypical PKC (aPKC) activity in spinal segments containing the phrenic motor nucleus impaired ipsilateral iPMF, suggesting a key role for spinal TNFα and aPKC in iPMF following unilateral axon conduction block. A small phrenic burst amplitude facilitation was also observed contralateral to axon conduction block, indicating crossed spinal phrenic motor facilitation (csPMF). csPMF was independent of spinal TNFα and aPKC. Ipsilateral iPMF and csPMF following unilateral withdrawal of phrenic synaptic inputs were associated with proportional increases in phrenic responses to chemoreceptor stimulation (hypercapnia), suggesting iPMF and csPMF increase phrenic dynamic range. These data suggest that local, spinal mechanisms sense and respond to reduced synaptic inputs to phrenic motor neurons. We hypothesize that iPMF and csPMF may represent compensatory mechanisms that assure adequate motor output is maintained in a physiological system in which prolonged inactivity ends life.