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

Chronic, episodic nicotine exposure alters GABAergic synaptic transmission to hypoglossal motor neurons and genioglossus muscle function at a critical developmental age

Regulation of GABAergic signaling through nicotinic acetylcholine receptor (nAChR) activation is critical for neuronal development. Here, we test the hypothesis that chronic episodic developmental nicotine exposure (eDNE) disrupts GABAergic signaling, leading to dysfunction of hypoglossal motor neurons (XIIMNs), which innervate the tongue muscles. We studied control and eDNE pups at two developmentally vulnerable age ranges: postnatal days (P)1-5 and P10-12. The amplitude and frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs, mIPSCs) at baseline were not altered by eDNE at either age. In contrast, eDNE increased GABAAR-α1 receptor expression on XIIMNs and, in the older group, the postsynaptic response to muscimol (GABAA receptor agonist). Activation of nAChRs with exogenous nicotine increased the frequency of GABAergic sIPSCs in control and eDNE neurons at P1-5. By P10-12, acute nicotine increased sIPSC frequency in eDNE but not control neurons. In vivo experiments showed that the breathing-related activation of tongue muscles, which are innervated by XIIMNs, is reduced at P10-12. This effect was partially mitigated by subcutaneous muscimol, but only in the eDNE pups. Taken together, these data indicate that eDNE alters GABAergic transmission to XIIMNs at a critical developmental age, and this is expressed as reduced breathing-related drive to XIIMNs in vivo.NEW & NOTEWORTHY Here, we provide a thorough assessment of the effects of nicotine exposure on GABAergic synaptic transmission, from the cellular to the systems level. This work makes significant advances in our understanding of the impact of nicotine exposure during development on GABAergic neurotransmission within the respiratory network and the potential role this plays in the excitatory/inhibitory imbalance that is thought to be an important mechanism underlying neonatal breathing disorders, including sudden infant death syndrome.

Influence of developmental nicotine exposure on serotonergic control of breathing-related motor output

Serotonin plays an important role in the development of brainstem circuits that control breathing. Here, we test the hypothesis that developmental nicotine exposure (DNE) alters the breathing-related motor response to serotonin (5HT). Pregnant rats were exposed to nicotine or saline, and brainstem-spinal cord preparations from 1- to 5-day-old pups were studied in a split-bath configuration, allowing drugs to be applied selectively to the medulla or spinal cord. The activity of the fourth cervical ventral nerve roots (C4VR), which contain axons of phrenic motoneurons, was recorded. We applied 5HT alone or together with antagonists of 5HT1A, 5HT2A, or 5HT7 receptor subtypes. In control preparations, 5HT applied to the medulla consistently reduced C4VR frequency and this reduction could not be blocked by any of the three antagonists. In DNE preparations, medullary 5HT caused a large and sustained frequency increase (10 min), followed by a sustained decrease. Notably, the transient increase in frequency could be blocked by the independent addition of any of the antagonists. Experiments with subtype-specific agonists suggest that the 5HT7 subtype may contribute to the increased frequency response in the DNE preparations. Changes in C4VR burst amplitude in response to brainstem 5HT were uninfluenced by DNE. Addition of 5HT to the caudal chamber modestly increased phasic and greatly increased tonic C4VR activity, but there were no effects of DNE. The data show that DNE alters serotonergic signaling within brainstem circuits that control respiratory frequency but does not functionally alter serotonin signaling in the phrenic motoneuron pool.

Reevaluation of motoneuron morphology: diversity and regularity among motoneurons innervating different arm muscles along a proximal-distal axis

Dendritic fields of spinal motoneurons (MNs) are popularly believed to be stellate; however, variation in dendritic arborization, especially concerning the innervated muscle groups, has not been systematically studied. We addressed this problem by injecting Neurobiotin through patch-pipettes into single MNs (rat cervical cord slices) that had been retrogradely labelled from innervated muscle groups situated along a proximal (here named “scapular” including serratus anterior) to distal (forearm) axis. MNs had fairly straight dendrites with small numbers of mainly proximal branches that exhibited acute branch angles, leaving large areas around the cells with no dendrites. MNs in the same group of pools showed similar morphologies, but there were clear differences among groups. Forearm MNs (n = 35) showed hemidirectionally-extended multipolar dendrites, whereas scapular MNs (n = 15) had bipolar dendrites, and pectoralis MNs (n = 20) had an intermediate morphology. MNs thus showed a spectrum of morphologic characteristics along the axis. This may be because more distally-located forearm muscles are involved in finer movements and need a wider variety of inputs for neural control than proximally-located muscles. We also devised a quantitative method for evaluating the degree to which a cell’s dendritic field displays a symmetric spherical shape; only 20% of MNs tested reached this criterion.

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.

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.

Hydrogen sulfide ameliorates disorders in the parafacial respiratory group region of neonatal rats caused by prenatal cigarette smoke exposure via an antioxidative effect

We previously found that hydrogen sulfide (H₂S) ameliorated the dysfunction of central chemoreception caused by prenatal cigarette smoke exposure (CSE). In the present study, we further explored whether the parafacial respiratory group (pFRG) is involved in the protection of central chemoreception by H₂S against prenatal CSE-induced injury. We found that NaHS, a donor of H₂S, restored the expression of Phox2b, which was downregulated by prenatal CSE, in the pFRG region of neonatal rats. NaHS also relieved the prenatal CSE-induced excitatory synapse disturbance in the pFRG region of neonatal rats. Additionally, NaHS prevented the increase in the malondialdehyde level and suppression of antioxidase activity in the pFRG region of neonatal rats induced by prenatal CSE. Furthermore, NaHS prevented the downregulation of the expression of antioxidases and Nrf2 in the pFRG region of neonatal rats with prenatal CSE. These results suggest that H₂S can protect the pFRG of neonatal rats against prenatal CSE-induced injury via an antioxidative effect.

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.

Developmental plasticity of GABAergic neurotransmission to brainstem motoneurons

KEY POINTS

Critical homeostatic behaviours such as suckling, swallowing and breathing depend on the precise control of tongue muscle activity. Perinatal nicotine exposure has multiple effects on baseline inhibitory GABAergic neurotransmission to hypoglossal motoneurons (XIIMNs), consistent with homeostatic compensations directed at maintaining normal motoneuron output. Developmental nicotine exposure (DNE) alters how GABAergic neurotransmission is modulated by acute activation of nicotinic acetylcholine receptors, which may provide insight into mechanisms by which nicotine exposure alters motor function under conditions that result in increased release of GABA, such as hypoxia, or endogenous acetylcholine, as occurs in the transition from NREM to REM sleep, or in response to exogenous nicotine.

ABSTRACT

Nicotinic acetylcholine receptor (nAChR) signalling regulates neuronal differentiation and synaptogenesis. Here we test the hypothesis that developmental nicotine exposure (DNE) disrupts the development of GABAergic synaptic transmission to hypoglossal motoneurons (XIIMNs). GABAergic spontaneous and miniature inhibitory postsynaptic currents (sIPSCs/mIPSCs) were recorded from XIIMNs in brainstem slices from control and DNE rat pups of either sex, 1-5 days old, at baseline and following acute stimulation of nAChRs with nicotine. At baseline, sIPSCs were less frequent and smaller in DNE cells (consistent with decreased action potential-mediated GABA release), and mIPSCs were more frequent (consistent with increased vesicular GABA release from presynaptic terminals). Acute nicotine challenge increased sIPSC frequency in both groups, though the increase was greater in DNE cells. Acute nicotine challenge did not change the frequency of mIPSCs in either group, though mIPSC amplitude increased significantly in DNE cells, but not control cells. Stimulation of postsynaptic GABA_A receptors with muscimol caused a significantly greater chloride current in DNE cells than in control cells. The increased quantal release of GABA, coupled with the rise in the strength of postsynaptic inhibition may be homeostatic adjustments to the decreased action-potential-mediated input from GABAergic interneurons. However, this will exaggerate synaptic inhibition under conditions where the release of GABA (e.g. hypoxia) or ACh (sleep-wake transitions) is increased. These findings reveal a mechanism that may explain why DNE is associated with deficits in the ability to respond appropriately to chemosensory stimuli or to changes in neuromodulation secondary to changes in central nervous system state.

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.

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.