Developmental Nicotine Exposure And Its Effects On Morphology And Electrophysiology Of Hypoglossal Motoneurons In The Neonatal Rat
Developmental nicotine exposure (DNE) is known to cause deleterious effects in neonatal mammals through nicotine's actions on nicotinic acetylcholine receptors (nAChRs). In this work, we studied how DNE altered the structure and function of the hypoglossal motoneurons (XIIMNs) over the first few days post-parturition. Previous work in XIIMNs demonstrated an increase in cellular excitability (Pilarski et al., 2011), alterations in synaptic transmission among respiratory-related neurons (Wang et al., 2006; Pilarski et al., 2012; Jaiswal et al., 2013), and a reduction in inspiratory drive currents in DNE animals (Pilarski et al., 2011). Here we show that the effects of DNE extend to alterations in the spike-timing precision and reliability of XIIMNs, as well as spike-frequency adaptation. Additionally, simple morphological analysis of XIIMNs following nicotine exposure in utero has revealed a reduction in soma cross-sectional area. We were interested in studying the complete morphology of XIIMNs following DNE to discern its effects on more complex morphological parameters. We advanced this research using a combination of techniques in thin brainstem slices of neonatal rats, including whole cell patch clamp recordings and immunohistochemistry of intracellularly labeled hypoglossal motoneurons. Furthermore, morphological analysis revealed significant differences in the complexity of the dendritic arborization, showing that neurons from DNE animals had shorter dendrites that branched less often. We also used computational analysis to gain insight into mechanisms that may underlie the changes in spike-timing precision and reliability. In a single cell model of XIIMNs, decreases in potassium-dependent conductances such as the calcium-activated potassium current could potentially replicate the alterations seen in vitro. Finally, we also did a systems-level study of the hyoglossus muscle, a tongue retractor, to determine the relation between tongue retraction force and motor unit discharge characteristics. These experiments utilized adult, anesthetized rats to record single motor units, whole muscle electromyography (EMG) activity and tongue retraction force during spontaneous breathing. We determined that during inspiration-related tongue retractions in low and high force conditions, recruitment of motor units plays a crucial role in the control of tongue force output, whereas rate coding of single motor units is present, but appears to play a lesser role. Overall, this study shows that DNE effects the input-output properties of XIIMNs, potentially through changes in intrinsic channel properties; DNE also alters XIIMN morphology, particularly dendritic arborization; and that organization of a tongue retractor muscle depends primarily on recruitment, but also rate coding, to increase force output.
Development, Hypoglossal, Motoneuron, Motor Unit, Nicotine, Breathing, Physiological Sciences
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