Developmental Nicotine Exposure Alters AMPA Neurotransmission in the Hypoglossal Motor Nucleus and Pre-Bötzinger Complex of Neonatal Rats

Abstract

Developmental nicotine exposure (DNE) impacts central respiratory control in neonates born to smoking mothers. We previously showed that DNE enhances the respiratory motor response to bath application of AMPA to the brainstem, although it was unclear which brainstem respiratory neurons mediated these effects (Pilarski and Fregosi, 2009). Here we examine how DNE influences AMPA-type glutamatergic neurotransmission in the pre-Bötzinger complex (pre-BötC) and the hypoglossal motor nucleus (XIIMN), which are neuronal populations located in the medulla that are necessary for normal breathing. Using rhythmic brainstem slices from neonatal rats, we microinjected AMPA into the pre-BötC or the XIIMN while recording from XII nerve rootlets (XIIn) as an index of respiratory motor output. DNE increased the duration of tonic activity and reduced rhythmic burst amplitude after AMPA microinjection into the XIIMN. Also, DNE led to an increase in respiratory burst frequency after AMPA injection into the pre-BötC. Whole-cell patch-clamp recordings of XII motoneurons showed that DNE increased motoneuron excitability but did not change inward currents. Immunohistochemical studies indicate that DNE reduced the expression of glutamate receptor subunits 2 and 3 (GluR2/3) in the XIIMN and the pre-BötC. Our data show that DNE alters AMPAergic synaptic transmission in both the XIIMN and pre-BötC, although the mechanism by which this occurs is unclear. We suggest that the DNE-induced reduction in GluR2/3 may represent an attempt to compensate for increased cell excitability, consistent with mechanisms underlying homeostatic plasticity.

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

“…Such plasticity may also be present in the adult spinal cord since blocking hindlimb motor neuron axon conduction increases motor neuron synaptic properties (Gallego et al, 1979; Manabe et al, 1989); this “disuse-induced enhancement” remains for days following resumption of peripheral nerve conduction. Homeostatic plasticity has also been reported in the control of breathing in neonatal (Jaiswal et al, 2013) and adolescent (Kline et al, 2007) rats; in both cases, a prolonged increase in respiratory neural activity elicited adaptive changes in neuronal properties that decreased the overall network response.…”

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

“…In particular, it will be important to determine directly whether the number, locations or types of synaptic inputs to hypoglossal motoneurons are affected by DNE. We know, for example, that DNE increases the density of GABA A receptors (Jaiswal et al, 2015) and reduces the density of glutamate receptor subunits 2 and 3 (Jaiswal et al, 2013) in the hypoglossal motor nucleus. Both effects are consistent with changes in the efficacy of these major inhibitory and excitatory neurotransmitters due to DNE (Jaiswal et al, 2013; 2015; Luo et al, 2004, 2007; Pilarski and Fregosi, 2009).…”

“…We know, for example, that DNE increases the density of GABA A receptors (Jaiswal et al, 2015) and reduces the density of glutamate receptor subunits 2 and 3 (Jaiswal et al, 2013) in the hypoglossal motor nucleus. Both effects are consistent with changes in the efficacy of these major inhibitory and excitatory neurotransmitters due to DNE (Jaiswal et al, 2013; 2015; Luo et al, 2004, 2007; Pilarski and Fregosi, 2009). Given that the efficacy of nicotinic cholinergic agonists is reduced by DNE (Pilarski et al, 2012), it will be important to determine whether the density or subtype of AchRs is affected in the hypoglossal motor nucleus.…”

“…Respiratory motoneurons, the final output pathway of the respiratory system, display specific physiological consequences of DNE that likely contribute to the compromised respiratory function. Hypoglossal motoneurons, which control respiratory movements of the tongue, display reduced excitability (Pilarski et al ., 2011), greater spike frequency adaptation (Powell et al ., 2015), differences in the precision and reliability of spike timing (Powell et al ., 2015), reduced excitatory synaptic input (Pilarski et al ., 2011) and alterations in the efficacy of excitatory and inhibitory synaptic transmission (Jaiswal et al ., 2013; Jaiswal et al ., 2015) as a consequence of DNE.…”

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

“…Maternal tobacco smoking or exposure to environmental tobacco smoke are major risk factors for SIDS (Mitchell and Milerad, 2006) and infants born to mothers that smoke exhibit increased nicotinic acetylcholine receptor expression in brainstem nuclei that control normal respiration, arousal (Cohen et al, 2005;Duncan et al, 2008;Machaalani et al, 2011) and integrate information from cardiovascular, pulmonary, respiratory tract and gastrointestinal receptor afferents (Machaalani et al, 2011;Machaalani and Waters, 2008;Waters et al, 1999). Despite neuro-pathologic evidence of morphological and receptor binding abnormalities affecting multiple brainstem nuclei (Jaiswal et al, 2013;Machaalani et al, 2011;Slotkin et al, 1987;Smith et al, 2010), there is no consistent phenotype associated with gestational or developmental exposure to cigarette smoke or to nicotine. And, although SIDS victims show a higher incidence of respiratory abnormalities including apneas, delayed arousal responses and diminished ventilatory chemo-reflexes, the frequency of those events is low and of limited diagnostic and therapeutic utility.…”