Lila Wollman's Abstracts
Lila Wollman
Ph.D. Candidate
Physiological Sciences
Conference Summary
Society for Neuroscience
San Diego, CA
Lay Abstract
Developmental nicotine exposure alters the frequency of miniature inhibitory post-‐synaptic currents in hypoglossal motoneurons.
Prenatal exposure to nicotine is associated with adverse neonatal outcomes including impaired cardiovascular and respiratory control. Maternal smoking is currently the number one risk factor for Sudden Infant Death Syndrome (SIDS), and impairments in the cardiorespiratory response to a decrease in blood oxygen levels is thought to be an important mechanism in SIDS. Prenatal nicotine exposure alters multiple brain regions, including brainstem neurons important for the control of breathing. One group of neurons, called hypoglossal motoneurons (XIIMNs) control the muscles of the tongue to keep the upper airway open with each breath. Obstructive apneas, or cessations in breathing due to obstruction of the upper airway, are caused by abnormal activation of the tongue muscles and present as the main pathological feature seen in nicotine-‐exposed neonates. Prenatal exposure to nicotine with continued exposure through breast milk over the first week of life
(developmental nicotine exposure, DNE) is a widely used rodent model for studying the effects of nicotine exposure during development. Pregnant rats are implanted with an osmotic mini-‐pump that delivers nicotine, resulting in blood levels that are comparable to that seen in pregnant human smokers. With this model, pups are exposed to nicotine in utero through the placenta and after birth through breast milk. Previous work using this model shows that DNE affects the XIIMNs by enhancing their response to the inhibitory signaling molecule known as GABA, which is an inhibitory neurotransmitter, and increasing the number of GABA receptors present on these neurons. These post-‐synaptic changes (changes in the XIIMNs themselves) caused by DNE could be accompanied by pre-‐synaptic changes (changes to the neurons releasing GABA onto XIIMNs), but this has yet to be investigated. Either a post-‐synaptic or pre-‐synaptic change that results in increased inhibition of XIIMNs could explain the increase in obstructive apneas seen in DNE neonates. Here we test the hypothesis that DNE alters the release of GABA onto XIIMNs. To test this hypothesis, we studied DNE neonatal rats in the first 5 days of life using a brainstem slice that contains the XIIMNs. Using this preparation, we can visualize the electrical activity of individual XIIMNs. Many other neurons make synaptic contact with a single XIIMN and share information through the release of chemical neurotransmitters. The binding of a neurotransmitter molecule to its receptor on the XIIMN results in current flow across the cell membrane, which can be recorded and analyzed. Using pharmacological techniques, we isolated currents in XIIMNs that result from the pre-‐synaptic release of GABA. We analyzed the amplitude and frequency of these currents. A change in amplitude could indicate either a pre-‐synaptic or post-‐synaptic effect due to DNE, whereas a change in frequency indicates a pre-‐synaptic effect due to DNE. Our results indicate that: 1) DNE does not alter the amplitude of GABA currents in XIIMNs, 2) DNE does not alter the frequency of all GABA currents, but does alter the frequency of a subset of these currents, called “minis”, which represent the release of a single “package”, or quantum, of GABA onto XIIMNs. These results show that DNE has a pre-‐synaptic effect, which changes the probability of the quantal release of GABA onto XIIMNs by a currently unknown mechanism.
Abstract
Developmental nicotine exposure alters the frequency of miniature inhibitory post-‐synaptic currents in hypoglossal motoneurons.
LB Wollman1, RB Levine1,2, RF Fregosi1,2
Department of Physiology1 and Neuroscience2, University of Arizona, Tucson, AZ, 85724
Prenatal nicotine exposure with continued exposure through breast milk over the first week of life (developmental nicotine exposure, DNE) is known to cause complex changes in hypoglossal motoneurons (XII MNs) including altered responses to inhibitory neurotransmitters and increased density of GABAA receptors on these neurons. Here we test the hypothesis that DNE alters the amplitude and frequency of spontaneous, action potential mediated, and miniature GABA inhibitory post-‐synaptic currents (IPSCs) recorded from XII MNs in neonatal rats. We did whole cell patch clamp recordings from XII MNs in medullary slices (700 micron thick) from DNE pups and control pups. Cells were voltage clamped at -‐55mV and spontaneous GABA IPSCs (sIPSCs), including both action potential mediated events and miniature events, were recorded in the presence of the glutamate receptor antagonist 6-‐cyano-‐7-‐nitroquinoxaline-‐2,3-‐dione (CNQX) and the glycine receptor antagonist Strychnine. Miniature GABA IPSCs (mIPSCs), representative of random quantal release of neurotransmitter, were recorded in the presence of CNQX, Strychnine, and the voltage-‐gated sodium channel antagonist Tetrodotoxin (TTX). Our results are as follows: 1) DNE does not alter the amplitude (Control {n=4} 21.4±2.7 pA, DNE {n=4} 22±2.1 pA) or frequency (Control {n=4} 366.4±107.6 counts/min, DNE {n=4} 311.5±82.8 counts/min) of GABA sIPSCs. 2) DNE does not alter the amplitude
(Control {n=6} 14.6±1.6 pA, DNE {n=6} 15.3±1.4 pA) of GABA mIPSCs. 3) DNE did increase the frequency of GABA mIPSCs (Control {n=6} 26.5±9.1 counts/min, DNE
{n=6} 59.2±21.3 counts/min). These results show that DNE alters the random, quantal release of GABA onto XII MNs by a currently unknown mechanism.