Sudden Infant Death Syndrome likely results from sleep related cardiorespiratory abnormalities, particularly a dysregulation of parasympathetic heart rate control. However, little is known about the interactions between sleep/wake systems and cardioinhibitory vagal neurons (CVNs) located in the nucleus ambiguus that control cardiac parasympathetic output. Previous work has demonstrated that α2-adrenergic agonists significantly decreased GABAergic IPSC frequency in CVNs, but this does not explain norepinphrine's (NE) role in arousal related tachycardia. Therefore, the present study hypothesized that α1-adrenergic receptor activation would increase the frequency of GABAergic IPSCs to CVNs. In rat pups, CVNs were labeled by retrograde tracing and synaptic events were recorded by whole cell voltage clamp techniques in vitro. For all experiments, α2-adrenergic activity was prevented by the antagonist, atipamezole (1μM). Bath application of NE (20μM), NE with propranolol (a general β receptor antagonist, 10μM) or the specific α1-adrenergic receptor agonist, phenylephrine (50μM), all significantly increased the frequency of GABAergic inhibitory neurotransmission. Since augmented GABAergic neurotransmission to CVNs inhibits parasympathetic activity and increases heart rate, these data demonstrate a brainstem mechanism where NE release would increase heart rate during arousal from sleep. Supported by NIH grant: HL49965, 59895, 72006 to DM

Exaggeration of the noradrenergic system within the brain is a potential source of autonomic imbalance in which sympathetic activity dominates parasympathetic control. Such dysregulation may play a role in the observed increase in heart rate and blood pressure seen in post-traumatic stress disorder (PTSD). While current research focuses on the enhanced sympathoadrenal activity, little is known about the potentially equally important vagal withdrawal in PTSD. Parasympathetic innervation to the heart originates in premotor cardioinhibitory vagal neurons (CVNs) located in the nucleus ambiguus and the activity of these CVNs largely determine resting heart rate. This study examines the ability of the catecholaminergic neurotransmitter, norepinephrine (NE), to alter glycinergic synaptic inputs to CVNs via α1 adrenergic receptors. Bath application of either NE or phenylephrine, an α1 agonist, significantly increased inhibitory glycinergic inputs to CVNs. These data indicate that NE activates α1 receptors to increase glycinergic neurotransmission to CVNs, resulting in a likely withdrawal of parasympathetic activity to the heart and subsequent increase in heart rate. These results provide a cellular mechanism by which increased NE activity in the brainstem can lead to the increases in heart rate and blood pressure that occur with PTSD and other conditions of exaggerated noradrenergic activity. Supported by NIH grants HL 49965, 59895, 72006 to DM.

Human-based studies have concluded that SO2 exposure results in decreased parasympathetic activity (increased resting heart rate and decreased heart rate variability), however the mechanisms responsible for the adverse effects of SO2 are not well understood. The EPA has stated that there is a lack of research focusing on the health effects of prenatal and neonatal SO2 exposure. To address this, an exposure chamber was designed to expose pregnant Sprague-Dawley rats to 5 ppm SO2 for one hour daily during pregnancy and one week after giving birth. In neonatal rats (postnatal days 2–7), premotor cardioinhibitory vagal neurons (CVNs) in the nucleus ambiguus were identified in an in vitro brainstem slice preparation retaining rhythmic respiratory activity. Individual CVNs were whole-cell patch clamped and isolated for either spontaneous glutmatergic neurotransmission or inspiratory-related glycinergic and GABAergic neurotransmission. Preliminary results indicate that inspiratory-related glycinergic and GABAergic neurotransmission to CVNs is unchanged, while glutamatergic neurotransmission to CVNs is significantly diminished. This suggests that a cellular target of SO2 exposure is a decreased excitatory glutamatergic neurotransmission to CVNs, causing decreased parasympathetic control of resting heart rate, autonomic imbalance, and tachycardia. Research funded by GW Institute for Sustainability Research, Education, and Policy Research Award.

β adrenergic receptors (βAR) are a frequent target for drugs used to treat cardiac arrhythmias, hypertension, as well as other heart conditions. One of the most commonly used drugs for treatment, β antagonists, act by blocking catecholamine binding sites in the heart, autonomic system as well as the kidneys. Despite the highly beneficial effect of β blockers in lowering heart rate and blood pressure, very little is known about the role of βAR in modulating parasympathetic activity that controls heart rate. In this study we examined, using patch clamp electrophysiology in-vitro, the effect of βAR on cardiac vagal neurons (CVNs), which are the neurons that dominate parasympathetic control of heart rate and originate in the brainstem nucleus ambiguus. Application of the β1 receptor subtype specific agonist dobutamine significantly decreased the frequency of excitatory postsynaptic currents (EPSCs) in CVNs. To determine the role of β2 receptors in altering glutamatergic EPSCs, albuterol, a β2 receptor agonist was applied. Albuterol did not evoke any significant change in excitatory glutamatergic neurotransmission to CVNs. These results suggest CVNs are inhibited by β1 but not β2 receptor activation, and that the beneficial effect of β receptor blockers in treating arrhythmias and tachycardia may include a disinhibition and increase in cardioprotective parasympathetic activity to the heart.

In hypertension, there is an autonomic imbalance in which sympathetic activity dominates over parasympathetic control. Parasympathetic activity to the heart originates from cardiac vagal neurons located in the nucleus ambiguus. Presympathetic neurons that project to sympathetic neurons in the spinal cord are located in the ventral brainstem in close proximity to cardiac vagal neurons, and many of these presympathetic neurons are catecholaminergic. In addition to their projection to the spinal cord, many of these presympathetic neurons have axon collaterals that arborize into neighboring cardiorespiratory locations and likely release norepinephrine onto nearby neurons. Activation of α2-adrenergic receptors in the central nervous system evokes a diverse range of physiological effects, including reducing blood pressure. This study tests whether clonidine, an α2-adrenergic receptor agonist, alters excitatory glutamatergic, and/or inhibitory GABAergic or glycinergic synaptic neurotransmission to cardiac vagal neurons in the nucleus ambiguus. Cardiac vagal neurons were identified in an in vitro brainstem slice preparation, and synaptic events were recording using whole cell voltage clamp methodologies. Clonidine significantly inhibited GABAergic neurotransmission but had no effect on glycinergic or glutamatergic pathways to cardiac vagal neurons. This diminished inhibitory GABAergic neurotransmission to cardiac vagal neurons would increase parasympathetic activity to the heart, decreasing heart rate and blood pressure. The results presented here provide a cellular substrate for the clinical use of clonidine as a treatment for hypertension as well as a role in alleviating posttraumatic stress disorder by evoking an increase in parasympathetic cardiac vagal activity, and a decrease in heart rate and blood pressure.