Heart rate is determined primarily by the influence of the parasympathetic nervous system on the heart. Parasympathetic cardiac regulation is predominantly mediated by the activity of premotor cardioinhibitory vagal neurons (CVNs), which are principally located in the ventrolateral brainstem in the nucleus ambiguus (NA). The axons of these neurons project down the vagus nerve and synapse upon post-ganglionic parasympathetic neurons in the cardiac ganglia. Fibers from the cardiac ganglia synapse upon pacemaker cells in the sino-atrial and atrioventricular nodes, which directly influence the rate and strength of cardiac contraction. The overall research interest of our lab is the neural control of heart rate and cardiac function. Specifically, we focus on the neurobiology of CVNs in the NA.

 

 

Anatomy of parasympathetic innervation of the heart. Preganglionic cardioinhibitory vagal neurons originate primarily in the nucleus ambiguus. These neurons innervate post-ganglionic neurons in the cardiac ganglia that surround the heart.

 

 

 

The nucleus ambiguus is a heterogenous group of cells--in addition to containing neurons that control heart rate, it also contains neurons responsible for the control of other visceral functions. To identify CVNs in brain slices, we apply the non-toxic fluorescent tracer XRITC to the efferent terminals of CVN axons. This tracer is actively transported to the cells bodies of CVNs. We can then view the nucleus with infrared illumination, and under fluorescent illumination with an infrared sensitive cooled charged-coupled device camera. Neurons containing the fluorescent tracer can be identified by superimposing the fluorescent and infrared images on a video monitor.

Identification of premotor cardioinhibitory vagal neurons in vitro.CVNs can be identified by illumination of the non-toxic fluorescent tracer XRITC, and visualized using infrared wavelengths.

Using patch-clamp electrophysiological techniques we have determined that premotor parasympathetic cardiac neurons do not possess pacemaker activity and the activity of cardiac vagal neurons in the brainstem is controlled by the activation and modulation of three major synaptic inputs; glutamatergic, GABAergic and glycinergic neurotransmission to CVNs.

Synaptic inputs to CVNs.Excitatory input from the Nucleus Tractus Solitarius activates both NMDA and non-NMDA receptors, whereas the inhibitory input from the NTS activates GABAA receptors. CVNs also receive inhibitory glycinergic inputs from an unknown source. Acetylcholine activates alpha4 beta2 nicotinic receptors, which facilitates both GABAergic and glycinergic neurotransmission to CVNs. In addition, acetylcholine activates presynaptic alpha7 nicotinic receptors in inputs to CVNs that facilitate the release of glutamate from presynaptic terminals, and post-synaptic nicotinic receptors that elicit a depolarizing current, and augment post synaptic non-NMDA currents in CVNs.

For example, with each respiratory cycle the heart beats more rapidly in inspiration, which is referred to as respiratory sinus arrhythmia. Respiratory sinus arrhythmia is mediated in part by a reduction of parasympathetic cardiac activity during inspiration. Recent work from this laboratory has shown inspiratory activity evokes an increase in both inhibitory GABAergic and glycinergic activity to cardiac vagal neurons, and furthermore that activation of alpha4 beta2 nicotinic receptors are responsible for the increase in GABAergic, but not glycinergic neurotransmission to cardiac vagal neurons during inspiration.

CVNs are inhibited during inspiratory activity. Lower trace: Rhythmic inspiratory-related activity recorded from the hypoglossal nerve rootlet. Middle trace: Rectified and adjacent averaged hypoglossal rootlet activity (50msec bins). Upper trace: Current clamp recording from an identified CVN.

Whereas respiratory sinus arrhythmia benefits pulmonary gas exchange by improving ventilation-to-perfusion ratios, respiratory dysfunction presents a major challenge to the cardiorespiratory system. Hypoxia and hypercapnia change eupnic respiratory activity to gasping which is accompanied by a pronounced bradycardia mediated by increases in parasympathetic cardiac activity. While it is known respiratory neurons in the brainstem directly respond to hypoxia and hypercapnia there is a scarcity of information concerning whether central hypoxia and hypercapnia can alter the activity of parasympathetic cardiac vagal neurons. One major project in the laboratory is to test the hypothesis that central hypoxia and hypercapnia recruit a respiratory related excitatory pathway and diminish inhibitory respiratory related synaptic neurotransmission to cardiac vagal neurons in the brainstem.

A second major focus of the lab is to characterize the role of nicotinic receptors in mediating the respiratory modulation of cardiac vagal neurons. We are testing the hypothesis that chronic fetal nicotine exposure exaggerates these cardiorespiratory responses and alters nicotinic modulation of neurotransmission to cardiac vagal neurons. This project not only addresses hypotheses fundamental to understanding the basis and mechanisms of cardiorespiratory rhythms in the medulla, but will also suggest which receptors and processes could be altered by fetal exposure to nicotine which increases the risk of cardiorespiratory diseases such as sudden death syndrome (SIDS).

While most cardiorespiratory reflexes act to maintain blood pressure, heart rate and respiratory within normal limits and promote survival. In some hypotensive cardiovascular challenges, such as septic and hemorrhagic shock, there is a paradoxical increase in cardiac vagal activity and bradycardia, which is detrimental to survival. This abnormal increase in cardiac vagal activity involves activation of central opioid receptors. Another project in our laboratory focuses on the cellular modulation of cardiac vagal neurons by opioid receptors. We are testing the hypothesis, at the cellular level, that activation of opioid receptors increases the excitability of cardiac vagal neurons by modulating critical excitatory and inhibitory synaptic inputs as well as directly altering voltage gated currents in cardiac vagal neurons. Furthermore we will identify the source and characterize the electrophysiological properties of the opioid-containing neurons that innervate cardiac vagal neurons utilizing a novel transsynaptic virus that evokes expression of green fluorescent protein (GFP) which identifies the neurons that project to cardiac vagal neurons in-vitro without altering their electrophysiological properties.