Identification and dissociation of cardiovascular neurons from the medulla for patch clamp analysis

Sex-steroid-dependent plasticity of brain-stem autonomic circuits

In the central nervous system (CNS), nuclei of the brain stem play a critical role in the integration of peripheral sensory information and the regulation of autonomic output in mammalian physiology. The nucleus tractus solitarius of the brain stem acts as a relay center that receives peripheral sensory input from vagal afferents of the nodose ganglia, integrates information from within the brain stem and higher central centers, and then transmits autonomic efferent output through downstream premotor nuclei, such as the nucleus ambiguus, the dorsal motor nucleus of the vagus, and the rostral ventral lateral medulla. Although there is mounting evidence that sex and sex hormones modulate autonomic physiology at the level of the CNS, the mechanisms and neurocircuitry involved in producing these functional consequences are poorly understood. Of particular interest in this review is the role of estrogen, progesterone, and 5α-reductase-dependent neurosteroid metabolites of progesterone (e.g., allopregnanolone) in the modulation of neurotransmission within brain-stem autonomic neurocircuits. This review will discuss our understanding of the actions and mechanisms of estrogen, progesterone, and neurosteroids at the cellular level of brain-stem nuclei. Understanding the complex interaction between sex hormones and neural signaling plasticity of the autonomic nervous system is essential to elucidating the role of sex in overall physiology and disease.

A case report of clonidine induced syncope: a review of central actions of an old cardiovascular drug

Background

Clonidine is an imidazoline sympatholytic, acting on both α₂-adrenergic and imidazoline receptors in the brainstem to induce antihypertensive and negative chronotropic effects in the vasculature and heart respectively.

Case presentation

A 69-year-old gentleman with hypertension presented to the emergency department after multiple syncopal episodes over the past 12 months. Electrocardiogram demonstrated sinus bradycardia with a heart rate of 42 beats per minute. It was hypothesized that the antihypertensive agent clonidine was responsible for inducing symptomatic bradycardia. Clonidine was thus gradually tapered and then discontinued over five days restoring normal sinus rhythm rates while avoiding hypertensive rebound related to sympathetic surge. His heart rate and blood pressure remained within normal limits after the clonidine taper and subsequent adjustments to his other hypertensive medications and he was discharged.

Conclusions

While clonidine has fallen out of favor for its indication as an antihypertensive, it remains a viable option for the use of opioid withdrawal, chronic pain, and smoking cessation, necessitating the appropriate clinical and pharmacological competencies for a physician to prescribe. A discussion of the clinical effects of clonidine brainstem receptor activation follows.

Cardiac neuroanatomy - Imaging nerves to define functional control

The autonomic nervous system regulates normal cardiovascular function and plays a critical role in the pathophysiology of cardiovascular disease. Further understanding of the interplay between the autonomic nervous system and cardiovascular system holds promise for the development of neuroscience-based cardiovascular therapeutics. To this end, techniques to image myocardial innervation will help provide a basis for understanding the fundamental underpinnings of cardiac neural control. In this review, we detail the evolution of gross and microscopic anatomical studies for functional mapping of cardiac neuroanatomy.

Neurotransmission to parasympathetic cardiac vagal neurons in the brain stem is altered with left ventricular hypertrophy-induced heart failure

Hypertension, cardiac hypertrophy, and heart failure (HF) are widespread and debilitating cardiovascular diseases that affect nearly 23 million people worldwide. A distinctive hallmark of these cardiovascular diseases is autonomic imbalance, with increased sympathetic activity and decreased parasympathetic vagal tone. Recent device-based approaches, such as implantable vagal stimulators that stimulate a multitude of visceral sensory and motor fibers in the vagus nerve, are being evaluated as new therapeutic approaches for these and other diseases. However, little is known about how parasympathetic activity to the heart is altered with these diseases, and this lack of knowledge is an obstacle in the goal of devising selective interventions that can target and selectively restore parasympathetic activity to the heart. To identify the changes that occur within the brain stem to diminish the parasympathetic cardiac activity, left ventricular hypertrophy was elicited in rats by aortic pressure overload using a transaortic constriction approach. Cardiac vagal neurons (CVNs) in the brain stem that generate parasympathetic activity to the heart were identified with a retrograde tracer and studied using patch-clamp electrophysiological recordings in vitro. Animals with left cardiac hypertrophy had diminished excitation of CVNs, which was mediated both by an augmented frequency of spontaneous inhibitory GABAergic neurotransmission (with no alteration of inhibitory glycinergic activity) as well as a diminished amplitude and frequency of excitatory neurotransmission to CVNs. Opportunities to alter these network pathways and neurotransmitter receptors provide future targets of intervention in the goal to restore parasympathetic activity and autonomic balance to the heart in cardiac hypertrophy and other cardiovascular diseases.

Optogenetic stimulation of locus ceruleus neurons augments inhibitory transmission to parasympathetic cardiac vagal neurons via activation of brainstem α1 and β1 receptors

Locus ceruleus (LC) noradrenergic neurons are critical in generating alertness. In addition to inducing cortical arousal, the LC also orchestrates changes in accompanying autonomic system function that compliments increased attention, such as during stress, excitation, and/or exposure to averse or novel stimuli. Although the association between arousal and increased heart rate is well accepted, the neurobiological link between the LC and parasympathetic neurons that control heart rate has not been identified. In this study, we test directly whether activation of noradrenergic neurons in the LC influences brainstem parasympathetic cardiac vagal neurons (CVNs). CVNs were identified in transgenic mice that express channel-rhodopsin-2 (ChR2) in LC tyrosine hydroxylase neurons. Photoactivation evoked a rapid depolarization, increased firing, and excitatory inward currents in ChR2-expressing neurons in the LC. Photostimulation of LC neurons did not alter excitatory currents, but increased inhibitory neurotransmission to CVNs. Optogenetic activation of LC neurons increased the frequency of isolated glycinergic IPSCs by 27 ± 8% (p = 0.003, n = 26) and augmented GABAergic IPSCs in CVNs by 21 ± 5% (p = 0.001, n = 26). Inhibiting α1, but not α2, receptors blocked the evoked responses. Inhibiting β1 receptors prevented the increase in glycinergic, but not GABAergic, IPSCs in CVNs. This study demonstrates LC noradrenergic neurons inhibit the brainstem CVNs that generate parasympathetic activity to the heart. This inhibition of CVNs would increase heart rate and risks associated with tachycardia. The receptors activated within this pathway, α1 and/or β1 receptors, are targets for clinically prescribed antagonists that promote slower, cardioprotective heart rates during heightened vigilant states.

Chronic intermittent hypoxia-hypercapnia blunts heart rate responses and alters neurotransmission to cardiac vagal neurons

Patients with obstructive sleep apnoea experience chronic intermittent hypoxia-hypercapnia (CIHH) during sleep that elicit sympathetic overactivity and diminished parasympathetic activity to the heart, leading to hypertension and depressed baroreflex sensitivity. The parasympathetic control of heart rate arises from pre-motor cardiac vagal neurons (CVNs) located in nucleus ambiguus (NA) and dorsal motor nucleus of the vagus (DMNX). The mechanisms underlying diminished vagal control of heart rate were investigated by studying the changes in blood pressure, heart rate, and neurotransmission to CVNs evoked by acute hypoxia-hypercapnia (H-H) and CIHH. In vivo telemetry recordings of blood pressure and heart rate were obtained in adult rats during 4 weeks of CIHH exposure. Retrogradely labelled CVNs were identified in an in vitro brainstem slice preparation obtained from adult rats exposed either to air or CIHH for 4 weeks. Postsynaptic inhibitory or excitatory currents were recorded using whole cell voltage clamp techniques. Rats exposed to CIHH had increases in blood pressure, leading to hypertension, and blunted heart rate responses to acute H-H. CIHH induced an increase in GABAergic and glycinergic neurotransmission to CVNs in NA and DMNX, respectively; and a reduction in glutamatergic neurotransmission to CVNs in both nuclei. CIHH blunted the bradycardia evoked by acute H-H and abolished the acute H-H evoked inhibition of GABAergic transmission while enhancing glycinergic neurotransmission to CVNs in NA. These changes with CIHH inhibit CVNs and vagal outflow to the heart, both in acute and chronic exposures to H-H, resulting in diminished levels of cardioprotective parasympathetic activity to the heart as seen in OSA patients.

Structural remodeling of the heart and its premotor cardioinhibitory vagal neurons following T(5) spinal cord transection

Midthoracic spinal cord injury (SCI) is associated with enhanced cardiac sympathetic activity and reduced cardiac parasympathetic activity. The enhanced cardiac sympathetic activity is associated with sympathetic structural plasticity within the stellate ganglia, spinal cord segments T1-T4, and heart. However, changes to cardiac parasympathetic centers rostral to an experimental SCI are relatively unknown. Importantly, reduced vagal activity is a predictor of high mortality. Furthermore, this autonomic dysregulation promotes progressive left ventricular (LV) structural remodeling. Accordingly, we hypothesized that midthoracic spinal cord injury is associated with structural plasticity in premotor (preganglionic parasympathetic neurons) cardioinhibitory vagal neurons located within the nucleus ambiguus as well as LV structural remodeling. To test this hypothesis, dendritic arborization and morphology (cholera toxin B immunohistochemistry and Sholl analysis) of cardiac projecting premotor cardioinhibitory vagal neurons located within the nucleus ambiguus were determined in intact (sham transected) and thoracic level 5 transected (T5X) rats. In addition, LV chamber size, wall thickness, and collagen content (Masson trichrome stain and structural analysis) were determined. Midthoracic SCI was associated with structural changes within the nucleus ambiguus and heart. Specifically, following T5 spinal cord transection, there was a significant increase in cardiac parasympathetic preganglionic neuron dendritic arborization, soma area, maximum dendritic length, and number of intersections/animal. This parasympathetic structural remodeling was associated with a profound LV structural remodeling. Specifically, T5 spinal cord transection increased LV chamber area, reduced LV wall thickness, and increased collagen content. Accordingly, results document a dynamic interaction between the heart and its parasympathetic innervation.

Parasympathetic preganglionic cardiac motoneurons labeled after voluntary diving

A dramatic bradycardia is induced by underwater submersion in vertebrates. The location of parasympathetic preganglionic cardiac motor neurons driving this aspect of the diving response was investigated using cFos immunohistochemistry combined with retrograde transport of cholera toxin subunit B (CTB) to double-label neurons. After pericardial injections of CTB, trained rats voluntarily dove underwater, and their heart rates (HR) dropped immediately to 95 ± 2 bpm, an 80% reduction. After immunohistochemical processing, the vast majority of CTB labeled neurons were located in the reticular formation from the rostral cervical spinal cord to the facial motor nucleus, confirming previous studies. Labeled neurons caudal to the rostral ventrolateral medulla were usually spindle-shaped aligned along an oblique line running from the dorsal vagal nucleus to the ventrolateral reticular formation, while those more rostrally were multipolar with extended dendrites. Nine percent of retrogradely-labeled neurons were positive for both cFos and CTB after diving and 74% of these were found rostral to the obex. CTB also was transported transganglionically in primary afferent fibers, resulting in large granular deposits in dorsolateral, ventrolateral, and commissural subnuclei of the nucleus tractus solitarii (NTS) and finer deposits in lamina I and IV-V of the trigeminocervical complex. The overlap of parasympathetic preganglionic cardiac motor neurons activated by diving with those activated by baro- and chemoreceptors in the rostral ventrolateral medulla is discussed. Thus, the profound bradycardia seen with underwater submersion reinforces the notion that the mammalian diving response is the most powerful autonomic reflex known.

Nesfatin-1 activates cardiac vagal neurons of nucleus ambiguus and elicits bradycardia in conscious rats

Nesfatin-1, a peptide whose receptor is yet to be identified, has been involved in the modulation of feeding, stress, and metabolic responses. More recently, increasing evidence supports a modulatory role for nesfatin-1 in autonomic and cardiovascular activity. This study was undertaken to test if the expression of nesfatin-1 in the nucleus ambiguus, a key site for parasympathetic cardiac control, may be correlated with a functional role. As we have previously demonstrated that nesfatin-1 elicits Ca²⁺ signaling in hypothalamic neurons, we first assessed the effect of this peptide on cytosolic Ca²⁺ in cardiac pre-ganglionic neurons of nucleus ambiguus. We provide evidence that nesfatin-1 increases cytosolic Ca²⁺ concentration via a Gi/o-coupled mechanism. The nesfatin-1-induced Ca²⁺ rise is critically dependent on Ca²⁺ influx via P/Q-type voltage-activated Ca²⁺ channels. Repeated administration of nesfatin-1 leads to tachyphylaxis. Furthermore, nesfatin-1 produces a dose-dependent depolarization of cardiac vagal neurons via a Gi/o-coupled mechanism. In vivo studies, using telemetric and tail-cuff monitoring of heart rate and blood pressure, indicate that microinjection of nesfatin-1 into the nucleus ambiguus produces bradycardia not accompanied by a change in blood pressure in conscious rats. Taken together, our results identify for the first time that nesfatin-1 decreases heart rate by activating cardiac vagal neurons of nucleus ambiguus. Our results indicate that nesfatin-1, one of the most potent feeding peptides, increases cytosolic Ca²⁺ by promoting Ca²⁺ influx via P/Q channels and depolarizes nucleus ambiguus neurons; both effects are Gi/o-mediated. In vivo studies indicate that microinjection of nesfatin-1 into nucleus ambiguus produces bradycardia in conscious rats. This is the first report that nesfatin-1 increases the parasympathetic cardiac tone.

Prenatal nicotine exposure enhances the trigeminocardiac reflex via serotonin receptor facilitation in brainstem pathways

In this study we used a rat model for prenatal nicotine exposure to test whether clinically relevant concentrations of brain nicotine and cotinine are passed from dams exposed to nicotine to her pups, whether this changes the trigeminocardiac reflex (TCR), and whether serotonergic function in the TCR brainstem circuitry is altered. Pregnant Sprague-Dawley dams were exposed to 6 mg·kg(-1)·day(-1) of nicotine via osmotic minipumps for the duration of pregnancy. Following birth dams and pups were killed, blood was collected, and brain nicotine and cotinine levels were measured. A separate group of prenatal nicotine-exposed pups was used for electrophysiological recordings. A horizontal brainstem slice was obtained by carefully preserving the trigeminal nerve with fluorescent identification of cardiac vagal neurons (CVNs) in the nucleus ambiguus. Stimulation of the trigeminal nerve evoked excitatory postsynaptic current in CVNs. Our data demonstrate that prenatal nicotine exposure significantly exaggerates both the TCR-evoked changes in heart rate in conscious unrestrained pups, and the excitatory neurotransmission to CVNs upon trigeminal afferent nerve stimulation within this brainstem reflex circuit. Application of the 5-HT1A receptor antagonist WAY 100635 (100 μM) and 5-HT2A/C receptor antagonist ketanserin (10 μM)significantly decreased neurotransmission, indicating an increased facilitation of 5-HT function in prenatal nicotine-exposed animals. Prenatal nicotine exposure enhances activation of 5-HT receptors and exaggerates the trigeminocardiac reflex.

Activation of D2-like dopamine receptors inhibits GABA and glycinergic neurotransmission to pre-motor cardiac vagal neurons in the nucleus ambiguus

The parasympathetic control of heart rate arises from premotor cardiac vagal neurons (CVNs) located in the nucleus ambiguus (NA). Previous microinjection studies in NA show that dopamine evokes a decrease in heart rate, but the underlying mechanisms responsible for these responses were not identified. This study tested whether dopamine modulates inhibitory GABAergic and glycinergic and/or excitatory glutamatergic neurotransmission to CVNs. Retrogradely labeled CVNs were identified in an in vitro rat brainstem slice preparation and synaptic events were recorded using whole cell voltage clamp techniques. Bath application of dopamine (100 μM) had no effect on excitatory synaptic events, but reversibly inhibited the frequency (but not amplitude) of GABAergic inhibitory postsynaptic currents (IPSCs) in CVNs. Similarly, dopamine (10 μM and 100 μM) inhibited glycinergic IPSC frequency by ~50% and 70% respectively. The reduction in inhibitory neurotransmission to CVNs by dopamine was prevented by the sodium channel blocker TTX (1μM) indicating that the dopamine mediated effects were action potential dependent. Dopamine evoked responses were mimicked by the D2-like receptor agonist, Quinpirole but not D1-like receptor agonist, SKF 38393. In addition, the dopamine mediated depression of inhibitory synaptic responses were prevented by the D2-like receptor antagonist sulpiride, but not by D1-like or adrenergic or serotonergic receptor antagonists, suggesting that these responses were D2-like receptor mediated and not D1-like or adrenergic or 5-HT receptor mediated. These data suggest that dopamine acts via dis-inhibition, and diminishes inhibitory GABAergic and glycinergic neurotransmission to CVNs, which would be predicted to increase parasympathetic activity to the heart and evoke a bradycardia.

Cardiorespiratory coupling in health and disease

Cardiac and respiratory activities are intricately linked both functionally as well as anatomically through highly overlapping brainstem networks controlling these autonomic physiologies that are essential for survival. Cardiorespiratory coupling (CRC) has many potential benefits creating synergies that promote healthy physiology. However, when such coupling deteriorates autonomic dysautonomia may ensue. Unfortunately there is still an incomplete mechanistic understanding of both normal and pathophysiological interactions that respectively give rise to CRC and cardiorespiratory dysautonomia. Moreover, there is also a need for better quantitative methods to assess CRC. This review addresses the current understanding of CRC by discussing: (1) the neurobiological basis of respiratory sinus arrhythmia (RSA); (2) various disease states involving cardiorespiratory dysautonomia; and (3) methodologies measuring heart rate variability and RSA.

Urocortin 3 elevates cytosolic calcium in nucleus ambiguus neurons

Urocortin 3 (also known as stresscopin) is an endogenous ligand for the corticotropin-releasing factor receptor 2 (CRF(2)). Despite predominant G(s) coupling of CRF(2), promiscuous coupling with other G proteins has been also associated with the activation of this receptor. As urocortin 3 has been involved in central cardiovascular regulation at hypothalamic and medullary sites, we examined its cellular effects on cardiac vagal neurons of nucleus ambiguus, a key area for the autonomic control of heart rate. Urocortin 3 (1 nM-1000 nM) induced a concentration-dependent increase in cytosolic Ca(2+) concentration that was blocked by the CRF(2) antagonist K41498. In the case of two consecutive treatments with urocortin 3, the second urocortin 3-induced Ca(2+) response was reduced, indicating receptor desensitization. The effect of urocortin 3 was abolished by pre-treatment with pertussis toxin and by inhibition of phospolipase C with U-73122. Urocortin 3 activated Ca(2+) influx via voltage-gated P/Q-type channels as well as Ca(2+) release from endoplasmic reticulum. Urocortin 3 promoted Ca(2+) release via inositol 1,4,5 trisphosphate receptors, but not ryanodine receptors. Our results indicate a novel Ca(2+) -mobilizing effect of urocortin 3 in vagal pre-ganglionic neurons of nucleus ambiguus, providing a cellular mechanism for a previously reported role for this peptide in parasympathetic cardiac regulation.

Synaptic and intrinsic activation of GABAergic neurons in the cardiorespiratory brainstem network

GABAergic pathways in the brainstem play an essential role in respiratory rhythmogenesis and interactions between the respiratory and cardiovascular neuronal control networks. However, little is known about the identity and function of these GABAergic inhibitory neurons and what determines their activity. In this study we have identified a population of GABAergic neurons in the ventrolateral medulla that receive increased excitatory post-synaptic potentials during inspiration, but also have spontaneous firing in the absence of synaptic input. Using transgenic mice that express GFP under the control of the Gad1 (GAD67) gene promoter, we determined that this population of GABAergic neurons is in close apposition to cardioinhibitory parasympathetic cardiac neurons in the nucleus ambiguus (NA). These neurons fire in synchronization with inspiratory activity. Although they receive excitatory glutamatergic synaptic inputs during inspiration, this excitatory neurotransmission was not altered by blocking nicotinic receptors, and many of these GABAergic neurons continue to fire after synaptic blockade. The spontaneous firing in these GABAergic neurons was not altered by the voltage-gated calcium channel blocker cadmium chloride that blocks both neurotransmission to these neurons and voltage-gated Ca²⁺ currents, but spontaneous firing was diminished by riluzole, demonstrating a role of persistent sodium channels in the spontaneous firing in these cardiorespiratory GABAergic neurons that possess a pacemaker phenotype. The spontaneously firing GABAergic neurons identified in this study that increase their activity during inspiration would support respiratory rhythm generation if they acted primarily to inhibit post-inspiratory neurons and thereby release inspiration neurons to increase their activity. This population of inspiratory-modulated GABAergic neurons could also play a role in inhibiting neurons that are most active during expiration and provide a framework for respiratory sinus arrhythmia as there is an increase in heart rate during inspiration that occurs via inhibition of premotor parasympathetic cardioinhibitory neurons in the NA during inspiration.

β adrenergic receptor modulation of neurotransmission to cardiac vagal neurons in the nucleus ambiguus

β-adrenergic receptors are a class of G protein-coupled receptors that have essential roles in regulating heart rate, blood pressure, and other cardiorespiratory functions. Although the role of β adrenergic receptors in the peripheral nervous system is well characterized, very little is known about their role in the central nervous system despite being localized in many brain regions involved in autonomic activity and regulation. Since parasympathetic activity to the heart is dominated by cardiac vagal neurons (CVNs) originating in the nucleus ambiguus (NA), β adrenergic receptors localized in the NA represent a potential target for modulating cardiac vagal activity and heart rate. This study tests the hypothesis that activation of β adrenergic receptors alters the membrane properties and synaptic neurotransmission to CVNs. CVNs were identified in brainstem slices, and membrane properties and synaptic events were recorded using the whole-cell voltage-clamp technique. The nonselective β agonist isoproterenol significantly decreased inhibitory GABAergic and glycinergic as well as excitatory glutamatergic neurotransmission to CVNs. In addition, the β(1)-selective receptor agonist dobutamine, but not β(2) or β(3) receptor agonists, significantly decreased inhibitory GABAergic and glycinergic and excitatory glutamatergic neurotransmission to CVNs. These decreases in neurotransmission to CVNs persisted in the presence of tetrodotoxin (TTX). These results provide a mechanism by which activation of adrenergic receptors in the brainstem can alter parasympathetic activity to the heart. Likely physiological roles for this adrenergic receptor activation are coordination of parasympathetic-sympathetic activity and β receptor-mediated increases in heart rate upon arousal.

Anaesthetics differentially modulate the trigeminocardiac reflex excitatory synaptic pathway in the brainstem

The trigeminocardiac reflex (TCR) occurs upon excitation of the trigeminal nerve with a resulting bradycardia and hypotension. While several anaesthetics and analgesics have been reported to alter the incidence and strength of the TCR the mechanisms for this modulation are unclear. This study examines the mechanisms of action of ketamine, isoflurane and fentanyl on the synaptic TCR responses in both neurones in the spinal trigeminal interpolaris (Sp5I) nucleus and cardiac vagal neurones (CVNs) in the Nucleus Ambiguus (NA). Stimulation of trigeminal afferent fibres evoked an excitatory postsynaptic current (EPSC) in trigeminal neurones with a latency of 1.8 ± 0.1 ms, jitter of 625 μs, and peak amplitude of 239 ± 45 pA. Synaptic responses further downstream in the reflex pathway in the CVNs occurred with a latency of 12.1 ± 1.1 ms, jitter of 0.8-2 ms and amplitude of 57.8 ± 7.5 pA. The average conduction velocity to the Sp5I neurones was 0.94 ± 0.18 mm ms(-1) indicating a mixture of A-δ and C fibres. Stimulation-evoked EPSCs in both Sp5I and CVNs were completely blocked by AMPA/kainate and NMDA glutamatergic receptor antagonists. Ketamine (10 μm) inhibited the peak amplitude and duration in Sp5I as well as more distal synapses in the CVNs. Isoflurane (300 μm) significantly inhibited, while fentanyl (1 μm) significantly enhanced, EPSC amplitude and area in CVNs but had no effect on the responses in Sp5l neurones. These findings indicate glutamatergic excitatory synaptic pathways are critical in the TCR, and ketamine, isoflurane and fentanyl differentially alter the synaptic pathways via modulation of both AMPA/kainate and NMDA receptors at different synapses in the TCR.

α1-adrenergic receptors facilitate inhibitory neurotransmission to cardiac vagal neurons in the nucleus ambiguus

The cholinergic cardiac vagal neurons (CVNs), located in the nucleus ambiguus, are the origin of cardioinhibitory parasympathetic activity. Catecholaminergic neurons in nearby regions of the brainstem, including the C1 and C2 cell groups, are thought to play a key role in both arousing from sleep and maintaining wakefulness. Because norepinephrine (NE) could play an important role in influencing the activity of CVNs, particularly in response to sleeping/waking and arousal states, the present study investigated the contribution of α(1)-adrenergic receptor activation to augment inhibitory and/or blunt excitatory neurotransmission to CVNs. To test the effects of α(1)-adrenergic receptor activation, CVNs were labeled in rats by retrograde tracing and synaptic events were recorded by whole cell voltage clamp techniques in vitro. Prazosin, an inverse agonist of α(1)-adrenergic receptor, significantly decreased the frequency of both GABAergic and glycinergic neurotransmission to CVNs. Activation of α(1)-adrenergic receptors by the α(1)-adrenergic receptor agonists NE or phenylephrine (PE) both significantly increased GABAergic and glycinergic inhibitory event frequency. This effect was prevented by the sodium channel blocker tetrodotoxin (TTX). Activation of α(1)-adrenergic receptors did not alter glutamatergic neurotransmission to CVNs. This study indicates that α(1)-adrenergic receptor activation in the brainstem can facilitate inhibitory GABAergic and glycinergic neurotransmission so as to reduce CVN activity; this synaptic modulation may play a role in the tachycardia seen during NE-dependent behavioral arousal.

Respiratory modulation of premotor cardiac vagal neurons in the brainstem

The respiratory and cardiovascular systems are highly intertwined, both anatomically and physiologically. Respiratory and cardiovascular neurons are often co-localized in the same brainstem regions, and this is particularly evident in the ventral medulla which contains presympathetic neurons in the rostral ventrolateral medulla, premotor parasympathetic cardioinhibitory neurons in the nucleus ambiguus, and the ventral respiratory group, which includes the pre-Botzinger complex. Anatomical studies of respiratory and cardiovascular neurons have demonstrated that many of these neurons have projections and axon collateral processes which extend into their neighboring cardiorespiratory regions providing an anatomical substrate for cardiorespiratory interactions. As other reports in this Special Issue of Respiratory Physiology & Neurobiology focus on interactions between the respiratory network and baroreceptors, neurons in the nucleus tractus solitarius, presympathetic neurons and sympathetic activity, this report will focus on the respiratory modulation of parasympathetic activity and the neurons that generate parasympathetic activity to the heart, cardiac vagal neurons.

GLP-1 receptor stimulation depresses heart rate variability and inhibits neurotransmission to cardiac vagal neurons

AIMS

glucagon-like peptide 1 (GLP-1) is an incretin hormone released from the gut in response to food intake. Whereas GLP-1 acts in the periphery to inhibit glucagon secretion and stimulate insulin release, it also acts in the central nervous system to mediate autonomic control of feeding, body temperature, and cardiovascular function. Because of its role as an incretin hormone, GLP-1 receptor analogs are used as a treatment for type 2 diabetes. Central or peripheral administration of GLP-1 increases blood pressure and heart rate, possibly by activating brainstem autonomic nuclei and increasing vagus nerve activity. However, the mechanism(s) by which GLP-1 receptor stimulation affects cardiovascular function are unknown. We used the long-lasting GLP-1 receptor agonist Exendin-4 (Ex-4) to test the hypothesis that GLP-1 signalling modulates central parasympathetic control of heart rate.

METHODS AND RESULTS

using a telemetry system, we assessed heart rate in mice during central Ex-4 administration. Heart rate was increased by both acute and chronic central Ex-4 administration. Spectral analysis indicated that the high frequency and low frequency powers of heart rate variability were diminished by Ex-4 treatment. Finally, Ex-4 decreased both excitatory glutamatergic and inhibitory glycinergic neurotransmission to preganglionic parasympathetic cardiac vagal neurons.

CONCLUSION

these data suggest that central GLP-1 receptor stimulation diminishes parasympathetic modulation of the heart thereby increasing heart rate.

Endogenous inhibition of the trigeminally evoked neurotransmission to cardiac vagal neurons by muscarinic acetylcholine receptors

Stimulation of the nasal mucosa by airborne irritants or water evokes a pronounced bradycardia accompanied by peripheral vasoconstriction and apnea. The dive response, which includes the trigeminocardiac reflex, is among the most powerful autonomic responses. These responses slow the heart rate and reduce myocardial oxygen consumption. Although normally cardioprotective, exaggeration of this reflex can be detrimental and has been implicated in cardiorespiratory diseases, including sudden infant death syndrome (SIDS). An essential component of the diving response and trigeminocardiac reflex is activation of the parasympathetic cardiac vagal neurons (CVNs) in the nucleus ambiguus that control heart rate. This study examined the involvement of cholinergic receptors in trigeminally evoked excitatory postsynaptic currents in CVNs in an in vitro preparation from rats. CVNs were identified using a retrograde tracer injected into the fat pads at the base of the heart. Application of the acetylcholinesterase inhibitor neostigmine significantly decreased the amplitude of glutamatergic neurotransmission to CVNs on stimulation of trigeminal fibers. Whereas nicotine did not have any effect on the glutamatergic responses, the muscarinic acetylcholine receptor (mAChR) agonist bethanechol significantly decreased the excitatory neurotransmission. Atropine, an mAChR antagonist, facilitated these responses indicating this trigeminally evoked brain stem pathway in vitro is endogenously inhibited by mAChRs. Tropicamide, an m4 mAChR antagonist, prevented the inhibitory action of the muscarinic agonist bethanechol. These results indicate that the glutamatergic synaptic neurotransmission in the trigeminally evoked pathway to CVNs is endogenously inhibited in vitro by m4 mAChRs.

Clonidine, an alpha2-receptor agonist, diminishes GABAergic neurotransmission to cardiac vagal neurons in the nucleus ambiguus

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 alpha(2)-adrenergic receptors in the central nervous system evokes a diverse range of physiological effects, including reducing blood pressure. This study tests whether clonidine, an alpha(2)-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.

Nicotinic receptors partly mediate brainstem autonomic dysfunction evoked by the inhaled anesthetic isoflurane

BACKGROUND

Isoflurane is one of the most commonly used volatile anesthetics, yet the cardiorespiratory depression that occurs with its use remains poorly understood. In this study, the author examined isoflurane modulation of postsynaptic gamma-aminobutyric acid (GABA) receptors in parasympathetic cardiac vagal neurons (CVNs) and alterations of GABAergic function by targeting nicotinic acetylcholine receptors on GABAergic presynaptic terminals.

METHODS

Rhythmic inspiratory-related activity was recorded from the hypoglossal rootlet of 800 microm medullary sections. CVNs were identified by retrograde fluorescent labeling, and GABAergic neurotransmission to CVNs were examined using patch-clamp electrophysiological techniques.

RESULTS

Isoflurane at concentrations of >50 microM significantly suppressed inspiratory bursting frequency, amplitude, and duration. Isoflurane dose-dependently decreased the frequency and increased the decay time of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) in CVNs. To test whether the inhibition of GABAergic activity to CVNs was mediated by presynaptic nicotinic receptors, the nicotinic antagonist, dihydro-beta-erythroidine in an alpha(4)beta(2)-selective concentration (3 microM), was used. Dihydro-beta-erythroidine (3 microM) prevented the isoflurane-evoked depression of spontaneous GABAergic IPSC frequency, yet isoflurane still increased the IPSC decay time.

CONCLUSIONS

These results suggest clinically relevant concentrations of isoflurane inhibit brainstem respiratory rhythmogenesis, prolong inhibitory GABAergic postsynaptic currents and reduce GABA activity in CVNs. The decrease of GABAergic IPSCs frequency is dependent upon inhibition of presynaptic alpha(4)beta(2) nicotinic receptors.

5HT2 receptor activation facilitates P2X receptor mediated excitatory neurotransmission to cardiac vagal neurons in the nucleus ambiguus

Parasympathetic preganglionic cardiac vagal neurons (CVNs) which dominate the control of heart rate are located within the nucleus ambiguus (NA). Serotonin (5HT), and in particular 5HT2 receptors, play an important role in cardiovascular function in the brainstem. However, there is a lack of information on the mechanisms of action of 5HT2 receptors in modulating parasympathetic cardiac activity. This study tests whether activation of 5HT2 receptors alters excitatory glutamatergic and purinergic neurotransmission to CVNs. Application of alpha-methyl-5-hydroxytryptamine (alpha-Me-5HT), a 5HT2 agonist, reversibly increased both the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) in CVNs. Similar responses were obtained with alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine hydrochloride (BW723C86), and m-chlorophenylpiperazine (m-CPP), 5HT2B and 5HT2B/C receptor agonists, respectively. The facilitation evoked by alpha-Me-5HT was prevented by the 5HT2B/C receptor antagonist SB206553 hydrochloride (SB206553). Interestingly, the blockage of both NMDA and non-NMDA glutamatergic receptors did not prevent alpha-Me-5HT-evoked facilitation of mEPSCs, however, the responses were blocked by the P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). The responses evoked by alpha-Me-5HT were mimicked by application of alpha,beta-methylene ATP (alpha,beta-Me-ATP), a P2X receptor agonist, which were also blocked by PPADS. In summary, these results indicate activation of 5HT2 receptors facilitates excitatory purinergic, but not glutamatergic, neurotransmission to CVNs.

Recruitment of excitatory serotonergic neurotransmission to cardiac vagal neurons in the nucleus ambiguus post hypoxia and hypercapnia

Inhibitory GABAergic and glycinergic neurotransmission to cardioinhibitory cardiac vagal neurons (CVNs) increase during inspiratory activity and likely mediate respiratory sinus arrhythmia, while the frequency of excitatory postsynaptic currents (EPSCs) in CVNs are unaltered during the different phases of respiration. However, following hypoxia and hypercapnia (H/H), the parasympathetic activity to the heart increases and thus far, identification of the pathways and neurotransmitters that are responsible for exciting CVNs post H/H are unclear. This study identifies different excitatory pathways to CVNs recruited post H/H. Spontaneous and inspiratory-related EPSCs were recorded in CVNs before, during, and after 10 min of H/H in an in vitro slice preparation that retains rhythmic respiratory activity. Before and during H/H, EPSCs in CVNs were completely blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and d(-)-2-amino-5-phosphonopentanoic acid (AP5), selective AMPA/kainate and N-methyl-d-apartate (NMDA) receptor blockers, respectively. However, after H/H, there was a significant increase in EPSCs during each inspiratory burst. While some of the inspiratory-related EPSCs were blocked by the broad purinergic receptor antagonist pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (PPADS) and the specific P2X receptor antagonist 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate monolithium trisodium salt (TNP-ATP) a P2X receptor blocker, most of the recruited excitatory neurotransmission to CVNs is serotonergic because odansetron, a selective 5-HT3 antagonist, abolished the majority of the spontaneous and inspiratory-related EPSCs evoked during recovery from H/H. The results from this study suggest that following episodes of H/H, two nonglutamatergic excitatory pathways, purinergic and serotonergic, activating P2X and 5-HT3 receptors, respectively, are recruited to excite CVNs in the post H/H recovery period.

Cholecystokinin-8s excites identified rat pancreatic-projecting vagal motoneurons

It is known that cholecystokinin (CCK) acts in a paracrine fashion to increase pancreatic exocrine secretion via vagal circuits. Recent evidence, however, suggests that CCK-8s actions are not restricted to afferent vagal fibers, but also affect brain stem structures directly. Within the brain stem, preganglionic neurons of the dorsal motor nucleus of the vagus (DMV) send efferent fibers to subdiaphragmatic viscera, including the pancreas. Our aims were to investigate whether DMV neurons responded to exogenously applied CCK-8s and, if so, the mechanism of action. Using whole cell patch-clamp recordings we show that perfusion with CCK-8s induced a concentration-dependent excitation in approximately 60% of identified pancreas-projecting DMV neurons. The depolarization was significantly reduced by tetrodotoxin, suggesting both direct (on the DMV membrane) and indirect (on local synaptic circuits) effects. Indeed, CCK-8s increased the frequency of miniature excitatory currents onto DMV neurons. The CCK-A antagonist, lorglumide, prevented the CCK-8s-mediated excitation whereas the CCK-B preferring agonist, CCK-nonsulfated, had no effect, suggesting the involvement of CCK-A receptors only. In voltage clamp, the CCK-8s-induced inward current reversed at -106 +/- 3 mV and the input resistance increased by 150 +/- 15%, suggesting an effect mediated by the closure of a potassium conductance. Indeed, CCK-8s reduced both the amplitude and the time constant of decay of a calcium-dependent potassium conductance. When tested with pancreatic polypeptide (which reduces pancreatic exocrine secretion), cells that responded to CCK-8s with an excitation were, instead, inhibited by pancreatic polypeptide. These data indicate that CCK-8s may control pancreas-exocrine secretion also via an effect on pancreas-projecting DMV neurons.

Glucagon-like peptide-1 excites pancreas-projecting preganglionic vagal motoneurons

Glucagon-like peptide-1 (GLP-1) increases pancreatic insulin secretion via a direct action on pancreatic beta-cells. A high density of GLP-1-containing neurons and receptors is also present in brain stem vagal circuits; therefore, the aims of the present study were to investigate 1) whether identified pancreas-projecting neurons of the dorsal motor nucleus of the vagus (DMV) respond to exogenously applied GLP-1, 2) the mechanism(s) of action of GLP-1, and 3) whether the GLP-1-responsive neurons (putative modulators of endocrine secretion) could be distinguished from DMV neurons responsive to peptides that modulate pancreatic exocrine secretion, specifically pancreatic polypeptide (PP). Whole cell recordings were made from identified pancreas-projecting DMV neurons. Perfusion with GLP-1 induced a concentration-dependent depolarization in approximately 50% of pancreas-projecting DMV neurons. The GLP-1 effects were mimicked by exendin-4 and antagonized by exendin-(9-39). In approximately 60% of the responsive neurons, the GLP-1-induced depolarization was reduced by tetrodotoxin (1 microM), suggesting both pre- and postsynaptic sites of action. Indeed, the GLP-1 effects were mediated by actions on potassium currents, GABA-induced currents, or both. Importantly, neurons excited by GLP-1 were unresponsive to PP and vice versa. These data indicate that 1) GLP-1 may act on DMV neurons to control pancreatic endocrine secretion, 2) the effects of GLP-1 on pancreas-projecting DMV neurons are mediated both via a direct excitation of their membrane as well as via an effect on local circuits, and 3) the GLP-1-responsive neurons (i.e., putative endocrine secretion-controlling neurons) could be distinguished from neurons responsive to PP (i.e., putative exocrine secretion-controlling neurons).

NO Differentially Regulates Neurotransmission to Premotor Cardiac Vagal Neurons in the Nucleus Ambiguus

NO is involved in the neural control of heart rate, and NO synthase expressing neurons and terminals have been localized in the nucleus ambiguus where parasympathetic cardiac vagal preganglionic neurons are located; however, little is known about the mechanisms by which NO alters the activity of premotor cardiac vagal neurons. This study examines whether the NO donor sodium nitroprusside ([SNP] 100 μmol/L) and precursor, l -arginine (10 mmol/L), modulate excitatory and inhibitory synaptic neurotransmission to cardiac vagal preganglionic neurons. Glutamatergic, GABAergic, and glycinergic activity to cardiac vagal neurons was examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation in rats. Both SNP, as well as l -arginine, increased the frequency of GABAergic neurotransmission to cardiac vagal preganglionic neurons but decreased the amplitude of GABAergic inhibitory postsynaptic currents. In contrast, both l -arginine and SNP inhibited the frequency of glutamatergic and glycinergic synaptic events in cardiac vagal preganglionic neurons. SNP and l -arginine also decreased glycinergic inhibitory postsynaptic current amplitude, and this response persisted in the presence of tetrodotoxin. Inclusion of the NO synthase inhibitor 7-nitroindazole (100 μmol/L) prevented the l -arginine–evoked responses. These results demonstrate that NO differentially regulates excitatory and inhibitory neurotransmission, facilitating GABAergic and diminishing glutamatergic and glycinergic neurotransmission to cardiac vagal neurons.

Reactive oxygen species mediate central cardiorespiratory network responses to acute intermittent hypoxia

Although oxidative stress and reactive oxygen species generation is typically associated with localized neuronal injury, reactive oxygen species have also recently been shown to act as a physiological signal in neuronal plasticity. Here we define an essential role for reactive oxygen species as a critical stimulus for cardiorespiratory reflex responses to acute episodic hypoxia in the brain stem. To examine central cardiorespiratory responses to episodic hypoxia, we used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and synaptic neurotransmission to cardioinhibitory vagal neurons. We show that whereas continuous hypoxia does not stimulate excitatory neurotransmission to cardioinhibitory vagal neurons, acute intermittent hypoxia of equivalent duration incrementally recruits an inspiratory-evoked excitatory neurotransmission to cardioinhibitory vagal neurons during intermittent hypoxia. This recruitment was dependent on the generation of reactive oxygen species. Further, we demonstrate that reactive oxygen species are incrementally generated in glutamatergic neurons in the ventrolateral medulla during intermittent hypoxia. These results suggest a neurochemical basis for the pronounced bradycardia that protects the heart against injury during intermittent hypoxia and demonstrates a novel role of reactive oxygen species in the brain stem.

The polyvagal perspective

The polyvagal theory introduced a new perspective relating autonomic function to behavior, that included an appreciation of the autonomic nervous system as a "system," the identification of neural circuits involved in the regulation of autonomic state, and an interpretation of autonomic reactivity as adaptive within the context of the phylogeny of the vertebrate autonomic nervous system. The paper has two objectives: first, to provide an explicit statement of the theory; and second, to introduce the features of a polyvagal perspective. The polyvagal perspective emphasizes how an understanding of neurophysiological mechanisms and phylogenetic shifts in neural regulation leads to different questions, paradigms, explanations, and conclusions regarding autonomic function in biobehavioral processes than peripheral models. Foremost, the polyvagal perspective emphasizes the importance of phylogenetic changes in the neural structures regulating the autonomic nervous system and how these phylogenetic shifts provide insights into the adaptive function and the neural regulation of the two vagal systems.

Differential control of central cardiorespiratory interactions by hypercapnia and the effect of prenatal nicotine

Hypercapnia evokes a strong cardiorespiratory response including gasping and a pronounced bradycardia; however, the mechanism responsible for these survival responses initiated in the brainstem is unknown. To examine the effects of hypercapnia on the central cardiorespiratory network, we used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and inhibitory synaptic neurotransmission to cardioinhibitory vagal neurons (CVNs). Hypercapnia differentially modulated inhibitory neurotransmission to CVNs; whereas hypercapnia selectively depressed spontaneous glycinergic IPSCs in CVNs without altering respiratory-related increases in glycinergic neurotransmission, it decreased both spontaneous and inspiratory-associated GABAergic IPSCs. Because maternal smoking is the highest risk factor for sudden infant death syndrome (SIDS) and prenatal nicotine exposure is proposed to be the link between maternal smoking and SIDS, we examined the cardiorespiratory responses to hypercapnia in animals exposed to nicotine in the prenatal and perinatal period. In animals exposed to prenatal nicotine, hypercapnia evoked an exaggerated depression of GABAergic IPSCs in CVNs with no significant change in glycinergic neurotransmission. Hypercapnia altered inhibitory neurotransmission to CVNs at both presynaptic and postsynaptic sites. Although the results obtained in this study in vitro cannot be extrapolated with certainty to in vivo responses, the results of this study provide a likely neurochemical mechanism for hypercapnia-evoked bradycardia and the dysregulation of this response with exposure to prenatal nicotine, creating a higher risk for SIDS.

Differential sensitivity of GABAergic and glycinergic inputs to orexin-A in preganglionic cardiac vagal neurons of newborn rats

AIM

To test the effect of orexin-A (hypocretin-1), a neuropeptide synthesized in the lateral hypothalamus and the perifornical area, on the glycinergic inputs and the GABAergic inputs of cardiac vagal neurons (CVN).

METHODS

The effects of orexin-A at three concentrations (20 nmol/L, 100 nmol/L, 500 nmol/L) on the glycinergic inputs and the GABAergic inputs were investigated by using retrograde fluorescent labeling of cardiac neurons (CVN) in the nucleus ambiguus (NA) and the voltage patch-clamp technique.

RESULTS

Orexin-A dose-dependently increased the frequency of both the glycinergic and the GABAergic spontaneous inhibitory postsynaptic currents (sIPSC). However, at a lower concentration (20 nmol/L) of orexin-A, although the frequency of the glycinergic sIPSC was significantly increased, the frequency of the GABAergic sIPSC was not significantly changed.

CONCLUSION

The glycinergic inputs and the GABAergic inputs have different sensitivities to orexin-A, which suggests that the two kinds of inhibitory inputs might play different roles in the synaptic control of cardiac vagal functions.

Prenatal nicotine exposure recruits an excitatory pathway to brainstem parasympathetic cardioinhibitory neurons during hypoxia/hypercapnia in the rat: implications for sudden infant death syndrome

Maternal cigarette smoking and prenatal nicotine exposure increase the risk for sudden infant death syndrome (SIDS) by 2- to 4-fold, yet despite adverse publicity, nearly one of four pregnant women smoke tobacco. Infants who succumb to SIDS typically experience a severe bradycardia that precedes or is accompanied by centrally mediated life-threatening apneas and gasping. Although the causes of the apnea and bradycardia prevalent in SIDS victims are unknown, it has been hypothesized that these fatal events are exaggerated cardiorespiratory responses to hypoxia or hypercapnia. Changes in heart rate are primarily determined by the activity of cardiac vagal neurons (CVNs) in the brainstem. In this study, we tested whether hypoxia/hypercapnia evokes synaptic pathways to CVNs and whether these cardiorespiratory interactions are altered by prenatal exposure to nicotine. Spontaneous rhythmic inspiratory-related activity was recorded from the hypoglossal rootlet of 700- to 800-microm medullary sections. CVNs were identified in this preparation by retrograde fluorescent labeling, and excitatory synaptic inputs to CVNs were isolated and studied using patch-clamp electrophysiologic techniques. Hypoxia/hypercapnia did not elicit an increase in excitatory neurotransmission to CVNs in unexposed animals, but in animals that were exposed to nicotine in the prenatal period, hypoxia/hypercapnia recruited an excitatory neurotransmission to CVNs. This study establishes a likely neurochemical mechanism for the exaggerated decrease in heart rate in response to hypoxia/hypercapnia that occurs in SIDS victims.

Effects of pancreatic polypeptide on pancreas-projecting rat dorsal motor nucleus of the vagus neurons

We investigated the pre- and postsynaptic effects of pancreatic polypeptide (PP) on identified pancreas-projecting neurons of the rat dorsal motor nucleus of the vagus in thin brain stem slices. Perfusion with PP induced a TTX- and apamin-sensitive, concentration-dependent outward (22% of neurons) or inward current (21% of neurons) that was accompanied by a decrease in input resistance; PP was also found to affect the amplitude of the action potential afterhyperpolarization. The remaining 57% of neurons were unaffected. PP induced a concentration-dependent inhibition in amplitude of excitatory (n = 22 of 30 neurons) and inhibitory (n = 13 of 17 neurons) postsynaptic currents evoked by electrical stimulation of the adjacent nucleus of the solitary tract, with an estimated EC(50) of 30 nM for both. The inhibition was accompanied by an alteration in the paired pulse ratio, suggesting a presynaptic site of action. PP also decreased the frequency, but not amplitude, of spontaneous excitatory (n = 6 of 11 neurons) and inhibitory currents (n = 7 of 9 neurons). In five neurons, chemical stimulation of the area postrema (AP) induced a TTX-sensitive inward (n = 3) or biphasic (outward and inward) current (n = 2). Superfusion with PP reversibly reduced the amplitude of these chemically stimulated currents. Regardless of the PP-induced effect, the vast majority of responsive neurons had a multipolar somata morphology with dendrites projecting to areas other than the fourth ventricle or the central canal. These results suggest that pancreas-projecting rat dorsal motor nucleus of the vagus neurons are heterogeneous with respect to their response to PP, which may underlie functional differences in the vagal modulation of pancreatic functions.

Misidentification of cardiac vagal pre-ganglionic neurons after injections of retrograde tracer into the pericardial space in the rat

We evaluated whether pericardial injections of the retrograde tracers cholera toxin subunit B (CTb) or Fast Blue (FB) reliably labelled cardiac vagal pre-ganglionic neurons. Injections of CTb into the pericardial space of the rat labelled neurons in both the external and compact formations of the nucleus ambiguus. Most labelled neurons were found in the compact formation of the nucleus ambiguus, and the majority of these, and only these, expressed immunoreactivity for calcitonin gene-related peptide. This distribution of labelled neurons and their immunohistochemical properties is characteristic of oesophageal motoneurons. Examination of the oesophagus following intra-pericardial CTb applications revealed strong labelling of motor end plates within the skeletal muscle of the thoracic but not the abdominal oesophagus. When a second retrograde tracer, FB, was injected into the abdominal oesophagus, labelled somata were found adjacent to CTb-labelled neurons in the compact formation of the nucleus ambiguus. No co-localisation of tracers was found, but identical proportions of calcitonin gene-related peptide (CGRP) immunoreactivity were observed in both groups of neurons. FB injected into the pericardial space labelled intra-cardiac neurons but not brainstem neurons. We conclude that intra-pericardial, and perhaps sub-epicardial, injections of some retrograde tracers are likely to label a subset of oesophageal, as well as cardiac, vagal motor neurons in the brainstem.

Hypocretin-1 (orexin-A) facilitates inhibitory and diminishes excitatory synaptic pathways to cardiac vagal neurons in the nucleus ambiguus

Hypocretin-1 is a neuropeptide recently shown to be involved in autonomic regulation. Hypocretin-1 is expressed by hypothalamic neurons, which project to many regions of the central nervous system, including the nucleus ambiguus. One possible site of action of hypocretin-1 could be cardioinhibitory parasympathetic vagal neurons within the nucleus ambiguus. This study examines whether hypocretin-1 modulates inhibitory and excitatory postsynaptic currents in cardiac vagal neurons in the rat nucleus ambiguus. GABAergic, glycinergic, and glutamatergic activity to cardiac vagal neurons was examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation. Hypocretin-1 (1 microM) produced a significant increase in the frequency and amplitude of both GABAergic and glycinergic inhibitory postsynaptic currents and a significant decrease in the frequency of glutamatergic excitatory postsynaptic currents. Application of tetrodotoxin (0.5 microM) blocked all of the responses to hypocretin-1, indicating the changes in neurotransmission with hypocretin-1 do not occur at presynaptic terminals but rather occur at the preceding GABAergic, glycinergic, and glutamatergic neurons that project to cardiac vagal neurons. The increase in GABAergic and glycinergic inhibitory postsynaptic currents, and the decrease in glutamatergic excitatory postsynaptic currents, could be mechanisms by which hypocretin-1 affects heart rate and cardiac function.

Characterization of pancreas-projecting rat dorsal motor nucleus of vagus neurons

The electrophysiological and morphological properties of rat dorsal motor nucleus of the vagus (DMV) neurons innervating the pancreas were examined by using whole cell patch clamp recordings from brain stem slices and postfixation morphological reconstructions of Neurobiotin-filled neurons. Recordings were made from 178 DMV neurons whose projections had been identified by previous apposition of the fluorescent neuronal tracer DiI to the body of the pancreas. DMV neurons projecting to the pancreas had an input resistance of 434 +/- 14 M omega, an action potential duration of 3 +/- 0.1 ms, and an afterhyperpolarization of 18 +/- 0.4 mV amplitude and 108 +/- 7 ms time constant of decay; these electrophysiological properties resembled those of gastric-projecting neurons but were significantly different from those of intestinal-projecting neurons. Interestingly, 14 of 178 pancreas-projecting neurons showed the presence of a slowly developing afterhyperpolarization whose presence was not reported in DMV neurons projecting to any other gastrointestinal area. The morphological characteristics of pancreas-projecting neurons (soma area 274 +/- 12 microm2; soma diameter of 25 +/- 0.7 microm; soma form factor 0.74 +/- 0.01; segments 9.7 +/- 0.41), however, were similar to those of intestinal- but differed from those of gastric-projecting neurons. In summary, these results suggest that pancreas-projecting rat DMV neurons are heterogeneous with respect to some electrophysiological and morphological properties. These differences might underlie functional differences in the vagal modulation of pancreatic functions.

Prenatal nicotine exposure alters central cardiorespiratory responses to hypoxia in rats: implications for sudden infant death syndrome

Maternal cigarette smoking and prenatal nicotine exposure are the highest risk factors for sudden infant death syndrome (SIDS). During hypoxia, respiratory frequency and heart rate transiently increase and subsequently decrease. These biphasic cardiorespiratory responses normally serve to prolong survival during hypoxia by reducing the metabolic demands of cardiac and respiratory muscles. However, exaggerated responses to hypoxia may be life threatening and have been implicated in SIDS. Heart rate is primarily determined by the activity of brainstem preganglionic cardioinhibitory vagal neurons (CVNs) in the nucleus ambiguus. We developed an in vitro rat brainstem slice preparation that maintains rhythmic inspiratory-related activity and contains fluorescently labeled CVNs. Synaptic inputs to CVNs were examined using patch-clamp electrophysiological techniques. Hypoxia evoked a biphasic change in the frequency of both GABAergic and glycinergic IPSCs in CVNs, comprised of an initial increase followed by a decrease in IPSC frequency. Prenatal exposure to nicotine changed the GABAergic response to hypoxia from a biphasic response to a precipitous decrease in spontaneous GABAergic IPSC frequency. This study establishes a likely neurochemical mechanism for the heart rate response to hypoxia and a link between prenatal nicotine exposure and an exaggerated bradycardia during hypoxia that may contribute to SIDS.

Fentanyl inhibits GABAergic neurotransmission to cardiac vagal neurons in the nucleus ambiguus

Fentanyl citrate is a synthetic opiate analgesic often used clinically for neonatal anesthesia. Although fentanyl significantly depresses heart rate, the mechanism of inducing bradycardia remains unclear. One possible site of action is the cardioinhibitory parasympathetic vagal neurons in the nucleus ambiguus (NA), from which originates control of heart rate and cardiac function. Inhibitory synaptic activity to cardiac vagal neurons is a major determinant of their activity. Therefore, the effect of fentanyl on GABAergic neurotransmission to parasympathetic cardiac vagal neurons was studied using whole-cell patch clamp electrophysiology. Application of fentanyl induced a reduction in both the frequency and amplitude of GABAergic IPSCs in cardiac vagal neurons. This inhibition was mediated at both pre- and postsynaptic sites as evidenced by a dual decrease in the frequency and amplitude of spontaneous miniature IPSCs. Application of the selective micro-antagonist CTOP abolished the fentanyl-mediated inhibition of GABAergic IPSCs. These results demonstrate that fentanyl acts on micro-opioid receptors on cardiac vagal neurons and neurons preceding them to reduce GABAergic neurotransmission and increase parasympathetic activity. The inhibition of GABAergic effects may be one mechanism by which fentanyl induces bradycardia.

Voltage gated P/Q and N-type calcium channels mediate the nicotinic facilitation of GABAergic and glycinergic inputs to cardiac vagal neurons

Previous work has shown endogenous cholinergic activity facilitates both GABAergic and glycinergic neurotransmission to premotor cardiac vagal neurons. Exogenous application of nicotine increases the frequency of glycinergic and GABAergic inhibitory postsynaptic currents (IPSCs) and miniature IPSCs (mIPSCs) to cardiac vagal neurons. In this study we examined whether the nicotine evoked facilitation of GABAergic and glycinergic neurotransmission to cardiac vagal neurons is dependent or independent of activation of voltage dependent calcium channels. Nicotine evoked increases in GABAergic and glycinergic mIPSCs in cardiac vagal neurons which were blocked by the non-specific calcium channel antagonist cadmium (100 microM). Application of the L (Cav 1) type calcium channel antagonist nimodipine (10 microM) had no effect. However, the increase in both GABAergic and glycinergic mIPSCs elicited by nicotine was abolished by the P/Q (Cav 2.1) voltage gated calcium channel antagonist omega-agatoxin IVA (100 nM). Omega-conotoxin GVIA (1 microM), a specific blocker of N (Cav 2.2) type voltage gated calcium currents, inhibited the nicotine elicited augmentation of GABA and abolished the increase in glycine mIPSC frequency. This work demonstrates that the nicotine evoked facilitation of GABAergic and glycinergic neurotransmission to cardiac vagal neurons is dependent upon activation of P/Q (Cav 2.1) and N (Cav 2.2) type calcium channels.

Respiratory sinus arrhythmia: endogenous activation of nicotinic receptors mediates respiratory modulation of brainstem cardioinhibitory parasympathetic neurons

The heart rate increases during inspiration and decreases during expiration. This respiratory sinus arrhythmia (RSA) occurs by modulation of premotor cardioinhibitory parasympathetic neuron (CPN) activity. However, RSA has not been fully characterized in rats, and despite the critical role of CPNs in the generation of RSA, little is known about the mechanisms that mediate this cardiorespiratory interaction. This study demonstrates that RSA in conscious rats is similar to that in other species. The mechanism of RSA was then examined in vitro. Rhythmic inspiratory-related activity was recorded from the hypoglossal rootlet of 700- to 800-microm medullary sections. CPNs were identified by retrograde fluorescent labeling, and neurotransmission to CPNs was examined using patch-clamp electrophysiological techniques. During inspiratory bursts, the frequency of both spontaneous gamma-aminobutyric acidergic (GABAergic) and spontaneous glycinergic synaptic events in CPNs was significantly increased. Focal application of the nicotinic antagonist dihydro-beta-erythroidine in an alpha4beta2-selective concentration (3 micromol/L) abolished the respiratory-evoked increase in GABAergic frequency. In contrast, the increase in glycinergic frequency during inspiration was not altered by nicotinic antagonists. Prenatal nicotine exposure exaggerated the increase in GABAergic frequency during inspiration and enhanced GABAergic synaptic amplitude both between and during inspiratory events. Glycinergic synaptic frequency and amplitude were unchanged by prenatal nicotine exposure. This study establishes a neurochemical link between neurons essential for respiration and CPNs, reveals a functional role for endogenous acetylcholine release and the activation of nicotinic receptors in the generation of RSA, and demonstrates that this cardiorespiratory interaction is exaggerated in rats prenatally exposed to nicotine.

Glycinergic inputs to cardiac vagal neurons in the nucleus ambiguus are inhibited by nociceptin and mu-selective opioids

Most parasympathetic regulation of heart rate originates from preganglionic cardiac vagal neurons within the nucleus ambiguus. Little is known regarding the modulation of glycinergic transmission to these neurons. However, the presence of mu-opioid receptors and opioid-receptor-like (ORL1) receptors within the ambiguus, together with the presence of endogenous ligands for both receptor types in the same area, suggests opioids may modulate synaptic transmission to cardiac vagal neurons. This study therefore examined the effects of endomorphin-1 and endomorphin-2 (the mu-selective endogenous peptides), DAMGO (a synthetic, mu-selective agonist), and nociceptin (the ORL1-selective endogenous peptide) on spontaneous glycinergic inhibitory postsynaptic currents (IPSCs) in rat cardiac parasympathetic neurons. All four of the opioids used in this study decreased spontaneous IPSCs. At concentrations of 100 microM, the amplitude of the IPSCs was reduced significantly by nociceptin (-56.6%), DAMGO (-46.5%), endomorphin-1 (-45.1%), and endomorphin-2 (-26%). IPSC frequency was also significantly reduced by nociceptin (-61.1%), DAMGO (-69.9%), and endomorphin-1 (-40.8%) but not endomorphin-2. Lower concentrations of nociceptin and DAMGO (10-30 microM) also effectively decreased IPSC amplitude and frequency. The inhibitory effects of DAMGO were blocked by d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2 (C-TOP; 10 microM), a selective mu-receptor antagonist. Neither nociceptin nor DAMGO inhibited the postsynaptic responses evoked by exogenous application of glycine or affected TTX-insensitive glycinergic mini-IPSCs. These results indicate that mu-selective opioids and nociceptin act on preceding neurons to decrease glycinergic inputs to cardiac vagal neurons in the nucleus ambiguus. The resulting decrease in glycinergic transmission would increase parasympathetic activity to the heart and may be a mechanism by which opioids induce bradycardia.

Neuropeptide Y and peptide YY inhibit excitatory synaptic transmission in the rat dorsal motor nucleus of the vagus

Pancreatic polypeptides (PPs) such as neuropeptide Y (NPY) and peptide YY (PYY) exert profound, vagally mediated effects on gastrointestinal (GI) motility and secretion. Whole-cell patch clamp recordings were made from brainstem slices containing identified GI-projecting rat dorsal motor nucleus of the vagus (DMV) neurons to determine the mechanism of action of PPs. Electrical stimulation of nucleus tractus solitarii (NTS) induced excitatory postsynaptic currents (EPSCs) that were reduced in a concentration-dependent manner by NPY and PYY (both at 0.1-300 nM) in 65 % of the neurons. An increase in the paired-pulse ratio without changes in the postsynaptic membrane input resistance or EPSC rise and decay time suggested that the effects of PPs on EPSCs were due to actions at presynaptic receptors. The Y1 and Y2 receptor selective agonists [Leu31,Pro34]NPY and NPY(3-36) (both at 100 nM) mimicked the inhibition of NPY and PYY on the EPSC amplitude. The effects of 100 nM NPY, but not PYY, were antagonized partially by the Y1 receptor selective antagonist BIBP3226 (0.1 micro M). In addition, the inhibition of the EPSC amplitude induced by NPY, but not PYY, was attenuated partially by pretreatment with the alpha2 adrenoceptor antagonist yohimbine (10 micro M), and occluded partially by the alpha2 adrenoceptor agonist UK14,304 (10 micro M) as well as by pretreatment with reserpine. Pretreatment with a combination of BIBP3226 and yohimbine almost completely antagonized the NPY-mediated effects on EPSCs. Contrary to the inhibition of EPSCs, perfusion with PPs had no effect on the amplitude of inhibitory postsynaptic currents (IPSCs) and a minimal effect on a minority of DMV neurons. Differences in the receptor subtypes utilized and in the mechanism of action of NPY and PYY may indicate functional differences in their roles within the circuitry of the dorsal vagal complex (DVC).

Endogenous acetylcholine and nicotine activation enhances GABAergic and glycinergic inputs to cardiac vagal neurons

The heart slows during expiration and heart rate increases during inspiration. This cardiorespiratory interaction is thought to occur by increased inhibitory synaptic events to cardiac vagal neurons during inspiration. Since cholinergic receptors have been suggested to be involved in this cardiorespiratory interaction, we tested whether endogenous cholinergic activity modulates GABAergic and glycinergic neurotransmission to cardiac vagal neurons in the nucleus ambiguus, whether nicotine can mimic this facilitation, and we examined the nicotinic receptors involved. Cardiac vagal neurons in the rat were labeled with a retrograde fluorescent tracer and studied in an in vitro slice using patch-clamp techniques. Application of neostigmine (10 microM), an acetylcholinerase inhibitor, significantly increased the frequency of both GABAergic and glycinergic inhibitory postsynaptic currents (IPSCs) in cardiac vagal neurons. Exogenous application of nicotine increased the frequency and amplitude of both GABAergic and glycinergic IPSCs. The nicotinic facilitation of both GABAergic and glycinergic IPSCs were insensitive to 100 nM alpha-bungarotoxin but were abolished by dihydro-beta-erythrodine (DHbetaE) at a concentration (3 microM) specific for alpha4beta2 nicotinic receptors. In the presence of TTX, nicotine increased the frequency of GABAergic and glycinergic miniature synaptic events, which were also abolished by DHbetaE (3 microM). This work demonstrates that there is endogenous cholinergic facilitation of GABAergic and glycinergic synaptic inputs to cardiac vagal neurons, and activation of alpha4beta2 nicotinic receptors at presynaptic terminals facilitates GABAergic and glycinergic neurotransmission to cardiac vagal neurons. Nicotinic facilitation of inhibitory neurotransmission to premotor cardiac parasympathetic neurons may be involved in generating respiratory sinus arrhythmia.

Distribution of mu-opioid receptors in rat visceral premotor neurons

Agonists of the mu-opioid receptor (MOR) can modulate the activity of visceral premotor neurons, including cardiac premotor neurons. Neurons in brainstem regions containing these premotor neurons also contain dense concentrations of the MOR1. This study examined the distribution of MOR1 within two populations of visceral premotor neurons: one located in the dorsal motor nucleus of the vagus and the other in the nucleus ambiguus. Visceral premotor neurons contained the retrograde tracer Fluoro-Gold following injections of the tracer into the pericardiac region of the thoracic cavity. MOR1 was localized using immunogold detection of an anti-peptide antibody. Visceral premotor neurons in both regions contained MOR1 at somatic and dendritic sites, although smaller dendrites were less likely to contain the receptor than larger dendrites, suggesting there may be selective trafficking of MOR1 within these neurons. MOR1 labeling in nucleus ambiguus neurons was more likely to be localized to plasma membrane sites, suggesting that ambiguus neurons may be more responsive to opioid ligands than neurons in the dorsal motor nucleus of the vagus. In addition, many of the dendrites of visceral premotor neurons were in direct apposition to other dendrites. MOR1 was often detected at these dendro-dendritic appositions that may be gap junctions. Together these findings indicate that the activity of individual visceral premotor neurons, as well as the coupling between neurons, may be regulated by ligands of the MOR.

Nicotinic ACh receptor subtypes on gastrointestinally projecting neurones in the dorsal motor vagal nucleus of the rat

To determine the predominant nicotinic ACh receptor (nAChR) located on neurones in the dorsal motor nucleus of the vagus (DMV) that project to the gastrointestinal tract, we used the rat brainstem slice preparation and whole-cell recordings of DMV neurones identified by retrograde DiI tracing to pharmacologically characterize nAChRs. Pressure ejection of acetylcholine (ACh, 250 microM for 200 ms) from a patch pipette placed approximately 10-20 microm from the surface of the recorded cell produced an inward current in most DMV neurones sampled. The average currents for neurones projecting to the fundus, antrum and caecum were 149 +/- 38 (n = 25), 115 +/- 18 (n = 29) and 117 +/- 23 pA (n = 6), respectively. Blockade of the alpha7 subtype of nAChR with either alpha-bungarotoxin (alpha-BGT) or methyllycaconitine (MLA) counteracted 60-75 % of the ACh-evoked current in DMV neurones projecting to the fundus, antrum and caecum. In neurones projecting to the fundus and the antrum, currents resistant to alpha-BGT were significantly blocked by dihydro-beta-erythroidine (10-20 nM), an antagonist of the alpha4beta2 subtype of nAChR. In neurones projecting to the caecum, currents resistant to alpha-BGT were significantly depressed by a low concentration of mecamylamine (1 microM). Cytisine (100 microM), an agonist of nAChRs that contain the alpha7 or the beta4 subunit, evoked significant currents in caecum-projecting neurones that were previously exposed to alpha-BGT. In contrast, cytisine had no effect on DMV neurones previously exposed to alpha-BGT that project to the fundus or antrum. Our data indicate that the prevailing nAChR subtype in DMV neurones projecting to the GI tract is the alpha7 subtype. In addition, we obtained evidence for the co-expression of the alpha4beta2 nAChR subtype on DMV neurones projecting to the fundus and antrum, and the alpha3beta4 nAChR subtype on DMV neurones projecting to the caecum.

Endomorphin-2 inhibits GABAergic inputs to cardiac parasympathetic neurons in the nucleus ambiguus

The nucleus ambiguus is an area containing cardiac vagal neurons, from which originates most of the parasympathetic control regulating heart rate and cardiac function. GABAergic pathways to these neurons have recently been described, yet modulation of this GABAergic input and its impact upon cardiac vagal neurons is unknown. The nucleus ambiguus has been shown to contain mu-opioid receptors and endomorphin-1 and endomorphin-2, the endogenous peptide ligands for the mu-receptor, whilst microinjections of opioids in the ambiguus area evoke bradycardia. The present study therefore examined the effects of endomorphin-1, endomorphin-2 and DAMGO (a synthetic, mu-selective agonist) on spontaneous GABAergic IPSCs in cardiac parasympathetic neurons. Only endomorphin-2 (100 microM) produced a significant inhibition, of both the frequency (-22.8%) and the amplitude (-30.5%) of the spontaneous IPSCs in cardiac vagal neurons. The inhibitory effects of endomorphin-2 were blocked by naloxonazine (10 microM), a selective mu(1) receptor antagonist. Naloxonazine alone (10 microM) had a potentiating effect on the frequency of the GABAergic IPSCs (+161.43%) but not on the amplitude, indicating that GABA release to cardiac vagal neurons may be under tonic control of opioids acting at the mu(1) receptor. Endomorphin-2 did not reduce the responses evoked by exogenous application of GABA. These results indicate that endomorphin-2 acts on mu(1) receptors located on precedent neurons to decrease GABAergic input to cardiac vagal neurons located in the nucleus ambiguus. The subsequent increase in parasympathetic outflow to the heart may be one mechanism by which mu-selective opioids act to induce bradycardia.

Patch clamp electrophysiology in nodose ganglia of adult rat

The patch clamp technique is widely utilized for studying the electrophysiological properties of enzymatically isolated sensory neurons. Unfortunately, dissociation of the ganglion severs the afferent fibers. As a result, isolated neurons can only be broadly categorized according to somatic action potential waveforms, ion channel subtypes, chemical sensitivities and cell diameter. Such restricted classifications contrast with the continuum of conduction velocities (CVs), discharge patterns, sensory modalities and functional properties of visceral and spinal afferents. Previous reports of patch clamp recordings using intact ganglion have been limited to young animal preparations. This raises concerns regarding postnatal development and impedes the use of chronic models of disease or injury, which often necessitate the use of a more mature animal preparation. Here, we present a methodology for preparing nodose ganglion from adult rat (250-400 g) for study using the patch clamp technique. Successful whole cell recordings were obtained from approximately 50% of the cells selected for study, the majority of which had intact afferent fibers. Measures of somatic discharge and afferent fiber CV at both room and physiological temperatures were consistent with investigations using sharp microelectrodes. Voltage clamp recordings of whole cell Na(+), Ca(2+) and K(+) ion channel currents were comparable to those obtained using isolated neuron preparations. The ability to classify voltage- and ligand-gated ion channel type with afferent fiber CV in an adult preparation adds a valuable new dimension to cellular investigations of the diverse functional and chemical properties of the peripheral afferent nervous system.

Nociceptin inhibits gamma-aminobutyric acidergic inputs to cardiac parasympathetic neurons in the nucleus ambiguus

Studies have shown that nociceptin, the endogenous ligand for the opioid receptor-like receptor (ORL(1)), modulates central control of cardiovascular activity. The nucleus ambiguus, an area containing cardiac parasympathetic neurons, contains both ORL(1) receptors and neurons that contain nociceptin itself. Although previous work has shown that nociceptin acts to increase parasympathetic outflow to the heart, the mechanisms by which this is achieved are unknown. In the present study, the effects of nociceptin on spontaneous gamma-aminobutyric acidergic (GABAergic) input to cardiac parasympathetic neurons (IPSCs) was examined. At 100 microM, nociceptin inhibited both the frequency (-35.6%) and the amplitude (-49.5%) of spontaneous GABAergic IPSCs in cardiac vagal neurons. Nociceptin also caused a novel postsynaptic inhibition of the responses evoked by exogenous application of GABA. These results indicate that nociceptin acts both on neurons precedent to cardiovascular neurons to decrease the activity of GABAergic neurons that synapse upon cardiovascular neurons and directly, inhibiting the postsynaptic currents evoked by GABA. This inhibition by nociceptin would increase parasympathetic outflow to the heart, thus providing a possible mechanism for nociceptin-induced bradycardia.