Transcriptional response of the heart to vagus nerve stimulation

Heart failure is a major clinical problem, with treatments involving medication, devices, and emerging neuromodulation therapies such as vagus nerve stimulation (VNS). Considering the ongoing interest in using VNS to treat cardiovascular disease, it is important to understand the genetic and molecular changes developing in the heart in response to this form of autonomic neuromodulation. This experimental animal (rat) study investigated the immediate transcriptional response of the ventricular myocardium to selective stimulation of vagal efferent activity using an optogenetic approach. Vagal preganglionic neurons in the dorsal motor nucleus of the vagus nerve were genetically targeted to express light-sensitive chimeric channelrhodopsin variant ChIEF and stimulated using light. RNA sequencing of the left ventricular myocardium identified 294 differentially expressed genes (false discovery rate < 0.05). Qiagen Ingenuity Pathway Analysis (IPA) highlighted 118 canonical pathways that were significantly modulated by vagal activity, of which 14 had a z score of ≥2/≤-2, including EIF-2, IL-2, integrin, and NFAT-regulated cardiac hypertrophy. IPA revealed the effect of efferent vagus stimulation on protein synthesis, autophagy, fibrosis, autonomic signaling, inflammation, and hypertrophy. IPA further predicted that the identified differentially expressed genes were the targets of 50 upstream regulators, including transcription factors (e.g., MYC and NRF1) and microRNAs (e.g., miR-335-3p and miR-338-3p). These data demonstrate that the vagus nerve has a major impact on the myocardial expression of genes involved in the regulation of key biological pathways. The transcriptional response of the ventricular myocardium induced by stimulation of vagal efferents is consistent with the beneficial effect of maintained/increased vagal activity on the heart.NEW & NOTEWORTHY This experimental animal study investigated the immediate transcriptional response of the ventricular myocardium to selective stimulation of vagal efferent activity. Vagal stimulation induced significant transcriptional changes in the heart involving the pathways controlling autonomic signaling, inflammation, fibrosis, and hypertrophy. This study provides the first direct evidence that myocardial gene expression is modulated by the activity of the autonomic nervous system.

Immediate and sustained increases in the activity of vagal preganglionic neurons during exercise and after exercise training

AIMS

The brain controls the heart by dynamic recruitment and withdrawal of cardiac parasympathetic (vagal) and sympathetic activity. Autonomic control is essential for the development of cardiovascular responses during exercise, however, the patterns of changes in the activity of the two autonomic limbs, and their functional interactions in orchestrating physiological responses during exercise, are not fully understood. The aim of this study was to characterize changes in vagal parasympathetic drive in response to exercise and exercise training by directly recording the electrical activity of vagal preganglionic neurons in experimental animals (rats).

METHODS AND RESULTS

Single unit recordings were made using carbon-fibre microelectrodes from the populations of vagal preganglionic neurons of the nucleus ambiguus (NA) and the dorsal vagal motor nucleus of the brainstem. It was found that (i) vagal preganglionic neurons of the NA and the dorsal vagal motor nucleus are strongly activated during bouts of acute exercise, and (ii) exercise training markedly increases the resting activity of both populations of vagal preganglionic neurons and augments the excitatory responses of NA neurons during exercise.

CONCLUSIONS

These data show that central vagal drive increases during exercise and provide the first direct neurophysiological evidence that exercise training increases vagal tone. The data argue against the notion of exercise-induced central vagal withdrawal during exercise. We propose that robust increases in the activity of vagal preganglionic neurons during bouts of exercise underlie activity-dependent plasticity, leading to higher resting vagal tone that confers multiple health benefits associated with regular exercise.

Distinct pro-vigilant profile induced in rats by the mGluR5 potentiator LSN2814617

While treatment options are available, excessive daytime sleepiness (EDS) remains a significant unmet medical need for many patients. Relatively little rodent behavioural pharmacology has been conducted in this context to assess potential pro-vigilant compounds for their ability to restore functional capacity following experimentally induced sleep loss. Male Wistar rats were prepared for electroencephalographic (EEG) recording and subject to 11 h of sleep restriction using a biofeedback-induced cage rotation protocol. A simple response latency task (SRLT) was used to behaviourally index sleep restriction and the effects of pro-vigilant compounds: modafinil, D-amphetamine, caffeine, and the mGlu5-positive allosteric modulator LSN2814617. Sleep restriction resulted in a consistent, quantified loss of non-rapid eye movement (NREM) and REM sleep that impaired SRLT performance in a manner suggestive of progressive task disengagement. In terms of EEG parameters, all compounds induced wakefulness. Amphetamine treatment further decreased SRLT performance capacity, whereas the other three compounds decreased omissions and allowed animals to re-engage in the task. Caffeine and modafinil also significantly increased premature responses during this period, an effect not observed for LSN2814617. While all compounds caused compensatory sleep responses, the magnitude of compensation observed for LSN2814617 was much smaller than would be predicted to result from the prolongation of wakefulness exhibited. Using simple response latencies to index performance, an mGlu5 PAM dramatically increased wakefulness and improved functional capacity of sleep-restricted animals, without eliciting a proportionate compensatory sleep response. This effect was qualitatively distinct from that of amphetamine, caffeine and modafinil.

Oxytocin-immunoreactive innervation of identified neurons in the rat dorsal vagal complex

BACKGROUND

Oxytocin (OXT) has been implicated in reproduction and social interactions and in the control of digestion and blood pressure. OXT-immunoreactive axons occur in the dorsal vagal complex (DVC; nucleus tractus solitarius, NTS, dorsal motor nucleus of the vagus, DMV, and area postrema, AP), which contains neurons that regulate autonomic homeostasis. The aim of the present work is to provide a systematic investigation of the OXT-immunoreactive innervation of dorsal motor nucleus of the vagus (DMV) neurons involved in the control of gastrointestinal (GI) function.

METHODS

We studied DMV neurons identified by (i) prior injection of retrograde tracers in the stomach, ileum, or cervical vagus or (ii) induction of c-fos expression by glucoprivation with 2-deoxyglucose. Another subgroup of DMV neurons was identified electrophysiologically by stimulation of the cervical vagus and then juxtacellularly labeled with biotinamide. We used two- or three-color immunoperoxidase labeling for studies at the light microscopic level.

KEY RESULTS

Close appositions from OXT-immunoreactive varicosities were found on the cell bodies, dendrites, and axons of DMV neurons that projected to the GI tract and that responded to 2-deoxyglucose and juxtacellularly labeled DMV neurons. Double staining for OXT and choline acetyltransferase revealed that OXT innervation was heavier in the caudal and lateral DMV than in other regions. OXT-immunoreactive varicosities also closely apposed a small subset of tyrosine hydroxylase-immunoreactive NTS and DMV neurons.

CONCLUSIONS & INFERENCES

Our results provide the first anatomical evidence for direct OXT-immunoreactive innervation of GI-related neurons in the DMV.

Memory consolidation in the cerebellar cortex

Several forms of learning, including classical conditioning of the eyeblink, depend upon the cerebellum. In examining mechanisms of eyeblink conditioning in rabbits, reversible inactivations of the control circuitry have begun to dissociate aspects of cerebellar cortical and nuclear function in memory consolidation. It was previously shown that post-training cerebellar cortical, but not nuclear, inactivations with the GABA_A agonist muscimol prevented consolidation but these findings left open the question as to how final memory storage was partitioned across cortical and nuclear levels. Memory consolidation might be essentially cortical and directly disturbed by actions of the muscimol, or it might be nuclear, and sensitive to the raised excitability of the nuclear neurons following the loss of cortical inhibition. To resolve this question, we simultaneously inactivated cerebellar cortical lobule HVI and the anterior interpositus nucleus of rabbits during the post-training period, so protecting the nuclei from disinhibitory effects of cortical inactivation. Consolidation was impaired by these simultaneous inactivations. Because direct application of muscimol to the nuclei alone has no impact upon consolidation, we can conclude that post-training, consolidation processes and memory storage for eyeblink conditioning have critical cerebellar cortical components. The findings are consistent with a recent model that suggests the distribution of learning-related plasticity across cortical and nuclear levels is task-dependent. There can be transfer to nuclear or brainstem levels for control of high-frequency responses but learning with lower frequency response components, such as in eyeblink conditioning, remains mainly dependent upon cortical memory storage.

Involvement of central 5-HT7 receptors in modulation of cardiovascular reflexes in awake rats

This study evaluated the role of 5-HT7 receptors within the central nervous system in modulating cardiovascular responses to the activation of chemo-, baro- and cardiopulmonary reflexes and in the regulation of mean arterial pressure and heart rate, using intracisternal (i.c.) application of the selective 5-HT7 receptor antagonist SB-269970 in awake rats. Experiments were performed on male Wistar rats (300-320 g). At 4 days before the experiment, rats were anesthetized and placed in a stereotaxic frame implantation of a guide cannula in the direction of the cisterna magna to be used for microinjection of saline or SB-269970 (100 microg/kg). On the day before the experiments a femoral artery and vein were cannulated to record arterial pressure and heart rate and to inject drugs to activate cardiovascular reflexes, respectively. The chemo-, baro- and cardiopulmonary reflexes were activated in different experimental groups before and after i.c. injection of saline or SB-269970. The antagonism of 5-HT7 receptors reduced: (a) the pressor (50+/-4 vs. 19+/-9 mm Hg) and bradycardic (-247+/-13 vs. -69+/-27 bpm) responses to chemoreflex activation; (b) the fall in MAP (-54+/-4 vs. -20+/-6 mm Hg) and the bradycardia (-294+/-12 vs. -98+/-34 bpm) in response to cardiopulmonary reflex activation; and (c) the gain of the baroreflex (-2.3+/-0.1 to -0.9+/-0.2 bpm/mm Hg). Intracisternal application of SB-269970 increased significantly baseline MAP in those rats previously submitted to the activation of a cardiovascular reflex but in naïve rats produced no changes in the baseline MAP were observed. The fact that cardiovascular responses to all reflexes tested were attenuated by the antagonism of 5-HT7 receptors suggests that brainstem 5-HT7 receptors brainstem facilitate the processing of the autonomic responses to cardiovascular reflex activation and that a 5-HT-containing pathway to the brainstem provides a normalizing input during challenges produced by cardiovascular reflex activation which seems to be mediated by 5-HT7 receptors.

Effect of 5-HT depletion on cardiovascular vagal reflex sensitivity in awake and anesthetized rats

Antagonism of central 5-HT1A and 5-HT7 receptors inhibits reflex-evoked vagal bradycardias indicating that 5-HT is released during these reflexes. The present experiments examined the effect of 5-HT depletion with para-chlorophenylalanine (p-CPA) on the cardiac vagal baroreflex and cardiopulmonary reflex in awake and anesthetized rats. Immunocytochemistry and neurochemical detection showed that p-CPA depleted the brainstem of 5-HT, but not of norepinephrine or dopamine. Depletion of 5-HT was associated with an increase in mean arterial pressure (MAP) in awake rats. This difference was abolished by anesthesia, which reduced MAP in both groups of animals. The baroreflex gain, whether calculated from the rise in pressure induced by phenylephrine or the fall in pressure evoked by sodium nitroprusside, was significantly attenuated in depleted rats compared to controls. This attenuation of the baroreflex gain was unaffected by subsequent anesthesia. 5-HT depletion also attenuated the cardiopulmonary reflex vagal bradycardias but this only reached statistical significance when the rats were anesthetized. The data support the view that 5-HT is released in the reflex activation of the cardiac vagal pathway.

The role of central 5-HT3 receptors in vagal reflex inputs to neurones in the nucleus tractus solitarius of anaesthetized rats

Brainstem 5-hydroxytryptamine (5-HT, serotonin)-containing neurones modulate cardiovascular reflex responses but the differing roles of the many 5-HT receptors have not been thoroughly investigated. The present experiments on anaesthetized rats investigated the role of 5-HT3 receptors in modulating vagal afferent evoked activity of nucleus tractus solitarius (NTS) neurones. Recordings were made from 301 NTS neurones receiving an input at long (> 20 ms) minimum onset latency from stimulation of the vagus nerve. These included 140 neurones excited by activating non-myelinated cardiopulmonary afferents by right atrial injection of phenylbiguanide (PBG). Ionophoretic application of PBG, a highly selective 5-HT3 receptor agonist, significantly increased activity (from 2.4 +/- 0.4 to 5.5 +/- 0.8 spikes s(-1)) in 96 of 106 neurones tested and in all 17 neurones tested the increase in activity (3.4 +/- 1.1 to 7.0 +/- 1.9 spikes s(-1)) was significantly attenuated (3.0 +/- 0.9 to 3.8 +/- 1.1 spikes s(-1)) by the selective 5-HT3 receptor antagonist granisetron. Ionophoretic application of PBG potentiated responses to vagus nerve and cardiopulmonary afferent stimulation, and granisetron significantly attenuated this cardiopulmonary input (20.2 +/- 5.7 to 10.6 +/- 4.1 spikes burst(-1)) in 9 of 10 neurones tested. Ionophoretic application of AMPA and NMDA also excited NTS neurones and these excitations could be selectively antagonized by the non-NMDA and NMDA receptor antagonists DNQX and AP-5, respectively. At these selective currents, DNQX and AP-5 also attenuated PBG- and cardiopulmonary input-evoked increases in NTS activity. These data are consistent with the hypothesis that vagal inputs, including non-myelinated cardiopulmonary inputs to the NTS, utilize a 5-HT-containing pathway which activates 5-HT3 receptors. This excitatory response to 5-HT3 receptor activation may be partly a direct postsynaptic action but part may also be due to facilitation of the release of glutamate which in turn acts on either non-NMDA or NMDA receptors to evoke excitation.

Central 5-HT7 receptors are critical for reflex activation of cardiac vagal drive in anaesthetized rats

5-Hydroxytryptamine (5-HT; serotonin)-containing neurones contribute to reflex activation of parasympathetic outflow in a number of species, but the 5-HT receptors mediating these effects have yet to be fully determined. The present experiments demonstrate that central 5-HT7 receptors are involved in the vagal bradycardia evoked during the cardiopulmonary reflex, baroreflexes and the chemoreflex, as well as other autonomic changes caused by these reflexes. The experiments examined the effects of the selective 5-HT7 receptor antagonists SB-269970 and SB-656104 on these reflexes. For the cardiopulmonary reflex, when compared to time-matched vehicle control experiments, intracisternal application of SB-269970 (30-300 microg kg(-1), i.c.) dose-dependently attenuated the evoked bradycardia. At the highest dose, SB-269970 also attenuated the reflex hypotension and sympathoinhibition. The structurally different 5-HT7 receptor antagonist SB-656104 (100 microg kg(-1), i.c.) similarly attenuated the reflex bradycardia and hypotension. SB-269970 (100 microg kg(-1), i.c.) also attenuated the bradycardias evoked by electrical stimulation of aortic nerve afferents and the baroreflex evoked by the pressor response to phenylephrine (3-25 microg kg(-1), i.v.). The gain of the baroreflex was also significantly attenuated (0.15 +/- 0.06 versus 0.34 +/- 0.06 ms mmHg(-1)). Finally, SB-269970 (100 microg kg(-1), i.c.) significantly attenuated both the bradycardia and sympathoexcitation evoked by the chemoreflex. These data indicate that central 5-HT7 receptors play an important facilitatory role in the reflex activation of vagal outflow to the heart.