Cardiorespiratory effects of intertrigeminal area stimulation in vagotomized rats

The Mammalian Diving Response: Inroads to Its Neural Control

The mammalian diving response (DR) is a remarkable behavior that was first formally studied by Laurence Irving and Per Scholander in the late 1930s. The DR is called such because it is most prominent in marine mammals such as seals, whales, and dolphins, but nevertheless is found in all mammals studied. It consists generally of breathing cessation (apnea), a dramatic slowing of heart rate (bradycardia), and an increase in peripheral vasoconstriction. The DR is thought to conserve vital oxygen stores and thus maintain life by directing perfusion to the two organs most essential for life-the heart and the brain. The DR is important, not only for its dramatic power over autonomic function, but also because it alters normal homeostatic reflexes such as the baroreceptor reflex and respiratory chemoreceptor reflex. The neurons driving the reflex circuits for the DR are contained within the medulla and spinal cord since the response remains after the brainstem transection at the pontomedullary junction. Neuroanatomical and physiological data suggesting brainstem areas important for the apnea, bradycardia, and peripheral vasoconstriction induced by underwater submersion are reviewed. Defining the brainstem circuit for the DR may open broad avenues for understanding the mechanisms of suprabulbar control of autonomic function in general, as well as implicate its role in some clinical states. Knowledge of the proposed diving circuit should facilitate studies on elite human divers performing breath-holding dives as well as investigations on sudden infant death syndrome (SIDS), stroke, migraine headache, and arrhythmias. We have speculated that the DR is the most powerful autonomic reflex known.

Anatomo-Physiologic Basis for Auricular Stimulation

Introduction: Stimulation of cranial nerves modulates central nervous system (CNS) activity via the extensive connections of their brainstem nuclei to higher-order structures. Clinical experience with vagus-nerve stimulation (VNS) demonstrates that it produces robust therapeutic effects, however, posing concerns related to its invasiveness and side-effects. Discussion: Trigeminal nerve stimulation (TNS) has been recently proposed as a valid alternative to VNS. The ear presents afferent vagus and trigeminal-nerve distribution; its innervation is the theoretical basis of different reflex therapies, including auriculotherapy. An increasing number of studies have shown that several therapeutic effects induced by invasive VNS and TNS, can be reproduced by noninvasive auricular-nerve stimulation. However, the sites and neurobiologic mechanisms by which VNS and TNS produce their therapeutic effects are not clear yet. Conclusions: Accumulating evidence suggests that VNS and TNS share multiple levels and mechanisms of action in the CNS.

Intracerebroventricular injections of dronabinol, a cannabinoid receptor agonist, does not attenuate serotonin-induced apnea in Sprague-Dawley rats

Background

Evidence suggests that vagal nerve activity may play a role in sleep apnea induction. In anesthetized rats, dronabinol, a cannabinoid (CB) receptor agonist, injected into the nodose ganglia attenuates reflex apnea and increases genioglossus activity, and reflex apnea attenuation is blocked by systemic pre-treatment with cannabinoid type 1 and/or type 2 receptor antagonists. However, it is unclear whether dronabinol has similar effects in the central nervous system; CB receptors are widely distributed in the brain, especially on neuronal circuitry important for respiration and upper airway activation. Here, we examine the effects of intracerebroventricular (ICV) injection of dronabinol on serotonin (5-HT)-induced apnea.

Methods

Adult male Sprague-Dawley rats were anesthetized and instrumented with bilateral electrodes to monitor genioglossi EMG and with a piezoelectric strain gauge to monitor respiratory pattern. Serotonin was intravenously infused into a femoral vein to induce reflex apnea. After baseline recordings, rats were placed in a stereotaxic apparatus. A unilateral osteotomy was made to allow access for injection to the right lateral ventricle, and the dura were carefully removed. Dronabinol (100, 10, 1, or 0.1 μg/3 μl DMSO) or control (3 μl DMSO) was injected into the right lateral ventricle and 5-HT infusion was repeated. Data (mean ± SEM) were analyzed using a mixed model analysis with a repeated/fixed measure.

Results

There was no main effect in 5-HT-induced apnea or breath duration, or in breath instability, between ICV dronabinol injected and ICV vehicle control injected groups. Moreover, there was no main effect in phasic or tonic genioglossus activity between ICV dronabinol injected and ICV vehicle control injected groups.

Conclusion

Our data show that ICV injection of dronabinol did not decrease 5-HT-induced apneas, and did not increase genioglossus activity. This in contrast to published results of dronabinol’s effect on apnea via the vagus nerve. Our results suggest that the effects of dronabinol on reflex apneas are peripherally mediated via suppression of vagal nerve activity.

The mammalian diving response: an enigmatic reflex to preserve life?

The mammalian diving response is a remarkable behavior that overrides basic homeostatic reflexes. It is most studied in large aquatic mammals but is seen in all vertebrates. Pelagic mammals have developed several physiological adaptations to conserve intrinsic oxygen stores, but the apnea, bradycardia, and vasoconstriction is shared with those terrestrial and is neurally mediated. The adaptations of aquatic mammals are reviewed here as well as the neural control of cardiorespiratory physiology during diving in rodents.

Persistence of the nasotrigeminal reflex after pontomedullary transection

Most behaviors have numerous components based on reflexes, but the neural circuits driving most reflexes rarely are documented. The nasotrigeminal reflex induced by stimulating the nasal mucosa causes an apnea, a bradycardia, and variable changes in mean arterial blood pressure (MABP). In this study we tested the nasotrigeminal reflex after transecting the brainstem at the pontomedullary junction. The nasal mucosae of anesthetized rats were stimulated with ammonia vapors and their brainstems then were transected. Complete transections alone induced an increase in resting heart rate (HR; p<0.001) and MABP (p<0.001), but no significant change in ventilation. However, the responses to nasal stimulation after transection were similar to those seen prior to transection. HR still dropped significantly (p<0.001), duration of apnea remained the same, as did changes in MABP. Results from rats whose transection were incomplete are discussed. These data implicate that the neuronal circuitry driving the nasotrigeminal reflex, and indirectly the diving response, is intrinsic to the medulla and spinal cord.

Hypocapnia-dependent facilitation of augmented breaths: observations in awake vs. anesthetized rats

We investigated whether commonly used injectable laboratory anesthetics alter the regulation of augmented breaths (ABs) in different respiratory backgrounds. Male rats were studied on three separate experimental days, receiving one of three injections in randomized order: ethyl carbamate ('urethane'; 1.2mgkg(-1)), ketamine/xylazine (ket/xyl; 80/10mgkg(-1)), or normal saline. Following each of the three interventions, breathing was monitored during 15min exposures to normoxia (room air), hypoxia (10% O(2)) and hypoxia+CO(2) (10% O(2), 5% CO(2)). Urethane anesthesia completely eliminated ABs from the breathing rhythm in room air conditions (p<0.001), and decreased the hypocapnia-dependent component of this response (p<0.001). ket/xyl left the normal incidence of ABs in room air breathing intact but significantly suppressed the hypoxia-induced facilitation of ABs (p=0.0015). These results provide the first clear evidence that laboratory anesthesia can profoundly alter the regulation of ABs including the hypocapnia-dependent component of their facilitation.

Impact of the vagal feedback on cardiorespiratory coupling in anesthetized rats

Cardiorespiratory coupling can be significantly influenced by both pontine and vagal modulation of medullary motor and premotor areas. We investigated influences of the pontine intertrigeminal region (ITR) and peripheral vagal pathways on the coupling between systolic blood pressure (SBP) and respiration in 9 anesthetized rats. Glutamate injection into the ITR perturbed both respiration and SBP and decreased SBP-respiratory coherence (0.95±0.01 vs 0.89±0.02; (p=0.01). Intravenous infusion of serotonin (5-HT) produced apnea and hypertension and also decreased SBP-respiratory coherence (0.95±0.01 vs 0.72±0.06; p=0.04). Bilateral vagotomy eliminated the cardiorespiratory coherence perturbations induced by central (glutamate injection into the ITR: 0.89±0.03 vs 0.86±0.03; p=0.63) and peripheral (5-HT infusion: 0.89±0.03 vs 0.88±0.02; p=0.98) pharmacologic manipulations. Glutamate stimulation of the ITR postvagotomy increased the relative spectral power density of SBP in the respiratory frequency range (0.25±0.08 vs 0.55±0.06; p=0.01). The data suggest that SBP-respiratory coupling is largely mediated within the central nervous system, with vagal systems acting in a way that disrupts coherence during transient cardiorespiratory disturbances. Although decreased cardiorespiratory coherence may increase cardiac work during perturbations, this may be physiologically advantageous in restoring homeostatic equilibrium of respiration and blood pressure.