BLOCKADE OF 5-HT 1A RECEPTORS IN THE PHRENIC NUCLEUS OF THE RAT ATTENUATED RAPHE INDUCED ACTIVATION OF THE PHRENIC NERVE ACTIVITY

INTRODUCTION A group of neurons in the medullary raphe nuclei is involved in respiratory control. Raphe neurons are known to be the major source of serotonergic projections to other respiratory areas of the brain stem and spinal cord (1-3). Previous studies have shown that stimulation of the neurons in the raphe pallidus (RP) produces excitatory effects on respiratory activity The basic respiratory behavior of anesthetized animals is attributed to phrenic nerve activity. The phrenic motor nucleus (PMN) receives descending serotonergic projections originating from the RP (3). These projections make important contributions to the changes in discharge patterns of the phrenic nerve activity. It has been shown that raphe induced respiratory facilitation of the phrenic nerve activity is attenuated following intravenous administration of the serotonergic receptor antagonist methysergide (3). Therefore, we hypothesized that chemical stimulation of RP would produce excitatory responses that are mediated through 5-HT 1A receptors in the PMN. The present study was performed to investigate changes in phrenic nerve activity with chemical stimulation of RP by the synaptic excitant, D,L-homocysteic acid (DLH). Additionally, this study examined the role of 5-HT 1A receptors in the PMN on the excitatory response elicited from RP. METHODS The protocol for this study was approved by the Ethical Committee for Biomedical Research of the University of Split School of Medicine, Split, Croatia. All experiments were carried out in accordance with the National Research Council's guide for the care and use of laboratory animals. General procedures Experiments were performed on adult male Sprague-Dawley rats weighing 280-330 g. Anesthesia was performed with intraperitoneal injection of 20% solution of urethane in 0.9% saline (1.2 g/kg; supplemental dose 0.2 g/kg). The adequacy of anesthesia was assessed by the absence of a withdrawal reflex after noxious paw pinch. The femoral vein and artery were cannulated for intravenous drug delivery, blood pressure monitoring, and sampling of arterial blood. Blood samples were taken at regular intervals, and arterial blood gasses were maintained within physiological limits by infusion of bicarbonate solution. The trachea was cannulated through midline incision. All animals were vagotomized bilaterally. End-JOURNAL OF PHYSIOLOGY AND PHARMACOLOGY 2009, 60, 3, 167-172 www.jpp. Stimulation of the raphe pallidus nucleus produces facilitatory effects on respiratory activity. Numerous serotonergic projections from the raphe pallidus have been shown to terminate in the phrenic nucleus. This study was undertaken to examine the role of 5-hydroxytryptamine 1A (5-HT 1A ) receptors in the phrenic nucleus on the excitatory response of the phrenic nerve activity elicited from the raphe pallidus. We hypothesized that blockade of 5-HT 1A receptors in the phrenic nucleus will attenuate raphe-induced facilitation of the phrenic nerve. Chemical stimulation of the raphe pallidus by synaptic excitant D,L-homocysteic acid produced increase in the amplitude of the phrenic nerve activity. After microinjection of the specific 5-HT 1A receptor antagonist WAY, N-(2-(4,2-methoxyphenyl)-1-piperazinyl)ethyl)-N-2-pyridinyl-cyclohexane-carboxamide maleate into the phrenic nucleus, the raphe-induced facilitation of the phrenic nerve was attenuated. These data suggest that excitation of the phrenic nerve activity elicited by activation of the neurons in the raphe pallidus is mediated by 5-HT 1A receptors in the phrenic nucleus. K e y w o r d s : breathing, serotonin, phrenic nerve, ra

Exogenous Nitric Oxide and Bubble Formation in Divers

. Purpose: Prevention of bubble formation is a central goal in standard decompression procedures. Previously we have shown that exercise 20 Y 24 h prior to a dive reduces bubble formation and increases survival in rats exposed to a simulated dive. Furthermore, we have demonstrated that nitric oxide (NO) may be involved in this protection; blocking the production of NO increases bubble formation while giving rats a long-lasting NO donor 20 h and immediately prior to a dive reduces bubble formation. This study determined whether a short-lasting NO donor, nitroglycerine, reduced bubble formation after standard dives and decompression in man. Methods: A total of 16 experienced divers were randomly assigned into two groups. One group performed two dives to 30 m of seawater (msw) for 30 min breathing air, and performed exercise at an intensity corresponding to 30% of maximal oxygen uptake during the bottom time. The second group performed two simulated dives to 18 msw for 80 min breathing air in a hyperbaric chamber, and remained sedentary during the bottom period. The first dive for each diver served as the control dive, whereas the divers received 0.4 mg of nitroglycerine by oral spray 30 min before the second dive. Following the dive, gas bubbles in the pulmonary artery were recorded using ultrasound. Results: The open-water dive resulted in significantly more gas bubbles than the dry dive (0.87 T 1.3 vs 0.12 T 0.23 bubbles per square centimeter). Nitroglycerine reduced bubble formation significantly in both dives from 0.87 T 1.3 to 0.32 T 0.7 in the in-water dive and from 0.12 T 0.23 to 0.03 T 0.03 bubbles per square centimeter in the chamber dive. Conclusion: The present study demonstrates that intake of a short-lasting NO donor reduces bubble formation following decompression after different dives

Ventilation-perfusion inequality in the human lung is not increased following no-decompression-stop hyperbaric exposure

Venous gas bubbles occur in recreational SCUBA divers in the absence of decompression sickness, forming venous gas emboli (VGE) which are trapped within pulmonary circulation and cleared by the lung without overt pathology. We hypothesized that asymptomatic VGE would transiently increase ventilation-perfusion mismatch due to their occlusive effects within the pulmonary circulation. Two sets of healthy volunteers (n = 11, n = 12) were recruited to test this hypothesis with a single recreational ocean dive or a baro-equivalent dry hyperbaric dive. Pulmonary studies (intrabreath VA/Q (iV/Q), alveolar dead space, and FVC) were conducted at baseline and repeat 1- and 24-h after the exposure. Contrary to our hypothesis VA/Q mismatch was decreased 1-h post-SCUBA dive (iV/Q slope 0.023 ± 0.008 ml−1 at baseline vs. 0.010 ± 0.005 NS), and was significantly reduced 24-h post-SCUBA dive (0.000 ± 0.005, p < 0.05), with improved VA/Q homogeneity inversely correlated to dive severity. No changes in VA/Q mismatch were observed after the chamber dive. Alveolar dead space decreased 24-h post-SCUBA dive (78 ± 10 ml at baseline vs. 56 ± 5, p < 0.05), but not 1-h post dive. FVC rose 1-h post-SCUBA dive (5.01 ± 0.18 l vs. 5.21 ± 0.26, p < 0.05), remained elevated 24-h post SCUBA dive (5.06 ± 0.2, p < 0.05), but was decreased 1-hr after the chamber dive (4.96 ± 0.31 L to 4.87 ± 0.32, p < 0.05). The degree of VA/Q mismatch in the lung was decreased following recreational ocean dives, and was unchanged following an equivalent air chamber dive, arguing against an impact of VGE on the pulmonary circulation

Central chemoreflex sensitivity and sympathetic neural outflow in elite breath-hold divers

Repeated hypoxemia in obstructive sleep apnea patients increases sympathetic activity, thereby promoting arterial hypertension. Elite breath-holding divers are exposed to similar apneic episodes and hypoxemia. We hypothesized that trained divers would have increased resting sympathetic activity and blood pressure, as well as an excessive sympathetic nervous system response to hypercapnia. We recruited 11 experienced divers and 9 control subjects. During the diving season preceding the study, divers participated in 7.3 +/- 1.2 diving fish-catching competitions and 76.4 +/- 14.6 apnea training sessions with the last apnea 3-5 days before testing. We monitored beat-by-beat blood pressure, heart rate, femoral artery blood flow, respiration, end-tidal CO(2), and muscle sympathetic nerve activity (MSNA). After a baseline period, subjects began to rebreathe a hyperoxic gas mixture to raise end-tidal CO(2) to 60 Torr. Baseline MSNA frequency was 31 +/- 11 bursts/min in divers and 33 +/- 13 bursts/min in control subjects. Total MSNA activity was 1.8 +/- 1.5 AU/min in divers and 1.8 +/- 1.3 AU/min in control subjects. Arterial oxygen saturation did not change during rebreathing, whereas end-tidal CO(2) increased continuously. The slope of the hypercapnic ventilatory and MSNA response was similar in both groups. We conclude that repeated bouts of hypoxemia in elite, healthy breath-holding divers do not lead to sustained sympathetic activation or arterial hypertension. Repeated episodes of hypoxemia may not be sufficient to drive an increase in resting sympathetic activity in the absence of additional comorbidities

Glossopharyngeal insufflation induces cardioinhibitory syncope in apnea divers

Apnea divers increase intrathoracic pressure voluntarily by taking a deep breath followed by glossopharyngeal insufflation. Because apnea divers sometimes experience hypotension and syncope during the maneuver, they may serve as a model to study the mechanisms of syncope. We recorded changes in hemodynamics and sympathetic vasomotor tone with microneurography during breath holding with glossopharyngeal insufflation. Five men became hypotensive and fainted during breath holding with glossopharyngeal insufflation within the first minute. In four divers, heart rate dropped suddenly to a minimum of 38 +/- 4 beats/min. Therefore, cardioinhibitory syncope was more common than low cardiac output syncope

Cardiovascular regulation during apnea in elite divers

Involuntary apnea during sleep elicits sustained arterial hypertension through sympathetic activation; however, little is known about voluntary apnea, particularly in elite athletes. Their physiological adjustments are largely unknown. We measured blood pressure, heart rate, hemoglobin oxygen saturation, muscle sympathetic nerve activity, and vascular resistance before and during maximal end-inspiratory breath holds in 20 elite divers and in 15 matched control subjects. At baseline, arterial pressure and heart rate were similar in both groups. Maximal apnea time was longer in divers (1.7+/-0.4 versus 3.9+/-1.1 minutes; P5-fold greater muscle sympathetic nerve activity increase (P<0.01) with a massively increased pressor response compared with control subjects (9+/-5 versus 32+/-15 mm Hg; P<0.001). Vascular resistance increased in both groups, but more so in divers (79+/-46% versus 140+/-82%; P<0.01). Heart rate did not change in either group. The rise in muscle sympathetic nerve activity correlated with oxygen desaturation (r(2)=0.26; P<0.01) and with the increase in mean arterial pressure (r(2)=0.40; P<0.0001). In elite divers, breath holds for several minutes result in an excessive chemoreflex activation of sympathetic vasoconstrictor activity. Extensive sympathetically mediated peripheral vasoconstriction may help to maintain adequate oxygen supply to vital organs under asphyxic conditions that untrained subjects are not able to tolerate voluntarily. Our results are relevant to conditions featuring periodic apnea