Serotonin is necessary for short-term modulation of the exercise ventilatory response

Inaugural Review Prize 2023: The exercise hyperpnoea dilemma: A 21st-century perspective

During mild or moderate exercise, alveolar ventilation increases in direct proportion to metabolic rate, regulating arterial CO2 pressure near resting levels. Mechanisms giving rise to the hyperpnoea of exercise are unsettled despite over a century of investigation. In the past three decades, neuroscience has advanced tremendously, raising optimism that the 'exercise hyperpnoea dilemma' can finally be solved. In this review, new perspectives are offered in the hope of stimulating original ideas based on modern neuroscience methods and current understanding. We first describe the ventilatory control system and the challenge exercise places upon blood-gas regulation. We highlight relevant system properties, including feedforward, feedback and adaptive (i.e., plasticity) control of breathing. We then elaborate a seldom explored hypothesis that the exercise ventilatory response continuously adapts (learns and relearns) throughout life and ponder if the memory 'engram' encoding the feedforward exercise ventilatory stimulus could reside within the cerebellum. Our hypotheses are based on accumulating evidence supporting the cerebellum's role in motor learning and the numerous direct and indirect projections from deep cerebellar nuclei to brainstem respiratory neurons. We propose that cerebellar learning may be obligatory for the accurate and adjustable exercise hyperpnoea capable of tracking changes in life conditions/experiences, and that learning arises from specific cerebellar microcircuits that can be interrogated using powerful techniques such as optogenetics and chemogenetics. Although this review is speculative, we consider it essential to reframe our perspective if we are to solve the till-now intractable exercise hyperpnoea dilemma.

Short-term modulation of the ventilatory response to exercise is preserved in obstructive sleep apnea

BACKGROUND

The ventilatory response to exercise can be transiently adjusted in response to environmentally (e.g., breathing apparatus) or physiologically altered conditions (e.g., respiratory disease), maintaining constant relative arterial P_CO2 regulation from rest to exercise (Mitchell and Babb, 2006); this augmentation is called short-term modulation (STM) of the exercise ventilatory response. Obesity and/or obstructive sleep apnea could affect the exercise ventilatory response and the capacity for STM due to chronically increased mechanical and/or ventilatory loads on the respiratory system, and/or recurrent (chronic) intermittent hypoxia experienced during sleep. We hypothesized that: (1) the exercise ventilatory response is augmented in obese OSA patients compared with obese non-OSA adults, and (2) the capacity for STM with added dead space is diminished in obese OSA patients.

METHODS

Nine obese adults with OSA (age: 39±6 yr, BMI: 40±5kg/m², AHI: 25±24 events/h [range 6-73], mean±SD) and 8 obese adults without OSA (age: 38±10 yr, BMI: 37±6kg/m², AHI: 1±2) completed three, 20-min bouts of constant-load submaximal cycling exercise (8min rest, 6min at 10 and 30W) with or without added external dead space (200 or 400mL; 20min rest between bouts). Steady-state measurements were made of ventilation (V˙_E)_, oxygen consumption V˙_O2), carbon dioxide production (V˙_CO2), and end-tidal P_CO2 (PET_CO2). The exercise ventilatory response was defined as the slope of the V˙E-V˙_CO2 relationship (ΔV˙E/ΔV˙_CO2).

RESULTS

In control (i.e. no added dead space), the exercise ventilatory response was not significantly different between non-OSA and OSA groups (ΔV˙E/ΔV˙_CO2 slope: 30.5±4.2 vs 30.5±3.8, p>0.05); PET_CO2 regulation from rest to exercise did not differ between groups (p>0.05). In trials with added external dead space, ΔV˙E/ΔV˙_CO2 increased with increased dead space (p < 0.05) and the PET_CO2 change from rest to exercise remained small (<2mmHg) in both groups, demonstrating STM. There were no significant differences between groups.

CONCLUSIONS

Contrary to our hypotheses: (1) the exercise ventilatory response is not increased in obese OSA patients compared with obese non-OSA adults, and (2) the capacity for STM with added dead space is preserved in obese OSA and non-OSA adults.

Control of breathing during exercise

During exercise by healthy mammals, alveolar ventilation and alveolar-capillary diffusion increase in proportion to the increase in metabolic rate to prevent PaCO2 from increasing and PaO2 from decreasing. There is no known mechanism capable of directly sensing the rate of gas exchange in the muscles or the lungs; thus, for over a century there has been intense interest in elucidating how respiratory neurons adjust their output to variables which can not be directly monitored. Several hypotheses have been tested and supportive data were obtained, but for each hypothesis, there are contradictory data or reasons to question the validity of each hypothesis. Herein, we report a critique of the major hypotheses which has led to the following conclusions. First, a single stimulus or combination of stimuli that convincingly and entirely explains the hyperpnea has not been identified. Second, the coupling of the hyperpnea to metabolic rate is not causal but is due to of these variables each resulting from a common factor which link the circulatory and ventilatory responses to exercise. Third, stimuli postulated to act at pulmonary or cardiac receptors or carotid and intracranial chemoreceptors are not primary mediators of the hyperpnea. Fourth, stimuli originating in exercising limbs and conveyed to the brain by spinal afferents contribute to the exercise hyperpnea. Fifth, the hyperventilation during heavy exercise is not primarily due to lactacidosis stimulation of carotid chemoreceptors. Finally, since volitional exercise requires activation of the CNS, neural feed-forward (central command) mediation of the exercise hyperpnea seems intuitive and is supported by data from several studies. However, there is no compelling evidence to accept this concept as an indisputable fact.

Obesity: challenges to ventilatory control during exercise--a brief review

Obesity is a national health issue in the US. Among the many physiological changes induced by obesity, it also presents a unique challenge to ventilatory control during exercise due to increased metabolic demand of moving larger limbs, increased work of breathing due to extra weight on the chest wall, and changes in breathing mechanics. These challenges to ventilatory control in obesity can be inconspicuous or overt among obese adults but for the most part adaptation of ventilatory control during exercise in obesity appears remarkably unnoticed in the majority of obese people. In this brief review, the changes to ventilatory control required for maintaining normal ventilation during exercise will be examined, especially the interaction between respiratory neural drive and ventilation. Also, gaps in our current knowledge will be discussed.

Short-term modulation of the exercise ventilatory response in younger and older women

The exercise ventilatory response (EVR; defined as the slope of the relationship between ventilation and CO(2) production) is reversibly augmented within a single exercise trial with increased respiratory dead space (DS) in both younger (Wood, H.E., Mitchell, G.S., Babb, T.G., 2008. Short-term modulation of the exercise ventilatory response in young men. J. Appl. Physiol. 104, 244-252) and older (Wood, H.E., Mitchell, G.S., Babb, T.G., 2010. Short-term modulation of the exercise ventilatory response in older men. Respir. Physiol. Neurobiol. 173, 37-46) men. The neural mechanism accounting for this augmentation is known as short-term modulation (STM) of the EVR. Since the effects of female sex hormones on STM are unknown, we examined the capacity for STM in healthy adult women of two age groups; nine younger (29±3 yrs, eumenorrheic) and seven older (69±3 yrs, postmenopausal) women were studied at rest and during cycle exercise (10 W, 30 W; not randomized) in control conditions and with added external DS (200 mL, 400 mL; randomized). Within groups, the main effects of DS and work rate on EVR were analyzed with a two-way repeated measures ANOVA; EVR comparisons between groups were made with unpaired t-tests. In both groups, EVR increased progressively with increasing DS volume (e.g. at 10 W 31±4 and 35±6 in control, 40±11 and 40±6 with 200 mL, 48±12 and 49±11 with 400 mL DS in younger and older women, respectively). In younger women, the effects of DS on EVR differed between work rates (significant interaction, p<0.05), although this was not the case for older women. In both groups, [Formula: see text] regulation was similar between DS and control; hence, increased EVR was not due to altered chemoreceptor feedback from rest to exercise. EVR with and without added DS did not differ between age groups. We conclude that the capacity for STM of the EVR with added DS is similar in healthy younger and older women.

Short- and long-term modulation of the exercise ventilatory response

The importance of adaptive control strategies (modulation and plasticity) in the control of breathing during exercise has become recognized only in recent years. In this review, we discuss new evidence for modulation of the exercise ventilatory response in humans, specifically, short- and long-term modulation. Short-term modulation is proposed to be an important regulatory mechanism that helps maintain blood gas homeostasis during exercise.

Acute and chronic treatment with serotonin reuptake inhibitors exert opposite effects on respiration in rats: possible implications for panic disorder

Prompted by the suggested importance of respiration for the pathophysiology of panic disorder, we studied the influence of serotonin reuptake inhibitors (SRIs) as well as other serotonin-modulating compounds on respiration in freely moving rats. The effect on respiration after acute administration of compounds enhancing synaptic levels of serotonin, that is, the serotonin reuptake inhibitors paroxetine and fluoxetine, the serotonin-releasing agents m-chlorophenylpiperazine and d-fenfluramine, and the selective 5-HT1A antagonist WAY-100635, were investigated. All serotonin-releasing substances decreased respiratory rate in unrestrained, awake animals, suggesting the influence of serotonin on respiratory rate under these conditions to be mainly inhibitory. In line with a previous study, rats administered fluoxetine for 23 days or more, on the other hand, displayed an enhanced respiratory rate. The results reinforce the assumption that the effect of subchronic administration of a serotonin reuptake inhibitor on certain serotonin-regulated parameters may be opposite to that obtained after acute administration. We suggest that our observations may be of relevance for the fact that acute administration of SRIs, d-fenfluramine, or m-chlorophenylpiperazine often is anxiogenic in panic disorder patients, and that weeks of administration of an SRI leads to a very effective prevention of panic.

Short-term modulation of the exercise ventilatory response in older men

During exercise with added dead space (DS), the exercise ventilatory response (DeltaV(E)/ DeltaV(CO(2))) is augmented in younger men, via short-term modulation (STM) of the exercise ventilatory response. We hypothesized that STM would be diminished or absent in older men due to age-related changes in respiratory function and ventilatory control. Men were studied at rest and during cycle exercise with and without added DS. DeltaV(E)/ DeltaV(CO(2)) increased progressively with increasing DS volume (p<0.01), such that CO(2) was not retained with added DS versus without. Hence, the increase in DeltaV(E)/ DeltaV(CO(2)) was not due to increased chemoreceptor feedback from rest to exercise. Increasing exercise intensity diminished the DeltaV(E)/ DeltaV(CO(2)) (p<0.01), and the size of this effect varied by DS volume (p<0.05). We conclude that STM of the exercise ventilatory response is robust in older men; hence, despite age-related changes in lung function and ventilatory control, the exercise ventilatory response can still adapt to increased DS, in order to maintain isocapnia during exercise relative to rest.

Spinal serotonin receptor activation modulates the exercise ventilatory response with increased dead space in goats

Small increases in respiratory dead space (VD) augment the exercise ventilatory response by a serotonin-dependent mechanism known as short-term modulation (STM). We tested the hypotheses that the relevant serotonin receptors for STM are in the spinal cord, and are of the 5-HT2-receptor subtype. After preparing adult female goats with a mid-thoracic (T6-T8) subarachnoid catheter, ventilation and arterial blood gases were measured at rest and during treadmill exercise (4.8 km/h; 5% grade) with and without an increased VD (0.2-0.3 L). Measurements were made before and after spinal or intravenous administration of a broad-spectrum serotonin receptor antagonist (methysergide, 1-2mg total) and a selective 5-HT2-receptor antagonist (ketanserin, 5-12 mg total). Although spinal methysergide had no effect on the exercise ventilatory response in control conditions, the augmented response with increased VD was impaired, allowing Pa(CO)(2) to increase from rest to exercise. Spinal methysergide diminished both mean inspiratory flow and frequency responses to exercise with increased VD. Spinal ketanserin impaired Pa(CO)(2) regulation with increased VD, although its ventilatory effects were less clear. Intrathecal dye injections indicated CSF drug distribution was caudal to the upper cervical spinal cord and intravenous drugs at the same total dose did not affect STM. We conclude that spinal 5-HT2 receptors modulate the exercise ventilatory response with increased VD in goats.

Short-term modulation of the exercise ventilatory response in young men

Arterial isocapnia is a hallmark of moderate exercise in humans and is maintained even when resting arterial Pco(2) (Pa(CO(2))) is raised or lowered from its normal level, e.g., with chronic acid-base changes or acute increases in respiratory dead space. When resting ventilation and/or Pa(CO(2)) are altered, maintenance of isocapnia requires active adjustments of the exercise ventilatory response [slope of the ventilation (Ve)-CO(2) production (Vco(2)) relationship, DeltaVe/DeltaVco(2)]. On the basis of animal studies, it has been proposed that a central neural mechanism links the exercise ventilatory response to the resting ventilatory drive without need for changes in chemoreceptor feedback from rest to exercise, a mechanism referred to as short-term modulation (STM). We tested the hypothesis that STM is elicited by increased resting ventilatory drive associated with added external dead space (DS) in humans. Twelve young men were studied in control conditions and with added DS (200, 400, and 600 ml; randomized) at rest and during mild-to-moderate cycle exercise. DeltaVe/DeltaVco(2) increased progressively as DS volume increased (P < 0.0001). While resting end-tidal Pco(2) (Pet(CO(2))) increased with DS, the change in Pet(CO(2)) from rest to exercise was not increased, indicating that increased chemoreceptor feedback from rest to exercise cannot account for the greater exercise ventilatory response. We conclude that STM of the exercise ventilatory response is induced in young men when resting ventilatory drive is increased with external DS, confirming the existence of STM in humans.

Lesions in the cerebellar fastigial nucleus have a small effect on the hyperpnea needed to meet the gas exchange requirements of submaximal exercise

The purpose of this study was to test the hypothesis that an intact cerebellar fastigial nucleus (CFN) is necessary for the hyperpnea to meet the gas exchange needs of submaximal exercise. Bilateral stainless steel microtubules were implanted in the cerebellum inside (n = 12) or outside (n = 2) the CFN for injection (0.5 to 10 microl) of the neurotoxin ibotenic acid. All goats had difficulty maintaining normal posture and walking for up to 1 mo after the implantation of the microtubules and again for hours or days after the neurotoxin was injected. Postmortem histology indicated there were 55% fewer living neurons (P < 0.001, n = 9, 3,720 +/- 553 vs. 1,670 +/- 192) in the CFN of the experimental goats compared with a control group of goats. As is typical for goats before implantation of the microtubules, the decrease in arterial Pco(2) from rest during mild and moderate treadmill exercise was 2.0 +/- 0.39 and 3.5 +/- 0.45 Torr, respectively. Implantation of the microtubules did not significantly change this exercise hyperventilation. However, neurotoxic lesioning with 10 mul ibotenic acid significantly (P < 0.05) attenuated the decrease in arterial Pco(2) by 1.3 and 2.8 Torr at the first and second workload, respectively. The modest attenuation of the exercise hypocapnia at both workloads in CFN-lesioned goats suggests that the CFN is part of the control system that enables the ventilatory response to meet the gas exchange requirements of submaximal exercise.

Layers of exercise hyperpnea: modulation and plasticity

Despite the fundamental biological significance of the ventilatory response to mild or moderate physical activity (the exercise hyperpnea), we still know remarkably little concerning its underlying mechanisms. Part of the difficulty in revealing those mechanisms may arise due to confusion between multiple mechanistic layers, each contributing to the impressive degree of regulation achieved. The primary, feedforward exercise stimulus or stimuli increase ventilation in approximate proportion to changes in metabolic rate. Chemoreceptor feedback then minimizes deviations from optimal blood gas regulation, most often preventing excessive hypocapnia in non-human mammals. Recent evidence has accumulated, suggesting that adaptive control strategies including modulation and plasticity may adjust the feedforward and/or feedback contributions when blood gas homeostasis proves inadequate. In this review, we present evidence from a goat model of exercise hyperpnea concerning the existence of modulation and plasticity, and specifically mechanisms known as short-term and long-term modulation of the exercise ventilatory response. Throughout the review, we consider available evidence concerning the relevance of these mechanisms to humans. Plasticity is a property only recently recognized in the neural system subserving respiratory control, and the application of these concepts to the exercise ventilatory response in humans is in its infancy. Modulation and plasticity may confer an ability of individuals to adapt their exercise ventilatory response so that it remains appropriate in the face of life-long changes in endogenous (e.g. development, aging, onset of disease) or exogenous (e.g. altitude, wearing a breathing apparatus during physical exertion) physiological conditions.

Neuroplasticity in respiratory motor control

Although recent evidence demonstrates considerable neuroplasticity in the respiratory control system, a comprehensive conceptual framework is lacking. Our goals in this review are to define plasticity (and related neural properties) as it pertains to respiratory control and to discuss potential sites, mechanisms, and known categories of respiratory plasticity. Respiratory plasticity is defined as a persistent change in the neural control system based on prior experience. Plasticity may involve structural and/or functional alterations (most commonly both) and can arise from multiple cellular/synaptic mechanisms at different sites in the respiratory control system. Respiratory neuroplasticity is critically dependent on the establishment of necessary preconditions, the stimulus paradigm, the balance between opposing modulatory systems, age, gender, and genetics. Respiratory plasticity can be induced by hypoxia, hypercapnia, exercise, injury, stress, and pharmacological interventions or conditioning and occurs during development as well as in adults. Developmental plasticity is induced by experiences (e.g., altered respiratory gases) during sensitive developmental periods, thereby altering mature respiratory control. The same experience later in life has little or no effect. In adults, neuromodulation plays a prominent role in several forms of respiratory plasticity. For example, serotonergic modulation is thought to initiate and/or maintain respiratory plasticity following intermittent hypoxia, repeated hypercapnic exercise, spinal sensory denervation, spinal cord injury, and at least some conditioned reflexes. Considerable work is necessary before we fully appreciate the biological significance of respiratory plasticity, its underlying cellular/molecular and network mechanisms, and the potential to harness respiratory plasticity as a therapeutic tool.

Repeated exercise paired with "imperceptible" dead space loading does not alter VE of subsequent exercise in humans

We employed an associative learning paradigm to test the hypothesis that exercise hyperpnea in humans arises from learned responses forged by prior experience. Twelve subjects undertook a "conditioning" and a "nonconditioning" session on separate days, with order of performance counterbalanced among subjects. In both sessions, subjects performed repeated bouts of 6 min of treadmill exercise, each separated by 5 min of rest. The only difference between sessions was that all the second-to-penultimate runs of the conditioning session were performed with added dead space in the breathing circuit. Cardiorespiratory responses during the first and last runs (the "control" and "test" runs) were compared for each session. Steady-state exercise end-tidal PCO(2) was significantly lower (P = 0.003) during test than during control runs for both sessions (dropping by 1.8 +/- 2 and 1.4 +/- 3 Torr during conditioning and nonconditioning sessions, respectively). This and all other test-control run differences tended to be greater during the first session performed regardless of session type. Our data provide no support for the hypothesis implicating associative learning processes in the ventilatory response to exercise in humans.

p-Chlorophenylalanine eliminates long-term modulation of the exercise ventilatory response in goats

Repeated hypercapnic exercise augments future exercise ventilatory responses, an effect termed long-term modulation. We hypothesized that serotonin depletion with p-chlorophenylalanine (PCPA, 100mg kg(-1) i.v.) would attenuate long-term modulation. Ventilation, CO(2) production and arterial blood gases were measured at rest and during exercise (4kmh(-1), 5% grade) in goats before and after training (14 hypercapnic exercise trials). Six post-training exercise trials were performed. Trials 1-3 and 4-6 were grouped for analysis (post-training 1 and 2, respectively). Without PCPA, training exaggerated the Pa(CO(2)) decrease from rest to exercise (pre-training: 1.4+/-3mmHg; post-training 1: 3.1+/-3mmHg; post-training 2: 2.3+/-3mmHg; P<0.05), indicative of long-term modulation. The Pa(CO(2)) decrease from rest to exercise was unaffected by training following PCPA (pre-training: 1.4+/-1mmHg; post-training 1: 1.4+/-3mmHg; post-training 2: 1.1+/-5mmHg; P>0.05). Thus, PCPA abolishes long-term modulation, implicating serotonin in its underlying mechanism.

Effect of 8-OH DPAT and ketanserin on the ventilatory acclimatization to hypoxia in awake goats

We have previously reported that broad-spectrum serotonergic blockade increased the acute hypoxic ventilatory response in awake goats. The purpose of the present study was to examine the putative serotonin (5-HT) receptor subtype(s) that may have contributed to this response. Following the administration of the selective 5-HT(1A)-receptor agonist, 8-hydroxy-(2-di-n-propylamino) tetralin (8-OH DPAT, 0.1 mg x kg(-1)i.v.), there was an increase in normoxic expired minute ventilation (V(E)) that was due to an increased breathing frequency. V(E) increased during hypoxia but the change in V(E) (Delta V(E)) associated with hypoxic exposure was not different from the Delta V(E) of saline treated goats. The combination of 8-OH DPAT and a selective 5-HT(2A/2C) receptor antagonist, ketanserin (0.1 and 1.0 mg x kg(-1)i.v., respectively), also increased normoxic V(E) but did not alter the hypoxia induced Delta V(E). Both 8-OH DPAT alone and in combination with ketanserin attenuated the change in V(E) associated with sustained hypoxia but neither was able to attenuate the increased hypoxic ventilatory response that occurs following acclimatization. The augmented acute hypoxic ventilatory response that we previously reported does not appear to be mediated via the activation of the 5-HT(1A) receptor or through the combination of 5-HT(1A) activation and 5-HT(2A/2C) blockade. The results of this study further suggest that while 5-HT may modulate hypoxic ventilation it does not appear to be necessary for the development of ventilatory acclimatization to hypoxia.

Short-term modulation of the exercise ventilatory response in goats: effects of 8-OH-DPAT and MPPI

Increased respiratory dead space increases the exercise ventilatory response, a response known as short-term modulation (STM). We hypothesized that STM results from a spinal, serotonin (5-HT)-dependent mechanism. Because 5-HT(1A) autoreceptors on caudal brain stem raphe neurons inhibit 5-HT release, we hypothesized that 5-HT(1A)-receptor agonists would inhibit, whereas 5-HT(1A)-receptor antagonists would enhance, STM. Ventilatory and arterial blood-gas measurements were made at rest and during exercise (4.0-4.5 km/h, 5% grade) in goats with the respiratory mask alone or with increased dead space (0.20-0.25 liter), before and after intravenous administration of the 5-HT(1A)-receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.1 mg/kg) or the antagonist 4-iodo-N-(2-[4-(methoxyphenyl)-1-piperazinyl]ethyl)-N-2-pyridinylbenz amide (MPPI; 0.08 mg/kg). 8-OH-DPAT increased the slope of the arterial PCO(2) vs. metabolic CO(2) production relationship and decreased the ventilation vs. metabolic CO(2) production relationship during exercise with increased dead space (not with the mask alone), indicating an impairment of STM. In contrast, MPPI had minimal effects on any measured variable. Although nonspecific effects of 8-OH-DPAT cannot be ruled out, impaired STM is consistent with the hypothesis that STM requires active raphe serotonergic neurons and 5-HT release.

Serotonin reuptake inhibition does not enhance short term modulation of the exercise ventilatory response

Increased respiratory dead space causes a serotonin (5-HT) dependent augmentation of the exercise ventilatory response known as short term modulation (STM). Contrary to predictions, 5-HT reuptake inhibition with fluoxetine failed to enhance, and even impaired STM with large dead space volumes (0.4-0.6 L). In this study, we tested the hypotheses that: (1) fluoxetine similarly impairs STM with smaller dead space volumes (0.2 L); whereas (2) acute 5-HT release and reuptake inhibition with fenfluramine would enhance STM. Ventilatory and blood gas measurements were made on five goats (37-58 kg) during rest and exercise, with the mask alone or with increased dead space (0.2 L). STM protocols were performed following chronic fluoxetine (>/=21 days, 1 mg/kg, SQ, SID) and acute fenfluramine administration (1 mg/kg, IV). Following fluoxetine, STM was partially impaired. Fenfluramine had no detectable effects on STM. The data suggest that: (1) chronic fluoxetine diminishes STM, possibly via down-regulation of relevant 5-HT receptors, and (2) drugs that release 5-HT acutely do not enhance STM.

Methysergide augments the acute, but not the sustained, hypoxic ventilatory response in goats

Ventilatory acclimatization to hypoxia (VAH) is the time-dependent increase in ventilation that occurs during sustained hypoxia. As serotonin (5-HT) has been reported to be an important modulator of respiratory output, 5-HT may also play a role in VAH. Methysergide (a broad-spectrum 5-HT antagonist), was given to awake goats (1 mg kg(-1) i.v.) 30 min prior to being exposed to 4 h of isocapnic hypoxia. Although methysergide slightly decreased arterial pH, presumably due to a non-significant increase in arterial P(CO2), it did not alter normoxic ventilation. Following methysergide, the expired minute ventilation (VE) was significantly elevated above the control (saline) response after 30 min of hypoxia, but methysergide did not otherwise alter VAH. We repeated the study in the same goats using ketanserin, a specific 5-HT2A/2C receptor antagonist (1.2 mg kg(-1) i.v.). Ketanserin had no effect on the acute hypoxic ventilatory response, or on VAH. We conclude that while 5-HT modulates the acute hypoxic ventilatory response in goats, it does not appear to act through the 5-HT2A/2C receptor subtypes.

Effects of serotonin re-uptake inhibition on ventilatory control in goats

Fluoxetine (Prozac) inhibits serotonin (5-HT) re-uptake. thereby enhancing serotonergic effects. Since serotonin is known to affect ventilation in a variety of circumstances, we investigated the effects of chronic serotonin re-uptake inhibition with fluoxetine on selected ventilatory responses including: (1) eupnea; (2) the hypercapnic ventilatory response at rest; (3) the exercise ventilatory response and (4) repeated trials of hypercapnic exercise. Ventilatory and arterial blood gases were measured in goats (n = 5) at rest, during steady-state treadmill exercise, and during successive rest/exercise trials with increased respiratory dead space (0.4-0.6 L). Fluoxetine was administered (> or = 4 weeks, 1 mg/kg, SQ, SID) and protocols were repeated. Following fluoxetine, PaCO2 was increased in most conditions studied; however, no differences were seen in exercise PaCO2 regulation or ventilatory responses pre- versus post-fluoxetine. We conclude that chronic fluoxetine slightly depresses respiratory control at rest, but, has minimal effects during exercise or with mild hypercapnia during rest or exercise in goats.

Modulation of ventilatory control during exercise

The control of ventilatory responses to mild or moderate dynamic exercise has been the subject of considerable debate for over a century. The prevailing view has been that the ventilatory response to exercise is stereotypical and rather unmalleable. However, paradigms involving novel associations of stimulus inputs have been shown to modulate breathing in short and longer time scales. The scope of this review includes examples of modified ventilatory responses to exercise which have been investigated in terms of neural mechanisms. An attempt to synthesise the available data into a model of neuromodulation is presented.

Influence of body temperature on responses to hypoxia and hypercapnia: implications for SIDS

1. This paper reviews current knowledge regarding interactions between body temperature and the respiratory responses to hypoxia and/or hypercapnia, with special emphasis on how these interactions might predispose towards sudden infant death syndrome (SIDS). 2. Use has been made of an adult rat model in which body core temperature is fixed by means of an intra-abdominal heat exchanger. Initial studies indicated that hyperthermia (Tb approximately 41 degrees C) enhanced the ventilatory response to hypercapnia, whereas hypothermia (Tb approximately 35 degrees C) interacted with hypoxia to depress respiration. 3. Studies involving hypothalamic lesions in urethane-anaesthetized rats have implicated the posterior hypothalamic area in the hypoxia/hypothermia interaction. Further studies are directed towards examining the role played by more caudal areas, including the raphe nuclei. 4. It has been shown that not only does the hypoxia/hypothermia interaction depress breathing but it also reduces, or sometimes eliminates, the ventilatory response to hypercapnia, which under normal circumstances provides one of the most powerful excitatory inputs to the respiratory centres. This implies that an expected reversal of the respiratory depression by build up of CO2 levels may not occur, which in turn has important implications for SIDS. 5. The literature dealing with the effects of hyperthermia on hypoxic and hypercapnic responses is also reviewed. It is concluded that environmental heat stress may only become a significant problem when it accompanies a febrile infection, under which circumstances it may seriously compromise thermoregulatory ability and alter breathing responses to chemical stimuli.

Long-term modulation of the exercise ventilatory response in goats

1. To test the hypothesis that repeated associations of exercise and increased respiratory dead space elicit mechanisms that augment future ventilatory responses to exercise alone, experiments were conducted on normal adult goats familiarized with experimental procedures. 2. Measurements of ventilation, arterial blood gases and CO2 production were made at rest, during mild steady-state exercise (4 km h-1; 5% grade) and with increased dead space at rest in seven goats before and after training. In Series I experiments, training consisted of fourteen to twenty exercise trials explicitly paired with increased dead space (0.8 l) over 2 days. Increased dead space predominantly represents a CO2 chemoreceptor stimulus with only mild hypoxic stimulation. Post-training measurements were made 1-6 h and 1 week after training was completed. 3. The same goats repeated a slightly modified protocol several months later (Series II; 6 trials per day for 4 days) to determine if responses were both repeatable and reversible, and to investigate training effects on dynamic ventilatory responses at the onset of exercise. 4. In Series I experiments, resting minute ventilation and breathing frequency were elevated 1-6 h post-training compared to baseline (44 and 74% respectively), whereas resting tidal volume decreased (14%). One week post-training, resting values had returned to baseline. Series II training had no significant effects on resting measurements. 5. Relative to baseline, arterial partial pressure of CO2 (Pa,CO2) values decreased significantly more from rest to exercise 1-6 h post-training in both Series I (2.7 +/- 0.2 vs. 1.8 +/- 0.9 mmHg) and Series II (3.4 +/- 0.6 vs. 2.0 +/- 0.6 mmHg). The exercise ventilatory response increased 25-28% 1-6 h post-training (both series), largely due to a greater exercise frequency response, but returned to baseline 1 week post-training. Training had no effect on ventilatory responses to CO2 at rest, suggesting that decreases in CO2 chemoreceptor responsiveness did not cause the greater exercise ventilatory response. Model estimates indicate that the net feedforward exercise ventilatory stimulus was increased 40-50% by training. 6. Training had no discernable effects on ventilatory dynamics at the onset of exercise. However, post-training differences in Pa,CO2 regulation and ventilation were established early in exercise, prior to steady state. 7. Collectively, these experiments suggest a previously unsuspected degree of repeatable and reversible plasticity in the control system subserving the exercise ventilatory response. Such plasticity may contribute to the development of normal exercise hyperpnoea and to adaptive responses of the ventilatory control system in adult animals.