Evidence for a dopamine interaction with the central respiratory control system in the rat

The hypoxic respiratory response of the pre-Bötzinger complex

Since the discovery of the pre-Bötzinger Complex (preBötC) as a crucial region for generating the main respiratory rhythm, our understanding of its cellular and molecular aspects has rapidly increased within the last few decades. It is now apparent that preBötC is a highly flexible neuronal network that reconfigures state-dependently to produce the most appropriate respiratory output in response to various metabolic challenges, such as hypoxia. However, the responses of the preBötC to hypoxic conditions can be varied based on the intensity, pattern, and duration of the hypoxic challenge. This review discusses the preBötC response to hypoxic challenges at the cellular and network level. Particularly, the involvement of preBötC in the classical biphasic response of the respiratory network to acute hypoxia is illuminated. Furthermore, the article discusses the functional and structural changes of preBötC neurons following intermittent and sustained hypoxic challenges. Accumulating evidence shows that the preBötC neural circuits undergo substantial changes following hypoxia and contribute to several types of the respiratory system's hypoxic ventilatory responses.

L-DOPA Improves Ventilation but Not the Ventilatory Response to Hypercapnia in a Reserpine Model of Parkinson's Disease

Parkinson's disease (PD) is a neurological disorder characterized by progressive degeneration of the substantia nigra that affects mainly movement control. However, pathological changes associated with the development of PD may also alter respiration and can lead to chronic episodes of hypoxia and hypercapnia. The mechanism behind impaired ventilation in PD is unclear. Therefore, in this study, we explore the hypercapnic ventilatory response in a reproducible reserpine-induced (RES) model of PD and parkinsonism. We also investigated how dopamine supplementation with L-DOPA, a classic drug used to treat PD, would affect the breathing and respiratory response to hypercapnia. Reserpine treatment resulted in decreased normocapnic ventilation and behavioral changes manifested as low physical activity and exploratory behavior. The respiratory rate and the minute ventilation response to hypercapnia were significantly higher in sham rats compared to the RES group, while the tidal volume response was lower. All of this appears to be due to reduced baseline ventilation values produced by reserpine. L-DOPA reversed reduced ventilation, indicating a stimulatory effect of DA on breathing, and showed the potency of DA supplementation in restoring normal respiratory activity.

Acute hydrogen sulfide-induced neurochemical and morphological changes in the brainstem

Hydrogen sulfide (H₂S) is a toxin affecting the cardiovascular, respiratory, and central nervous systems. Acute H₂S exposure is associated with a high rate of mortality and morbidity. The precise pathophysiology of H₂S-induced death is a controversial topic; however, inhibition of the respiratory center in the brainstem is commonly cited as a cause of death. There is a knowledge gap on toxicity and toxic mechanisms of acute H₂S poisoning on the brainstem, a brain region responsible for regulating many reflective and vital functions. Serotonin (5-HT), dopamine (DA), and γ-aminobutyric acid (GABA) play a role in maintaining a normal stable respiratory rhythmicity. We hypothesized that the inhibitory respiratory effects of H₂S poisoning are mediated by 5-HT in the respiratory center of the brainstem. Male C57BL/6 mice were exposed once to an LCt₅₀ concentration of H₂S (1000 ppm). Batches of surviving mice were euthanized at 5 min, 2 h, 12 h, 24 h, 72 h, and on day 7 post-exposure. Pulmonary function, vigilance state, and mortality were monitored during exposure. The brainstem was analyzed for DA, 3,4-dehydroxyphenyl acetic acid (DOPAC), 5-HT, 5-hydroxyindoleatic acid (5-HIAA), norepinephrine (NE), GABA, glutamate, and glycine using HPLC. Enzymatic activities of monoamine oxidases (MAO) were also measured in the brainstem using commercial kits. Neurodegeneration was assessed using immunohistochemistry and magnetic resonance imaging. Results showed that DA and DOPAC were significantly increased at 5 min post H₂S exposure. However, by 2 h DA returned to normal. Activities of MAO were significantly increased at 5 min and 2 h post-exposure. In contrast, NE was significantly decreased at 5 min and 2 h post-exposure. Glutamate was overly sensitive to H₂S-induced toxicity manifesting a time-dependent concentration reduction throughout the 7 day duration of the study. Remarkably, there were no changes in 5-HT, 5-HIAA, glycine, or GABA concentrations. Cytochrome c oxidase activity was inhibited but recovered by 24 h. Neurodegeneration was observed starting at 72 h post H₂S exposure in select brainstem regions. We conclude that acute H₂S exposure causes differential effects on brainstem neurotransmitters. H₂S also induces neurodegeneration and biochemical changes in the brainstem. Additional work is needed to fully understand the implications of both the short- and long-term effects of acute H₂S poisoning on vital functions regulated by the brainstem.

Embryonic Thermal Manipulation Affects Ventilation, Metabolism, Thermal Control and Central Dopamine in Newly Hatched and Juvenile Chicks

The first third of incubation is critical for embryonic development, and environmental changes during this phase can affect the physiology and survival of the embryos. We evaluated the effects of low (LT), control (CT), and high (HT) temperatures during the first 5 days of incubation on ventilation (V._E), body temperature (Tb), oxygen consumption (V.O₂), respiratory equivalent (V._E/V.O₂), and brain monoamines on 3-days-old (3d) and 14-days-old (14d) male and female chickens. The body mass of LT animals of both ages and sexes was higher compared to HT and CT animals (except for 3d males). The heart mass of 14d HT animals was higher than that of CT animals. Thermal manipulation did not affect V._E, V.O₂ or V._E/V.O₂ of 3d animals in normoxia, except for 3d LT males V._E, which was lower than CT. Regarding 14d animals, the HT females showed a decrease in V._E and V.O₂ compared to CT and LT groups, while the HT males displayed a lower V.O₂ compared to CT males, but no changes in V._E/V.O₂. Both sexes of 14d HT chickens presented a greater Tb compared to CT animals. Thermal manipulations increased the dopamine turnover in the brainstem of 3d females. No differences were observed in ventilatory and metabolic parameters in the 3d animals of either sexes, and 14d males under 7% CO₂. The hypercapnic hyperventilation was attenuated in the 14d HT females due to changes in V.O₂, without alterations in V._E. The 14d LT males showed a lower V._E, during hypercapnia, compared to CT, without changes in V.O₂, resulting in an attenuation in V._E/V.O₂. During hypoxia, 3d LT females showed an attenuated hyperventilation, modulated by a higher V.O₂. In 14d LT and HT females, the increase in V._E was greater and the hypometabolic response was attenuated, compared to CT females, which resulted in no change in the V._E/V.O₂. In conclusion, thermal manipulations affect hypercapnia-induced hyperventilation more so than hypoxic challenge, and at both ages, females are more affected by thermal manipulation than males.

Oleic Derivatives of Dopamine and Respiration

Ventilatory inhibition is considered an undesirable pharmacological side effect of pharmacotherapy in neurodegenerative conditions underlain by brain dopamine deficiency. In this context, oleic derivatives of dopamine or N-acyl-dopamines are novel substances that may be of high therapeutic interest as having the ability to cross the blood-brain barrier and acting in dopamine-like manner. In the present study we seek to define the influence of N-acyl-dopamines on lung ventilation and its hypoxic responses in the rat. We found that N-oleoyl-dopamine decreased both normoxic and peak hypoxic ventilation in response to 8% acute hypoxia, on average, by 31% and 41%, respectively. Its metabolite, 3'-O-methyl-N-oleoyl-dopamine, caused a 15% ventilatory decrease each, whereas an oleic ester derivative, 3'-O-oleoyl-N-oleoyl-dopamine, caused 11% and 19% ventilatory decreases, respectively. All three N-acyl-dopamines investigated displayed an inhibitory effect on ventilation. The findings indicate that 3'-O-methyl-N-oleoyl-dopamine and 3'-O-oleoyl-N-oleoyl-dopamine performed better than N-oleoyl-dopamine in term of less ventilatory suppression, albeit the differences among the three compounds were modest. We conclude that N-acyl-dopamines are worthy of intensified explorations as potential carriers of dopamine molecule in view of the lack of clinically effective methods of dopamine delivery into the brain in neurodegenerative conditions.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

Long-term ventilatory adaptation and ventilatory response to hypoxia in plateau pika (Ochotona curzoniae): role of nNOS and dopamine

We assessed ventilatory patterns and ventilatory responses to hypoxia (HVR) in high-altitude (HA) plateau pikas, repetitively exposed to hypoxic burrows, and control rats. We evaluated the role of neuronal nitric oxide synthase (nNOS) and dopamine by using S-methyl-l-thiocitrulline (SMTC) inhibitor and haloperidol antagonist, respectively. Ventilation (Vi) was measured using a whole body plethysmograph in conscious pikas (n = 9) and low-altitude (LA) rats (n = 7) at different Pi(O(2)) (56, 80, 111, 150, and 186 mmHg) and in HA acclimatized rats (n = 9, 8 days at 4,600 m) at two different Pi(O(2)) (56 and 80 mmHg). The effects of NaCl, SMTC, and haloperidol on ventilatory patterns were assessed in pikas at Pi(O(2)) = 56 and 80 mmHg. We observed a main species effect with larger Vi, tidal volume (VT), inspiratory time/total time (T(i)/T(tot)), and a lower expiratory time in pikas than in LA rats. Pikas had also a larger VT and lower respiratory frequency compared with HA rats in hypoxia. HVR of pikas and rats were not statistically different. In pikas, SMTC induced a significant increase in Vi and VT for a Pi(O(2)) of 56 mmHg, but had no effect for a PiO(2) of 80 mmHg, i.e., the living altitude of pikas. In pikas, haloperidol injection had no effect on any ventilatory parameter. Long-term ventilatory adaptation in pikas is mainly due to an improvement in respiratory pattern (VT and T(i)/T(tot)) with no significant improvement in HVR. The sensitivity to severe acute hypoxia in pikas seems to be regulated by a peripheral nNOS mechanism.

Brain monoaminergic neurons and ventilatory control in vertebrates

Monoamines (noradrenaline (NA), adrenaline (AD), dopamine (DA) and serotonin (5-HT) are key neurotransmitters that are implicated in multiple physiological and pathological brain mechanisms, including control of respiration. The monoaminergic system is known to be widely distributed in the animal kingdom, which indicates a considerable degree of phylogenetic conservation of this system amongst vertebrates. Substantial progress has been made in uncovering the participation of the brain monoamines in the breathing regulation of mammals, since they are involved in the maturation of the respiratory network as well as in the modulation of its intrinsic and synaptic properties. On the other hand, for the non-mammalian vertebrates, most of the knowledge of central monoaminergic modulation in respiratory control, which is actually very little, has emerged from studies using anuran amphibians. This article reviews the available data on the role of brain monoaminergic systems in the control of ventilation in terrestrial vertebrates. Emphasis is given to the comparative aspects of the brain noradrenergic, adrenergic, dopaminergic and serotonergic neuronal groups in breathing regulation, after first briefly considering the distribution of monoaminergic neurons in the vertebrate brain.

Hypoxia transduction by carotid body chemoreceptors in mice lacking dopamine D(2) receptors

Hypoxia-induced dopamine (DA) release from carotid body (CB) glomus cells and activation of postsynaptic D(2) receptors have been proposed to play an important role in the neurotransmission process between the glomus cells and afferent nerve endings. To better resolve the role of D(2) receptors, we examined afferent nerve activity, catecholamine content and release, and ventilation of genetically engineered mice lacking D(2) receptors (D(2)(-/-) mice). Single-unit afferent nerve activities of D(2)(-/-) mice in vitro were significantly reduced by 45% and 25% compared with wild-type (WT) mice during superfusion with saline equilibrated with mild hypoxia (Po(2) approximately 50 Torr) or severe hypoxia (Po(2) approximately 20 Torr), respectively. Catecholamine release in D(2)(-/-) mice was enhanced by 125% in mild hypoxia and 75% in severe hypoxia compared with WT mice, and the rate of rise was increased in D(2)(-/-) mice. We conclude that CB transduction of hypoxia is still present in D(2)(-/-) mice, but the response magnitude is reduced. However, the ventilatory response to acute hypoxia is maintained, perhaps because of an enhanced processing of chemoreceptor input by brain stem respiratory nuclei.

Dopaminergic mechanisms of neural plasticity in respiratory control: transgenic approaches

Data supporting the hypothesis that dopamine-2 receptors (D(2)-R) contribute to time-dependent changes in the hypoxic ventilatory response (HVR) during acclimatization to hypoxia are briefly reviewed. Previous experiments with transgenic animals (D(2)-R 'knockout' mice) support this hypothesis (J. Appl. Physiol. 89 (2000) 1142). However, those experiments could not determine (1) if D(2)-R in the carotid body, the CNS, or both were involved, or (2) if D(2)-R were necessary during the acclimatization to hypoxia versus some time prior to chronic hypoxia, e.g. during a critical period of development. Additional experiments on C57BL/6J mice support the idea that D(2)-R are critical during the period of exposure to hypoxia for normal ventilatory acclimatization. D(2)-R in carotid body chemoreceptors predominate under control conditions to inhibit normoxic ventilation, but excitatory effects of D(2)-R, presumably in the CNS, predominate after acclimatization to hypoxia. The inhibitory effects of D(2)-R in the carotid body are reset to operate primarily under hypoxic conditions in acclimatized rats, thereby optimizing O(2)-sensitivity.

Hormones and breathing

A number of hormones, including hypothalamic neuropeptides acting as neurotransmitters and neuromodulators in the CNS, are involved in the physiologic regulation of breathing and participate in adjustment of breathing in disease. In addition to central effects, some hormones also control breathing at peripheral chemoreceptors or have local effects on the lungs and airways. Estrogen and progesterone seem to protect from sleep-disordered breathing, whereas testosterone may predispose to it. Progesterone and thyroxine have long been known to stimulate respiration. More recently, several hormones such as corticotropin-releasing hormone and leptin have been suggested to act as respiratory stimulants. Somatostatin, dopamine, and neuropeptide Y have a depressing effect on breathing. Animal models and experimental human studies suggest that also many other hormones may be involved in respiratory control.

Dopamine modulates synaptic transmission in the nucleus of the solitary tract

10.1152/jn.00224.2002. Dopamine (DA) modulates the cardiorespiratory reflex by peripheral and central mechanisms. The aim of this study was to examine the role of DA in synaptic transmission of the nucleus tractus solitarius (NTS), the major integration site for cardiopulmonary reflexes. To examine DA's role, we used whole cell, voltage-clamp recordings in a rat horizontal brain stem slice. Solitary tract stimulation evoked excitatory postsynaptic currents (EPSCs) that were reduced to 70 +/- 5% of control by DA (100 microM). The reduction in EPSCs by DA was accompanied by a decrease in the paired pulse depression ratio with little or no change in input resistance or EPSC decay, suggesting a presynaptic mechanism. The D1-like agonist SKF 38393 Br (30 microM) did not alter EPSC amplitude, whereas the D2-like agonist, quinpirole HCl (30 microM), depressed EPSCs to 73 +/- 4% of control. The D2-like receptor antagonist, sulpiride (20 microM), abolished DA modulation of EPSCs. Most importantly, sulpiride alone increased EPSCs to 131 +/- 10% of control, suggesting a tonic D2-like modulation of synaptic transmission in the NTS. Examination of spontaneous EPSCs revealed DA reversibly decreased the frequency of events from 9.4 +/- 2.2 to 6.2 +/- 1.4 Hz. Sulpiride, however, did not alter spontaneous events. Immunohistochemistry of NTS slices demonstrated that D2 receptors colocalized with synaptophysin and substance P, confirming a presynaptic distribution. D2 receptors also localized to cultured petrosal neurons, the soma of presynaptic afferent fibers. In the petrosal neurons, D2 was found in cells that were TH-immunopositive, suggesting they were chemoreceptor afferent fibers. These results demonstrate that DA tonically modulates synaptic activity between afferent sensory fibers and secondary relay neurons in the NTS via a presynaptic D2-like mechanism.

Central dopamine modulates anapyrexia but not hyperventilation induced by hypoxia

Hypoxia causes hyperventilation and decreases body temperature (T(b)) and metabolism [O(2) consumption (VO(2))]. Because dopamine (DA) is released centrally in response to peripheral chemoreceptor stimulation, we tested the hypothesis that central DA mediates the ventilatory, thermal, and metabolic responses to hypoxia. Thus we predicted that injection of haloperidol (a DA D(2)-receptor antagonist) into the third ventricle would augment hyperventilation and attenuate the drop in T(b) and VO(2) in conscious rats. We measured ventilation, T(b), and VO(2) before and after intracerebroventricular injection of haloperidol or vehicle (5% DMSO in saline), followed by a 30-min period of hypoxia exposure. Haloperidol did not change T(b) or VO(2) during normoxia; however, breathing frequency was decreased. During hypoxia, haloperidol significantly attenuated the falls in T(b) and VO(2), although hyperventilation persisted. The present study shows that central DA participates in the thermal and metabolic responses to hypoxia without affecting hyperventilation, showing that DA is not a common mediator of this interaction.

Dopaminergic modulation of ventilation in obese Zucker rats

To investigate the hypothesis that the impaired respiratory drive noted in morbid obesity was attributable to altered dopaminergic mechanisms acting on peripheral and/or central chemoreflex sensitivity, seven obese and seven lean Zucker rats were studied at 11 wk of age. Ventilation (VE) was measured by the barometric technique during hyperoxic (100% O(2)), normoxic (21% O(2)), hypoxic (10% O(2)), and hypercapnic (7% CO(2)) exposures after the administration of vehicle (control), haloperidol [Hal, 1 mg/kg, a central and peripheral dopamine (Da) receptor antagonist], or domperidone (Dom, 0.5 mg/kg, a peripheral Da receptor antagonist). In both lean and obese rats, Hal increased tidal volume and decreased respiratory frequency during hyperoxia or normoxia, resulting in an unchanged VE. In contrast, Dom did not affect tidal volume, frequency, or VE during hyperoxia or normoxia. During hypoxia, however, VE significantly increased from 1,132 +/- 136 to 1,348 +/- 98 ml. kg(-1). min(-1) (P < 0.01) after the administration of Dom in obese rats, whereas no change was observed in lean rats. Hal significantly decreased VE during hypoxia compared with control in lean but not obese rats. In both lean and obese rats, Hal decreased VE in response to hypercapnia, whereas Dom had no effect. Our major findings suggest that peripheral chemosensitivity to hypoxia in obese Zucker rats is reduced as a result of an increased dopaminergic receptor modulation in the carotid body.

Changes in dopamine D(2)-receptor modulation of the hypoxic ventilatory response with chronic hypoxia

Modulation of the hypoxic ventilatory response (HVR) by dopamine D(2)-receptors (D(2)-R) in the carotid body (CB) and central nervous system (CNS) are hypothesized to contribute to ventilatory acclimatization to hypoxia. We tested this with blockade of D(2)-R in the CB or CNS in conscious rats after 0, 2 and 8 days of hypoxia. On day 0, CB D(2)-R blockade significantly increased VI and frequency (fR) in hyperoxia (FI(O(2))=0.30), but not hypoxia (FI(O(2))=0.10). CNS D(2)-R blockade significantly decreased fR in hypoxia only. On day 2, neither CB nor CNS D(2)-R blockade affected VI or fR. On day 8, CB D(2)-R blockade significantly increased hypoxic VI and fR. CNS D(2)-R blockade significantly decreased hypoxic VI and fR. CB and CNS D(2)-R modulation of the HVR decreased after 2 days of hypoxia, but reappeared after 8 days. Changes in the opposing effects of CB and CNS D(2)-R on the HVR during chronic hypoxia cannot completely explain ventilatory acclimatization in rats.

Ventilatory responses to acute and chronic hypoxia in mice: effects of dopamine D(2) receptors

We used genetically engineered D(2) receptor-deficient [D(2)-(-/-)] and wild-type [D(2)-(+/+)] mice to test the hypothesis that dopamine D(2) receptors modulate the ventilatory response to acute hypoxia [hypoxic ventilatory response (HVR)] and hypercapnia [hypercapnic ventilatory response (HCVR)] and time-dependent changes in ventilation during chronic hypoxia. HVR was independent of gender in D(2)-(+/+) mice and significantly greater in D(2)-(-/-) than in D(2)-(+/+) female mice. HCVR was significantly greater in female D(2)-(+/+) mice than in male D(2)-(+/+) and was greater in D(2)-(-/-) male mice than in D(2)-(+/+) male mice. Exposure to hypoxia for 2-8 days was studied in male mice only. D(2)-(+/+) mice showed time-dependent increases in "baseline" ventilation (inspired PO(2) = 214 Torr) and hypoxic stimulated ventilation (inspired PO(2) = 70 Torr) after 8 days of acclimatization to hypoxia, but D(2)-(-/-) mice did not. Hence, dopamine D(2) receptors modulate the acute HVR and HCVR in mice in a gender-specific manner and contribute to time-dependent changes in ventilation and the acute HVR during acclimatization to hypoxia.

Central nervous system mechanisms of ventilatory acclimatization to hypoxia

Ventilatory acclimatization to hypoxia is the time-dependent increase in ventilation that occurs with chronic exposure to hypoxia. Despite decades of research, the physiological mechanisms that increase the hypoxic ventilatory response during chronic hypoxia are not well understood. This review focuses on adaptations within the central nervous system (CNS) that increase the hypoxic ventilatory response. Although an increase in CNS responsiveness had been proposed many years ago, only recently has strong experimental evidence been provided for an increase in the CNS gain in the rat, which has proved to be a good model of VAH in humans. Within the CNS, several neuroanatomical sites could be involved as well as changes in various neurotransmitters, neuromodulators or signalling mechanisms within any of those sites. Lastly, adaptations within the CNS could involve both direct effects of decreased P(O(2)) and indirect effects of increased afferent nerve activity due to chronic stimulation of the peripheral arterial chemoreceptors.

Catecholaminergic modulation of respiratory rhythm in an in vitro turtle brain stem preparation

An in vitro brain stem preparation from adult turtles was used to determine effects of dopamine (DA) and norepinephrine (NE) on the pattern of respiratory motor output recorded from hypoglossal nerve roots (XII). Bath-applied DA (10-200 microM) increased the frequency of respiratory bursts (peaks) from 0.9 +/- 0.2 to 2.4 +/- 0.3 (SE) peaks/min, resulting in a 99 +/- 9% increase in neural minute activity. R[+]-SCH-23390 (10 microM, D1 antagonist) and eticlopride (20 microM, D2 antagonist) attenuated the DA-mediated increase in peak frequency by 52 and 59%, respectively. On the other hand, the DA-receptor agonists apomorphine (D1, D2), quinelorane (D2), and SKF-38393 (D1) had no effect on peak frequency. Prazosin, an alpha1-adrenergic antagonist (250 nM) abolished the DA-mediated frequency increase. Although NE (10-200 microM) and phenylephrine (10-200 microM, alpha1-adrenergic agonist) increased peak frequency from 0.5 +/- 0.1 to 1.2 +/- 0.3 peaks/min and from 0.6 +/- 0.1 to 1. 0 +/- 0.2 peaks/min, respectively, these effects were not as large as that with DA alone. The data suggest that both dopaminergic and adrenergic receptor activation in the brain stem increase respiratory frequency in turtles, but the DA receptor-mediated increase is dependent on coactivation of alpha1-adrenergic receptors.

Dopamine and nicotine, but not serotonin, modulate the crustacean ventilatory pattern generator

Dopamine (DA) causes a dose-dependent increase in the frequency of motor neuron bursts [virtual ventilation (fR)] produced by deafferented crab ventilatory pattern generators (CPGv). Domperidone, a D2-specific DA antagonist, by itself reversibly depresses fR and also blocks the stimulatory effects of DA. Serotonin (5HT) has no direct effects on this CPGv. Nicotine also causes dramatic dose-dependent increases in the frequency of motor bursts from the CPGv. The action is triphasic, beginning with an initial reversal of burst pattern typical of reversed-mode ventilation, followed by a 2- to 3-min period of depression and then a long period of elevated burst rate. Acetylcholine chloride (ACh) alone is ineffective, but in the presence of eserine is moderately stimulatory. The inhibitory effects of nicotine are only partially blocked by curare. The excitatory action of nicotine is blocked by prior perfusion of domperidone, but not by SKF-83566.HCl, a D1-specific DA antagonist. SKF-83566 had no effects on the ongoing pattern of firing. These observations support the hypothesis that dopaminergic pathways are involved in the maintenance of the CPGv rhythm and that the acceleratory effects of nicotine may involve release of DA either directly or via stimulation of atypical ACh receptors at intraganglionic sites.

Effects of domperidone and medroxyprogesterone acetate on ventilation in man

If endogenous dopamine acts as an inhibitory neurotransmitter in the carotid bodies in man, domperidone (DP), a selective dopamine D-2 receptor antagonist should stimulate carotid bodies and augment ventilation. Furthermore, the combination of a central ventilatory stimulant, medroxyprogesterone acetate (MPA), with a peripheral ventilatory stimulant, DP, may produce an additive/synergistic ventilatory effect. We conducted a double-blind, placebo-controlled (P), cross-over trial comparing MPA 20 mg three times daily (TID) and DP (20 mg TID) alone and together in 8 healthy male human subjects. Drug effects were measured after 7 days, and a two-week drug washout period was allowed. MPA significantly increased alveolar ventilation (VA), and slopes of hypercapnic and hypoxic ventilatory responses. Domperidone alone significantly increased the slope of the hypoxic response; however, VA and PaCO2 did not change significantly. The combination of MPA and DP resulted in ventilatory changes similar to MPA alone. We conclude that in man endogenous dopamine acts as a modulator of chemoreception during hypoxemia, but plays no major role tonically in control of ventilation during normoxemia and normocapnia. Lack of additive effect with combined DP and MPA suggests that these drugs may share the same final common pathway in the process of chemoreception.

Brain sulfide levels in anaesthesia: a comparison with hydrogen sulfide intoxication

Although sublethal concentrations of hydrogen sulfide produce a state not unlike anaesthesia, measurement of rat brain sulfide levels by gas dialysis and IC with electrochemical detection after either 1.5 g/kg urethane or 42 mg/kg pentobarbital failed to demonstrate any changes as compared with endogenous brain sulfide levels of saline-injected controls. This suggests that the mechanisms underlying anaesthesia are not directly linked to endogenous cerebral sulfide levels.

Differentiation of phencyclidine and sigma receptor types affecting the central inspiratory termination mechanism in cat

The effects of 1) the phencyclidine receptor ligand TCP, 2) sigma receptor ligands (+)3-PPP and DTG, and 3) N-methyl-D-aspartate receptor blockers MK-801 and dextrorphan were determined on a brainstem mechanism which controls the termination of the inspiratory phase of the breathing cycle. Inspiratory bursts were recorded from the phrenic nerve in decerebrate paralyzed cats ventilated by means of a phrenic driven servoventilator. The central mechanism which terminates inspiration was tested by withholding lung inflation, thus suppressing the contribution of the vagal feedback from the lungs to inspiratory termination. TCP increased the duration of test inspiration (tTi) by 17% at 0.03 mg/kg and by 14-fold (from 1.6 to 23 s) at 1 mg/kg. With dextrorphan, tTi was significantly increased at 3 mg/kg. In contrast, (+)3-PPP and DTG did not increase tTi at doses up to 10 mg/kg, although MK-801 (0.03 mg/kg), given after the sigma ligands, increased tTi by 59-90%. It is concluded that phencyclidine but not sigma receptor ligands block the central mechanism which terminates inspiration and that the likely site of action is the NMDA receptor complex.

Dopaminergic control of the cough reflex as demonstrated by the effects of apomorphine

The effect of apomorphine on the cough reflex induced by electrical stimulation of the superior laryngeal nerve was studied in cats anesthetized with pentobarbital. Intravenous administration of apomorphine in doses which ranged from 0.1 to 1.0 mg/kg decreased the number of coughs in a dose-dependent manner. Haloperidol (0.3 mg/kg i.v.) did not significantly change the number of coughs. However, haloperidol administered 15 min prior to administration of apomorphine abolished the apomorphine-induced decrease in the number of coughs. These results suggest that dopaminergic mechanisms could have an important role in the regulation of the cough reflex.

Changes in cerebrospinal fluid homovanillic acid in children with Ondine's curse

The cerebrospinal fluid (CSF) concentrations of three acid monoamine metabolites, two purines, and a group of amino acids were determined in two children with chronic central alveolar hypoventilation (Ondine's curse). The levels of all assayed neuroactive substances, metabolites, and amino acids, with one exception, were normal compared to an age-matched group of neurologically healthy children. The levels of the dopamine metabolite homovanillic acid in the children with Ondine's curse were approximately 2.4 times higher than expected for age range. The present findings may indicate a link between central nervous system dopamine activity and chronic central alveolar hypoventilation. Among other possible explanations, the changes seen might represent a primary alteration in dopamine activity or may reflect a change in dopamine turnover resulting from the chronic hypoventilation.

Involvement of central serotonergic mechanisms in the cough reflex

To determine the role of central serotonergic systems in modulating the cough reflex, the effects of serotonergic agonists on the respiration and the cough reflex were comparatively studied. Male and female cats were anesthetized with sodium pentobarbital. Respiration and cough reflex were measured using a pneumotachograph via a cannula inserted into the trachea. The cough reflex was elicited by electrical stimuli to the superior laryngeal nerve. Tranylcypromine, a MAO inhibitor, in a dose of 5 mg/kg, i.v., increased the respiration, but depressed the cough reflex. The serotonin precursor 5-hydroxytryptophan (5 mg/kg, i.v.) depressed the respiration and the cough reflex. Haloperidol (2 mg/kg, i.v.) abolished the tranylcypromine-stimulated respiratory responses, and it intensified the tranylcypromine induced cough depression. It is concluded that the increase in serotonin levels in the brain has a depressant influence on the central generating mechanisms of the cough reflex. Furthermore, central dopaminergic mechanisms seem to play a modulating role on the cough reflex.

Domperidone-induced potentiation of ventilatory responses in awake goats

Dopamine has been implicated in maintaining tonic inhibition of carotid body activity. We tested this hypothesis by assessing the ventilatory effects of a peripheral dopamine antagonist, domperidone. The effects of this agent on the ventilatory responses to hypoxia and hypercapnia were also examined. The study was performed in awake carotid body intact and carotid body denervated goats. Resting minute ventilation increased while PaCO2 decreased (4 Torr) following domperidone administration (0.5 mg/kg, I.V.) in carotid body intact goats. This response did not occur in carotid body denervated goats supporting the hypothesis that endogenous dopamine provides tonic inhibition in the carotid body. Hypoxic and hypercapnic ventilatory responses were significantly augmented following domperidone administration in the carotid body intact goats. This supports the concept of dopaminergic modulation of the response of the carotid body to stimuli. Domperidone allows study of carotid chemoreceptor dopaminergic activity in awake animals because of its high affinity for carotid body D2 dopamine receptors and its lack of CNS effects.

Effects of apomorphine and haloperidol in fetal lambs

Injections of 100 micrograms apomorphine (I.A.) in intact unanaesthetized fetal lambs resulted in the onset of low-voltage electrocortical activity if not already present, onset or an increase in amplitude of fetal breathing movements, and in about 50% of experiments, onset or an increase in skeletal muscle activity. These responses also occurred during isocapnic hypoxia, though the stimulation of breathing was less than in normoxic lambs. Apomorphine had no consistent effects on the blood gases, pH, heart rate or blood pressure. In fetal lambs with the carotid nerves and vago-sympathetic trunks sectioned, apomorphine had the same effects as in intact lambs, except that there was a small fall in arterial O2 pressure (Pa,O2). Haloperidol (0.5-1 mg I.V.) had no effect on spontaneous breathing movements or electrocortical activity, but blocked all responses to apomorphine for several hours. Dopamine (up to 4 mg I.V.) caused an increase in arterial pressure accompanied by bradycardia, but had no effect on breathing movements or electrocortical activity. After suprapontine brain-stem transection, the electrocortical response to apomorphine was reduced. The respiratory response was lost or reversed, with apomorphine causing a reduction in amplitude of breathing. Haloperidol reduced the incidence of breathing movements for several hours in brain-stem-transected fetuses. We conclude that there is a central pathway, including at least one dopaminergic synapse and with components above the pons, which can stimulate fetal motor activity and breathing. It does not appear to be tonically active and its normal function is unknown.

Dopaminergic control of respiration as shown by effects of 4-aminopyridine

The dopaminergic control of respiration in conscious and urethane-anaesthetized rabbits, was studied by comparing the respiratory effects of 4-aminopyridine alone (4-AP; 1 mg/kg i.v.) and those after the administration of dopamine antagonists (domperidone and haloperidol; 1 mg/kg each). The respiratory rate in conscious rabbits was increased by 4-AP. After domperidone this increase was reduced and preceded by a transient decrease. In spontaneously breathing, anesthetized rabbits there was a transient reduction after which the respiratory rate was increased by 4-AP; tidal volume was affected in an inverse manner. After domperidone, the excitatory effect of 4-AP on respiratory rate and the inhibitory effect on tidal volume were blocked. The effects of 4-AP on respiratory rate were prevented by vagotomy. In anesthetized, vagotomized, paralyzed and artificially ventilated rabbits (VPV animals) the peak amplitude of the integrated phrenic nerve activity ("phrenic activity') was increased by 4-AP. After pretreatment with haloperidol this effect of 4-AP on phrenic activity was reduced while the respiratory rate was now increased. In VPV animals with denervated carotid bodies the excitatory effect of 4-AP on phrenic activity was strongly enhanced and respiratory rate was increased. These effects were slightly reduced but not blocked by haloperidol. It is concluded that endogenous dopamine is involved in the control of respiration through effects on peripheral mechanisms (inhibition of inspiratory activity and enhancement of respiratory rate) as well as on central mechanisms (stimulation of inspiratory activity and reduction of respiratory rate).

A pharmacological study on respiratory rhythm in the isolated brainstem-spinal cord preparation of the newborn rat

An in vitro brainstem-spinal cord preparation of the newborn rat was used to examine the effects of neurotransmitters and transmitter candidates on respiratory frequency. Spontaneous periodic depolarization of the spinal ventral roots of the 4th or 5th cervical segment was observed at a frequency of 5-15 min-1 constantly for more than 5 h. The frequency of this depolarization was monitored as an index of the respiratory frequency. An elevation of the concentration of Ca2+ or Mg2+ caused a decrease in the respiratory frequency, whereas an elevation of K+ concentration caused an increase. The frequency was also increased by a reduction of pH. The highest frequency was observed at 27-28 degrees C. Dopamine, 5-hydroxytryptamine, histamine, acetylcholine, glutamic acid, substance P, and thyrotropin releasing hormone accelerated the respiratory frequency when applied by perfusion to the brainstem, whereas noradrenaline, gamma-aminobutyric acid, glycine, and [Met5] enkephalin and [Leu5] enkephalin slowed the frequency. Experiments with antagonists suggested that the stimulant effect of acetylcholine on respiratory frequency was mediated mainly by muscarinic receptors and the depressant effect of noradrenaline was mediated by alpha-adrenoceptors.

Gammahydroxybutyric acid: central biochemical and behavioral effects in neonatal rats

Administration of gammahydroxybutyric acid (GHBA) to 4 days old animals caused a dose dependent decrease in locomotor activity. GHBA also induced a marked hypoventilation, irregular breathing and finally apnea, while heart rate was slightly increased. Changes in monoamine neurotransmitter turnover indicated an inhibition of dopamine (DA) neurotransmission. It is concluded that GHBA mechanisms in the neonatal rat brain are biochemically as well as functionally mature at an early age and that the effects on locomotor activity and respiratory regulation at least partly may involve interactions with central DA neurotransmission.

Differential effects on phrenic output of two dopamine agonists, apomorphine and bromocriptine

Effects of dopamine agonists upon phrenic output were studied in decerebrate, vagotomized, paralyzed, carotid-body denervated dogs. Intravenous administration of dopamine was without effect in these dogs. Apomorphine (APO) increased phrenic minute activity while bromocriptine (BRO) decreased phrenic minute activity following intravenous injection at doses of 100 micrograms/kg for each drug. BRO-induced decreases in phrenic minute activity were associated with decreases in phrenic amplitude; burst frequency remained unchanged. Conversely, APO-induced increases in phrenic minute activity resulted from significant increases in both frequency and amplitude. Both APO and BRO significantly prolonged inspiratory duration and shortened expiratory duration. Responses to APO and BRO were abolished with haloperidol. These results confirm previous evidence for the existence of dopaminergic receptors within the central nervous system, which are inaccessible to exogenous dopamine, and which can alter timing as well as amplitude characteristics of the integrated phrenic neurogram. Both excitation and inhibition of phrenic activity following administration of APO and BRO are mediated by mechanisms within or below the caudal brainstem.

Blockade of dopamine-induced chemosensory inhibition by domperidone

The sensory discharges from the carotid body chemoreceptors of the cat are transiently inhibited by dopamine (DA) injections. This chemosensory inhibition was effectively blocked by domperidone, a selective antagonist of D2 dopaminoceptors. The basal frequency of spontaneous chemosensory impulses was immediately and sustainly increased after domperidone, suggesting the withdrawal of a tonic inhibition of chemosensory discharges by endogenous DA released from glomus cells. The peripheral dopaminergic modulation of chemoreflexes may be separately blocked by domperidone, a drug unable to cross the blood-brain barrier.

Central respiratory and cardiovascular effects in the rat of some putative neurotransmitter amino acids

Respiratory performance was studied in halothane anesthetized rats after intracerebroventricular (i.c.v.) injection of beta-alanine, taurine or glycine (0.01--1 mg). The amino acids induced a marked decrease in both respiratory frequency (f) and tidal volume (VT), which was immediate and longlasting. The respiratory depressant action of glycine could readily be reversed by strychnine, a glycine antagonist. Measurement of respiratory time intervals, inspiratory time (TI), expiratory time (TE) and total cycle duration (TTOT), after administration of the putative neurotransmitter amino acids revealed that the effects on f were due to prolongation of the duration of expiration. The duration of inspiration was principally unaltered, but mean inspiratory flow (VT/TI) and respiratory timing (TI/TTOT) decreased. In experiments employing the occluded breath technique, P0.1 was reduced in the same magnitude as the mean inspiratory flow (VT/TI). The results also showed a change in central (bulbopontine) setting for TE, while the setting to TI was unaltered. An inert amino acid, valine, which was administered i.c.v. in the same doses, had no effects on respiratory parameters. Apart from the effects on basal ventilation of beta-alanine, taurine and glycine, the CO2 induced respiratory response was blunted. These three amino acids also depressed heart rate and mean arterial pressure. Although relatively high doses were used to induce the respiratory effects, it may be hypothetised that the putative neurotransmitters beta-alanine, taurine and glycine may have a physiological role in the central regulation of breathing.

Effects of TRH and TRH analogues on the central regulation of breathing in the rat

Respiratory activity was studied in rats during light halothane anesthesia. Thyrotropin releasing hormone (TRH) and two TRH analogues: the desamidated form (TRH-OH) and gamma-butyrolactone-gamma-carbonyl-L-histidyl-L-prolinamide citrate (DN 1417) were administered intracerebroventricularly. TRH 0.5-5 micrograms induced a marked tachypnoea with a rapid onset and a duration of at least 20 min. DN 1417, a potent analogue of TRH with a very low TSH (thyroid stimulating hormone) releasing activity was more effective in stimulating respiratory frequency, while TRH-OH, regarded to have neither TSH releasing nor extra hypothalamic effects, at equimolar doses was unable to induce any changes in the respiratory pattern. When TRH was given into the fourth ventricle the dose response curve was slightly shifted to the left. In experiments employing the occluded breath technique, P0.1 was increased in the same magnitude as the mean inspiratory flow (VT/T1). The results also indicated an increase in the gain of the inflation reflex loop whereas the central bulbopontine setting for T1 and TTOT were not significantly changed. Local injection of TRH into the nucleus tractus solitarii induced a stimulation of respiratory frequency which was slower in onset compared to the response seen after injection into the lateral or fourth ventricles. Concomitantly to the respiratory changes, i.c.v. TRH injection induced a hypocarbia and an alkalosis. No changes in blood pressure or heart rate were seen. The respiratory stimulant effect of TRH could be potentiated by pretreatment with naloxone, methylatropine or a low dose of GABA. Haloperidol or propranolol did not significantly change the respiratory effects of TRH, while reserpine pretreatment seemed to blunt some of the ventilatory effects of TRH. It seems likely that TRH has few direct effects on brain stem neurones involved in the central regulation of respiration, but the main effects seem to be elicited in areas rostral to the brain stem. The respiratory stimulating effect of TRH is unrelated to TSH. Furthermore, other neurotransmitter systems might also be involved in modulation of the respiratory stimulation evoked by TRH.