Serotonergic control of phrenic motoneuronal activity at the level of the spinal cord of the rabbit

Daily acute intermittent hypoxia enhances phrenic motor output and stimulus-evoked phrenic responses in rats

Plasticity is a hallmark of the respiratory neural control system. Phrenic long-term facilitation (pLTF) is one form of respiratory plasticity characterized by persistent increases in phrenic nerve activity following acute intermittent hypoxia (AIH). Although there is evidence that key steps in the cellular pathway giving rise to pLTF are localized within phrenic motor neurons (PMNs), the impact of AIH on the strength of breathing-related synaptic inputs to PMNs remains unclear. Further, the functional impact of AIH is enhanced by repeated/daily exposure to AIH (dAIH). Here, we explored the effects of AIH vs. 2 weeks of dAIH preconditioning on spontaneous and evoked responses recorded in anesthetized, paralyzed (with pancuronium bromide) and mechanically ventilated rats. Evoked phrenic potentials were elicited by respiratory cycle-triggered lateral funiculus stimulation at C2 delivered prior to- and 60 min post-AIH (or an equivalent time in controls). Charge-balanced biphasic pulses (100 µs/phase) of progressively increasing intensity (100 to 700 µA) were delivered during the inspiratory and expiratory phases of the respiratory cycle. Although robust pLTF (~60% from baseline) was observed after a single exposure to moderate AIH (3 x 5 min; 5 min intervals), there was no effect on evoked phrenic responses, contrary to our initial hypothesis. However, in rats preconditioned with dAIH, baseline phrenic nerve activity and evoked responses were increased, suggesting that repeated exposure to AIH enhances functional synaptic strength when assessed using this technique. The impact of daily AIH preconditioning on synaptic inputs to PMNs raises interesting questions that require further exploration.

Aripiprazole-induced hiccups: a case report

Hiccups can arise from idiopathic, psychogenic and organic causes. The use of therapeutic drugs forms one of the important causes of hiccups. Although the exact pathophysiological processes involved have not yet been established, the neurotransmitters dopamine, serotonin and gamma amino butyric aid (GABA) have been documented to play a significant role in the generation of hiccups. We report a patient of organic bipolar affective disorder who developed hiccups with the atypical antipsychotic aripiprazole. The possible underlying neurotransmitter mechanisms, predisposing factors and clinical implications of this rare adverse event are discussed.

Serotonergic raphe magnus cell discharge reflects ongoing autonomic and respiratory activities

Serotonergic cells are located in a restricted number of brain stem nuclei, send projections to virtually all parts of the CNS, and are critical to normal brain function. They discharge tonically at a rate modulated by the sleep-wake cycle and, in the case of medullary serotonergic cells in raphe magnus and the adjacent reticular formation (RM), are excited by cold challenge. Yet, beyond behavioral state and cold, endogenous factors that influence serotonergic cell discharge remain largely mysterious. The present study in the anesthetized rat investigated predictors of serotonergic RM cell discharge by testing whether cell discharge correlated to three rhythms observed in blood pressure recordings that averaged >30 min in length. A very slow frequency rhythm with a period of minutes, a respiratory rhythm, and a cardiac rhythm were derived from the blood pressure recording. Cross-correlations between each of the derived rhythms and cell activity revealed that the discharge of 38 of the 40 serotonergic cells studied was significantly correlated to the very slow and/or respiratory rhythms. Very few serotonergic cells discharged in relation to the cardiac cycle and those that did, did so weakly. The correlations between serotonergic cell discharge and the slow and respiratory rhythms cannot arise from baroreceptive input. Instead we hypothesize that they are by-products of ongoing adjustments to homeostatic functions that happen to alter blood pressure. Thus serotonergic RM cells integrate information about multiple homeostatic activities and challenges and can consequently modulate spinal processes according to the most pressing need of the organism.

Neural plasticity of respiratory control system: modeling perspectives

Classical models of respiratory control assume a hardwired system architecture with reflex regulation of respiratory rhythm and total ventilation. Recent experimental studies, however, reveal a much more pliable architecture with varying forms of neural plasticity in the afferent and efferent pathways. Here, mathematical models of several types of neural plasticity are proposed and their computational roles in respiratory neural processing are discussed.

Involvement of medullary GABAergic and serotonergic raphe neurons in respiratory control: electrophysiological and immunohistochemical studies in rats

In the present study we first examined the possible involvement of the putative neurotransmitters gamma-aminobutyric acid (GABA) and serotonin (5-HT) in raphe-induced facilitatory or inhibitory effects on the respiratory activity of rats. Secondly, we investigated the possibility of spinal projections of GABAergic and serotonergic neurons from the medullary raphe nuclei to the phrenic motor nucleus (PMN). We observed that an intravenous (i.v.) injection of (+)-bicuculline, a GABA(A) receptor antagonist, significantly reduced respiratory inhibition induced by electrical stimulation of the raphe magnus (RM) or the raphe obscurus (RO). On the other hand, an i.v. injection of methysergide, a broad-spectrum 5-HT receptor antagonist, significantly reduced the respiratory facilitation induced by electrical stimulation of the raphe pallidus (RP) or RO. By using a combined method of retrograde tracing with Texas Red injected into the PMN region at segments C4 and C5 and immunohistochemical labeling, we observed that glutamic acid decarboxylase (GAD; a GABA synthesizing enzyme) immunopositive and Texas Red double labeled neurons were predominantly localized in the RM, and additionally in the RO. However 5-HT immunopositive and Texas Red double-labeled neurons were predominantly localized in the RP, and additionally in the RO and RM. These findings suggest that RM-, or RO-induced inhibitory effects, are transmitted, at least in part, to the PMN via a direct GABAergic descending pathway. The RP-, or RO-induced facilitatory effects in rats however, are transmitted via a serotonergic descending pathway.

Purinergic modulation of respiration via medullary raphe nuclei in rats

The involvement of P2X receptors in raphe nuclei in respiratory control was investigated. Experiments were done on urethane anesthetized, spontaneously breathing or paralyzed and artificially ventilated adult rats. We found that microinjection of ATP (0.1-0.2 M, 10-70 nl) into raphe magnus (RM) caused dose-dependent decreases in integrated phrenic amplitude and respiratory frequency, whereas injection of ATP into raphe pallidus (RP) caused dose-dependent increases in phrenic amplitude and respiratory frequency. Microinjection of pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (0.02 M, 50 nl), a broad-spectrum P2X receptor antagonist, into the RM or RP did not cause any significant change in respiration, but partially blocked the respiratory effects of ATP that was subsequently injected into the same sites within the RM or RP. These findings indicate that the ATP-P2X mediated neurotransmission could contribute to the respiratory control by affecting the activities of raphe nuclei.

Effects of electrical stimulation of the medullary raphe nuclei on respiratory movement in rats

The present study was undertaken to examine the effects of electrical stimulation of the medullary raphe nuclei on respiration in rats anesthetized with ketamine and xylazine. Train pulse stimuli (100 Hz, 10-30 microA) were applied in the regions of the caudal raphe nuclei: the raphe magnus (RM), raphe pallidus (RP) and raphe obscurus (RO). Stimulation of the RM depressed inspiratory movements measured by means of an abdominal pneumograph, whereas stimulation of the RP augmented inspiratory movements. It was revealed that stimulation of the RO induced either inhibitory or facilitatory effects on respiratory movements depending on the stimulation sites. These findings confirm and extend previous studies concerning the effects of raphe stimulation on respiratory activity in cats. The present results demonstrate that in rats the caudal raphe nuclei are involved in respiratory control.

The crossed phrenic phenomenon: a model for plasticity in the respiratory pathways following spinal cord injury

Hemisection of the cervical spinal cord rostral to the level of the phrenic nucleus interrupts descending bulbospinal respiratory pathways, which results in a paralysis of the ipsilateral hemidiaphragm. In several mammalian species, functional recovery of the paretic hemidiaphragm can be achieved by transecting the contralateral phrenic nerve. The recovery of the paralyzed hemidiaphragm has been termed the "crossed phrenic phenomenon." The physiological basis for the crossed phrenic phenomenon is as follows: asphyxia induced by spinal hemisection and contralateral phrenicotomy increases central respiratory drive, which activates a latent crossed respiratory pathway. The uninjured, initially latent pathway mediates the hemidiaphragm recovery by descending into the spinal cord contralateral to the hemisection and then crossing the midline of the spinal cord before terminating on phrenic motoneurons ipsilateral and caudal to the hemisection. The purpose of this study is to review work conducted on the crossed phrenic phenomenon and to review closely related studies focusing particularly on the plasticity associated with the response. Because the review deals with recovery of respiratory muscles paralyzed by spinal cord injury, the clinical relevance of the reviewed studies is highlighted.

Theophylline-induced respiratory recovery following cervical spinal cord hemisection is augmented by serotonin 2 receptor stimulation

Cervical spinal cord hemisection leads to a disruption of bulbospinal innervation of phrenic motoneurons resulting in paralysis of the ipsilateral hemidiaphragm. We have previously demonstrated separate therapeutic roles for theophylline, and more recently serotonin (5-HT) as modulators to phrenic nerve motor recovery; mechanisms that likely occur via adenosine A1 and 5-HT2 receptors, respectively. The present study was designed to specifically determine if concurrent stimulation of 5-HT2 receptors may enhance motor recovery induced by theophylline alone. Adult female rats (250-350 g; n=7 per group) received a left cervical (C2) hemisection that resulted in paralysis of the ipsilateral hemidiaphragm. Twenty-four hours later rats were given systemic theophylline (15 mg/kg, i.v.), resulting in burst recovery in the ipsilateral phrenic nerve. Theophylline-induced recovery was enhanced with the 5-HT2A/2C receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI; 1.0 mg/kg). DOI-evoked augmentation of theophylline-induced recovery was attenuated following subsequent injection of the 5-HT2 receptor antagonist, ketanserin (2.0 mg/kg). In a separate group, rats were pretreated with ketanserin, which did not prevent subsequent theophylline-induced respiratory recovery. However, pretreatment with ketanserin did prevent DOI-induced augmentation of the theophylline-evoked phrenic nerve burst recovery. Lastly, using immunocytochemistry and in situ hybridization, we showed for the first time a positive co-localization of adenosine A1 receptor mRNA and immunoreactivity with phrenic motoneurons of the cervical ventral horns. Taken together, the results of the present study suggest that theophylline may induce motor recovery likely at adenosine A1 receptors located at the level of the spinal cord, and the concurrent stimulation of converging 5-HT2 receptors may augment the response.

Serotonin(2) receptors mediate respiratory recovery after cervical spinal cord hemisection in adult rats

The aim of the present study was to specifically investigate the involvement of serotonin [5-hydroxytryptamine (5-HT(2))] receptors in 5-HT-mediated respiratory recovery after cervical hemisection. Experiments were conducted on C(2) spinal cord-hemisected, anesthetized (chloral hydrate, 400 mg/kg ip), vagotomized, pancuronium- paralyzed, and artificially ventilated female Sprague-Dawley rats in which CO(2) levels were monitored and maintained. Twenty-four hours after spinal hemisection, the ipsilateral phrenic nerve displayed no respiratory-related activity indicative of a functionally complete hemisection. Intravenous administration of the 5-HT(2A/2C)-receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) induced respiratory-related activity in the phrenic nerve ipsilateral to hemisection under conditions in which CO(2) was maintained at constant levels and augmented the activity induced under conditions of hypercapnia. The effects of DOI were found to be dose dependent, and the recovery of activity could be maintained for up to 2 h after a single injection. DOI-induced recovery was attenuated by the 5-HT(2)-receptor antagonist ketanserin but not with the 5-HT(2C)-receptor antagonist RS-102221, suggesting that 5-HT(2A) and not necessarily 5-HT(2C) receptors may be involved in the induction of respiratory recovery after cervical spinal cord injury.

Distribution of Serotonin 2A and 2C Receptor mRNA Expression in the Cervical Ventral Horn and Phrenic Motoneurons Following Spinal Cord Hemisection

Cervical spinal cord injury leads to a disruption of bulbospinal innervation from medullary respiratory centers to phrenic motoneurons. Animal models utilizing cervical hemisection result in inhibition of ipsilateral phrenic nerve activity, leading to paralysis of the hemidiaphragm. We have previously demonstrated a role for serotonin (5-HT) as one potential modulator of respiratory recovery following cervical hemisection, a mechanism that likely occurs via 5-HT2A and/or 5-HT2C receptors. The present study was designed to specifically examine if 5-HT2A and/or 5-HT2C receptors are colocalized with phrenic motoneurons in both intact and spinal-hemisected rats. Adult female rats (250-350 g; n = 6 per group) received a left cervical (C2) hemisection and were injected with the fluorescent retrograde neuronal tracer Fluorogold into the left hemidiaphragm. Twenty-four hours later, animals were killed and spinal cords processed for in situ hybridization and immunohistochemistry. Using (35)S-labeled cRNA probes, cervical spinal cords were probed for 5-HT2A and 5-HT2C receptor mRNA expression and double-labeled using an antibody to Fluorogold to detect phrenic motoneurons. Expression of both 5-HT2A and 5-HT2C receptor mRNA was detected in motoneurons of the cervical ventral horn. Despite positive expression of both 5-HT2A and 5-HT2C receptor mRNA-hybridization signal over phrenic motoneurons, only 5-HT2A silver grains achieved a signal-to-noise ratio representative of colocalization. 5-HT2A mRNA levels in identified phrenic motoneurons were not significantly altered following cervical hemisection compared to sham-operated controls. Selective colocalization of 5-HT2A receptor mRNA with phrenic motoneurons may have implications for recently observed 5-HT2A receptor-mediated regulation of respiratory activity and/or recovery in both intact and injury-compromised states.

5-Hydroxytryptophan-induced respiratory recovery after cervical spinal cord hemisection in rats

The present study investigates the role of serotonin in respiratory recovery after spinal cord injury. Experiments were conducted on C(2) spinal cord hemisected, anesthetized, vagotomized, paralyzed, and artificially ventilated rats in which end-tidal CO(2) was monitored and maintained. Before drug administration, the phrenic nerve ipsilateral to hemisection showed no respiratory-related activity due to the disruption of the descending bulbospinal respiratory pathways by spinal cord hemisection. 5-Hydroxytryptophan (5-HTP), a serotonin precursor, was administrated intravenously. 5-HTP induced time- and dose-dependent increases in respiratory recovery in the phrenic nerve ipsilateral to hemisection. Although the 5-HTP-induced recovery was initially accompanied by an increase in activity in the contralateral phrenic nerve, suggesting an increase in descending respiratory drive, the recovery persisted well after activity in the contralateral nerve returned to predrug levels. 5-HTP-induced effects were reversed by a serotonin receptor antagonist, methysergide. Because experiments were conducted on animals subjected to C(2) spinal cord hemisection, the recovery was most likely mediated by the activation of a latent respiratory pathway spared by the spinal cord injury. The results suggest that serotonin is an important neuromodulator in the unmasking of the latent respiratory pathway after spinal cord injury. In addition, the results also suggest that the maintenance of 5-HTP-induced respiratory recovery may not require a continuous enhancement of central respiratory drive.

Plasticity of cardiorespiratory neural processing: classification and computational functions

Neural plasticity, or malleability of neuronal structure and function, is an important attribute of the mammalian forebrain and is generally thought to be a kernel of biological intelligence. In this review, we examine some reported manifestations of neural plasticity in the cardiorespiratory system and classify them into four functional categories, integral; differential; memory; and statistical-type plasticity. At the cellular and systems level the myriad forms of cardiorespiratory plasticity display emergent and self-organization properties, use- and disuse-dependent and pairing-specific properties, short-term and long-term potentiation or depression, as well as redundancy in series or parallel structures, convergent pathways or backup and fail-safe surrogate pathways. At the behavioral level, the cardiorespiratory system demonstrates the capability of associative and nonassociative learning, classical and operant conditioning as well as short-term and long-term memory. The remarkable similarity and consistency of the various types of plasticity exhibited at all levels of organization suggest that neural plasticity is integral to cardiorespiratory control and may subserve important physiological functions.

Neuropharmacology of control of respiratory rhythm and pattern in mature mammals

This review summarizes the current understanding of the neurotransmitters and neuromodulators that are involved, firstly, in respiratory rhythm and pattern generation, where glutamate plays an essential role in the excitatory mechanisms and glycine and gamma-aminobutyric acid mediate inhibitory postsynaptic effects, and secondly, in the transmission of input signals from the central and peripheral chemoreceptors and of motor outputs to respiratory motor neurons. Finally, neuronal mechanisms underlying respiratory modulations caused by respiratory depressants and excitants, such as general anesthetics, benzodiazepines, opioids, and cholinergic agents, are described.

Regional differences in serotonergic input to canine parasternal intercostal motoneurons

There is a mediolateral gradient in activation of the parasternal intercostal (PI) muscle during inspiration. In the present study, we tested the hypotheses that serotonergic [5-hydroxytryptamine (5-HT)] input from descending central drive and/or intrinsic size-related properties of PI motoneurons leads to the differential activation of PI muscles. In dogs, PI motoneurons innervating the medial and lateral regions of the PI muscles at the T(3)-T(5) interspaces were retrogradely labeled by intramuscular injection of cholera toxin B subunit. After a 10-day survival period, PI motoneurons and 5-HT terminals were visualized by using immunohistochemistry and confocal imaging. There were no differences in motoneuron morphology among motoneurons innervating the medial and lateral regions of the PI muscle. However, the number of 5-HT terminals and the 5-HT terminal density (normalized for surface area) were greater in motoneurons innervating the medial region of the PI muscle compared with the lateral region. These results suggest that differences in distribution of 5-HT input may contribute to regional differences in PI muscle activation during inspiration and that differences in PI motoneuron recruitment do not relate to size.

Effects of the serotonin synthesis inhibitor p-CPA on the expression of the crossed phrenic phenomenon 4 h following C2 spinal cord hemisection

The present study assesses the effects of para-chlorophenylalanine (p-CPA), a serotonin-depleting drug, on the recovery of respiratory-related activity in the phrenic nerve induced by asphyxia 4 h following ipsilateral C2 hemisection in young adult rats. HPLC analysis was used to quantify levels of serotonin (5-HT), dopamine (DA), norepinephrine, and the 5-HT metabolite, 5-hydroxyindoleacetic acid, in the C4 segment of the spinal cord, all of which were significantly lower in p-CPA-treated hemisected rats compared to hemisected controls receiving saline. Hemisection alone was found to significantly increase 5-HT levels and significantly decrease DA levels compared to normal controls. Eight of eight saline-injected rats expressed recovery of respiratory-related activity in the ipsilateral phrenic nerve during asphyxia 4 h following hemisection, while only 4/8 rats in the p-CPA-treated group expressed recovery in the ipsilateral nerve. Quantification of integrated phrenic nerve wave-forms indicated that the mean amplitude of respiratory-related activity in the ipsilateral phrenic nerve was significantly lower in p-CPA-treated rats than in saline controls. In addition, saline controls demonstrated significant increases in mean respiratory frequency and mean amplitude of contralateral phrenic nerve activity during asphyxia, compared to normocapnia. However, p-CPA-treated rats did not express significant differences in either mean respiratory frequency or mean amplitude of integrated respiratory wave-forms during asphyxia, compared to normocapnia. The results suggest that p-CPA treatment attenuates the recovery of respiratory-related activity in the phrenic nerve 4 h following ipsilateral C2 hemisection and attenuates asphyxia-induced increases in respiratory frequency and respiratory burst amplitude recorded from the contralateral phrenic nerve.

Effects of serotonin on crossed phrenic nerve activity in cervical spinal cord hemisected rats

The present study investigates the effect of 5-hydroxytryptophan (5-HTP), a serotonin precursor, on crossed phrenic nerve activity (CPNA) in rats subjected to a left C2 spinal cord hemisection. Electrophysiological experiments were conducted on anesthetized, vagotomized, paralyzed, and artificially ventilated rats to assess phrenic nerve activity. The left phrenic nerve lost rhythmic activity due to the disruption of the bulbospinal respiratory pathways following spinal cord hemisection. Activity was induced in the left phrenic nerve (CPNA) by temporary asphyxia. 5-HTP administration increased CPNA during asphyxia in the left phrenic nerve in a dose-dependent fashion. Specifically, in a group of eight animals, application of 5-HTP at 0.5, 1.0, and 2.0 mg/kg significantly increased CPNA by 102.2+/-18.5%, 200.8+/-58.1%, and 615.0+/-356.9% compared with predrug control values, respectively. 5-HTP-induced increases in CPNA were reversed by methysergide (2-6 mg/kg, i.v.), a serotonin receptor antagonist. The results suggest that serotonin is involved in the modulation of crossed phrenic nerve activity following spinal cord injury.

Modulation of vomiting by the medullary midline

Studies were conducted to investigate the possible role of the brainstem midline region as a source of facilitatory input for the vomiting process. Experiments were conducted using the "fictive vomiting' model in decerebrate, paralysed cats. Dysfunction of the medullary midline region produced by localized injections of the neurotoxin kainic acid abolished or greatly attenuated fictive vomiting. In addition, some respiratory-related midline neurons were found to fire in synchrony with co-active phrenic and abdominal nerve discharge during fictive vomiting. These experiments demonstrate the importance of the medullary midline for the normal occurrence of the vomiting process. An explanation for the post-lesion elimination of vomiting is that the lesions remove an important source of facilitatory input to spinal respiratory motoneurons and/or to the brainstem circuitry that mediates vomiting.

Hypoxia-induced long-term facilitation of respiratory activity is serotonin dependent

Repeated isocapnic hypoxia evokes long-term facilitation (LTF) of phrenic nerve activity in rats. We wished to determine: (1) whether hypoxia-induced LTF is serotonin dependent; and (2) whether hypoxia-induced LTF is a property of upper airway motoneurons. Phrenic and hypoglossal nerve activities were recorded in urethane anesthetized, vagotomized, paralyzed and artificially ventilated rats (n = 7). Rats were exposed to three, 5-min hypoxic episodes (FIo2 = 0.10) separated by 5 min of hyperoxia (FIo2 = 0.50). One hour after the final hypoxic episode, integrated phrenic and hypoglossal amplitudes and burst frequency were increased above control values (63 +/- 17%, 78 +/- 26% and 9.6 +/- 2.1 bursts/min, respectively: p < 0.05). In rats pretreated with methysergide (n = 7; 4 mg/kg), no changes in phrenic or hypoglossal activity from pre-stimulus control values were observed at any time post-stimulation. The results indicate that hypoxia-induced LTF requires 5-HT receptors and is characteristic of both hypoglossal and phrenic motor output.

Mediation of serotonin-induced hyperventilation via 5-HT3-receptor in European eel Anguilla anguilla

The effects of serotonin (5-hydroxytryptamine) on ventilation were investigated by continuous measurements of intrabuccal pressure in unrestrained eel. Intravenous administration of 5-hydroxytryptamine (30 micrograms.kg-1) caused a large increase in ventilatory frequency (+ 100%) and amplitude (+ 140%). The 5-hydroxytryptamine-induced hyperventilation was blocked by the 5-HT3-receptor antagonists metoclopramide (1.0 mg.kg-1) or MDL72222 (1.0 mg.kg-1), and was insensitive to the 5-HT1/2-receptor antagonist methysergide (3.0 mg.kg-1) and to the 5-HT4-receptor antagonist DAU 6285 CL (3.0 mg.kg-1). The hyperventilatory response to 5-hydroxytryptamine could be mimicked by the 5-HT3 receptor agonist 1-phenylbiguanide (300 micrograms.kg-1). These results strongly implicate the 5-HT3-receptor as the mediator of the 5-hydroxytryptamine-induced hyperventilation in eel.

Effects of serotonin receptor blockade by methysergide on loaded breathing in the rabbit

Previous works demonstrated that the excitatory role of serotonin (5-hydroxytryptamine, 5-HT) on ventilation is mediated by 5-HT1,2 receptors stimulation. We hypothesized that load-induced hypoventilation could be minimized by a central release of 5-HT. Conversely, blockade of 5-HT receptors should accentuate hypoventilation. We compared the ventilatory effects of methysergide (MS), a 5-HT1,2 receptors antagonist, in 3 groups of anesthetized, spontaneously breathing rabbits: (1) a group of animals breathing for 60 min through a 370 cm H2O.liter-1.s inspiratory resistive load (IRL group), whose paCO2 increased with IRL; (2) a Control group; (3) a Control + CO2 group, made hypercapnic to assess the possible effect of this stimulus on the ventilatory responses to MS. In the 3 groups, i.v. injection of MS induced the same ventilatory changes, characterized by a rapid shallow breathing with a shorter integrated diaphragmatic activity. This confirms the tonic facilitatory effect of 5-HT on ventilation and suggests that IRL would not increase 5-HT release in the sites close to the respiratory nuclei.

Long-term facilitation of inspiratory intercostal nerve activity following carotid sinus nerve stimulation in cats

1. Repeated carotid sinus nerve (CSN) stimulation evokes a serotonin-dependent long-term facilitation (LTF) of phrenic nerve activity in cats. To determine whether CSN stimulation-evoked LTF is a general property of spinal inspiratory motoneurones, phrenic and inspiratory internal intercostal (IIC) nerve activities were recorded in nine cats (eight anaesthetized; one decerebrate), which were vagotomized, paralysed, thoracotomized and ventilated with O2; airway CO2 was controlled by means of of a servo-respirator. Baseline conditions were established by setting the arterial CO2 pressure (Pa,CO2) at approximately 2 mmHg above the threshold for IIC activity. One CSN was stimulated (3 times threshold, 25 Hz, 0.5 ms duration) with five (2 min) trains, each separated by 5 min. 2. The peak integrated phrenic activity was elevated by 33% whereas IIC activity was elevated by 226% above baseline, 90 min post-stimulation (P < 0.05). The results were similar when expressed as a percentage of the maximal neural activities (elicited by combined hypercapnia and CSN stimulation), although differences between the nerves were less pronounced. The burst frequency was not change following stimulation. 3. In five additional cats that were pretreated with the serotonin receptor antagonist, methysergide maleate (0.5-1 mg kg-1, I.V.), the CO2 thresholds of the phrenic (12 mmHg) and IIC nerves (22 mmHg) were increased (P < 0.05), and LTF could not be elicited in either neurogram. 4. Successive CSN stimulation episodes evoked a previously undescribed phenomenon. Although the peak integrated phrenic activity was unchanged (90-95% of maximal), IIC activity increased progressively during successive stimulus episodes (66-90% of maximal; P < 0.05). However, after methysergide treatment, the initial stimulus-evoked phrenic response decreased to 58% of maximal and both neurograms exhibited progressive augmentation of the stimulus-evoked response. As stimulus-evoked augmentation does not require serotonin, it is independent of LTF. 5. We conclude that CSN stimulation-evoked LTF of IIC activity exceeds that of phrenic activity. Since LTF requires the neuromodulator serotonin and is expressed predominantly by changes in burst pattern formation versus rhythm generation, serotonin may exert a greater influence on IIC relative to phrenic respiratory motor output. A unique mechanism is described whereby successive CSN stimulus episodes cause progressively increasing responses in both neurograms.

Serotonin reveals ineffective spinal pathways to contralateral phrenic motoneurons in spinally hemisected rats

Serotonin reveals ineffective (subthreshold) pathways from the C2 lateral funiculus to ipsilateral phrenic motoneurons in spinalized rats. The objective of the present study was to investigate serotonergic modulation of crossed-spinal pathways to contralateral phrenic motoneurons. Rats (n = 10) were anesthetized (urethane), paralyzed, vagotomized, and artificially ventilated. The spinal cord was hemisected at C1-C2 and, on the intact side, a tungsten stimulating electrode was placed ventral to the C2 dorsal root entry zone in the dorsolateral (approximately 1.1 mm) or the ventrolateral funiculus (approximately 2.2 mm depth). Single shocks (100-750 microA, 0.1-0.5 ms, 2 Hz) elicited a short-latency (approximately 1.0 ms to peak) excitation in the ipsilateral phrenic nerve, but usually evoked little or no response in the contralateral phrenic nerve at either stimulus site. Following systemic injection of the monoamine oxidase inhibitor pargyline (25 mg/kg) and the serotonin precursor 5-hydroxytryptophan (5-10 mg/kg), complex responses were revealed in the contralateral phrenic nerve, including: (1) spontaneous tonic activity; (2) a short-latency (approximately 1.0 ms to peak) evoked excitation; and (3) two long-latency (approximately 2.2 and 7.8 ms to peak) evoked excitations. The longest latency excitation was expressed only when the stimulating electrode was positioned in the dorsolateral funiculus. Contralateral evoked responses were blocked by systemic methysergide (2-6 mg/kg), a broad-spectrum serotonin receptor antagonist. These results indicate that serotonin converts ineffective crossed phrenic pathways in the spinal cord to effective pathways. It remains to be determined whether serotonin is both necessary and sufficient in this modulatory process, or if it is a nonspecific result of increased phrenic motoneuron excitability.

Characteristics of slow bursting activities recorded in cervical ventral roots in the in vitro brainstem-spinal cord preparation of the neonatal rat

The aim of the present work was to disclose, through pharmacological activation of an isolated central nervous system maintained in vitro, spinal locomotor and respiratory-like activities inferred from an in vivo rabbit preparation. In a brainstem-spinal cord preparation in neonatal rats (0-3 days old), medullary respiratory activity occurred spontaneously in the cervical ventral roots. During 5-hydroxytryptophan (5-HTP) superfusion (0.2 mM), a slower rhythm with longer burst duration developed in the same ventral roots, with the pre-existing long-lasting slow bursting (LLSB) activity. At the same time, locomotor bursts were recorded from lumbar ventral roots. The LLSB activity was mainly recorded in cervical ventral roots, but they could also be encountered at the lumbar level, where they were eliminated after thoracic transection. The LLSB activity and the locomotor bursting were maintained after a C1 or C2 spinal transection, whereas medullary activity disappeared. Bilateral recording of the three types of rhythmic activity demonstrated that the LLSB activity and the medullary respiratory bursting typically displayed a synchronous bilateral coupling, whereas at caudal levels an alternate bilateral pattern was the rule for locomotor activity. Lactic acid could reinduce LLSB activity if introduced after it had just disappeared during the washout phase following 5-HTP superfusion. These results strongly suggest that the LLSB activity that originates from cervical generators belongs to the respiratory system, and not to locomotor activity. Finally, similar results in an in vivo rabbit preparation have been obtained through pharmacological activation. This preparation appears to be a suitable model for the analysis of this cervical burst generator and for the study of interactions among the different pattern generators.

Effects of serotonin and substance P on bulbar respiratory neurons in vivo

We studied the effects of separate or co-applications by microiontophoresis of serotonin (5-HT) and substance P(4-11) onto brainstem respiratory neurons in anesthetized or decerebrate cats. 5-HT either produced an excitation (36%, n = 10) or an inhibition (43%, n = 12) or had no effect (21%, n = 6). SP(4-11) had predominantly an excitatory effect (84%, n = 26) or no effect. Fifteen respiratory neurons responded to both 5-HT and SP(4-11). Test applications of 5-HT were made during a long application of SP(4-11). We obtained 'additive effects' when the inhibitory effect of 5-HT was superimposed to the excitation of SP(4-11) with slight modification (n = 1) or without any modification (n = 2). In other cases, called 'non-additive effects', we observed a great modification of the responsiveness of the neuron to the inhibitory effect of 5-HT (n = 2) or a complete blockade of the excitatory effect of 5-HT (n = 2) during co-application. The remaining results presented a potentiation of 5-HT effect by SP(4-11) or a biphasic response to 5-HT during SP(4-11) application. The results indicate that both 5-HT and SP receptors coexist on the membrane of the same respiratory-related neurons in the brainstem of cat and suggest an interaction between both substances in vivo in the central respiratory system.

Regional distribution of serotonin and substance P co-existing in nerve fibers and terminals in the brainstem of the rat

Two-color fluorescence immunohistochemistry was used to identify and map the distribution of nerve processes immunoreactive for both serotonin and substance P in the rat brainstem. Doubly labeled fibers were observed throughout the brainstem, but tended to be densest in cranial nerve motor nuclei and in reticular regions of the ventral medulla. In the trigeminal motor nucleus, the facial nucleus and the spinal accessory nucleus, the majority of serotonergic varicosities also appeared to contain substance P; in the occulomotor nucleus and the hypoglossal nucleus the numbers of double-labeled and single-labeled serotonergic varicosities were roughly equal. Thus, co-existence of substance P with serotonin was common in many cell groups innervating skeletal muscle. The proportion of double-labeled varicosities was significantly lower in the nucleus of the solitary tract, wherein single-labeled varicosities were much more common. Double-labeled fibers and varicosities were also significantly less common in the spinal trigeminal nucleus. In addition, double-labeling appeared to be uncommon in regions involved in the processing of special sensory information (e.g. auditory, vestibular and visual pathways). These results demonstrate a subpopulation of serotonergic fibers immunoreactive for substance P in the brainstem of the rat. The consistently high density of double-labeled processes in cranial nerve motor nuclei suggests that, as may be the case in the spinal cord, neurons containing serotonin and substance P regulate the activity of motoneurons that innervate skeletal muscle. In addition, they may be involved in other aspects of the function of the brainstem.

Modulation of respiratory activity of neonatal rat phrenic motoneurones by serotonin

1. The effects of serotonin on phrenic motoneurones were studied in an in vitro preparation of the isolated brainstem and spinal cord from neonatal rats. 2. Serotonin (5-HT; > or = 5-10 microM) increased inspiratory-modulated phrenic nerve activity and produced a small amount of tonic activity during expiration. Inspiratory-modulated activity of the fourth cervical ventral root also increased, but was accompanied by robust tonic activity, which often obscured the rhythmic activity. 3. Serotonin, in both normal and tetrodotoxin-containing medium, depolarized phrenic motoneurones and increased cell input resistance. Serotonin also increased inspriatory-modulated firing as well as the response of phrenic motoneurones to injected current. The y-intercept of the relationship between firing frequency and injected current (f-I) was increased, but the slope was not affected. There was no bistable firing behaviour. 4. Under voltage clamp conditions, 5-HT produced a tonic inward current of 0.07-0.37 nA. This current increased with less negative holding potentials and decreased with more negative holding potentials (-75 to -90 mV) but did not reverse. 5. In addition, 5-HT decreased inspiratory-modulated synaptic current by 23 +/- 6%. The degree of attenuation was not affected by holding potential. The time course of the decrease in inspiratory-modulated synaptic current was similar to the changes seen in tonic inward current and input resistance. 6. Depolarization, tonic inward current, and shift in the f-I relationship produced by 5-HT were antagonized by the 5-HT2/1C receptor antagonist ketanserin and mimicked by the 5-HT2/1C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI). However, the 5-HT induced decrease in inspiratory-modulated synaptic current was not reduced by ketanserin nor mimicked by DOI. 7. We conclude that exogenously applied 5-HT simultaneously increases cell excitability and decreases inspiratory-modulated synaptic current in phrenic motoneurones via different types of receptors. When these responses occurred simultaneously, the increase in excitability predominated and the net effect was an augmentation of inspiratory-modulated phrenic motoneurone activity.

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

The exercise ventilatory response is augmented during conditions of increased respiratory dead space (delta Vd), a phenomenon that we refer to as short term modulation (STM). To test the hypothesis that serotonin is necessary in the mechanism underlying STM, experiments were conducted on ten awake goats. Ventilation, CO2 production and PaCO2 were measured at rest and during treadmill exercise (4 km/h, 5% grade), with and without delta Vd (0.25 L), before and after systemic administration of the serotonin receptor antagonist, methysergide maleate (n = 6; 1 mg/kg, i.v.), or the tryptophan hydroxylase inhibitor, p-chlorophenylalanine (PCPA; n = 4; 100 mg/kg, i.v.). Pre-methysergide: (1) PaCO2 decreased from rest to exercise to a similar degree with (-1.9 mmHg) and without (-1.8 mmHg) delta Vd; (2) the exercise ventilatory response increased 59% +/- 13% (P < 0.01) with delta Vd, accounting for similar exercise PaCO2 regulation and demonstrating STM; and (3) effects of delta Vd on exercise tidal volume and frequency responses were inconsistent. Post-methysergide: (1) there were no significant effects on ventilation or PaCO2 at rest or during exercise in control (mask) conditions; (2) the exercise ventilatory response was unaffected by delta Vd, thereby allowing PaCO2 to increase 4.1 +/- 3.0 mmHg from rest to exercise (P < 0.05); and (3) with delta Vd during exercise, the tidal volume response was increased, but was offset by a decreased frequency response. Following PCPA (16-24 h): (1) hyperventilation was evident at rest and during exercise; (2) the exercise ventilatory response was augmented, indicating STM; and (3) the exercise ventilatory response with delta Vd was not affected further, allowing PaCO2 to increase from rest to exercise and indicating an inability to elicit further STM. These data suggest that serotonin is necessary for short term modulation of the exercise ventilatory response with increased respiratory dead space, although the location of relevant serotonin receptors is not yet clear.

5-Hydroxytryptophan (5-HTP) augments spontaneous and evoked phrenic motoneuron discharge in spinalized rats

Experiments on anesthetized, spinalized rats were conducted to determine the effects of systemic 5-hydroxytryptophan (5-HTP) administration on: (1) spontaneous phrenic nerve activity and (2) evoked phrenic responses to short latency, non-serotonergic synaptic inputs elicited by electrical stimulation of lateral funiculus. 5-HTP augmented spontaneous phrenic activity and allowed expression of a second, longer latency evoked response. Both effects were antagonized by methysergide. Our results suggest that spinal serotonin increases the efficacy of synaptic inputs to phrenic motoneurons.

Serotonergic excitatory drive to hypoglossal motoneurons in the decerebrate cat

In decerebrate, paralyzed, vagotomized and artificially ventilated cats, serotonin (5-HT) and its analogues, microinjected into the hypoglossal (XII) motor nucleus, altered the activity of the genioglossal branch of XII nerve. 5-HT, carboxamidotryptamine maleate (5-CT) and DOI (1-5 mM) increased the activity by over 200%. Methysergide reversed this increase. Methysergide, mianserin, or ketanserin (100-250 nl, 1 mM) reduced the spontaneous hypoglossal activity by 20-50%. Buspirone, 8-OH-DPAT and (-)-propranolol were without effect. Thus, 5-HT provides a substantial tonic excitatory drive to XII motoneurons. The 5-HT receptors involved are likely to be type 1C or 2, but uncertainty regarding the affinity profiles of the drugs used in in vivo conditions in the cat precludes a definite identification.