Effects of different monoamine oxidase inhibitors on respiratory activity in rats with chronically impaired central serotonergic function

Ventilatory response to hypercapnia and hypoxia after extensive lesion of medullary serotonergic neurons in newborn conscious piglets

Acute inhibition of serotonergic (5-HT) neurons in the medullary raphé (MR) using a 5-HT(1A) receptor agonist had an age-dependent impact on the "CO(2) response" of piglets (33). Our present study explored the effect of chronic 5-HT neuron lesions in the MR and extra-raphé on the ventilatory response to hypercapnia and hypoxia in piglets, with possible implications on the role of 5-HT in the sudden infant death syndrome. We established four experimental groups. Group 1 (n = 11) did not undergo any treatment. Groups 2, 3, and 4 were injected with either vehicle or the neurotoxin 5,7-dihydroxytryptamine in the cisterna magna during the first week of life (group 2, n = 9; group 4, n = 11) or second week of life (group 3, n = 10). Ventilation was recorded in response to 5% CO(2) (all groups) and 12% O(2) (group 2) during wakefulness and sleep up to postnatal day 25. Surprisingly, the piglets did not reveal changes in their CO(2) sensitivity during early postnatal development. Overall, considerable lesions of 5-HT neurons (up to 65% decrease) in the MR and extra-raphé had no impact on the CO(2) response, regardless of injection time. Postlesion raphé plasticity could explain why we observed no effect. 5,7-Dihydroxytryptamine-treated males, however, did present a lower CO(2) response during sleep. Hypoxia significantly altered the frequency during sleep in lesioned piglets. Further studies are necessary to elucidate the role of plasticity, sex, and 5-HT abnormalities in sudden infant death syndrome.

Altered respiratory motor drive after spinal cord injury: supraspinal and bilateral effects of a unilateral lesion

Because some bulbospinal respiratory premotor neurons have bilateral projections to the phrenic nuclei, we investigated whether changes in contralateral phrenic motoneuron function would occur after unilateral axotomy via C(2) hemisection. Phrenic neurograms were recorded under baseline conditions and during hypercapnic and hypoxic challenge in C(2) hemisected, normal, and sham-operated rats at 1 and 2 months after injury. The rats were anesthetized, vagotomized, and mechanically ventilated. No group differences were seen in contralateral neurograms at 1 month after injury. At 2 months, however, there was a statistically significant decrease in respiratory rate (RR) at normocapnia, an elevated RR during hypoxia, and an attenuated increase in phrenic neurogram amplitude during hypercapnia in the C(2)-hemisected animals. To test whether C(2) hemisection had induced a supraspinal change in respiratory motor drive, we recorded ipsilateral and contralateral hypoglossal neurograms during hypercapnia. As with the phrenic motor function data, no change in hypoglossal output was evident until 2 months had elapsed when hypoglossal amplitudes were significantly decreased bilaterally. Last, the influence of serotonin-containing neurons on the injury-induced change in phrenic motoneuron function was examined in rats treated with the serotonin neurotoxin, 5,7-dihydroxytryptamine. Pretreatment with 5,7-dihydroxytryptamine prevented the effects of C(2) hemisection on contralateral phrenic neurogram amplitude and normalized the change in RR during hypoxia. The results of this study show novel neuroplastic changes in segmental and brainstem respiratory motor output after C(2) hemisection that coincided with the spontaneous recovery of some ipsilateral phrenic function. Some of these effects may be modulated by serotonin-containing neurons.

Monoaminergic and GABAergic terminations in phrenic nucleus of rat identified by immunohistochemical labeling

The termination patterns of axons in the phrenic nucleus immunoreactive to synthetic enzymes for catecholamines and for serotonin and GABA were studied in rats. Spinal cord tissue in which phrenic motoneurons were retrogradely labeled with horseradish peroxidase was incubated with antisera against dopamine beta-hydroxylase, phenylethanolamine-N-methyltransferase, 5-hydroxytryptamine, and GABA to identify presumptive terminations of monoaminergic and GABAergic neurons onto identified phrenic motoneurons. In the C3 to C5 spinal cord, 5-hydroxytryptamine-, dopamine beta-hydroxylase- and GABA-like positive terminals with varicosities formed a dense network, with presumptive synaptic contacts on dendrites and somas of phrenic motoneurons. A similar pattern of terminations was also observed in adjacent (non-respiratory muscle) motoneuron pools. There were fewer phenylethanolamine-N-methyltransferase-positive terminal arborizations in the cervical spinal cord compared to thoracic spinal cord; phenylethanolamine-N-methyltransferase terminals were not seen in the vicinity of phrenic motoneurons. These results suggest that phrenic motoneuronal activity is influenced by multiple supraspinal inputs utilizing different neurotransmitters. These transmitters also mediate inputs to other (nearby) spinal motoneurons and thus are not unique for signal transmission to phrenic motoneurons.

p-Chloroamphetamine: effect on sleep and respiration in the rat

Rats, fitted with chronic EEG and EMG electrodes and a thoracic pneumograph, were monitored electrophysiologically for three successive days before and after an IP injection of p-chloroamphetamine (PCA) (2 mg/kg). During the 12 hours post PCA treatment, sleep onset was delayed, the percentage of Rapid Eye Movement (REM) sleep was decreased and the breathing rate during both the Non-REM (NREM) and REM sleep states was reduced. By 24 and 48 hours after the PCA injection, the sleep pattern and NREM respiratory rate had returned to control values; however, respiratory rate during REM sleep still tended to be decreased. The results suggest that PCA, at this dose, is capable of inducing insomnia and reducing REM sleep acutely without chronically altering the sleep pattern. The data also suggest that respiratory rate during sleep may decrease following PCA treatment.

Dopaminergic modulation of respiratory timing mechanisms in carotid body-denervated dogs

The role of central dopaminergic mechanisms in ventilatory control was investigated by monitoring phrenic motorneuronal output of anesthetized dogs which were vagotomized, paralyzed and artificially ventilated and in which bilateral carotid body denervation had been performed. Intravenous administration of dopamine (DA) had no effect on phrenic output in these dogs. In contrast, apomorphine (APO), a potent dopaminergic agonist, which unlike dopamine, crosses the blood-brain barrier consistently and significantly prolonged inspiratory duration and shortened expiratory duration without altering phrenic amplitude. The dopaminergic antagonist haloperidol produced a frequency-dependent decrease in basal phrenic minute activity and reversed or abolished APO-induced changes in the phrenic profile. These data verify the specificity of APO for dopamine receptors and further suggest that DA receptors in the central nervous system exert a tonic effect on central respiratory control mechanisms. In contrast, domperidone, a dopaminergic antagonist which poorly penetrates the blood-brain barrier did not alter basal phrenic characteristics. Moreover, with the exception of inspiratory duration, domperidone did not antagonize APO-induced alterations in the phrenic profile. We conclude that dopaminergic mechanisms within the brain or spinal cord modulate timing relationships of central respiratory output.