A comparison of the effects of monoamines on neurones of the bulbar reticular formation

Evolving concepts of sleep cycle generation: From brain centers to neuronal populations

As neurophysiological investigations of sleep cycle control have provided an increasingly detailed picture of events at the cellular level, the concept that the sleep cycle is generated by the interaction of multiple, anatomically distributed sets of neurons has gradually replaced the hypothesis that sleep is generated by a single, highly localized neuronal oscillator.

Cell groups that discharge during rapid-eye-movement (REM) sleep (REM-on) and neurons that slow or cease firing during REM sleep (REM-off) have long been thought to comprise at least two neurochemically distinct populations. The fact that putatively cholinoceptive and/or cholinergic (REM-on) and putatively aminergic (REM-off) cell populations discharge reciprocally over the sleep cycle suggests a causal interdependence.

In some brain stem areas these cell groups are not anatomically segregated and may instead be neurochemically mixed (interpenetrated). This finding raises important theoretical and practical issues not anticipated in the original reciprocal-interaction model. The electrophysiological evidence concerning the REM-on and REM-off cell groups suggests a gradient of sleep-dependent membrane excitability changes that may be a function of the connectivity strength within an anatomically distributed neuronal network. The connectivity strength may be influenced by the degree of neurochemical interpenetration between the REM-on and REM-offcells. Recognition of these complexities forces us to revise the reciprocal-interaction model and to seek new methods to test its tenets.

Cholinergic microinjection experiments indicate that some populations of REM-on cells can execute specific portions of the REM sleep syndrome or block the generation of REM sleep. This observation suggests that the order of activation within the anatomically distributed generator populations may be critical in determining behavioral outcome. Support for the cholinergic tenets of the reciprocal-interaction model has been reinforced by observations from sleep-disorders medicine.

Specific predictions of the reciprocal-interaction model and suggestions for testing these predictions are enumerated for future experimental programs that aim to understand the cellular and molecular basis of the mammalian sleep cycle.

Extracellular levels of serotonin in the medial pontine reticular formation in relation to sleep-wake cycle in cats: a microdialysis study

Extracellular levels of endogenous serotonin (5-hydroxytryptamine; 5-HT) and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA) were measured in the medial pontine reticular formation (medial PRF) of intact cats. A microdialysis probe was inserted through a guide cannula into the medial PRF. At least 12 h after the probe insertion, in vivo brain microdialysis was initiated. The perfusion rate was 1 microliters/min, and perfusate fractions at regular intervals of 20 min were collected. Changes in extracellular 5-HT levels were compared across sleep-wake states of the animals, such as waking (W), slow wave sleep (SWS) and rapid eye movement (REM) sleep. To assess sleep-wake states, EEG, EMG, EOG and PGO waves were simultaneously recorded in parallel with microdialysis of the medial PRF. Extracellular 5-HT levels were highest (20-28 fmol/20 microliters) during W. As the animals entered SWS, 5-HT levels decreased to about 90% of those during W. The state of REM sleep usually interrupted SWS for 3-8 min. During the longer periods of REM sleep, during the 20 min periods in which the perfusates were collected, we observed the lowest 5-HT levels (60-50%).

Response of obstructive sleep apnea to fluoxetine and protriptyline

Protripyline is the pharmacologic agent most commonly used to treat obstructive sleep apnea (OSA); however, its anticholinergic side effects make it intolerable to many patients. Because serotonin may be a central respiratory stimulant and because the serotonin-uptake inhibitor, fluoxetine, is usually well tolerated, we wanted to try fluoxetine in the treatment of OSA. Therefore, we compared the effect of fluoxetine to that of protriptyline in 12 patients with OSA. Both drugs significantly decreased the proportion of REM sleep time and decreased the number of apneas or hypopneas in NREM sleep. The response to fluoxetine was equivalent to that of protriptyline; however, for the group as a whole, there was no significant improvement in the number of arterial oxygen desaturation events, the level of arterial oxygen desaturation, or the number of arousals with either agent. Although there was wide variability in the response to each medication, six of the 12 patients had good responses, including improvement in oxygenation, to either fluoxetine or protriptyline. Three patients could not complete the trial of protriptyline. We conclude that fluoxetine is beneficial to some, but not all, patients with OSA. Fluoxetine was better tolerated than protriptyline.

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.

Centrally and peripherally administered 5-HTP have opposite effects on respiration

5-Hydroxytryptophan (5-HTP) caused a prolonged increase of respiratory (phrenic nerve) activity when given into the third ventricle of paralyzed, vagotomized and glomectomized cats. In contrast, intravenous 5-HTP always led to a prolonged inhibition. These findings must be due to different modes of action of the same drug given by different routes and illustrates that caution must be used in interpreting experimental results following administration of exogenous agents.

Evidence for the existence of alpha- and beta-adrenoceptors on neurones and glial cells of cultured rat central nervous system--an autoradiographic study

The cellular localization of the binding of radioactive noradrenaline and alpha- and beta-adrenoceptor antagonists was studied in organotypic cultures of rat cerebellum, brain stem and spinal cord using autoradiography. In cerebellar cultures, many neurones, which appeared to be Purkinje cells, were labelled by [3H]noradrenaline and by the beta-antagonists [3H]dihydroalprenolol and [3H]carazolol, whereas no binding of the alpha-antagonists [3H]prazosin and [3H]rauwolscine was detected. In cultures of spinal cord and brain stem, [3H]noradrenaline and the beta-antagonists were bound to many large neurones. Binding of [3H] alpha-antagonists was observed to a small number of brain stem and spinal neurones, the labelling being much weaker than that produced by the [3H] beta-antagonists. The antidepressant [3H]desmethylimipramine was bound to many neurones and glial cells in cerebellar, brain stem and spinal cord cultures. Glial cells also possessed binding sites for [3H]noradrenaline and alpha- and beta-adrenoceptor antagonists, findings that are consistent with recent electrophysiological observations which indicate the existence of alpha- and beta-adrenoceptors on cultured astrocytes.

Transmitter sensitivities among various types of cultured brain stem neurons

We prepared cultures of dissociated cells from the lower brain stem of 14- to 15- day-old rat fetuses, and studied how neuronal activities of various types of neurons were influenced by application of various transmitters or transmitter-like substances. Unit discharges of the cultured neurons were recorded extracellularly, using suction micropipette, while the test substances were applied iontophoretically to the surface membrane of the cell body. We selected for testing 4 types of neurons with the typical appearance, i.e. multipolar, pyramidal, polygonal and fusiform one. Each type of neuron had specific sensitivities to acetylcholine and leucine-enkephalin. Amino acids and substance P acted to 4 types of neurons as either unanimous excitant or inhibitor. Monoamines and leucine-enkephalin had both inhibitory and facilitatory effect on unit discharges, although the inhibitory effect was dominant. These findings suggest that there is a specific relation between morphological properties of the brain stem neurons and their sensitivities to transmitters.

Biphasic responses to acetylcholine in mammalian reticulospinal neurons

Acetylcholine (ACh) responses were elicited by ionophoresis from neurons, located in the medial pontine reticular formation, which were antidromically identified as having axons projecting in the reticulospinal tracts. Most neurons were silent at rest and could be caused to discharge at a regular, slow rate by a constant application of glutamate. ACh altered this slow rate of firing in 28 of 29 cells but showed three different patterns of effect: approximately one-third were excited, one-third were inhibited, and one-third showed biphasic inhibition-excitation. The ACh responses were not sensitive to atropine. These observations suggest that reticulospinal neurons have ACh receptors mediating both inhibition and excitation, perhaps located on different portions of the same neuron.

Prolonged stimulation of respiration by endogenous central serotonin

We have recently reported a new neural brainstem mechanism which is uniquely activated by stimulation of carotid body afferent input to the brain and which facilitates respiration for hours after the immediate affects of the stimulation have dissipated (Millhorn, Eldridge and Waldrop, 1980). In the present study respiratory responses to carotid body or carotid sinus nerve stimulation were measured in vagotomized, anesthetized, and paralyzed cats whose end-tidal PCO2 and temperature were servo-controlled and kept constant. The responses of animals pretreated with various serotonin antagonists and a dopamine-norepinephrine antagonist were compared to the responses of untreated control animals. All three differently acting serotonin antagonists (methysergide, parachlorophenylalanine, and 5, 7-dihydroxytryptamine) either prevented or significantly reduced the magnitude of the long-lasting respiratory response whereas the dopamine-norepinephrine antagonist (alpha-methyltyrosine) failed to alter it. We conclude that the long-lasting increase of respiratory activity following stimulation of carotid body afferents is due to activation of an endogenous central serotoninergic mechanism which facilitates respiration.

Inhibition from nucleus raphe magnus of tooth pulp responses in medial reticular neurones of the cat can be antagonized by bicuculline

In decerebrate cats stimulation in nucleus raphe magnus (NRM) inhibited the response of neurones in the medial brain stem reticular formation to electrical stimulation of the tooth pulp. These evoked responses were also reduced by iontophoretically applied GABA. The inhibition from NRM was blocked by bicuculline applied by iontophoresis in 10 out of 16 neurones and the actions of GABA were also antagonized. Inhibition following stimulation in the contralateral reticular formation was not blocked by bicuculline. The results suggest that GABA is involved as a neurotransmitter in mediating the effects of NRM stimulation on reticular neurones.

Catecholaminergic depressant effects on bulbar respiratory mechanisms

On the basis of histochemical and pharmacological studies, catecholamines have been implicated in central mechanisms controlling respiration. This hypothesis was tested in iontophoretic studies on neurones located in bulbar respiratory centres. Adrenaline and noradrenaline had a predominantly depressant effect on respiratory as well as on closely situated non-respiratory units. These depressions were mimicked by the application of isoproterenol and clonidine; acetylcholine and serotonin had inconsistent effects on these neurones. In control experiments, microinjections, using a Hamilton syringe, were made in the area of bulbar respiratory centres: noradrenaline, but not serotonin, depressed the central respiratory activity reflected in the phrenic nerve discharge. These results suggest that specific adrenergic and noradrenergic depressant mechanisms could affect both respiratory and other physiological centres at the bulbar level.

Effects of excitatory and inhibitory amino acids on phasic respiratory neurons

The responsiveness of phasically active brainstem respiratory neurons to several amino acids was investigated in cats under Dial anesthesia. Four-barreled microelectrodes were used to extrude iontophoretically the putative neurotransmitters L-glutamate, L-asparatate, glycine, and gamma-aminobutyric acid (GABA), L-glutamate and L-aspartate caused increased activity when applied to either inspiratory or expiratory neurons and appeared to be equal in efficacy. Likewise, GABA and glycine depressed ongoing phasic neural activity of both inspiratory and expiratory units. In this case, however, the dosage of GABA required to produce a given depression was significantly less than the required dosage of glycine. These findings support the hypothesis that L-glutamate and/or L-aspartate may act as excitatory neurotransmitter agents at the synapses of brainstem respiratory neurons and conversely, GABA may act as the natural inhibitory neurotransmitter.