The medullary projections of afferent bronchopulmonary C fibres in the cat as shown by antidromic mapping

Behaviour of raphe cells projecting to the dorsomedial medulla during carbachol-induced atonia in the cat

1. The activity of most brainstem serotonergic cells is suppressed during sleep, particularly the rapid eye movement (REM) phase. Thus, they may play a major role in state-dependent changes in CNS functioning. Our main goal was to search for medullary raphe cells having axonal branches in the region of the hypoglossal (XII) motor nucleus and assess their behaviour during the atonia produced by microinjections of a cholinergic agonist, carbachol, into the dorsal pontine tegmentum. In chronic animals, such microinjections evoke a desynchronized sleep-like state similar to natural REM sleep; in decerebrate animals, they produce eye movements and a motor suppression similar to the postural atonia of REM sleep. 2. In decerebrate, paralysed, vagotomized and artificially ventilated cats, we recorded extracellularly from medullary raphe cells antidromically activated from the XII nucleus region. Forty-five cells recorded in the raphe obscurus and pallidus nuclei were antidromically activated with latencies characteristic of non-myelinated fibres (4.4-42.0 ms). For thirty-three of the forty-five cells, we found one or more axonal branches within or just below the XII nucleus. The remaining twelve cells, in addition to the XII nucleus, had axonal ramifications in the medial nucleus of the solitary tract (NTS) and/or the dorsal motor nucleus of the vagus (DMV). 3. A subset of fourteen spontaneously active cells with identified axonal projections were held long enough to be recorded during the carbachol-induced atonia, and eight of these also during the subsequent recovery and a systemic administration of the serotonergic 1A receptor agonist (+/-)8-hydroxy-2-(di-N-propylamino)tetrealin hydrobromide (8-OH-DPAT). All but one were suppressed during the atonia in parallel to the suppression of XII, phrenic and postural nerve activities (firing rate, 1.3 +/- 0.7 Hz before and 0.1 +/- 0.2 Hz after carbachol (means +/- S.D.)). Following the recovery from the atonia, the firing rates of the eight cells increased to the pre-carbachol level (1.6 +/- 1.0 Hz). Subsequently, all were silenced by 8-OH-DPAT. 4. These cells fulfil most physiological criteria for serotonergic cells and have the potential to modulate, in a state-dependent manner, activities in the motor XII nucleus, visceral sensory NTS, and DMV. The decrements in serotonergic neuronal activity that occur during the carbachol-induced atonia suggest that a similar withdrawal of serotonergic input may occur during REM sleep and contribute to the characteristic reductions in upper airway motor tone.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Fos-determined distribution of neurons activated during the Bezold-Jarisch reflex in the medulla oblongata in conscious rabbits and rats

Experiments were performed in unanaesthetized rabbits and rats to investigate the distribution, within the medulla oblongata, of neurons activated during the Bezold-Jarisch reflex. Repeated intravenous injections of phenylbiguanide evoked depressor and bradycardic responses in both rabbits and rats. Fos-positive neurons were present in the nucleus tractus solitarius and in the caudal ventrolateral medulla oblongata. Double-label tyrosine hydroxylase (TH) immunohistochemical studies in the ventrolateral medulla showed that most Fos-positive neurons in the caudal ventrolateral medulla were TH-negative neurons scattered between A1 noradrenaline cells, in the rabbit and in the rat. Approximately 20% of neurons in the caudal ventrolateral medulla in rabbits, and 50% in rats, were immunoreactive for both Fos and TH. Some Fos-positive, TH-negative neurons in the caudal ventrolateral medulla were retrogradely labelled with cholera toxin B-Gold after injection of this tracer into the sympathoexcitatory region of the rostral ventrolateral medulla. Our data suggests that neurons in the nucleus tractus solitarius, and rostrally projecting TH-negative neurons in the caudal ventrolateral medulla, are part of the pathway by which stimulation of cardiopulmonary receptors inhibits sympathetic vasomotor tone to decrease blood pressure during the Bezold-Jarisch reflex.

CNS innervation of airway-related parasympathetic preganglionic neurons: a transneuronal labeling study using pseudorabies virus

The CNS cell groups that innervate the tracheal parasympathetic preganglionic neurons were identified by the viral retrograde transneuronal labeling method. Pseudorabies virus (PRV) was injected into the tracheal wall of C8 spinal rats and after 4 days survival, brain tissue sections from these animals were processed for immunohistochemical detection of PRV. Retrogradely labeled parasympathetic preganglionic neurons were seen in three sites in the medulla: the compact portion of the nucleus ambiguus, the area ventral to the nucleus ambiguus, and the rostralmost portion of the medial nucleus tractus solitarius (NTS); this labeling pattern correlated well with the retrograde cell body labeling seen following cholera toxin beta-subunit injections in the tracheal wall. PRV transneuronally labeled neurons were found throughout the CNS with the most abundant labeling concentrated in the ventral medulla oblongata. Labeled neurons were identified along the ventral medullary surface, and in nearby areas including the parapyramidal, retrotrapezoid, gigantocellular and lateral paragigantocellular reticular nuclei as well as the caudal raphe nuclei (raphe pallidus, obscurus, and magnus). Serotonin (5-HT) neurons of the caudal raphe complex (B1-B3 cell groups) and ventromedial medulla were labeled as well as a few C1 adrenergic neurons. The A5 cell group was the major noradrenergic area labeled although a small number of locus coeruleus neurons were also labeled. Several NTS regions contained labeled cells including the commissural, intermediate, medial, central, ventral, and ventrolateral subnuclei. PRV infected neurons were present in the Kölliker-Fuse and Barrington's nuclei. In the rostral mesencephalon, the precommissural nucleus of the dorsal periventricular gray matter was labeled. Labeling was present in the dorsal, lateral and paraventricular hypothalamic nuclei. In summary, the airway parasympathetic preganglionic neurons are innervated predominantly by a network of lower brainstem neurons that lie in the same regions known to be involved in respiratory and cardiovascular regulation. These findings are discussed in relationship to some of the potential CNS mechanisms that may be operative in airway disorders as well as potentially involved in certain fatal respiratory conditions such as Ondine's curse and sudden infant death syndrome (SIDS).

Projections from the commissural subnucleus of the solitary tract onto catecholamine cell groups of the ventrolateral medulla

Efferent projections of the commissural subnucleus of the solitary tract (COM) to the ventrolateral medulla were studied in the cat using anterograde labeling with biocytin combined with dopamine beta-hydroxylase immunohistochemistry. COM neurons were observed to send their axons densely to the areas of distribution of respiration-related neurons in the ventrolateral medulla, e.g. ventral respiratory group, Bötzinger complex. Axon terminals from COM neurons were further found in the areas of distribution of catecholamine neurons (C1 and A1 cell groups), that were distributed in the close vicinity of the reported respiration-related areas in the ventrolateral medulla. Putative synaptic contacts of axon terminals from COM neurons with catecholamine neurons were often observed in the C1 area.

Projections from the commissural subnucleus of the nucleus of the solitary tract: an anterograde tracing study in the cat

The commissural subnucleus (COM) of the nucleus of the solitary tract (NTS) is known to receive primary afferents from the lungs and other viscera innervated by the vagus nerve, and thus to participate in central autonomic and respiratory control. The aim of the present study was to identify the areas of terminal arborizations of COM neurons in order to examine brainstem sites which may be involved in reflex responses mediated by these neurons. The projections were studied in cats, using biocytin as an anterograde tracer. Labeled fibers and terminal boutons were visualized by horseradish-peroxidase histochemistry, 2-3 days after microinjection of the tracers into the COM 1-2 mm caudal to the obex. Labeled axons were examined in the brainstem from the rostral pons to the caudal medulla and were found bilaterally, with an ipsilateral predominance, mainly in the following regions: (1) The dorsolateral rostral pons. Terminal boutons were observed in the lateral and medial parabrachial nuclei, Kölliker-Fuse nucleus, and around the mesencephalic trigeminal tract. This area corresponds to the pontine respiratory group also known as the "pneumotaxic center." (2) The pontine area dorsolateral to the superior olivary nucleus. This region contains the A5 noradrenergic cell group; (3) Near the ventral surface, below the facial nucleus. This area overlaps with the 'retrotrapezoid nucleus.' (4) Respiration-related areas of the medulla, including the dorsal and ventral respiratory groups, and the Bötzinger complex. (5) The dorsal motor nucleus of the vagus. These results suggest that the COM is involved in reflex arcs, which have both respiratory functions and autonomic functions. The pathway to the dorsolateral pons, which has been identified in our recent electrophysiological study is likely to play a role in mediating respiratory responses from pulmonary rapidly adapting receptors. Other pathways may represent additional projections from second-order neurons receiving input from this group of lung receptors, or projections from as yet unidentified neurons that relay information from different afferents terminating in the COM.

Efferent projections of pulmonary rapidly adapting receptor relay neurons in the cat

Axonal projections of second order neurons activated by vagal afferent fibers originating from pulmonary rapidly adapting receptors (RARs) were studied electrophysiologically in Nembutal-anesthetized, paralyzed and artificially ventilated cats. Extracellular recordings from these neurons (referred to as 'RAR-cells') were made in the commissural subnucleus (COM) of the nucleus of the solitary tract (NTS). They were identified by a combination of stimuli including stimulation of the vagus nerve(s), hyperinflation and deflation of the lungs, and a brief period of ammonia vapor inhalation. A total of 80 RAR-cells were tested for axonal projections to a respiration-related area in the brain-stem, either the dorsolateral rostral pons or the dorsal respiratory group (DRG) or the ventral respiratory group (VRG) or the Bötzinger complex (BOT) and/or the spinal cord. Twenty-two of the 47 (47%) RAR-cells tested for ipsilateral pontine projection could be antidromically activated, and in 8 cases evidence for axonal arborization was obtained. Only 1 of the 11 RAR-cells tested for DRG projection, and 1 of the 10 RAR-cells tested for VRG projection, were antidromically activated. No RAR-cells were activated from the BOT (n = 8) or from the C3-C4 segments of the spinal cord (n = 11). Bilateral lesions of the COM abolished the reflex responses induced by ammonia inhalation or hyperinflation of the lungs, but not the Hering-Breuer reflex. These results indicate that a pathway from the COM to the rostral pons forms part of the reflex arc originating from RARs.

Neurones in commissural nucleus tractus solitarii required for full expression of the pulmonary C fibre reflex in rat

1. The pulmonary C fibre reflex, triggered by activating pulmonary C fibre endings in the lung, consists of rapid shallow breathing (which may be preceded by apnoea), bradycardia, and hypotension. The purpose of this work was to identify proximal synapses in this reflex. From pilot data, we hypothesized that neurones in a discrete region of the commissural nucleus in the nucleus tractus solitarii (NTS) are required for full expression of the pulmonary C fibre reflex. Studies were carried out in urethane-anaesthetized, unilaterally vagotomized, spontaneously breathing rats, in which diaphragm electromyogram, arterial pressure, and blood gases were measured. Phenyldiguanide (PDG) was injected in the right atrium to elicit the pulmonary C fibre reflex. Unilateral NTS injections were made through multibarrelled pipettes containing DL-homocysteic acid (DLH) to mimic the reflex, cobalt chloride to reversibly impair the reflex, and/or dye to mark the injection sites. 2. PDG (5-16 micrograms kg-1) injected in the right atrium of twenty-six rats produced the classic pulmonary C fibre reflex: a vagally mediated, rapid onset of rapid shallow breathing, bradycardia and hypotension. 3. Injection of DLH (3-12 nl of 20 mM for a total of 60-240 pmol) in the dorsomedial aspect of the commissural nucleus of the NTS in thirty rats mimicked the pulmonary C fibre reflex, producing rapid shallow breathing, hypotension, and a slight bradycardia. 4. Interruption of neuronal transmission by injecting cobalt chloride (15-30 nl of 100 mM) in the site where DLH produced rapid shallow breathing, reversibly impaired the rapid shallow breathing and bradycardia produced by right atrial injections of PDG in fifteen rats. The commissural region where DLH produced rapid shallow breathing and cobalt impaired the pulmonary C fibre reflex extended from 720-1100 microns caudal to the obex, 30-200 microns lateral to mid-line, and 200-600 microns ventral to the dorsal surface of the brain stem within the NTS. 5. Taken together, the results suggest that neurones within a discrete region in the dorsomedial commissural nucleus in caudal NTS are required for full expression of the pulmonary C fibre reflex.