Distribution and classification of respiratory neurones in the nucleus tractus solitarius of the rat medulla and their responses to central and periphereal stimuli

Abstract

In the cat the dorsal respiratory group comprising the nucleus tractus solitarius (NTS) is regarded as an important respiratory centre. However the relevance of this nucleus in the control of respiration in the rat is still unclear. The present work aimed to identify and localize the various neurons involved in respiration in the nucleus tractus solitarius region of the rat medulla. Further, we intended to study the responses of these identified respiratory neurons to central and peripheral stimulation, in particular: 1)effect of stimulation of midbrain periaqueductal gray (PAG) matter and, 2)effect of stimulation of the vagus (vagal feedback). Nembutal-anaesthetised (80 mg/Kg), spontaneously-breathing rats (n = 60, 300–400 g) were used for the study. Neurons firing in relation to diaphragm electromyogram (dEMG) were located between 0.6–1.5 mm lateral to the midline, extending 0.5 mm caudal to 1.8 mm rostral to the posterior end of the area postrema (AP). Four different populations of respiratory-related neurons were recorded. They have been classified (with reference to dEMG) as: expiratory-inspiratory phase-spanning cells, early inspiratory neurons (eI-cells), cells that fire only in phase with diaphragm (I-all cells), and expiratory cells. The phase-spanning and eI-cell populations were located both in the ventrolateral and ventromedial NTS rostral to the area postrema, while the other cells were found predominantly in the more lateral regions. eI-cells were observed to start firing up to 20 ms prior to dEMG and ceased firing about one third of the way through diaphragm activity. They also showed a decrementing discharge pattern. Excitatory amino acid (DLH, 15–60 nl of 0.2 M) stimulation of PAG along its dorso-ventral axis produced dose-dependent changes in the cardiorespiratory parameters: increased respiratory frequency, decreased inspiratory and expiratory durations, increased blood pressure (BP) and heart rate. PAG stimulation elicited increases in activity in both phase-spanning and eI-neuronal populations in the NTS. The PAG sites evoking an increase in the phase-spanning cell activity were located more laterally than the PAG sites that led to increased eI-cell activity. The increase in the responses of the phase-spanning neurons was not accompanied by any changes in the respiratory frequency, whereas the PAG stimulation giving rise to an increase in eI-neuronal activity also elicited simultaneous increases in dEMG and the respiratory frequency. The central end of the cut vagus was electrically stimulated to activate pulmonary stretch-receptor afferents. Vagal stimulation caused depression of inspiratory cell activity along with bradycardia and a fall in BP. Both eI-neurons and I-all cells were inhibited. These cells exhibited a 'vagal escape' phenomenon ie, resuming activity whilst vagal stimulation continued. Phase-spanning cells and expiratory cells are yet to be tested for responses to vagal stimulation. The behaviours of these respiratory-related cells of the NTS, their temporal relationships with the dEMG, firing patterns, their responses to central and peripheral stimuli are discussed and a possible role for each in the brainstem control of respiration is proposed. Similar studies have also been carried out to elucidate the roles of ventrolateral column (VLM)/ventral respiratory group (VRG) neurons in the control of respiration in the rat. The findings of these studies are presented in an accompanying abstract.