Rhythmic bursting of pre‐ and post‐inspiratory neurones during central apnoea in mature mice

Abstract

Stimulation of pulmonary vagal C fibres (PCFs) inhibits inspiration but the response pattern of respiratory rhythm‐generating neurones is unknown. This study provides the first description of the effects of PCF stimulation on six different types of respiratory neurones located in the ventrolateral medulla of the mature mouse. Studies were performed in both urethane‐anaesthetized (1.5 g kg−1 I.P.) mature mice and in an arterially perfused working heart‐brainstem preparation (WHBP). In both preparations the respiratory motor pattern of phrenic and recurrent laryngeal nerves were comparable. Stimulation of PCFs, using phenylbiguanide (2–5 μg) injected into the right atrium, evoked a similar respiratory and cardiac response pattern in both anaesthetized and perfused mice, which included: (i) a significant prolongation of the inter‐inspiratory interval; (ii) an increase in the duration and amplitude of post‐inspiratory (ii) activity; and (iii) an atropine‐sensitive bradycardia (50–260 beats min−1). In the WHBP, PCF stimulation evoked a depolarization (11 ± 1 mV) and high frequency tonic discharge (up to 64 Hz) in ten out of twenty‐one PI neurones. During the PCF‐induced prolongation of PI activity all other PI neurones (n= 11), as well as pre‐inspiratory neurones (PreI; n= 11), displayed oscillations in membrane potential and/or rhythmic bursting at a similar frequency of 0.7‐1.0 Hz. Other respiratory neurones recorded, including stage II expiratory neurones (n= 7), early‐ (n= 6), ramp‐ (n= 16) and late‐inspiratory neurones (n= 4), ceased firing rhythmically during PCF stimulation. The firing behaviour of PI and PreI neurones was assessed after switching to a low Ca2+ (0.2 mm)‐high Mg2+ (5.25 mm) perfusate to block synaptic transmission in the WHBP. In the absence of synaptic transmission, PreI neurones (n= 7/8) continued to discharge rhythmically, whereas all other respiratory cell types (including PI neurones, n= 5) fired tonically. In conclusion, stimulation of PCFs elicits a reflex‐evoked prolongation of the PI phase of the respiratory cycle and excitation of PI neurones including rhythmic discharging. It is suggested that this rhythmic bursting depends on inhibitory connections from PreI neurones. The functional significance of these central ‘apnoeic rhythms’ are discussed.

Specific Respiratory Neuron Types Have Increased Excitability That Drive Presympathetic Neurones in Neurogenic Hypertension

“…14 The latter is consistent with findings of increased minute ventilation, 59 the larger carotid bodies in SH rats with smaller diameter arteries, 60 and enhanced carotid sinus nerve discharge during hypoxia. 61,62 Reflexly, carotid body stimulation increases both pre-I and post-I neuronal activities, and the chemoreflex-evoked sympathoexcitation is larger in SH than Wistar rats, 63 whereas carotid sinus nerve denervation reduced both respiratory modulation of iCSN and lumbar sympathetic nerve in SH rats 64 and arterial pressure in conscious SH rats. 14,64 In summary, prehypertensive in situ SH rats have enhanced respiratory-modulated sympathetic activity at cervical, thoracic, and lumbar levels.…”

Tonic pulmonary stretch receptor feedback modulates both eupnea and gasping in an in situ rat preparation

“…These experiments conform with the "Guiding Principles for Research Involving Animals and Human Beings" of the American Physiological Society. Only minor changes have been made from procedures described previously (15)(16)(17)(18)28). Briefly, rats were anesthetized with halothane (5% in O 2), bisected below the diaphragm, immersed in ice-cold artificial cerebral spinal fluid (aCSF; described below), and decerebrated at a precollicular level.…”

Electrophysiology on Isolated Brainstem-spinal Cord Preparations from Newborn Rodents Allows Neural Respiratory Network Output Recording

“…This allows a precise study of the setting up of the central respiratory drive at birth, which is very interesting and clinically relevant, but it focuses on these early ages and cannot be extended to later ages. Older ages can be used in arterially perfused working heart-brainstem preparations 25 , but with substantial protocol modifications (see below). An isolated brainstem-spinal cord preparation from adult guinea pigs has been developed by Morin-Surun et al 26 , in which the brainstem is perfused via the basilar artery and the rhythm is recorded in the XII cranial nerve.…”

“…c-fos analysis in the preparations can also be realized in order to identify the neural populations involved in specific respiratory responses. Finally, the preparation can be modified to keep other organs such as the heart 25 , ribs 6 , or physiological artery perfusion with other physiological rhythms 33 . However, these very important modifications would require other dissection protocols.…”