Elsevier

Brain Research

Volume 809, Issue 2, 2 November 1998, Pages 204-213
Brain Research

Research report
The pre-Bötzinger complex and phase-spanning neurons in the adult rat

https://doi.org/10.1016/S0006-8993(98)00872-5Get rights and content

Abstract

To characterise respiratory neurons in the pre-Bötzinger complex of adult rats, extracellular recordings were made from 302 respiratory neurons in the ventral respiratory group of sodium pentobarbitone anaesthetised adult rats. Neurons were located 0 to 1.6 mm caudal to the facial nucleus, and ventral to the nucleus ambiguus. The pre-Bötzinger complex comprised expiratory neurons (22%, 22/100), inspiratory neurons (37%, 37/100) and phase-spanning neurons (41%, 41/100). In contrast, 80% (125/157) of Bötzinger neurons were expiratory, and 80% (36/45) of rostral ventral respiratory group neurons were inspiratory. Rostrocaudally, the pre-Bötzinger complex extended about 400 μm, starting at the caudal pole of the nucleus ambiguus compact formation. The pre-Bötzinger complex was also characterised by a predominance of propriobulbar neurons (81%, 13/16). Furthermore, 68% (33/48) of expiratory–inspiratory neurons found were located within the pre-Bötzinger complex. The variety of neuronal subtypes in the pre-Bötzinger complex, including many firing during the expiratory–inspiratory transition is consistent with the hypothesis that this nucleus plays a key role in respiratory rhythm generation in the adult rat.

Introduction

The pre-Bötzinger complex in neonatal rats is thought to be a critical site for the generation of respiratory rhythmicity 28, 33. This proposal is based on studies showing that: (1) elimination of the pre-Bötzinger area by microsection abolishes rhythmic motor output in either the hypoglossal, or the C5, nerve roots; (2) neurons in the pre-Bötzinger complex display bursts of activity that increase or decrease, depending on their average resting membrane potential within a certain range; and (3) pre-Bötzinger neurons continue to produce rhythmical bursts of action potentials under low-Ca2+/high-Mg2+ solution that blocks synaptic transmission 6, 16, 33, 34.

Pre-Bötzinger neurons may also be responsible for generating respiratory rhythms in adult mammals. In the adult cat, spike-triggered averaging studies revealed that some inspiratory neurons found directly caudal to the Bötzinger complex had widespread excitatory and inhibitory monosynaptic connections with other respiratory neurons in the rostral and caudal ventral respiratory groups (rVRG and cVRG; 8, 11, 31). More recently, in the adult cat, a transition zone also termed the `pre-Bötzinger complex' has been identified. This site contains a high density of phase-spanning neurons, which contrasts with the more rostrally placed Bötzinger complex that contains mainly expiratory neurons, and the more caudally placed rVRG, which contains mainly inspiratory neurons 4, 30.

Although considerable information is available about the pre-Bötzinger complex in neonatal rats, there is very limited electrophysiological evidence to show that the pre-Bötzinger area is a distinct sub-region between the Bötzinger complex and the rVRG in the adult rat. Previous pharmacological studies revealed that inhibitory synaptic mechanisms are important in the generation of rhythmic breathing activity in the adult rat, suggesting that network interactions between excitatory and inhibitory neurons are essential for respiratory rhythm generation 15, 22, 23, 29. Anatomical studies in adult rats demonstrate a distinct region (which corresponds to the pre-Bötzinger complex in the neonatal rat) containing more propriobulbar neurons than the Bötzinger complex or the rVRG 7, 9. However, other details about these propriobulbar neurons, such as their firing patterns and neuronal connections are unavailable. The aim of the present study was to examine respiratory neurons in the ventral medulla by systematic exploration with microelectrodes suitable for extracellular recording. Since propriobulbar neurons that fire through the respiratory phase transition are hypothesised to be critically involved in respiratory rhythmogenesis, our attention was directed to (1) whether or not there is a transition zone between Bötzinger expiratory and rVRG inspiratory neurons, (2) the distribution of propriobulbar respiratory neurons in the ventral respiratory group, and (3) the location of respiratory neurons that fire during the transition from expiration to inspiration (EI) or from inspiration to expiration (IE).

Section snippets

Animal preparation

Experiments were conducted on 13 adult, bilaterally vagotomised Sprague Dawley rats (350–450 g). Anaesthesia was induced with sodium pentobarbital (75 mg/kg i.p.). Atropine (0.4 mg/kg i.p.) was given to reduce bronchial secretions. The level of anaesthesia was regularly checked by monitoring blood pressure and phrenic nerve discharge in response to nociceptive stimuli before administration of additional doses of sodium pentobarbitone (3 mg/kg i.v.). Body temperature (36–37°C), end-tidal CO2

Results

Extracellular recordings were made from respiratory neurons located in the ventral medulla in an area extending 1.6 mm caudally from the caudal pole of the facial nucleus and between 1.7 and 2.0 mm lateral to the midline. To avoid recording from the more dorsally located respiratory motoneurons, the nucleus ambiguus was mapped by stimulation of the cervical vagal and superior laryngeal nerves (see Section 2), and recording of intramedullary antidromic field potentials. Tracking was carried out

Discussion

In the present study, the pre-Bötzinger complex was identified as a transition zone between the Bötzinger complex and rVRG. The term `transition' emphasises several key features about this area. First, is the heterogeneous range of firing patterns compared with the more rostral Bötzinger complex populations that are mainly expiratory modulated, and the more caudal rVRG that are mainly inspiratory. Second, is the high proportion of units in the pre-Bötzinger complex that fire during the phase

Conclusion

The present study demonstrates that (1) there is a transition zone (pre-Bötzinger Complex) in the adult rat, that contains a heterogeneous mixture of expiratory, inspiratory and phase-spanning neurons. This is in contrast to the predominantly expiratory neurons in the Bötzinger Complex and inspiratory neurons in the rVRG; (2) most of the neurons that fire during the EI phase transition are located within the pre-Bötzinger Complex; (3) propriobulbar respiratory neurons that fire with an EI or IE

Acknowledgements

We thank V. Arkell for her excellent technical assistance. Our work is supported by grants from the Garnett Passe and Rodney Williams Memorial Foundation, the National Heart Foundation, the North Shore Heart Research Foundation and the National Health and Medical Research Council of Australia.

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