Research reportThe pre-Bötzinger complex and phase-spanning neurons 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.
References (35)
- et al.
Pre-Bötzinger complex in cats: respiratory neuronal discharge patterns
Brain Res.
(1992) - et al.
Electrophysiological study of dorsal respiratory neurons in the medulla obongata of the rat
Brain Res.
(1994) - et al.
Involvement of the rostral ventrolateral medulla in respiratory rhythm genesis during the peri-natal period: an in vitro study in newborn and fetal rats
Dev. Brain Res.
(1994) - et al.
Brainstem connections of the rostral ventral respiratory group of the rat
Brain Res.
(1990) - et al.
Excitation and inhibition of medullary inspiratory neurons by two types of burst inspiratory neurons in the cat
Neurosci. Lett.
(1989) - et al.
Distribution of medullary respiratory neurons in the rat
Brain Res.
(1988) - et al.
Location and axonal projection of early-onset decrementing expiratory neurons in the cat
Neurosci. Lett.
(1993) - et al.
The role of inhibitory amino acids in control of respiratory motor output in an arterially perfused rat
Respir. Physiol.
(1992) Antidromic activation of neurons as an analytical tool in the study of the central nervous system
J. Neurosci., Methods
(1981)- et al.
Origin of the expiratory inhibition of nucleus tractus solitarius inspiratory neurones
Brain Res.
(1983)
Mechanisms of respiratory rhythm generation changes profoundly during early life in mice and rats
Neurosci. Lett.
Viscerotopic representation of the upper alimentary tract in the medulla oblongata in the rat: the nucleus ambiguus
J. Comp. Neurol.
Expiratory neurons of the Bötzinger complex in the rat: a morphological study following intracellular labelling with biocytin
J. Comp. Neurol.
Discharge patterns of brain-stem respiratory neurons during Hering–Breuer reflex evoked by lung inflation
J. Neurophysiol.
Brainstem network controlling descending drive to phrenic motoneurons in rat
J. Comp. Neurol.
The propriobulbar respiratory neurons in the cat
Exp. Brain Res.
Decrementing expiratory neurons of the Bötzinger complex: II. Direct inhibitory synaptic linkage with ventral respiratory group neurons
Exp. Brain Res.
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