Identification of central nervous system neurons innervating the respiratory muscles of the mouse: a transneuronal tracing study
Introduction
The nervous control of respiration that has been mainly studied in vivo in adult mammals (rats, cats, dogs, rabbits) [2], is now studied in neonatal rodents 7, 8. However, little information is available on the adult mouse 1, 7, 8. Substantial evidence indicates that central nervous system (CNS) regions involved in regulation of behavioral and emotional responses also play a role in the modulation of autonomic functions [2]. From a comparative point of view, it is generally accepted that the respiratory control system possesses a similar anatomical organization among different rodent species 2, 11. However, differences between the rat and mouse respiratory system should be emphasized 7, 8. First, the vagal control of the respiratory rhythm is stronger in the mouse than in the rat [7]. Second, respiratory bursts mainly occur in phrenic roots in mouse preparations, whereas they occur in all the cervical rootlets in rat preparations [7]. Finally, pontine control of the medullary respiratory rhythm generator may differ between the mouse and the rat. Respiratory bursts occur spontaneously in rat pontomedullary preparations, but the pons must be removed in the mouse preparation to obtain similar activity [7]. We have studied the motor and premotor components of the anatomical network of respiratory control in mice by means of a transneuronal labeling technique using rabies virus 1, 6.
Section snippets
Material and methods
In order to characterize the morphology and the location of the respiratory motoneurons and their serially connected chains of neurons, the Challenge Virus Standard of rabies virus was used. This tracer is amplified across the synapses [1] and has the ability to invade cell dendrites as well as somata, thereby labeling all the neuronal arborizations. Experiments were carried out in adult mice of the CH3/HeJ strain, at least 3 months old, and weighing 36 ± 2 g. The animals were anesthetized by a
Results
Labeled neurons were found bilaterally throughout the CNS (x̄ = mean number of labeled cells in each slice and in each experiment). At the spinal level, phrenic (x̄ = 11; range, 8–15) and intercostal (x̄ = 9; range, 8–13) motoneurons, as well as interneurons (x̄ = 6; range, 3–13) were labeled. The cells were organized in clusters of neurons in the ventral horn (Fig. 1A) and were located inside the layers 6–9. However, in one of the experiments, cells were also found in the layers 1–3.
At the
Discussion
These preliminary results show labeled cells related to the control of breathing at different levels of the CNS. However, some of the labeled neurons may originate from virus infection through muscular afferents, because the dorsal roots were intact.
Our findings show labeled phrenic and intercostal motoneurons and interneurons in the spinal cord. The latter may have an important role in respiratory integration. In mammals, it is postulated that the respiratory rhythm is generated by the
Acknowledgements
Supported by the Acciones Integradas Hispano-Francesas/Picasso Program, IFR Sciences du Cerveau and CNRS-ATIPE “Virologie”. The authors wish to thank Dr W. E. Cameron for the comments made to improve the manuscript.
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