Central Respiratory Circuits That Control Diaphragm Function in Cat Revealed by Transneuronal Tracing

Previous transneuronal tracing studies in the rat and ferret have identified regions throughout the spinal cord, medulla, and pons that are synaptically linked to the diaphragm muscle; however, the extended circuits that innervate the diaphragm of the cat have not been well defined. The N2C strain of rabies virus has been shown to be an effective transneuronal retrograde tracer of the polysynaptic circuits innervating a single muscle. Rabies was injected throughout the left costal region of the diaphragm in the cat to identify brain regions throughout the neuraxis that influence diaphragm function. Infected neurons were localized throughout the cervical and thoracic spinal cord with a concentration of labeling in the vicinity of the phrenic nucleus where diaphragm motoneurons are known to reside. Infection was also found throughout the medulla and pons particularly around the regions of the dorsal and ventral respiratory groups and the medial and lateral reticular formations but also in several other areas including the caudal raphe nuclei, parabrachial nuclear complex, vestibular nuclei, ventral paratrigeminal area, lateral reticular nucleus, and retrotrapezoid nucleus. Infection was also localized in fastigial and dentate deep cerebellar nuclei. Additionally infected neurons were observed in the midbrain, particularly in the periaqueductal grey matter and mesencephalic reticular nucleus but also in other regions including the pedunculopontine nucleus, cuneiform nucleus, Edinger-Westphal nucleus, nucleus v locus coeruleus, and the red nucleus. Diencephalic labeling was most concentrated within the regions of the perifornical area and the medial parvocellular division of the paraventricular nucleus of the hypothalamus but was also found in several other areas. Infected neurons were also observed in the cruciate sulcus of the cerebral cortex as well as in prefrontal and insular cortices. This data suggests that the central circuits innervating the diaphragm muscle in the cat are highly complex and integrated with the central circuits controlling a variety of other non-respiratory behaviors.