High cervical spinal cord injury (SCI) interrupts bulbospinal respiratory pathways innervating phrenic motoneurons, and induces an inactivation of phrenic nerves (PN) and diaphragm. We have previously shown that the ipsilateral (ipsi) PN was inactivated following a lateral C2 SCI, but was spontaneously partially reactivated 7 days post-SCI. This phrenic reactivation depended on contralateral (contra) descending pathways, located laterally, that cross the spinal midline. We analysed here whether long-term post-lesional changes may occur in the respiratory network. We showed that ipsi PN reactivation was greater at 3 months compared with 7 days post-SCI, and that it was enhanced after acute contra phrenicotomy (Phx), which also induced a substantial reactivation of the ipsi diaphragm (not detected at 7 days post-SCI). At 3 months post-SCI (compared with 7 days post-SCI), ipsi PN activity was only moderately affected by ipsi Phx or by gallamine treatment, a nicotinic neuromuscular blocking agent, indicating that it was less dependent on ipsi sensory phrenic afferents. After an additional acute contra SCI (C1) performed laterally, ipsi PN activity was abolished in rats 7 days post-SCI, but persisted in rats 3 months post-SCI. This activity thus depended on new functional descending pathways located medially rather than laterally. These may not involve newly recruited neurons as retrograde labelling showed that ipsi phrenic motoneurons were innervated by a similar number of medullary respiratory neurons after a short and long post-lesional time. These results show that after a long post-lesional time, phrenic reactivation is reinforced by an anatomo-functional reorganization of spinal respiratory pathways.

Heart rate (HR) was continuously monitored during successive 24-hr periods in 19 healthy subjects and 26 major depressed patients (DSM III-R). Recordings were performed after a 2-week wash-out period and the morningness or eveningness typology of each subject was determined. The chronobiological parameters and rhythm percentage (RP) were calculated by the single cosinor method from the smoothed HR curves of each subject. In normal subjects, HR follows a circadian rhythm (RP greater than 65%) with the lowest values at night. Morning type subjects have an earlier peak time (13:30) than evening type subjects (17:30). Major depressive patients were split into two groups; in the first one HR circadian rhythm was still present (RP greater than 63%) with a decrease in amplitude (24%) while in the second group, no circadian rhythm of HR could be detected (RP less than 25%, decrease in amplitude greater than 70%). In the group of patients with a persisting HR circadian rhythm, no veritable phase advance was observed. Our results suggest that circadian HR rhythm, which can be easily studied with non-invasive methods, might represent a chronobiological marker of some depressions. Given the lag that exists between the rhythms of morning type and evening type subjects, our study also stresses the importance of taking into account this behavioural trait in chronobiological studies.

Heart rate (HR) was continuously monitored during successive 24-hr periods in 19 healthy subjects and 26 major depressed patients (DSM III-R). Recordings were performed after a 2-week wash-out period and the morningness or eveningness typology of each subject was determined. The chronobiological parameters and rhythm percentage (RP) were calculated by the single cosinor method from the smoothed HR curves of each subject. In normal subjects, HR follows a circadian rhythm (RP greater than 65%) with the lowest values at night. Morning type subjects have an earlier peak time (13:30) than evening type subjects (17:30). Major depressive patients were split into two groups; in the first one HR circadian rhythm was still present (RP greater than 63%) with a decrease in amplitude (24%) while in the second group, no circadian rhythm of HR could be detected (RP less than 25%, decrease in amplitude greater than 70%). In the group of patients with a persisting HR circadian rhythm, no veritable phase advance was observed. Our results suggest that circadian HR rhythm, which can be easily studied with non-invasive methods, might represent a chronobiological marker of some depressions. Given the lag that exists between the rhythms of morning type and evening type subjects, our study also stresses the importance of taking into account this behavioural trait in chronobiological studies.

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