RESPIRATORY-RELATED DISCHARGE PATTERNS OF CAUDAL RAPHE NEURONS PROJECTING TO THE UPPER THORACIC SPINAL-CORD IN THE RAT

Sympathetic activity is modulated by central respiratory drive. Bulbos pinal neurones arising in the ventrolateral medulla and A5 region prob ably contribute to this modulation. In the present investigation the i nvolvement of caudal raphe-spinal neurones in relaying respiratory-rel ated inputs to sympathetic preganglionic neurones was investigated. Ex periments were carried out on anaesthetized, vagotomized, paralysed an d artificially ventilated rats. Extracellular recordings were made fro m the cell bodies of 53 caudal raphe neurones activated antidromically by stimulating the spinal cord between T1 and T3. Their axonal conduc tion velocities ranged from 0.7-9.1 m/s (median = 3.8 m/s). Thirty-six of 53 neurones (consisting of neurones with on-going activity and qui escent neurones activated with glutamate) were held long enough for de tailed analysis. Of those recorded 26 were in the region of raphe obsc urus, nine in raphe pallidus and one in raphe magnus. Twenty-five of 3 6 neurones had firing patterns related to phrenic nerve discharge. Of the four firing patterns defined: seven neurones had the highest proba bility of firing during inspiration (inspiratory-related), 10 neurones had the highest probability of firing during expiration (expiratory-r elated), 3 had the highest probability of firing during post-inspirati on (post-inspiratory-related) and 5 had lowest levels of firing during early- and post-inspiratory phases (early and post-inspiratory depres sed). Of 27 neurones with axonal projections through or to the region of the intermediolateral cell column in the upper thoracic cord 19 had a respiratory-related discharge pattern. For respiratory-modulated ne urones with on-going activity the median of the modal inter-spike inte rvals was 0.08 s. None of the neurones had an ECG-related firing patte rn. The findings of this study also indicate a species difference betw een rats and cats regarding the physiological properties of some raphe -spinal neurones; i.e., an absence of ECG-related activity in the rats . The characteristics of the neurones recorded in this study are not t hose of 'typical' 5-HT-containing neurones with reference to axonal co nduction velocities and discharge characteristics.