EXCITATORY AND DEPRESSANT RESPIRATORY RESPONSES TO CHEMICAL-STIMULATION OF THE ROSTRAL VENTROLATERAL MEDULLA IN THE CAT

The rostral ventrolateral medulla (rVLM) is known to play an important role in cardiorespiratory control. In the rVLM an 'apnoea region', in which unilateral focal blocks induce strong depressant effects on ins piratory activity up to complete apnoea, has been described. This stud y was designed to systematically investigate the effects provoked by u nilateral micro-injections (10-30 nl) of D,L-homocysteic acid 160 mm i nto this region on respiratory activity and arterial blood pressure in pentobarbitone anaesthetized, vagotomized, paralyzed and artificially ventilated cats. Micro-injections into the rostral portion of this ar ea caused depressant respiratory responses up to complete apnoea, whil e micro-injections into more caudally located sites induced excitatory respiratory responses. Similar effects were observed in the activity of phrenic nerves and inspiration-related medullary neurons of both th e dorsal and ventral respiratory group. The respiratory responses coul d be accompanied by marked increases in blood pressure (greater-than-o r-equal-to 30 mmHg), especially at locations ventral to the retrofacia l and facial nucleus; however, they could also occur in the absence of appreciable changes or even in association with slight decreases in b lood pressure. Similar respiratory and pressor effects were observed i n carotid sinus denervated cats. The results indicate that two distinc t rVLM neuronal populations, one located more rostrally and the other more caudally, may have an important role in the genesis and/or mainte nance of respiratory rhythm by exerting respectively inhibitory and ex citatory influences on inspiratory activity. Furthermore, they support the hypothesis that different neural substrates of the rVLM are invol ved in the regulation of respiratory and cardiovascular functions.