A fall in arterial blood pressure produced by inhibition of the caudalmost ventrolateral medulla: the caudal pressor area.

The caudal edge of the ventrolateral medulla was mapped to localize sites where microinjections of L-glutamate (L-glu) produce pressor responses in paralyzed and artificially ventilated urethane-anesthetized rats. Pressor responses ranging from 15 to 65 mmHg were obtained when L-Glu (0.25 M, 200 nl) was microinjected in the ventral medullary surface within an area localized between the rootlets of the XII and first cervical nerves, lateral to the pyramids and just medial to the spinal roots of the XI cranial nerve. This area has been called the caudal pressor area (CPA). Inhibition of the CPA by microinjection of GABA or glycine resulted in marked falls (15-45 mmHg) of arterial blood pressure (AP). Hypotension in response to CPA inhibition was also obtained in unanesthetized decerebrate animals. Cardiovascular responses to CPA stimulation or inhibition depend on the activity of neurons in the rostral ventrolateral medulla (RVLM). During hypotension provoked by RVLM inhibition, pressor responses to CPA stimulation were abolished. Conversely, pressor responses to RVLM stimulation were maintained during hypotension produced by inhibition of CPA. Pressor response to bilateral carotid occlusion were not reduced by CPA inhibition. We conclude that cells in the caudal most ventrolateral medulla exert a tonic pressor activity that contributes to maintenance of basal levels of the vasomotor tone and arterial blood pressure, its inhibition, however, does not prevent the pressor response to carotid occlusion.