Vagal cardioinhibitory area in and around the caudal inferior olivary nucleus of cats. I. Localization and connection.

Bradycardia was produced by electrical stimulation and injection of glutamate (200 nl, 1 M) to the regions in and around the extremely caudal parts of the inferior olivary nucleus (ION) in chloralose-urethane anesthetized cats. Either medullary transection at 4 mm rostral to the obex, spinal transection at C2-3 or both transections together did not abolish the ION-induced bradycardia. Bradycardia induced by stimulating one ION was slightly reduced and completely abolished by unilateral and bilateral vagotomies, respectively. The ION-evoked bradycardia was also slightly reduced by a raphe lesion. This lesion together with unilateral vagotomy abolished only the bradycardia induced from the ION on the vagotomized side. In 28 left vagotomized cats, the left ION-induced bradycardia was subjected to the effects of destroying the right ION, dorsal motor/solitary nucleus (DM/SN) and ambiguous nucleus (AN). The bradycardia was completely abolished by ION lesion (7/7), greatly reduced (7/13) or completely abolished (6/13) by DM/SN lesion, and slightly increased (5/8) or decreased (3/8) by AN lesion. We concluded that cells and/or fibers in and around the ION may project fibers to the ipsilateral DM/SN and through the raphe nucleus to the contralateral ION which may send fibers to the other DM/SN.