Characteristics of caudal ventrolateral medullary neurons antidromically activated from rostral ventrolateral medulla in the rabbit.

We made extracellular recordings from 107 spontaneously active neurons in the caudal ventrolateral medulla, after identifying the cells by antidromically activating them from the rostral ventrolateral medulla, in urethane-anesthetized rabbits. We tested the response of these neurons to inputs from baroreceptors and chemoreceptors. The median conduction velocity for antidromically activated neurons was 0.84 m/s. Raising blood pressure with intravenous noradrenaline excited 22% of 96 neurons tested, inhibited 61%, and had no effect on the remaining 17%. The spontaneous discharge rate of neurons excited by an increase in blood pressure was 1.6 +/- 0.3 spikes/s, lower than the discharge rate of neurons inhibited by this procedure (4.9 +/- 0.5 spikes/s). Excitation of chemoreceptors by hypoxia increased the discharge rate of 14/16 neurons tested in the group excited by baroreceptor inputs. In the group inhibited by baroreceptor inputs 21/35 neurons tested were excited and 12/35 neurons were inhibited by chemoreceptor inputs. Neurons excited by an increase in blood pressure were located in the previously defined caudal vasodepressor region and in a region just rostral to the obex, intermediate between the vasodepressor region and the rostral sympathoexcitatory region. These neurons may form part of the central inhibitory link in the baroreceptor-vasomotor pathway. Other antidromically activated neurons in the vasodepressor region may be inhibitory vasomotor cells with a function relatively independent of baroreceptor inputs, or they may be A1 catecholamine neurons, with axons passing through the rostral medulla en route to the forebrain.