Tonically rhythmic neurons within a cardiorespiratory region of the nucleus tractus solitarii of the rat.

1. Accumulated evidence from the literature led us to investigate whether centrally generated activity was present within a central neuronal network for cardiovascular control. An in vitro approach using a brain stem slice preparation was employed to study the cardiorespiratory region of the nucleus of the solitary tract (NTS) in the rat. 2. We have discovered rhythmically active neurons within a restricted part of the cardiorespiratory NTS. These neurons were localized to regions directly medial and dorsomedial to the solitary tract (ts) at levels 0.2 mm rostral to obex extending caudally to the rostral part of the commissural subnucleus, an area considered to be concerned with cardiovascular regulation. Although other subnuclei were explored for neurons with ongoing activity (i.e., dorsolateral, dorsal, and interstitial) at levels 1.5 mm caudal to 0.75 mm rostral to obex, we failed to find similarly tonically active cells. 3. Intra- or extracellular recordings were made from 85 neurons with a mean firing rate of 5.1 +/- 0.3 (SE) Hz (range 1-15). The majority of these (n = 75) received an excitatory synaptic input from the ipsilateral ts, with latencies ranging between 4 and 20 ms. 4. To determine whether the tonically rhythmic cells were dependent on synaptic excitatory drives or were inherent to the cell, we tested, in 45 neurons recorded extracellularly, the effect of blocking synaptic inputs mediated by excitatory amino acids by applying either DL-2-amino-5-phosphonovaleric acid [APV; N-methyl-D-aspartate (NMDA) antagonist] or MK-801 (NMDA antagonist) with kynurenic acid (Kyn; NMDA, quisqualate, and kainate receptor blocker) to the bath. After bath application of APV and Kyn or MK-801 and Kyn, two different responses were observed. In 19 cells ongoing rhythmic activity was unperturbed, but firing was completely silenced in 26 neurons. In all cases neurons failed to respond to glutamate delivered locally, and the synaptic input evoked from the ts was blocked. This evidence indicates the existence of two cell types: autoactive (AA) or pacemaker-like neurons, the discharge pattern of which depends on intrinsic properties, and synaptically driven (SD) neurons, the activity of which is driven by synaptic inputs. 5. Cobalt chloride (Co) was used to block synaptic effects and was found to increase the discharge rate of AA neurons by 9.9 Hz on average (i.e., cells resistant to APV and Kyn or MK-801 and Kyn). However, the rhythmic activity of cells previously silenced with excitatory amino acid antagonists (i.e., SD cells) was also abolished in the presence of Co.(ABSTRACT TRUNCATED AT 400 WORDS)