RESPONSES OF NEURONS IN THE NUCLEUS-TRACTUS-SOLITARIUS TO STIMULATIONOF HEART AND LUNG RECEPTORS IN THE RAT

To characterize central integration of reflex responses to stimulation of mechanically and chemically sensitive receptors in the heart and l ung, male rats (350 to 425 g) were anesthetized (pentobarbital, 50 mg/ kg IF) and paralyzed (gallamine triethiodide, 25 mg/kg IV), and then t hey underwent bilateral sinoaortic denervation. Extracellular activity of neurons in the nucleus tractus solitarius (NTS) was recorded in re sponse to bolus intra-atrial saline (50, 100, 200, or 300 mu L) or phe nylbiguanide (PBG, 16 mu g/kg in 100 mu L) administered in random sequ ence. Changes in mean arterial pressure (MAP), mean right atrial press ure, and right atrial systolic pressure (RASP) were measured as correl ates of stimulus intensity, and heart rate (HR) and renal sympathetic nerve activity (RSNA) were used to assess efferent reflex effects of c ardiac and pulmonary receptor stimulation. NTS neurons with possible a fferent input from stretch and chemically sensitive receptors were ide ntified by an excitatory evoked response to electrical stimulation of the ipsilateral vagus nerve (1 Hz, 500 mu A, 1-millisecond duration). Thirty-eight vagus nerve-evoked NTS units with onset latencies of 25.3 +/-0.9 milliseconds displayed excitatory or inhibitory responses to sa line or PBG injections or to both interventions. Saline administration elicited volume-dependent transient increases in MAP and RASP, which were followed by reflex decreases in MAP, HR, and RSNA. PBG injections also evoked hypotension, bradycardia, and sympathoinhibition. In cont rast to the graded effects of graded saline injections on MAP, right a trial pressure, HR, and RSNA, the mean change in peak NTS cell activit y after saline injections did not correlate with the volume injected; rather, the cells responded at one volume threshold and did not alter firing when stimulus intensity (ie, the volume injected) was increased . Although the number of NTS units with firing thresholds at the respe ctive volumes was fairly evenly distributed, the total number of cells responding at any given volume was greater as the volume injected inc reased. Thus, the graded reflex effects observed during volume expansi on may involve recruitment of a progressively greater number of respon sive neurons and not graded increases in the discharge frequency of an y particular NTS cell. These data indicate a high degree of neural int egration in the NTS, which may be critical for the reflex adjustments to stimulation of mechanically and chemically sensitive receptors.