SIMULTANEOUS ENCODING OF CAROTID-SINUS PRESSURE AND DP DT BY NTS TARGET NEURONS OF MYELINATED BARORECEPTORS/

1. We seek to understand the baroreceptor signal processing that occur s centrally, beginning with the transformation of the signal at the fi rst stage of processing. Because quantitative descriptions of the enco ding of mean arterial pressure and its derivative with respect to time by baroreceptive second-order neurons have been unavailable, we chara cterized the responses of nucleus tractus solitarius (NTS) neurons tha t receive direct myelinated baroreceptor inputs to combinations of the se two stimulus variables. 2. In anesthetized, paralyzed, artificially ventilated rabbits, the carotid sinus was vascularly isolated and the carotid sinus nerve was dissected free from surrounding tissue. Singl e-unit extracellular recordings were made from NTS neurons that receiv ed direct (with the use of physiological criteria) synaptic inputs fro m carotid sinus baroreceptors with myelinated axons. The vast majority of these neurons did not receive ipsilateral aortic nerve convergent inputs. With the use of a computer-controlled linear motor, a piecewis e linear pressure waveform containing 32 combinations of pressure and its rate of change with respect to time (dP/dt) was delivered to the i psilateral carotid sinus. 3. The average NTS firing frequency during t he different stimulus combinations of pressure and dP/dt was a nonline ar and interdependent function of both variables. Most notable was the ''extinctive'' encoding of carotid sinus pressure by these neurons. T his was characterized by an increase in firing frequency going from lo w to medium mean pressures (analyzed at certain positive dP/dt values) followed by a decrease in activity during high-pressure stimuli. All second-order neurons analyzed had their maximal firing rates when dP/d t was positive. 4. All neurons had their maximal firing frequency loca tions (''receptive field centers'') at just 3 of 32 possible pressure- dP/dt coordinates. The responses of a small population of neurons were used to generate a composite description of the encoding of pressure and dP/dt. When combined as a composite of individually normalized val ues, the encoding of carotid sinus pressure and dP/dt may be approxima ted with the use of two-dimensional Gaussian functions. 5. We conclude that the population of NTS neurons recorded most faithfully encodes t he rate and direction of (mean) pressure change, as opposed to providi ng the CNS with an unambiguous encoding of absolute pressure. Instead, the activity of these neurons, individually or as a population, serve s as an estimate for the first derivative of the myelinated barorecept or signal's encoding of mean pressure. We therefore speculate that the output of these individual neurons is useful in dynamic, rather than static, arterial pressure control.