Cardiorespiratory responses following electrical stimulation of caudal sites in the rat medulla

A limited occipital craniotomy was conducted on urethane-anesthetized rats to expose the caudal medulla in the region of the obex. Discrete bipolar electrical stimulation was administered at sites in the dorsal medulla of spontaneously breathing rats in the vicinity of the caudal nucleus tractus solitarius (NTS) and adjacent reticular formation. Cardiorespiratory responses were recorded during microstimulation at three separate stimulating frequencies to examine the functional interaction of cardiovascular and respiratory-related neuronal elements in the NTS. Microstimulation was conducted at sites in the dorsal and medial regions of the NTS beginning at the level of the area postrema and extending posteriorly through the rostrocaudal course of the NTS; microstimulation was also conducted at midline sites in the commissural region of the NTS and the ventral and ventrolateral regions of the caudal NTS. Microstimulation of loci in the reticular formation adjacent to these NTS sites did not elicit any cardiorespiratory responses whereas stimulation of individual NTS regions elicited specific patterns of cardiorespiratory responses. Specifically, microstimulation of the dorsal and medial NTS at the level of the area postrema elicited pressor responses associated with apneic/hypopneic responses whereas stimulation of midline sites in the commissural region, dorsomedial sites caudal to the area postrema and the ventral and ventrolateral areas of the caudal regions of the NTS elicited depressor responses associated with bradycardic and apneic/hypopneic responses. The most profound respiratory effects (i.e., apnea) and heart rate responses (i.e., bradycardia) were seen following stimulation of the ventral and ventrolateral regions of the caudal NTS. These findings support the notion that the caudal NTS is a major site for coordinating cardiorespiratory afferent information in the rat and it is also apparent from this study that specific regions of the caudal NTS demonstrate a functional coexistence of cardiovascular and respiratory-related neurons. Finally, the results from this study showing the regional specificity and frequency-dependent characteristics of cardiorespiratory response patterns elicited by microstimulation suggest that the local microcircuitry and intrinsic neuronal networks in the more caudal regions of the rat NTS are more complex and heterogeneous than hitherto revealed.