Human Pavlovian HR-decelerative conditioning with negative tilt as US: evidence of vagal and sympathetic influences on the UR in dogs.

Following a review of studies employing negative tilt in human Pavlovian conditioning of heart rate (HR) deceleration (Furedy et al, in press), this paper reports data based on animal subjects on such physiological aspects of the decelerative unconditioned response (UR) as the degree of vagal involvement. Five anesthetized dogs underwent 90 degrees negative body tilts pre- and postbilateral vagotomy, while interbeat interval (IBI), left ventricular pressure (LVP) and its first derivative, d(LVP)/dt, which is a measure of sympathetic cardiac drive, were recorded. Consistent with the vagal interpretation of the tilt-induced decelerative UR, the results indicated that vagotomy markedly changed the tilt-induced bradycardic reflex from a fast-recruiting, large-magnitude (over 45%), and sustained (throughout the 20-27-s tilt) IBI increase, to slower-recruiting, and markedly smaller (less than 5%) IBI increase. However, there was also evidence of an initial sympathetic excitation of about 5 s, as indicated by a 45% increase in d(LVP)/dt, which returned to baseline level by 9 s following tilt onset. Vagotomy increased this tilt-induced sympathetic excitation to about 100%, and it remained at above 70% throughout the tilt. Prevagotomy LVP showed a slight (about 10%) and delayed (about 6 s following tilt onset) depressor response, which was eliminated by vagotomy. Finally, unaveraged data from individual dogs suggested that prevagotomy, LVP changes preceded IBI changes. Regarding implications of these results for human HR deceleration-inducing preparations, we conclude that the different physiological mechanisms that accompany and/or produce a given change in HR need continuing investigation with multiple dependent physiological variables (which are assessed for topographical differences), and in both human and animal preparations.