Baroreceptor involvement in the immobility reflex of the frog: Evidence for a cross-species mechanism

Abstract

Baroreceptor activity has been implicated in the initiation of the immobility reflex in rabbits (Hatton, Lanthorn, Webster, & Meyer, 1978). In order to test the generality of the effect to other species, frogs, with the carotid labyrinth denervated or the glossopharyngeal nerve sectioned, were tested for the immobility reflex. The results indicate that sectioning of the glossopharyngeal nerve significantly reduced the incidence of immobility relative to controls, while frogs with the carotid labyrinth denervated were intermediate in suscept­ ibility to immobility in comparison to sectioned and control animals. The immobility reflex shares a number of common behavioral and physiological characteristics with the baroreceptor reflex. As pointed out by Gellhom (1967), increased pressure in the sino-aortic area or stimulation of the sinus nerve leads to a fall in blood pressure and heart rate, a loss in tone of striated muscle, and a tendency to sleep. In addition, spindling occurs in the cortical EEG and there is a reduced responsiveness to external stimuli. These effects are not present after denervation of the sino-aortic area. Likewise, during the immobility reflex in rabbits, there is marked hypoten­ sion of the muscles (Carli, 1969) and an initial decrease in blood pressure and heart rate, followed by a rapid increase and gradual decrease in these values over the course of the immobility episode (Carli, 1974; Hatton, Note 1); there is also a reduced responsiveness to external stimuli, and synchronization occurs in the corti­ cal EEG (Gerebtzoff, 1941; Hatton, Woodruff, & Meyer, 1975). As with the baroreceptor reflex, the incidence of occurrence of immobility is significantly reduced in rabbits following denervation of the carotid sinus or sectioning of the sinus nerve (Hatton et al., 1978). The present study was designed to examine the generality of the effect of denervation of the carotid sinus on the immobility reflex in another species.