Role of the cardiac nerve in the effect of a novel innocuous stimulus on the heart rate of Megalobulimus mogianensis

Abstract

The effects of a brief jet of water delivered to the anterior portion of body-head on the heart rate of Megalobulimus mogianensis were determined in a group of intact snails (N = 8), previously prepared for electrocardiogram recording. The heart rate was significantly increased following stimulation. Nevertheless, with repetition of the stimulus there was a significant decrease in the magnitude of the heart rate variation and in the time for the basal heart rate to recover (first stimulus, 7.4 ± 1.2 bpm and 15.5 ± 1.8 min; second stimulus, 4.8 ± 1.0 bpm and 10.6 ± 1.5 min; third stimulus, 5.0 ± 0.3 bpm and 11.1 ± 1.8 min), indicating that this behavioral response undergoes early habituation. To determine the role of the cardiac nerve in mediating the heart rate alterations induced by the jet of water two other groups were tested: denervated animals (N = 8) and sham-operated control animals (N = 8). Although the innocuous stimulus caused the heart rate to increase significantly in both experimental groups, the mean increase in heart rate in denervated animals (3.2 ± 0.4 bpm) was 41% of the value obtained in sham-operated animals (7.8 ± 1.5 bpm), indicating that the cardiac nerve is responsible for 59% of the cardioacceleration induced by the innocuous stimulus. The increase in heart rate observed in denervated animals may be due to an increase in venous return promoted by the intense muscular activity associated with the retraction-protraction of the anterior part of the body induced by the jet of water. Correspondence

Role of the cardiac nerve in the adaptive changes of heart rate in response to an aversive stimulus in Megalobulimus mogianensis

“…In contrast, Romero and Hoffmann Aversive stimulus, heart rate and cardiac denervation www.bjournal.com.br (14) showed that repetition of an innocuous stimulus induces a gradual decrease in the magnitude of the heart rate increase in a series of three stimulations. Their findings indicate that this behavioral response undergoes early habituation, a form of learning common to all animals (7).…”

“…These studies have also established the relative contributions of intrinsic and neural control to heart rate (2,3,12,13). Recently, we described the effects of a brief jet of water delivered to the anterior portion of the body-head on the heart rate of M. mogianensis (14).…”

“…Several animals tended to become inactive or retract into their shell after stimulation. It was necessary to spray these animals with water to keep them active or make them emerge from the shell between stimulations (14). To prevent probable anticipatory cardiac responses, a variable interval between stimulations corresponding to the time until basal heart rate recovery was employed.…”

“…It is well-known that mollusk cardiac nerves, which originate from the visceral ganglion, contain inhibitory as well as excitatory fibers. In their work on intact specimens of M. mogianensis, Romero and Hoffmann (14) showed that the delivery of the first innocuous stimulus (a jet of water) induced a heart rate increase of 7.4 ± 1.2 bpm associated with a recovery time of 15.5 ± 1.8 min. Our present data showed a similar increase in the heart rate of snails submitted to an aversive thermal stimulus (8.7 ± 1.6 bpm) but a faster recovery of the basal heart rate (5.7 ± 0.9 min).…”

“…A jet of water, similar to the rain after the drought, has an alerting effect. It induces an increase in the general activity level, which is reflected in the tendency of snails to accelerate their movements after the withdrawal reaction (14). On the other hand, the aversive thermal stimulus may be similar to contact with a hot rock surface (20); if a stimulus of this nature persists, it may threaten the survival of a cold-blooded animal.…”