Amphibian Prey-catching Behavior: a Behavioral Model for Studying the Role of Stress Hormones in Learning
James A. Carr
Department of Biological Sciences
Texas Tech University, Lubbock, TX
Performing an appropriate behavior (prey catching, for example) in response to a stimulus (moving worm) can be a life and death matter for animals living in their natural environment. Restraining behavioral responses to inappropriate stimuli (such as a moving leaf or twig) can be equally as important. Although behavioral responses such as prey-catching require both motor and sensory components of the nervous system, the endocrine status of an organism plays a critical role in modulating the effectiveness of various stimuli to elicit the behavior. This is particularly true during stress. Stress hormones not only prepare an organism's internal environment for confrontation but have profound impacts on how behavioral stimuli are interpreted.
We have been studying the effects of stress-related hormones on a simple form of learning associated with prey-catching behavior in toads. Exposure to a moving prey item elicits a sequence of behavior reactions that include orienting toward the prey-item. If the orienting movements are continuously released by the same prey-stimulus over a period of time the animal eventually habituates to the stimulus. This is an attractive model for studying hormonal control of visuomotor processing because of the simplicity of the toad brain. My laboratory has shown that peptides related to melanocyte-stimulating hormone (MSH) facilitate the acquisition of habituation in this model. A physiological role for these peptides is suggested by the distribution of melanocortin nerve terminals in visuomotor areas of the toad brain. Corticotropin-releasing hormone, which regulates ACTH secretion during stress, also appears to influence prey-catching behavior by reducing the motivation of toads to respond to a moving prey-item. Thus, changes in endocrine status during stress appear have complex effects on prey-catching behavior, influencing both motivation and learning. Melanocortins, by reducing the effectiveness of non-relevant stimuli to release prey-catching, may reduce exposure to potential predators, a role similar to the one they play in background adaptation. Together, CRH and melanocortins may be important in helping animals gather and process information about their environment during stress.