Date of Award
Spring 3-31-2026
Document Type
Honors Thesis
Department/Major
Basic Biomedical Science
First Advisor
Brian Burrell
Second Advisor
Cliff Summers
Third Advisor
Bernard Wone
Keywords
injury, stress, behavioral modulation, medicinal leech
Abstract
Many studies have focused on trying to discover the fundamental processes of pain and stress. This is rather difficult for pain especially, since pain-relate behaviors include multiple components of which some are emotional while others are sensory. In this study, we use an operant conditioning assay and phototaxis assay to determine the behavioral effects of injury and stress within Hirudo verbana. Operant conditioning involved placing the animals in a two-chamber arena, with one chamber being illuminated and the other being completely dark with a pea gravel barrier in the connecting section of the arena. Normal animals improve their latency over 5 trials of training and retain this memory for 2 hours. Injured animals exhibit no learning or memory across 5 trials as their escape response is already sensitized. However, their capacity learning this task and and retaining their memory recovered by day 12 post injury. Upon looking at multiple factors, it was discovered that the animal’s negative phototaxis was being increased and causing the decrease in escape latency. Therefore, we looked at how hunger and differing stressors affect the animal’s behavior in the operant conditioning and phototaxis assay. In normal animals, those that underwent 1 and 5 min of drying stress had low escape latency with no change in latency between two trials separated by 30 mins. However, the animals that underwent 3 minutes of stress had a higher escape latency and had a significant decrease in escape latency between the 2 trials, possibly indicating improved learning of how to escape the lit chamber the second time. Interestingly enough, when unfed animals underwent drying stress, all groups had high escape latencies, and it appeared that hunger masked the effects seen previously with drying stress. Also, when unfed animals underwent operant conditioning, they had a plateau of significantly higher escape latencies for trials 1-5 with no memory; however, their learning and memory reappear by 4 weeks post feeding. Altogether, this study indicates that injury and stress enhance finding shelter by enhancing negative phototaxis. Hunger on the other hand seems to diminish the need to shelter and blunts the effects of stress on enhancing negative phototaxis.
Recommended Citation
Dockendorf, Ella, "MODULATION OF BEHAVIOR IN THE MEDICINAL LEECH DUE TO INJURY AND STRESS" (2026). Honors Thesis. 414.
https://red.library.usd.edu/honors-thesis/414