Animals have evolved several chemosensory systems for detecting potentially dangerous foods in the environment. Activation of specific sensory cells within these chemosensory systems usually elicits an aversive behavioral response, leading to avoidance of the noxious foods. Blowflies respond to sugars, salts and water through the activation of specific chemoreceptor neurons in the antennal, labellar and tarsal chemosensillae. These insects also detect deterrent stimuli with the so called fifth or deterrent cell. Using forensically important flies (blowflies) as a model organism, the question was if these flies have the ability to detect the nutritional value of corpses when injected with different doses of morphine. In the attempt to gain information on the mechanisms underlying reception of noxious and repellent compounds, electrophysiological and behavioral experiments have been performed to confirm the hypothesis that morphine sulfate has a repellent effect on fly attraction to corpse. Electrophysiological and behavioral results indicate that morphine sulfate activate the fifth cell in the chemosensillae. In field behavioral test, carrions injected with doses of morphine sulfate, are colonized later with flies than morphine-free carrions. This finding is in accordance with the spike frequency elevation observed for the fifth cell activity. The prevailing activation of the deterrent cell by morphine sulfate is directly coupled with a coherent behavioral output. Therefore, comparison of behavioral and electrophysiological data, affirm that blowfly identify morphine sulfate as a deterrent stimuli by activation of the fifth cell.
