Neurotransmitter basis of mollusc behavior: control of choice between the orienting and the defense response to the presentation of an unfamiliar object.

The role of the serotoninergic and opioid systems in the choice between a passive defensive and an orienting response of the mollusc Lymnaea stagnalis to the presentation of an unfamiliar object was studied. Under the influence of 5-HTP, a metabolic precursor of serotonin, orienting and investigatory behavior was activated; the number of protective reactions decreased. Under the influence of naloxone, an antagonist of opiate receptors, the proportion of orienting responses decreased, and the number of passive defensive avoidance and freezing reactions increased. The influence of the agents investigated on the attitude of the snail to a novel object was found to be well coordinated with their influence on other forms of behavior and the behavioral state of the mollusc as a whole.