Behavioral effects of opiates on the land snail Helix aspersa.

The terrestrial snail Helix aspersa appears to have an endogenous opiate system involved with regulating its locomotor activities as well as its responses to tactile stimulations. In animals which were repeatedly injected with the same dose of morphine, tolerance developed. Injection of the opiate antagonist, naloxone, resulted in a gradual reversal of the morphine induced behavioral characteristics. In addition, the effect of morphine on locomotor activity can be considered biphasic. At higher doses morphine induced a state of muscular rigidity in the organisms whereas at lower doses the animals were hyperactive.