Serotonin and NMDA glutamate receptor antagonists selectively impair the reactivation of associative memory in the common snail.

Experiments on the common snail were performed to study the influences of serotonin and glutamate receptor antagonists on the processes of reactivation of an associative habit consisting of refusing a particular type of food. Twenty-four hours after training, animals were injected with the non-selective serotonin receptor antagonist methiothepin (0.1 mg/snail) or the NMDA glutamate receptor antagonist MK-801 (0.005 mg/snail), after which they were presented with a "reminder" stimulus (the "conditioned reflex" foodstuff, a banana) and tested for retention of the habit. Three hours after antagonist injections and the "reminding" procedure, snails showed impairments in the reproduction of the acquired habit, which persisted for more than two weeks. Furthermore, animals with amnesia after treatment with methiothepin/reminding showed facilitation of repeated acquisition of the aversive habit to banana. Repeat training of animals which had shown amnesia after MK-801/reminding did not result in acquisition of the habit. It is suggested that serotonin receptors are involved in the mechanisms underlying extraction of the memory trace of the aversive habit to the foodstuff in snails, while NMDA glutamate receptors are involved in memory trace storage processes.