We investigated the influence of the protein synthesis blocker anisomycin on contextual memory in the terrestrial snail Helix. Prior to the training session, the behavioral responses in two contexts were similar. Two days after a session of electric shocks (5 d) in one context only, the context conditioning was observed as the significant difference of behavioral response amplitudes in two contexts. On the day following testing of context learning, a session of "reminding" was performed, immediately after which the snails were injected with anisomycin or vehicle. Testing of long-term context memory has shown that only anisomycin injections impaired the context conditioning. In control series, the snails were injected after the training session with anisomycin/saline without reminding, and no impairment of the long-term context memory was observed, while injection of anisomycin during the training session completely abolished the long-term memory. No effects of anisomycin on the short-term memory were observed. Surprisingly, injection of anisomycin after the reminding combined with reinforcing stimuli elicited no effect on the context memory. Differences between single-trial and multisession learning are discussed.
Membrane mechanisms of conditioning of the defensive reflex in the snails Helix pomatia and H. lucorum were investigated. Tapping on the shell was used as a conditioned stimulus, which under normal conditions produces no defensive reaction. A light blow of air into the pneumostome, called the defensive closure reaction, was used as an unconditioned stimulus. When a combination of conditioned and unconditioned stimuli were presented with 2-4 min interval, the reflex developed over a period of 3 days. The separate conditioned and unconditioned stimuli presented randomly were used as an active control. The electrical characteristics of identified interneurons involved in this defensive behavior were then measured in an isolated preparation. There was shown to be a decrease in the threshold of action potential generation from 20.5 to 16.3 mV and depolarizing shift of membrane potential from -62.1 to -57.0 mV. The electrical characteristics of withdrawal interneurons of active control snails did not differ from those in intact animals. All results show an increase in excitability of withdrawal interneurons after associative learning.
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