The effects of protein synthesis inhibitors on the reactivation of an associative skill consisting of refusing a particular food by common snails were studied. Animals were given single injections of a protein synthesis inhibitor (cycloheximide at 0.6 mg/snail or anisomycin at 0.4 mg) 24 h after three days of training, and were then presented with a "reminding" stimulus (the "conditioned reflex" food-banana) and tested for retention of the skill. Observations revealed an impairment of reproduction of the acquired skill 2.5 h after the "reminder," with spontaneous restoration at 4.5-5.5 h. Other snails were given single 1.8-mg doses of cycloheximide or three 0.6-mg doses with intervals of 2 h. "Reminders" were presented after each injection. In these conditions, impairment of reproduction of the conditioned reflex also appeared 2.5 h after the first "reminder," though amnesia lasted at least 30 days and repeat training of the animals produced only partial recovery of the skill. Thus, we have provided the first demonstration that recovery of a long-term memory "trace" on exposure to relatively low doses of protein synthesis inhibitors produces transient and short-lived amnesia, lasting 2-3 h, while long-term, irreversible amnesia occurs after longer-lasting or more profound suppression of protein synthesis. These results suggest that the "reminding" process induces reconsolidation of the " initial" memory, suppression of which by protein synthesis inhibitors leads to "erasure" of the memory "trace" and impairs consolidation on repeat training.
Experiments on defensive behavior command neurons in common snails showed that synaptic facilitation in the responses of nerve cells to sensory stimulation occurs 50-60 min after the onset of application of serotonin (10 microM) to the CNS. The properties of neuron electrogenic membranes (membrane potential, membrane excitability) did not change after exposure to serotonin. Along with synaptic facilitation, serotonin (100 microM) increased the excitability and produced minimal depolarization of the membranes of command neurons. Serotonin had selective effects on the reactions of neurons to different sensory stimuli: facilitation of neuron responses to tactile stimulation of the head lasted 1 h, while responses to application of dilute quinine solution lasted 2-3 h; serotonin facilitated neuron responses to tactile stimulation only of the snail's head, and did not alter the responses to stimulation of the foot or the mantle ridge. The time course of the electrophysiological effects of serotonin coincided with changes in bound calcium (Cab) levels in command neurons. This set of serotonin-induced neurophysiological effects is similar to the effects resulting from the development of nociceptive sensitization. It is suggested that serotonin is involved in the mechanisms of transient changes and consolidation of long-term plastic rearrangements in command neurons which underlie sensitization.
Changes in defense and alimentary behavior as well as in the reactions of command and motor neurons of these types of behavior during the development of sensitization were investigated in edible snails. Following a one-time exposure of the head of the mollusc to a 50% solution of quinine, brief (50-70 min) and prolonged (hours or days) facilitation of the defense reactions of the animals and of the responses of the command neurons of defense behavior in response to tactile and chemical sensory stimuli was discovered. The alimentary behavior of the snails and the reactions of the modulatory neurons of alimentary behavior in response to the presentation of carrot juice was suppressed in sensitized snails. Differences were observed in the dynamics of defense responses to tactile and chemical sensory stimulations in one and the same sensitized animals. Brief facilitation of the responses during sensitization correlated primarily with the depolarization of the membrane potential of the neurons of defense behavior and with an increase in excitability of the plasma membrane. Prolonged facilitation of the responses was determined primarily by change in the efficiency of synaptic transmission. The described model of the development of sensitization may be the basis for the study of the molecular and cellular mechanisms underlying learning.
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