Consolidation of memory for odor-reward association: beta-adrenergic receptor involvement in the late phase

Abstract

Experimentally naive rats can learn rapidly to discriminate among three odors to obtain food reinforcement. After three massed trials, they show almost errorless performance. This task has proved to be useful in studying time-dependent postacquisition intracellular processes necessary for long-term memory. The present experiments evaluated the temporal dynamics of the role of beta-noradrenergic receptors in long-term consolidation. Rats were implanted with intracerebroventricular cannulae and trained in a single session to find reinforcement in a hole in a sponge impregnated with a particular odor. Injections of the beta-receptor antagonist timolol were made at 5 min, 1, 2, or 5 hr after training. Memory and relearning ability were evaluated 48 hr later. Rats treated with timolol 2 hr after training showed a memory deficit at the retention test, but were able to relearn the task normally. Injections at the earlier or later time points were ineffective. The results reinforce previous observations with systemic injections that beta-noradrenergic receptors are involved in the late phase of memory consolidation and suggest a critical time window during which they are necessary. The time window is compatible with the current view that long-term memory depends on late involvement of the cAMP cascade leading to new protein synthesis necessary for synaptic reorganization.

Figure 1

Acquisition of odor–reward association. (A) Latency to make the nose-poke response to the target odor. (B) Errors during acquisition trials. An error was scored when the rat made a nose-poke into a nonreinforced hole. There was a significant decrease in the time to emit the nose-poke response and consume the reinforcement over the three trials and a significant decrease in the number of errors. Most rats had an errorless performance at the third trial.

Figure 2

Latency for correct response, which was a nose-poke into sponge with the reinforced odor. Independent groups were treated at 5 min, 1, 2, or 5 hr after training with saline (solid lines) or timolol (dotted lines). (T) test score; (R) retraining score. Testing and retraining took place in a single session, 48 hr after initial training. (*) Significantly different from the saline control group; (●) different from timolol 5 min; () 1 hr and (♦) 5 hr groups (all P < 0.01). There were no other significant orthogonal comparisons.

Figure 3

Errors (nose pokes to the nontarget odor) at test (T) and retraining (R) for independent groups treated at 5 min, 1, 2, and 5 hr after acquisition, with saline or timolol; test and retraining took place in a single session, 48 hr after initial training. (*) Significantly different from saline control group; (●) significantly different from 5 min, () 1 hr; (♦) 5 hr timolol groups (P < 0.01). There were no other significant orthogonal comparisons.