Short- and long-term memory are differentially regulated by monoaminergic systems in the rat brain

Rats with cannulae implanted in the dorsal CA1 region of the hippocampus or in the entorhinal cortex (EC) were trained in one-trial step-down inhibitory avoidance and tested 1.5 or 24 h later, in order to measure short-term memory (STM) and long-term memory (LTM) respectively. Several drugs infused immediately post-training inhibited STM without altering LTM: the D1 receptor agonist SKF38393 (7.5 microgram) given into either CA1 or EC, the beta blocker timolol (0.3 microgram) given into EC, the 5HT1A receptor agonist 8-HO-DPAT (2.5 microgram) given into CA1, and the 5HT1A antagonist NAN-190 (2.5 microgram) given into EC. These findings indicate that STM is not a necessary step toward LTM. Intraentorhinal 8-HO-DPAT enhanced STM and depressed LTM. The D1 antagonist SCH23390 (0.5 microgram) enhanced STM without affecting LTM when given into CA1, and blocked LTM without affecting STM when given into EC. Intraentorhinal norepinephrine (0.3 microgram) enhanced both STM and LTM, and the same drug when given into CA1 enhanced LTM selectively. None of the drugs had any effect on retrieval of either STM or LTM when given prior to testing. The data indicate that STM and LTM are differentially modulated by D1, beta, and 5HT1A receptors in CA1 and EC.