Memory improvement by post-trial injection of lidocaine into the tuberomammillary nucleus, the source of neuronal histamine

Brain histamine is exclusively contained within and released from neurons w hose cell bodies are clustered in the tuberomammillary nucleus (TM) of the posterior hypothalamus. This experiment examined the effects of a transient inactivation of the TM on inhibitory avoidance learning. Rats with chronic ally implanted cannulae were tested on a 1-trial step-through avoidance tas k. Immediately following training, the rats received unilateral intra-TM in fusions (0.5 mu I) of lidocaine (5 or 20 mu g). Control groups included veh icle-injected rats and a group given an injection of 20 mu g lidocaine 5 h after training. When tested 24 h later, rats treated with 20 mu g lidocaine exhibited longer step-through latencies than vehicle-treated controls, ind icative of superior learning of the task. The failure of the delayed post-t rial injection of lidocaine to significantly influence step-through latenci es indicates that the compound influenced learning by modulating memory sto rage processes rather than by acting on performance variables during retrie val of the task. Thus, inactivation of the TM by lidocaine can exert facili tatory effects on mnemonic processing, which might be related to a temporar y reduction of histaminergic activity during the early phase of memory cons olidation, (C) 1999 Academic Press.