Cholinergic mechanisms in a simple test of olfactory learning in the rat

Male rats were tested in a simple olfactory habituation and discrimination paradigm. Untreated rats habituated their responses over three trials to one odour but were capable of recognising a novel odour presented on the fourth trial. Administration (SC) of either scopolamine or N-methylscopolamine before trials commenced produced a decrease in overall responding. Scopolamine, but not N-methylscopolamine, also blocked habituation to the first odour and recognition to the second, novel odour. Administration of scopolamine after trial 3 did not block the ability of the animals to respond differentially to a novel odour, although again overall levels of responding were decreased. Electrolytic lesions of the medial septal area increased overall levels of responding but lesioned animals still habituated their response over trials and were capable of recognising a novel odour. Therefore although cholinergic mechanisms appear to be involved in this type of learning, these effects are unlikely to be mediated via the septohippocampal system.