RECOGNITION MEMORY IN RATS - III - NEUROCHEMICAL SUBSTRATES

In the first part of three overviews on recognition memory in the rat, we discussed the tasks employed to study recognition memory. In the s econd part, we discussed the neuroanatomical systems thought to be of importance for the mediation of recognition memory in the rat. In part icular, we delineated two parallel-distributed neuronal networks, one thai is essential for the processing of non-spatial/item recognition m emory processes and incorporates the cortical association areas such a s TE1, TE2 and TE3, the rhinal cortices, the mediodorsal thalamic nucl eus and prefrontal cortical areas (Network 1), the other comprising of the hippocampus, mamillary bodies, anterior thalamic nuclei and media l prefrontal areas (Network 2), suggested to be pivotal for the proces sing of spatial recognition memory. The next step will progress to the level of the neurotransmitters thought to be involved. Current data s uggest that the majority of drugs have non-specific, i.e. delay-indepe ndent effects in tasks measuring recognition memory. This may be due t o attentional, motivational or motoric changes. Alternatively, delay-i ndependent effects may result from altered acquisition/encoding rather than from altered retention. Furthermore, the neurotransmitter system s affected by these drugs could be important as modulators rather than as mediators of recognition memory pei se, It could, of course, also be the case that systemic treatment induces non-specific effects which overshadow any specific, delay-dependent, effect. This possibility re ceives support from lesion experiments (For example, of the septohippo campal cholinergic system) or studies employing local intracerebral in fusion techniques. However, it is evident that those delay-dependent e ffects are relatively subtle and more readily seen in delayed response paradigms, which tax spatial recognition memory. One interpretation o f these results could be that some neurotransmitter systems are more i nvolved in spatial than in item recognition memory processes. However, performance in delayed response tasks can be aided by mediating strat egies. Drugs or lesions can alter those strategies, which could equall y explain some of the (delay-dependent) drug effects on delayed respon ding. Thus, it is evident that neither of the neurotransmitter systems reviewed (glutamate, GABA, acetylcholine, serotonin, dopamine and nor adrenaline) can be viewed as being directly and exclusively concerned with storage/retention. Rather, our model of recognition memory sugges ts that information about previously encountered items is differential ly processed by distinct neural networks and is not mediated by a sing le neuro transmitter type. (C) 1998 Elsevier Science Ltd.