Differential hippocampal and cortical cholinergic activation during the acquisition, retention, reversal and extinction of a spatial discrimination in an 8-arm radial maze by mice

Abstract Possible differentiation of the intervention of cholinergic septohippocampal and magnocellular forebrain (NBM) projections to cortex during learning and memory processes has been investigated directly using mice. High-affinity choline uptake velocities in the hippocampus and cortex were analyzed, in parallel, at various periods during the acquisition, over 8 days, as were the subsequent retention, reversal and extinction of a spatial discrimination in an 8-arm radial maze. Initial acquisition induced an immediate (30 s) and long-lasting (approx. 3 h) increase in mean hippocampal (+ 33%) and cortical (+ 23%) cholinergic activities. The time course of this activation was structure-dependent and correlations of hippocampal-cortical cholinergic activities showed large and consistent alterations as a function of time after training. Cholinergic activation in both brain regions was observed immediately following each daily training session with amplitudes which did not vary significantly in spite of a progressive daily increment in performance. Following acquisition mice were tested for retention, reversal and extinction: 30 s following the retention session, cholinergic activation was observed in both cortex and hippocampus, with magnitudes similar to those observed at the end of acquisition. However, in the reversal and extinction groups, a treatment-dependent attenuation of cholinergic activation was observed which was accompanied by a significant loss of correlation of cholinergic activity between these two brain regions. The results are discussed in relation to the concepts of reference and working memory and also to novelty, stress, arousal and frustrative non-reward. The data constitute direct experimental evidence for a differential involvement of cholinergic septohippocampal and NBM-cortical projections in learning and memory processes.