ИСТИНА

A common way to study memory of neuronal representations in the rodent neocortex is Pavlovian conditioning to a discrete sensory stimulus. However, in natural environment associative memories are complex and involve integrated conditioned stimuli (CS) consisting of different sensory modalities. Very little is known about how neuronal populations in neocortex code complex stimuli during long-term memory formation and retrieval. Previously we have developed a behavioral paradigm for multisensory compound CS fear conditioning in mice that mimics complexity of natural learning. In the present study, we examined encoding of different components of the compound CS by neurons of the mouse parietal cortex. To map neuronal responses we used transgenic mice with the expression of the EGFP controlled by the c-fos promoter. We trained mice in a fear-conditioning task to a compound cue that consisted of auditory (tone) and visual (blinking light) components. Control mice were presented with the compound CS, without the footshock. A week later, we performed three sequential retrieval sessions during which mice received separate auditory and visual components of the compound CS and the compound CS itself. To monitor retrieval-induced neuronal activation we performed two-photon in vivo imaging of fos-EGFP expression in the parietal associative cortex 90 minutes after each test. Based on the intensity of EGFP fluorescence, all registered neurons were divided into the two groups - high- and low-expressing. The number of high-expressing neurons increased in the trained mice compared to the control mice in all retrieval sessions, while the number of low-expressing neurons decreased in all the sessions. Next, we analyzed neurons that showed high fos-EGFP expression only in one of the retrieval session. We found three activation specificities of such neurons: light-related, sound-related and compound CS-related neurons. The number of light-dependent high-expressing neurons increased in the trained mice compared to the control mice. The number of sound-dependent and compound CS-dependent neurons was equal in the trained and control mice. Taken together our data suggests that coding of complex associative memory in the parietal cortex involves at least three neuronal assemblies with different response specificity to the components of the compound CS.