Molecular neurobiology of olfactory learning in the land slug

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We examined whether the PC is necessary for olfactory learning. In the first series of experiments, slugs were aversively conditioned by pairing vegetable odor with quinidine 7 days after ablation of bilateral PCs. The slugs with PC lesions showed significantly lower scores of learning compared to sham-operated slugs. In the second series of experiments, slugs were first conditioned, and at various time points, the PCs were ablated. When tested 3 days after PC lesion, these slugs also showed severe impairment of memory. These results indicate that the PC is necessary at least for the retention or retrieval of aversive odor memories.We then investigated whether the central synapses in the slug display synaptic plasticity. A single electrical stimulation of the tentacle nerve evoked a potential in the procerebrum (PC) with at least 3 components, and this stimulus was used as the test stimulus. After a high frequency stimulation of the tentacle nerve, the positive component of the evoked potential increased, and this potentiation lasted for more than two hours. That synaptic plasticity found in the PC may be involved in olfactory learning. We also screened genes in the brain that are upregulated by neural activity. Dissecting the brain results in transient up-regulation of neural activity, and this induces transcription of several genes. By differential display analyses using 72 pairs of primers, approximately 10,000 bands were displayed and 28 showed reproducible changes in the expression levels, and 10 of these were cloned for further analysis.