The neural circuitry underlying this response was elucidated

with laser ablation and comprises the mechanosensory cells ALM, AVM, PLM, and PVD and interneurons AVD, AVA, AVB, PVC, and DVA [32]. In 1990, Rankin et al. [5] showed that the tap-withdrawal response decrements with repeated stimulation, and that the decrement is readily reversed with electric shock and therefore cannot be explained by sensory adaptation or fatigue, matching the classic definition of habituation, a non-associative form of learning [33]. Habituation can be short term or long-term; long-term habituation is sensitive to the stimulation protocol and worms tapped 80 learn more times at a 60-s interval can remember training for more than 24 h, but only if the taps are distributed

into see more four blocks with 1 h rest periods 34 and 35. This memory is dependent on CREB-mediated protein-synthesis and AMPA-type glutamate receptor trafficking 35 and 36•. If worms are mass-trained with no rest periods, they do not display long-term memory at 24 h, but do show decremented responding for at least 12 h [37••]. This intermediate memory was not dependent on glutamate signaling or CREB activity, but was found to induce accumulation of a synaptic vesicle marker in the terminals of the body touch cells, suggesting a possible role for neuropeptide signaling [37••]. Indeed, the mechanosensory neurons express several neuropeptides and loss of one of them, FLP-20, was shown to disrupt intermediate memory, as well as the accumulation of synaptic vesicles. Restoring flp-20 expression in the body touch cells rescued the learning deficit of the mutant, suggesting repeated activation recruits dense-core vesicles to the synaptic terminals, which leads to increased FLP-20 release and smaller reversal responses [37••]. Neuropeptides play an important role

in mediating learning and memory behavior in C. elegans. Insulin signaling has emerged with an especially prominent role likely because of the use VAV2 of feeding state as an unconditioned stimulus in many assays. There are, however, dozens of uncharacterized neuropeptide receptors and future research will undoubtedly implicate many of them in behavioral plasticity. Despite its reproducible synaptic connections, C. elegans display a remarkable capacity for learning and memory, which makes the stereotyped nervous system a huge asset as researchers can study the same neuron in every animal of a genetically homogeneous population. In a detailed study of orthologs between C. elegans and Homo sapiens Shaye and Greenwald [38] found 7663 C. elegans genes that had direct orthologs in H. Sapiens; the ortholist they generated included almost all of the components of known conserved signaling pathways, and many essential components of five major pathways they examined more closely (WNT, TGF-beta, Insulin, Notch and RTK/Ras/MapK).