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Nathaniel J. Himmel, Atit A. Patel, and Daniel N. Cox

Nociception is a protective mechanism that mediates behavioral responses to a range of potentially damaging stimuli, including noxious temperature, chemicals, and mechanical stimulation. Nociceptive mechanisms are found throughout metazoans. Noxious stimuli are transduced by specialized, high-threshold peripheral nociceptors, which fire action potentials to elicit adaptive behavioral responses. Nociception is essential for survival and provides a mechanism for sensory perception of noxious stimuli, which alerts the organism to potential environmental dangers. When coupled with pain sensation and complex behavioral responses, this mechanism protects the organism from incipient damage. Moreover, acute and chronic pain may manifest as altered nociception in neuropathic pain states. Elucidating the neural bases of nociception is therefore important for identifying and implementing novel strategies for the treatment of neuropathic pain, as well as uncovering the mechanistic bases by which the nervous system integrates information to produce specific behaviors in response to a range of noxious stimuli. Invertebrate organisms, such as Drosophila melanogaster and Caenorhabditis elegans, have emerged as powerful, genetically tractable platforms for exploring these questions. Here, we concisely review the current state of knowledge regarding the cells, molecules, neural circuits, and behaviors associated with invertebrate nociception in the fruit fly and nematode worm.


James S.H. Wong and Catharine H. Rankin

The nematode, Caenorhabditis elegans (C. elegans), is an organism useful for the study of learning and memory at the molecular, cellular, neural circuitry, and behavioral levels. Its genetic tractability, transparency, connectome, and accessibility for in vivo cellular and molecular analyses are a few of the characteristics that make the organism such a powerful system for investigating mechanisms of learning and memory. It is able to learn and remember across many sensory modalities, including mechanosensation, chemosensation, thermosensation, oxygen sensing, and carbon dioxide sensing. C. elegans habituates to mechanosensory stimuli, and shows short-, intermediate-, and long-term memory, and context conditioning for mechanosensory habituation. The organism also displays chemotaxis to various chemicals, such as diacetyl and sodium chloride. This behavior is associated with several forms of learning, including state-dependent learning, classical conditioning, and aversive learning. C. elegans also shows thermotactic learning in which it learns to associate a particular temperature with the presence or absence of food. In addition, both oxygen preference and carbon dioxide avoidance in C. elegans can be altered by experience, indicating that they have memory for the oxygen or carbon dioxide environment they were reared in. Many of the genes found to underlie learning and memory in C. elegans are homologous to genes involved in learning and memory in mammals; two examples are crh-1, which is the C. elegans homolog of the cAMP response element-binding protein (CREB), and glr-1, which encodes an AMPA glutamate receptor subunit. Both of these genes are involved in long-term memory for tap habituation, context conditioning in tap habituation, and chemosensory classical conditioning. C. elegans offers the advantage of having a very small nervous system (302 neurons), thus it is possible to understand what these conserved genes are doing at the level of single identified neurons. As many mechanisms of learning and memory in C. elegans appear to be similar in more complex organisms including humans, research with C. elegans aids our ever-growing understanding of the fundamental mechanisms of learning and memory across the animal kingdom.