The insulin-like signaling pathway is known to regulate fat metabolism, dauer formation, and longevity in Caenorhabditis elegans. Here, we report that this pathway is also involved in salt chemotaxis learning, in which animals previously exposed to a chemoattractive salt under starvation conditions start to show salt avoidance behavior. Mutants of ins-1, daf-2, age-1, pdk-1, and akt-1, which encode the homologs of insulin, insulin/IGF-I receptor, PI 3-kinase, phosphoinositide-dependent kinase, and Akt/PKB, respectively, show severe defects in salt chemotaxis learning. daf-2 and age-1 act in the ASER salt-sensing neuron, and the activity level of the DAF-2/AGE-1 pathway in this neuron determines the extent and orientation of salt chemotaxis. On the other hand, ins-1 acts in AIA interneurons, which receive direct synaptic inputs from sensory neurons and also send synaptic outputs to ASER. These results suggest that INS-1 secreted from AIA interneurons provides feedback to ASER to generate plasticity of chemotaxis.

Caenorhabditis elegans exhibits a food-associated behavior that is modulated by the past cultivation temperature. Mutations in INS-1, the homolog of human insulin, caused the defect in this integrative behavior. Mutations in DAF-2/insulin receptor and AGE-1/phosphatidylinositol 3 (PI-3)-kinase partially suppressed the defect of ins-1 mutants, and a mutation in DAF-16, a forkhead-type transcriptional factor, caused a weak defect. In addition, mutations in the secretory protein HEN-1 showed synergistic effects with INS-1. Expression of AGE-1 in any of the three interneurons, AIY, AIZ, or RIA, rescued the defect characteristic of age-1 mutants. Calcium imaging revealed that starvation induced INS-1-mediated down-regulation of AIZ activity. Our results suggest that INS-1, in cooperation with HEN-1, antagonizes the DAF-2 insulin-like signaling pathway to modulate interneuron activity required for food-associated integrative behavior. The secreted peptide hormone insulin modulates neural plasticity. Insulin and insulin receptors are expressed in several regions of the rat brain (Havrankova et al. 1978a,b), insulin receptors localize to post-synapses (Abbott et al. 1999), and insulin can produce long-term depression (LTD) of synaptic transmission through endocytosis of ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in rat hippocampal CA1 neurons (Man et al. 2000). In addition, Phosphatidylinositol 3 (PI-3)-kinase that functions in the insulin signaling pathway is thought to induce long-term potentiation (LTP) of synaptic transmission in the dentate gyrus of rat (Kelly and Lynch 2000). These alterations by proteins of the insulin signaling pathway may be involved in learning and memory, but what kind of behavior the insulin signaling pathway modulates has been largely unknown.The nematode Caenorhabditis elegans is well suited for the analysis of the molecular and cellular mechanisms underlying neural plasticity because of its accessible genetics, stereotyped behavioral responses, and its simple nervous system consisting of 302 neurons whose connections are entirely known (White et al. 1986). Recently, physiological analysis of the neural circuit in live worms has become possible by the use of cameleon, a genetically encodable calcium indicator, to measure Ca 2+ concentration changes (Miyawaki et al. 1997;Kimura et al. 2004).C. elegans exhibits thermotaxis, an integrative behavior in which well-fed animals in a thermal gradient are attracted to their cultivation temperature, whereas starved animals avoid it (Hedgecock and Russell 1975;Mohri et al. 2005;Rankin 2005). This food-associated behavioral plasticity, regarded the most complex behavior in C. elegans, is an ideal behavioral paradigm for comprehensive study of neural plasticity at the molecular, physiological, and behavioral levels. In this study, we show that in cooperation with a secreted protein HEN-1, an insulin homolog INS-1, and insulin-like signaling pathway modulate neuronal activity of interneurons to execute thermotaxis behavior in...

The phosphatidylinositol 3-kinase (PI3K) pathway regulates many cellular functions, but its roles in the nervous system are still poorly understood. We found that a newly discovered insulin receptor isoform, DAF-2c, is translocated from the cell body to the synaptic region of the chemosensory neuron in Caenorhabditis elegans by a conditioning stimulus that induces taste avoidance learning. This translocation is essential for learning and is dependent on the mitogen-activated protein kinase-regulated interaction of CASY-1 (the calsyntenin ortholog) and kinesin-1. The PI3K pathway is required downstream of the receptor. Light-regulated activation of PI3K in the synaptic region, but not in other parts of the cell, switched taste-attractive behavior to taste avoidance, mimicking the effect of conditioning. Thus, synaptic PI3K is crucial for the behavioral switch caused by learning.

Oda S, Tomioka M, Iino Y. Neuronal plasticity regulated by the insulin-like signaling pathway underlies salt chemotaxis learning in Caenorhabditis elegans. J Neurophysiol 106: 301-308, 2011. First published April 27, 2011 doi:10.1152/jn.01029.2010.-Quantification of neuronal plasticity in a living animal is essential for understanding learning and memory. Caenorhabditis elegans shows a chemotactic behavior toward NaCl. However, it learns to avoid NaCl after prolonged exposure to NaCl under starvation conditions, which is called salt chemotaxis learning. Insulin-like signaling is important for this behavioral plasticity and functions in one of the salt-sensing sensory neurons, ASE right (ASER). However, how neurons including ASER show neuronal plasticity is unknown. To determine the neuronal plasticity related to salt chemotaxis learning, we measured Ca 2ϩ response and synaptic release of individual neurons by using in vivo imaging techniques. We found that response of ASER increased whereas its synaptic release decreased after prolonged exposure to NaCl without food. These changes in the opposite directions were abolished in insulin-like signaling mutants, suggesting that insulinlike signaling regulates these plasticities in ASER. The response of one of the downstream interneurons, AIB, decreased profoundly after NaCl conditioning. This alteration in AIB response was independent of the insulin-like signaling pathway. Our results suggest that information on NaCl is modulated at the level of both sensory neurons and interneurons in salt chemotaxis learning. learning and memory; in vivo imaging LEARNING AND MEMORY ARE CRUCIAL for animals to cope with a constantly changing environment. Previous studies suggested that insulin is involved in learning and memory in mammals (Dou et al. 2005; Zhao et al. 1999). Indeed, studies using mammalian cultured neurons or Xenopus tadpoles suggested that insulin regulates neuronal plasticities such as long-term depression (LTD), internalization of DL-␣-amino-3-hydroxy-5-methylisoxazole-propionic acid (AMPA) receptors, and changes in the number of synapses (Chiu et al. 2008;Man et al. 2000). However, how insulin actually regulates learning and memory is still obscure.Caenorhabditis elegans also shows learning and memory such as thermotaxis learning, food-odor associative learning, and salt chemotaxis learning (Mori et al. 2007;Nuttley et al. 2002; Saeki et al. 2001;Tomioka et al. 2006). The molecular mechanisms underlying these behavioral plasticities have been studied well. Insulin-like signaling, for instance, regulates several types of starvation-associated learning (Kodama et al. 2006;Lin et al. 2010;Tomioka et al. 2006). However, the plasticities of neuronal activity underlying these behavioral plasticities are mostly unknown.Salt chemotaxis learning is one of the starvation-associated learning types. In this behavioral plasticity, worms show aversive behavior toward the attractant NaCl after prolonged exposure to NaCl for 10 -60 min without food. However, in the presence of f...