Effects of tap withdrawal response habituation on other withdrawal behaviors: the localization of habituation in the nematode Caenorhabditis elegans

A novel vibration-induced exercise paradigm improves fitness and lipid metabolism of Caenorhabditis elegans

Exercise has been known to reduce the risk of obesity and metabolic syndrome, but the mechanisms underlying many exercise benefits remain unclear. This is, in part, due to a lack of exercise paradigms in invertebrate model organisms that would allow rapid mechanistic studies to be conducted. Here we report a novel exercise paradigm in Caenorhabditis elegans (C. elegans) that can be implemented under standard laboratory conditions. Mechanical stimulus in the form of vibration was transduced to C. elegans grown on solid agar media using an acoustic actuator. One day post-exercise, the exercised animals showed greater physical fitness compared to the un-exercised controls. Despite having higher mitochondrial reactive oxygen species levels, no mitohormetic adaptations and lifespan extension were observed in the exercised animals. Nonetheless, exercised animals showed lower triacylglycerides (TAG) accumulation than the controls. Among the individual TAG species, the most significant changes were found in mono- and polyunsaturated fatty acid residues. Such alteration resulted in an overall lower double bond index and peroxidation index which measure susceptibility towards lipid peroxidation. These observations are consistent with findings from mammalian exercise literature, suggesting that exercise benefits are largely conserved across different animal models.

Mechanisms of plasticity in a Caenorhabditis elegans mechanosensory circuit

Despite having a small nervous system (302 neurons) and relatively short lifespan (14–21 days), the nematode Caenorhabditis elegans has a substantial ability to change its behavior in response to experience. The behavior discussed here is the tap withdrawal response, whereby the worm crawls backwards a brief distance in response to a non-localized mechanosensory stimulus from a tap to the side of the Petri plate within which it lives. The neural circuit that underlies this behavior is primarily made up of five sensory neurons and four pairs of interneurons. In this review we describe two classes of mechanosensory plasticity: adult learning and memory and experience dependent changes during development. As worms develop through young adult and adult stages there is a shift toward deeper habituation of response probability that is likely the result of changes in sensitivity to stimulus intensity. Adult worms show short- intermediate- and long-term habituation as well as context dependent habituation. Short-term habituation requires glutamate signaling and auto-phosphorylation of voltage-dependent potassium channels and is modulated by dopamine signaling in the mechanosensory neurons. Long-term memory (LTM) for habituation is mediated by down-regulation of expression of an AMPA-type glutamate receptor subunit. Intermediate memory involves an increase in release of an inhibitory neuropeptide. Depriving larval worms of mechanosensory stimulation early in development leads to fewer synaptic vesicles in the mechanosensory neurons and lower levels of an AMPA-type glutamate receptor subunit in the interneurons. Overall, the mechanosensory system of C. elegans shows a great deal of experience dependent plasticity both during development and as an adult. The simplest form of learning, habituation, is not so simple and is mediated and/or modulated by a number of different processes, some of which we are beginning to understand.

Olfactory jump reflex habituation in Drosophila and effects of classical conditioning mutations

Habituation is a nonassociative learning mechanism, in which an initial response toward repeated stimuli gradually wanes. This is amongst the simplest and most widespread forms of behavioral plasticity. So far, neither the underlying molecular mechanisms nor the precise neural networks of habituation are well understood. We have developed a novel paradigm to quantify habituation of the olfactory jump reflex in Drosophila. We present data demonstrating several behavioral properties of this phenomenon, generally observed in other species. We also show that the dunce and rutabaga memory mutants behave abnormally in this assay, suggesting that this assay might be used in behavioral screens for new mutants with defects in this simpler form of behavioral plasticity.

Decreased sensory stimulation reduces behavioral responding, retards development, and alters neuronal connectivity in Caenorhabditis elegans

Activity-dependent plasticity is a critical component of nervous systems. We show that in Caenorhabditis elegans, worms raised in isolation made smaller responses to mechanosensory stimulation and were smaller and slower to begin laying eggs than age-matched group-raised worms. The glutamate receptor gene GLR-1 was critical for the observed alterations in behavior but not in size, whereas the cGMP-dependent protein kinase gene EGL-4 was critical for the observed changes in size but not the changes in behavior. Mechanosensory stimulation during development reversed the effects of isolation on behavior and began to reduce the effects of isolation on size. In C. elegans, the six mechanosensory touch neurons synapse onto the four pair of command interneurons for forward and backward movement. Touch (mechanosensory) neurons of worms raised in isolation expressed lower levels of green fluorescent protein (GFP)-tagged synaptobrevin than touch neurons of worms raised in colonies. Command interneurons of worms raised in isolation expressed lower levels of GFP-tagged glutamate receptors than command interneurons of worms raised in groups. Brief mechanical stimulation during larval development rescued the expression of GFP-tagged glutamate receptors but not GFP-tagged synaptobrevin. Together, these results indicate that the level of stimulation experienced by C. elegans during development profoundly affects the development of neuronal connectivity and has widespread cellular and behavioral consequences.

Mutations of the caenorhabditis elegans brain-specific inorganic phosphate transporter eat-4 affect habituation of the tap-withdrawal response without affecting the response itself

The studies reported here were designed to investigate the role of the mutation eat-4 in the response to tap and in habituation in the nematode Caenorhabditis elegans. In C. elegans eat-4 has been found to affect a number of glutamatergic pathways. It has been hypothesized to positively regulate glutaminase activity and therefore glutamatergic neurotransmission. In the eat-4(ky5) loss-of-function worms, there is presumably insufficient glutamate available for sustained transmission. In the experiments reported here eat-4 worms showed no differences from wild-type in the magnitude of response to a single tap, indicating that the neural circuit underlying the response was intact and functional in the mutant worms. However, when eat-4 worms were given repeated taps the resulting habituation was different from that seen in wild-type worms: eat-4 worms habituate more rapidly and recover more slowly than wild-type worms at all interstimulus intervals tested. In addition, eat-4 worms do not show dishabituation. The same transgene rescues pharyngeal activity defects and both the habituation and dishabituation deficits seen in the eat-4 worms. Our results suggest that neurotransmitter regulation plays a role in habituation and may play a role in dishabituation.