Retention time of attenuated response to diacetyl after pre-exposure to diacetyl in Caenorhabditis elegans

Sleep Analysis in Adult C. elegans Reveals State-Dependent Alteration of Neural and Behavioral Responses

Sleep, a state of quiescence associated with growth and restorative processes, is conserved across species. Invertebrates including the nematode Caenorhabditis elegans exhibit sleep-like states during development, satiety, and stress. Here, we describe behavior and neural activity during sleep and awake states in adult C. elegans hermaphrodites using new microfluidic methods. We observed effects of fluid flow, oxygen, feeding, odors, and genetic perturbations on long-term sleep behavior over 12 h. We developed a closed-loop sleep detection system to automatically deliver chemical stimuli to assess sleep-dependent changes to evoked neural responses in individual animals. Sleep increased the arousal threshold to aversive stimulation, yet the associated sensory neuron and first-layer interneuron responses were unchanged. This localizes adult sleep-dependent neuromodulation within interneurons presynaptic to the premotor interneurons, rather than afferent sensory circuits. However, sleep prolonged responses in appetitive chemosensory neurons, suggesting that sleep modulates responsiveness specifically across sensory systems rather than broadly damping global circuit activity.

SIGNIFICANCE STATEMENT Much is known about molecular mechanisms that facilitate sleep control. However, it is unclear how these pathways modulate neural circuit-level sensory processing or how misregulation of neural activity contributes to sleep disorders. The nematode Caenorhabditis elegans provides the ability to study neural circuitry with single-neuron resolution, and recent studies examined sleep states between developmental stages and when stressed. Here, we examine an additional form of spontaneous sleep in adult C. elegans at the behavioral and neural activity levels. Using a closed-loop system, we show that delayed behavioral responses to aversive chemical stimulation during sleep arise from sleep-dependent sensorimotor modulation localized presynaptic to the premotor circuit, rather than early sensory circuits.

BRCA1 and BARD1 mediate apoptotic resistance but not longevity upon mitochondrial stress in Caenorhabditis elegans

Interventions that promote healthy aging are typically associated with increased stress resistance. Paradoxically, reducing the activity of core biological processes such as mitochondrial or insulin metabolism promotes the expression of adaptive responses, which in turn increase animal longevity and resistance to stress. In this study, we investigated the relation between the extended Caenorhabditis elegans lifespan elicited by reduction in mitochondrial functionality and resistance to genotoxic stress. We find that reducing mitochondrial activity during development confers germline resistance to DNA damage-induced cell cycle arrest and apoptosis in a cell-non-autonomous manner. We identified the C. elegans homologs of the BRCA1/BARD1 tumor suppressor genes, brc-1/brd-1, as mediators of the anti-apoptotic effect but dispensable for lifespan extension upon mitochondrial stress. Unexpectedly, while reduced mitochondrial activity only in the soma was not sufficient to promote longevity, its reduction only in the germline or in germline-less strains still prolonged lifespan. Thus, in animals with partial reduction in mitochondrial functionality, the mechanisms activated during development to safeguard the germline against genotoxic stress are uncoupled from those required for somatic robustness and animal longevity.

Food Search Strategy Changes in Caenorhabditis elegans under Chronic Starvation Conditions

Starvation is a primary threat to survival in nature. This study investigated the effects of starvation on animal behavior and neural function using a nematode model. Nematodes exhibit chemotactic responses to various compounds, including diacetyl produced by food bacteria. Locomotion, chemotactic behavior, and olfactory adaptation were measured following chronic starvation. Our results revealed a starvation-dependent reduction in locomotor activity. Chemotaxis response to the odorant diacetyl was attenuated after 2-38 hr of starvation. However, chemotactic behavior increased significantly after 48 hr of starvation compared with that after 38 hr of starvation, suggesting that food search behavior was enhanced after 48 hr of starvation. Inhibition of diacetyl adaptation was observed in the nematodes after 48 hr of starvation. However, exogenous exposure to serotonin during 48 hr of starvation caused the inhibition of diacetyl adaptation to be attenuated in following 24 hr period of normal feeding.Therefore, the inhibitory effects of starvation on olfactory adaptation may reduce chemotaxis response to the odorant diacetyl in a manner mediated by serotonin.

Extension of the established period of diacetyl adaptation by oxygen intermediates in the nematode Caenorhabditis elegans

After pre-exposure to the odorant diacetyl, the nematode Caenorhabditis elegans showed a decline in chemotactic responses to diacetyl, a phenomenon known as diacetyl adaptation. In the present study, we found that the established period of diacetyl adaptation in nematodes increased with the breeding temperature. When wild-type (N2) nematodes were bred at 15°C, adaptation was observed from the young adult (YA) to the 3-day-old adult that is reached 3 days after the YA stage. On breeding nematodes at 20°C and 25°C, adaptation was observed between the YA and 5-day-old adult and between the YA and the 7-day-old adult, respectively. Breeding temperature has been shown to correlate with the rate of aging in nematodes, which is related to the level of oxygen consumption. Accordingly, long-lived isp-1 and clk-1 mutants that demonstrate decreased levels of oxygen consumption showed a shorter established period of adaptation than N2 nematodes, whereas short-lived gas-1 and mev-1 mutants that have a hypersensitive response to oxygen showed a longer period of adaptation than the N2. Moreover, the established period of diacetyl adaptation in N2 nematodes was shortened by the antioxidant α-lipoic acid. These results suggest that oxygen intermediates, which are produced by oxygen consumption, play a significant role in diacetyl adaptation. Although this is only one of many factors that regulate diacetyl adaptation, such as the release of neurotransmitters and changes in intracellular conditions, the acquisition of this adaptation requires an increase in the intensity of moderate oxygen signals.

Effect of temperature pre-exposure on the locomotion and chemotaxis of C. elegans

The effect of temperature pre-exposure on locomotion and chemotaxis of the soil-dwelling nematode Caenorhabditis elegans has been extensively studied. The behavior of C. elegans was quantified using a simple harmonic curvature-based model. Animals showed increased levels of activity, compared to control worms, immediately after pre-exposure to 30 °C. This high level of activity in C. elegans translated into frequent turns by making 'complex' shapes, higher velocity of locomotion, and higher chemotaxis index (CI) in presence of a gradient of chemoattractant. The effect of pre-exposure was observed to be persistent for about 20 minutes after which the behavior (including velocity and CI) appeared to be comparable to that of control animals (maintained at 20 °C). Surprisingly, after 30 minutes of recovery, the behavior of C. elegans continued to deteriorate further below that of control worms with a drastic reduction in the curvature of the worms' body. A majority of these worms also showed negative chemotaxis index indicating a loss in their chemotaxis ability.

Sensory interaction between attractant diacetyl and repellent 2-nonanone in the nematode Caenorhabditis elegans

In the nematode Caenorhabditis elegans, the odorant diacetyl is sensed by AWA sensory neurons in the amphid sensory organ and elicits an attractive response, whereas 2-nonanone is sensed by AWB amphid sensory neurons and elicits an avoidance response. In the present study, we report that nematodes exhibit a sensory interaction between the attractant diacetyl and repellent 2-nonanone. In the presence of food, the chemotactic response to 0.01% diacetyl in nematodes preexposed to 0.1% diacetyl was greater than that in nonexposed naive nematodes (P < 0.05). The response to diacetyl was also greater in nematodes preexposed to 3% 2-nonanone in the presence of food than that in naive nematodes (P < 0.01). In the absence of food, the response to diacetyl in nematodes preexposed to diacetyl or 2-nonanone was significantly lower than that in nonexposed control nematodes (P < 0.01). The avoidance response to 10% 2-nonanone in nematodes preexposed to each odorant in the presence or absence of food was lower than that in nonexposed nematodes (P < 0.05). To confirm the validity of our results, the chemotactic responses to diacetyl and 2-nonanone were observed using che-3, odr-4, and odr-10 mutants, which exhibited defective sensitivity to diacetyl or 2-nonanone. From the results of our experiments, we conclude that nematodes exhibit a sensory interaction between diacetyl and 2-nonanone and speculate that this interaction is driven by higher-level neuronal circuits that underlie sensory integration.

Enhancement of chemotactic response to sodium acetate in the nematode Caenorhabditis elegans

In this study, we investigated the chemotactic response of a wild-type (N2) nematode (Caenorhabditis elegans) to a water-soluble attractant, sodium acetate, after pre-exposure to the chemical. The chemotactic response to 1.0 M sodium acetate of the non-exposed control nematodes was lower than that of the nematodes that were pre-exposed to 1.0 M sodium acetate for 90 min (p < 0.05). The increase in the response to sodium acetate was observed up to 6 hr, but not at 12 hr after exposure. To clarify the mechanism of this enhancement of the chemotactic response, several mutants were used. The chemotactic response of pre-exposed tph-1 and bas-1 mutants, whose main defect was serotonin secretion, was enhanced in comparison with that of the control mutants (p < 0.01). However, cat-1 and cat-2 mutants, which are respectively defective in serotonin and dopamine secretion and dopamine secretion only, showed no enhancement of the chemotactic response to sodium acetate, even when pre-exposed to this chemical. When the cat-1 and cat-2 mutants were pre-exposed to sodium acetate and bred in the presence of 40 mM dopamine, these mutants showed enhanced chemotactic response to sodium acetate (p < 0.05). These results suggest that the enhancement of chemotactic response to sodium acetate after pre-exposure to this chemical is modulated by dopaminergic neurotransmission.