Tonic receptors convey stimulus duration and intensity and are implicated in homeostatic control. However, how tonic homeostatic signals are generated, and how they reconfigure neural circuits and modify animal behavior is poorly understood. Here we show that C. elegans O2-sensing neurons are tonic receptors that continuously signal ambient [O2] to set the animal’s behavioral state. Sustained signalling relies on a Ca2+ relay involving L-type voltage-gated Ca2+ channels, the ryanodine and the IP3 receptors. Tonic activity evokes continuous neuropeptide release, which helps elicit the enduring behavioral state associated with high [O2]. Sustained O2 receptor signalling is propagated to downstream neural circuits, including the hub interneuron RMG. O2 receptors evoke similar locomotory states at particular [O2], regardless of previous d[O2]/dt. However, a phasic component of the URX receptors’ response to high d[O2]/dt, as well as tonic-to-phasic transformations in downstream interneurons, enable transient reorientation movements shaped by d[O2]/dt. Our results highlight how tonic homeostatic signals can generate both transient and enduring behavioral change.

[Ca]1 to the same amount as occurred during the pace-maker cycle also produced an outward current of sufficient magnitude to account for the post-burst hyperpolarization.8. Depolarization of the cell membrane by extrinsic current during the burst increased and prolonged the change in dye absorbance as well as the post-burst hyperpolarization.9. It is suggested that Ca2+ enters during the pace-maker cycle, thereby increasing[Ca]1, and that this increase is sufficient to activate an outward current carried by K+ ions which causes or contributes to the post-burst hyperpolarization.

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2. Dye absorbance increased linearly for depolarizing pulse durations up to 100 msec and approximately linearly between 100 and 300 msec, but for longer durations the absorbance change decreased.3. The absorbance change vs. voltage relation increased steeply between -20 and 0 mV (e-fold per 8-5 mV), peaked at + 36 mV and declined non-linearly to an estimated null or suppression potential of about + 139 mV.4. TTX (5 x 10-5 M) had no effect on the change in dye absorbance produced by brief or long duration stimuli whereas Ca2+ free ASW abolished all changes in dye absorbance.5