Optical inactivation of a proprioceptor in an insect by non-genetic tools.

BACKGROUND: The specific role of sensory organs in locomotor pattern generation is traditionally investigated by means of mechanical ablation in arthropods that currently do not allow genetic manipulation. Mechanical ablation is irreversible, and may lead to injury discharges and changes in the structural integrity of the cuticle. NEW METHOD: Here, we present a new method to temporarily or permanently deprive parts of an insect nervous system of sensory feedback from leg proprioceptors by means of blue light application. We illuminated campaniform sensilla (CS) with a blue LED (420-480 nm) or a 473 nm laser at different light intensities to optically eliminate sensory and motor neuron responses to mechanical stimulation. RESULTS: We were able to eliminate all stimulus-evoked responses of CS. Individual CS groups were precisely and selectively inactivated without affecting nearby proprioceptors, using an optical fiber (Ø 200 µm) to guide the light. Our results demonstrated that lower light intensities significantly increase the required exposure time, but also the chance for recovery, thus making the effect reversible. COMPARISON WITH EXISTING METHODS: In contrast to mechanical ablation, optical inactivation of individual sensory organs is non-invasive and does not affect the behavioral state of the animal, nor does it induce escape behavior. This is especially relevant in non-model system experimental animals where optogenetic manipulation cannot be used, due to a lack of established methods of access. CONCLUSION: Our results show that the proposed method is a reliable alternative to mechanical ablation and can be successfully applied to the CS, as it fulfills all requirements regarding selectivity, efficiency, and reproducibility.

Gradients in mechanotransduction of force and body weight in insects.

Posture and walking require support of the body weight, which is thought to be detected by sensory receptors in the legs. Specificity in sensory encoding occurs through the numerical distribution, size and response range of sense organs. We have studied campaniform sensilla, receptors that detect forces as strains in the insect exoskeleton. The sites of mechanotransduction (cuticular caps) were imaged by light and confocal microscopy in four species (stick insects, cockroaches, blow flies and Drosophila). The numbers of receptors and cap diameters were determined in projection images. Similar groups of receptors are present in the legs of each species (flies lack Group 2 on the anterior trochanter). The number of receptors is generally related to the body weight but similar numbers are found in blow flies and Drosophila, despite a 30 fold difference in their weight. Imaging data indicate that the gradient (range) of cap sizes may more closely correlate with the body weight: the range of cap sizes is larger in blow flies than in Drosophila but similar to that found in juvenile cockroaches. These studies support the idea that morphological properties of force-detecting sensory receptors in the legs may be tuned to reflect the body weight.

Identification of the origin of force-feedback signals influencing motor neurons of the thoraco-coxal joint in an insect.

Force feedback from Campaniform sensilla (CS) on insect limbs helps to adapt motor outputs to environmental conditions, but we are only beginning to reveal the neural control mechanisms that mediate these influences. We studied CS groups that affect control of the thoraco-coxal joint in the stick insect Carausius morosus by applying horizontal and vertical forces to the leg stump. Motor effects of ablation of CS groups were evaluated by recording extracellularly from protractor (ProCx) and retractor (RetCx) nerves. Extracellular recordings showed that the effects of stimulating the sensilla were consistent with their broad ranges of directional sensitivity: for example, RetCx firing in response to posterior bending could be reduced by ablating several groups of trochanteral CS, whereas ablation of tibial and femoral sensilla had little effect. In contrast, ProCx motor neuron activity upon anteriorly directed stimuli was affected mainly by ablating a single CS group (G2). Dye fills of trochanteral, femoral and tibial CS groups with fluorescent dyes revealed a common projection pattern with little group specificity. These findings support the idea that the influences of CS feedback are determined by the activities of pre-motor interneurons, facilitating fast and task-dependent adaptation to changing environmental conditions.