Genetically encoded calcium indicators (GECIs) can be used to image activity in defined neuronal populations. However, current GECIs produce inferior signals compared to synthetic indicators and recording electrodes, precluding detection of low firing rates. We developed a single-wavelength GECI based on GCaMP2 (GCaMP3), with increased baseline fluorescence (3x), dynamic range (3x), and higher affinity for calcium (1.3x). GCaMP3 fluorescence changes triggered by single action potentials were detected in pyramidal cell dendrites, with signal-to-noise ratio and photostability significantly better than GCaMP2, D3cpVenus, and TN-XXL. In Caenorhabditis elegans chemosensory neurons and the Drosophila melanogaster antennal lobe, sensory stimulation-evoked fluorescence responses were significantly enhanced with the new indicator (4–6x). In somatosensory and motor cortical neurons in the intact mouse, GCaMP3 detected calcium transients with amplitudes linearly dependent on action potential number. Long-term imaging in the motor cortex of behaving mice revealed large fluorescence changes in imaged neurons over months.

Fruit flies are attracted by a diversity of odors that signal the presence of food, potential mates, or attractive egg-laying sites. Most Drosophila olfactory neurons express two types of odorant receptor genes: Or83b, a broadly expressed receptor of unknown function, and one or more members of a family of 61 selectively expressed receptors. While the conventional odorant receptors are highly divergent, Or83b is remarkably conserved between insect species. Two models could account for Or83b function: it could interact with specific odor stimuli independent of conventional odorant receptors, or it could act in concert with these receptors to mediate responses to all odors. Our results support the second model. Dendritic localization of conventional odorant receptors is abolished in Or83b mutants. Consistent with this cellular defect, the Or83b mutation disrupts behavioral and electrophysiological responses to many odorants. Or83b therefore encodes an atypical odorant receptor that plays an essential general role in olfaction.

Drosophila melanogster is a model organism rich in genetic tools to manipulate and identify neural circuits involved in specific behaviors. Here we present a novel technique for two-photon calcium imaging in the central brain of head-fixed Drosophila walking on an air-supported ball. The ball’s motion is tracked at high resolution and can be treated as a proxy for the fly’s own movements. We used the genetically encoded calcium sensor, GCaMP3.0, to record from important elements of the motion-processing pathway, the horizontal-system (HS) lobula plate tangential cells (LPTCs) in the fly optic lobe. We presented motion stimuli to the tethered fly and found that calcium transients in HS-neurons correlated with robust optomotor behavior during walking. Our technique allows an entirely new set of questions to be addressed by monitoring behavior and physiology in identified neurons in a powerful genetic model organism with an extensive repertoire of walking behaviors.

Changes in behavioral state modify neural activity in many systems. In some vertebrates such modulation has been observed and interpreted in the context of attention and sensorimotor coordinate transformations. Here we report state-dependent activity modulations during walking in a visual-motor pathway of Drosophila. We used two-photon imaging to monitor intracellular calcium activity in motion-sensitive lobula plate tangential cells (LPTCs) in head-fixed Drosophila walking on an air-supported ball. Cells of the horizontal system (HS)--a subgroup of LPTCs--showed stronger calcium transients in response to visual motion when flies were walking rather than resting. The amplified responses were also correlated with walking speed. Moreover, HS neurons showed a relatively higher gain in response strength at higher temporal frequencies, and their optimum temporal frequency was shifted toward higher motion speeds. Walking-dependent modulation of HS neurons in the Drosophila visual system may constitute a mechanism to facilitate processing of higher image speeds in behavioral contexts where these speeds of visual motion are relevant for course stabilization.