Eavesdropping wires: Recording activity in axons using genetically encoded calcium indicators

Abstract

Neurons broadcast electrical signals to distal brain regions through extensive axonal arbors. Genetically encoded calcium sensors permit the direct observation of action potential activity at axonal terminals, providing unique insights on the organization and function of neural projections. Here, we consider what information can be gleaned from axonal recordings made at scales ranging from the summed activity extracted from multi-cell axon projections to single boutons. In particular, we discuss the application of different recently developed multi photon and fiber photometry methods for recording neural activity in axons of rodents. We define experimental difficulties associated with imaging approaches in the axonal compartment and highlight the latest methodological advances for addressing these issues. Finally, we reflect on ways in which new technologies can be used in conjunction with axon calcium imaging to address current questions in neurobiology.

Keywords: Axons; Calcium imaging; Fiber photometry; Genetically-encoded calcium indicators; Neural circuits; Neural projections; Protein engineering; Rodents; Two-photon imaging.

Fig. 1.

Differences between recordings of calcium signals in axons using fiber photometry and two-photon microscopy. A, Both methods allow recording activity in specific projections. Fiber photometry (left) in not image forming and is amenable to recordings during freely moving behaviors and from regions at any depth in the brain. Two-photon microscopy (right) results in an image of the recorded axons but it is limited to head-fixed animals and, unless tissue is removed, to superficial areas. B, Neurons in a given area can project to different targets. Their activity (black ticks) is locked to different behavioral events (magenta ticks). Calcium activity in axons in the target area reflects somatic activity and is correlated across boutons of the same axons, albeit with different amplitude. Left, fiber photometry-based recordings reflect the average population activity in the target region. Right, using two-photon microscopy, calcium activity can be extracted from individual boutons. Given the high correlation across boutons of the same axon, activity be quantified both in axons and boutons.