Axonal correlates of striatal calcium transients measured by fiber photometry

Fiber photometry is a widely used technique in neuroscience for monitoring neural activity in specific cell populations of freely behaving animals.¹ Utilizing genetically encoded calcium indicators (GECIs) like GCaMP, this method detects changes in intracellular calcium levels, which reflect neuronal activity.^1,2,3,4 Traditionally, it has been assumed that fiber photometry signals primarily represent action-potential-induced calcium influx in neuronal somas. However, recent studies have questioned this view, with the observation that a significant portion of the recorded calcium signal originates from the neuropil.^5,6 Because neuropil calcium flux can be induced by other biological phenomena, namely subthreshold events that are uncorrelated with action-potential firing, this finding has consequences for the interpretation of the fiber photometry signal. Thus, whether fiber photometry is a reliable reporter of action-potential firing is still an open question. To investigate this, we conducted simultaneous recordings from indirect and direct pathway spiny projection neurons (SPNs) in the ventrolateral striatum (VLS) and their axonal projections to the globus pallidus externus (GPe) and substantia nigra reticulata (SNr) in freely behaving mice. Our findings revealed a robust correlation between calcium signals in the cell bodies and axons of SPNs, indicating that fiber photometry captures activity propagating along neuronal projections. Understanding the precise origins of these signals is crucial for the accurate interpretation of neuronal dynamics and for designing experiments that utilize fiber photometry to study brain function.