Improved two-photon imaging of GPCR-based optogenetic neurotransmitter sensors using orthogonally polarized excitation

Fluorescent proteins such as GFP are best excited by light that is polarized parallel to the dipole axis of the fluorophore. In most cases, fluorescent proteins are randomly oriented, resulting in unbiased images even when polarized light is used for excitation, e.g. in two-photon microcopy. Here we reveal a surprisingly strong polarization sensitivity in a class of GPCR-based neurotransmitter sensors where the fluorophore is anchored on both ends. In tubular structures such as dendrites, this effect led to a complete loss of membrane signal in dendrites running parallel to the polarization direction of the excitation beam. Our data reveal a major problem for two-photon measurements of neurotransmitter concentration that has not been recognized by the neuroscience community. To remedy the sensitivity to dendritic orientation, we designed an optical device that generates interleaved pulse trains of orthogonal polarization, removing the orientation bias from images. The passive device, which we inserted in the beam path of an existing two-photon microscope, also removed the strong direction bias in second harmonic generation (SHG) images. We conclude that for optical measurements of transmitter concentration with GPCR-based sensors, orthogonally polarized excitation is essential.