Functional optical imaging from bat inferior colliculus using a micro-endoscope

Abstract

We developed a confocal micro-endoscopic system that monitors responses of multiple neurons as fluorescence changes reflecting calcium activation derived from the calcium fluorescence indicator dye, Oregon green BAPTA-1, AM. We recorded calcium response from different depths (0 - 1300 μm from dorsal surface) of the inferior colliculus (IC) in two species of bats (Carollia perspicillata and Eptesicus fuscus) to white noise bursts and to FM sweeps. Full horizontal-vertical video image scanning of the entire fiber optic bundle’s face (normal-scan) was limited to 18.9 frames/s by the scanning mirror, but isolation and recording of successive individual horizontal scans (line-scan), done by freezing the vertical scan, yielded 7,550 lines/s, or 132 microseconds for 1 line. Line-scanning confirmed that the calcium responses were slow, in the 70-130 ms range. Peak latency of the calcium responses were negatively correlated with recording depth (presumably tonotopic organization) for all stimuli and for both bat species. There were significant differences between peak latencies of calcium responses for noise vs FM sounds in both bats. The slow time-course of calcium responses matches the slow inhibitory events that determine response latencies in the IC, not the fast time-course of responses that encode biosonar information itself.