Odour-evoked [Ca²⁺] transients in mitral cell dendrites of frog olfactory glomeruli

Abstract

We measured Ca²⁺ concentration, [Ca²⁺], transients in mitral cell distal apical dendritic tufts produced by physiological odour stimulation of the olfactory epithelium and electrical stimulation of the olfactory nerve (ON) using two-photon scanning and conventional wide-field microscopy of Ca²⁺-Green-1 dextran in an in vitro frog nose–brain preparation. Weak or strong ON shock-evoked fluorescence transients always had short latency with an onset 0–10 ms after the onset of the bulb local field potential, rapidly increasing to a peak of up to 25% fractional fluorescence change (ΔF/F) in 10–30 ms, were blocked by 10 µm CNQX, decaying with a time constant of about 1 s. With stronger ON shocks that activated many receptor axons, an additional, delayed, sustained AP5-sensitive component (peak at ≈ 0.5 s, up to 40% ΔF/F maximum) could usually be produced. Odour-evoked [Ca²⁺] transients sometimes displayed a rapid onset phase that peaked within 50 ms but always had a sustained phase that peaked 0.5–1.5 s after onset, regardless of the strength of the odour or the amplitude of the response. These were considerably larger (up to 150% ΔF/F) than those evoked by ON shock. Odour-evoked [Ca²⁺] transients were also distinguished from ON shock-evoked transients by tufts in different glomeruli responding with different delays (time to onset differed by up to 1.5 s between different tufts for the same odour). Odour-evoked [Ca²⁺] transients were increased by AMPA-kainate receptor blockade, but substantially blocked by AP5. Electrical stimulation of the lateral olfactory tract (5–6 stimuli at 10 Hz) that evoked granule cell feedback inhibition, blocked 60–100% of the odour-evoked [Ca²⁺] transient in tufts when delivered within about 0.5 s of the odour. LOT-mediated inhibition was blocked by 10 µm bicuculline.