Resolving different presynaptic activity patterns within single olfactory glomeruli of Xenopus laevis larvae

Area of Science:

• Neuroscience
• Olfactory System Research
• Sensory Processing

Background:

• Olfactory receptor neurons (ORNs) typically converge into glomeruli, presumed to be uniform functional units.
• The degree of axonal activity homogeneity within a glomerulus and the number of converging ORNs remain unclear.
• Visualizing individual ORN axon activity in live imaging presents challenges.

Purpose of the Study:

• To investigate the activity of ORN axon terminals within olfactory glomeruli.
• To determine the homogeneity of presynaptic calcium activity among ORN axons responding to the same odor.
• To explore methods for visually distinguishing ORN axon subgroups within glomeruli.

Main Methods:

• Electrophysiological recordings and rapid 4D calcium imaging of ORN activity.
• Analysis of spontaneous and stimulus-evoked calcium activity at the axonal level.
• Application of olfactory stimuli with varying amino acid concentrations.
• Automated region-of-interest detection and correlation analysis for visual distinction of axon subgroups.

Main Results:

• Presynaptic calcium activity in olfactory glomeruli is homogeneous across ORN axons responsive to the same odor, despite somatic response diversity.
• Correlated spontaneous calcium activity, observed postsynaptically, was not detected presynaptically.
• Spatio-temporal inhomogeneities in presynaptic responses were induced by olfactory stimuli trains.
• At least two distinct axon subgroups per glomerulus, differing in odor sensitivity, were visually identified.

Conclusions:

• Olfactory glomeruli, at the presynaptic axon terminal level, exhibit functional homogeneity under uniform stimulation.
• Specific stimulation protocols can reveal functional subdivisions within glomeruli, indicating distinct axon subgroups.
• This study provides a method to visually separate and analyze functional differences among ORN axon populations within olfactory glomeruli.