Coincidence Detection within the Excitable Rat Olfactory Bulb Granule Cell Spines

Area of Science:

• Neuroscience
• Cellular Neuroscience
• Computational Neuroscience

Background:

• Mammalian olfactory bulb granule cells (GCs) possess reciprocal synapses with mitral and tufted cells.
• GCs exhibit excitable spines capable of generating local action potentials (APs) or 'spine spikes'.
• GCs can also fire global APs that propagate dendritically, influencing synaptic integration.

Purpose of the Study:

• To investigate the summation rules of local spine Ca2+ entry and global APs in GC dendrites.
• To elucidate the role of voltage-gated channels and temporal dynamics in synaptic integration.
• To validate computational models with experimental data in rat olfactory bulb slices.

Main Methods:

• Developed a compartmental GC model to simulate Ca2+ summation under varying temporal separations (Δt) of local and global signals.
• Experimentally validated simulations using two-photon glutamate uncaging at spines and somatic current injection to evoke APs.
• Imaged spine Ca2+ signals in acute slices of juvenile rat brain (both sexes).

Main Results:

• Simulations revealed strongly sublinear Ca2+ summation for coincident (Δt = 0 ms) local and global signals, with efficiency rising for non-coincident inputs.
• Sublinear summation at coincidence depends on voltage-gated Na+ channels in the spine head, not NMDARs.
• Experimental data confirmed simulated summation efficiency and showed that synaptically evoked global APs and EPSPs occur at Δt ≈ 10 ms, explaining observed linear summation.

Conclusions:

• GC spine Ca2+ signaling exhibits sublinear summation for coincident local and global inputs due to spine head voltage-gated Na+ channels.
• The temporal dynamics of GC activation explain the integration of synaptic inputs and global APs.
• Findings support the existence and functional significance of the GC spine spike mechanism in olfactory processing.