Local postsynaptic signalling on slow time scales in reciprocal olfactory bulb granule cell spines matches asynchronous release

Abstract

In the vertebrate olfactory bulb (OB), axonless granule cells (GC) mediate self- and lateral inhibitory interactions between mitral/tufted cells via reciprocal dendrodendritic synapses. Locally triggered release of GABA from the large reciprocal GC spines occurs on both fast and slow time scales, possibly enabling parallel processing during olfactory perception. Here we investigate local mechanisms for asynchronous spine output.To reveal the temporal and spatial characteristics of postsynaptic ion transients, we imaged spine and adjacent dendrite Ca2+- and Na+-signals with minimal exogenous buffering by the respective fluorescent indicator dyes upon two-photon uncaging of DNI-glutamate in OB slices from juvenile rats. Both postsynaptic fluorescence signals decayed slowly, with average half durations in the spine head of t1/2_Δ[Ca2+]i ~500 ms and t1/2_Δ[Na+]i ~1000 ms. We also analysed the kinetics of already existing data of postsynaptic spine Ca2+-signals in response to glomerular stimulation in OB slices from adult mice, either WT or animals with partial GC glutamate receptor deletions (NMDAR: GluN1 subunit; AMPAR: GluA2 subunit). In a large subset of spines the fluorescence signal had a protracted rise time (average time to peak ~400 ms, range 20 ms - >1000 ms). This slow rise was independent of Ca2+ entry via NMDARs, since similarly slow signals occurred in ΔGluN1 GCs. Additional Ca2+ entry in ΔGluA2 GCs (with AMPARs rendered Ca2+-permeable), however, resulted in larger ΔF/Fs that rose yet more slowly.Thus GC spines appear to dispose of several local mechanisms to promote asynchronous GABA release, which are reflected in the time course of mitral/tufted cell recurrent inhibition.

Presynaptic NMDARs cooperate with local spikes toward GABA release from the reciprocal olfactory bulb granule cell spine

“…In any case, substantial asynchronous release from the GC spine on yet longer time scales (detected at up to 500 ms post TPU) was frequently observed, in line with earlier studies on recurrent inhibition that localized the origin of asynchronous signaling within GCs ( Schoppa et al, 1998 ; Chen et al, 2000 ; Isaacson, 2001 ). This time course of asynchronous release also matches with the duration of physiological postsynaptic GC spine Ca 2+ transients ( Ona Jodar et al, 2020 ). Thus, the microcircuit can operate across a wide range of latencies, which might contribute to glomerulus-specific GC global AP firing latencies ( Kapoor and Urban, 2006 ), and also can generate combined synchronous and asynchronous output.…”

“…In addition, TPU-evoked signals can be expected to be localized to the stimulated GC spine: there is no Ca 2+ influx into the adjacent dendritic shaft upon TPU, as we have shown in GC Ca 2+ imaging experiments that were performed previously (dye 100 µM OGB-1, average amplitude ratio ∆F/F spine:dendrite 46:2 Bywalez et al, 2015 ) and also interleaved with the MC recordings in this study, using a low-affinity dye (100 µM OGB-6F, ratio ∆F/F spine:dendrite 24:2, Ona Jodar et al, 2020 ). Finally, we also tested via Ca 2+ imaging whether neighboring GC spines of the same GC could sense uncaged glutamate (n = 11 spine pairs, average distance between stimulated and non-stimulated spine heads 4.1 ± 2.2 µm, range 1.5–6.8 µm, Figure 1—figure supplement 1C,D,E ).…”