Calcium buffering in rodent olfactory bulb granule cells and mitral cells

Abstract

In the mammalian olfactory bulb, axonless granule cells (GCs) mediate self- and lateral inhibitory interactions between mitral cells (MCs) via reciprocal dendrodendritic synapses. Calcium signals in the GC dendrites and reciprocal spines appear to decay unusually slowly, hence GC calcium handling might contribute to the known asynchronous release at this synapse. By recording fluorescence transients of different Ca(2+)-sensitive dyes at variable concentrations evoked by backpropagating action potentials (APs) and saturating AP trains we extrapolated Ca(2+) dynamics to conditions of zero added buffer for juvenile rat GC apical dendrites and spines and MC lateral dendrites. Resting [Ca(2+)] was at approximately 50 nM in both GC dendrites and spines. The average endogenous GC buffer capacities (kappa(E)) were within a range of 80-90 in the dendrites and 110-140 in the spines. The extrusion rate (gamma) was estimated as 570 s(-1) for dendrites and 870 s(-1) for spines and the decay time constant as approximately 200 ms for both. Single-current-evoked APs resulted in a [Ca(2+)] elevation of approximately 250 nM. Calcium handling in juvenile and adult mouse GCs appeared mostly similar. In MC lateral dendrites, we found AP-mediated [Ca(2+)] elevations of approximately 130 nM with a similar decay to that in GC dendrites, while kappa(E) and gamma were roughly 4-fold higher. In conclusion, the slow GC Ca(2+) dynamics are due mostly to sluggish Ca(2+) extrusion. Under physiological conditions this slow removal may well contribute to delayed release and also feed into other Ca(2+)-dependent mechanisms that foster asynchronous output from the reciprocal spine.

Figure 4. Ca²⁺ handling in juvenile rat mitral cells. Extrapolation to conditions of 0 added buffer

A, two-photon scan of rat mitral cells (PND 14) filled with 100 μm OGB-1 and somatic voltage recordings of the left cell's reponse to 500 ms depolarizing pulses (white trace). Scaling of the voltage trace similar to traces in B. B, somatic voltage recordings and simultaneous fluorescence transients ΔF/F imaged across the lateral dendrite shown in A, at the positions labelled 1 and 2. Depiction as in Fig. 1B. C, data points from recordings with 20, 50 and 100 μm OGB-1. Upper graph: extrapolation of decay half-duration data vs. buffering capacity κ_B in mitral cell lateral dendrites. Lower graph: inverse of absolute Δ[Ca²⁺] per single somatic AP. Continuous lines, linear fit; dotted lines, 2σ confidence interval; dashed lines, 1σ confidence interval. These intervals do not account for the uncertainty in the data points themselves. All error bars represent s.e.m. The errors in x-direction (s.e.m. of mean κ) are very small and thus barely visible.

Figure 1. Fluorescence transients ΔF/F in granule cells in response to single-current-evoked APs and AP trains. Fast equilibration

A, two-photon scan of a granule cell filled with 100 μm OGB-1 and somatic voltage recordings of this cell's response to 500 ms depolarizing pulses (white traces at the bottom). Scaling of the voltage trace similar to traces in B. B, somatic voltage recordings and corresponding averaged fluorescence transients ΔF/F imaged in line-scan mode across the proximal dendrite and a spine of the cell shown in A. The decay half-duration τ_1/2 was 650 ms in the dendrite and 430 ms in the spine. Left, responses to single APs (ΔF/F)_AP; on the right, responses to trains of 15 APs at 50 and 80 Hz, all evoked by somatic current injection. Scaling of all traces as indicated by the bars; note the condensed scaling of the ΔF/F responses to AP trains. C, examples of averaged (ΔF/F)_AP responses from two other granule cell dendrites, filled with 50 μm and 20 μm OGB-1 respectively, and recorded at the indicated times after break-in. Scaling of the voltage recordings and (ΔF/F)_AP transients similar to above.

Figure 5. Influence of background levels on [Ca²⁺]₀, peak [Ca²⁺] and κ_B

Measurements of [Ca²⁺]₀, peak [Ca²⁺] and κ_B for OGB-1 in individual locations in GC dendrites (upper row) and spines (lower row) versus the fluorescence background measured next to these locations. The lines indicate the linear fits, r the Spearman correlation coefficient and P the significance of the difference between the determined r and r= 0 (no correlation). The shown data include all GC OGB-1 measurements at 20, 50 and 100 μm; to heighten the sensitivity to possible correlations, peak [Ca²⁺] and κ_B were normalized to their average value at the concentration at hand and pooled.

Figure 2. Extrapolation to conditions of 0 added buffer for granule cell dendrites and spines; data with low-affinity dye

A, upper graphs: extrapolation of decay half-duration data versus exogenous buffering capacity κ_B in dendrites and spines; lower graphs: inverse of absolute change Δ[Ca²⁺] per single somatic AP. Continuous lines, linear fit; dotted lines, 2σ confidence interval; dashed lines, 1σ confidence interval. These intervals do not account for the uncertainty in the data points themselves. All error bars represent s.e.m. The errors in x-direction (s.e.m. of mean κ_B) are very small and thus not visible. B, relation of spine to dendrite Δ[Ca²⁺]_AP (grey filled diamonds) and of τ_1/2 (black filled triangles) of individual pairs of spines and their parent dendrites versus the concentration of OGB-1 (9 pairs for each data point). The open symbols represent data from mouse (16 pairs). The lines represent linear fits of the rat data, the error bars again represent s.e.m. C, representative data for the low-affinity dye OGB-6F. About 20 averaged traces for ΔF/F. The decay half-duration was τ_1/2≈ 200 ms for the dendrite and τ_1/2≈ 240 ms for the adjacent spine. The inset shows a scan of the respective granule cell with the scale bar corresponding to 10 μm; the data shown were recorded from the upper spine next to the GC soma.

Figure 3. Ca²⁺ handling in mouse granule cells

A, two-photon scan of a mouse granule cell (PND 36) filled with 100 μm OGB-1 and somatic voltage recordings of this cell's reponse to 500 ms depolarizing pulses (white trace). Scaling of the voltage trace similar to traces in B. B, somatic voltage recordings and simultaneous fluorescence transients ΔF/F imaged in line-scan mode across the proximal dendrite and a spine of the cell shown in A. Depiction as in Fig. 1B. C, comparison between mouse and rat data for dendrites (D) and spines (S); all using 100 μm OGB-1. The rat data were normalized to 100%. The mouse data are shown as relative fractions and included 24 GCs of mice aged PNDs 16–43.

Figure 6. Estimated average Ca²⁺ dynamics under physiological conditions in granule cells, mitral cell lateral dendrites and hippocampal CA1 cells

This figure illustrates the estimated mean dimensions and the extrapolated Ca²⁺ dynamics of the investigated structures (GC spine and dendrite, MC lateral dendrite; Tables 1 and 2) and a spine on a fine hippocampal pyramidal cell dendrite for comparison. The olfactory bulb anatomical data were in part taken from Woolf et al. (1991a; see Methods and Results). The hippocampal data were taken from Harris & Stevens (1989) and Sabatini et al. (2002; their Table 1), respectively. The black traces show the mean Ca²⁺ transient, the grey band denotes the s.d. with respect to Δ[Ca²⁺]_AP and τ. B shows the extrapolated buffering capacities and extrusion rates for GC spines and dendrites and MC lateral dendrites under physiological conditions.