Figure 4.
Membrane Properties of Small-Diameter Dorsal Root Ganglion Neurons (DRGNs) from WT and DEF Mice
Current-clamp recordings were performed on DRGNs 6-month-old mice. Membrane input resistance (Ri) was determined by evaluating membrane voltage changes in response to negative and positive current injection. The ohmic relations were fitted with linear regression and the Ri derived from the slope.
(A) Representative traces of displacement-clamp currents recorded using CsCl/NMG-based pipette solution in response to ~250-ms mechanical displacement steps of ~0.42 μm to WT small-diameter DRGN (shown as inset). DRGNs were held at −70 mV. Summary data of displacement-response relationship of mechanically activated (MA) currents (IMA) represented as the I/Imax or open channel probability (Po) against displacement (X) fitted with single Boltzmann function. Data from WT DRGNs (shown in black symbols and fitted with sigmoidal curve in black) and the one-half maximum displacements (X1/2) are 1.1 ± 0.1 μm and 0.3 ± 0.1 μm (n = 11). Data from DEF DRGNs (shown in blue symbols and fitted with sigmoidal curve in blue) and the one-half maximum displacements (X1/2) are 1.5 ± 0.1 μm and 0.4 ± 0.1 μm (n = 7).
(B) Among the small-diameter neurons, there were three distinct classes: fast, medium, and slow adapting. Exemplary plots from fast-adapting DRGNs from WT mice (shown in black, mean Ri in MΩ; 100 ± 6; n = 15), and in DEF (shown in blue, mean Ri in MΩ; 45 ± 5; n = 17: p< 0.0001). The inset is an example of data used to generate the plots. For medium-adapting neurons the Ri (in MW) were as follows: WT (306 ± 23; n = 9) and DEF (162 ± 29; n = 11: p = 0.0014). For slow-adapting neurons the Ri were WT (626 ± 47; n = 13) and DEF (395 ± 38; n = 11: p = 0.0012).
(C) Brief (~5 ms) stepwise positive current was injected to elicit subthreshold (shown in different color codes) and threshold depolarization (WT in black and DEF in blue). The threshold currents are indicated. The threshold voltage was determined, using a dV/dt loop plot (inset, right).
(D and E) Typical voltage response from slow-adapting DRGNs recorded from WT and DEF mice.
(F and G) Action potentials generated using varying pulse durations from fast-adapting DRGNs in WT (F) and DEF (G) mice.
(H) Plots of the relations between threshold potential and pulse duration in WT (in black) and DEF (in blue) mice. The insets show the dV/dt versus membrane potential (V) loops used to determine the thresholds.