Divergent Changes in PBN Excitability in a Mouse Model of Neuropathic Pain

After discharges do not reflect intrinsic membrane properties of PBN neurons. A, Experimental design for investigating afterdischarges. CCI-ION was used as a model for peripheral nerve injury and electrophysiological data were acquired from horizontal sections through the PBN. B, CCI-ION results in mechanical hypersensitivity in both sexes that persisted for at least 2 weeks after injury (C). Summary of the number of neurons from sham and CCI mice tested with different current injection protocols in attempts to evoke afterdischarges. We recorded from a total of 12 neurons from sham and 11 neurons from CCI mice, but some PBN neurons were tested with multiple protocols. Examples of each protocol include square pulses of fixed duration and increasing magnitude (D), depolarizing sinusoidal currents (E), and square pulses of fixed intensity and increasing duration (F). Afterdischarges, such as the one following the first depolarizing square pulse in F, were rarely observed and could not be consistently evoked, in any PBN neuron. Mixed-effects analysis with Sidak's multiple-comparisons test, *p < 0.05.

Phasic GABA_B inhibition of inhibitory afferents in PBN is enhanced after nerve injury. A, Experimental schematic. B, Samples of electrically evoked IPSCs in a PBN neuron from a sham (top) and CCI (bottom) mouse. Each depicts an overlay of 20 responses to paired-pulse stimulation (50 ms interpulse interval). All traces have been baseline corrected. Neither the amplitude (C) or PPR (D) was affected by CCI. E, Sample recordings from sham (top) and CCI neurons (bottom) before and after bath application of the GABA_B agonist baclofen (1 μM, red trace). F, As a population, baclofen had no effect on PPR in neurons from sham mice but significantly increased PPR in neurons from CCI-ION mice. In contrast, the GABA_B antagonist CGP55845 (1 μM) had no effect on PPR from either group (G,H). Each trace in (E) and (G) are the average of 20 baseline-corrected individual trials, and the stimulus artifact has been truncated vertically for clarity. Wilcoxon matched-pairs test: *p < 0.05.

Spontaneous firing frequencies and action potential properties are unaltered in a neuropathic injury model. A, Example traces of spontaneous firing in neurons from sham (black) and cuff (magenta) animals. B, Example action potentials from panel A indicated by horizontal bars. There were no differences between neurons from sham and cuff cells in (C) capacitance, (D) input resistance, (E) firing frequency, (F) action potential threshold, (G) peak, (H) rise time from threshold to peak, (I) decay from peak to 90% of threshold, or (J) total action potential width.

Heterogeneous firing types of PBN neurons. A, Representative traces of RS, LF, and reluctant neurons. B, Representative traces of neurons that did not fit the main firing types. C, Input–frequency and (D) rheobase for each firing type. Intrinsic properties, including (E) capacitance, (F) input resistance, and (G) resting membrane potential were all different between firing types. Data for spontaneous neurons are replicated from Figure 2 for comparison. Each datapoint represents a cell. Bonferroni corrected t test: *p < 0.05, **p < 0.01, ****p < 0.0001. Main effect from mixed-effects analysis: ^{^^}p < 0.01, ^{^^^^}p < 0.0001.