Fig 1.
MRL/Lpr mice develop thermal hyperalgesia in the hind paw.
Line plots show the average (± SD) of the paw withdrawal latency to radiant heat stimuli between the age of 8 weeks to 16 weeks in MRL/Lpr (N = 12) and control MRL mice (N = 9). Comparisons between data at the age 8 weeks to each of the other weeks in MRL/Lpr mice are labeled with #. Comparisons between MRL/Lpr mice and the control mice at each time point are labeled with *. One symbol: P < 0.05; three symbols: P < 0.001.
Fig 2.
Nociceptive sensory neurons in the DRG from MRL/Lpr mice have increased excitability.
Nociceptive sensory neurons in the DRGs from lupus mice have elevation of resting membrane potentials, smaller membrane capacitances, lower rheobases, and more negative action potential thresholds. Raw data (A and B) show samples of membrane potential responses (left) to steps of intracellular current injection (center), and measurements of AP amplitude, AP half duration, and AP thresholds (right) in control (A) and lupus (B) mice. Note significantly different resting membrane potentials, rheobases, and action potential thresholds between lupus mice and control mice. Bar graphs show the average (+ SD) of resting membrane potential (C), input resistance (D), membrane capacitance (E), rheobase (F), action potential threshold (G), action potential amplitude (H), and action potential half duration (I) in nociceptive DRG neurons from control (26 neurons from 15 mice) and lupus mice (24 neurons from 10 mice). (J) shows percentage of neurons with multiple APs in the MRL/Lpr group (24 neurons from 10 mice) and the control group (26 neurons from 15 mice). Data obtained from individual neurons are shown in the scatter plot. AP: action potential; RMP: resting membrane potential. * P < 0.05; *** P < 0.001; ns: no significance.
Fig 3.
Cation inward currents, IA, and IK currents are not altered in MRL/Lpr mice.
(A): Raw data show voltage steps of 1s test pulses between -90 mV to +10 mV in 10 mV increments (Top) applied to a neuron, and the current responses from the neuron (Bottom). Inward and outward currents were evoked. The outward current had two components: the inactivating outward component at the initial stage (IA current) and the non-inactivating outward component (IK current). Line plots show summaries (average ± SD) of the inward currents (B), IA currents (C), and IK currents (D) obtained from control (26 neurons from 15 mice) and MRL/Lpr mice (24 neurons from 10 mice).
Fig 4.
MRL/Lpr mice have no changes in Nav1.7 protein expression but increased protein expression of TNAα, IL-1β, phosphorylated ERK1/2, and suppression of AMPA activity in the DRGs.
Bar graphs show protein expression (mean + SD) ratios of Nav1.7, TNFα, IL-1β to GAPDH, the ratio of phosphorylated ERK (pERK) to ERK, the ratio of phosphorylated AMPK to total AMPK in the DRGs from four MRL/Lpr mice and four control MRL mice. Data obtained from individual animals are shown in the scatter plot. Samples of each protein molecule expression in each group are shown below. * P < 0.05; ** P < 0.01; *** P < 0.001.
Fig 5.
Suppression of the AMPK activity leads to thermal hyperalgesia while activation of AMPK reduces thermal hyperalgesia.
(A): Line plots show summaries (means ± SD) of the paw withdrawal latency to radiant heat stimuli in control mice immediately before (baseline) and after intraplantar injection of Compound C (concentration: 25 μMol; volume: 25 μl; N = 12) or saline (25 μl, N = 12) was made. (B): Line plots show summaries (means ± SD) of the paw withdrawal latency to radiant heat stimuli in MRL/Lpr mice immediately before (baseline) and after intraplantar injection of AICAR (concentration: 1 mMol, volume: 25 μl; N = 10) or saline (25 μl, N = 10). Comparisons between data before and different time points after injection of compound C (A) or AICAR (B) are labeled with #. Comparisons of saline treated group and Compound C treated group (A) or AICAR treated group (B) are labeled with *. One symbol: P < 0.05; Two symbols: P < 0.01; Three symbols: P < 0.001.
Fig 6.
Pharmacological suppression of AMPK activity in normal nociceptive DRG neurons recapitulates membrane properties found in MRL/Lpr mice with thermal hyperalgesia.
Raw data (A and B) show samples of membrane potential responses (left) to steps of intracellular current injection (middle), and measurements of AP amplitude, AP half duration, and AP thresholds (right) before (baseline) (A) and during perfusion of compound C (concentration in the bath: 10 μMol) (B). Note different resting membrane potentials, rheobases, and action potential thresholds between baseline, and during perfusion of compound C. Bar graphs show the average (+ SD) of resting membrane potential (C), input resistance (D), membrane capacitance (E), rheobase (F), action potential threshold (G), action potential amplitude (H), and action potential half duration (I) in 14 nociceptive DRG neurons from 9 control mice before (baseline), during perfusion, and washout of compound C. Data obtained from individual neurons are shown in the scatter plot. AP: action potential; RMP: resting membrane potential. * P < 0.05; ** P < 0.01; ns: no significance.