The spinal dorsal horn comprises heterogeneous populations of interneurons and projection neurons, which form neuronal circuits crucial for processing of primary sensory information. Although electrophysiological analyses have uncovered sensory stimulation-evoked neuronal activity of various spinal dorsal horn neurons, monitoring these activities from large ensembles of neurons is needed to obtain a comprehensive view of the spinal dorsal horn circuitry. In the present study, we established in vivo calcium imaging of multiple spinal dorsal horn neurons by using a two-photon microscope and extracted three-dimensional neuronal activity maps of these neurons in response to cutaneous sensory stimulation. For calcium imaging, a fluorescence resonance energy transfer (FRET)-based calcium indicator protein, Yellow Cameleon, which is insensitive to motion artifacts of living animals was introduced into spinal dorsal horn neurons by in utero electroporation. In vivo calcium imaging following pinch, brush, and heat stimulation suggests that laminar distribution of sensory stimulation-evoked neuronal activity in the spinal dorsal horn largely corresponds to that of primary afferent inputs. In addition, cutaneous pinch stimulation elicited activities of neurons in the spinal cord at least until 2 spinal segments away from the central projection field of primary sensory neurons responsible for the stimulated skin point. These results provide a clue to understand neuronal processing of sensory information in the spinal dorsal horn.

Photobiomodulation has analgesic effects via inhibition of nerve activity, but few reports have examined the effects on the spinal dorsal horn, the entry point for nociceptive information in the central nervous system. In this study, we evaluated the effects of laser irradiation of peripheral nerve axons, which are conduction pathways for nociceptive stimuli, on the neuronal firing in lamina II of the spinal dorsal horn of a rat evoked by mechanical stimulation with von Frey filaments (vFF). In order to record neuronal firing, electrodes were inserted into lamina II of the exposed rat spinal dorsal horn. The exposed sciatic nerve axons were irradiated with an 808 nm laser. The 26.0 g vFF-evoked firing frequency was inhibited from 5 min after laser irradiation and persisted for 3 h. Sham irradiation did not alter the firing frequency. Laser irradiation selectively inhibited 15.0 and 26.0 g vFF-evoked firing, which corresponded to nociceptive stimuli. Histopathological evaluation revealed no damage to the sciatic nerve due to laser irradiation. These results indicate that neuronal firing is inhibited in lamina II of the spinal dorsal horn, suggesting that laser irradiation inhibits Aδ and/or C fibers that conduct nociceptive stimuli.

Background: Although the widely used single L-enantiomers of local anesthetics have less toxic effects on the cardiovascular and central nervous systems, the mechanisms mediating their antinociceptive actions are not well understood. The authors hypothesized that significant differences in the ion channel blocking abilities of the enantiomers of bupivacaine would be identified. Methods: The authors performed electrophysiologic analysis on rat dorsal root ganglion neurons in vitro and on spinal transmissions in vivo. Results: In the dorsal root ganglion, these anesthetics decreased the amplitudes of action potentials. The half-maximum inhibitory concentrations of D-enantiomer D-bupivacaine were almost equal for Aβ (29.5 μM), Aδ (29.7μM), and C (29.8 μM) neurons. However, the half-maximum inhibitory concentrations of L-bupivacaine was lower for Aδ (19.35 μM) and C (19.5 μM) neurons than for A β (79.4 μM) neurons. Moreover, D-bupivacaine almost equally inhibited tetrodotoxin-resistant (mean ± SD: 15.8 ± 10.9% of the control, n = 14, P < 0.001) and tetrodotoxin-sensitive (15.4 ± 15.6% of the control, n = 11, P = 0.004) sodium currents. In contrast, L-bupivacaine suppressed tetrodotoxin-resistant sodium currents (26.1 ± 19.5% of the control, n = 18, P < 0.001) but not tetrodotoxin-sensitive sodium currents (74.5 ± 18.2% of the control, n = 11, P = 0.477). In the spinal dorsal horn, L-bupivacaine decreased the area of pinch-evoked excitatory postsynaptic currents (39.4 ± 11.3% of the control, n = 7, P < 0.001) but not touch-evoked responses (84.2 ± 14.5% of the control, n = 6, P = 0.826). In contrast, D-bupivacaine equally decreased pinch- and touch-evoked responses (38.8 ± 9.5% of the control, n = 6, P = 0.001, 42.9 ± 11.8% of the control, n = 6, P = 0.013, respectively). Conclusions: These results suggest that the L-enantiomer of bupivacaine (L-bupivacaine) effectively inhibits noxious transmission to the spinal dorsal horn by blocking action potential conduction through C and Aδ afferent fibers. Editor’s Perspective: What We Already Know about This Topic: What This Article Tells Us That Is New:

The expression of hormonal receptors in pain-processing regions complicates understanding the hormonal effects on pain mechanisms. This study investigates androgen receptor (AR) involvement in pain sensitivity and sex differences in pain perception. Mechanical pain thresholds were higher in normal male mice compared to gonadectomized (GDX) male and normal female mice, correlating with serum testosterone levels. In the dorsal root ganglia (DRG), AR was expressed in normal males but undetectable in GDX males and normal females. Androgen receptor overlapped with NeuN, a neuronal nuclei marker, indicating androgen signaling activation in sensory neurons. In male sensory neuron–selective AR conditional knockout (AR-cKO) mice, mechanical pain thresholds were significantly lower than in wild-type males, with the greatest AR depletion in calcitonin gene–related peptide (CGRP) ⁺ neurons. Electrophysiological analyses revealed increased excitability of spinal dorsal horn neurons in both GDX males and AR-cKO males. In female mice, administration of testosterone propionate or dihydrotestosterone significantly raised mechanical pain thresholds, accompanied by increased AR expression in the DRG. This effect was abolished in AR-cKO females, where AR depletion was most prominent in CGRP ⁺ neurons, consistent with male findings. These results indicate that primary sensory neurons, particularly CGRP ⁺ neurons, are critical targets of androgen in regulating mechanical pain sensitivity. Therefore, manipulating androgen signaling in sensory neurons may offer a promising approach to managing mechanical pain.

Photobiomodulation is an effective treatment for pain. We previously reported that the direct laser irradiation of the exposed sciatic nerve inhibited firing in the rat spinal dorsal horn evoked by mechanical stimulation, corresponding to the noxious stimulus. However, percutaneous laser irradiation is used in clinical practice, and it is unclear whether it can inhibit the firing of the dorsal horn. In this study, we investigated whether the percutaneous laser irradiation of the sciatic nerve inhibits firing. Electrodes were inserted into the lamina II of the dorsal horn, and mechanical stimulation was applied using von Frey filaments (vFFs) with both pre and post laser irradiation. Our findings show that percutaneous laser irradiation inhibited 26.0 g vFF-evoked firing, which corresponded to the noxious stimulus, but did not inhibit 0.6 g and 8.0 g vFF-evoked firing. The post- (15 min after) and pre-irradiation firing ratios were almost the same as those for direct and percutaneous irradiation. A photodiode sensor implanted in the sciatic nerve showed that the power density reaching the sciatic nerve percutaneously was attenuated to approximately 10% of that on the skin. The relationship between the laser intensity reaching the nerve and its effect could be potentially useful for a more appropriate setting of laser conditions in clinical practice.