Korean J Pain.  2018 Jan;31(1):10-15. 10.3344/kjp.2018.31.1.10.

Allopregnanolone suppresses mechanical allodynia and internalization of neurokinin-1 receptors at the spinal dorsal horn in a rat postoperative pain model

• Fujita M
• Fukuda T
• Sato Y
• Takasusuki T
• Tanaka M

• KMID: 2400903
• DOI: http://doi.org/10.3344/kjp.2018.31.1.10

Abstract

BACKGROUND
To identify a new strategy for postoperative pain management, we investigated the analgesic effects of allopregnanolone (Allo) in an incisional pain model, and also assessed its effects on the activities of the primary afferent fibers at the dorsal horn.
METHODS
In experiment 1, 45 rats were assigned to Control, Allo small-dose (0.16 mg/kg), and Allo large-dose (1.6 mg/kg) groups (n = 15 in each). The weight bearing and mechanical withdrawal thresholds of the hind limb were measured before and at 2, 24, 48, and 168 h after Brennan's surgery. In experiment 2, 16 rats were assigned to Control and Allo (0.16 mg/kg) groups (n = 8 in each). The degree of spontaneous pain was measured using the grimace scale after the surgery. Activities of the primary afferent fibers in the spinal cord (L6) were evaluated using immunohistochemical staining.
RESULTS
In experiment 1, the withdrawal threshold of the Allo small-dose group was significantly higher than that of the Control group at 2 h after surgery. Intergroup differences in weight bearing were not significant. In experiment 2, intergroup differences in the grimace scale scores were not significant. Substance P release in the Allo (0.16 mg/kg) group was significantly lower than that in the Control group.
CONCLUSIONS
Systemic administration of Allo inhibited mechanical allodynia and activities of the primary afferent fibers at the dorsal horn in a rat postoperative pain model. Allo was proposed as a candidate for postoperative pain management.

Keyword

Allopregnanolone; Mechanical allodynia; Neurokinin-1 receptor; Postoperative pain; Spinal dorsal horn; Substance P

MeSH Terms

Animals
Extremities
Hyperalgesia*
Pain, Postoperative*
Pregnanolone*
Rats*
Receptors, Neurokinin-1*
Spinal Cord
Spinal Cord Dorsal Horn*
Substance P
Weight-Bearing
Pregnanolone
Receptors, Neurokinin-1
Substance P

Figure

• 

  Fig. 1 Summary of withdrawal thresholds to von Frey filaments before and after surgery. The results are expressed as medians with 1st and 3rd quartiles, and 10th and 90th percentiles. White column: Control group, Light gray column: Allopregnanolone 0.16 mg/kg group, Dark gray column: Allopregnanolone 1.6 mg/kg group. *P < 0.05 (Control vs Allopregnanolone 0.16 mg/kg group) by Kruskal-Wallis test and Steel's test. #P < 0.05 (vs pre-incision values) by Wilcoxon's signed rank test.

• 

  Fig. 2 Positive cell number of neurokinin-1 receptor (Nk-1r) internalization (gray column) and total observation cell number (gray + white column) at 45 minutes after surgery. Cont: Control group, Allo: allopregnanolone (0.16 mg/kg) group. *P < 0.05 vs Control group, #P < 0.05 vs Contralateral side by Fisher exact test.

• 

  Fig. 3 Representative confocal fluorescent microscopic images of neurokinin-1 receptor (NK-1r) and NeuN at L6 lumbar spinal dorsal horn from incision model rats. Green: NK-1r, Red: NeuN. (A) An image of NK-1r in the lamina I. Notice the presence of NK-1r on a neuron membrane and the lack of NK-1r containing endosomes in the cytoplasm. (B) An image of incision-induced NK-1r internalization. Notice the presence of NK-1r containing endosomes in the cytoplasm. Scale bar is 10 µm.

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