Expression and Kinetics of Endogenous Cannabinoids in the Brain and Spinal Cord of a Spare Nerve Injury (SNI) Model of Neuropathic Pain

Abstract

The role of endogenous cannabinoids in neuropathic pain has been actively studied, among which 2-arachidonoyl glycerol (2-AG) has received the most attention. However, owing to its chemical properties, direct detection of 2-AG distribution in tissues is difficult. Moreover, although desorption electrospray ionization mass spectrometry imaging (DESI-MSI) has enabled the detection of 2-AG, its distribution in the brain and spinal cord of neuropathic pain models has not been reported. In this study, the expression and distribution of 2-AG in the brain and spinal cord of a spare nerve injury (SNI) mice model of neuropathic pain was examined using DESI-MSI. The brain and lumbar spinal cord were collected and analyzed on days 3, 7, and 21 after treatment. On days 3 and 7 after treatment, 2-AG expression in the SNI model was decreased in the hypothalamus, midbrain, and especially in the periaqueductal gray (PAG) region but increased in the lumbar spinal cord. On day 21, the SNI model showed decreased 2-AG expression in the hypothalamus, but the difference from the control was not significant. Furthermore, there were no differences in 2-AG expression between the lumbar spinal cord, midbrain, or PAG. These data suggest that 2-AG might be involved in pain control.

Keywords: 2-arachidonoyl glycerol; DESI-MSI; endogenous cannabinoids; hypothalamus; lumbar spinal cord; periaqueductal gray; spare nerve injury.

Figure 1

Time series results of the von Frey behavior test. The vertical axis represents the minimum force (g) at which mice responded positively in behavioral tests. The baseline is the value of behavioral tests before SNI or Sham treatment. Behavioral tests on Days 3, 7, and 21 were performed before the sacrifice, respectively.

Figure 2

Distribution of 2-AG at the thalamic level in the brain cross-sections of SNI mice. (A) On day 3 post-treatment, 2-AG level in the whole brain was reduced in the SNI group. The difference in 2-AG level was particularly pronounced in the hypothalamus, showing a decrease in the SNI group. (B) On day 7 post-treatment, 2-AG level in the whole brain was reduced in the SNI group. Furthermore, 2-AG level in the hypothalamus was decreased in the SNI group, similar to the result on day 3 post-treatment. (C) On day 21 post-treatment, the SNI group also showed a similar decrease in the 2-AG level, but the difference was smaller. A similar trend was observed in the hypothalamus. *: p < 0.05, **: p < 0.01.

Figure 3

Detection of 2-AG in the PAG region of mice brains. (A) On day 3 post-treatment, 2-AG level in the whole brain was reduced in the SNI group. The difference was particularly pronounced in the midbrain; 2-AG levels in the superior colliculus (SC) and reticular formation (RN) were lower in the SNI group than in the sham group. 2-AG level in the PAG was also decreased in the SNI group. (B) On day 7 post-treatment, 2-AG level in the whole brain was reduced in the SNI group. In the midbrain, however, there was a difference in 2-AG between the two groups: 2-AG levels in the SC and RN were lower in the SNI group than in the sham group, similar to the result on day 3 post-treatment. 2-AG level in the PAG was also decreased in the SNI group. (C) On day 21 post-treatment, there was no difference in 2-AG level in the whole brain between the SNI and sham groups. There was also no difference in 2-AG levels in the midbrain and PAG between the two groups. *: p < 0.05,**: p < 0.01.

Figure 4

2-AG distribution in the lumbar spinal cord cross-section. (A) On day 3 post-treatment, 2-AG level was increased by 1.34-fold (p = 0.002) in the SNI group compared to the sham group. (B) On day 7 post-treatment, 2-AG level was increased by 1.39-fold (p = 0.008) in the SNI group compared to that in the sham group. (C) On day 21 post-treatment, 2-AG level increased by 1.08-fold in the SNI group compared to that in the sham group (p = 0.211). *: p < 0.05,**: p < 0.01.