Investigation of noradrenergic receptor system in anti-nociception using formalin test in the naked mole rat (Heterocephalus glaber)

Abstract

The naked mole rat (NMR) is a rodent that has gained importance as a biomedical research model for various conditions like hypoxic brain injury, cancer and nociception. This study was designed to investigate possible involvement of the noadrenergic receptor system in antinoception in the NMR, using the alpha-2 adrenergic receptor specific ligands clonidine (agonist) and yohimbine (antagonist) in the formalin test. Formalin test followed 30 min after intraperitoneal administration of ligands or control. A total of 96 naked mole rats were used. A significant reduction in nociceptive behaviours was demonstrated after administration of clonidine in the doses 1,3,10 and 30 μg/kg (n = 8 per group). Doses of clonidine above 30 μg/kg caused loss of motor and proprietion skills exhibited by prostration and failure to turn over when placed on their backs. The antinociception by 3 μg/kg clonidine was reversed by administration of 30 μg/kg of yohimbine. The present study demonstrates that the noradrenergic receptor system is present and involved in formalin test-related antinociceptive mechanisms in the NMR, similar to other mammals. Given the increasing importance of the NMR as a model for pain and nociception, the species may prove useful as an animal model for noradrenergic mechanisms in pain modulation.

Keywords: Animal behavior; Animal science; Cell biology; Clonidine; Formalin test; Naked mole rats; Neuroscience; Nociception; Noradrenergic; Pharmaceutical science; Statistics; Yohimbine.

Figure 1

The effects of subcutaneous injection of 20μl 10% formalin (unfilled circle) or saline (unfilled triangle pointed down) dorsally on right hind paw on pain behaviours in the naked mole rat. Significance difference in pain behavior were observed in the time intervals 0–5 min (p < 0.05) and 40–60 min (P < 0.001). Shown as a∗∗ = P < 0.05 and a∗∗∗ = P < 0.001. Data are shown as mean ± SEM. Number of animals (n) = 8 for both saline and formalin. Data were analyzed using two way ANOVA with Bonferroni's post hoc test.

Figure 2

The effect of intra-peritoneal (i. p) injection of clonidine in concentrations of 1.0, 3.0, or 10, 30 μg/kg compared to a control group of i. p saline injection followed by a formalin test after 30 min of treatment (clonidine) or control (saline) administration. Significant differences in pain behavior were found in the periods; p < 0.001 (a∗∗∗) during 35–60 min between control and clonidine dosages 30 and 10 μg/kg; p < 0.001 (b∗∗∗) during 35–45 and 50–60 min and p < 0.05 (b∗∗) during 45–50 min between control and clonidine dosages 3 and 1 μg/kg; p < 0.05 (c∗∗) during 50–60 min between clonidine dosages 30 and 3 μg/kg; p < 0.01 (d∗) during 45–50 and 55–60 min between clonidine dosages 30 and 1 μg/kg; Data are shown as mean time (±SEM). Number of animals (n) = 8 for control and clonidine dosages 1, 3, 10 and 30 μg/kg respectively. Data were analyzed using two way ANOVA with Bonferroni's post hoc test.

Figure 3

The effect of intra-peritoneal (i. p) injection of 20 μl water for injection (solvent for yohimbine hydrochloride powder) and 30 μg/kg yohimbine followed by a formalin test after 30 min of treatment (yohimbine) or control (water for injection) administration. Data are shown as mean ± SEM. Sample size (n) = 8 for both control and yohimbine. No significant difference in pain behavior was found between the two treatments.

Figure 4

The effect of intra-peritoneal (i. p) injection of control, clonidine 3 μg/kg and co administration of yohimbine 30 μg/kg with 3 μg/kg clonidine. Significant differences in pain behavior were found in the periods; p < 0.05 (a∗∗) during 45–50 min between control and clonidine 3 μg/kg; p < 0.05 (b∗∗) during 45–55 min between clonidine 3 μg/kg and co-administered yohimbine 30 μg/kg & clonidine 3 ug/kg. Data are shown as mean ± SEM. Number of animals (n) = 8 for clonidine and control while n = 7 for yohimbine co-administered with clonidine. Data were analyzed using two way ANOVA with Bonferroni's post hoc test.