Effects of Salmon Calcitonin on the Concentrations of Monoamines in Periaqueductal Gray in Formalin Test

Abstract

Background: The receptors of salmon calcitonin, located on certain areas of the brain such as the periaqueductal gray matter, are responsible for pain modulation.

Aims: The effects of intracerebroventricular injection of salmon calcitonin on the behavioral response to pain and on the levels of monoamines in the periaqueductal gray were explored using a biphasic animal model of pain.

Study design: Animal experiment.

Methods: A total of 45 male rats were divided into four groups (n=6). Salmon calcitonin was injected into the lateral ventricle of the brain (1.5 nmol, with a volume of 5 μL). After 20 min, 2.5% formalin was subcutaneously injected into the right leg claw, and pain behavior was recorded on a numerical basis. At the time of the formalin test, the periaqueductal gray area was microdialized. High-performance liquid chromatography method was used to gauge the levels of monoamines and their metabolites.

Results: Intracerebroventricular injections of salmon calcitonin resulted in pain reduction in the formalin test (p<0.05). The dialysate concentrations of serotonin, dopamine, norepinephrine, 5-hydroxyindoleacetic acid, 3,4-dihydroxyphenylacetic, and 4-hydroxy-3-methoxyphenylglycol increased in the periaqueductal gray area in different phases of the formalin pain test (p<0.05).

Conclusion: Salmon calcitonin reduced pain by increasing the concentrations of monoamines and the metabolites derived from them in the periaqueductal gray area.

Keywords: formalin test; microdialysis; Chromatography; high-performance liquid; monoamine oxidase; salmon calcitonin.

Figure 1

The nociceptive score in different groups. [Group 1: Normal saline was injected intracerebroventricularly, and normal saline was injected subcutaneously in the hind paw; Group 2: normal saline was injected intracerebroventricularly, and 2.5% formalin was injected subcutaneously; Group 3: Salmon calcitonin with a dose of 1.5 nmol was injected intracerebroventricularly, and normal saline was injected subcutaneously; Group 4: Salmon calcitonin with a dose of 1.5 nmol was injected intracerebroventricularly, and 2.5% formalin was injected subcutaneously]; *Significant differences between groups 2 and 4 (p<0.05) (mean ± standard deviation)

Figure 2

Concentrations of serotonin in different groups. [S1 to S8 represent the collected samples of dialysis from the periaqueductal gray at different times. Base sample without medication effect (S1), base sample with medication effect (S2), four samples related to different times of the formalin test (S3-S6), and two samples after completion of formalin test (S7, S8)]; *Significant differences between groups 2 and 4 (p<0.05) (mean ± standard deviation)

Figure 3

Concentrations of 5-hydroxyindoleacetic acid in different groups. [S1 to S8 represent the collected samples of dialysis from the periaqueductal gray at different times. Base sample without medication effect (S1), base sample with medication effect (S2), four samples related to different times of the formalin test (S3-S6), and two samples after completion of formalin test (S7, S8)]; *Significant differences between groups 2 and 4 (p<0.05) (mean ± standard deviation)

Figure 4

Concentrations of dopamine in different groups. [S1 to S8 represent the collected samples of dialysis from the periaqueductal gray at different times. Base sample without medication effect (S1), base sample with medication effect (S2), four samples related to different times of the formalin test (S3-S6), and two samples after completion of formalin test (S7, S8)]; *Significant differences between groups 2 and 4 (p<0.05) (mean ± standard deviation)

Figure 5

Concentrations of 3,4-dihydroxyphenylacetic in different groups. [S1 to S8 represent the collected samples of dialysis from the periaqueductal gray at different times. Base sample without medication effect (S1), base sample with medication effect (S2), four samples related to different times of the formalin test (S3-S6), and two samples after completion of formalin test (S7, S8)]; *Significant differences between groups 2 and 4 (p<0.05) (mean ± standard deviation)

Figure 6

Concentrations of norepinephrine in different groups. [S1 to S8 represent the collected samples of dialysis from the periaqueductal gray at different times. Base sample without medication effect (S1), base sample with medication effect (S2), four samples related to different times of the formalin test (S3-S6), and two samples after completion of formalin test (S7, S8)]; *Significant differences between groups 2 and 4 (p<0.05) (mean ± standard deviation)

Figure 7

Concentrations of 4-hydroxy-3-methoxyphenylglycol in different groups. [S1 to S8 represent the collected samples of dialysis from the periaqueductal gray at different times. Base sample without medication effect (S1), base sample with medication effect (S2), four samples related to different times of the formalin test (S3-S6), and two samples after completion of formalin test (S7, S8)]; *Significant differences between groups 2 and 4 (p<0.05) (mean ± standard deviation)