The effect of intracerebroventricular injection of CGRP on pain behavioral responses and monoamines concentrations in the periaqueductal gray area in rat

Abstract

Objectives: Calcitonin gene related peptide (CGRP) receptors are widely distributed in the central nervous system. The aim of this study was to investigate the effects of intracerebroventricular (ICV) injection of CGRP on pain behavioral responses and on levels of monoamines in the periaqueductal gray area (PAG) during the formalin test in rats.

Materials and methods: Twenty-four male rats were studied in four groups (n=6). CGRP was injected into the left cerebral ventricle (1.5 nmol, 5 µl). After 20 min, formalin (2.5%) was subcutaneously injected into the right hind paw. Behavior nociceptive score was recorded up to 60 min. During the formalin test, the PAG was subjected to microdialysis and levels of norepinephrine, 3-methoxy-4-hydroxyphenyl-glycol (HMPG), dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin and 5-hydroxyindole-acetic acid (HIAA) were measured by HPLC.

Results: ICV injection of CGRP lead to a significant pain reduction in acute, middle and chronic phases of the formalin test. Dialysate concentrations of norepinephrine, HMPG, dopamine, DOPAC, serotonin and HIAA in the PGA area showed an increase in acute phase, middle phase and beginning of the chronic phase of the formalin test.

Conclusion: CGRP significantly reduced pain by increased concentrations of monoamines and their metabolites in dialysates from PAG when injected ICV to rats.

Keywords: CGRP; Formalin test; HPLC; Microdialysis; Monoamines; Periaqueductal gray area.

Figure 1

The nociceptive score in different groups. (Group 1: ACSF was ICV injected and normal saline was injected subcutaneously (SC) in the hind paw. Group 2: ACSF was ICV injected and formalin 2.5% was injected SC. Group 3: CGRP with a dose of 1.5 nmol was ICV injected and normal saline was injected SC. Group 4: CGRP with a dose of 1.5 nmol was ICV injected and formalin 2.5% was injected SC) * Significant differences between group 2 and group 4 (P<0.05)

Figure 2

Concentrations of norepinephrine in different groups. (Group 1: ACSF was ICV injected and normal saline was injected subcutaneously (SC) in the hind paw. Group 2: ACSF was ICV injected and formalin 2.5% was injected SC. Group 3: CGRP with a dose of 1.5 nmol was ICV injected and normal saline was injected SC. Group 4: CGRP with a dose of 1.5 nmol was ICV injected and formalin 2.5% was injected SC). [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 group 2 and group 4 (P<0.05)

Figure 3

Concentrations of HMPG in different groups. [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 group 2 and group 4 (P<0.05)

Figure 4

concentration of dopamine in test and control groups. [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 group 2 and group 4 (P<0.05)

Figure 5

Concentration of DOPAC in test and control groups. [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)]

Figure 6

Concentrations of serotonin in different groups. [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 group 2 and group 4 (P<0.05)

Figure 7

Concentrations of HIAA in different groups. [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 group 2 and group 4 (P<0.05)