Antinociception induced by chronic glucocorticoid treatment is correlated to local modulation of spinal neurotransmitter content

Abstract

Background: While acute effects of stress on pain are well described, those produced by chronic stress are still a matter of dispute. Previously we demonstrated that chronic unpredictable stress results in antinociception in the tail-flick test, an effect that is mediated by increased levels of corticosteroids. In the present study, we evaluated nociception in rats after chronic treatment with corticosterone (CORT) and dexamethasone (DEX) in order to discriminate the role of each type of corticosteroid receptors in antinociception.

Results: Both experimental groups exhibited a pronounced antinociceptive effect after three weeks of treatment when compared to controls (CONT); however, at four weeks the pain threshold in CORT-treated animals returned to basal levels whereas in DEX-treated rats antinociception was maintained. In order to assess if these differences are associated with altered expression of neuropeptides involved in nociceptive transmission we evaluated the density of substance P (SP), calcitonin gene-related peptide (CGRP), somatostatin (SS) and B2-gamma-aminobutiric acid receptors (GABAB2) expression in the spinal dorsal horn using light density measurements and stereological techniques. After three weeks of treatment the expression of CGRP in the superficial dorsal horn was significantly decreased in both CORT and DEX groups, while GABAB2 was significantly increased; the levels of SP for both experimental groups remained unchanged at this point. At 4 weeks, CGRP and SP are reduced in DEX-treated animals and GABAB2 unchanged, but all changes were restored to CONT levels in CORT-treated animals. The expression of SS remained unaltered throughout the experimental period.

Conclusion: These data indicate that corticosteroids modulate nociception since chronic corticosteroid treatment alters the expression of neuropeptides involved in nociceptive transmission at the spinal cord level. As previously observed in some supraspinal areas, the exclusive GR activation resulted in more profound and sustained behavioural and neurochemical changes, than the one observed with a mixed ligand of corticosteroid receptors. These results might be of relevance for the pharmacological management of certain types of chronic pain, in which corticosteroids are used as adjuvant analgesics.

Figures

Figure 1

Nociceptive behaviour. Tail (A, B) and paw (C, D) withdrawal latency after chronic corticosteroid treatment for 21 (1) and 28 (2) days with CORT and DEX. Both CORT and DEX groups display higher TF latencies after 21 days of treatment (A, B) although this effect is only sustained by DEX group at the end of the experiment (B); note that only DEX induces an increase in hind-paw latency and only after 28 days of treatment (D). (*p < 0.05, **p < 0.01 and ***p < 0.001).

Figure 2

Neuropeptide and receptor expression in the spinal dorsal horn. Immunoreactive content in the dorsal horn of the spinal cord after 21 and 28 days of chronic corticosteroid treatment. (CGRP(A, E), SP(B, F), SS(C, G) and GABAB2(D, H); *p < 0.05, **p < 0.01 and ***p < 0.001).

Figure 3

Pearson correlation between densitometry and stereology quantification methods. Pearson correlation for validation of densitometric versus stereologic quantification methods on days 21(A-D) and 28(E-H). (SS(A, E), SP(B, F), CGRP(C, G) and GABAB2(D, H)).

Figure 4

Time course of testing and sampling sessions throughout the four week experimental period. Within a testing session, tail-flick and hot-plate tests were performed according to the time course example for day 7.

Figure 5

Photomicrographs of superficial dorsal horn sections. Examples of photomicrographs of superficial dorsal horn sections immunoreacted for CGRP (A) on days 21 (A1–3) and 28 (A4–6) for CONT (A1,4), DEX (A2,5) and CORT (A3,6).

Figure 6

Photomicrographs of superficial dorsal horn sections. Examples of photomicrographs of superficial dorsal horn sections immunoreacted for GABAB2 (B) on days 21 (B1–3) and 28 (B4–6) for CONT (B1,4), DEX (B2,5) and CORT (B3,6).