Stress in the adult rat exacerbates muscle pain induced by early-life stress

Abstract

Background: Early-life stress and exposure to stressful stimuli play a major role in the development of chronic widespread pain in adults. However, how they interact in chronic pain syndromes remains unclear.

Methods: Dams and neonatal litters were submitted to a restriction of nesting material (neonatal limited bedding [NLB]) for 1 week. As adults, these rats were exposed to a painless sound stress protocol. The involvement of sympathoadrenal catecholamines interleukin 6 (IL-6) and tumor necrosis factor alpha (TNFα) in nociception was evaluated through behavioral and enzyme-linked immunosorbent assays, surgical interventions, and intrathecal antisense treatments.

Results: Adult NLB rats exhibited mild muscle hyperalgesia, which was markedly aggravated by sound stress (peaking 15 days after exposure). Adrenal medullectomy did not modify hyperalgesia in NLB rats but prevented its aggravation by sound stress. Sustained administration of epinephrine to NLB rats mimicked sound stress effect. Intrathecal treatment with antisense directed to IL-6 receptor subunit gp130 (gp130), but not to tumor necrosis factor receptor type 1 (TNFR1), inhibited hyperalgesia in NLB rats. However, antisense against either gp130 or TNFR1 inhibited sound stress-induced enhancement of hyperalgesia. Compared with control rats, NLB rats exhibit increased plasma levels of IL-6 but decreased levels of TNFα, whereas sound stress increases IL-6 plasma levels in control rats but not in NLB rats.

Conclusions: Early-life stress induces a persistent elevation of IL-6, hyperalgesia, and susceptibility to chronic muscle pain, which is unveiled by exposure to stress in adults. This probably depends on an interaction between adrenal catecholamines and proinflammatory cytokines acting at muscle nociceptor level.

Keywords: IL-6; TNFα; myalgia; neonatal limited bedding; nociceptors; sound stress.

Figure 1. Sound stress enhances muscle hyperalgesia exhibited by NLB rats

As previously reported (Green et al., 2011), rats submitted to the neonatal limited bedding protocol exhibit as adults (Day 0) decreased in muscle mechanical nociceptive threshold (~20% lower compared to control rats). As shown in the timeline inset, a 4-day cycle of sound stress consists of 30 min of intermittent exposure to sound stress administered on days 1, 3, and 4. Nociceptive threshold measurements were taken on post-stress days 1, 3, 5, 7, 10, 12, 15 and 22. Ten days after exposure to the sound stress protocol, the muscle hyperalgesia is significantly increased, persisting unattenuated for at least 12 days. ***P < 0.001.

Figure 2. Adrenal catecholamines play a role in sound stress-induced enhancement of muscle hyperalgesia exhibited by NLB rats

(A) While surgical excision of the adrenal medulla (AdMdx, open bars) in NLB rats did not modify the muscle hyperalgesia (Day 0), it prevented its enhancement by sound stress (Days 10–15). (B) Comparison of the effects of sustained administration of epinephrine or sound stress on mechanical nociceptive threshold in NLB rats. NLB rats implanted with epinephrine-releasing pumps (open bars), but not those exposed to sound stress (solid bars), exhibited a significant increase of muscle hyperalgesia by day 7 after implant. Both interventions produced a comparable increase in muscle hyperalgesia between days 10 and 15 after intervention. ***P < 0.001.

Figure 3. IL-6 contributes to baseline muscle hyperalgesia observed in NLB rats and its enhancement by sound stress

(A) Antisense (AS, open bars), but not mismatch (MM, solid bars) treatment directed to the IL-6 receptor subunit gp130 inhibited baseline muscle hyperalgesia exhibited by NLB rats. (B) This intervention was also able to inhibit the enhancement of muscle hyperalgesia produced by sound stress. ***P < 0.001.

Figure 4. TNFα contributes to the enhancement of muscle hyperalgesia by sound stress in NLB rats

(A)Intrathecal treatment with either antisense (AS, open bars) or mismatch (MM, solid bars) oligodeoxynucleotides directed to the TNFR1 mRNA did not modified baseline muscle hyperalgesia exhibited by NLB rats. (B) In contrast, antisense (AS, open bars), but not mismatch (MM, solid bars) treatment directed to the TNFR1 mRNA was able to reverse the enhancement of muscle hyperalgesia produced by sound stress. **P < 0.01; ***P < 0.001.

Figure 5. Effects of sound stress on TNFα and IL-6 plasma levels

(A) Blood samples were obtained pre (Day 0) and post (Day 15) exposure to sound stress in control (open bars) and NLB (solid bars) rats, which represent time points of baseline and enhanced muscle hyperalgesia values in NLB rats. (B) While NLB rats exhibited lower plasma levels of TNFα with respect to control rats, at baseline, sound stress did not modify these values in either control or NLB rats. (C) Before exposure to sound stress, NLB rats exhibited higher plasma levels of IL-6 with respect to control rats. Fifteen days after sound stress control rats, but not NLB rats, exhibited increased plasma levels of IL-6. *P < 0.05; ***P < 0.001.