Consistent with our hypothesis, the present study demonstrates an inverse association between serum allopregnanolone and ratings of low back pain and chest pain in subjects who served in the U.S. military since September 11, 2001. These data support the possibility that there may be antinociceptive effects for this neurosteroid in humans. Future studies are needed to replicate these findings and to definitively evaluate the precise roles of allopregnanolone in pain disorders. Also, the finding that DHEA demonstrates an inverse association with a pain measure (muscle soreness) in this cohort raises the possibility of a more general antinociceptive feature for neurosteroids as a class.
To date, the existing literature contains few studies examining the relationship between neurosteroid levels and pain in clinical populations. When the field is narrowed further to only allopregnanolone, a neurosteroid with considerable evidence of analgesic properties in rodents (26
), only one human study remains and this was conducted in a pain-free healthy volunteer cohort rather than a population that included subjects with pain symptoms (42
). This previous study reported that individuals who showed higher endogenous allopregnanolone levels in response to cold pressor, thermal heat, or tourniquet ischemia pain tests were less likely to report an increase in pain tolerance. In addition, this study reported that baseline allopregnanolone levels correlated significantly with a decrease in pain tolerance during pain testing in one ethnic group in the study sample (i.e., non-Hispanic whites).
Several experimental differences from the present study, however, should be noted when comparing the findings of the Mechlin et al. and the current study. Perhaps most importantly, these studies may involve different pathophysiological mechanisms, as the present study examines correlations in baseline perceived pain levels, whereas the study by Mechlin et al. examines effects on pain threshold and tolerance via several acute stressor paradigms. Whereas the present study included a clinical population reporting pain complaints, the Mechlin et al. study focused on healthy volunteers free of chronic pain conditions. Further, while the present study included only males, Mechlin et al. included both males and females, thereby potentially influencing mean baseline allopregnanolone levels since this neurosteroid fluctuates across the menstrual cycle (54
). This point may be especially relevant in light of the fact that the reported pain tolerances were gender specific in the previous study (42
). Methodologically, it also should be noted that Mechlin et al. determined allopregnanolone levels via radioimmunoassay, a technique that may identify several GABAergic neurosteroids simultaneously based on an 84–169% cross-reactivity of the method with 3α-hydroxy-4-pregnen-20-one (55
) and 6.6% cross-reactivity with pregnanolone (55
), potentially also contributing to divergent results.
While the present study finds that higher endogenous allopregnanolone levels correlate with decreased reporting of some types of pain symptoms, the mechanisms responsible for potential analgesic effects of this neurosteroid remain to be determined. Some possible explanations are currently supported in the literature, including a role for allopregnanolone in anti-inflammatory actions. For example, in both mice and rats, allopregnanolone administration reduces levels of inflammatory cytokines (58
). Longer term, the effects of allopregnanolone may result from protection against cell degeneration and/or cell death, as considerable evidence exists implicating allopregnanolone in several neuroprotective actions. For example, in a mouse model of the fatal, autosomal recessive neurodegenerative Niemann-Pick type C disease, there is a substantial reduction in the synthesis of allopregnanolone at birth (60
), and neonatal administration of allopregnanolone delays neurological symptom onset and doubles lifespan (61
). Further, neonatal administration of allopregnanolone increases neuronal myelination (62
), a process previously demonstrated to involve GABAA
). Also, allopregnanolone inhibits apoptosis in rat models of TBI, decreasing caspase-3 activity and DNA fragmentation (64
), decreasing glial fibrillary acid protein-positive astrocytes, and improving performance in spatial learning tasks (65
). Multiple mechanisms may thus contribute to a possible analgesic and neuroprotective effects of this neurosteroid in humans.
Examination of additional neurosteroids yielded no association of either pregnenolone or progesterone with any of the pain measures, but findings did emerge for both DHEA and DHEAS. While we found that DHEA levels in serum were inversely associated with muscle soreness, levels of its sulfated form were positively associated with chest pain. Prior evidence has yielded inconsistent analgesic profiles for both DHEA and DHEAS (39
) in humans. However, the differences in findings for each of these neurosteroids have been between inverse correlations with pain vs. no correlations at all. Thus, the positive correlation between DHEAS and chest pain in the present study is surprising both in that the response differs from that of DHEA, and in its incongruity with previous studies. Why these two neurosteroids demonstrate opposite serum profiles vis-a-vis pain ratings in this subject population is not yet clear. Further examination of various pain types and severities is needed to determine whether these findings are specific to muscle soreness or chest pain, as most previous studies reporting no correlation or a negative correlation for DHEAS have examined other types of pain (e.g., joint pain). Also, an examination of the same neurosteroids and pain measures in women could provide important information about possible gender-specific associations, as a negative correlation between DHEAS and musculoskeletal pain has been reported in a subject population of 57 women undergoing the menopausal transition (39
Finally, while age, current smoking, and history of alcohol use are not associated with any of the pain measures in the present study, a history of TBI is associated with muscle soreness. This association is not a surprising one (66
), especially given the high rates of polytrauma in this cohort of returning veterans (70
). However, the lack of an association with other pain measures such as headache is unexpected. It is possible that a larger sample size or the use of a more comprehensive pain assessment may be required to capture these additional associations.
Limitations of this exploratory pilot study must be acknowledged. Our R2 values were relatively low, with a maximum result of 0.17. We also conducted four separate analyses, thereby potentially increasing the likelihood of Type I error. Nevertheless, with the exception of TBI, only neurosteroid variables were significant in our analyses, supporting our hypothesis for a neurosteroid role in pain modulation. Further, the instrument utilized does not query the specific type of headache, potentially limiting assessments of relationships between neurosteroid levels or other findings and headache. Results will clearly require replication in a larger cohort, but these initial findings are promising, as they are consistent with the preclinical literature and provide data for future hypothesis testing.
In summary, the present study demonstrates that increased levels of endogenous allopregnanolone are associated with decreased self-reported low back pain and chest pain in subjects who served in the U.S. military after September 11, 2001. In addition, increased levels of DHEA are associated with decreased muscle soreness pain, and higher DHEAS levels are associated with increased chest pain in this population. This analysis used exploratory techniques (stepwise linear regression) and results should be replicated in an independent sample. These findings, particularly if supported by future replication studies, suggest that modulation of allopregnanolone or other neurosteroid levels may be logical targets for therapeutic intervention in pain disorders.