Prenatal alcohol exposure is a risk factor for adult neuropathic pain via aberrant neuroimmune function

Abstract

BackgroundClinical studies show that prenatal alcohol exposure (PAE) results in effects that persist into adulthood. Experimental animal models of moderate PAE demonstrate that young adults with PAE display potentiated sensitivity to light touch, clinically termed allodynia, following sciatic nerve chronic constriction injury (CCI) that coincides with heightened spinal glial, spinal macrophage, and peripheral immune responses. However, basal touch sensitivity and corresponding glial and leukocyte activation are unaltered. Therefore, the current study explored whether the enduring pathological consequences of moderate PAE on sensory processing are unmasked only following secondary neural insult.MethodsIn middle-aged (1 year) Long Evans rats that underwent either prenatal saccharin exposure (control) or moderate PAE, we modified the well-characterized model of sciatic neuropathy, CCI, to study the effects of PAE on neuro-immune responses in adult offspring. Standard CCI manipulation required 4 chromic gut sutures, while a mild version applied a single suture loosely ligated around one sciatic nerve. Spinal glial immunoreactivity was examined using immunohistochemistry. The characterization and functional responses of leukocyte populations were studied using flow cytometry and cell stimulation assays followed by quantification of the proinflammatory cytokines interleukin-1beta (IL-1β) and tumor necrosis factor-alpha (TNF-α). Data were statistically analyzed by ANOVA and unpaired t tests.ResultsThe current report demonstrates that mild CCI generates robust allodynia only in PAE rats, while the pathological effects of PAE following the application of a standard CCI are revealed by enhanced allodynia and elevated spinal glial activation. Additionally, mild CCI increases spinal astrocyte activation but not microglia, suggesting astrocytes play a larger role in PAE-induced susceptibility to aberrant sensory processing. Leukocyte populations from PAE are altered under basal conditions (i.e., prior to secondary insult), as the distribution of leukocyte populations in lymphoid organs and other regions are different from those of controls. Lastly, following in vitro leukocyte stimulation, only PAE augments the immune response to antigen stimulation as assessed by heightened production of TNF-α and IL-1β.ConclusionsThese studies demonstrate PAE may prime spinal astrocytes and peripheral leukocytes that contribute to enduring susceptibility to adult-onset neuropathic pain that is not apparent until a secondary insult later in life.

Lifelong Impacts of Moderate Prenatal Alcohol Exposure on Neuroimmune Function

“…Additional evidence exists supporting the possibility that PAE augments astrocyte and microglial activation in the dorsal horn of the spinal cord in neuropathic adult male rats. That is augmented expression of Iba-1, a well-characterized microglial activation marker, and GFAP (Figure 1 B) were observed in PAE rats with CCI ( 52 , 53 ). In line with these observations, analysis of cytokine levels in sham-treated PAE rats revealed that DRG IL-10 protein levels are remarkably suppressed, with IL-10 suppression greatest in PAE neuropathic rats compared with control-treated rats ( 52 ).…”

“…While speculative, glial cells may remain primed following an initial stimulation during gestational development (i.e., PAE), with classic activation markers returning to basal levels. However, these previously stimulated glia may develop an increased propensity for enhanced responses following subsequent immune stimulation ( 52 , 53 ). That is, it is possible under some circumstances that classic glial activation markers are uncoupled from their primed state such that basal levels of these markers are observed, yet these same glia over-respond to normal stimuli.…”

“…However, when CCI is reduced to a minor injury with only a single suture, robust allodynia was observed in PAE adults while touch sensitivity in control animals remained entirely unaltered ( 53 ). Surprisingly, neuropathic PAE animals from minor injury did not reveal significant spinal cord microglial activation by day 10 after CCI, as revealed by immunoreactivity for Iba-1 and transmembrane protein 119, both markers of microglial proliferation and/or activation, in the dorsal horn of the lumbar spinal cord ( 53 , 83 ). Conversely, significant increases in astrocyte activation, as examined by GFAP immunoreactivity were observed in the spinal cord compared with non-neuropathic control rats.…”

“…While, it is possible that increased microglial activation occurs earlier or later than the time points examined, these data suggest that spinal astrocytes and not microglia are playing a greater role in mediating the PAE-associated risks for developing allodynia following minor injury. Thus, PAE-induced pathological neuroimmune responses underlying abnormal CNS sensory processing can be unmasked by minor injury ( 53 ).…”

The Transgenerational Consequences of the Interaction Between Humans and Molecules: Alcohol as a Cultural Artifact

“…Finally, prenatal alcohol can increase susceptibility to substance abuse via indirect mechanisms. For instance, FASD are characterized by a higher vulnerability to stress, depression/anxiety disorders (Hellemans et al, 2010), and aberrant pain sensitivity (Sanchez et al, 2017).…”