Our lab recently completed a study funded by the Louisiana Board of Regents (grant #LEQSF(2019-20)-RD-A-28) investigating whether early-life exposure to the common antipyretic acetaminophen (APAP; aka paracetamol) interacts with interleukin-1β-induced inflammation to influence behavioral outcomes relevant to neurodevelopmental disorders in a C57BL/6J mouse model (Harshaw & Warner, 2022). The study was motivated both by (1) epidemiological studies, which have reported associations between early-life APAP and neurodevelopmental disorders, including autism spectrum disorders (ASDs) and attention-deficit hyperactivity disorder (ADHD; see Bauer et al., 2021), and (2) rodent studies, which have found that early-life exposure to APAP can have long-term effects on brain and behavior in mice and rats (e.g., Viberg et al., 2014; Blecharz-Klin et al., 2017; Philippot et al., 2017; Klein et al., 2020). Prior studies attempting to model the effects of early-life APAP in rodents had, nevertheless, largely neglected the fact that APAP is typically administered in a context of immune activation and/or fever. This was a critical shortcoming given both the fundamental issue of confounding by indication in epidemiological studies and the fact that early-life infection and sickness are also risk factors for developmental disorders, including ASDs (see Estes & Mcallister, 2016).
In our study, we attempted to mimic the repeated infections that are often typical in infancy by injecting pups with a small dose of interleukin-1β (IL-1β) on three occasions during early postnatal development (i.e., P5, P8, and P11). We intentionally used the minimal dose that had previously been found to reliably result in fever in neonatal rats. In a given litter, half of the pups were given an IL-1β injection and the other half were given a saline vehicle (Veh) injection. After a 45 min delay, to allow time for an immune response to develop, half of the pups received an injection of APAP and the other half a Veh injection. The dose of APAP used was allometrically scaled (i.e., to compensate for the large difference in surface-to-mass ratio and metabolism) to equal the dose typical for infants and young children. As described in our paper (see Harshaw & Warner, 2022), we found a number of distinct effects of IL-1β and APAP, with many of these being sex specific (e.g., an increase in social caution in males following early-life APAP). Nevertheless, we also found a number of significant interactions between IL-1β and APAP, including a significant increase in social avoidance in males treated with both IL-1β and APAP (see below).
We emphasize that, though provocative, our results do not support a simple conclusion regarding the relative danger vs. safety of APAP early in life. First, our study has a number of limitations and warrants both replication and refinement. Second, we documented a significantly protective effect of APAP against a novel IL-1β-induced morphological change in these same animals (see Harshaw & Warner, 2021). Given that previous studies have reported neuroprotective effects of APAP in specific contexts and populations of brain cells (e.g., Bisaglia et al., 2002; Locke et al., 2008; Tripathy & Grammas, 2009) future studies must investigate potential beneficial effects of APAP against changes in the developing brain induced by early-life inflammation. That is, it is important to emphasize that specific types of inflammation and infection early in life are also significant risk factors for neurodevelopmental disorders, including autism. As stated at the conclusion of our paper, “A key implication of our findings is that no simple conclusion regarding the relative safety vs. danger of APAP early in life is yet possible. In fact, it may be that inflammation and APAP constitute a developmental Scylla and Charybdis. Further research is needed, however, to ascertain the veracity and boundaries of this claim, including the conditions—genetic, epigenetic, and experiential—that may interact to canalize atypical developmental trajectories in response to these common early life exposures” (Harshaw & Warner, 2022).
Ultimately, our study adds valuable information about how inflammatory exposures may interact with APAP during early development to influence outcomes of potential significance to neurodevelopmental disorders. Our results showing significantly greater anxiety in females and higher levels of social avoidance in males with exposure to both inflammation and APAP are particularly promising. Future studies must nevertheless explore a broader range of both antipyretics and sources of early-life immune activation (e.g., LPS, Poly(I:C), live pathogen). In addition, other sources of potential interaction with APAP (e.g., early-life stress, the early-life hormonal milieu) of likely neurodevelopmental significance must be investigated.