2023 MSC Poster Contest Presentation: Contribution of macrophages and iNOS to migraine-like behaviors induced by dural prolactin and repeated stress

The 2023 MSC Emerging Science Contest for Early-Career Investigators took place on December 13, 2023. Below is a written summary of one of the presentations from the contest. Read about other presentations from the event in our Early-Career Science Library.

Category: Trigeminal pain and headache

Runner up: Hao-Ruei Mei, PhD candidate, UT Dallas, US

Title: Contribution of macrophages and iNOS to migraine-like behaviors induced by dural prolactin and repeated stress.

Hypothesis, methodology, findings and conclusions.

In this study, we found macrophages and iNOS (inducible nitric oxide synthase) are necessary for prolatin- and stress-indued migraine-like behaviors. We applied von Frey filaments to assess the periorbital mechanical threshold, a preclinical measure of headache-like behavior, and used two different models to induce periorbital hypersensitivity. First, we used repetitive restraint stress in mice (placing mice into a tail vein injection tube) to induce periorbital hypersensitivity and priming to the nitric oxide donor sodium nitroprusside. Second, we used non-invasive dural injections (delivering prolactin directly to the dura mater without surgery) to induce periorbital hypersensitivity.

We found that deletion of prolactin receptors in sensory neurons partially but significantly blocked the periorbital hypersensitivity caused by dural application of prolactin to female mice. This indicates that responses to dural prolactin are not mediated exclusively by sensory neurons. We thus hypothesized that prolactin acts in part through non-neuronal cells in the dura. Since prolactin is known to act on immune cells such as macrophages, we tested a role for these cells in this model. Depletion of macrophages using clodronate liposome injections before dural prolactin significantly blocked the prolactin responses. We further investigated whether prolactin induces responses through the activation of iNOS, an enzyme highly expressed in macrophages. Injection of the iNOS inhibitor AR-C102222 (AR-C) significantly blocked the dural prolactin-induced responses.

These findings indicate that both macrophages and iNOS contribute to the behavioral responses to dural prolactin. We have shown previously that the involvement of prolactin in repeated stress induces periorbital hypersensitivity and priming. To determine whether macrophages and iNOS contribute to repetitive stress-induced periorbital hypersensitivity and priming to sodium nitroprusside, stressed or control mice received clodronate liposomes or AR-C injection before repetitive stress exposure. Macrophage depletion in stressed mice significantly inhibited stress-induced periorbital hypersensitivity in both males and females. However, AR-C only blocked stress-induced migraine-like behaviors in females but not in males.

In conclusion, this study demonstrates the involvement of prolactin receptors on sensory neurons, as well as a role for macrophages and iNOS in dural prolactin-induced periorbital hypersensitivity. In response to repeated stress, macrophages contribute to behavioral responses in both males and females while iNOS only plays a role in females.

Implications for understanding migraine disease and/or its comorbidities, or how the research holds promise as a new avenue of future migraine study.

Recently, researchers have been investigating the link between immune cells and migraine, with indications that the immune system may contribute to the development and progression of migraine. Our findings highlight the crucial roles of macrophages and iNOS in stress and prolactin-induced periorbital hypersensitivity. Macrophages may be an essential source for NO, an important molecule implicated in migraine pathophysiology. Understanding these interactions provides new avenues for developing targeted therapies for migraine prevention and treatment.

The prevalence of migraine is generally higher in females than in males. Understanding sex-related mechanisms is crucial for developing effective treatments. In this study, we found sex-specific differences in iNOS activity following stress, demonstrating an additional mechanism of migraine that may differ between males and females. Targeting iNOS could be a novel therapeutic strategy for migraine in females.