New kid on the block: does histamine get along with inflammation in amyotrophic lateral sclerosis?

Results from amyotrophic lateral sclerosis (ALS) patients and pre-clinical studies strongly suggest that systemic and CNS-intrinsic immune activation plays a central role in ALS pathogenesis. Microglial cells are emerging in this context as master regulators with a bi-functional role in the progression of the pathological response. They foster a pro-inflammatory setting through the production of cytotoxic cytokines and chemokines (M1 phenotype), after an aborted effort to sustain an anti-inflammatory environment for motor neurons through the release of beneficial cytokines and growth factors (M2 phenotype). In this review, we gather information meant to propose that histamine and ATP, which are released from mast cells, microglia and damaged neurons at sites of injury where they function as transmitters, have to be considered as new players in the ALS neuroinflammatory arena. After all, abnormal histamine and ATP signalling in the brain are already documented in neurodegenerative/neuroinflammatory conditions such as multiple sclerosis, Alzheimer and Parkinson's disease and, at present, histamine- as well as ATP-related compounds are in clinical trial for these same pathologies. Concerning ALS, while emerging data are now available about purinergic mechanisms, the involvement of histamine is basically unexplored. The circumstantial evidence that we present here thus constitutes a solid background for formulating novel hypotheses, stimulating a scientific debate and, most of all, inspiring future research. We deem that a new potential role of histamine in the setting of ALS neuroinflammation might find a fertile ground where to thrive. ALS is still a disease without a cure: why not to play with a new kid on the block?