Progressive orexin A positive neuron loss in the 6-OHDA mouse model of Parkinson's disease: a pilot study with validation of an orexin A immunohistochemistry protocol for paraffin-embedded tissue.

The aim of this pilot study was to assess a time-dependent loss of orexin neurons in the unilateral intrastriatal 6-OHDA mouse model and to optimise the immunohistochemistry protocol for formalin-fixed, paraffin-embedded mouse brain tissue. A progressive loss of orexin A positive neurons in the lateral hypothalamus was observed in the lesioned side relative to the non-lesioned side. A significant reduction of orexin A positive neurons in the lateral hypothalamus was observed at 4 weeks with further loss at 6-8 weeks post 6-OHDA induction. Orexin neuron loss in the lateral hypothalamus has been reported in post-mortem Parkinson's disease patients and multiple animal studies, particularly in rats; however, evidence in neurotoxic mouse models remains limited. Orexin A positive neuron loss occurred alongside transient motor deficits, olfactory impairments, subtle cognitive changes and gastrointestinal dysfunction. Additionally, an orexin A immunohistochemistry protocol was successfully established for formalin-fixed, paraffin-embedded mouse brain tissue, enabling reproducible and reliable detection of orexin neurons as an alternative to frozen sections. This pilot study serves as a proof of concept that the unilateral intrastriatal 6-OHDA mouse model can successfully recapitulate the progressive loss of orexin A positive neurons and validates this model's suitably for investigating the longitudinal mechanisms of PD symptoms and pathology.