Abstract

Objective: We have previously shown changes in protein, immune cell, nitric oxide a neurotrophic factor content and distribution in the olfactory bulb of an endotoxin-treated rat model, as all these factors have been shown to be involved in neurodegeneration. We expanded our studies by fluorescently imaging smooth muscle actin and endothelial nitric oxide synthase (eNOS), both markers of blood vessels, to investigate loss of vasculature content, as well as imaging locations and quantities of immune cells. The work was performed to shed further light on associations between vessel integrity and immune cell initiated endothelial disruption. Our goal was to demonstrate that cytokine production, NOS induction and immune cell increases, are likely part of the process that leads to a loss of olfaction and dopaminergic signaling and includes vascular perturbations.

Methods: Rats were sacrificed following lipopolysaccharide (LPS) treatment. Olfactory bulbs were harvested, sectioned from top to bottom to include the tract and sensory neurons, and probed for markers of inflammation. Inducible nitric oxide synthase (iNOS), neuronal nitric oxide synthase (nNOS), eNOS, interleukin-1 beta (IL-1β), TNF-α, interleukin-6, glial cell derived neurotrophic factor (GDNF) and circulating nitric oxide (NO) were imaged together with tagged macrophages, T-cells, B-cells and neutrophils.

Results: Serum NO levels indicated that an inflammatory episode had occurred, being significantly higher in treated animals, with tissue levels of NOS elevated for an extended period of time. Immune cell clusters were seen in a number of areas and the localization of NOS isomers suggests that they have divergent roles in neurodegeneration. For instance, eNOS was associated with blood vessels, iNOS with glial and matrix cells and nNOS with glial cells and neurons. T and B-cell numbers showed a sustained increase; neutrophil numbers rapidly increased then returned to baseline levels; macrophage numbers increased and remained high; LAMP positive cell numbers (NK-cells) increased and remained high; GDNF content increased; IL-6, TNF-α and IL-1β levels all rapidly increased, before dropping to untreated levels, while circulating, NO levels increased dramatically. Of interest, the images of vascular content, immune cell content, eNOS and smooth muscle actin, allowed us to show detrimental interactions between cells, factors and vessels. Our data show that the majority of the vessels were intact, though sections of interest were ‘extracted’ to reveal possible leaky areas. Specific sites of IL-6 positive lymphocyte clustering were noted around vessels, suggesting that interactions are occurring that lead to disruptions of blood vessel tunicae, allowing the internalization of circulating cells and subsequent cytokine-initiated endothelial cell death.

Conclusion: Our findings suggest that protective GDNF and eNOS, which maintains vascular tone, are possibly synthesized too late to combat cytokine initiated neuron damage, glial activation and chronic loss of vascular integrity.

Keywords: Neurodegeneration; Olfaction; Blood-brain-barrier; Cytokines; Nitric oxide