Effect of intranigral injection of endotoxin (ET) on neuronal homeostasis in the hippocampus

Abstract

Background Neurogenesis in the hippocampus has been shown to be prone to alterations in response to a variety of physiological and pathological factors, one of which is neural inflammation. Inflammation has been implicated in changes observed in neurodegenerative diseases such as Parkinson’s and Alzheimer’s. Previous studies are in favor of the hypothesis that overt or discrete brain inflammation can result in reduced neurogenesis and-or neuronal death associated with cognitive decline and other functional disorders. We aim to show that mild inflammation induced by unilateral Endotoxin (ET) injection in the substantia nigra (SN) and ventral tegmental area (VTA) can alter neurogenesis in the Dentate Gyrus (DG) of the hippocampus. For this purpose, 2ug-2ul of Endotoxin were stereotaxically injected into the right SN of adult rats. The effect of ET administration on lateralized motor function was assessed using the Rota Rod and the Cylinder test. The effect on neurogenesis in the hippocampus was assessed by quantifying the number of proliferating cells in the Dentate Gyrus (DG). Methods Adult Sprague Dawley female rats (250-300g) received unilateral injection of Endotoxin (2ug-2ul saline) or sterile saline (2ul) in the right SN. Rats then received 3 consecutive intraperitoneal injections of 5’-Bromo-2’-deoxyuridine (BrdU) (66mg-kg-injection) separated by 2 hours intervals. The rats were perfused on day 3, 6, or 9 post injection. Behavioral observations included cylinder test and Rotarod tests. The fractionator method was used together with confocal immunofluorescent analysis to probe for BrdU and NeuN positive cells in the left and right DG. Moreover, Tyrosine hydroxylase (TH) and Ox-42 immunostaining were performed to check for the effect on dopaminergic neurons and microglial activation, respectively. The number of TH cells was quantified and the intensity of Ox-42 was recorded in the right and left SN. Results The cylinder test showed significant ipsilateral bias (with reference to