Modulation of blood-brain barrier permeability by microglia during the transition from pre- to hypertensive phase in spontaneously hypertensive rats

Abstract

The blood-brain barrier (BBB) rigidly controls the transit of substances between the plasma and the interstitium of brain areas. Microglia do not structure the BBB, but interfere with its functioning through the synthesis of pro-inflammatory cytokines. Experimental evidence has demonstrated in chronic hypertension microglial activation and BBB dysfunction with access of angiotensin II (Ang II) to the parenchyma of brain areas related to the autonomic control of circulation (paraventricular nucleus of the hypothalamus, PVN; nucleus tractus solitarius, NTS; region rostroventrolateral medulla, RVLM), but its absence in prehypertensive individuals. It is likely that Ang II and microglia interfere with BBB dysfunction during the onset of hypertension, but we do not know the triggering stimulus and the temporal sequence in which these changes are processed, which we intend to investigate in the present work. Therefore, spontaneous hypertensive rats (SHR) and Wistar (controls) were submitted to the evaluation of the temporal sequence of the basal hemodynamics, the alteration of the BBB permeability (i.a. administration of fluorescent dyes of high and low molecular weight) and the expression/activation of microglia (immunohistochemistry), from the prehypertensive phase (4 weeks of age) to the chronic phase of hypertension (12 weeks of age). In this study, we saw that physiological hemodynamic changes and the onset of hypertension in SHRs occur significantly from the 8th week of SHR onwards. However, the exponential increase in BBB permeability in the PVN of SHRs is evident as early as the 6th week. The same pattern was seen in the integrated density analyses, with the beginning of a progressive increase in IBA-1 immunoreactivity also from the 6th week of age of the SHRs, which did not apply to the controls, which maintained similar values throughout their development . Qualitative evaluations of microglia immunohistochemical photomicrographs demonstrate, in SHR, changes in their phenotype compatible with the M1 pathway (activation of pro-inflammatory microglial activity) already seen in other studies, characterized by the reduction of extensions and ramifications. Morphometric analyzes of the microglia present in the PVN of SHR showed a numerical reduction with an exponential curve over age, which is not seen in the controls, which remain without significant changes from the beginning to the end of the experiments. Despite these alterations, when evaluating the variation in the number of cells between ages and experimental groups, it is noted that it does not occur, a strong indication that it is a structural and non-numerical modification of the microglia, along with age and elevation from PAM. Temporal changes in MAP, BBB permeability and the presence of microglial activation during the onset of hypertension strongly suggest that the installation of the secretory phenotype by activated microglia with synthesis of pro-inflammatory cytokines, contributes to the installation of BBB dysfunction throughout development of spontaneous hypertension. For the continuity of this work, our new objectives are: 1 - to evaluate if these results obtained in the PVN are reproduced in the NTS, RVLM and non-autonomic areas of SHR and controls (Wistar); 2 - to evaluate the presence and temporal evolution of neuroinflammation by measuring cytokines present in homogenates of autonomic areas of SHR and its controls; 3 - we intend to inhibit microglial cells in 12-week SHR (chronic hypertension), redoing the other tests (hemodynamics, BBB permeability and evaluation of microglia activation), to understand the effects of microglia on this succession of events obtained so far. (AU)