Astrocytes are glial cells prone to morphological changes associated with age. The aim of the study was to investigate the immunoreactivity of glial fibrillary acidic protein (GFAP) in astrocytes of the periaqueductal gray matter (PAG) of the midbrain in adult and old male rats to demonstrate morphological changes associated with age and to assess morphometrically the number of astrocytes and the digital immunostaining intensity of the examined protein in PAG astrocytes of both groups of animals. In the study, 10 male Wistar rats in two age groups were used. The first group consisted of five 100-day-old animals, whereas the second comprised five 3-year-old rats. After euthanasia, the midbrain, containing PAG, was collected and embedded in paraffin blocks. Immnunohistochemical peroxidase-antiperoxidase reaction was carried out on coronal tissue sections with the use of the specific primary antibody against GFAP, goat anti-mouse IgG, peroxidase-antiperoxidase complex, and diaminobenzidine chromogen. GFAP-immunopositive PAG astrocytes were observed under a light microscope and subjected to morphometric analysis to determine their number and digital immunostaining intensity for the protein examined. GFAP-immunoreactive PAG astrocytes in 100-day-old rats showed uniform distribution. Numerous processes branching into secondary ones protruded from intensely GFAP-immunostained stellate cells. In contrast, in 3-year-old rats a significantly lower number of glial cells of different morphology was observed compared to young animals. Astrocytes had fewer primary processes without secondary branches. Morphometric analysis confirmed microscopic observations. Our findings indicate that PAG astrocytes are prone to quantitative and morphological changes with age, which, in turn, can cause disorders in emotional, pain, and defensive reactions.
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