The aim of this study was to evaluate the glial fibrillary acidic protein immunoreactive (GFAP-IR) astrocytes
in the periaqueductal gray matter (PAG) of rats treated with monosodium glutamate (MSG). This study was
performed on PAG sections of rats treated subcutaneously with MSG (2 g/kg b.w. (group I) and 4 g/kg b.w.
(group II)) and saline solution (group C). Immunohistochemical reactions were performed by peroxidaseantiperoxidase method using an antibody against GFAP protein. In animals receiving MSG in dorsomedial
(dm) and dorsolateral (dl) parts of PAG the majority of astrocytes had GFAP-IR bodies and few thin, poorly
branched processes. In lateral (l) and ventrolateral (vl) parts of PAG cells with numerous, branched processes
with GFAP expression were observed. Some astrocytes in vl PAG of group II rats were characterized by strongly
stained cytoplasmic bodies and thick, multiple branched processes. Statistically significant differences in the
number of GFAP-IR astrocytes were shown between control animals and group II in dm PAG and dl PAG.
All MSG-treated animals revealed statistically significantly fewer immunonegative structures for GFAP as well
as all cells in the PAG. In the PAG of rats receiving MSG, especially at high dose, there is an increase in GFAP
immunoreactivity in astrocytes. The hypertrophy of GFAP-IR cells and the probable lack of their proliferation
may indicate isomorphic astroglia in response to increased extracellular glutamate concentrations. However,
the decrease in their number in dm PAG and dl PAG may be related to MSG cytotoxicity.