Animal Model of Autism Induced by Prenatal Exposure to Valproate: Changes in Hipocampal Glial Parameters

Background:  Different theories have emerged trying to solve “the autism puzzle” and, although research in autism has evolved, its etiology remains unknown lacking an objective diagnostic criteria and treatment options. Environmental conditions have emerged as strong risk factors after epidemiological studies evidencing that maternal exposure to infections, ethanol and valproic acid (VPA) could lead to autism. Although most studies focus on neuronal parameters, glial cells have been gaining more attention in the last years. Astrocytes also have emerged as sophisticated cells which participate in a variety of functions vital for normal brain development, adult physiology and pathology and there are evidences showing the involvement of glial cells in ASD, such as increased GFAP expression, alterations in astrocytic aquaporins and connexins and mutant glial adhesion molecules. Therefore we focus on the glial activity, related to glutamate metabolism, on the animal model of autism induced by VPA.

Objectives: By means of the VPA-induced animal model of autism we aimed to characterize some astrocytic parameters, such as glutamate uptake, glutamine syntethase (GS) activity and glutathione (GSH) content in the hippocampus from young and adult male rats prenatally exposed to VPA.

 Methods: Females received a single intraperitoneal injection of valproic acid (600 mg/kg, 250 mg/mL diluted in NaCl 0.9%) in the 12.5th day of pregnancy. Male litters were killed by decapitation at post natal day 15 (P15 group) or 120 (P120 group), had their brains removed and the hippocampal samples were kept in −80°C until the assays (GS activity and GSH content) were performed, except the glutamate uptake, which were carried out with fresh tissue. Data are presented as mean±SE and were analyzed statistically by Student`s t test. P<0.05 were considered as statistically significant. All analyses were carried out using the Statistical Package for Social Sciences (SPSS) software.

Results:  Glutamate uptake was not statistically different at P15. At P120 there was a significantly increase in glutamate uptake in VPA group (0.99±0.07 nmol/mg protein/min), compared to the control samples (0.38±0.05 nmol/mg protein/min).  The next question aimed to understand a possible destiny to the intracellular glutamate, particularly at P120 which had a higher glutamate uptake. Glutamine synthetase activity assessed in hippocampal samples obtained from P15 and P120 rats. At P15, GS activity increased 42.8% in VPA group when compared to the control group. However, the opposite effect occurred at P120 rats, where VPA group decreased 26.8% the activity of GS when compared to the control group. Other metabolic destiny of glutamate is GSH, which content was evaluated in hippocampal structures obtained from 15 and 120 days old rats. There was no difference in GSH content at P15 but at P120 rats, VPA group had 26.8% increase in GSH content as compared to the control group.

Conclusions:  Once were found glial alteration on glutamate metabolism of VPA rats, the present data highlights that astrocyte clearance of glutamate from the synaptic cleft and its metabolic destiny are important aspects to be considered from both physiopathologic and pharmacological approaches in ASD.