Long-Term High-Fat Diet Consumption Depletes Glial Cells and Tyrosine Hydroxylase-Containing Neurons in the Brain of Middle-Aged Rats.

Epidemiologic studies have indicated that dyslipidemia may facilitate the progression of neuronal degeneration. However, the effects of chronic dyslipidemia on brain function, especially in older individuals, remain unclear. In this study, middle-aged 37-week-old male Wistar-Kyoto rats were fed a normal diet (ND) or a 45% high-fat diet (HFD) for 30 weeks (i.e., until 67 weeks of age). To study the effects of chronic dyslipidemia on the brain, we analyzed spontaneous locomotor activity, cognitive function, and brain tissues in both groups of rats after 30 weeks. Compared with age-matched rats fed a ND, Wistar-Kyoto rats fed a HFD had dyslipidemia and showed decreased movement but normal recognition of a novel object. In our brain analyses, we observed a significant decrease in astrocytes and tyrosine hydroxylase-containing neurons in the substantia nigra and locus coeruleus of rats fed a HFD compared with rats fed a ND. However, hippocampal pyramidal neurons were not affected. Our findings indicate that the long-term consumption of a HFD may cause lipid metabolism overload in the brain and damage to glial cells. The decrease in astrocytes may lead to reduced protection of the brain and affect the survival of tyrosine hydroxylase-containing neurons but not pyramidal neurons of the hippocampus.

Upregulation of Glutamatergic Receptors in Hippocampus and Locomotor Hyperactivity in Aged Spontaneous Hypertensive Rat.

Epidemiologic studies have indicated that chronic hypertension may facilitate the progression of abnormal behavior, such as emotional irritability, hyperactivity, and attention impairment. However, the mechanism of how chronic hypertension affects the brain and neuronal function remains unclear. In this study, 58-week-old male spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) control rats were used. Their locomotor activity and neuronal function were assessed by the open field test, novel object, and Y maze recognition test. Moreover brain tissues were analyzed. We found that the aged SHR exhibited significant locomotor hyperactivity when compared to the WKY rats. However, there was no significant difference in novel object and novel arm recognition between aged SHR and the WKY rats. In the analysis of synaptic membrane protein, the expression of glutamatergic receptors, such as the N-methyl-D-aspartate (NMDA) receptor receptors subunits 2B (GluN2B) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor 1 (GluA1) in the hippocampus of SHR were significantly higher than those of WKY rats. In addition, in the synaptic membrane of SHR's hippocampus and medial prefrontal cortex (mPFC), a down-regulation of astrocytes was found, though the excitatory amino acid transporter 2 (EAAT2) remained constant. Moreover, a down-regulation of microglia in the hippocampus and mPFC was seen in the SHR brain. Long-term exposure to high blood pressure causes upregulation of glutamate receptors. The upregulation of glutamatergic receptors in hippocampus may contribute to the hyper-locomotor activity of aged rodents and may as a therapeutic target in hypertension-induced irritability and hyperactivity.