ABSTRACT: Mechanisms contributing to age-related cognitive decline are poorly defined. Thus, we used canine microarrays to compare gene expression profiles of brain tissue from geriatric and young adult dogs. Cerebral cortex samples were collected from 6 geriatric (12 yr-old) and 6 young adult (1 yr-old) female beagles after being fed one of two diets (animal protein-based versus plant-protein based) for 12 months. RNA samples were hybridized to Affymetrix GeneChip Canine Genome Arrays. Statistical analyses indicated that the age had the greatest impact on gene expression, with 963 transcripts differentially expressed in geriatric dogs. Although not as robust as age, diet affected mRNA abundance of 140 transcripts. As demonstrated in aged rodents and humans, geriatric dogs had increased expression of genes associated with inflammation, stress response, and calcium homeostasis and decreased expression of genes associated with neuropeptide signaling and synaptic transmission. In addition to its existing strengths, availability of gene sequence information and commercial microarrays make the canine a powerful model for studying the effects of aging on cognitive function. Keywords: age; diet Six geriatric (11.1 yr old) and 6 weanling (8 wk old) female beagles were used. Three dogs of each age were assigned to one of two dietary treatments and fed for 12 months. Diets tested in this experiment were previously shown to manipulate energy metabolism. One diet was an animal-protein based diet (APB) and was composed primarily of highly digestible ingredients and animal-derived protein and fat sources (brewer’s rice, poultry by-product meal, poultry fat) and was formulated to contain 28% protein, 23% fat, and 5% dietary fiber. The other diet was a plant-protein based diet (PPB) and was composed primarily of moderately digestible plant-derived ingredients (corn, soybean meal, wheat middlings, and meat and bone meal) and was formulated to contain 26% protein, 11% fat, and 15% dietary fiber. Although the two diets were very different in terms of ingredient and chemical composition, both were formulated to meet or exceed all nutrient requirements for canine growth according to the Association of American Feed Control Officials. Young dogs were fed ad libitum to allow for adequate growth, while geriatric dogs were fed to maintain baseline BW throughout the experiment. To produce the desired metabolic effects, the PPB diet was formulated to contain a lower caloric density (APB = 5.38 kcal/g; PPB = 4.75 kcal/g) and have a lower nutrient digestibility than the APB diet. Thus, dogs fed the PPB diet needed to consume a greater (P<0.05) quantity of food (237 g/d; 1123 kcal/d) than dogs fed the APB diet (166 g/d; 893 kcal/d) to grow (young) or maintain BW (geriatrics). Even though metabolic indices were altered, mean BW among dietary treatments was not different at any time over the course of the study for young or geriatric dogs. After 12 months on experiment, animals were fasted for 12 hr and then given a lethal dose (130 mg/kg BW) of sodium pentobarbital (Euthasol, Virbac Corp., Fort Worth, TX) intravenously into the left forearm. Death was confirmed by lack of respiration and a corneal reflex, and absence of a heartbeat detected with a stethoscope placed under the left elbow. Cerebral cortex samples were collected immediately after death was confirmed, flash frozen using liquid nitrogen, and stored at -80oC until further analysis.
