Food quality and motivation: a refined low-fat diet induces obesity and impairs performance on a progressive ratio schedule of instrumental lever pressing in rats.

INTRODUCTION: Purified high-fat diet (HFD) feeding causes deleterious metabolic and cognitive effects when compared with unrefined low-fat diets in rodent models. These effects are often attributed to the diet's high content of fat, while less attention has been paid to other mechanisms associated with the diet's highly refined state. Although the effects of HFD feeding on cognition have been explored, little is known about the impact of refined vs. unrefined food on cognition. We tested the hypothesis that a refined low-fat diet (LFD) increases body weight and adversely affects cognition relative to an unrefined diet. MATERIALS AND METHODS: Rats were allowed ad libitum access to unrefined rodent chow (CON, Lab Diets 5001) or a purified low-fat diet (REF, Research Diets D12450B) for 6 months, and body weight and performance on an instrumental lever pressing task were recorded. RESULTS: After six months on their respective diets, group REF gained significantly more weight than group CON. REF rats made significantly fewer lever presses and exhibited dramatically lower breaking points than CON rats for sucrose and water reinforcement, indicating a chronic reduction of motivation for instrumental performance. Switching the rats' diet for 9 days had no effect on these measures. CONCLUSIONS: Diet-induced obesity produces a substantial deficit in motivated behavior in rats, independent of dietary fat content. This holds implications for an association between obesity and motivation. Specifically, behavioral traits comorbid with obesity, such as depression and fatigue, may be effects of obesity rather than contributing causes. To the degree that refined foods contribute to obesity, as demonstrated in our study, they may play a significant contributing role to other behavioral and cognitive disorders.

Mining for allosteric information: natural mutations and positional sequence conservation in pyruvate kinase.

Although the amino acid sequences and the structures of pyruvate kinase (PYK) isozymes are highly conserved, allosteric regulations differ. This suggests that amino acids with low conservation play important roles in the allosteric mechanism. The current work exploits a 'natural screen'- the 122 point mutations identified in the human gene encoding the erythrocyte PYK isozyme and associated with nonspherocytic hemolytic anemia - to learn what amino acid positions in PYK may be important for allosteric regulations. In addition to the mutations, we consider the conservation of each amino acid position across 241 PYK sequences. Three groups of residue positions have been created, those with: (1) no disease causing mutation identified; (2) a disease causing mutation identified and high conservation across isozymes; and (3) a disease causing mutation identified and low conservation. Mutations at positions not identified in the natural screen are likely to be tolerated with minimal loss of function. Mutations at highly conserved positions are more likely to disrupt properties common to all PYK isozymes (e.g., structure, catalysis). Residues in the third group are likely to be involved in roles that are necessary for function but not common to all isozymes (e.g., allostery). Many of the Group 3 residues are located in the C-domain and to a lesser extent the A domain.