Overweight and obesity result from an imbalance between caloric intake
and energy expenditure, including expenditure from spontaneous physical activity
(SPA). Changes in SPA and resulting changes in non-exercise activity
thermogenesis (NEAT) likely interact with diet to influence risk for obesity.
However, previous research on the relationship between diet, physical activity,
and energy expenditure has been mixed. The neuropeptide orexin is a driver of
SPA, and orexin neuron activity can be manipulated using DREADDs (Designer
Receptors Exclusively Activated by Designer Drugs). We hypothesized that HFD
decreases SPA and NEAT, and that DREADD-mediated activation of orexin neuron
signaling would abolish this decrease and produce an increase in NEAT instead.
To test these ideas, we characterized behaviors to determine the extent to which
access to a high-fat diet (HFD) influences the proportion and probability of
engaging in food intake and activity. We then measured NEAT following access to
HFD and following a DREADD intervention targeting orexin neurons. Two cohorts of
orexin-cre male mice were injected with an excitatory DREADD virus into the
caudal hypothalamus, where orexin neurons are concentrated. Mice were then
housed in continuous metabolic phenotyping cages (Sable Promethion). Food
intake, indirect calorimetry, and SPA were automatically measured every second.
For cohort 1 (n=8), animals were given access to chow, then switched to
HFD. For cohort 2 (n=4/group), half of the animals were given access to
HFD, the other access to chow. Then, among animals on HFD, orexin neurons were
activated following injections of clozapine n-oxide (CNO). Mice on HFD spent
significantly less time eating (p<0.01) and more time inactive compared
to mice on chow (p<0.01). Following a meal, mice on HFD were
significantly more likely to engage in periods of inactivity compared to those
on chow (p<0.05). NEAT was decreased in animals on HFD, and was
increased to the NEAT level of control animals following activation of orexin
neurons with DREADDs. Food intake (kilocalories) was not significantly different
between mice on chow and HFD, yet mice on chow expended more energy per unit of
SPA, relative to that in mice consuming HFD. These results suggest that HFD
consumption reduces SPA and NEAT, and increases inactivity following a meal.
Together, the data suggest a change in the efficiency of energy expenditure
based upon diet, such that SPA during HFD burns fewer calories compared to SPA
on a standard chow diet.