Agouti-related protein increases food hoarding more than food intake in Siberian hamsters

Abstract

Agouti-related protein (AgRP), an endogenous melanocortin 3/4 receptor antagonist, appears to play an important role in the control of food intake and energy balance because exogenous administration in rats and overexpression in mice result in hyperphagia and body mass gain. Furthermore, arcuate nucleus AgRP mRNA is increased with fasting in laboratory rats and mice and is decreased with refeeding. In Siberian hamsters, fasting also increases arcuate nucleus AgRP mRNA, but these animals increase food hoarding, rather than food intake with refeeding. Therefore, we tested whether exogenous AgRP increased food hoarding in this species. Hamsters were trained in a hoarding/foraging apparatus to run a programmed number of wheel revolutions to earn food pellets. Four doses of AgRP-(83-132) or vehicle were injected into the third ventricle at the beginning of the dark phase, and food hoarding, food intake, and foraging were measured at various time points subsequently. Overall, food hoarding was stimulated as much as 10 times more than food intake, and both responses occurred as early as 1 h after injection. Food hoarding was increased the greatest at the lowest dose (0.1 nmol), whereas food intake was increased the greatest at the second lowest dose (1 nmol). Food intake and especially food hoarding were increased up to seven days after the AgRP injections. Foraging was increased at all AgRP doses except the highest dose (100 nmol). These results suggest that AgRP triggers the search for food in this species, and once they find it, hoarding predominates over eating.

MTII attenuates ghrelin- and food deprivation-induced increases in food hoarding and food intake

“…For example, icv NPY stimulates consumption of sucrose when an animal has to move to a sipper tube to drink (eat), but does not do so when the sucrose is delivered via an intraoral catheter (no appetitive response; (Seeley et al, 1995)). Food intake after food deprivation or ghrelin injections in the present study, and after icv NPY (Day et al, 2005) or AgRP (Day and Bartness, 2004) in our previous studies using this model, are maximal when animals are foraging for their food (10 revolutions/pellet). In laboratory rats, if the appetitive effort (bar pressing requirement) is too energetically demanding, the ability of icv NPY to stimulate food intake is less than when food is freely available (Jewett et al, 1992).…”

“…We previously demonstrated that antagonism (inverse agonism) of MC3/4-R by intracerebroventricularly injected AgRP stimulates foraging and especially food hoarding, with a significantly smaller stimulation of food intake (Day and Bartness, 2004). Consistent with the notion of involvement of MC3/4-Rs in these appetitive and consummatory ingestive behaviors is the inhibition of AgRP-triggered increase in food hoarding by prior injection of MTII (Day and Bartness, unpublished observations).…”

“…These data are consistent with our initial study of ghrelin effects on ingestive behaviors in this species (Keen-Rhinehart and These interesting, albeit sometimes complicated findings, suggest several points. The foraging effort-dependent effects of peptidergic control of food hoarding is a frequent finding in our studies of food hoarding stimulation by third ventricular injection of NPY or a Y1 or Y5 agonist (Day et al, 2005), or AgRP (Day and Bartness, 2004) or systemic injection of ghrelin (KeenRhinehart and . The existence of peptide effects on ingestive behavior that are dependent on the presence of an appetitive behavioral component has been reported previously.…”

“…Cannulae were stereotaxically implanted into the third ventricle as described previously (Day and Bartness, 2004). Briefly, the animals were anesthetized with isoflurane and the fur at the top of the head was removed to expose the area to be incised.…”

Feeding behavior, obesity, and neuroeconomics

“…This suggests that optimal energy levels are influential in hoarding as well as food intake that occurs while foraging. Cerebroventricular injection of the orexigenic peptide, AgRP, increased the amount of food consumed and hoarded, but the stimulatory effect on hoarding occurred at doses lower than those that increase food intake [51]. Similar results were found with acute injection of two other orexigenic agents, peripherally-injected ghrelin [52] and centrally-injected NPY [53].…”