Impacts of metformin and aspirin on life history features and longevity of crickets: trade-offs versus cost-free life extension?

Abstract

We examined the impacts of aspirin and metformin on the life history of the cricket Acheta domesticus (growth rate, maturation time, mature body size, survivorship, and maximal longevity). Both drugs significantly increased survivorship and maximal life span. Maximal longevity was 136 days for controls, 188 days (138 % of controls) for metformin, and 194 days (143 % of controls) for aspirin. Metformin and aspirin in combination extended longevity to a lesser degree (163 days, 120 % of controls). Increases in general survivorship were even more pronounced, with low-dose aspirin yielding mean longevity 234 % of controls (i.e., health span). Metformin strongly reduced growth rates of both genders (<60 % of controls), whereas aspirin only slightly reduced the growth rate of females and slightly increased that of males. Both drugs delayed maturation age relative to controls, but metformin had a much greater impact (>140 % of controls) than aspirin (~118 % of controls). Crickets maturing on low aspirin showed no evidence of a trade-off between maturation mass and life extension. Remarkably, by 100 days of age, aspirin-treated females were significantly larger than controls (largely reflecting egg complement). Unlike the reigning dietary restriction paradigm, low aspirin conformed to a paradigm of “eat more, live longer.” In contrast, metformin-treated females were only ~67 % of the mass of controls. Our results suggest that hormetic agents like metformin may derive significant trade-offs with life extension, whereas health and longevity benefits may be obtained with less cost by agents like aspirin that regulate geroprotective pathways.

Appendix

Determining doses of metformin and aspirin was guided by several goals. Starting with human doses, we adjusted for body mass and metabolic rates to scale cricket doses roughly to that respective to humans. This was intended to identify a range likely tolerable for crickets. It was logistically intractable to match doses to growing crickets, particularly since males and females differ in mass and treatments were anticipated to diverge with time as well. Compensatory feeding or reductions associated with reduced growth could not be anticipated. We aimed to maintain a single dose for each treatment and across ages to provide a unified experimental framework. We also wanted to ensure that the doses were high enough to strongly impact crickets as there was uncertainty of biological differences between species (i.e., higher than the conservative human doses). We calculated the dose such that a juvenile cricket eating 30 mg of food would obtain that scaled equivalent of a human (i.e., adjusted for mass and metabolic rate). This ensured that the experiment began with a relatively high dose. Initial pilot studies did not detect any tolerance issues and results illustrate that all treatments obtained increased survivorship over controls across all ages (Fig. 1).

Diets for all treatments were prepared similarly. Aspirin and metformin were obtained from Sigma-Aldrich. Doses of drugs were based on recommendations for humans adjusted for differences in the mass and metabolic rate of crickets. The mass ratio of crickets (0.5386 g) to humans (~80 kg) was 6.7325 × 10⁻⁶. The ratio of the metabolic rate of crickets (14.38 ml O²/g/day; Hack 1997) to humans (4.62 ml O²/g/day; Nieman et al. 2003; Rizzo et al. 2005) was 3.11. Combining these conversion factors obtained a global conversion factor of 2.09 × 10⁻⁵. For metformin, the maximum recommended human dose is ~2.55 g/day (FDA guidelines). Applying the conversion factor yields a rough estimate of 2.09 × 10⁻⁵ × 2.55 = 5.3294 × 10⁻⁵ g of metformin/cricket. A juvenile cricket eats ~30 mg of food/day so the concentration of metformin in food that would deliver a high dose was 5.329 × 10⁻⁵ g metformin/0.03 g food or 1.78 × 10⁻³ g metformin/g diet. Anisimov (2013) reviewed dosages used in studies of metformin. The dose of metformin obtaining health and survival benefits in C57BL/6 mice was 0.1 % (w/w) in food (e.g., 1.0 × 10⁻³ g/g) (Martin-Montalvo et al. 2013)—very close to ours.

The recommended high dose of aspirin for humans is 325 mg, and a low dose (25 %) is 81 mg (pharmacy products). The high dose per cricket obtained by applying the global conversion factor was 2.09 × 10⁻⁵ × 325 = 6.79 × 10⁻³ mg/cricket. Given that a juvenile cricket eats ~30 mg of food/day, the concentration for the diet was 6.79 × 10⁻⁶/0.03 = 2.263 × 10⁻⁴ g aspirin/g food for the high dose. The low dose was 25 % of the high dose (5.65 × 10⁻⁵ g aspirin/g food). The metformin-aspirin combination was made with the above dosage of metformin and the high-dose aspirin.

Drugs were stirred into the mixture of agar and water after it had cooled well below 50 °C. Metformin is stable at temperatures as high as 70 °C (Sharma et al. 2010), but aspirin loses stability above 50 °C (Snavely et al. 1993; Al-Gohary and Al-Kassas 2000). When the mixture reached room temperature, it was poured into a blender and 30 g of guinea pig food was thoroughly blended in. The agar was then set and stored in a dark refrigerator. New diet was prepared weekly.