Fathead minnow exposed to environmentally relevant concentrations of metformin for one life cycle show no adverse effects

Abstract

Metformin is a glucose-lowering drug taken for diabetes. It is excreted by humans in urine and detected in municipal wastewater effluents and rivers. Fathead minnows ( Pimephales promelas) were exposed over a life cycle to measured concentrations of metformin: 3.0, 31, and 322 μg/L. No significant changes were observed in survival, maturation, growth, condition factor, or liver size. Relative ovary size of females exposed to 322 μg/L metformin was significantly larger than controls. There was no induction of vitellogenin in plasma of minnows, and gonad maturation was not statistically different from controls. The start of breeding was delayed by 9–10 d in the mid- and high metformin treatments (statistically significant only in the mid-concentration), but numbers and quality of eggs were not statistically different from controls. There were no effects of metformin on survival or growth of offspring. Exposure to metformin at environmentally relevant concentrations (i.e., 3.0 and 31 μg/L metformin) caused no adverse effects in fathead minnows exposed for a life cycle, with the exception of a delay in time to first breeding (that did not impact overall egg production). The results of the study are important to help understand whether metformin concentrations in rivers and lakes can harm fishes.

Effects of Metformin and its Metabolite Guanylurea on Fathead Minnow (Pimephales promelas) Reproduction

“…Because the occurrence of intersex in male fathead minnows is uncommon and has thus far not been replicated in other exposures to metformin, the weight of evidence does not indicate metformin as a primary cause of intersex gonad development. Based on our data as well as that of Parrott et al (2021), metformin does not appear to be a potent reproductive toxicant in fish.…”

“…Elsewhere, juvenile fathead minnows (75–85 dpf) exposed to 1.0, 10, or 100 µg metformin/L for 7 days had elevated whole‐body expression of four genes associated with reproduction and development ( vtg , esr1 , gonadotropin‐releasing hormone 3 [g nrh3 ], and cyp3A ), although similar changes were not observed in adult male livers following a 7‐day exposure at the same concentrations (Crago et al, 2016). Most recently, a study exposing fathead minnows to 3.0, 31, or 322 µg/L metformin over a full life cycle identified no adverse effect on fecundity and zero incidence of oocytes in testes (Parrott et al, 2021). Gene expression in testes was not investigated, but a lack of effect on gonad histology and spawning suggests that treated individuals would not differ significantly from controls.…”

“…However, most recently, a study of fathead minnow embryos exposed to varying concentrations (3.4–269 µg/L) of metformin for 21 days showed no significant impact on hatchability, survival, or growth (Parrott et al, 2022). Likewise, a full life cycle exposure of fathead minnows to metformin (3.0–300 µg/L) demonstrated no impacts on survival, growth, or fecundity (Parrott et al, 2021). Assessment of gonad histology showed no significant impacts on males or females, and no incidence of intersex was observed.…”

Are Fish Populations at Risk? Metformin Disrupts Zebrafish Development and Reproductive Processes at Chronic Environmentally Relevant Concentrations

“…At 20 dpf, a significant decrease in survival was also observed for larvae exposed to 2 929 and 14 423 ng/L MET, when compared to the control (45.2 and 24.4% survival, respectively) (Table ). As expected, after 20 dpf, mortality was very low until adulthood, since even under normal raising conditions, zebrafish are more sensitive up to 20 dpf, considering that during this period they develop from one cell stage into a swimming larvae and change from vitellogenic reserves to external feeding. , Other recent studies with zebrafish are in agreement with the present findings, reporting that MET at concentrations from 1 to 10 μg/L significantly impacts zebrafish survival at the early phases of development. , However, these effects in mortality are not supported by other studies examining the effects of MET in other fish species, such as brown trout ( Salmo trutta ) and fathead minnows ( Pimephales promelas ), where MET exposure did not impact mortality at different development phases, including the larval stage. ,, Analyzing the few available studies, we hypothesize that zebrafish, at least in the early life stages of development, are potentially more sensitive than the other few fish species tested so far that present longer periods of embryonic development.…”

“…Despite the short-term reproductive studies referenced above, the assumption that pairwise breeding is a better alternative to the tank breeding approach to assess MET effects in fish reproductive success is largely in agreement with the two full life cycle studies that addressed the reproductive output of P. promelas exposed to MET. 15,25 Even though Parrot et al…”