The importance of pH: how aquarium water is affecting behavioural responses to drug exposure in larval zebrafish

There has been rapid growth in the use of larval zebrafish as a complementary vertebrate model for drug discovery, abuse liability and pharmacological toxicology, resulting in a huge increase in zebrafish facilities worldwide. However, many research groups working with zebrafish do not typically report the pH of husbandry conditions in methodologies, nor are the pH of drug treatments reported in many research articles. This unknown factor can be a major contributor in the differential effects of drug treatments. Therefore, as a case study, we tested the impact of altering pH of several drugs of abuse and assessed locomotor changes associated with a single drug concentration delivered at different pHs. We found that a change of a single pH unit, within the pH ranges commonly used in zebrafish husbandry, was enough to alter locomotor activity at a fixed drug concentration. Many pharmacological agents are dependent on environmental factors, such as pH, to determine bioavailability. Efficaciousness for many classes of drug is dependent on their ionization state in which shifts towards uncharged species can influence the easy of a drug crossing biological membranes. Thus, we urge users to report pH in husbandry methods and drug treatments to improve replicability and inter-study comparisons.