Zebrafish larvae are insensitive to stimulation by cocaine: importance of exposure route and toxicokinetics

Zebrafish (Danio rerio) larvae have been suggested as vertebrate model to complement or even replace mammals for rapidly assessing behavioral effects of psychoactive drugs. Yet, divergent responses have been reported in mammals and fish despite the conservation of many drug targets. Cocaine, eg, acts as stimulant in mammals but no such response has been documented for zebrafish larvae. We hypothesized that differences in exposure routes (inhalation or injection in mammals vs waterborne in fish) may be a reason for differences in behavioral responses. We characterized cocaine toxicokinetics by liquid chromatography-mass spectrometry and found its rapid uptake into larvae. We used Matrix-assisted laser desorption ionization-mass spectrometry imaging for the first time to characterize internal distribution of cocaine in zebrafish larvae. Surprisingly, eyes accumulated the highest amount of cocaine and retained most of it even after 48 h depuration. We attribute this to trapping by pigment melanin, a thus far little explored mechanism that may also be relevant for other basic drugs. Cocaine also reached the brain but with levels similar to those in trunk indicating simple passive diffusion as means of distribution which was supported by toxicokinetic models. Although brain levels covered those known to cause hyperactivity in mammals, only hypoactivity (decreased locomotion) was recorded in zebrafish larvae. Our results therefore point to cocaine’s anesthetic properties as the dominant mechanism of interaction in the fish: upon entry through the fish skin and gills, it first acts on peripheral nerves rapidly overriding any potential stimulatory response in the brain