Exposure of Zebrafish Embryos to Morphine and Cocaine Induces Changes in the Levels of Dopamine and of Proteins Related to the Reward Pathway

Morphine, a drug of abuse used to treat moderate-to-severe pain, elicits its actions by binding to the opioid receptors. Cocaine is an example of a recreational drug that inhibits dopamine reuptake. The molecular effects of morphine and cocaine have been described in different specific brain regions. However, the systemic outcome of these drugs on the whole organism has not been fully addressed. The aim of this study is to analyse the global effects of morphine (10 μM) and cocaine (15 μM) in the expression of proteins related to the reward pathway. Zebrafish embryos were exposed to these drugs from 5 hpf (hours post-fertilisation) to 6 dpf (days post-fertilisation). Dopamine levels were determined by ELISA, and the expression of Fos proteins, Creb, its activated form p-Creb and tyrosine hydroxylase (Th) were examined by Western blot. Both drugs decreased Th levels at 72 hpf and 6 dpf and modified the expression of Fos family members, pCreb and Creb in a time-dependent manner. Morphine and cocaine exposure differentially modified dopamine levels in 72 hpf and 6 dpf zebrafish embryos. Our results indicate that drugs of abuse modify the expression of several proteins and molecules related to the activation of the reward pathway.