When going to the beach for a swim, probably the last thing on your mind is whether you are swimming in a thin soup of drug traces. Whether prescribed over the counter or illegal, many drugs taken by people wind up in our waterways. These drugs cannot be removed by wastewater treatment plants and thus get released back into the environment. A famous coastal region in Brazil, the Bay of All Saints (Baía de Todos os Santos in Portuguese), has been the subject of multiple environmental contaminant studies and researchers have found measurable quantities of cocaine in the seawater, sediment and mussels (Perna perna) of the bay. Although the environmental concentrations are too low to affect humans, whether cocaine at these levels has an effect on the nervous system of aquatic species that reside in the bay was unknown. So, Mayana Fontes and a team of researchers from Universidade Estadual Paulista Júlio de Mesquita Filho, Brazil, and other universities in Brazil investigated how levels of cocaine in the seawater affect the nervous system of P. perna mussels.
Fontes and colleagues placed adult mussels in tanks containing seawater with two different concentrations of cocaine that were realistic for the Bay of All Saints (0.2 and 2 μg l−1) for 2, 4 or 7 days. Then, they collected the adductor muscles, which open and close the shell, and gonads of these mussels. In the adductor muscle, they measured the activity of acetylcholine esterase, an enzyme essential for a functioning nervous system. In the gonads, they measured the levels of two major neurotransmitters, serotonin and dopamine, the activity of the enzyme that breaks down these neurotransmitters, monoamine oxidase, and the activity of cyclooxygenase, an enzyme that is a sign of inflammation and is important for egg release in mussels. In addition, the authors also measured how much energy the mussels spent during the exposure to cocaine by measuring mitochondrial electron transport activity, and how much energy they set aside by measuring the level of total lipids in the gonads.
The team found that mussels exposed to cocaine had a decreased acetylcholine esterase activity in the adductor muscle, which shows that cocaine disrupted the normal activity of the nervous system. They also found that mussels exposed to cocaine had raised levels of dopamine and serotonin. These neurotransmitters are essential for reproduction, and changes could lead to an unsustainable population in the wild. The mussels also increased cyclooxygenase activity in the gonads, which shows an inflammatory response to low doses of cocaine. In addition, the scientists found changes in the mitochondrial electron transport chain – which converts the energy from oxygen into ATP in the mitochondria – directly increasing the amount of energy the mussels spent after exposure to the drug, which decreases the amount of energy they can access for reproduction as energy is limited. Finally, they also found that there was an increase in levels of total lipid, which act as a convenient and easy to access energy source for the animals.
Fontes and colleagues have shown that the current concentration of cocaine in the seawater around the Bay of All Saints can negatively impact the area's mussels in complex ways and this may put them at a greater risk from other chemicals that we dump into the environment.