I'm not sure that nalmefene can be used in treating cocaine abuse. Coke has a dopaminergic effect which modulates neurotransmission, although it is also linked to the opioidergic system.

Nalmefene has a molecular shape that allows it 'block' neuronal endorphin receptors (like a fake key stuck in a lock that can't open the door). If endorphins enter the receptors of firing neurons, they promote new dendritic growth to neighbouring neurons that were also firing at the time of reception. That's how behaviour becomes hardwired - literally.

When neuronal receptors for endorphins are blocked, the process is 'reversed'. i.e. if the stimulation of a neuron by endorphin does not happen because of the blocking, then existing dendritic connections are weakened over time. That is essentially what the nalmefene treatment entails - 'forgetting' behaviour'. Although such behaviour can be quickly 'remembered' again by unblocked stimulation.

Whereas (if I understand it correctly) stimulated dopamine receptors modulate transmission across the synaptic gap. Which is a whole different kettle of students.

I don't know if dopaminergic antagonists have yet been discovered.

You can't be me, I'm taken