Nerve cell membrane binding sites that are activated by dopamine molecules. Some are excitatory and some inhibitory, but both classes contain many subtypes. Some of these receptors mediate behavioural effects.
There is good evidence that prolactin-secreting cells in the pituitary gland are under inhibitory control of dopamine, and that these cells can show striking proliferation under conditions of physiologic demand [such as during pregnancy] or when dopamine receptors are blocked.
He said it's the same type of mechanism that's at work in drug and alcohol addiction, which hijacks the same dopamine pathways and pushes a person to use more and more of the substance as dopamine receptors shut down.