bremelanotide PT-141 why does this increases libido?

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I have always tought libido was a mix of testosterone and dopamine.
These new class of drugs called melanocortins, which work in the mind, increasing sexual desire. Deep inside the brain, the substance stirs passion by activating hypothalamic and limbic areas

My question is: why are melanocortins receptors and not the dopamine ones, implicated with sex?