No, animal organisms use these receptors for their internal regulation, e.g. opiate receptors are used in a reward system which pushes the animal behavior towards doing things which are good for the procreation of its genes – like having sex, eating nutritious food and so on. Plants like opium poppy produce some substances for their own purposes (like defense against parasites), and the effect of these substances on the animal brain receptors is a coincidence.
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