If the brain already has opiate receptors, doesn`t that mean it can naturally provide pain relief?

如果大脑已经有了阿片受体，那不就意味着它可以自然地缓解疼痛吗？

After being injected, the drug crosses the blood-brain barrier, where it binds to opiate receptors and causes a brief high described as being similar to that of heroin.

注射后，这种药物会穿过血脑屏障，在那里它与阿片受体结合， 并引起一种类似于海洛因的短暂快感。