''There are strong reasons for considering opioids, at very low doses, as antidepressants. Although many people may not realize it, we are all born with a natural—or, as researchers say, “endogenous”—opioid system. Our bodies manufacture various opioid molecules and our cells are studded with keyhole-like structures called receptors that are specifically designed to fit these naturally occurring opioid “keys.” There are four types of receptors that accept different opioid molecules. They are very common in brain cells, but also in the spinal cord, the digestive tract and in peripheral nerves. “It has long been understood that the endogenous opioid system that we have is responsive to stress and mood—it helps regulate them,” explains Irwin Lucki, Ph.D''.