Dopamine and endorphins are released, which are often associated with pleasure. When laughing the brain also releases endorphins that can relieve some physical pain. It can also boost the immune system and release endorphins to relieve pain. The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin. This effectively blocks the receptor, preventing the body from responding to opioids and endorphins. Exorphins are exogenous opioid peptides, distinguished from endorphins (or endogenous opioid peptides). These endorphins likely operate on the same opioid receptors that naloxone blocks. Neurons of the PAG are excited by endorphins and by opiate analgesics. Endorphins are produced in the pituitary gland. Naloxone has been shown to block the action of pain-lowering endorphins the body produces naturally. The principal function of endorphins is to inhibit the communication of pain signals. Increasing endurance has been proven to release endorphins resulting in a positive mind. Such peptides may be produced by the body itself, for example endorphins. Soon after they provided evidence that the endorphins system was involved in sexual pleasure. All three types of endorphins are fragments of the precursor protein proopiomelanocortin (POMC).