There are three types of opioid receptors: Mu (μ-opioid receptors), delta, and kappa (κ-opioid receptor). Endogenous opioids (enkephalins, dynorphin, endorphin) do not bind specifically to any one opioid receptor. Receptor binding of the opioid causes a cascade leading to the channel opening and hyperpolarization of the neuron. The opioid receptors have the following channel types: Mu, K+ channel; l Delta, K+ channel; Kappa, Ca2+ channel. Hyperpolarization can lead to post-synaptic neural inhibition and presynaptic inhibition of neurotransmitter release. Post-synaptic neural inhibition can reduce analgesia and central hyperactivity may reduce its efficacy.