Activation of both the GHB receptor and GABAB is responsible for the addictive profile of GHB. GHB's effect on dopamine release is biphasic. Low concentrations stimulate dopamine release via the GHB receptor. Higher concentrations inhibit dopamine release via GABAB receptors as do other GABAB agonists such as baclofen and phenibut. After an initial phase of inhibition, dopamine release is then increased via the GHB receptor. Both the inhibition and increase of dopamine release by GHB are inhibited by opioid antagonists such as naloxone and naltrexone. Dynorphin may play a role in the inhibition of dopamine release via kappa opioid receptors.