<P> Purification of the receptor further verified its existence . The first attempt to purify the receptor involved the use of a novel opioid receptor antagonist called chlornaltrexamine that was demonstrated to bind to the opioid receptor . Caruso later purified the detergent - extracted component of rat brain membrane that eluted with the specifically bound H - chlornaltrexamine . </P> <P> There are four major subtypes of opioid receptors . OGFr was originally discovered and named as a new opioid receptor zeta (ζ). However it was subsequently found that it shares little sequence similarity with the other opioid receptors, and has quite different function . </P> <Table> <Tr> <Th> Receptor </Th> <Th> Subtypes </Th> <Th> Location </Th> <Th> Function </Th> </Tr> <Tr> <Td> delta (δ) DOR OP </Td> <Td> δ, δ </Td> <Td> <Ul> <Li> brain <Ul> <Li> pontine nuclei </Li> <Li> amygdala </Li> <Li> olfactory bulbs </Li> <Li> deep cortex </Li> </Ul> </Li> <Li> peripheral sensory neurons </Li> </Ul> </Td> <Td> <Ul> <Li> analgesia </Li> <Li> antidepressant effects </Li> <Li> convulsant effects </Li> <Li> physical dependence </Li> <Li> may modulate μ - opioid receptor - mediated respiratory depression </Li> </Ul> </Td> </Tr> <Tr> <Td> kappa (κ) KOR OP </Td> <Td> κ, κ, κ </Td> <Td> <Ul> <Li> brain <Ul> <Li> hypothalamus </Li> <Li> periaqueductal gray </Li> <Li> claustrum </Li> </Ul> </Li> <Li> spinal cord <Ul> <Li> substantia gelatinosa </Li> </Ul> </Li> <Li> peripheral sensory neurons </Li> </Ul> </Td> <Td> <Ul> <Li> analgesia </Li> <Li> anticonvulsant effects </Li> <Li> depression </Li> <Li> dissociative / hallucinogenic effects </Li> <Li> diuresis </Li> <Li> dysphoria </Li> <Li> miosis </Li> <Li> neuroprotection </Li> <Li> sedation </Li> <Li> stress </Li> </Ul> </Td> </Tr> <Tr> <Td> mu (μ) MOR OP </Td> <Td> μ, μ, μ </Td> <Td> <Ul> <Li> brain <Ul> <Li> cortex (laminae III and IV) </Li> <Li> thalamus </Li> <Li> striosomes </Li> <Li> periaqueductal gray </Li> <Li> rostral ventromedial medulla </Li> </Ul> </Li> <Li> spinal cord <Ul> <Li> substantia gelatinosa </Li> </Ul> </Li> <Li> peripheral sensory neurons </Li> <Li> intestinal tract </Li> </Ul> </Td> <Td> μ: <Ul> <Li> analgesia </Li> <Li> physical dependence </Li> </Ul> <P> μ: </P> <Ul> <Li> respiratory depression </Li> <Li> miosis </Li> <Li> euphoria </Li> <Li> reduced GI motility </Li> <Li> physical dependence </Li> </Ul> <P> μ: </P> <Ul> <Li> possible vasodilation </Li> </Ul> </Td> </Tr> <Tr> <Td> Nociceptin receptor NOR OP </Td> <Td> ORL </Td> <Td> <Ul> <Li> brain <Ul> <Li> cortex </Li> <Li> amygdala </Li> <Li> hippocampus </Li> <Li> septal nuclei </Li> <Li> habenula </Li> <Li> hypothalamus </Li> </Ul> </Li> <Li> spinal cord </Li> </Ul> </Td> <Td> <Ul> <Li> anxiety </Li> <Li> depression </Li> <Li> appetite </Li> <Li> development of tolerance to μ - opioid agonists </Li> </Ul> </Td> </Tr> <Tr> <Td> zeta (ζ) ZOR </Td> <Td> </Td> <Td> <Ul> <Li> heart </Li> <Li> liver </Li> <Li> skeletal muscle </Li> <Li> kidney </Li> <Li> brain </Li> <Li> pancreas </Li> <Li> fetal tissue <Ul> <Li> liver </Li> <Li> kidney </Li> </Ul> </Li> </Ul> </Td> <Td> <Ul> <Li> tissue growth <Ul> <Li> embryonic development </Li> <Li> regulation of cancer cell proliferation </Li> </Ul> </Li> </Ul> </Td> </Tr> </Table> <Tr> <Th> Receptor </Th> <Th> Subtypes </Th> <Th> Location </Th> <Th> Function </Th> </Tr>

Where are opioid receptors located in the brain
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