MINDのＩＭＡＧＥの形成は、頭脳神経細胞システムの「自動運転」の結果。
MINDのＩＭＡＧＥの形成は、頭脳神経細胞システムの「自動運転」の結果。
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以下はウイキペディア参照。
By the mid-1960s, it had become apparent from pharmacologic studies that opioids were likely to exert their actions at specific receptor sites, and that there were likely to be multiple such sites.[4] Early studies had indicated that opiates appeared to accumulate in the brain.[5] The receptors were first identified as specific molecules through the use of binding studies, in which opiates that had been labeled with radioisotopes were found to bind to brain membrane homogenates. The first such study was published in 1971, using 3H-levorphanol.[6] In 1973, Candace Pert and Solomon H. Snyder published the first detailed binding study of what would turn out to be the μ opioid receptor, using 3H-naloxone.[7] That study has been widely credited as the first definitive finding of an opioid receptor, although two other studies followed shortly after.
Mechanism of activation
Opioid receptors are a type of G protein–coupled receptor (GPCR). These receptors are distributed throughout the central nervous system and within the peripheral tissue of neural and non-neural origin. They are also located in high concentrations in the Periaqueductal gray, Locus coeruleus, and the Rostral ventromedial medulla.[43] The receptors consist of an extracellular amino acid N-terminus, seven trans-membrane helical loops, three extracellular loops, three intracellular loops, and an intracellular carboxyl C-terminus. Three GPCR extracellular loops provide a compartment where signaling molecules can attach to generate a response. Heterotrimeric G protein contain three different sub-units, which include an alpha (α) subunit, a beta (β) subunit, and a gamma (γ) sub-unit.[44] The gamma and beta sub-units are permanently bound together, producing a single Gβγ sub-unit. Heterotrimeric G proteins act as ‘molecular switches’, which play a key role in signal transduction, because they relay information from activated receptors to appropriate effector proteins. All G protein α sub-units contain palmitate, which is a 16-carbon saturated fatty acid, that is attached near the N-terminus through a labile, reversible thioester linkage to a cysteine amino acid. It is this palmitoylation that allows the G protein to interact with membrane phospholipids due to the hydrophobic nature of the alpha sub-units. The gamma sub-unit is also lipid modified and can attach to the plasma membrane as well. These properties of the two sub-units, allow the opioid receptor's G protein to permanently interact with the membrane via lipid anchors.[45]
When an agonistic ligand binds to the opioid receptor, a conformational change occurs, and the GDP molecule is released from the Gα sub-unit. This mechanism is complex, and is a major stage of the signal transduction pathway. When the GDP molecule is attached, the Gα sub-unit is in its inactive state, and the nucleotide-binding pocket is closed off inside the protein complex. However, upon ligand binding, the receptor switches to an active conformation, and this is driven by intermolecular rearrangement between the trans-membrane helices. The receptor activation releases an ‘ionic lock’ which holds together the cytoplasmic sides of transmembrane helices three and six, causing them to rotate. This conformational change exposes the intracellular receptor domains at the cytosolic side, which further leads to the activation of the G protein. When the GDP molecule dissociates from the Gα sub-unit, a GTP molecule binds to the free nucleotide-binding pocket, and the G protein becomes active. A Gα(GTP) complex is formed, which has a weaker affinity for the Gβγ sub-unit than the Gα(GDP) complex, causing the Gα sub-unit to separate from the Gβγ sub-unit, forming two sections of the G protein. The sub-units are now free to interact with effector proteins; however, they are still attached to the plasma membrane by lipid anchors.[46] After binding, the active G protein sub-units diffuses within the membrane and acts on various intracellular effector pathways. This includes inhibiting neuronal adenylate cyclase activity, as well as increasing membrane hyper-polarisation. When the adenylyl cyclase enzyme complex is stimulated, it results in the formation of Cyclic Adenosine 3', 5'-Monophosphate (cAMP), from Adenosine 5' Triphosphate (ATP). cAMP acts as a secondary messenger, as it moves from the plasma membrane into the cell and relays the signal.[47]
cAMP binds to, and activates cAMP-dependent protein kinase A (PKA), which is located intracellularly in the neuron. The PKA consists of a holoenzyme - it is a compound which becomes active due to the combination of an enzyme with a coenzyme. The PKA enzyme also contains two catalytic PKS-Cα subunits, and a regulator PKA-R subunit dimer. The PKA holoenzyme is inactive under normal conditions, however, when cAMP molecules that are produced earlier in the signal transduction mechanism combine with the enzyme, PKA undergoes a conformational change. This activates it, giving it the ability to catalyse substrate phosphorylation.[48] CREB (cAMP response element binding protein) belongs to a family of transcription factors and is positioned in the nucleus of the neuron. When the PKA is activated, it phosphorylates the CREB protein (adds a high energy phosphate group) and activates it. The CREB protein binds to cAMP response elements CRE, and can either increase or decrease the transcription of certain genes. The cAMP/PKA/CREB signalling pathway described above is crucial in memory formation and pain modulation.[49] It is also significant in the induction and maintenance of long-term potentiation, which is a phenomenon that underlies synaptic plasticity - the ability of synapses to strengthen or weaken over time.
Voltage-gated dependent calcium channel, (VDCCs), are key in the depolarisation of neurons, and play a major role in promoting the release of neurotransmitters. When agonists bind to opioid receptors, G proteins activate and dissociate into their constituent Gα and Gβγ sub-units. The Gβγ sub-unit binds to the intracellular loop between the two trans-membrane helices of the VDCC. When the sub-unit binds to the voltage-dependent calcium channel, it produces a voltage-dependent block, which inhibits the channel, preventing the flow of calcium ions into the neuron. Embedded in the cell membrane is also the G protein-coupled inwardly-rectifying potassium channel. When a Gβγ or Gα(GTP) molecule binds to the C-terminus of the potassium channel, it becomes active, and potassium ions are pumped out of the neuron.[50] The activation of the potassium channel and subsequent deactivation of the calcium channel causes membrane hyperpolarization. This is when there is a change in the membrane's potential, so that it becomes more negative. The reduction in calcium ions causes a reduction neurotransmitter release because calcium is essential for this event to occur.[51] This means that neurotransmitters such as glutamate and substance P cannot be released from the presynaptic terminal of the neurons. These neurotransmitters are vital in the transmission of pain, so opioid receptor activation reduces the release of these substances, thus creating a strong analgesic effect.
Cadet P, Mantione KJ and Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J.Immunol.,170,5118–23. PMID 12734358