Dopamine receptor D2, also known as D2R, is a protein that, in humans, is encoded by the DRD2gene. After work from Paul Greengard's lab had suggested that dopamine receptors were the site of action of antipsychotic drugs, several groups, including those of Solomon H. Snyder and Philip Seeman used a radiolabeled antipsychotic drug to identify what is now known as the dopamine D2 receptor.[5] The dopamine D2 receptor is the main receptor for most antipsychotic drugs. The structure of DRD2 in complex with the atypical antipsychotic risperidone has been determined.[6][7]
The long form (D2Lh) has the "canonical" sequence and functions as a classic post-synaptic receptor.[17] The short form (D2Sh) is pre-synaptic and functions as an autoreceptor that regulates the levels of dopamine in the synaptic cleft.[17]Agonism of D2sh receptors inhibits dopamine release; antagonism increases dopaminergic release.[17] A third D2(Longer) form differs from the canonical sequence where 270V is replaced by VVQ.[18]
Active and inactive forms
D2R conformers are equilibrated between two full active (D2HighR) and inactive (D2LowR) states, while in complex with an agonist and antagonist ligand, respectively.
The monomeric inactive conformer of D2R in binding with risperidone was reported in 2018 (PDB ID: 6CM4). However, the active form which is generally bound to an agonist, is not available yet and in most of the studies the homology modeling of the structure is implemented. The difference between the active and inactive of G protein-coupled receptor is mainly observed as conformational changes at the cytoplasmic half of the structure, particularly at the transmembrane domains (TM) 5 and 6. The conformational transitions occurred at the cytoplasmic ends are due to the coupling of G protein to the cytoplasmic loop between the TM 5 and 6.[19]
It was observed that either D2R agonist or antagonist ligands revealed better binding affinities inside the ligand-binding domain of the active D2R in comparison with the inactive state. It demonstrated that ligand-binding domain of D2R is affected by the conformational changes occurring at the cytoplasmic domains of the TM 5 and 6. In consequence, the D2R activation reflects a positive cooperation on the ligand-binding domain.
In drug discovery studies in order to calculate the binding affinities of the D2R ligands inside the binding domain, it's important to work on which form of D2R. It's known that the full active and inactive states are recommended to be used for the agonist and antagonist studies, respectively.
Any disordering in equilibration of D2R states, which causes problems in signal transferring between the nervous systems, may lead to diverse serious disorders, such as schizophrenia,[20]autism[citation needed] and Parkinson's disease.[citation needed] In order to assist in the management of these conditions, equilibration between the D2R states is controlled by implementing of agonist and antagonist D2R ligands.[citation needed] In most cases, it was observed that the problems regarding the D2R states may have genetic roots and are controlled by drug therapies.[citation needed] So far, there is no certain treatment for these mental disorders.
Allosteric pocket and orthosteric pocket
There is an orthosteric binding site (OBS), as well as a secondary binding pocket (SBP) on the dopamine 2 receptor, and interaction with the SBP is a requirement for allosteric pharmacology. The compound SB269652 is a negative allosteric modulator of the D2R.[21]
Oligomerization of D2R
It was observed that D2R exists in dimeric forms or higher order oligomers.[22] There are some experimental and molecular modeling evidences that demonstrated the D2R monomers cross link from their TM 4 and TM 5 to form dimeric conformers.[23][24]
-141C insertion/deletion[26] The polymorphisms have been investigated with respect to association with schizophrenia.[27]
Some researchers have previously associated the polymorphism Taq 1A (rs1800497) to the DRD2 gene.
However, the polymorphism resides in exon 8 of the ANKK1 gene.[28] DRD2 TaqIA polymorphism has been reported to be associated with an increased risk for developing motor
fluctuations but not hallucinations in Parkinson's disease.[29][30] A splice variant in Dopamine receptor D2(rs1076560) was found to be associated with limb truncal tardive dyskinesia and diminished expression factor of Positive and Negative Syndrome Scale (PANSS) in schizophrenia subjects.[31]
Ligands
Most of the older antipsychotic drugs such as chlorpromazine and haloperidol are antagonists for the dopamine D2 receptor, but are, in general, very unselective, at best selective only for the "D2-like family" receptors and so binding to D2, D3 and D4, and often also to many other receptors such as those for serotonin and histamine, resulting in a range of side-effects and making them poor agents for scientific research. In similar manner, older dopamine agonists used for Parkinson's disease such as bromocriptine and cabergoline are poorly selective for one dopamine receptor over another, and, although most of these agents do act as D2 agonists, they affect other subtypes as well. Several selective D2ligands are, however, now available, and this number is likely to increase as further research progresses.
LSD – in vitro, LSD was found to be a partial agonist and potentiates dopamine-mediated prolactin secretion in lactotrophs.[35] LSD is also a 5-HT2A agonist.
OSU-6162 – also 5-HT2A partial agonist, acts as "dopamine stabilizer"
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^Grandy DK, Miller GM, Li JX (February 2016). ""TAARgeting Addiction"--The Alamo Bears Witness to Another Revolution: An Overview of the Plenary Symposium of the 2015 Behavior, Biology and Chemistry Conference". Drug and Alcohol Dependence. 159: 9–16. doi:10.1016/j.drugalcdep.2015.11.014. PMC4724540. PMID26644139. This original observation of TAAR1 and DA D2R interaction has subsequently been confirmed and expanded upon with observations that both receptors can heterodimerize with each other under certain conditions ... Additional DA D2R/TAAR1 interactions with functional consequences are revealed by the results of experiments demonstrating that in addition to the cAMP/PKA pathway (Panas et al., 2012) stimulation of TAAR1-mediated signaling is linked to activation of the Ca++/PKC/NFAT pathway (Panas et al.,2012) and the DA D2R-coupled, G protein-independent AKT/GSK3 signaling pathway (Espinoza et al., 2015; Harmeier et al., 2015), such that concurrent TAAR1 and DA DR2R activation could result in diminished signaling in one pathway (e.g. cAMP/PKA) but retention of signaling through another (e.g., Ca++/PKC/NFA)
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