Dopamine receptor signaling — D1-like Golf/Gs and D2-like Gi/o control of adenylyl cyclase, the cAMP/PKA/DARPP-32/PP1 switch, and GRK/arrestin/RGS termination in striatal neurons

Dopamine signals through five G protein-coupled receptors split into two classes with opposite effects on adenylyl cyclase. The D1-like receptors (DRD1 and DRD5) couple through the stimulatory G alpha subunits — in the striatum chiefly the Golf variant GNAL rather than ubiquitous Gs — to activate adenylyl cyclase type 5 (ADCY5), raising cAMP and activating cAMP-dependent protein kinase (PKA). The D2-like receptors (DRD2, DRD3 and DRD4) couple through the inhibitory G alpha subunits GNAI2 and GNAO1 to inhibit ADCY5 and lower cAMP. The signal converges on a single bistable integrator, the phosphoprotein DARPP-32 (PPP1R1B). When PKA phosphorylates DARPP-32 at threonine 34 it becomes a potent inhibitor of protein phosphatase 1 (PP1, the PPP1CA/PPP1CB/PPP1CC catalytic subunits), amplifying phosphorylation of PP1 substrates downstream of D1 signalling. The switch is governed by a network of kinases and phosphatases acting on distinct residues: CDK5 phosphorylates threonine 75, converting DARPP-32 into a competitive inhibitor of PKA; a PKA-activated PP2A holoenzyme with the B56-delta regulatory subunit (PPP2R5D) and a Ca2+-activated PP2A holoenzyme with the PR72 subunit (PPP2R3A) dephosphorylate threonine 75; the Ca2+/calmodulin phosphatase calcineurin (PPP3CA/PPP3CB/PPP3CC) dephosphorylates threonine 34; and casein kinase 1 (CSNK1A1) phosphorylates serine 137 to protect phospho-threonine 34 from calcineurin. Receptor output is terminated by G protein-coupled receptor kinases (GRK2, GRK3, GRK5, GRK6), which phosphorylate agonist-occupied receptors so that beta-arrestins (ARRB1, ARRB2) uncouple them from G proteins and drive internalisation, and by regulators of G protein signalling, most prominently the striatal RGS9-2/G-beta-5/R7BP GAP complex (RGS9, GNB5, RGS7BP) that accelerates GTP hydrolysis on the D2-coupled G alpha subunits. Dopamine signalling also reaches the ERK cascade through DARPP-32-dependent control of the striatal tyrosine phosphatase STEP (PTPN5), and D2 receptors engage a G protein-independent arrestin arm in which ARRB2 scaffolds PP2A onto AKT.

Derived from the RGD Pathway Ontology diagram PW:0000394 through the RGD_PATHWAY_ONTOLOGY project. The RGD diagram and its long record text supplied the boundary, the two receptor classes and their opposite coupling, the cAMP/PKA/DARPP-32/PP1 core, the DARPP-32 phospho-switch, the STEP/ERK branch and the GRK/arrestin/RGS termination machinery. Molecular functions were taken from each protein's experimental GO annotations in QuickGO and the primary literature, verified against go-basic (releases/2026-07-26). Core versus regulation. The module core is the canonical cascade: D1-like receptors (DRD1 GO:0001588 IDA, DRD5 GO:0001588 IDA) -> Golf/Gs (GNAL GO:0003925) -> ADCY5 (GO:0004016 IDA/EXP) -> cAMP -> PKA catalytic subunits (PRKACA/PRKACB/PRKACG GO:0004691 IDA/IBA) -> DARPP-32 phospho-Thr34 (PPP1R1B GO:0004864 protein phosphatase inhibitor activity, IDA PMID:6087160) inhibiting PP1 (PPP1CA/PPP1CB/PPP1CC GO:0004722), and the opposing D2-like arm (DRD2/DRD3/DRD4 GO:0001591 IDA) -> Gi/o (GNAI2, GNAO1 GO:0003925) inhibiting ADCY5. Everything else is regulation: the DARPP-32 phospho-switch, GRK/arrestin desensitisation, the RGS GAP step, the STEP/ERK branch and the non-canonical arrestin-AKT arm. These are modelled as separate REGULATORY_STEP parts and linked with POSITIVELY_REGULATES/NEGATIVELY_REGULATES connections rather than folded into the core reaction chain. DARPP-32 as a participant, not a mere substrate. DARPP-32 is a PKA/CDK5 substrate, but it does the work of the step it is annotated to: phospho-Thr34-DARPP-32 is itself the active inhibitor of PP1 (GO:0004864, IDA PMID:6087160) and phospho-Thr75-DARPP-32 is itself the active inhibitor of PKA (PMID:10604473), so it is a legitimate functional participant and the hub of the switch, not a passive input. PP2A and calcineurin are modelled as holoenzyme complexes, because specificity for DARPP-32 is set by the regulatory subunit. B56-delta/PPP2R5D (GO:0019888, IDA) directs the cAMP/PKA-driven Thr75 dephosphorylation (PMID:17301223) and PR72/PPP2R3A (GO:0019888) directs the Ca2+-driven Thr75 dephosphorylation (PMID:17535922); the catalytic subunits PPP2CA/PPP2CB carry protein serine/threonine phosphatase activity (GO:0004722). Calcineurin (catalytic PPP3CA/PPP3CB/PPP3CC, GO:0033192 IDA) dephosphorylates Thr34. The regulatory subunits that are not in the RGD gene list (PP2A scaffold PPP2R1A, calcineurin B PPP3R1) are omitted; their absence is a deliberate scope choice, not a claim they are uninvolved. CSNK1A1/CK1 and Ser137. The RGD diagram lists CSNK1A1 among the DARPP-32 kinases. The Ser137 mechanism (phospho-Ser137 blocks Thr34 dephosphorylation by calcineurin) is shown in PMID:7708705 for casein kinase I, which did not resolve which CK1 gene product is responsible; the CSNK1A1-level assignment therefore rests on the RGD curation plus CSNK1A1's experimental protein serine/threonine kinase activity (GO:0004674 EXP/IDA), and the isoform caveat is flagged in a knowledge gap. RGS9-2/G-beta-5/R7BP. The striatal GAP for the D2-coupled G alpha subunits is the obligate RGS9-2 (RGS9 GO:0005096 GTPase activator activity, IMP/IBA) / G-beta-5 (GNB5 GO:0005096, IDA, the atypical beta subunit) / R7BP (RGS7BP, the palmitoylated membrane anchor) trimer (PMID:23857581, PMID:19332565). RGS7BP carries no GO molecular function (it is a scaffold/anchor), so it is a functionless active unit, kept visible because it sets the complex's localisation and stability. RGS2, RGS4 and RGS7 (GO:0005096) are the additional GAPs the RGD text names under "RGS in DA signaling" and are modelled as an alternative variant within the same termination part. STEP (PTPN5) and SRC. PTPN5/STEP is retained because it has a defined, directly supported role: it dephosphorylates and inactivates ERK (GO:0004725 protein tyrosine phosphatase activity, IBA/TAS; PMID:12483215, PMID:15608059), and DARPP-32/PP1 controls its phosphorylation state, so it is the mechanistic link from the cAMP/PP1 core to the ERK branch drawn on the RGD diagram. SRC is on RGD's wider human gene list but is not drawn on the diagram and has only a broad, indirect role (a ubiquitous tyrosine kinase implicated in D1/NMDA potentiation); it is excluded as peripheral rather than asserted with a dopamine-specific function it does not have in GOA. Non-canonical arrestin-AKT arm. The RGD record text states that Arrb2 in complex with Drd2 and PP2 inactivates PI3K/Akt and activates GSK3. This G protein-independent arm (PMID:16051150, GO:0160213 beta-arrestin-dependent dopamine receptor signaling pathway) is modelled as a regulatory part: ARRB2 (GO:0030674 protein-macromolecule adaptor activity, IDA) scaffolds PP2A (GO:0004722) onto AKT1, lowering AKT activity and disinhibiting GSK3B. AKT1 and GSK3B are not on the RGD curated gene-link list but are named in the RGD description and have completed gene reviews, so they are included as the targets of this arm. Deliberately outside the module. Dopamine biosynthesis (TH, DDC and partners; RGD subpathway PW:0000802) is upstream ligand supply, not receptor signalling, and is kept in modules/catecholamine_biosynthesis.yaml rather than duplicated (the human GO-CAM gomodel:69c59f8a00000117 models that step). This module is the concrete striatal instantiation of the generic Gs-coupled and Gi-coupled GPCR-cAMP motif in modules/gpcr_camp_pka_signaling.yaml; the Gq/PLC/calcium/PKC branch of dopamine signalling (proposed for a D1-D2 heteromer) is the generic motif in modules/gpcr_plc_calcium_pkc_signaling.yaml and is NOT included here, because the existence of a Gq-coupled D1/D2 heteromer is contested: PMID:17194762 reports it while PMID:25560761 was unable to detect Gq/G11 coupling to D1/D2 homomers or heteromers and challenges the complex in vivo. This is recorded as a knowledge gap rather than asserted. GO-CAM grounding. The curator-built human DRD1-DRD5 Gs and DRD2-DRD4 Gi/Go dopamine signalling models (gomodel:69c59f8a*) match this module's receptor and G protein activities and are linked under gocam_associations. Those models type the G protein with G protein activity (GO:0003925) and the cyclase with adenylate cyclase activity (GO:0004016); they use ADCY6 as the cyclase exemplar where this module uses the striatal ADCY5 named by the RGD diagram and PMID:12665504. The striatal DARPP-32 switch, the PP2A and calcineurin steps, and the RGS/GRK/arrestin termination machinery are not covered by any cached production GO-CAM, so those molecular functions were taken from QuickGO and the primary literature directly. No gene reviews exist for the receptors, G proteins, ADCY5, PKA, PP1, DARPP-32, GRKs, arrestins or RGS proteins; reviews exist for CDK5, PPP2CA, PPP2CB, PPP3CA, PPP3CB, AKT1 and GSK3B and were used for those molecular functions, except that AKT1 is given GO:0106310 protein serine kinase activity, the specific child of its review's GO:0004674, because the step modelled here is AKT's inhibitory serine phosphorylation of GSK3B.