D2/D3 Receptor Agonist Occupancy
trigger
A dopamine agonist drug binds and activates D2-subfamily receptors directly. The class is selective for this subfamily over adrenergic and serotonergic receptors, and within it the non-ergot agents have preferential affinity for D3.
Downstream
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Direct Postsynaptic D2/D3 Signalling Independent of Presynaptic Dopamine
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Lactotroph D2 Activation Suppressing Prolactin Secretion
Show evidence (2 references)
"First, it has high selectivity for interacting with dopamine D2 subfamily receptors (D2, D3, and D4 receptor subtypes) and has little interaction with adrenergic or serotonergic receptors."
Establishes that the drug class acts on the D2 subfamily rather than on other aminergic receptors, which is what makes this node receptor-specific.
"Second, within the D2 subfamily, it has preferential affinity for the D3 receptor subtype, which, according to preclinical studies, could contribute additional efficacy for treatment of both motor and psychiatric syndromes in Parkinson's disease."
Supports binding both DRD2 and DRD3 on this node rather than DRD2 alone.
Direct Postsynaptic D2/D3 Signalling Independent of Presynaptic Dopamine
central effector
Agonist occupancy drives Gi/Go-coupled signalling at the receptor itself, so the response does not depend on presynaptic dopamine synthesis, storage or release. This is the step that distinguishes the class from levodopa, which must first be taken up and decarboxylated by surviving neurons, and it is why agonists retain effect as the presynaptic terminal is lost.
Downstream
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Fewer Motor Complications but Less Symptomatic Benefit Than Levodopa
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Impulse Control Disorders on Dopamine Agonist Therapy
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Somnolence and Peripheral Oedema on Dopamine Agonist Therapy
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Nausea and Vomiting on Dopamine Agonist Therapy
Show evidence (1 reference)
"Third, it has full intrinsic activity at dopamine D2 subfamily receptors."
Full intrinsic activity at the receptor is what makes the downstream signalling independent of how much endogenous dopamine is available.
Fewer Motor Complications but Less Symptomatic Benefit Than Levodopa
consequence
Beginning treatment with an agonist rather than levodopa halves the risk of dyskinesia and reduces wearing off, and costs symptomatic control: at four years the levodopa arm had the better motor score. Both halves of that trade-off are this node, because a module that recorded only the benefit would misrepresent the class.
Show evidence (2 references)
"RESULTS: Initial pramipexole treatment resulted in a significant reduction in the risk of developing dyskinesias (24.5% vs 54%; hazard ratio, 0.37; 95% confidence interval [CI], 0.25-0.56; P<.001) and wearing off (47% vs 62.7%; hazard ratio, 0.68; 95% CI, 0.49-0.63; P =.02)."
The motor-complication-sparing half of the trade-off, from the randomized comparison against levodopa initiation.
"The mean improvement in the total Unified Parkinson's Disease Rating Scale score from baseline to 48 months was greater in the levodopa group than in the pramipexole group (2 +/- 15.4 points vs -3.2 +/- 17.3 points, P =.003)."
The cost half of the same trade-off: agonist initiation gave less symptomatic benefit than levodopa over four years.
Impulse Control Disorders on Dopamine Agonist Therapy
adverse effect
Gambling, compulsive sexual behaviour, compulsive buying and binge eating are about two and a half times as common on an agonist as off one. The rate is similar for pramipexole and ropinirole, so this reads as a class effect of D2-subfamily agonism rather than a property of one drug.
Show evidence (2 references)
"Impulse control disorders were more common in patients treated with a dopamine agonist than in patients not taking a dopamine agonist (17.1% vs 6.9%; odds ratio [OR], 2.72; 95% confidence interval [CI], 2.08-3.54; P < .001)."
Quantifies the association between agonist exposure and impulse control disorders in 3090 patients.
"Impulse control disorder frequency was similar for pramipexole and ropinirole (17.7% vs 15.5%; OR, 1.22; 95% CI, 0.94-1.57; P = .14)."
Supports describing this as a class effect, and argues against attributing it to one drug's receptor-subtype profile.
Somnolence and Peripheral Oedema on Dopamine Agonist Therapy
adverse effect
Daytime somnolence and peripheral oedema are both substantially more frequent on agonist than on levodopa initiation, and are the usual reasons for dose reduction or withdrawal.
Show evidence (1 reference)
"Somnolence (36% vs 21%, P =.005) and edema (42% vs 15%, P<.001) were more common in pramipexole-treated subjects than in levodopa-treated subjects."
Direct randomized comparison establishing both adverse effects against a levodopa control.
Nausea and Vomiting on Dopamine Agonist Therapy
adverse effect
Nausea and vomiting are dose-limiting on initiation and are why parenteral apomorphine is given with an antiemetic. No anatomical site is bound on this node: see the module notes.
Show evidence (1 reference)
"In terms of safety analyses from single-armed studies, apomorphine treatment was associated with several AES, including nausea, drowsiness, vomiting, dizziness, and excessive sweating."
Establishes nausea and vomiting as adverse effects of direct D2-subfamily agonism, pooled across studies.
Lactotroph D2 Activation Suppressing Prolactin Secretion
effector
D2 receptors on pituitary lactotrophs tonically restrain prolactin release, and an agonist reinstates that restraint pharmacologically. This is the same receptor on the same cell type whose tonic restraint a D2 antagonist removes, with the sign reversed.
Downstream
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Normoprolactinaemia and Restored Gonadal Function
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"Cabergoline is a long-acting dopamine-agonist drug that suppresses prolactin secretion and restores gonadal function in women with hyperprolactinemic amenorrhea."
States the suppression of prolactin secretion by a dopamine agonist, which is this node.
Normoprolactinaemia and Restored Gonadal Function
consequence
Sustained prolactin suppression returns prolactin to the reference range and restores ovulatory cycles, which is the clinical endpoint dopamine agonists are prescribed for in hyperprolactinaemia.
Show evidence (2 references)
"RESULTS: Stable normoprolactinemia was achieved in 186 of the 223 women treated with cabergoline (83 percent) and 138 of the 236 women treated with bromocriptine (59 percent, P < 0.001)."
The biochemical endpoint, from the randomized comparison of two dopamine agonists.
"Seventy-two percent of the women treated with cabergoline and 52 percent of those treated with bromocriptine had ovulatory cycles or became pregnant during treatment (P < 0.001)."
The gonadal endpoint that follows from prolactin normalization.