There are five subtypes of dopamine receptors, D1, D2, D3, D4, and D5. The D1 and D5 receptors are members of the D1-like family of dopamine receptors, whereas the D2, D3 and D4 receptors are members of the D2-like family. There is also some evidence that suggests the existence of possible D6 and D7 dopamine receptors, but such receptors have not been conclusively identified.
D2-like activation is coupled to the G protein Gαi, which subsequently increases phosphodiesterase activity. Phosphodiesterases break down cAMP, producing an inhibitory effect in neurons.
D2 (DRD2). There is a short version of D2 (D2Sh) and a long version of D2 (D2Lh):
The D2Sh are pre-synaptic situated, having modulatory functions (called autoreceptor, they regulate the neurotransmission by feed-back mechanisms, i.e., synthesis, storage and release of dopamine into the synaptic cleft).
The D2Lh may have the classic function of a post-synaptic receptor, i.e., keep going on the neurotransmission (excitatory or inhibitory) once blocked by a receptor antagonist or stimulated by the endogenous neurotransmitter itself or a synthetic full or partial agonist.
D3 (DRD3) Maximum expression of dopamine D3 receptors is noted in the islands of Calleja and nucleus accumbens.
D4 (DRD4). The D4 receptor has the following variants D4.2, D4.3a, D4.3b, D4.4a, D4.4b, D4.4c, D4.4d, D4.4e, D4.5a, D4.5b, D4.6a, D4.6b, D4.7a, D4.7b, D4.7c, D4.7d, D4.8, D4.10. These variants differ in a variable number tandem repeat domain present within the coding sequence of exon 3. Some of these alleles are associated with greater incidence of certain diseases. For example, the D4.7 alleles have an established association with attention-deficit hyperactivity disorder.
Role of dopamine receptors in the central nervous system
Dopamine receptors control neural signaling that modulates many important behaviors, such as spatial working memory.
Non-CNS dopamine receptors
In humans, the pulmonary artery expresses D1, D2, D4, and D5 and receptor subtypes, which may account for vasorelaxive effects of dopamine in the blood. In rats, D1-like receptors are present on the smooth muscle of the blood vessels in most major organs.
D4 receptors have been identified in the atria of rat and human hearts. Dopamine increases myocardial contractility and cardiac output, without changing heart rate, by signaling through dopamine receptors.
Dopamine receptors are present along the nephron in the kidney, with proximal tubule epithelial cells showing the highest density. In rats, D1-like receptors are present on the juxtaglomerular apparatus and on renal tubules, while D2-like receptors are present on the renal tubules, glomeruli, postganglionic sympathetic nerve terminals, and zona glomerulosa cells of the renal cortex. Dopamine signaling affects diuresis and natriuresis.
Dopamine receptors in disease
Dysfunction of dopaminergic neurotransmission in the CNS has been implicated in a variety of neuropsychiatric disorders, including Tourette's syndrome, Parkinson's disease, schizophrenia,neuroleptic malignant syndrome Attention-deficit hyperactivity disorder (ADHD), and drug and alcohol dependence.
Attention-deficit hyperactivity disorder
Dopamine receptors have been recognized as important components in the etiology of ADHD for many years. Drugs used to treat ADHD, including methylphenidate and amphetamine, have significant effects on dopamine signaling in the brain. Studies of gene association have implicated several genes within dopamine signaling pathways; in particular, the D4.7 variant of D4 has been consistently shown to be more frequent in ADHD patients. ADHD patients with the 4.7 allele also tend to have better cognitive performance and long-term outcomes compared to ADHD patients without the 4.7 allele, suggesting that the allele is associated with a more benign form of ADHD.
Dopamine is the primary neurotransmitter involved in the reward pathways in the brain. Thus, drugs that increase dopamine signaling may produce euphoric effects. Many recreational drugs, such as cocaine and methamphetamine, alter the functionality of the dopamine transporter (DAT), the protein responsible for removing dopamine from the neural synapse. When DAT activity is blocked, the synapse floods with dopamine and increases dopaminergic signaling. When this occurs, particularly in the nucleus accumbens, increased D1 and D2 receptor signaling mediates the "rewarding" stimulus of drug intake. Reward pathway signaling can affect other regions of the brain as well, inducing long-term changes in regions such as the nucleus accumbens and frontal cortex; these changes can strengthen drug craving and alter cognitive pathways, with drug abuse potentially creating drug addiction and drug dependence.
Main article: Dopamine hypothesis of schizophrenia
While there is evidence that the dopamine system is involved in schizophrenia, the theory that hyperactive dopaminergic signal transduction induces the disease is controversial. Psychostimulants, such as amphetamine and cocaine, induce dramatic changes in dopamine signaling; large doses and prolonged usage can induce symptoms that resemble schizophrenia. Additionally, many antipsychotic drugs target dopamine receptors, especially D2 receptors.
Dopamine receptor mutations can cause genetic hypertension in humans. This can occur in animal models and humans with defective dopamine receptor activity, particularly D1.
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