5-HT receptors, 5-hydroxytryptamine receptors, or serotonin receptors, are a group of G protein-coupled receptor and ligand-gated ion channels found in the central and peripheral nervous systems. [1] [2] [3] They mediate both excitatory and inhibitory neurotransmission. The serotonin (i.e., 5-hydroxytryptamine, hence "5-HT") receptors are activated by the neurotransmitter serotonin, which acts as their natural ligand.
The serotonin receptors modulate the release of many neurotransmitters, including glutamate, GABA, dopamine, epinephrine / norepinephrine, and acetylcholine, as well as many hormones, including oxytocin, prolactin, vasopressin, cortisol, corticotropin, and substance P, among others. Serotonin receptors influence various biological and neurological processes such as aggression, anxiety, appetite, cognition, learning, memory, mood, nausea, sleep, and thermoregulation. They are the target of a variety of pharmaceutical and recreational drugs, including many antidepressants, antipsychotics, anorectics, antiemetics, gastroprokinetic agents, antimigraine agents, hallucinogens, and entactogens. [4]
Serotonin receptors are found in almost all animals and are even known to regulate longevity and behavioral aging in the primitive nematode, Caenorhabditis elegans . [5] [6]
5-hydroxytryptamine receptors or 5-HT receptors, or serotonin receptors are found in the central and peripheral nervous systems. [1] [2] They can be divided into 7 families of G protein-coupled receptors which activate an intracellular second messenger cascade to produce an excitatory or inhibitory response. The exception to this is the 5-HT3 receptor which is a ligand-gated ion channel. In 2014, a novel 5-HT receptor was isolated from the small white butterfly, Pieris rapae, and named pr5-HT8. It does not occur in mammals and shares relatively low similarity to the known 5-HT receptor classes. [7]
Family | Type | Mechanism | Potential |
---|---|---|---|
5-HT1 | Gi/Go-protein coupled. | Decreasing cellular levels of cAMP. | Inhibitory |
5-HT2 | Gq/G11-protein coupled. | Increasing cellular levels of IP3 and DAG. | Excitatory |
5-HT3 | Ligand-gated Na+ and K+ cation channel. | Depolarizing plasma membrane. | Excitatory |
5-HT4 | Gs-protein coupled. | Increasing cellular levels of cAMP. | Excitatory |
5-HT5 | Gi/Go-protein coupled. [8] | Decreasing cellular levels of cAMP. | Inhibitory |
5-HT6 | Gs-protein coupled. | Increasing cellular levels of cAMP. | Excitatory |
5-HT7 | Gs-protein coupled. | Increasing cellular levels of cAMP. | Excitatory |
The 7 general serotonin receptor classes include a total of 14 known serotonin receptors. [9] The 15th receptor 5-HT1P has been distinguished on the basis of functional and radioligand binding studies, its existence has never been definitely affirmed or refuted. [10] [11] The specific types have been characterized as follows: [12] [13] [14]
Information on serotonin receptors (human isoforms if nothing else is stated) | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Receptor | First cloned – PDB entries | Gene(s) | Distribution | Function | Agonists | Antagonists | Uses of drugs that act on this receptor | |||||
Blood vessels | CNS | GI Tract | Platelets | PNS | Smooth Muscle | |||||||
5-HT1A | 1987 –
| Yes | Yes | No | No | No | No |
| Selective (for 5-HT1A over other 5-HT receptors)
Nonselective
|
| ||
5-HT1B | 1992 –
| Yes | Yes | No | No | No | No |
| ||||
5-HT1D | 1991 –
| Yes | Yes | No | No | No | No |
| ||||
5-HT1E | 1992 –
| Yes | Yes | No | No | No | No |
| ||||
5-HT1F | 1993 –
| No | Yes | No | No | No | No |
|
| |||
5-HT1P | Not cloned | No | No | Yes | No | No | No |
| ||||
5-HT2A | 1988 | Yes | Yes | Yes | Yes | Yes | Yes |
| ||||
5-HT2B | 1992 | Yes | Yes | Yes | Yes | Yes | Yes |
|
| |||
5-HT2C | 1988 | Yes | Yes | Yes | Yes | Yes | Yes |
|
| |||
5-HT3 | 1993 | No | Yes | Yes | No | Yes | No |
| ||||
5-HT4 | 1995 | No | Yes | Yes | No | Yes | No |
| ||||
5-HT5A | 1994 | No | Yes | No | No | No | No |
|
|
| ||
5-HT5B | 1993 | No | No | No | No | No | No | Functions in rodents, |
| |||
5-HT6 | 1993 | No | Yes | No | No | No | No |
|
| |||
5-HT7 | 1993 | Yes | Yes | Yes | No | No | No |
|
Note that there is no 5-HT1C receptor since, after the receptor was cloned and further characterized, it was found to have more in common with the 5-HT2 family of receptors and was redesignated as the 5-HT2C receptor. [107]
Very nonselective agonists of 5-HT receptor subtypes include ergotamine (an antimigraine), which activates 5-HT1A, 5-HT1D, 5-HT1B, D2 and norepinephrine receptors. [38] LSD (a psychedelic) is a 5-HT1A, 5-HT2A, 5-HT2C, 5-HT5A and 5-HT6 agonist. [38]
The genes coding for serotonin receptors are expressed across the mammalian brain. Genes coding for different receptors types follow different developmental curves. Specifically, there is a developmental increase of HTR5A expression in several subregions of the human cortex, paralleled by a decreased expression of HTR1A from the embryonic period to the post-natal one. [108]
A number of receptors were classed as "5-HT1-like" - by 1998 it was being argued that, since these receptors were "a heterogeneous population of 5-HT1B, 5-HT1D and 5-HT7" receptors the classification was redundant. [109]
The 5-HT2A receptor is a subtype of the 5-HT2 receptor that belongs to the serotonin receptor family and is a G protein-coupled receptor (GPCR). The 5-HT2A receptor is a cell surface receptor, but has several intracellular locations.
A serotonin receptor agonist is an agonist of one or more serotonin receptors. They activate serotonin receptors in a manner similar to that of serotonin, a neurotransmitter and hormone and the endogenous ligand of the serotonin receptors.
The 5-HT1 receptors are a subfamily of the 5-HT serotonin receptors that bind to the endogenous neurotransmitter serotonin (also known as 5-hydroxytryptamine, or 5-HT). The 5-HT1 subfamily consists of five G protein-coupled receptors (GPCRs) that share 40% to 63% overall sequence homology, including 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, and 5-HT1F. Receptors of the 5-HT1 type, specifically, the 5-HT1A and 5-HT1D receptor subtypes, are present on the cell bodies. Receptors of the 5-HT1 type, specifically, the 5-HT1B and 5-HT1D receptor subtypes, are also present on the nerve terminals. These receptors are broadly distributed throughout the brain and are recognized to play a significant part in regulating synaptic levels of 5-HT.
The serotonin 1A receptor is a subtype of serotonin receptors, or 5-HT receptors, that binds serotonin, also known as 5-HT, a neurotransmitter. 5-HT1A is expressed in the brain, spleen, and neonatal kidney. It is a G protein-coupled receptor (GPCR), coupled to the Gi protein, and its activation in the brain mediates hyperpolarization and reduction of firing rate of the postsynaptic neuron. In humans, the serotonin 1A receptor is encoded by the HTR1A gene.
5-hydroxytryptamine receptor 1B also known as the 5-HT1B receptor is a protein that in humans is encoded by the HTR1B gene. The 5-HT1B receptor is a 5-HT receptor subtype.
8-OH-DPAT is a research chemical of the aminotetralin chemical class which was developed in the 1980s and has been widely used to study the function of the 5-HT1A receptor. It was one of the first major 5-HT1A receptor full agonists to have been discovered.
5-Hydroxytryptamine receptor 2B (5-HT2B) also known as serotonin receptor 2B is a protein that in humans is encoded by the HTR2B gene. 5-HT2B is a member of the 5-HT2 receptor family that binds the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT). Like all 5-HT2 receptors, the 5-HT2B receptor is Gq/G11-protein coupled, leading to downstream activation of phospholipase C.
5-Hydroxytryptamine (serotonin) receptor 5A, also known as HTR5A, is a protein that in humans is encoded by the HTR5A gene. Agonists and antagonists for 5-HT receptors, as well as serotonin uptake inhibitors, present promnesic (memory-promoting) and/or anti-amnesic effects under different conditions, and 5-HT receptors are also associated with neural changes.
The 5HT6 receptor is a subtype of 5HT receptor that binds the endogenous neurotransmitter serotonin (5-hydroxytryptamine, 5HT). It is a G protein-coupled receptor (GPCR) that is coupled to Gs and mediates excitatory neurotransmission. HTR6 denotes the human gene encoding for the receptor.
The 5-HT7 receptor is a member of the GPCR superfamily of cell surface receptors and is activated by the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT). The 5-HT7 receptor is coupled to Gs (stimulates the production of the intracellular signaling molecule cAMP) and is expressed in a variety of human tissues, particularly in the brain, the gastrointestinal tract, and in various blood vessels. This receptor has been a drug development target for the treatment of several clinical disorders. The 5-HT7 receptor is encoded by the HTR7 gene, which in humans is transcribed into 3 different splice variants.
WAY-100635 is a piperazine drug and research chemical widely used in scientific studies. It was originally believed to act as a selective 5-HT1A receptor antagonist, but subsequent research showed that it also acts as potent full agonist at the D4 receptor. It is sometimes referred to as a silent antagonist at the former receptor. It is closely related to WAY-100135.
SB-242084 is a psychoactive drug and research chemical which acts as a selective antagonist for the 5HT2C receptor. It has anxiolytic effects, and enhances dopamine signalling in the limbic system, as well as having complex effects on the dopamine release produced by cocaine, increasing it in some brain regions but reducing it in others. It has been shown to increase the effectiveness of the selective serotonin reuptake inhibitor (SSRI) class of antidepressants, and may also reduce their side effects. In animal studies, SB-242084 produced stimulant-type activity and reinforcing effects, somewhat similar to but much weaker than cocaine or amphetamines.
5-Carboxamidotryptamine (5-CT) is a tryptamine derivative closely related to the neurotransmitter serotonin.
CP-94253 is a drug which acts as a potent and selective serotonin 5-HT1B receptor agonist, with approximately 25× and 40× selectivity over the closely related 5-HT1D and 5-HT1A receptors. It has a range of behavioral effects, based on animal testing. The effects include the following: promoting wakefulness by increasing dopamine release in the brain; reducing food intake and promoting satiety; enhancing the reinforcing effects of cocaine; and possible antidepressant effects. A recent study found that "Regardless of sex, CP94253 decreased cocaine intake after abstinence and during resumption of SA [self-administration] and decreased cue reactivity" suggesting that agonism of the inhibitory 5-HT2B receptors may diminish the cognitive reward of cocaine usage and increased use of the drug without a period of abstinence may be a product of test subjects trying to achieve a previously rewarding experience through larger dosages of cocaine.
Osemozotan (MKC-242) is a selective 5-HT1A receptor agonist with some functional selectivity, acting as a full agonist at presynaptic and a partial agonist at postsynaptic 5-HT1A receptors. 5-HT1A receptor stimulation influences the release of various neurotransmitters including serotonin, dopamine, norepinephrine, and acetylcholine. 5-HT1A receptors are inhibitory G protein-coupled receptor.
Pruvanserin is a selective 5-HT2A receptor antagonist which was under development by Eli Lilly and Company for the treatment of insomnia. It was in phase II clinical trials in 2008 but appears to have been discontinued as it is no longer in the company's development pipeline. In addition to its sleep-improving properties, pruvanserin has also been shown to have antidepressant, anxiolytic, and working memory-enhancing effects in animal studies.
1-(1-Naphthyl)piperazine (1-NP) is a drug which is a phenylpiperazine derivative. It acts as a non-selective, mixed serotonergic agent, exerting partial agonism at the 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, and 5-HT1F receptors, while antagonizing the 5-HT2A, 5-HT2B, and 5-HT2C receptors. It has also been shown to possess high affinity for the 5-HT3, 5-HT5A, 5-HT6, and 5-HT7 receptors, and may bind to 5-HT4 and the SERT as well. In animals it produces effects including hyperphagia, hyperactivity, and anxiolysis, of which are all likely mediated predominantly or fully by blockade of the 5-HT2C receptor.
SB-206553 is a drug which acts as a mixed antagonist for the 5-HT2B and 5-HT2C serotonin receptors. It has anxiolytic properties in animal studies and interacts with a range of other drugs. It has also been shown to act as a positive allosteric modulator of α7 nicotinic acetylcholine receptors. Modified derivatives of SB-206553 have been used to probe the structure of the 5-HT2B receptor.
SB-243213 is a research chemical which acts as a selective inverse agonist for the 5HT2C receptor and has anxiolytic effects. It has better than 100x selectivity for 5-HT2C over all other receptor subtypes tested, and a longer duration of action compared to older 5-HT2C antagonist ligands.
Hypidone (developmental code name YL-0919) is an investigational serotonergic antidepressant which is under development for the treatment of major depressive disorder. It acts as a serotonin reuptake inhibitor, 5-HT1A receptor partial agonist, and 5-HT6 receptor full agonist. It is used as the hydrochloride salt. As of January 2021, hypidone is in phase 2 clinical trials for major depressive disorder.
Similarly, little is known about which of serotonin's 14 known receptors must be activated to achieve an antidepressant response.