The 5-HT2C receptor is a subtype of the 5-HT2 receptor that binds the endogenous neurotransmitter serotonin (5-hydroxytryptamine, 5-HT). Like all 5-HT2 receptors, it is a G protein-coupled receptor (GPCR) that is coupled to Gq/G11 and mediates excitatory neurotransmission. HTR2C denotes the human gene encoding for the receptor, [4] [5] that in humans is located on the X chromosome. As males have one copy of the gene and females have one of the two copies of the gene repressed, polymorphisms at this receptor can affect the two sexes to differing extent.
At the cell surface the receptor exists as a homodimer. [6] The crystal structure has been known since 2018. [7]
5-HT2C receptors are located mainly in the choroid plexus, [8] and in rats is also found in many other brain regions in high concentrations, including parts of the hippocampus, anterior olfactory nucleus, substantia nigra, several brainstem nuclei, amygdala, subthalamic nucleus and lateral habenula. 5-HT2C receptors are also found on epithelial cells lining the ventricles. [9]
The 5-HT2C receptor is one of the many binding sites for serotonin. Activation of this receptor by serotonin inhibits dopamine and norepinephrine release in certain areas of the brain. [10]
5-HT2C receptors are claimed to significantly regulate mood, anxiety, feeding, and reproductive behavior. [11] 5-HT2C receptors regulate dopamine release in the striatum, prefrontal cortex, nucleus accumbens, hippocampus, hypothalamus, and amygdala, among others.
Research indicates that some suicide victims have an abnormally high number of 5-HT2C receptors in the prefrontal cortex. [12] Agomelatine, which is a 5-HT2C and 5-HT2B antagonist as well as a MT1 and MT2 agonist, is an effective antidepressant. [13] [14] It has been called a norepinephrine-dopamine disinhibitor because antagonism of 5-HT2C receptors by agomelatine results in an increase of dopamine and norepinephrine activity in the frontal cortex.[ citation needed ] Conversely, many SSRIs (but not fluoxetine, which is a 5-HT2C antagonist [15] ) indirectly stimulate 5-HT2C activity by increasing levels of serotonin in the synapse although the delayed mood elevation that is usually typical of SSRIs is usually paralleled by the downregulation of the 5-HT2C receptors. [16] Many atypical antipsychotics block 5-HT2C receptors, but their clinical use is limited by multiple undesirable actions on various neurotransmitters and receptors [ citation needed ]. Fluoxetine acts as a direct 5-HT2C antagonist in addition to inhibiting serotonin reuptake, however, the clinical significance of this action is variable. [15] Several tetracyclic antidepressants, including mirtazapine, are potent 5-HT2C antagonists; this action may contribute to their efficacy. [17] [18] [19]
An overactivity of 5-HT2C receptors may contribute to depressive and anxiety symptoms in a certain population of patients. Activation of 5-HT2C by serotonin is responsible for many of the negative side effects of SSRI and SNRI medications, such as sertraline, paroxetine, venlafaxine, and others. Some of the initial anxiety caused by SSRIs is due to excessive signalling at 5-HT2C. Over a period of 1–2 weeks, the receptor begins to downregulate, along with the downregulation of 5-HT2A, 5-HT1A, and other serotonin receptors. This downregulation parallels the onset of the clinical benefits of SSRIs. 5-HT2C receptors exhibit constitutive activity in vivo, and may retain the ability to influence neurotransmission in the absence of ligand occupancy. Thus, 5-HT2C receptors do not require binding by a ligand (serotonin) in order to exhibit influence on neurotransmission. Inverse agonists may be required to fully extinguish 5-HT2C constitutive activity, and may prove useful in the treatment of 5-HT2C-mediated conditions in the absence of typical serotonin activity. [16] In addition to the evidence for a role of 5-HT2C receptor stimulation in depressive symptoms there also is evidence that activation of 5-HT2C receptors may have beneficial effects upon certain aspects of depression, one group of researchers found that direct stimulation of 5-HT2C receptors with a 5-HT2C agonist reduced cognitive deficits in mice with a TPH2 loss-of-function mutation. [20]
5-HT2C receptors mediate the release and increase of extracellular dopamine in response to many drugs, [21] [22] including caffeine, nicotine, amphetamine, morphine, cocaine, and others. 5-HT2C antagonism increases dopamine release in response to reinforcing drugs, and many dopaminergic stimuli. Feeding, social interaction, and sexual activity all release dopamine subject to inhibition of 5-HT2C. Increased 5-HT2C expression reduces dopamine release in both the presence and absence of stimuli.
Conditions that increase cytokine levels in the human body may have potential to raise 5-HT2C gene expression in the brain. This could possibly comprise a link between viral infections and associated depression. Cytokine therapy has been shown to increase 5-HT2C gene expression, resulting in increased activity of 5-HT2C receptors in the brain [ citation needed ].
Serotonin is involved in basal and stress-induced regulation of hypothalamus and pituitary gland hormones such as prolactin, adrenocorticotropic hormone (ACTH), vasopressin and oxytocin, mainly via actions of receptor subtypes 5-HT2A and 5-HT2C. [23] Therefore, the 5-HT2C receptor is a significant modulator of the hypothalamic–pituitary–adrenal axis (HPA axis). [24] The HPA axis is the main controller of acute sympathetic stress responses related to fight-or-flight response. Prolonged activation and disturbances of the HPA axis contribute to depressive and anxiety symptoms seen in many psychopathological conditions.
Stimulation of 5-HT2C receptors leads to increase of corticotropin releasing hormone (CRH) and vasopressin mRNA in the paraventricular nucleus and proopiomelanocortin in the anterior pituitary lobe. In rats, restraint stress (which can produce depressive symptoms if being chronic) induces secretion of prolactin, ACTH, vasopressin and oxytocin which is partially mediated via 5-HT2C receptor. Responses during such conditions as dehydration or haemorrhage causes the release of oxytocin via serotonergic response that is partly mediated by 5-HT2C. In addition, peripheral release of vasopressin involves serotonergic response which is partially mediated via 5-HT2C.
Expression of the 5-HT2C receptor in the CNS is modulated by female sex hormones estradiol and progesterone. Combination of the hormones decrease the receptor concentration in the ventral hippocampus in rats and could thus affect mood. [25]
Many human polymorphisms have been identified influencing the expression of 5-HT2C. Significant correlations are suggested, specifically in relation to psychiatric disorders such as depression, OCD, and anxiety-related conditions. Polymorphisms also correlate with susceptibility to a number of conditions including substance use disorders and obesity. There are indications that the alternative splicing of the 5-HT2C receptor is regulated by a snoRNA called SNORD115, the deletion of which is associated with Prader–Willi syndrome. [26] [27] As the human gene is located in the X chromosome, males have only one copy of the gene whereas women have two, meaning that mutations in the gene affect the phenotype of men even when the allele would be recessive in nature. As women have two copies of the gene, but only one allele is expressed in each cell, they are a mosaic for polymorphisms, meaning that one genetic variant may be prevalent in one tissue and another variant will be prevalent in a different tissue (as with all other x-linked genetic variations).
Exogenous PAMs [34] binding at the receptor vestibule
The 5-HT2C receptor has been shown to interact with MPDZ. [37] [38]
5HT2CR pre-mRNA can be the subject of RNA editing. [39] It is the only serotonin receptor as well as the only member of the large family of 7 transmembrane receptors (7TMRs) known to be edited. Different levels of editing result in a variety of effects on receptor function.
The type of RNA editing that occurs in the pre-mRNA of the 5HT2CR is Adenosine to Inosine (A to I) editing.
A to I RNA editing is catalyzed by a family of adenosine deaminases acting on RNA (ADARs) that specifically recognize adenosines within double-stranded regions of pre-mRNAs and deaminate them to inosine. Inosines are recognised as guanosine by the cells translational machinery. There are three members of the ADAR family ADARs 1–3 with ADAR1 and ADAR2 being the only enzymatically active members. ADAR3 is thought to have a regulatory role in the brain. ADAR1 and ADAR2 are widely expressed in tissues while ADAR3 is restricted to the brain. The double stranded regions of RNA are formed by base-pairing between residues in the close to region of the editing site with residues usually in a neighboring intron but can be an exonic sequence. The region that base pairs with the editing region is known as an Editing Complementary Sequence (ECS).
ADARs bind interact directly with the dsRNA substrate via their double stranded RNA binding domains. If an editing site occurs within a coding sequence, it can result in a codon change. This can lead to translation of a protein isoform due to a change in its primary protein structure. Therefore, editing can also alter protein function. A to I editing occurs in a non coding RNA sequences such as introns, untranslated regions (UTRs), LINEs, SINEs ( especially Alu repeats) The function of A to I editing in these regions is thought to involve creation of splice sites and retention of RNAs in the nucleus amongst others.
Editing occurs in 5 different closely located sites within exon 5, which corresponds to the second intracellular loop of the final protein. The sites are known as A, B, C′ (previously called E), C and D, and are predicted to occur within amino acid positions 156, 158 and 160. Several codon changes can occur due to A-to-I editing at these sites. Thirty-two different mRNA variants can occur leading to 24 different protein isoforms.
These codon changes which can occur due to A to I editing at these sites can lead to a maximum of 32 different mRNA variants leading to 24 different protein isoforms. The number of protein isoforms is less than 32 since some amino acids are encoded by more than one codon. [40] Another editing site, site F has also been located in the exon complementary sequence (ECS) of intron 5. [41] The ECS required for formation of double stranded RNA structure is found within intron 5. [39]
RNA editing of this receptor occurs at 4 locations in the rat. [39] Editing also occurs in the mouse. [42] The initial demonstration of RNA editing in rat. [39] The predominant isoform in rat brain is VNV which differs from the most common type found in humans. [39] [43] The editing complementary sequence is known to be conserved across Mammalia.
The 5-HT2c receptor is the only serotonin receptor edited despite its close sequence similarities to other family members. [43] 5HT2CR is different due to possessing an imperfect inverted repeat at the end of exon 5 and the beginning of intron 5 allowing formation of an RNA duplex producing the dsRNA required by ADARs for editing. Disruption of this inverted repeat was demonstrated to cease all editing. [39] The different 5HT2CR mRNA isoforms are expressed differently throughout the brain, yet not all of the 24 have been detected perhaps due to tissue specific expression or low frequency editing of a particular type. Those isoforms that are not expressed at all or at a very low frequency are linked by being edited only at site C' and/or site B but not at site A. Some examples of differences in frequency of editing and site edited in different parts of the human brain of 5HT2CR include low frequency of editing in cerebellum and nearly all editing is at site D while in the hippocampus editing frequency is higher with site A being the main editing site. Site C' is only found edited in the thalamus. The most common isoform in human brain is the VSV isoform. [40] [43] [44]
Mice knock out and other studies have been used to determine which ADAR enzyme are involved in editing. Editing at A and B sites has been demonstrated to be due to ADAR1 editing. [45] [46] [47] Also since ADAR1 expression is increased in response to the presence of interferon α, it was also observed that editing at A and B sites was also increased because of this. [45] C' and D sites require ADAR2 and editing is decreased by the presence of ADAR1 with editing of C' site only observed in ADAR1 double knock out mice. [48] The C site has been shown to be mainly edited by ADAR2 but in presence of upregulated expression of ADAR1, there was an increase in editing of this site and the enzymes presence can also result in limited editing in ADAR 2 knock out mice. [45] [48] This demonstrates that there must be some form interaction between the two A to I editing enzymes. Also such interactions and tissue specific expression of ADARs interaction may explain the variety in editing patterns in different regions of the brain.
Second, the editing pattern controls the amount of the 5-HT2CR mRNA that leads to the expression of full-length protein through the modulation of alternative splice site selection 76,77. Among three alternative splice donor sites (GU1 to GU3; Fig. 4C), GU2 is the only site that forms the mature mRNA to produce the functional, full-length 5-HT2CR protein. Unedited pre-mRNAs tend to be spliced at the GU1 site, resulting in the truncated, non-functional protein if translated 76,77. However, most pre-mRNAs edited at more than one position are spliced at GU2 77. Thus, when editing is inefficient, increased splicing at GU1 may act as a control mechanism to decrease biosynthesis of the 5-HT2CR-INI and thereby limit serotonin response. Third, RNA editing controls the ultimate physiological output of constitutively active receptors by affecting the cell surface expression of the 5-HT2CR. The 5-HT2CR-VGV, which displays the lowest level of constitutive activity, is fully expressed at the cell surface under basal conditions and is rapidly internalized in the presence of agonist 78; additionally, in vitro, LSD shows negligible activity with this isoform. [49] In contrast, the 5-HT2CR-INI is constitutively internalized and accumulates in endosomes 78.
Structure
As mentioned editing results in several codon changes. The editing sites are found in the second intracellular domain of the protein which is also the receptors G protein coupling domain. Therefore, editing of these sites can affect the affinity of the receptor for G protein binding. [39]
Function
Editing results in reduced affinity for specific G proteins which in turn affects internal signalling via second messengers (Phospholipase C signalling system). The fully edited isoform, VGV, considerably reduces 5-HT potency, G-protein coupling and agonist binding, compared to the unedited protein isoform, INI. 72–76. Most evidence for the effect of editing on function comes from downstream measurements of receptor activity, radio ligand binding and functional studies. Inhibitory effects are linked to the extent of editing. Those isoforms with a higher level of editing require higher levels of serotonin to activate the phospholipase c pathway. Unedited INI form has a greater tendency to isomerise to an active form which can more easily interact with G proteins. This indicates that RNA editing here may be a mechanism for regulating neuronal excitability by stabilising receptor signalling. [39] [43]
Editing is also thought to function in cell surface expression of the receptor subtype. The fully edited VGV, which has the lowest level of constitutive activity, is fully expressed at the cell surface while the non-edited INI is internalised and accumulates in endosome. [50]
Editing is also thought to influence splicing. Three different spliced isoforms of the receptor exist. Editing regulates the amount of 5HT2CR mRNA which leads to translation of the full length protein selection of alternative splice sites. t76,77. These splice sites are termed Gu1, Gu2, GU3. Only GU2 site splicing results in translation of the full length receptor while editing at GU1 is known to result in translation of a truncated protein. This is thought to be a regulatory mechanism to decrease the amount of unedited isoform INI to limit serotonin response when editing is inefficient. Most of the pre-mRNAs which are edited are spliced at the GU2 site. [41] [44]
Dysregulation
Serotonin family of receptors are often linked to pathology of several human mental conditions such as Schizophrenia, anxiety, Bipolar disorder and major depression. [51] There have been several experimental investigations into the effects of alternative editing patterns of the 5HT2CR and these conditions with a wide variability in results especially those relating to schizophrenia. [52] Some studies have noted that there is an increase in RNA editing at site A in depressed suicide victims. [12] [52] E site editing was observed to be increased in individuals with major depression. [53] In rat models this increase is also observed and can be reversed with fluoxetine with some suggestion that E site editing maybe linked to major depression. [54] [55]
Serotonin or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex, touching on diverse functions including mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vasoconstriction.
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. They mediate both excitatory and inhibitory neurotransmission. The serotonin receptors are activated by the neurotransmitter serotonin, which acts as their natural ligand.
Aromatic L-amino acid decarboxylase, also known as DOPA decarboxylase (DDC), tryptophan decarboxylase, and 5-hydroxytryptophan decarboxylase, is a lyase enzyme, located in region 7p12.2-p12.1.
Histamine H3 receptors are expressed in the central nervous system and to a lesser extent the peripheral nervous system, where they act as autoreceptors in presynaptic histaminergic neurons and control histamine turnover by feedback inhibition of histamine synthesis and release. The H3 receptor has also been shown to presynaptically inhibit the release of a number of other neurotransmitters (i.e. it acts as an inhibitory heteroreceptor) including, but probably not limited to dopamine, GABA, acetylcholine, noradrenaline, histamine and serotonin.
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.
Desmetramadol, also known as O-desmethyltramadol (O-DSMT), is an opioid analgesic and the main active metabolite of tramadol. Tramadol is demethylated by the liver enzyme CYP2D6 to desmetramadol in the same way as codeine, and so similarly to the variation in effects seen with codeine, individuals who have a less active form of CYP2D6 will tend to have reduced analgesic effects from tramadol. Because desmetramadol itself does not need to be metabolized to induce an analgesic effect, it can be used in individuals with low CYP2D6 activity unlike tramadol.
Glutamate receptor 3 is a protein that in humans is encoded by the GRIA3 gene.
5-Hydroxytryptamine receptor 4 is a protein that in humans is encoded by the HTR4 gene.
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.
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.
5-hydroxytryptamine (serotonin) 1e receptor (5-HT1e) is a highly expressed human G-protein coupled receptor that belongs to the 5-HT1e receptor family. The human gene is denoted as HTR1E.
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.
Glutamate receptor, ionotropic, kainate 1, also known as GRIK1, is a protein that in humans is encoded by the GRIK1 gene.
Glutamate receptor 4 is a protein that in humans is encoded by the GRIA4 gene.
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.
RS-102221 is a drug developed by Hoffmann–La Roche, which was one of the first compounds discovered that acts as a potent and selective antagonist at the serotonin 5-HT2C receptor, with around 100× selectivity over the closely related 5-HT2A and 5-HT2B receptors. It has anxiolytic effects in animal studies, increases the effectiveness of SSRI antidepressants, and shows a complex interaction with cocaine, increasing some effects but decreasing others, reflecting a role for the 5-HT2C receptor in regulation of the dopamine signalling system in the brain.
α-Methylserotonin (αMS), also known as α-methyl-5-hydroxytryptamine (α-methyl-5-HT) or 5-hydroxy-α-methyltryptamine (5-HO-αMT), is a tryptamine derivative closely related to the neurotransmitter serotonin (5-HT). It acts as a non-selective serotonin receptor agonist and has been used extensively in scientific research to study the function of the serotonin system.
CGS-12066A is a drug which acts as a potent and selective agonist for the 5-HT1B receptor with lower affinity for the three 5-HT2 receptor subtypes. It is used for studying the role of the 5-HT1B receptor in various processes including perception of pain and the sleep-wake cycle.
5-HT2C receptor agonists are a class of drugs that activate 5-HT2C receptors. They have been investigated for the treatment of a number of conditions including obesity, psychiatric disorders, sexual dysfunction and urinary incontinence.
This article incorporates text from the United States National Library of Medicine, which is in the public domain.