Short description: Serotonin receptor protein distributed in the cerebrum and raphe nucleus
Generic protein structure example
The serotonin 1A receptor (or 5-HT1A 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.[1][2]
Activation of central 5-HT1A receptors triggers the release or inhibition of norepinephrine depending on species, presumably from the locus coeruleus, which then reduces or increases neuronal tone to the iris sphincter muscle by modulation of postsynaptic α2-adrenergic receptors within the Edinger-Westphal nucleus, resulting in pupil dilation in rodents, and pupil constriction in primates including humans.[11][12][13]
5-HT1A autoreceptor desensitization and increased 5-HT1A receptor postsynaptic activation via general increases in serotonin levels by serotonin precursor supplementation, serotonin reuptake inhibition, or inhibition of monoamine oxidase has been shown to be a major mediator in the therapeutic benefits of most mainstream antidepressant supplements and pharmaceuticals, including serotonin precursors like L-tryptophan and 5-HTP, SSRIs, serotonin-norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants (TCAs), tetracyclic antidepressants (TeCAs), and monoamine oxidase inhibitors (MAOIs).[25] 5-HT1A receptor activation likely plays a significant role in the positive effects of serotonin releasing agents (SRAs) like MDMA (commonly known as ecstasy) as well.[26][27]
5-HT1A receptors in the dorsal raphe nucleus are co-localized with neurokinin 1 (NK1) receptors and have been shown to inhibit the release of substance P. Their endogenous ligand.[28][29] In addition to being antidepressant and anxiolytic in effect, 5-HT1A receptor activation has also been demonstrated to be antiemetic[30][31] and analgesic,[32][33] and all of these properties may be mediated in part or full, depending on the property in question, by NK1 receptor inhibition. Consequently, novel NK1 receptor antagonists are now in use for the treatment of nausea and emesis, and are also being investigated for the treatment of anxiety and depression.[34]
5-HT1A receptor activation has been shown to increase dopamine release in the medial prefrontal cortex, striatum, and hippocampus, and may be useful for improving the symptoms of schizophrenia and Parkinson's disease.[35] As mentioned above, some of the atypical antipsychotics are 5-HT1A receptor partial agonists, and this property has been shown to enhance their clinical efficacy.[35][36][37] Enhancement of dopamine release in these areas may also play a major role in the antidepressant and anxiolytic effects as seen upon postsynaptic activation of the 5-HT1A receptor.[38][39]
The activation of 5-HT1A receptors has been demonstrated to impair certain aspects of memory (affecting declarative and non-declarative memory functions) and learning (due to interference with memory-encoding mechanisms), by inhibiting the release of glutamate and acetylcholine in various areas of the brain.[40] 5-HT1A activation is known to improve cognitive functions associated with the prefrontal cortex, possibly via inducing prefrontal cortex dopamine and acetylcholine release.[41] Conversely, the 5-HT1A antagonist, WAY100635, alleviated learning and memory impairments induced by glutamate blockade (with dizocilpine)[42] or hippocampal cholinergic denervation (by fornix transection)[43] in primates. Furthermore, 5-HT1A receptor antagonists such as lecozotan have been shown to facilitate certain types of learning and memory in rodents, and as a result, are being developed as novel treatments for Alzheimer's disease.[44]
Other effects of 5-HT1A activation that have been observed in scientific research include:
5-HT1A receptors can be located on the cell body, dendrites, axons, and both presynaptically and postsynaptically in nerve terminals or synapses. Those located on the soma and dendrites are referred to as somatodendritic, and those located presynaptically in the synapse are simply referred to as presynaptic. As a group, receptors that are sensitive to the neurotransmitter that is released by the neuron on which the receptors are located are known as autoreceptors; they typically constitute the key component of an ultra-short negative feedback loop whereby the neuron's release of neurotransmitter inhibits its further release of neurotransmitter. Stimulation of 5-HT1A autoreceptors inhibits the release of serotonin in nerve terminals. For this reason, 5-HT1A receptor agonists tend to exert a biphasic mode of action; they decrease serotonin release and postsynaptic 5-HT1A receptor activity in low doses, and further decrease serotonin release but increase postsynaptic 5-HT1A receptor activity at higher doses by directly stimulating the receptors in place of serotonin.
This autoreceptor-mediated inhibition of serotonin release has been theorized to be a major factor in the therapeutic lag that is seen with serotonergic antidepressants such as the SSRIs.[64] The autoreceptors must first desensitize before the concentration of extracellular serotonin in the synapse can become elevated appreciably.[64][65] Though the responsiveness of the autoreceptors is somewhat reduced with chronic treatment, they still remain effective at constraining large increases in extracellular serotonin concentrations.[64] For this reason, serotonin reuptake inhibitors that also have 5-HT1A receptor antagonistic or partial agonistic properties, such as vilazodone and SB-649,915, are being investigated and introduced as novel antidepressants with the potential for a faster onset of action and improved effectiveness compared to those currently available.[66]
Unlike most drugs that elevate extracellular serotonin levels like the SSRIs and MAOIs, SRAs such as fenfluramine and MDMA bypass serotonin autoreceptors such as 5-HT1A. They do this by directly acting on the release mechanisms of serotonin neurons and forcing release to occur regardless of autoreceptor-mediated inhibition.[67] As such, SRAs induce immediate and much greater increases in extracellular serotonin concentrations compared to other serotonin-elevating agents such as the SSRIs. [Note: This is questionable as the level of serotonin output from SRAs is still dose dependant and, while SRAs will initially bypass autoreceptors, the increase in serotonin they induce will then agonise autoreceptors.] In contrast to SRAs, SSRIs may decrease serotonin levels initially (especially at lower dosages due to the biphasic mode of action mentioned above) and require several weeks of chronic dosing before serotonin concentrations reach their maximal elevation (due to 1A autoreceptor desensitization) and full clinical benefits for conditions such as depression and anxiety are seen[68][69] (although other studies show an acute increase in 5-HT[70][71] which may account for initial worsening of symptoms in sensitive individuals[72]). For these reasons, selective serotonin releasing agents (SSRAs) such as MDAI and MMAI have been proposed as novel antidepressants with a putatively faster onset of action and improved effectiveness compared to current treatments.[68]
Similarly to SRAs, sufficiently high doses of 5-HT1A receptor agonists also bypass the 5-HT1A autoreceptor-mediated inhibition of serotonin release and therefore increase 5-HT1A postsynaptic receptor activation by directly agonizing the postsynaptic receptors in lieu of serotonin.
Ligands
The distribution of 5-HT1A receptors in the human brain may be imaged with the positron emission tomography using the radioligand [11C] WAY-100,635.[73]
For example, one study has found increased 5-HT1A binding in type 2 diabetes.[74] Another PET study found a negative correlation between the amount of 5-HT1A binding in the raphe nuclei, hippocampus and neocortex and a self-reported tendency to have spiritual experiences.[75] Labeled with tritium, WAY-100,635 may also be used in autoradiography.[76]
The 5-HT1A receptor is coded by the HTR1Agene. There are several human polymorphisms associated with this gene. A 2007 review listed 27 single nucleotide polymorphisms (SNP).[93] The most investigated SNPs are C-1019G (rs6295), C-1018G,[94] Ile28Val (rs1799921), Arg219Leu (rs1800044), and Gly22Ser (rs1799920).[93] Some of the other SNPs are Pro16Leu, Gly272Asp, and the synonymous polymorphism G294A (rs6294). These gene variants have been studied in relation to psychiatric disorders with no definitive results.[93]
Protein-protein interactions
The 5-HT1A receptor has been shown to interact with brain-derived neurotrophic factor (BDNF), which may play a major role in its regulation of mood and anxiety.[95][96]
↑"Localization of 5-HT1A receptors in the living human brain using [carbonyl-11C]WAY-100635: PET with anatomic standardization technique". Journal of Nuclear Medicine40 (1): 102–9. January 1999. PMID9935065.
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↑"Involvement of brain 5-HT1A receptors in the hypotensive response to urapidil". The American Journal of Cardiology64 (7): 7D–10D. August 1989. doi:10.1016/0002-9149(89)90688-7. PMID2569265.
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↑"Neue Wirkstoffe - Lecozotan" (in de). Österreichische Apothekerzeitung (17/2007): 805. August 13, 2008.
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↑"Low doses of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH DPAT) increase ethanol intake". Psychopharmacology115 (1–2): 173–9. June 1994. doi:10.1007/BF02244769. PMID7862892.
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↑"8-OH-DPAT and male rat sexual behavior: partial blockade by noradrenergic lesion and sexual exhaustion". Pharmacology, Biochemistry, and Behavior56 (1): 111–6. January 1997. doi:10.1016/S0091-3057(96)00165-7. PMID8981617.
↑"5-HT1A receptor agonists prevent in rats the yawning and penile erections induced by direct dopamine agonists". Psychopharmacology108 (1–2): 47–50. 1992. doi:10.1007/BF02245284. PMID1357709.
↑"Potentiation of fluoxetine-induced penile erections by combined blockade of 5-HT1A and 5-HT1B receptors". European Journal of Pharmacology321 (3): R11-3. March 1997. doi:10.1016/S0014-2999(97)00050-2. PMID9085055.
↑"8-Hydroxy-2-(di-n-propylamino)-tetralin inhibits food intake in fasted rats by an action at 5-HT1A receptors". Methods and Findings in Experimental and Clinical Pharmacology29 (4): 269–72. May 2007. doi:10.1358/mf.2007.29.4.1075362. PMID17609739.
↑"Dose-dependent effects of the 5-HT1A receptor agonist 8-OH-DPAT on sleep and wakefulness in the rat". Journal of Sleep Research1 (3): 169–175. September 1992. doi:10.1111/j.1365-2869.1992.tb00033.x. PMID10607047.
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↑ 59.059.1"A comparison of the oxytocin and vasopressin responses to the 5-HT1A agonist and potential anxiolytic drug alnespirone (S-20499)". Pharmacology, Biochemistry, and Behavior60 (3): 677–83. July 1998. doi:10.1016/S0091-3057(98)00025-2. PMID9678651.
↑ 60.060.1"Differential effects of serotonin (5-HT1A and 5-HT2) agonists and antagonists on renin and corticosterone secretion". Neuroendocrinology45 (4): 305–10. April 1987. doi:10.1159/000124754. PMID2952898.
↑"Stimulation of corticosterone and beta-endorphin secretion in the rat by selective 5-HT receptor subtype activation". European Journal of Pharmacology137 (1): 1–8. May 1987. doi:10.1016/0014-2999(87)90175-0. PMID2956114.
↑"Hormonal and temperature responses to flesinoxan in normal volunteers: an antagonist study". European Neuropsychopharmacology14 (2): 151–5. March 2004. doi:10.1016/S0924-977X(03)00108-1. PMID15013031.
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↑"SB-649915-B, a novel 5-HT1A/B autoreceptor antagonist and serotonin reuptake inhibitor, is anxiolytic and displays fast onset activity in the rat high light social interaction test". Neuropsychopharmacology32 (10): 2163–72. October 2007. doi:10.1038/sj.npp.1301341. PMID17356576.
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↑"First delineation of 5-HT1A receptors in human brain with PET and [11C]WAY-100635". European Journal of Pharmacology283 (1–3): R1-3. September 1995. doi:10.1016/0014-2999(95)00438-Q. PMID7498295.
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↑"[3H]WAY-100635 for 5-HT1A receptor autoradiography in human brain: a comparison with [3H]8-OH-DPAT and demonstration of increased binding in the frontal cortex in schizophrenia". Neurochemistry International30 (6): 565–74. June 1997. doi:10.1016/S0197-0186(96)00124-6. PMID9152998.
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↑"Partial agonist properties of rauwolscine and yohimbine for the inhibition of adenylyl cyclase by recombinant human 5-HT1A receptors". Biochemical Pharmacology45 (11): 2337–41. June 1993. doi:10.1016/0006-2952(93)90208-E. PMID8517875.
↑"Yohimbine and rauwolscine inhibit 5-hydroxytryptamine-induced contraction of large coronary arteries of calf through blockade of 5 HT2 receptors". Naunyn-Schmiedeberg's Archives of Pharmacology323 (2): 149–54. June 1983. doi:10.1007/BF00634263. PMID6136920.
↑"1A Receptor-Biased Agonists with Robust Antidepressant-like Activity". Journal of Medicinal Chemistry62 (5): 2750–2771. March 2019. doi:10.1021/acs.jmedchem.9b00062. PMID30721053.
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↑"Membrane cholesterol depletion enhances ligand binding function of human serotonin1A receptors in neuronal cells". Biochem Biophys Res Commun390 (1): 93–6. December 2009. doi:10.1016/j.bbrc.2009.09.072. PMID19781522.
↑ 93.093.193.2"5-HT1A gene variants and psychiatric disorders: a review of current literature and selection of SNPs for future studies". The International Journal of Neuropsychopharmacology11 (5): 701–21. August 2008. doi:10.1017/S1461145707008218. PMID18047755.
↑"Interaction between 5-HT1A and BDNF genotypes increases the risk of treatment-resistant depression". Journal of Neural Transmission114 (8): 1065–8. 2007. doi:10.1007/s00702-007-0705-9. PMID17401528.
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