Although a number of mood-stabilising atypical antipsychotics and antidepressants modulate serotonin type 7 receptor (5-HT7), the detailed contributions of 5-HT7 function to clinical efficacy and pathophysiology have not been fully understood. The mood-stabilising antipsychotic agent, lurasidone, and the serotonin partial agonist reuptake inhibitor, vortioxetine, exhibit higher binding affinity to 5-HT7 than other conventional antipsychotics and antidepressants. The initially expected rapid onset of antidepressant effects—in comparison with conventional antidepressants or mood-stabilising antipsychotics—due to 5-HT7 inhibition has not been observed with lurasidone and vortioxetine; however, several clinical studies suggest that 5-HT7 inhibition likely contributes to quality of life of patients with schizophrenia and mood disorders via the improvement of cognition. Furthermore, it reported that 5-HT7 inhibition might mitigate antipsychotic-induced weight gain and metabolic complication by blocking other monoamine receptors. Further preclinical studies for the development of 5-HT7 modulation against neurodevelopmental disorders and neurodegenerative diseases have been ongoing. Various findings from various preclinical studies indicate the possibility that 5-HT7 modifications can provide two independent strategies. The first is that 5-HT7 inhibition ameliorates the dysfunction of inter-neuronal transmission in mature networks. The other is that activation of 5-HT7 can improve transmission dysfunction due to microstructure abnormality in the neurotransmission network—which could be unaffected by conventional therapeutic agents—via modulating intracellular signalling during the neurodevelopmental stage or via loss of neural networks with aging.
Receptor | LUR | APZ | Brex | CLZ | OLZ | PMZ | QTP | RIS | ZTP | VTX | ||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
5-HT1A | 6.8 | 5.6 | 0.12 | 124 | >1000 | 650 | 432 | 423 | 471 | 15.0 | ||||
5-HT2A | 2.0 | 8.7 | 0.47 | 5.4 | 2.3 | 48.4 | 100 | 0.2 | 2.7 | |||||
5-HT3 | >1000 | 630 | 241 | 57 | >1000 | >1000 | >1000 | 472 | 3.7 | |||||
5-HT7 | 0.5 | 10.3 | 3.7 | 18.0 | 365 | 0.5 | 307 | 6.6 | 12.0 | 19.0 | ||||
H1 | >1000 | 27.6 | 19 | 1.13 | 1.2 | 692 | 11 | 20.1 | 3.21 | |||||
D1 | 262 | >1000 | 160 | 266 | 100 | >1000 | 712 | 244 | 71.0 | |||||
D2 | 1.7 | 3.3 | 0.3 | 157 | 52.3 | 0.3 | 245 | 3.6 | 25.0 | |||||
Reference | [27] | [28,29] | [28][29] | [30] | [31,32] | [31][32] | [33,49] | [33][43] | [34] | [35] | [29,36] | [29][36] | [37] | [38] |
The earliest identified physiological function of 5-HT7 was regulation of circadian rhythms [2]. In this context, 5-HT7 was found to be expressed in the suprachiasmatic nucleus [50][44], which is a major regulatory region of circadian rhythms [51][45]. The influence of 5-HT7 on sleep regulation is complicated because the 5-HT7 inverse agonist SB269970 [40,42,43,52][40][42][46][47] increased latency in the onset but decreased the total amount of time spent in rapid eye movement sleep [53][48]. A number of antidepressants also increased latency in the onset but decreased the total amount of time spent in rapid eye movement sleep, similar to SB269970 [54][49].
The 5-HT7 knockout mice displayed a reduction of immobility times in both a forced swim test and a tail suspension test, common pharmacological behaviour tests for the screening of antidepressants (decreasing immobility time is considered to correlate to antidepressant action in humans) [22,55][22][50]. Based on this finding, 5-HT7 inhibitors/antagonists were actively explored and developed as antidepressants in the early years of this century. SB269970, an inverse agonist of 5-HT7, has been shown to reduce immobility time in forced swim and tail suspension tests; as a result, it became a candidate for use as an antidepressant [22,23,56][22][23][51]. Additionally, the administration of subeffective concentration of SB269970 enhanced the anti-immobility action of subeffective doses of of desipramine, citalopram and imipramine in the forced swim and tail suspension tests [56,57,58][51][52][53]. Most notably, SB269970 generated significantly rapid onset of antidepressant-like effects in olfactory bulbectomised models, when compared to fluoxetine [59][54]. As a result, 5-HT7 inhibitors were anticipated to join the rapid onset antidepressant class, since it is recognised that one of the major problems of monoamine transporter-inhibiting antidepressants, a currently/commonly prescribed antidepressant class, is that they require 2-4 weeks for the onset of antidepressive effects. The target regions for antidepressant effects of 5-HT7 inhibitors remain controversial. Injection of SB269970 into the hippocampus generated antidepressant-like activity in the forced swim test [60][55]. Injection of SB269970 into the lateral habenular nucleus lesion model using 6-hydroxydoapmine also exhibited antidepressant-like activity, whereas injection into the medial prefrontal cortex lesion of a model, conversely, displayed the enhancement of depressive-like activity [61,62][56][57]. AS19, a 5-HT7 agonist, demonstrated the opposite region-dependent effect against SB269970 [61,62][56][57].Activation of 5-HT7 was a potential candidate target for relieving symptoms in patients with Rett syndrome. Rett syndrome is the second most common cause of mental retardation in females and plays a role in severe neurodevelopmental disorders such as breathing dysfunction, loss of coordination, abnormal eye and hand movements, epilepsy, aberrant sleeping behaviour and cognitive impairment [180][137]. The prime pathogenesis of Rett syndrome is known to be various genetic mutations in methyl CpG-binding protein 2 gene (MeCP2) on the X chromosome, cyclin-dependent kinase-like 5 (CDKL5), forkhead box G1 (FOXG1), WD repeat domain 45 (WDR45) or syntaxin binding protein 1 (STXBP1) [181,182][138][139]. Restoring MeCP2 function can normalise functional abnormalities of MeCP2 knockout mice, whereas overexpression of the MeCP2 gene led to neurological defects [183][140]. Therefore, recent preclinical studies explored the targets in MeCP2 downstream effectors and other signalling, including 5-HT7. Based on the reduced 5-HT7 expression in Rett syndrome models, the systemic administration of 5-HT7 agonist relieved the related symptoms, anxiety, environment-related exploratory behaviour and motor learning ability of Rett syndrome mice models [184,185][141][142].