Antagonist interaction with the human 5-HT(7) receptor mediates the rapid and potent inhibition of non-G-protein-stimulated adenylate cyclase activity: a novel GPCR effect

Discovery of G Protein-Biased Antagonists against 5-HT7R

5-HT₇R belongs to a family of G protein-coupled receptors and is associated with a variety of physiological processes in the central nervous system via the activation of the neurotransmitter serotonin (5-HT). To develop selective and biased 5-HT₇R ligands, we designed and synthesized a series of pyrazolyl-diazepanes 2 and pyrazolyl-piperazines 3, which were evaluated for binding affinities to 5-HTR subtypes and functional selectivity for G protein and β-arrestin signaling pathways of 5-HT₇R. Among them, 1-(3-(3-chlorophenyl)-1H-pyrazol-4-yl)-1,4-diazepane 2c showed the best binding affinity for 5-HT₇R and selectivity over other 5-HTR subtypes. It was also revealed as a G protein-biased antagonist. The self-grooming behavior test was performed with 2c in vivo with Shank3^-/- transgenic (TG) mice, wherein 2c significantly reduced self-grooming duration time to the level of wild-type mice. The results suggest that 5-HT₇R could be a potential therapeutic target for treating autism spectrum disorder stereotypy.

Contribution of 5-HT2 Receptors to the Control of the Spinal Locomotor System in Intact Rats

Applying serotonergic (5-HT) agonists or grafting of fetal serotonergic cells into the spinal cord improves locomotion after spinal cord injury. Little is known about the role of 5-HT receptors in the control of voluntary locomotion, so we administered inverse agonists of 5-HT₂ (Cyproheptadine; Cypr), 5-HT_2A neutral antagonist (Volinanserin; Volin), 5-HT_2C neutral antagonist (SB 242084), and 5-HT_2B/2C inverse agonist (SB 206553) receptors intrathecally in intact rats and monitored their effects on unrestrained locomotion. An intrathecal cannula was introduced at the low thoracic level and pushed caudally until the tip reached the L2/L3 or L5/L6 spinal segments. Locomotor performance was evaluated using EMG activity of hindlimb muscles during locomotion on a 2 m long runway. Motoneuron excitability was estimated using EMG recordings during dorsi- and plantar flexion at the ankle. Locomotion was dramatically impaired after the blockage of 5-HT_2A receptors. The effect of Cypr was more pronounced than that of Volin since in the L5/L6 rats Cypr (but not Volin) induced significant alteration of the strength of interlimb coordination followed by total paralysis. These agents significantly decreased locomotor EMG amplitude and abolished or substantially decreased stretch reflexes. Blocking 5-HT_2B/2C receptors had no effect either on locomotion or reflexes. We suggest that in intact rats serotonin controls timing and amplitude of muscle activity by acting on 5-HT_2A receptors on both CPG interneurons and motoneurons, while 5-HT_2B/2C receptors are not involved in control of the locomotor pattern in lumbar spinal cord.

Pharmacology and Therapeutic Potential of the 5-HT7 Receptor

It is well-documented that serotonin (5-HT) exerts its pharmacological effects through a series of 5-HT receptors. The most recently identified member of this family, 5-HT₇, was first identified in 1993. Over the course of the last 25 years, this receptor has been the subject of intense investigation, and it has been demonstrated that 5-HT₇ plays an important role in a wide range of pharmacological processes. As a result of these findings, modulation of 5-HT₇ activity has been the focus of numerous drug discovery and development programs. This review provides an overview of the roles of 5-HT₇ in normal physiology and the therapeutic potential of this interesting drug target.

The arylpiperazine derivatives N-(4-cyanophenylmethyl)-4-(2-diphenyl)-1-piperazinehexanamide and N-benzyl-4-(2-diphenyl)-1-piperazinehexanamide exert a long-lasting inhibition of human serotonin 5-HT7 receptor binding and cAMP signaling

We performed a detailed in vitro pharmacological characterization of two arylpiperazine derivatives, compound N-(4-cyanophenylmethyl)-4-(2-diphenyl)-1-piperazinehexanamide (LP-211) previously identified as a high-affinity brain penetrant ligand for 5-hydroxytryptamine (serotonin) type 7 (5-HT₇) receptors, and its analog N-benzyl-4-(2-diphenyl)-1-piperazinehexanamide (MEL-9). Both ligands exhibited competitive displacement of [³H]-(2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine ([³H]-SB-269970) radioligand binding and insurmountable antagonism of 5-carboxamidotryptamine (5-CT)-stimulated cyclic adenosine monophosphate (cAMP) signaling in human embryonic kidney (HEK293) cells stably expressing human 5-HT₇ receptors. They also inhibited forskolin-stimulated adenylate cyclase activity in 5-HT₇-expressing HEK293 cells but not in the parental cell line. The compounds elicited long-lasting (at least 24 h) concentration-dependent inhibition of radioligand binding at 5-HT₇-binding sites in whole-cell radioligand binding assays, after pretreatment of the cells with the compounds and subsequent compound removal. In cAMP assays, pretreatment of cells with the compounds rendered 5-HT₇ receptors unresponsive to 5-CT and also rendered 5-HT₇-expressing HEK293 cells unresponsive to forskolin. Compound 1-(2-biphenyl)piperazine (RA-7), a known active metabolite of LP-211 present in vivo, was able to partially inhibit 5-HT₇ radioligand binding in a long-lasting irreversible manner. Hence, LP-211 and MEL-9 were identified as high-affinity long-acting inhibitors of human 5-HT₇ receptor binding and function in cell lines. The detailed in vitro characterization of these two pharmacological tools targeting 5-HT₇ receptors may benefit the study of 5-HT₇ receptor function and it may lead to the development of novel selective pharmacological tools with defined functional properties at 5-HT₇ receptors.

Distribution of 5-ht(1E) receptors in the mammalian brain and cerebral vasculature: an immunohistochemical and pharmacological study

BACKGROUND AND PURPOSE

The 5-ht(1E) receptor is highly expressed in the human brain and its structure is conserved in humans, suggesting an important physiological role for 5-ht(1E) receptors. However, neither the function nor the distribution of this receptor has been characterized in the mammalian brain.

EXPERIMENTAL APPROACH

Rats and mice lack the 5-ht(1E) receptor gene; thus, we used guinea pig brain tissue and immunofluorescent staining techniques to provide the first specific localization of 5-ht(1E) receptors in the mammalian brain.

KEY RESULTS

High levels of 5-ht(1E) receptors are detected in olfactory bulb glomeruli as well as the molecular layer of the dentate gyrus (DG). In DG membranes, BRL54443, a 5-ht(1E) /5-HT(1F) agonist, selectively stimulated 5-ht(1E) receptors and potently inhibited forskolin-dependent cAMP production (IC₅₀ = 14 nM). The staining pattern of 5-ht(1E) receptors in brain tissue suggests that this receptor is expressed predominantly in neurons rather than in glia. Additionally, 5-ht(1E) receptors were detected in the adventitial layer of cerebral arteries but not in the microvasculature, venous tissue or other brain arteries.

CONCLUSIONS AND IMPLICATIONS

These observations should help to predict clinical effects of 5-ht(1E) -selective drugs. For example, the stimulation of 5-ht(1E) receptors and subsequent inhibition of adenylate cyclase activity in the DG suggests that 5-ht(1E) receptors may mediate regulation of hippocampal activity by 5-HT, making it a possible drug target for the treatment of neuropsychiatric disorders characterized by memory deficits (such as Alzheimer's disease) or as a target for the treatment of temporal lobe epilepsy.

Toward selective drug development for the human 5-hydroxytryptamine 1E receptor: a comparison of 5-hydroxytryptamine 1E and 1F receptor structure-affinity relationships

The 5-hydroxytryptamine (5-HT) 1E receptor is highly expressed in the human frontal cortex and hippocampus, and this distribution suggests the function of 5-HT(1E) receptors might be linked to memory. To test this hypothesis, behavioral experiments are needed. Because rats and mice lack a 5-HT(1E) receptor gene, knockout strategies cannot be used to elucidate this receptor's functions. Thus, selective pharmacological tools must be developed. The tryptamine-related agonist BRL54443 [5-hydroxy-3-(1-methylpiperidin-4-yl)-1H-indole] is one of the few agents that binds 5-HT(1E) receptors with high affinity and some selectively; unfortunately, it binds equally well to 5-HT(1F) receptors (K(i) ≈ 1 nM). The differences between tryptamine binding requirements of these two receptor populations have never been extensively explored; this must be done to guide the design of analogs with greater selectivity for 5-HT(1E) receptors versus 5-HT(1F) receptors. Previously, we determined the receptor binding affinities of a large series of tryptamine analogs at the 5-HT(1E) receptor; we now examine the affinities of this same series of compounds at 5-HT(1F) receptors. The affinities of these compounds at 5-HT(1E) and 5-HT(1F) receptors were found to be highly correlated (r = 0.81). All high-affinity compounds were full agonists at both receptor populations. We identified 5-N-butyryloxy-N,N-dimethyltryptamine as a novel 5-HT(1F) receptor agonist with >60-fold selectivity versus 5-HT(1E) receptors. There is significant overlap between 5-HT(1E) and 5-HT(1F) receptor orthosteric binding properties; thus, identification of 5-HT(1E)-selective orthosteric ligands will be difficult. The insights generated from this study will inform future drug development and molecular modeling studies for both 5-HT(1E) and 5-HT(1F) receptors.