WAY-100635 antagonist-induced plasticity of 5-HT receptors: regulatory differences between a stable cell line and an in vivo native system

Translational possibility of [18 F]Mefway to image serotonin 1A receptors in humans: Comparison with [18 F]FCWAY in rodents

PURPOSE

To compare the cerebral uptake and binding potential of [¹⁸ F]FCWAY and [¹⁸ F]Mefway in the rodent to assess their potential for imaging serotonin 1A (5-HT_1A ) receptors.

MATERIALS AND METHODS

In vitro liver microsomal studies were performed to evaluate the degree of defluorination. Dynamic positron emission tomography (PET) studies were then conducted for 2 h with or without an anti-defluorination agent. The regions of interest were the hippocampus and frontal cortex (5-HT_1A target regions) and the cerebellum (5-HT_1A nontarget region). The in vivo kinetics of the radioligands were compared based on the brain uptake values and target-to-nontarget ratio. We also performed a comparison of binding potential (BP_ND ) as a steady-state binding parameter. Finally, binding affinities to 5-HT_1A receptors were assessed in Chinese hamster ovary cells (CHO-K1) cells expressing human recombinant 5-HT_1A receptors.

RESULTS

The radiochemical yield of [¹⁸ F]Mefway was slightly higher than that of [¹⁸ F]FCWAY (19 vs. 15%). With regard to metabolic stability against defluorination, both compounds exhibited similar stability in rat liver microsomes, but [¹⁸ F]Mefway displayed higher stability in the human microsome (defluorination ratio at 30 min: 32 vs. 29 in rat liver microsomes, 31 vs. 64 in human liver microsomes for [¹⁸ F]Mefway and [¹⁸ F]FCWAY, respectively). There were no significant differences in brain uptake, the target-to-nontarget ratios, and the BP_ND (at hippocampus, peak brain uptakes: 6.9 vs. 8.5, target-to-nontarget ratios: 6.9 vs. 8.5, BP_ND : 5.2 vs. 6.2 for [¹⁸ F]Mefway and [¹⁸ F]FCWAY). The binding affinity of [¹⁸ F]Mefway was considerably higher than that of [¹⁸ F]FCWAY (IC₅₀ : 1.5 nM vs. 2.2 nM).

CONCLUSION

[¹⁸ F]Mefway exhibits favorable characteristics compared to [¹⁸ F]FCWAY in rodents, and may be a promising radioligand for use in human subjects. Synapse 68:595-603, 2014. © 2014 Wiley Periodicals, Inc.

Antipsychotics differ in their ability to internalise human dopamine D2S and human serotonin 5-HT1A receptors in HEK293 cells

Antipsychotic drugs act preferentially via dopamine D(2) receptor blockade, but interaction with serotonin 5-HT(1A) receptors has attracted interest as additional target for antipsychotic treatment. As receptor internalisation is considered crucial for drug action, we tested the propensity of antipsychotics to internalise human (h)D(2S) receptors and h5-HT(1A) receptors. Agonist-induced internalisation of hemaglutinin (HA)-tagged hD(2S) and HA-h5-HT(1A) receptors expressed in HEK293 cells was increased by coexpression of G-protein coupled receptor kinase 2 and beta-arrestin2. At the HA-hD(2S) receptor, dopamine, quinpirole and bromocriptine behaved as full agonists, while S(-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(-)-3PPP] and sarizotan were partial agonists. The typical antipsychotic, haloperidol, and the atypical compounds, olanzapine, nemonapride, ziprasidone and clozapine did not internalise HA-hD(2S) receptors, whereas aripiprazole potently internalised these receptors (>50% relative efficacy). Among antipsychotics with combined D(2)/5-HT(1A) properties, bifeprunox and (3-exo)-8-benzoyl-N-[[(2S)7-chloro-2,3-dihydro-1,4-benzodioxin-1-yl]methyl]-8-azabicyclo-[3.2.1]octane-3-methanamine (SSR181507) partially internalised HA-hD(2S) receptors, piperazine, 1-(2,3-dihydro-1,4-benzodioxin-5-yl)-4-[[5-(4-fluorophenyl)-3-pyridinyl]methyl (SLV313) and N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine (F15063) were inactive. At the HA-h5-HT(1A) receptor, serotonin, (+)-8-hydroxy-2-(di-n-propylamino)tetralin [(+)-8-OH-DPAT] and sarizotan were full agonists, buspirone acted as partial agonist. (-)-Pindolol showed little activity and no internalising properties were manifested for the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide (WAY100635). Most antipsychotics induced HA-h5-HT(1A) receptor internalisation, with an efficacy rank order: nemonapride>F15063>SSR181507>bifeprunox approximately SLV313 approximately ziprasidone>aripiprazole and potencies: SLV313>SSR181507 approximately F15063>bifeprunox approximately nemonapride approximately aripiprazole>ziprasidone. Interestingly, the internalisation induced by clozapine was only minimal, whereas aripirazole and bifeprunox were more potent for internalisation than for G-protein activation. These different profiles of antipsychotics for receptor internalisation may help to evaluate their potential therapeutic impact in the treatment of schizophrenia.

5-HT1A receptor-mediated apoptosis: death by JNK?

There is growing interest in the potential use of 5-HT(1A) receptor agonists as neuroprotective agents in stroke and traumatic brain injury. However, a new study using a recombinant 5-HT(1A) receptor cell line suggests that these agonists may promote as well as inhibit apoptotic responses. Because heterologously expressed receptors may couple promiscuously to inappropriate signal transduction pathways, the results should be interpreted with caution.

Antagonist-induced increase in 5-HT1A-receptor expression in adult rat hippocampus and cortex

Many receptor antagonists function as reverse agonists on the signaling transduction pathway, but little is known about the action of these drugs on the regulation of receptor expression. Serotonin 1A (5-HT1A) receptor expression in 5-HT and serum-free fetal hippocampal cultures is increased in the presence of a specific 5-HT1A-receptor antagonist N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635). To study the plasticity of postsynaptic 5-HT1A receptors in the presence of antagonist in vivo, adult Sprague Dawley rats were injected i.p. either once or twice daily with a dose of WAY 100635 (3 mg/kg) over a period of 3 days. The 5-HT1A receptor expression was detected by immunocytochemistry and light microscopy, and the receptor immunoreactivity (IR) in hippocampus subregions was quantitatively assessed by using a comparative computer-assisted morphometric analysis. Following the daily injections of WAY 100635, a significant increase in 5-HT1A receptor labeling in hippocampal neurons was recorded. This marked increase in 5-HT1A receptor expression, which occurred within 4 h after a single injection of WAY 100635, is evident on the somata membrane and dendritic processes of hippocampal and cortex layer V neurons. By contrast, no increase in 5-HT1A receptor-IR was observed after multiple daily injections at a low dose (1 mg/kg) of WAY 100635. Our study shows that a single or multiple daily injections of WAY 100635 can result in an increase in 5-HT1A receptor-IR. This increase in labeling is consistent with an enhanced expression of the receptor protein. The action of this "inverse agonist" may have clinical importance in disorders such as depression, epilepsy, and Alzheimer's disease in which 5-HT1A receptor levels are deficient.