Presynaptic Selectivity of a Ligand for Serotonin 1A Receptors Revealed by In Vivo PET Assays of Rat Brain

et al. (2012) Presynaptic Selectivity of a Ligand for Serotonin 1A Receptors Revealed by In Vivo PET Assays of Rat Brain. PLoS ONE 7(8): e42589. doi:10.1371/journal.pone.0042589 Presynaptic Selectivity of a Ligand for Serotonin 1A Receptors Revealed by In Vivo PET Assays of Rat Brain Takeaki Saijo Jun Maeda Takashi Okauchi Jun-ichi Maeda Yasunori Morio Yasuhiro Kuwahara Masayuki Suzuki Nobuharu Goto Toshimitsu Fukumura Tetsuya Suhara Makoto Higuchi Tadafumi Kato, RIKEN Brain Science Institution, Japan A novel investigational antidepressant with high affinity for the serotonin transporter and the serotonin 1A (5-HT1A) receptor, called Wf-516 (structural formula: (2S)-1-[4-(3,4-dichlorophenyl)piperidin-1-yl]-3-[2-(5-methyl-1,3,4-oxadiazol-2yl)benzo[b]furan-4-yloxy]propan-2-ol monohydrochloride), has been found to exert a rapid therapeutic effect, although the mechanistic basis for this potential advantage remains undetermined. We comparatively investigated the pharmacokinetics and pharmacodynamics of Wf-516 and pindolol by positron emission tomographic (PET) and autoradiographic assays of rat brains in order to elucidate their molecular interactions with presynaptic and postsynaptic 5-HT1A receptors. In contrast to the full receptor occupancy by pindolol in PET measurements, the binding of Wf-516 to 5-HT1A receptors displayed limited capacity, with relatively high receptor occupancy being achieved in regions predominantly containing presynaptic receptors. This selectivity was further proven by PET scans of neurotoxicant-treated rats deficient in presynaptic 5-HT1A receptors. In addition, [35S]guanosine 59-O-[c-thio]triphosphate autoradiography indicated a partial agonistic ability of Wf516 for 5-HT1A receptors. This finding has lent support to reports that diverse partial agonists for 5-HT1A receptors exert high sensitivity for presynaptic components. Thus, the present PET data suggest a relatively high capacity of presynaptic binding sites for partial agonists. Since our in vitro and ex vivo autoradiographies failed to illustrate these distinct features of Wf-516, in vivo PET imaging is considered to be, thus far, the sole method capable of pharmacokinetically demonstrating the unique actions of Wf-516 and similar new-generation antidepressants. - Funding: This work was supported in part by Grants-in-Aid for the Molecular Imaging Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan. No additional external funding received for this study. Competing Interests: TS, J-iM, YM, YK, MS and NG are affiliated with Mitsubishi Tanabe Pharma Corporation. Wf-516 is a compound in development by Mitsubishi Tanabe Pharma Corporation. No additional competing interests exist. This does not alter the authors adherence to all the PLoS ONE policies on sharing data and materials. . These authors contributed equally to this work. Selective serotonin reuptake inhibitors (SSRIs), such as fluvoxamine, fluoxetine and paroxetine, are currently the most frequently prescribed antidepressants [13]. SSRIs induce fewer adverse effects than classical tricyclic agents [4], thereby contributing to improved quality of life. These drugs increase serotonin (5-HT) concentration at synaptic clefts through inhibitory binding to 5-HT transporters (5-HTTs) responsible for 5-HT reuptake, thus enhancing serotonergic neurotransmissions and producing an antidepressant effect [5]. However, this serotonergic reinforcement does not take place immediately after the initiation of treatment, as increased 5-HT stimulates 5-HT 1A (5-HT1A) autoreceptors as negative feedback, inhibiting the release of 5-HT at presynaptic terminals [6,7]. The persistent rise of 5-HT levels following repeated SSRI administration subsequently induces desensitization of 5-HT1A autoreceptors, and the firing frequencies of 5-HT neurons gradually recover [8,9], resulting in the delayed appearance of antidepressant effects. In practice, this delayed therapeutic benefit of SSRIs has been a source of distress for both depressive patients and psychiatrists. Pindolol, a therapeutic agent used for the treatment of hypertension, antagonistically binds to not only b adrenergic receptors but also to central 5-HT1A receptors [10], and its antagonism for 5-HT1A receptors is assumed to interrupt the autoreceptor-mediated negative feedback. To date, several clinical trials have demonstrated that pindolol accelerates the alleviation of depressive symptoms following initiation of SSRI treatment [1114]. For this serotonergic modulation, it is necessary for pindolol to preferentially block presynaptic 5HT1A autoreceptors without profound suppression of postsynaptic receptors, since postsynaptic antagonism could counteract the indirect agonism by SSRIs [10]. This selective binding property of pindolol for 5-HT1A autoreceptors has been investigated using positron emission tomography (PET) with [11C]N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexane-carboxamide] ([11C]WAY-100635), a specific radioligand suitable for PET imaging of 5-HT1A receptors [15], thus enabling quantification of occupancies of these receptors by therapeutic agents. Several reports have supported the selectivity of pindolol for presynaptic receptors abundantly located in the pontine raphe nucleus [16,17], although such a binding preference has not been confirmed by other studies [18]. In a non-clinical PET study using [11C]WAY-100635, preferential binding of pindolol to 5-HT1A autoreceptors was observed [19], but this was inconsistent with the findings of an ex vivo autoradiography (ARG) study that used intravenous administration of [3H]WAY-100635 and identified nonselective binding of pindolol to 5-HT1A receptors in rat brain [20]. Wf-516 (structural formula: (2 S)-1-[4-(3,4-dichlorophenyl)piperidin-1-yl]-3-[2-(5-methyl-1,3,4-oxadiazol-2-yl)benzo[b]furan-4yloxy]propan-2-ol monohydrochloride), a novel investigational antidepressant with high affinity for 5-HTT and 5-HT1A receptors [21], has been shown to have more rapid antidepressant-like effects than the classical tricyclic antidepressant imipramine in a rat chronic mild stress model of depression [22]. Moreover, a recent in vivo electrophysiological study using rats has indicated that Wf-516 at low and medium doses was an antagonist for presynaptic but not postsynaptic 5-HT1A receptors [23]. Although our previous PET study of rats demonstrated in vivo binding of Wf-516 to central 5-HTTs in a dosedependent manner [24], the pharmacological mechanisms involved in the presynaptic/postsynaptic selectivity of binding of Wf-516 to 5-HT1A receptors in living brains had still remained to be clarified using neuroimaging assays. The present study was conducted in order to determine the properties of the interaction between Wf-516 and presynaptic and postsynaptic 5-HT1A receptors localized predominantly in the raphe nucleus and hippocampus, respectively. Occupancies of these receptors by Wf (...truncated)