Characterization of 4-(2-hydroxyphenyl)-1-[2'-[N-(2''-pyridinyl)-p-fluorobenzamido]ethyl]piperazine (p-DMPPF) as a new potent 5-HT1A antagonist

New Trends and Current Status of Positron-Emission Tomography and Single-Photon-Emission Computerized Tomography Radioligands for Neuronal Serotonin Receptors and Serotonin Transporter

The critical role of serotonin (5-hydroxytryptamine; 5-HT) and its receptors (5-HTRs) in the pathophysiology of diverse neuropsychiatric and neurodegenerative disorders render them attractive diagnostic and therapeutic targets for brain disorders. Therefore, the in vivo assessment of binding of 5-HT receptor ligands under a multitude of physiologic and pathologic scenarios may support more-accurate identification of disease and its progression and the patient's response to therapy as well as the screening of novel therapeutic strategies. The present Review aims to focus on the current status of radioligands used for positron-emission tomography (PET) and single-photon-emission computerized tomography (SPECT) imaging of human brain serotonin receptors. We further elaborate upon and emphasize the attributes that qualify a radioligand for theranostics on the basis of its frequency of use in clinics, its benefit to risk assessment in humans, and its continuous evolution, along with the major limitations.

Enhancing a CH-π Interaction to Increase the Affinity for 5-HT1A Receptors

An electrostatic interaction related to a favorable position of the distal phenyl ring and a phenylalanine residue in the binding pocket would explain the higher 5-HT1A affinity of a 4-phenyl-1,2,3,6-tetrahydropyridine (THP) analogue compared to the corresponding 4-phenylpiperazine analogue. To explore a possible reinforcement of this interaction to increase the affinity for 5-HT1A receptors, different 4-substituted-phenyl analogues were synthesized and tested. The most important increase of affinity is obtained with two electron-donating methyl groups in positions 3 and 5.

Mechanism of the medium-duration afterhyperpolarization in rat serotonergic neurons

Most serotonergic neurons display a prominent medium-duration afterhyperpolarization (mAHP), which is mediated by small-conductance Ca(2+) -activated K(+) (SK) channels. Recent ex vivo and in vivo experiments have suggested that SK channel blockade increases the firing rate and/or bursting in these neurons. The purpose of this study was therefore to characterize the source of Ca(2+) which activates the mAHP channels in serotonergic neurons. In voltage-clamp experiments, an outward current was recorded at -60 mV after a depolarizing pulse to +100 mV. A supramaximal concentration of the SK channel blockers apamin or (-)-bicuculline methiodide blocked this outward current. This current was also sensitive to the broad Ca(2+) channel blocker Co(2+) and was partially blocked by both ω-conotoxin and mibefradil, which are blockers of N-type and T-type Ca(2+) channels, respectively. Neither blockers of other voltage-gated Ca(2+) channels nor DBHQ, an inhibitor of Ca(2+)-induced Ca(2+) release, had any effect on the SK current. In current-clamp experiments, mAHPs following action potentials were only blocked by ω-conotoxin and were unaffected by mibefradil. This was observed in slices from both juvenile and adult rats. Finally, when these neurons were induced to fire in an in vivo-like pacemaker rate, only ω-conotoxin was able to increase their firing rate (by ~30%), an effect identical to the one previously reported for apamin. Our results demonstrate that N-type Ca(2+) channels are the only source of Ca(2+) which activates the SK channels underlying the mAHP. T-type Ca(2+) channels may also activate SK channels under different circumstances.