Synthesis and Evaluation of [11C]7-Halogen-2-Phenyl Isoindolone Derivatives: Potential PET Radioligands for in vivo Imaging of 5-HT2C Receptors

The serotonin 5-HT2C receptor (5-HT2CR) is abundantly expressed throughout the central nervous system, and involved in a variety of neuroendocrine and neurobehavioral processes. The development of a selective radioligand that will enable in vivo imaging and quantification of 5-HT2CR densities represents a significant technological advancement in understanding both the normal function and pathophysiology of the 5-HT2CR. Four 7-halogen-2-phenyl isoindolones (7-F, Cl, Br, I) were synthesized and displayed high affinities for 5-HT2CR and high selectivity over 5-HT2A and 5-HT2B. [11C]7-Chloro-2-[4-methoxy-3-[2-(4-methylpiperidin-1-yl)ethoxy]phenyl]isoindolin-1-one (6) and [11C]7-iodo-2-[4-methoxy-3-[2-(4-methylpiperidin-1-yl)ethoxy]phenyl]isoindolin-1-one (9) were synthesized in high radiochemical yield of 37–44% [n = 10, decay corrected from end of (11C)CH3I synthesis] with high radiochemical purity via O-methylation with [11C]CH3I, respectively. MicroPET imaging studies in male rats with or without 5-HT2C antagonist SB-242084 showed that [11C]6 and [11C]9 display specific bindings to 5-HT2CR in the choroid plexus and hippocampus. In vivo microPET brain imaging studies in rhesus monkeys demonstrated that [11C]6 and [11C]9 exhibit excellent blood-brain barrier penetration. The contrast of bindings to the choroid plexus and hippocampus compared to the cerebellum peaked at 2.7 and 1.6, respectively, for [11C]6, and 3.7 and 2.7, respectively, for [11C]9, which were reduced by administration of a dose of SB-242084. Our results support the candidacy of [11C]6 and [11C]9 for further study as radioligands for in vivo quantitation of 5-HT2C sites by PET.

A Subset of Purposeless Oral Movements Triggered by Dopaminergic Agonists Is Modulated by 5-HT2C Receptors in Rats: Implication of the Subthalamic Nucleus

Dopaminergic medication for Parkinson’s disease is associated with troubling dystonia and dyskinesia and, in rodents, dopaminergic agonists likewise induce a variety of orofacial motor responses, certain of which are mimicked by serotonin2C (5-HT2C) receptor agonists. However, the neural substrates underlying these communalities and their interrelationship remain unclear. In Sprague-Dawley rats, the dopaminergic agonist, apomorphine (0.03–0.3 mg/kg) and the preferential D2/3 receptor agonist quinpirole (0.2–0.5 mg/kg), induced purposeless oral movements (chewing, jaw tremor, tongue darting). The 5-HT2C receptor antagonist 5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxyl]-5-pyridyl]carbamoyl]-6-trifluoromethylindone (SB 243213) (1 mg/kg) reduced the oral responses elicited by specific doses of both agonists (0.1 mg/kg apomorphine; 0.5 mg/kg quinpirole). After having confirmed that the oral bouts induced by quinpirole 0.5 mg/kg were blocked by another 5-HT2C antagonist (6-chloro-5-methyl-1-[6-(2-methylpiridin-3-yloxy)pyridine-3-yl carbamoyl] indoline (SB 242084), 1 mg/kg), we mapped the changes in neuronal activity in numerous sub-territories of the basal ganglia using c-Fos expression. We found a marked increase of c-Fos expression in the subthalamic nucleus (STN) in combining quinpirole (0.5 mg/kg) with either SB 243213 or SB 242084. In a parallel set of electrophysiological experiments, the same combination of SB 243213/quinpirole produced an irregular pattern of discharge and an increase in the firing rate of STN neurons. Finally, it was shown that upon the electrical stimulation of the anterior cingulate cortex, quinpirole (0.5 mg/kg) increased the response of substantia nigra pars reticulata neurons corresponding to activation of the “hyperdirect” (cortico-subthalamonigral) pathway. This effect of quinpirole was abolished by the two 5-HT2C antagonists. Collectively, these results suggest that induction of orofacial motor responses by D2/3 receptor stimulation involves 5-HT2C receptor-mediated activation of the STN by recruitment of the hyperdirect (cortico-subthalamonigral) pathway.

Effect of cannabinoid-serotonin interactions in the regulation of neuropeptide Y1 receptors expression in rats: the role of CB1 and 5-HT2C receptor

Neuropeptide Y (NPY) is involved in a diversity of critical functions such as circadian rhythms, energy homeostasis, and appetite regulation in the hypothalamus. It has identified as a crucial participant in adjusting energy intake and energy storage as fat via central neuropeptide Y1 receptor (NPY1R), leading to obesity and metabolic disorders. The present study was expected to investigate the interaction between 2-AG (CB1R agonist), m-CPP (5HT2CR agonist), SB-242084 (5HT2CR antagonist), and SR-141716A (CB1R antagonist) by mediating through the NPY1R for treating or preventing obesity, metabolic disorders, and other abnormalities. The expression level of NPY1R mRNA has studied on the rat brain by real-time quantitative PCR assay. Based on our findings, intracerebroventricular (ICV) injection of combined 2-AG (1 μg) + m-CPP (2.5 μg) has antagonistic interaction in the expression of the NPY1R gene (P < 0.001). Moreover, the ICV co-injection of SB-242084 (3 μg) + SR-141716A (1 μg) has antagonistic interaction in the NPY1R gene expression (P < 0.001). Co-administration of 2-AG (1 μg) + SB-242084 (3 μg) amplified NPY1R gene expression (P < 0.001), while the ICV co-injection of m-CPP (2.5 μg) + SR-141716A (1 μg) decreased NPY1R gene expression in the hypothalamus (P < 0.001). These results revealed the interference in cannabinoid and serotonergic systems via CB1 and 5HT2C receptors in the expression of NPY1R mRNA in the hypothalamic area of rats.

Blockade of Serotonin 2C Receptors with SB-242084 Moderates Reduced Locomotor Activity and Rearing by Cannabinoid 1 Receptor Antagonist AM-251

The endocannabinoid and serotonin (5-HT) systems have key roles in the regulation of several physiological functions such as motor activity and food intake but also in the development of psychiatric disorders. Here we tested the hypothesis, whether blockade of serotonin 2C (5-HT2C) receptors prevents the reduced locomotor activity and other behavioral effects caused by a cannabinoid 1 (CB1) receptor antagonist. As a pretreatment, we administered SB-242084 (1 mg/kg, ip.), a 5-HT2C receptor antagonist or vehicle (VEH) followed by the treatment with AM-251 (5 or 10 mg/kg, ip.), a CB1 receptor antagonist or VEH. The effects of the two drugs alone or in co-administration were investigated in social interaction (SI) and elevated plus maze (EPM) tests in male Wistar rats. Our results show that AM-251 decreased the time spent with rearing in the SI test and decreased locomotor activity in EPM test. In contrast, SB-242084 produced increased locomotor activity in SI test and evoked anxiolytic-like effect in both SI and EPM tests. When applied the drugs in combination, these behavioral effects of AM-251 were moderated by SB-242084. Based on these findings, we conclude that certain unwanted behavioral effects of CB1 receptor antagonists could be prevented by pretreatment with 5-HT2C receptor antagonists.

New Findings on the Sensitivity of Free-Operant Timing Behaviour to 5-Hydroxytryptamine Receptor Stimulation

Timing performance maintained under the free-operant psychophysical procedure (FOPP) is sensitive to 5-hydroxytryptamine (5-HT)1A and 5-HT2A receptor stimulation. Agonists of these receptors displace the psychometric function towards shorter durations, reducing , the index of central tendency of timing. Here we report the effects of Ro-600175, a selective 5-HT2C receptor agonist, and mCPP, a 5-HT receptor agonist with high affinity for 5-HT2C receptors and lower affinity for 5-HT1A, 5-HT1B and 5-HT2A receptors, on timing behaviour. Rats were trained under the FOPP to press two levers (A and B) in 50-s trials in which reinforcers were provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic psychometric curves were fitted to the data from each rat under each treatment condition for derivation of timing indices [ (time corresponding to %B = 50%), Weber fraction]. The rats received systemic treatment with Ro-600175, mCPP, and mCPP in combination with antagonists of 5-HT1A (WAY-100635), 5-HT1B (isamoltane), 5-HT2A (MDL-100907) or 5-HT2C (SB-242084) receptors. mCPP (2.5 mg kg−1 i.p.), but not Ro-600175 (1, 2, 4 mg kg−1 i.p.), reduced . SB-242084 (0.6 mg kg−1 i.p.) potentiated mCPP’s effect on . mCPP’s effect on was not altered by isamoltane (8.0 mg kg−1 i.p.), but was attenuated by MDL-100907 (1.0 mg kg−1 i.p.) and WAY-100635 (0.1 mg kg−1 s.c.). The results suggest that mCPP’s effect on timing is mediated by an agonistic action at 5-HT1A and 5-HT2A, but not 5-HT1B, receptors. The role of 5-HT2C receptors is unclear, in view of SB-242084’s ability to potentiate the effect of mCPP, while Ro-600175 had no effect on . The possibility is considered that 5-HT2C receptors may counteract 5-HT1A and/or 5-HT2A receptor-mediated effects on timing performance.

Envolvimento de receptores 5-HT2C do hipocampo ventral em comportamentos de defesa de ratos no labirinto em cruz elevado

A ativação farmacológica dos receptores 5-HT2C induz comportamentos de defesa em modelos animais. O estudo busca investigar se o bloqueio seletivo de receptores 5-HT2C no hipocampo ventral (HV) previne comportamentos defensivos induzidos por um agonista de receptor 5-HT2C administrado perifericamente em ratos expostos ao labirinto em cruz elevado (LCE). Quinze minutos após injeções intraperitoniais (IP, 1ml/kg) do agonista 5-HT2C WAY-161503, ratos foram microinjetados bilateralmente no HV com o antagonista seletivo de receptores 5-HT2C SB-242084 (0, 0,1, 0,5 ou 1.5μg). Dez minutos após, cada animal foi exposto ao LCE para o registro de categorias de ansiedade. Injeções sistêmicas do WAY-161503 reduziram seletivamente as explorações nos braços abertos e aumentaram padrões de avaliação de risco. Esse efeito foi atenuado de maneira dose-dependente pela microinjeção de SB-242084 no HV, confirmando a ação ansiogênica de agonistas 5-HT2C e sugerindo que esse perfil comportamental seja mediado, pelo menos em parte, por receptores 5-HT2C do HV.