The Pharmacological Characterization of Novel Melatonin Receptor Type-selective Ligands

• Main Author: Lipinski, Jennie V
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2019
• Subjects: Neurosciences; Pharmacology
• ISBN: 9781687924148; 1687924147

The hormone melatonin is recognized primarily for its role in the regulation of circadian rhythms and sleep-wake cycles, responsible for relaying the signal of darkness. Synthesized and secreted in the pineal gland, melatonin binds to and exerts its effects via two high-affinity GPCRs, MT1 and MT2. The melatonin receptor types exhibit distinct pharmacological profiles, structures and chromosomal localization. The known competitive yet nonselective MT1/MT2 melatonin receptor antagonist, luzindole, has been shown to induce antidepressant-like efficacy in rodent models of behavioral despair through actions at the MT2. Seemingly via the MT1, luzindole has demonstrated effects on the circadian system, significantly delaying the rate of re-entrainment in C3H mouse wheel-running activity following an abrupt advance of dark onset. With minimal discovery of highly specific and selective melatonin receptor ligands, we propose the characterization of newly synthesized, novel melatonin analogues that hold a minimum of 50-fold selectivity for either the MT1 or MT2, with pharmacological profiles comparable to luzindole. Via standard assessment of structure-activity relationships, in vitro competition binding, quantitative receptor autoradiography, and behavioral tests quantifying antidepressant-like efficacy and modulation of circadian rhythms, our goal was to mimic the distinct receptor-mediated behaviors produced by luzindole, with higher efficacy by selectivity. We report the following novel analogues: a highly selective MT2 ligand and prospective antagonist, ATBT-23 (KiMT1/MT2 ratio: 189.8), two type-preferring MT1 inverse agonists, N32 (KiMT2/MT1 ratio: 4.2) and N30#2 (KiMT2/MT1 ratio: 4.8), and a selective MT2 agonist, N34 (KiMT1/MT2 ratio: 54), which compete for 2-[125I]-iodomelatonin binding at hMT1 and hMT2 receptors expressed in CHO cells and mimic individual properties encompassed by luzindole. N32 (KiSCN: 2.27 ± 0.78 µM, KiPT: 6.69 ± 3.4 µM), N34 (KiSCN: 5.86 ± 2.9 µM, KiPT: 7.62 ± 4.1 µM), and N30#2 (KiSCN: 4.71 ± 3.0 µM, KiPT: 4.16 ± 1.8 µM) displayed affinity at melatonin receptors expressed in the C3H/HeN mouse suprachiasmatic nucleus (SCN) and pars tuberalis (PT) brain regions, as determined by in vitro competition for 2-[125I]-iodomelatonin binding. In the forced swim test, MT2- selective test compounds N34 and ATBT-23 at 10 mg/kg i.p. did not exert an overall antidepressant-like effect in C3H wildtype, MT1KO, or MT2KO mice in comparison to vehicle treatment. In the re-entrainment paradigm, the MT1/MT2 antagonist, MT1 weak inverse agonist ATBT-23 at 1 mg/kg s.c. (10.9 ± 0.85 days) produced a deceleration of re-entrainment to a new light-dark cycle following a 6-hour dark advance in male C3H WT mice, when compared to vehicle (7.13 ± 0.35 days). Further emphasis placed on the innovation and characterization of high-fold, selective melatonin receptor ligands could result in the identification of new therapeutic treatments, with potential in depressed populations and those with desynchronized rhythms.