Somatodendritic targeting of M5 muscarinic receptor in the rat ventral tegmental area: implications for mesolimbic dopamine transmission

Recent Advances in the Development of Pyrimidine-based CNS Agents

BACKGROUND

In the past few decades, considerable progress has been made in CNS drug discovery, and various new CNS agents have been developed. Pyrimidine is an important scaffold in the area of medicinal chemistry. Recently, pyrimidine-containing compounds have been successfully designed as potent CNS agents. Substantial research has been carried out on pyrimidine-bearing compounds to treat different disorders of CNS in various animal models.

METHODS

Utilizing various databases, including Google Scholar, PubMed, Science Direct, and Web of Science, the literature review was conducted. The specifics of significant articles were discussed with an emphasis on the potency of pyrimidines derivatives possessing CNS activity.

RESULTS

Recent papers indicating pyrimidine derivatives with CNS activity were incorporated into the manuscript. (46) to (50) papers included different pyrimidine derivatives as 5-HT agonist/antagonists, (62) to (67) as adenosine agonist/antagonist, (70) to (75) as anticonvulsant agents, (80) to (83) as cannabinoid receptor agonists, (102) to (103) as nicotinic and (110) as muscarinic receptor agonists. The remaining papers (113) to (114) represented pyrimidine-based molecular imaging agents.

CONCLUSION

Pyrimidine and its derivatives have been studied in detail to evaluate their efficacy in overcoming multiple central nervous system disorders. The article covers the current updates on pyrimidine-based compounds as potent CNS and molecular imaging agents and will definitely provide a better platform for the development of potent pyrimidine-based CNS drugs in the near future.

Chronic escalating-dose and acute binge cocaine treatments change the hippocampal cholinergic muscarinic system on drug presence and after withdrawal

Cocaine addiction is a relapsing disorder with loss of control in limiting drug intake. Considering the involvement of acetylcholine in the neurobiology of the disease, our aim was to evaluate whether cocaine induces plastic changes in the hippocampal cholinergic muscarinic system. Male Swiss-Webster mice received saline or cocaine (ip) three times daily (60-min intervals) either acutely or in an escalating-dose binge paradigm for 14 days. Locomotor activity was measured in all treatment days. Dopaminergic and cholinergic muscarinic receptors (D₁R, D₂R, M₁-M_5, mAChRs), choline acetyltransferase (ChAT), vesicular acetylcholine transporter (VAChT) and acetylcholinesterase (AChE) were quantified in the hippocampus by immunoblotting one hour after the last injection (on drug) or after 14 days of abstinence (withdrawal). Escalating-dose group showed cocaine-induced locomotor sensitization from day 2. M₃ mAChR and ChAT significantly increased after the on-drug acute binge treatment. Escalating-dose on-drug group showed increased ChAT, M₁, M₅ mAChR and D₂R; and decreased D₁R. Acute-binge withdrawal group showed increased VAChT, M₂ mAChR, D₁R, and D₂R; and decreased M₁ mAChR. Escalating-dose withdrawal group presented increased D₁R and VAChT and decreased M₁ mAChR and D₂R. Locomotor activity was negatively correlated with M₁ mAChR and AChE in on-drug group and positively correlated with VAChT in withdrawal group. M₁ mAChR was positively correlated with M₂ mAChR and ChAT in on-drug group, whereas ChAT was positively correlated with M₅ mAChR in withdrawal group. The results indicate that cocaine induced an increase in the hippocampal cholinergic tone in the presence of the drug, whereas withdrawal causes a resetting in the system.

Development of : A Potent, Highly Selective, and Systemically Active Orthosteric Antagonist of the M5 Muscarinic Acetylcholine Receptor for the Treatment of Opioid Use Disorder

The muscarinic acetylcholine receptor (mAChR) subtype 5 (M₅) represents a novel potential target for the treatment of multiple addictive disorders, including opioid use disorder. Through chemical optimization of several functional high-throughput screening hits, VU6019650 (27b) was identified as a novel M₅ orthosteric antagonist with high potency (human M₅ IC₅₀ = 36 nM), M₅ subtype selectivity (>100-fold selectivity against human M_1-4) and favorable physicochemical properties for systemic dosing in preclinical addiction models. In acute brain slice electrophysiology studies, 27b blocked the nonselective muscarinic agonist oxotremorine-M-induced increases in neuronal firing rates of midbrain dopamine neurons in the ventral tegmental area, a part of the mesolimbic dopaminergic reward circuitry. Moreover, 27b also inhibited oxycodone self-administration in male Sprague-Dawley rats within a dose range that did not impair general motor output.

Functional Characterization of Cholinergic Receptors in Melanoma Cells

In the last two decades, the scientific community has come to terms with the importance of non-neural acetylcholine in light of its multiple biological and pathological functions within and outside the nervous system. Apart from its well-known physiological role both in the central and peripheral nervous systems, in the autonomic nervous system, and in the neuromuscular junction, the expression of the acetylcholine receptors has been detected in different peripheral organs. This evidence has contributed to highlight new roles for acetylcholine in various biological processes, (e.g., cell viability, proliferation, differentiation, migration, secretion). In addition, growing evidence in recent years has also demonstrated new roles for acetylcholine and its receptors in cancer, where they are involved in the modulation of cell proliferation, apoptosis, angiogenesis, and epithelial mesenchymal transition. In this review, we describe the functional characterization of acetylcholine receptors in different tumor types, placing attention on melanoma. The latest set of data accessible through literature, albeit limited, highlights how cholinergic receptors both of muscarinic and nicotinic type can play a relevant role in the migratory processes of melanoma cells, suggesting their possible involvement in invasion and metastasis.

Examining the role of muscarinic M5 receptors in VTA cholinergic modulation of depressive-like and anxiety-related behaviors in rats

Acetylcholine is implicated in mood disorders including depression and anxiety. Increased cholinergic tone in humans and rodents produces pro-depressive and anxiogenic-like effects. Cholinergic receptors in the ventral tegmental area (VTA) are known to mediate these responses in male rats, as measured by the sucrose preference test (SPT), elevated plus maze (EPM), and the forced swim test (FST). However, these effects have not been examined in females, and the VTA muscarinic receptor subtype(s) mediating the pro-depressive and anxiogenic-like behavioral effects of increased cholinergic tone are unknown. We first examined the behavioral effects of increased VTA cholinergic tone in male and female rats, and then determined whether VTA muscarinic M5 receptors were mediating these effects. VTA infusion of the acetylcholinesterase inhibitor physostigmine (0.5 μg, 1 μg and 2 μg/side) in males and females produced anhedonic-like, anxiogenic, pro-depressive-like responses on the SPT, EPM, and FST. In females, VTA administration of the muscarinic M5 selective negative allosteric modulator VU6000181 (0.68 ng, 2.3 ng, 6.8 ng/side for a 3 μM, 10 μM, 30 μM/side infusion) did not alter SPT, EPM nor FST behavior. However, in males intra-VTA infusion of VU6000181 alone reduced time spent immobile on the FST. Furthermore, co-infusion of VU6000181 with physostigmine, in male and female rats, attenuated the pro-depressive and anxiogenic-like behavioral responses induced by VTA physostigmine alone, in the SPT, EPM, and FST. Together, these data reveal a critical role of VTA M5 receptors in mediating the anhedonic, anxiogenic, and depressive-like behavioral effects of increased cholinergic tone in the VTA.

Acetylcholine receptors: Key players in cancer development

Acetylcholine (ACh) was first identified as a classic neuromodulator and transmit signals through two subgroups of receptors, namely muscarinic receptors (mAChRs) and nicotinic receptors (nAChRs). Apart from its well-established physiological role in central nervous system (CNS) and peripheral nervous system (PNS), autonomic nervous system and neuromuscular junction, the widely distributed expression of AChRs in different human organs suggests roles in other biological processes in addition to synaptic transmission. Accumulating evidence revealed that cancer cell processes such as proliferation, apoptosis, angiogenesis and even epithelial-mesenchymal transition (EMT) are mediated by overexpression of AChRs in different kinds of tumors. In breast cancer, α7-nAChR and α9-nAChR were reported to be oncogenic. On the other hand, research on the role of mAChRs in breast cancer tumorgenesis is limited and confined to M3 receptor only. Since AChRs distributed in both CNS and PNS even non-neuronal tissues, there is an urgent need for the development of subtype-specific AChR antagonist which inhibits cancer cell progression with minimal intervention on the normal acetylcholine-regulated system within human body.

Selective allosteric modulation of muscarinic acetylcholine receptors for the treatment of schizophrenia and substance use disorders

Muscarinic acetylcholine receptor (mAChRs) subtypes represent exciting new targets for the treatment of schizophrenia and substance use disorder (SUD). Recent advances in the development of subtype-selective allosteric modulators have revealed promising effects in preclinical models targeting the different symptoms observed in schizophrenia and SUD. M₁ PAMs display potential for addressing the negative and cognitive symptoms of schizophrenia, while M₄ PAMs exhibit promise in treating preclinical models predictive of antipsychotic-like activity. In SUD, there is increasing support for modulation of mesocorticolimbic dopaminergic circuitry involved in SUD with selective M₄ mAChR PAMs or M₅ mAChR NAMs. Allosteric modulators of these mAChR subtypes have demonstrated efficacy in rodent models of cocaine and ethanol seeking, with indications that these ligand may also be useful for other substances of abuse, as well as in various stages in the cycle of addiction. Importantly, allosteric modulators of the different mAChR subtypes may provide viable treatment options, while conferring greater subtype specificity and corresponding enhanced therapeutic index than orthosteric muscarinic ligands and maintaining endogenous temporo-spatial ACh signaling. Overall, subtype specific mAChR allosteric modulators represent important novel therapeutic mechanisms for schizophrenia and SUD.

Cholinergic Receptor Blockade in the VTA Attenuates Cue-Induced Cocaine-Seeking and Reverses the Anxiogenic Effects of Forced Abstinence

Drug relapse after periods of abstinence is a common feature of substance abuse. Moreover, anxiety and other mood disorders are often co-morbid with substance abuse. Cholinergic receptors in the ventral tegmental area (VTA) are known to mediate drug-seeking and anxiety-related behavior in rodent models. However, it is unclear if overlapping VTA cholinergic mechanisms mediate drug relapse and anxiety-related behaviors associated with drug abstinence. We examined the effects of VTA cholinergic receptor blockade on cue-induced cocaine seeking and anxiety during cocaine abstinence. Male Sprague-Dawley rats were trained to self-administer intravenous cocaine (~0.5 mg/kg/infusion, FR1 schedule) for 10 days, followed by 14 days of forced abstinence. VTA infusion of the non-selective nicotinic acetylcholine receptor antagonist mecamylamine (0, 10, and 30 μg/side) or the non-selective muscarinic receptor antagonist scopolamine (0, 2.4 and 24 μg /side) significantly decreased cue-induced cocaine seeking. In cocaine naïve rats, VTA mecamylamine or scopolamine also led to dose-dependent increases in open arm time in the elevated plus maze (EPM). In contrast, rats that received I.V. cocaine, compared to received I.V. saline rats, displayed an anxiogenic response on day 14 of abstinence as reflected by decreased open arm time in the EPM. Furthermore, low doses of VTA mecamylamine (10 μg /side) or scopolamine (2.4 μg /side), that did not alter EPM behavior in cocaine naive rats, were sufficient to reverse the anxiogenic effects of cocaine abstinence. Together, these data point to an overlapping role of VTA cholinergic mechanisms to regulate relapse and mood disorder-related responses during cocaine abstinence.

NMDA Receptor-Dependent Cholinergic Modulation of Mesolimbic Dopamine Cell Bodies: Neurochemical and Behavioral Studies

Substance abuse disorders are devastating, costly, and difficult to treat. Identifying the neurochemical mechanisms underlying reinforcement promises to provide critical information in the development of effective treatments. Several lines of evidence suggest that striatal dopamine (DA) release serves as a teaching signal in reinforcement learning, and that shifts in DA release from the primary reward to reward-predicting stimuli play a critical role in the self-administration of both natural and non-natural rewards. However, far less is known about the reinforcing effects of motivationally neutral sensory stimuli, or how these signals can facilitate self-administration behavior. Thus, we trained rats ( n = 7) to perform a visual stimulus-induced instrumental task, which involved lever pressing for activation of a stimulus light. We then microinfused vehicle (phosphate buffered saline), carbachol (acetylcholine receptor agonist), or carbachol in the presence of an N-methyl-d-aspartate (NMDA) receptor-specific drug (NMDA itself, or the antagonist, AP5) into the ventral tegmental area (VTA). This enabled us to directly evaluate how chemical modulation of dopamine cell bodies affects the instrumental behavior, as well as the nature of extracellular dopamine transients recorded in the nucleus accumbens shell (NAc shell) using fast-scan cyclic voltammetry (FSCV). Intra-VTA infusion of carbachol enhanced the magnitude and frequency of dopamine transients in the NAc shell and potentiated active lever responding without altering inactive lever responding, as compared to infusion of vehicle. Coinfusion of carbachol with AP5 abolished dopamine transients recorded in the NAc and attenuated active lever responding without altering inactive lever responding. Finally, coadministration of carbachol and NMDA into the VTA restored both lever pressing and dopaminergic signals recorded in the striatum. Together, these results suggest that acetylcholine and glutamate synergistically act at dopamine cells in the VTA to modulate VTA-NAc shell dopaminergic output, and this underlies motivation to lever press for a motivationally neutral visual stimulus.

Physiological roles of CNS muscarinic receptors gained from knockout mice

Because the five muscarinic acetylcholine receptor subtypes have overlapping distributions in many CNS tissues, and because ligands with a high degree of selectivity for a given subtype long remained elusive, it has been difficult to determine the physiological functions of each receptor. Genetically engineered knockout mice, in which one or more muscarinic acetylcholine receptor subtype has been inactivated, have been instrumental in identifying muscarinic receptor functions in the CNS, at the neuronal, circuit, and behavioral level. These studies revealed important functions of muscarinic receptors modulating neuronal activity and neurotransmitter release in many brain regions, shaping neuronal plasticity, and affecting functions ranging from motor and sensory function to cognitive processes. As gene targeting technology evolves including the use of conditional, cell type specific strains, knockout mice are likely to continue to provide valuable insights into brain physiology and pathophysiology, and advance the development of new medications for a range of conditions such as Alzheimer's disease, Parkinson's disease, schizophrenia, and addictions, as well as non-opioid analgesics. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Segregated cholinergic transmission modulates dopamine neurons integrated in distinct functional circuits

Dopamine neurons in the ventral tegmental area (VTA) receive cholinergic innervation from brainstem structures that are associated with either movement or reward. Whereas cholinergic neurons of the pedunculopontine nucleus (PPN) carry an associative/motor signal, those of the laterodorsal tegmental nucleus (LDT) convey limbic information. We used optogenetics and in vivo juxtacellular recording and labeling to examine the influence of brainstem cholinergic innervation of distinct neuronal subpopulations in the VTA. We found that LDT cholinergic axons selectively enhanced the bursting activity of mesolimbic dopamine neurons that were excited by aversive stimulation. In contrast, PPN cholinergic axons activated and changed the discharge properties of VTA neurons that were integrated in distinct functional circuits and were inhibited by aversive stimulation. Although both structures conveyed a reinforcing signal, they had opposite roles in locomotion. Our results demonstrate that two modes of cholinergic transmission operate in the VTA and segregate the neurons involved in different reward circuits.

Electron microscopic localization of M2-muscarinic receptors in cholinergic and noncholinergic neurons of the laterodorsal tegmental and pedunculopontine nuclei of the rat mesopontine tegmentum

Muscarinic m2 receptors (M2Rs) are implicated in autoregulatory control of cholinergic output neurons located within the pedunculopontine (PPT) and laterodorsal tegmental (LTD) nuclei of the mesopontine tegmentum (MPT). However, these nuclei contain many noncholinergic neurons in which activation of M2R heteroceptors may contribute significantly to the decisive role of the LTD and PPT in sleep-wakefulness. We examined the electron microscopic dual immunolabeling of M2Rs and the vesicular acetylcholine transporter (VAchT) in the MPT of rat brain to identify the potential sites for M2R activation. M2R immunogold labeling was predominately seen in somatodendritic profiles throughout the PPT/LTD complex. In somata, M2R immunogold particles were often associated with Golgi lamellae and cytoplasmic endomembrannes, but were rarely in contact with the plasma membrane, as was commonly seen in dendrites. Approximately 36% of the M2R-labeled somata and 16% of the more numerous M2R-labeled dendrites coexpressed VAchT. M2R and M2R/VAchT-labeled dendritic profiles received synapses from inhibitory- and excitatory-type axon terminals, over 88% of which were unlabeled and others contained exclusively M2R or VAchT immunoreactivity. In axonal profiles M2R immunogold was localized to plasmalemmal and cytoplasmic regions and showed a similar distribution in many VAchT-negative glial profiles. These results provide ultrastructural evidence suggestive of somatic endomembrane trafficking of M2Rs, whose activation serves to regulate the postsynaptic excitatory and inhibitory responses in dendrites of cholinergic and noncholinergic neurons in the MPT. They also suggest the possibility that M2Rs in this brain region mediate the effects of acetylcholine on the release of other neurotransmitters and on glial signaling. J. Comp. Neurol. 524:3084-3103, 2016. © 2016 Wiley Periodicals, Inc.