Selective potentiation of (α4)3(β2)2 nicotinic acetylcholine receptors augments amplitudes of prefrontal acetylcholine- and nicotine-evoked glutamatergic transients in rats

Levo-tetrahydropalmatine inhibits α4β2 nicotinic receptor response to nicotine in cultured SH-EP1 cells

Nicotine, a major component of tobacco, is highly addictive and acts on nicotinic acetylcholine receptors (nAChRs) to stimulate reward-associated circuits in the brain. It is well known that nAChRs play critical roles in mediating nicotine reward and addiction. Current FDA-approved medications for smoking cessation are the antidepressant bupropion and the nicotinic partial agonist varenicline, yet both are limited by adverse side effects and moderate efficacy. Thus, development of more efficacious medications with fewer side effects for nicotine addiction and smoking cessation is urgently needed. l-Tetrahydropalmatine (l-THP) is an active ingredient of the Chinese medicinal herb Corydalis ambigua that possesses rich neuropharmacological actions on dopamine (DA) receptors in the mesocorticolimbic dopaminergic reward pathway. L-THP has been explored as anti-addiction treatments for drug abuse including nicotine. However, the targets and mechanisms of l-THP-caused anti-nicotine effects are largely unknown. In this study we address this question by elucidating the effects of l-THP on human neuronal nAChRs using patch-clamp recordings. Human neuronal α4β2-nAChRs were heterologously expressed in SH-EP1 human epithelial cells. Bath application of nicotine (0.1-100 μM) induced inward currents, co-application of l-THP (3 μM) inhibited nicotine-induced currents in the transfected cells. L-THP-caused inhibition was concentration-dependent (the EC₅₀ values for inhibiting the peak and steady-state current were 18 and 2.1 μM, respectively) and non-competitive. Kinetic analysis of the whole-cell currents showed that l-THP slowed rising time and accelerated decay time constants. L-THP specifically modulated α4β2-nAChRs, as it did not affect α7-nAChRs or α1*-nAChRs (muscle type). Interestingly, two putative α4β2-nAChR isoforms, namely sazetidine A-activated, high-sensitive one (α4₂β2₃-nAChR) and cytisine-activated, low-sensitive one (α4₃β2₂-nAChR) were pharmacologically separated, and the low-sensitive one was more susceptible to l-THP inhibition than the high-sensitive one. In conclusion, we demonstrate that l-THP blocks neuronal α4β2-nAChR function, which may underlie its inhibition on nicotine addiction.

The retinoid X receptor: a nuclear receptor that modulates the sleep-wake cycle in rats

RATIONALE

The nuclear receptor retinoid X receptor (RXR) belongs to a nuclear receptor superfamily that modulates diverse functions via homodimerization with itself or several other nuclear receptors, including PPARα. While the activation of PPARα by natural or synthetic agonists regulates the sleep-wake cycle, the role of RXR in the sleep modulation is unknown.

OBJECTIVES

We investigated the effects of bexarotene (Bexa, a RXR agonist) or UVI 3003 (UVI, a RXR antagonist) on sleep, sleep homeostasis, levels of neurochemical related to sleep modulation, and c-Fos and NeuN expression.

METHODS

The sleep-wake cycle and sleep homeostasis were analyzed after application of Bexa or UVI. Moreover, we also evaluated whether Bexa or UVI could induce effects on dopamine, serotonin, norepinephrine epinephrine, adenosine, and acetylcholine contents, collected from either the nucleus accumbens or basal forebrain. In addition, c-Fos and NeuN expression in the hypothalamus was determined after Bexa or UVI treatments.

RESULTS

Systemic application of Bexa (1 mM, i.p.) attenuated slow-wave sleep and rapid eye movement sleep. In addition, Bexa increased the levels of dopamine, serotonin, norepinephrine epinephrine, adenosine, and acetylcholine sampled from either the nucleus accumbens or basal forebrain. Moreover, Bexa blocked the sleep rebound period after total sleep deprivation, increased in the hypothalamus the expression of c-Fos, and decreased NeuN activity. Remarkably, UVI 3003 (1 mM, i.p.) induced opposite effects in sleep, sleep homeostasis, neurochemicals levels, and c-Fos and NeuN activity.

CONCLUSIONS

The administration of RXR agonist or antagonist significantly impaired the sleep-wake cycle and exerted effects on the levels of neurochemicals related to sleep modulation. Moreover, Bexa or UVI administration significantly affected c-Fos and NeuN expression in the hypothalamus. Our findings highlight the neurobiological role of RXR on sleep modulation.

Desformylflustrabromine, a positive allosteric modulator of α4β2-containing nicotinic acetylcholine receptors, enhances cognition in rats

Rationale

The α4β2 nicotinic acetylcholine receptors (α4β2-nAChRs) may represent useful targets for cognitive improvement. It has been recently proposed that a strategy based on positive allosteric modulation of α4β2-nAChRs reveals several advantages over the direct agonist approach. Nevertheless, the procognitive effects of α4β2-nAChR positive allosteric modulators (PAMs) have not been extensively characterized.

Objectives

The aim of the present study was to evaluate the procognitive efficacy of desformylflustrabromine (dFBr), a selective α4β2-nAChR PAM.

Methods

Cognitive effects were investigated in the novel object recognition task (NORT) and the attentional set-shifting task (ASST) in rats.

Results

The results demonstrate that dFBr attenuated the delay-induced impairment in NORT performance and facilitated cognitive flexibility in the ASST. The beneficial effects of dFBr were inhibited by dihydro-β-erythroidine, a relatively selective α4β2-nAChR antagonist, indicating the involvement of α4β2-nAChRs in cognitive processes. The tested α4β2-PAM was also effective against ketamine- and scopolamine-induced deficits of object recognition memory. Moreover, procognitive effects were also observed after combined treatment with inactive doses of dFBr and TC-2403, a selective α4β2-nAChR agonist.

Conclusions

These findings indicate that dFBr presents procognitive activity, supporting the strategy based on α4β2-nAChR potentiation as a plausible therapy for cognitive impairment.

Electronic supplementary material

The online version of this article (10.1007/s43440-020-00092-4) contains supplementary material, which is available to authorized users.

Sleep and Neurochemical Modulation by DZNep and GSK-J1: Potential Link With Histone Methylation Status

Histone methylation/demethylation plays an important modulatory role in chromatin restructuring, RNA transcription and is essential for controlling a plethora of biological processes. Due to many human diseases have been related to histone methylation/demethylation, several compounds such as 3-deazaneplanocin A (DZNep) or 3-((6-(4,5-Dihydro-1H-benzo[d]azepin-3(2H)-yl)-2-(pyridin-2-yl)pyrimidin-4-yl)amino)propanoic acid; N-[2-(2-pyridinyl)-6-(1,2,4,5-tetrahydro-3H-3-benzazepin-3-yl)-4-pyrimidinyl]-β-Alanine (GSK-J1), have been designed to inhibit histone methylase or suppress histone demethylase, respectively. In the present study, we investigated the effects on the sleep-wake cycle and sleep-related neurochemical levels after systemic injections of DZNep or GSK-J1 given during the light or dark phase in rats. DZNep dose-dependently (0.1, 1.0, or 10 mg/kg, i.p.) prolonged wakefulness (W) duration while decreased slow wave sleep (SWS) and rapid eye movement sleep (REMS) time spent during the lights-on period with no changes observed in dark phase. In opposite direction, GSK-J1 (0.1, 1.0, or 10 mg/kg, i.p.) injected at the beginning of the lights-on period induced no statistical changes in W, SWS, or REMS whereas if administered at darkness, we found a diminution in W and an enhancement in SWS and REMS. Finally, brain microdialysis experiments in freely moving animals were used to evaluate the effects of DZNep or GSK-J1 treatments on contents of sleep-related neurochemicals. The results showed that DZNep boosted extracellular levels of dopamine, norepinephrine, epinephrine, serotonin, adenosine, and acetylcholine if injected at the beginning of the lights-on period whereas GSK-J1 exerted similar outcomes but when administered at darkness. In summary, DZNep and GSK-J1 may control the sleep-wake cycle and sleep-related neurochemicals through histone methylation/demethylation activity.

Distinctive single-channel properties of α4β2-nicotinic acetylcholine receptor isoforms

Central nervous system nicotinic acetylcholine receptors (nAChR) are predominantly of the α4β2 subtype. Two isoforms exist, with high or low agonist sensitivity (HS-(α4β2)2β2- and LS-(α4β2)2α4-nAChR). Both isoforms exhibit similar macroscopic potency and efficacy values at low acetylcholine (ACh) concentrations, mediated by a common pair of high-affinity α4(+)/(-)β2 subunit binding interfaces. However LS-(α4β2)2α4-nAChR also respond to higher concentrations of ACh, acting at a third α4(+)/(-)α4 subunit interface. To probe isoform functional differences further, HS- and LS-α4β2-nAChR were expressed in Xenopus laevis oocytes and single-channel responses were assessed using cell-attached patch-clamp. In the presence of a low ACh concentration, both isoforms produce low-bursting function. HS-(α4β2)2β2-nAChR exhibit a single conductance state, whereas LS-(α4β2)2α4-nAChR display two distinctive conductance states. A higher ACh concentration did not preferentially recruit either conductance state, but did result in increased LS-(α4β2)2α4-nAChR bursting and reduced closed times. Introduction of an α4(+)/(-)α4-interface loss-of-function α4W182A mutation abolished these changes, confirming this site's role in mediating LS-(α4β2)2α4-nAChR responses. Small or large amplitude openings are highly-correlated within individual LS-(α4β2)2α4-nAChR bursts, suggesting that they arise from distinct intermediate states, each of which is stabilized by α4(+)/(-)α4 site ACh binding. These findings are consistent with α4(+)/(-)α4 subunit interface occupation resulting in allosteric potentiation of agonist actions at α4(+)/(-)β2 subunit interfaces, rather than independent induction of high conductance channel openings.

Systemic Injections of Cannabidiol Enhance Acetylcholine Levels from Basal Forebrain in Rats

Cannabis sativa is a plant that contains more than 500 components, of which the most studied are Δ⁹-tetrahydrocannabinol (Δ⁹-THC) and cannabidiol (CBD). Several studies have indicated that CBD displays neurobiological effects, including wake promotion. Moreover, experimental evidence has shown that injections of CBD enhance wake-related compounds, such as monoamines (dopamine, serotonin, epinephrine, and norepinephrine). However, no clear evidence is available regarding the effects of CBD on additional wake-related neurochemicals such as acetylcholine (ACh). Here, we demonstrate that systemic injections of CBD (0, 5, 10 or 30 mg/kg, i.p.) at the beginning of the lights-on period, increase the extracellular levels of ACh collected from the basal forebrain and measured by microdialysis and HPLC means. Moreover, the time course effects on the contents of ACh were present 5 h post-injection of CBD. Altogether, these data demonstrate that CBD increases ACh levels in a brain region related to wake control. This study is the first to show the effects of ACh levels in CBD-treated rats and suggests that the basal forebrain might be a site of action of CBD for wakefulness modulation.

Designing selective modulators for the nicotinic receptor subtypes: challenges and opportunities

Nicotinic receptors are membrane proteins involved in several physiological processes. They are considered suitable drug targets for various CNS disorders or conditions, as shown by the large number of compounds which have entered clinical trials. In recent years, nonconventional agonists have been discovered: positive allosteric modulators, allosteric agonists, site-specific agonists and silent desensitizers are compounds able to modulate the receptor interacting at sites different from the orthodox one, or to desensitize the receptor without prior opening. While these new findings can further complicate the pharmacology of these proteins and the design and optimization of ligands, they undoubtedly offer new opportunities to find drugs for the many therapeutic indications involving nicotinic receptors.

Role of N-Arachidonoyl-Serotonin (AA-5-HT) in Sleep-Wake Cycle Architecture, Sleep Homeostasis, and Neurotransmitters Regulation

The endocannabinoid system comprises several molecular entities such as endogenous ligands [anandamide (AEA) and 2-arachidonoylglycerol (2-AG)], receptors (CB₁ and CB₂), enzymes such as [fatty acid amide hydrolase (FAHH) and monoacylglycerol lipase (MAGL)], as well as the anandamide membrane transporter. Although the role of this complex neurobiological system in the sleep–wake cycle modulation has been studied, the contribution of the blocker of FAAH/transient receptor potential cation channel subfamily V member 1 (TRPV1), N-arachidonoyl-serotonin (AA-5-HT) in sleep has not been investigated. Thus, in the present study, varying doses of AA-5-HT (5, 10, or 20 mg/Kg, i.p.) injected at the beginning of the lights-on period of rats, caused no statistical changes in sleep patterns. However, similar pharmacological treatment given to animals at the beginning of the dark period decreased wakefulness (W) and increased slow wave sleep (SWS) as well as rapid eye movement sleep (REMS). Power spectra analysis of states of vigilance showed that injection of AA-5-HT during the lights-off period diminished alpha spectrum across alertness in a dose-dependent fashion. In opposition, delta power spectra was enhanced as well as theta spectrum, during SWS and REMS, respectively. Moreover, the highest dose of AA-5-HT decreased wake-related contents of neurotransmitters such as dopamine (DA), norepinephrine (NE), epinephrine (EP), serotonin (5-HT) whereas the levels of adenosine (AD) were enhanced. In addition, the sleep-inducing properties of AA-5-HT were confirmed since this compound blocked the increase in W caused by stimulants such as cannabidiol (CBD) or modafinil (MOD) during the lights-on period. Additionally, administration of AA-5-HT also prevented the enhancement in contents of DA, NE, EP, 5-HT and AD after CBD of MOD injection. Lastly, the role of AA-5-HT in sleep homeostasis was tested in animals that received either CBD or MOD after total sleep deprivation (TSD). The injection of CBD or MOD increased alertness during sleep rebound period after TSD. However, AA-5-HT blocked this effect by allowing animals to display an enhancement in sleep across sleep rebound period. Overall, our findings provide evidence that AA-5-HT is an important modulator of sleep, sleep homeostasis and neurotransmitter contents.

Attenuation of nicotine taking and seeking in rats by the stoichiometry-selective alpha4beta2 nicotinic acetylcholine receptor positive allosteric modulator NS9283

RATIONALE

The rewarding and reinforcing effects of nicotine are produced, in large part, by activation of neuronal α4β2* nicotinic acetylcholine receptors (nAChRs), pentameric protein complexes comprised of different stoichiometries of α4 and β2 subunits. However, little is known about the functional role of distinct subtypes of α4β2* nAChRs in nicotine addiction.

OBJECTIVES

NS9283 represents a new class of stoichiometry-selective positive allosteric modulators (PAMs) that selectively bind to α4β2 nAChRs containing three α4 and two β2 subunits (3(α4)2(β2) nAChRs). The present experiments were designed to determine the effects of NS9283 on nicotine self-administration and the reinstatement of nicotine-seeking behavior, an animal model of smoking relapse. Parallel studies of sucrose self-administration and reinstatement were conducted in separate cohorts of rats to determine if the effects of NS9283 generalized to other reinforced behaviors.

RESULTS

Acute and repeated administration of NS9283 dose-dependently reduced nicotine self-administration and reinstatement in male Sprague Dawley rats. These effects were reinforcer specific as no effects of NS9283 on sucrose self-administration and reinstatement were noted. NS9283 also failed to substitute for nicotine in supporting self-administration behavior suggesting that, at the doses tested, NS9283 alone is not reinforcing.

CONCLUSION

Taken together, these results provide compelling evidence that stoichiometry-selective PAMs of 3(α4)2(β2) nAChRs attenuate nicotine taking and seeking in rats and suggest that targeting 3(α4)2(β2) nAChRs may represent a promising therapeutic strategy for preventing smoking relapse.

Analysis of neuronal nicotinic acetylcholine receptor α4β2 activation at the single-channel level

The neuronal nicotinic acetylcholine receptor α4β2 forms pentameric proteins with two alternate stoichiometries. The high-sensitivity receptor is related to (α4)2(β2)3 stoichiometry while the low-sensitivity receptor to (α4)3(β2)2 stoichiometry. Both subtypes share two binding sites at the α4((+))/β2((-)) interface with high affinity for agonists. (α4)3(β2)2 has an additional binding site at the α4((+))/α4((-)) interface with low affinity for agonists. We investigated activation kinetics of both receptor subtypes by patch-clamp recordings of single-channel activity in the presence of several concentrations of acetylcholine (0.5 to 300μM). We used kinetic software to fit these data with kinetic models. We found that the high-sensitivity subtype correlates with the low-conductance channel (g-70=29pS) and does not activate with high efficacy. On the contrary, the low-sensitivity subtype correlated with a high-conductance channel (g-70=44pS) and exhibited higher activation efficacy. Opening events of individual nAChRs at high agonist concentrations occurred in clusters, which allowed us to determine kinetic constants for the activation of the triliganded receptor. Our kinetic modeling identified an intermediate state, between resting and open conformation of the receptor. Binding of the third molecule increases the efficacy of receptor activation by favoring the transition between resting and intermediate state around 18 times. The low rate for this transition in the diliganded receptor explains the action of acetylcholine as partial agonist when it binds to the high-affinity sites. The presence of the third binding site emerges as a potent modulator of nicotinic receptor α4β2 activation which may display different functions depending on agonist concentration.

Methylomic analysis of salivary DNA in childhood ADHD identifies altered DNA methylation in VIPR2

BACKGROUND

Peripheral epigenetic marks hold promise for understanding psychiatric illness and may represent fingerprints of gene-environment interactions. We conducted an initial examination of CpG methylation variation in children with or without attention-deficit/hyperactivity disorder (ADHD).

METHODS

Children age 7-12 were recruited, screened, evaluated and assigned to ADHD or non-ADHD groups by defined research criteria. Two independent age-matched samples were examined, a discovery set (n = 92, all boys, half control, half ADHD) and a confirmation set (n = 20, half ADHD, all boys). 5-methylcytosine levels were quantified in salivary DNA using the Illumina 450 K HumanMethylation array. Genes for which multiple probes were nominally significant and had a beta difference of at least 2% were evaluated for biological relevance and prioritized for confirmation and sequence validation. Gene pathways were explored and described.

RESULTS

Two genes met the criteria for confirmation testing, VIPR2 and MYT1L; both had multiple probes meeting cutoffs and strong biological relevance. Probes on VIPR2 passed FDR correction in the confirmation set and were confirmed through bisulfite sequencing. Enrichment analysis suggested involvement of gene sets or pathways related to inflammatory processes and modulation of monoamine and cholinergic neurotransmission.

CONCLUSIONS

Although it is unknown to what extent CpG methylation seen in peripheral tissue reflect transcriptomic changes in the brain, these initial results indicate that peripheral DNA methylation markers in ADHD may be promising and suggest targeted hypotheses for future study in larger samples.

Differential α4(+)/(-)β2 Agonist-binding Site Contributions to α4β2 Nicotinic Acetylcholine Receptor Function within and between Isoforms

Two α4β2 nicotinic acetylcholine receptor (α4β2-nAChR) isoforms exist with (α4)2(β2)3 and (α4)3(β2)2 subunit stoichiometries and high versus low agonist sensitivities (HS and LS), respectively. Both isoforms contain a pair of α4(+)/(-)β2 agonist-binding sites. The LS isoform also contains a unique α4(+)/(-)α4 site with lower agonist affinity than the α4(+)/(-)β2 sites. However, the relative roles of the conserved α4(+)/(-)β2 agonist-binding sites in and between the isoforms have not been studied. We used a fully linked subunit concatemeric nAChR approach to express pure populations of HS or LS isoform α4β2*-nAChR. This approach also allowed us to mutate individual subunit interfaces, or combinations thereof, on each isoform background. We used this approach to systematically mutate a triplet of β2 subunit (-)-face E-loop residues to their non-conserved α4 subunit counterparts or vice versa (β2HQT and α4VFL, respectively). Mutant-nAChR constructs (and unmodified controls) were expressed in Xenopus oocytes. Acetylcholine concentration-response curves and maximum function were measured using two-electrode voltage clamp electrophysiology. Surface expression was measured with (125)I-mAb 295 binding and was used to define function/nAChR. If the α4(+)/(-)β2 sites contribute equally to function, making identical β2HQT substitutions at either site should produce similar functional outcomes. Instead, highly differential outcomes within the HS isoform, and between the two isoforms, were observed. In contrast, α4VFL mutation effects were very similar in all positions of both isoforms. Our results indicate that the identity of subunits neighboring the otherwise equivalent α4(+)/(-)β2 agonist sites modifies their contributions to nAChR activation and that E-loop residues are an important contributor to this neighbor effect.

An Accessory Agonist Binding Site Promotes Activation of α4β2* Nicotinic Acetylcholine Receptors

Neuronal nicotinic acetylcholine receptors containing α4, β2, and sometimes other subunits (α4β2* nAChRs) regulate addictive and other behavioral effects of nicotine. These nAChRs exist in several stoichiometries, typically with two high affinity acetylcholine (ACh) binding sites at the interface of α4 and β2 subunits and a fifth accessory subunit. A third low affinity ACh binding site is formed when this accessory subunit is α4 but not if it is β2. Agonists selective for the accessory ACh site, such as 3-[3-(3-pyridyl)-1,2,4-oxadiazol-5-yl]benzonitrile (NS9283), cannot alone activate a nAChR but can facilitate more efficient activation in combination with agonists at the canonical α4β2 sites. We therefore suggest categorizing agonists according to their site selectivity. NS9283 binds to the accessory ACh binding site; thus it is termed an accessory site-selective agonist. We expressed (α4β2)2 concatamers in Xenopus oocytes with free accessory subunits to obtain defined nAChR stoichiometries and α4/accessory subunit interfaces. We show that α2, α3, α4, and α6 accessory subunits can form binding sites for ACh and NS9283 at interfaces with α4 subunits, but β2 and β4 accessory subunits cannot. To permit selective blockage of the accessory site, α4 threonine 126 located on the minus side of α4 that contributes to the accessory site, but not the α4β2 sites, was mutated to cysteine. Alkylation of this cysteine with a thioreactive reagent blocked activity of ACh and NS9283 at the accessory site. Accessory agonist binding sites are promising drug targets.

Neuropharmacological modulation of the P3-like event-related potential in a rat two-tone auditory discrimination task with modafinil and NS9283, a positive allosteric modulator of α4β2 nAChRs

The P300 (P3) event-related potential (ERP) is a neurophysiological signal believed to reflect cognitive processing of salient cues, and is thus used as a measure of attention and working memory. Additionally, P3 amplitude and latency is altered in neurological diseases and can be pharmacologically modulated. As P3-like ERPs can be recorded in rodents, it may serve as a potential translational biomarker of value for drug discovery. Here we investigated whether a positive allosteric modulator of α4β2 nicotinic acetylcholine receptors, NS9283, and the psychostimulant modafinil could modulate P3-like ERPs in healthy adult rats performing an auditory oddball discrimination task. ERPs were recorded with electroencephalography electrodes implanted into mediodorsal (MD) thalamus, medial prefrontal cortex, hippocampus and auditory cortex (AC). P3-like ERPs were detected in all brain regions, displaying larger amplitudes in target trials compared to non-target trials. Administration of modafinil (64 mg/kg) decreased P3-like ERP latency in MD thalamus and AC, whereas NS9283 augmented P3-like ERP amplitude in MD thalamus at 0.3 mg/kg and in AC at 3.0 mg/kg. Additionally, N1 pre-attention peak amplitude in MD thalamus was increased with 0.3 mg/kg NS9283. Neither of the compounds enhanced task performance. Rather, modafinil lowered correct rejections in non-target trials. In summary, our findings reveal pharmacological modulation of the rat P3-like ERP in cortical and subcortical regions by modafinil and NS9283. These findings encourage further exploration of the rat P3-like ERP in order to promote the understanding of its meaning within cognition, as well as its applicability as a translatable biomarker in drug development.