Review: Neurotransmitter receptors VI

Neuronal nicotinic receptors: Molecular organization and regulations

Cation-π interactions arise as a result of strong attractive forces between positively charged entities and the π-electron cloud of aromatic groups. The physicochemical characteristics of cation-π interactions are particularly well-suited to the dual hydrophobic/hydrophilic environment of membrane proteins. As high-resolution structural data of membrane proteins bring molecular features into increasingly sharper view, cation-π interactions are gaining traction as essential contributors to membrane protein chemistry, function, and pharmacology. Here we review the physicochemical properties of cation-π interactions and present several prominent examples which demonstrate significant roles for this specialized biological chemistry.

Glycine receptors (GlyR) belong to the pentameric ligand-gated ion channel (pLGIC) superfamily and mediate fast inhibitory transmission in the vertebrate CNS. Disruption of glycinergic transmission by inherited mutations produces startle disease in man. Many startle mutations are in GlyRs and provide useful clues to the function of the channel domains. E103K is one of few startle mutations found in the extracellular agonist binding site of the channel, in loop A of the principal side of the subunit interface. Homology modeling shows that the side chain of Glu-103 is close to that of Arg-131, in loop E of the complementary side of the binding site, and may form a salt bridge at the back of the binding site, constraining its size. We investigated this hypothesis in recombinant human α1 GlyR by site-directed mutagenesis and functional measurements of agonist efficacy and potency by whole cell patch clamp and single channel recording. Despite its position near the binding site, E103K causes hyperekplexia by impairing the efficacy of glycine, its ability to gate the channel once bound, which is very high in wild type GlyR. Mutating Glu-103 and Arg-131 caused various degrees of loss-of-function in the action of glycine, whereas mutations in Arg-131 enhanced the efficacy of the slightly bigger partial agonist sarcosine (N-methylglycine). The effects of the single charge-swapping mutations of these two residues were largely rescued in the double mutant, supporting the possibility that they interact via a salt bridge that normally constrains the efficacy of larger agonist molecules.

Objective: We aimed to determine whether in utero exposure to smoking may influence the activity and dynamics of cardiac autonomic control in preterm infants. We hypothesized that cardiac autonomic control is altered in preterm infants exposed prenatally to smoking and that these effects may vary as a function of the sleep state. Methods: We studied healthy, preterm neonates born to mothers who had smoked throughout pregnancy but not since birth (n = 16). In utero-exposed neonates were matched with control preterm neonates born to non-smoking mothers (n = 18). Cardiac autonomic control was monitored as a function of the sleep state by assessing heart rate variability with both linear and non-linear methods. Results: Preterm neonates with in utero exposure to smoking displayed alterations (relative to control neonates) in short-term cardiac autonomic control in all sleep states. These alterations included low vagal activity, elevated sympathetic activity, and low complexity and adaptability in heart rate control dynamics. Conclusions: Our results constitute direct evidence that cardiac autonomic activity and control are altered in sleeping preterm infants exposed to smoking in utero. Significance: These alterations may place the affected infants at a higher risk of neurological and cardiovascular complications, which could conceivably persist throughout childhood and adulthood.

Triflumezopyrim, a newly commercialized molecule from DuPont Crop Protection, belongs to the novel class of mesoionic insecticides. This study characterizes the biochemical and physiological action of this novel insecticide. Using membranes from the aphid, Myzus persicae, triflumezopyrim was found to displace ³H-imidacloprid with a K_i value of 43 nM with competitive binding results indicating that triflumezopyrim binds to the orthosteric site of the nicotinic acetylcholine receptor (nAChR). In voltage clamp studies using dissociated Periplaneta americana neurons, triflumezopyrim inhibits nAChR currents with an IC₅₀ of 0.6 nM. Activation of nAChR currents was minimal and required concentrations ≥100 μM. Xenopus oocytes expressing chimeric nAChRs (Drosophila α2/chick β2) showed similar inhibitory effects from triflumezopyrim. In P. americana neurons, co-application experiments with acetylcholine reveal the inhibitory action of triflumezopyrim to be rapid and prolonged in nature. Such physiological action is distinct from other insecticides in IRAC Group 4 in which the toxicological mode of action is attributed to nAChR agonism.

Mesoionic insecticides act via inhibition of the orthosteric binding site of the nAChR despite previous beliefs that such action would translate to poor insect control. Triflumezopyrim is the first commercialized insecticide from this class and provides outstanding control of hoppers, including the brown planthopper, Nilaparvata lugens, which is already displaying strong resistance to neonicotinoids such as imidacloprid.

Accumulated evidence suggests that cardiovascular autonomic nervous system (ANS) dysfunction may be the underlying cause of many MS clinical presentations, including neurodegeneration and reduced response to immunomodulatory therapies, depression, fatigue and sleep disorders, migraine, osteoporosis, and chronic cerebrospinal venous insufficiency, the newer MS vascular etiology. We have recently described the genetic, epigenetic, and environmental factors with the potential influencing ANS activity, and the interactions among these factors.

This review expands upon previous ones, describing the pharmacological, non-pharmacological, and surgical strategies that could be adopted to prevent and minimize the deterioration in ANS function, promoting a state of sympathovagal balance. However, these strategies should not be applied as “one size fits all”, but should take into account the nature and the degree of ANS dysfunction. These strategies would be effective in improving ANS function not only in MS, but also in other autoimmune and neurodegenerative diseases, where the dysfunction of this system plays a role.

Smoking behavior is influenced by both genetic and environmental factors. Nicotine is the major addictive substance in cigarettes. Nicotinic acetylcholine receptors (nAChRs) are thought to play an important role in nicotine addiction of smokers. One of the genes, α-4 subunit of nicotinic acetylcholine receptor (CHRNA4) gene was reported to be associated with smoking behavior in many populations.

The aim of this study is to determine association between α-4 subunit of nicotinic acetylcholine receptor single nucleotide polymorphism (rs2236196 and rs2273502 loci) and smoking behavior among Malay Males.

The study was conducted in Malay smokers (n = 248) and non-smoking controls (n = 248). DNA was extracted from leucocytes and the two SNPs were determined by a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. The PCR product was digested with restriction enzymes AfeI and Sau96I, respectively.

We found that the AA genotype frequency for CHRNA4 rs2236196 polymorphism in the smoker group was 80.6% while in nonsmoker 77.0%. No mutation (GG genotype) was detected in both groups. The AG genotype for the smoker group was 19.4% while in the nonsmoker group 23.0%. There was no significant difference observed in the genotype (χ² = 5.106, p = 0.078) and allele frequencies between both study groups. On the other hand, no mutation of CHRNA4 rs2273502 (TT genotype) was detected in the non-smoker group while the frequencies of genotype CC and heterozygous CT in non-smokers were 75.8% and 24.2%, respectively. In the smoker group, the frequencies were 73.4%, 2.0% and 24.6%, for TT, CC and CT, respectively. There was no significant difference observed in rs2273502 (χ² = 5.16, p = 0.078) and smoking behavior of the subjects. In conclusion, the results revealed that CHRNA4 rs2273502 and rs2236196 gene polymorphisms are not statistically significantly associated with smoking behavior in our population.