Lipid Membrane State Change by Catalytic Protonation and the Implications for Synaptic Transmission

In cholinergic synapses, the neurotransmitter acetylcholine (ACh) is rapidly hydrolyzed by esterases to choline and acetic acid (AH). It is believed that this reaction serves the purpose of deactivating ACh once it has exerted its effect on a receptor protein (AChR). The protons liberated in this reaction, however, may by themselves excite the postsynaptic membrane. Herein, we investigated the response of cell membrane models made from phosphatidylcholine (PC), phosphatidylserine (PS) and phosphatidic acid (PA) to ACh in the presence and absence of acetylcholinesterase (AChE). Without a catalyst, there were no significant effects of ACh on the membrane state (lateral pressure change ≤0.5 mN/m). In contrast, strong responses were observed in membranes made from PS and PA when ACh was applied in presence of AChE (>5 mN/m). Control experiments demonstrated that this effect was due to the protonation of lipid headgroups, which is maximal at the pK (for PS: pKCOOH≈5.0; for PA: pKHPO4−≈8.5). These findings are physiologically relevant, because both of these lipids are present in postsynaptic membranes. Furthermore, we discussed evidence which suggests that AChR assembles a lipid-protein interface that is proton-sensitive in the vicinity of pH 7.5. Such a membrane could be excited by hydrolysis of micromolar amounts of ACh. Based on these results, we proposed that cholinergic transmission is due to postsynaptic membrane protonation. Our model will be falsified if cholinergic membranes do not respond to acidification.

A Novel and Functionally Diverse Class of Acetylcholine-gated Ion Channels

Fast cholinergic neurotransmission is mediated by pentameric acetylcholine-gated ion channels; in particular, cationic nicotinic acetylcholine receptors play well-established roles in virtually all nervous systems. Acetylcholine-gated anion channels have also been identified in some invertebrate phyla, yet their roles in the nervous system are less well-understood. Here we describe the functional properties of five previously-uncharacterized acetylcholine-gated anion channels from C. elegans, including four from a novel nematode specific subfamily known as the diverse group. In addition to their activation by acetylcholine, these diverse group channels are activated at physiological concentrations by other ligands; three, encoded by the lgc-40, lgc-57 and lgc-58 genes, are activated by choline, while lgc-39 encoded channels are activated by octopamine and tyramine. Intriguingly, these and other acetylcholine-gated anion channels show extensive co-expression with cation-selective nicotinic receptors, implying that many cholinergic synapses may have both excitatory and inhibitory potential. Thus, the evolutionary expansion of cholinergic ligand-gated ion channels may enable complex synaptic signalling in an anatomically compact nervous system.

Constructing and Tuning Excitatory Cholinergic Synapses: The Multifaceted Functions of Nicotinic Acetylcholine Receptors in Drosophila Neural Development and Physiology

Nicotinic acetylcholine receptors (nAchRs) are widely distributed within the nervous system across most animal species. Besides their well-established roles in mammalian neuromuscular junctions, studies using invertebrate models have also proven fruitful in revealing the function of nAchRs in the central nervous system. During the earlier years, both in vitro and animal studies had helped clarify the basic molecular features of the members of the Drosophila nAchR gene family and illustrated their utility as targets for insecticides. Later, increasingly sophisticated techniques have illuminated how nAchRs mediate excitatory neurotransmission in the Drosophila brain and play an integral part in neural development and synaptic plasticity, as well as cognitive processes such as learning and memory. This review is intended to provide an updated survey of Drosophila nAchR subunits, focusing on their molecular diversity and unique contributions to physiology and plasticity of the fly neural circuitry. We will also highlight promising new avenues for nAchR research that will likely contribute to better understanding of central cholinergic neurotransmission in both Drosophila and other organisms.

Physiology of cholinergic and adrenergic synapses. Role in pharmacology and practical significance

The transmission of signals between cholinergic neurons and from neurons to muscle cells (neuro-neuronal and neuromuscular transduction) occurs through synapses. They are formed by the membranes of two contacting cells, presynaptic and postsynaptic, which are separated by a narrow synaptic gap. The review article provides up-to-date information about the physiological processes in cholinergic and adrenergic synapses. The role of these synapses in pharmacology and their practical significance are presented. The transmission of excitation in cholinergic synapses occurs with the help of acetylcholine. The stages of synthesis, storage and release of acetylcholine are the same in all cholinergic neurons. The specific effects of acetylcholine mediated through cholinergic synapses depend mainly on the type of synaptic cholinergic receptors. In the system of efferent innervation, adrenergic synapses are formed by the endings of postganglionic sympathetic (adrenergic) fibers and cells of effector organs.

Postsynaptic plasticity of cholinergic synapses underlies the induction and expression of appetitive memories in Drosophila

In vertebrates, memory-relevant synaptic plasticity involves postsynaptic rearrangements of glutamate receptors. In contrast, previous work indicates that Drosophila and other invertebrates store memories using presynaptic plasticity of cholinergic synapses. Here, we provide evidence for postsynaptic plasticity at cholinergic output synapses from the Drosophila mushroom bodies (MBs). We find that the nicotinic acetylcholine receptor (nAChR) subunit α5 is required within specific MB output neurons (MBONs) for appetitive memory induction, but is dispensable for aversive memories. In addition, nAChR α2 subunits mediate memory expression downstream of α5 and the postsynaptic scaffold protein Dlg. We show that postsynaptic plasticity traces can be induced independently of the presynapse, and that in vivo dynamics of α2 nAChR subunits are changed both in the context of associative and non-associative memory formation, underlying different plasticity rules. Therefore, regardless of neurotransmitter identity, key principles of postsynaptic plasticity support memory storage across phyla.

Temporal regulation of nicotinic acetylcholine receptor subunits supports central cholinergic synapse development in Drosophila

The construction and maturation of the postsynaptic apparatus are crucial for synapse and dendrite development. The fundamental mechanisms underlying these processes are most often studied in glutamatergic central synapses in vertebrates. Whether the same principles apply to excitatory cholinergic synapses, such as those found in the insect central nervous system, is not known. To address this question, we investigated a group of projection neurons in the Drosophila larval visual system, the ventral lateral neurons (LNvs), and identified nAchRα1 (Dα1) and nAchRα6 (Dα6) as the main functional nicotinic acetylcholine receptor (nAchR) subunits in the larval LNvs. Using morphological analyses and calcium imaging studies, we demonstrated critical roles of these two subunits in supporting dendrite morphogenesis and synaptic transmission. Furthermore, our RNA sequencing analyses and endogenous tagging approach identified distinct transcriptional controls over the two subunits in the LNvs, which led to the up-regulation of Dα1 and down-regulation of Dα6 during larval development as well as to an activity-dependent suppression of Dα1. Additional functional analyses of synapse formation and dendrite dynamics further revealed a close association between the temporal regulation of individual nAchR subunits and their sequential requirements during the cholinergic synapse maturation. Together, our findings support transcriptional control of nAchR subunits as a core element of developmental and activity-dependent regulation of central cholinergic synapses.

Male pheromones modulate synaptic transmission at the C. elegans neuromuscular junction in a sexually dimorphic manner

SUMMARYThe development of functional synapses in the nervous system is important for animal physiology and behaviors. The synaptic transmission efficacy can be modulated by the environment to accommodate external changes, which is crucial for animal reproduction and survival. However, the underlying plasticity of synaptic transmission remains poorly understood. Here we show that in C. elegans, the male pheromone increases the hermaphrodite cholinergic transmission at the neuromuscular junction (NMJ), which alters hermaphrodites’ locomotion velocity and mating efficiency in a developmental stage-dependent manner. Dissection of the sensory circuits reveals that the AWB chemosensory neurons sense those male pheromones and further transduce the information to NMJ using cGMP signaling. Exposure of hermaphrodites to male pheromones specifically increases the accumulation of presynaptic CaV2 calcium channels and clustering of postsynaptic receptors at cholinergic synapses of NMJ, which potentiates cholinergic synaptic transmission. Thus, our study demonstrates a circuit mechanism for synaptic modulation by sexual dimorphic pheromones.

The Interplay of Nicotine and Social Stress Mediate Dopaminergic Neuron Firing in the Ventral Tegmental Area - Nucleus Accumbens Pathway, Contributing to Stress andDepressive Mood Disorder

Nicotine use and social stress have a complex interplay, which has been shown to be mediated by cholinergic neurons in the ventral tegmental area (VTA). Social stress is often comorbid with nicotine consumption, and the presence of either stress or nicotine use significantly increasesthe riskof developing the other. In fact, it has been shown in mice that nicotine injection is sufficient to increase susceptibility to social defeat, a reliable model for stress and anxiety-like behavior. Stressful events can molecularly remodel cholinergic synapses, inducing the production of more cholinergic transporters and increasing the number of nicotinic receptor binding sites. One way stress and nicotine remodel cholinergic synapses are through long-term potentiation (LTP) in cholinergic pathways in the VTA, enhancing the experience of stress and the effects of addiction. Despite both primarily acting on the alpha7subtype nicotine receptor, nicotine and stress induce LTP in vastly different ways: nicotine acts quickly via ligand-gated ion channels while stress activates a slower hormonal-induced G-protein coupled receptor pathway. These findings suggest that dopaminergic VTA neurons may be a useful therapeutic target for depression, anxiety, and other stress-related disorders. Deep brain stimulation has preliminarily shown to be a potential therapeutic treatment for untreatable depression, especially when it targets the medial forebrain bundle within the VTA-NAc pathway. Sleep patterns are also partially regulated by dopaminergic VTA neurons, and sleep deficits may contribute to social stress and other depressive symptoms. The role of nicotine dependence in stress-related mental illnesses is especially important to consider given the recent increase in adolescent nicotine use with the advent of vaping. Adolescents already have an increased risk for developing mental illnesses, and it is important that young people are made aware of the potential psychological harms of nicotine use.

The uses of botulinum toxin A in facial aesthetics

The demand for non-surgical facial rejuvenation procedures is rising, and they are more popular than ever, with the aesthetic uses of botulinum toxin dramatically changing the landscape of facial rejuvenation. Botulinum toxin is a neurotoxin that works within cholinergic synapses present at neuromuscular endplates, preventing the transmission of neurotransmitters, such as acetylcholine, from nerves to muscles. This interference with nerve impulses leads to the muscles being temporarily weakened (paralysis). Botulinum toxin A was approved by the US Food and Drug Administration (FDA) for use in the glabella in 2002, followed by crow's feet in 2013 and then the forehead in 2017, with other aesthetic uses being classed as off-license. Botulinum toxin A yields good results in carefully selected patients, and a thorough consultation should always take place. Consultations should include management of expectations and the explanation that botulinum toxin A works on dynamic lines, rather than static lines. Treatment areas can be split into the upper face (glabellar, transverse forehead lines and lateral orbicularis oculi); mid face (bunny lines and perioral vertical lip lines); and lower face (masseter hypertrophy, mentalis, platysmal bands and gummy smile). Each patient should be assessed individually to determine individual anatomy, including the size, strength and location of muscles, with doses being adjusted accordingly.

The HSPG Syndecan is a core organizer of cholinergic synapses in C. elegans

SUMMARYThe extracellular matrix has emerged as an active component of chemical synapses regulating synaptic formation, maintenance and homeostasis. The heparan sulfate proteoglycan syndecans are known to regulate cellular and axonal migration in the brain. They are also enriched at synapses, but their synaptic functions remain more elusive. Here we show that SDN-1, the sole ortholog of syndecan in C. elegans, is absolutely required for the synaptic clustering of homomeric α7-like N-acetylcholine receptors (AChR) and regulates the synaptic content of heteromeric L-AChRs. SDN-1 is concentrated at neuromuscular junctions (NMJs) by the neurally-secreted synaptic organizer Ce-Punctin/MADD-4, which also activates the transmembrane netrin receptor DCC. Those cooperatively recruit the FARP and CASK orthologues that localize N-AChRs at cholinergic NMJs through physical interactions. Therefore, SDN-1 stands at the core of the cholinergic synapse organization by bridging the extracellular synaptic determinants to the intracellular synaptic scaffold that controls the postsynaptic receptor content.