Ionotropic Receptors as a Driving Force behind Human Synapse Establishment

The origin of nervous systems is a main theme in biology and its mechanisms are largely underlied by synaptic neurotransmission. One problem to explain synapse establishment is that synaptic orthologs are present in multiple aneural organisms. We questioned how the interactions among these elements evolved and to what extent it relates to our understanding of the nervous systems complexity. We identified the human neurotransmission gene network based on genes present in GABAergic, glutamatergic, serotonergic, dopaminergic, and cholinergic systems. The network comprises 321 human genes, 83 of which act exclusively in the nervous system. We reconstructed the evolutionary scenario of synapse emergence by looking for synaptic orthologs in 476 eukaryotes. The Human–Cnidaria common ancestor displayed a massive emergence of neuroexclusive genes, mainly ionotropic receptors, which might have been crucial to the evolution of synapses. Very few synaptic genes had their origin after the Human–Cnidaria common ancestor. We also identified a higher abundance of synaptic proteins in vertebrates, which suggests an increase in the synaptic network complexity of those organisms.     

Association study of 37 genes related to serotonin and dopamine neurotransmission and neurotrophic factors in cocaine dependence

Cocaine dependence is a neuropsychiatric disorder in which both environmental and genetic factors are involved. Several processes, that include reward and neuroadaptations, mediate the transition from use to dependence. In this regard, dopamine and serotonin neurotransmission systems are clearly involved in reward and other cocaine‐related effects, whereas neurotrophic factors may be responsible for neuroadaptations associated with cocaine dependence. We examined the contribution to cocaine dependence of 37 genes related to the dopaminergic and serotoninergic systems, neurotrophic factors and their receptors through a case–control association study with 319 single nucleotide polymorphisms selected according to genetic coverage criteria in 432 cocaine‐dependent patients and 482 sex‐matched unrelated controls. Single marker analyses provided evidence for association of the serotonin receptor HTR2A with cocaine dependence [rs6561333; nominal P‐value adjusted for age = 1.9e?04, odds ratio = 1.72 (1.29–2.30)]. When patients were subdivided according to the presence or absence of psychotic symptoms, we confirmed the association between cocaine dependence and HTR2A in both subgroups of patients. Our data show additional evidence for the involvement of the serotoninergic system in the genetic susceptibility to cocaine dependence.     

Wnt signaling regulates experience-dependent synaptic plasticity in the adult nervous system

Comment on: Jensen M, et al. Cell 2012; 149:173-87.     

Differential effects of ethanol on regional glutamatergic and GABAergic neurotransmitter pathways in mouse brain

This study investigates the effects of ethanol on neuronal and astroglial metabolism using ¹H‐[¹³C]‐NMR spectroscopy in conjunction with infusion of [1,6‐¹³C₂]/[1‐¹³C]glucose or [2‐¹³C]acetate, respectively. A three‐compartment metabolic model was fitted to the ¹³C turnover of Glu_C3, Glu_C4, GABA_C2, GABA_C3, Asp_C3, and Gln_C4 from [1,6‐¹³C₂]glucose to determine the rates of tricarboxylic acid (TCA) and neurotransmitter cycle associated with glutamatergic and GABAergic neurons. The ratio of neurotransmitter cycle to TCA cycle fluxes for glutamatergic and GABAegic neurons was obtained from the steady‐state [2‐¹³C]acetate experiment and used as constraints during the metabolic model fitting. ¹H MRS measurement suggests that depletion of ethanol from cerebral cortex follows zero order kinetics with rate 0.18 ± 0.04 μmol/g/min. Acute exposure of ethanol reduces the level of glutamate and aspartate in cortical region. Gln_C4 labeling was found to be unchanged from a 15 min infusion of [2‐¹³C]acetate suggesting that acute ethanol exposure does not affect astroglial metabolism in naive mice. Rates of TCA and neurotransmitter cycle associated with glutamatergic and GABAergic neurons were found to be significantly reduced in cortical and subcortical regions. Acute exposure of ethanol perturbs the level of neurometabolites and decreases the excitatory and inhibitory activity differentially across the regions of brain.

     

In vivo quantification of neuro‐glial metabolism and glial glutamate concentration using 1H‐[13C] MRS at 14.1T

Astrocytes have recently become a major center of interest in neurochemistry with the discoveries on their major role in brain energy metabolism. An interesting way to probe this glial contribution is given by in vivo ¹³C NMR spectroscopy coupled with the infusion labeled glial‐specific substrate, such as acetate. In this study, we infused alpha‐chloralose anesthetized rats with [2‐¹³C]acetate and followed the dynamics of the fractional enrichment (FE) in the positions C4 and C3 of glutamate and glutamine with high sensitivity, using ¹H‐[¹³C] magnetic resonance spectroscopy (MRS) at 14.1T. Applying a two‐compartment mathematical model to the measured time courses yielded a glial tricarboxylic acid (TCA) cycle rate (V_g) of 0.27 ± 0.02 μmol/g/min and a glutamatergic neurotransmission rate (V_NT) of 0.15 ± 0.01 μmol/g/min. Glial oxidative ATP metabolism thus accounts for 38% of total oxidative metabolism measured by NMR. Pyruvate carboxylase (V_PC) was 0.09 ± 0.01 μmol/g/min, corresponding to 37% of the glial glutamine synthesis rate. The glial and neuronal transmitochondrial fluxes (V_x^g and V_xⁿ) were of the same order of magnitude as the respective TCA cycle fluxes. In addition, we estimated a glial glutamate pool size of 0.6 ± 0.1 μmol/g. The effect of spectral data quality on the fluxes estimates was analyzed by Monte Carlo simulations.

     

Motor neuropathy‐associated mutation impairs Seipin functions in neurotransmission

Gain‐of‐toxic‐function mutations in Seipin (Asparagine 88 to Serine (N88S) and Serine 90 to Leucine (S90L) mutations, both of which disrupt the N‐glycosylation) cause autosomal dominant motor neuron diseases. However, the mechanism of how these missense mutations lead to motor neuropathy is unclear. Here, we analyze the impact of disruption of N‐glycosylation of Seipin on synaptic transmission by over‐expressing mutant Seipin in cultured cortical neurons via lentiviral infection. Immunostaining shows that over‐expressed Seipin is partly colocalized with synaptic vesicle marker synaptophysin. Electrophysiological recordings reveal that the Seipin mutation significantly decreases the frequency, but not the amplitudes of miniature excitatory post‐synaptic currents and miniature inhibitory post‐synaptic currents. The amplitude of both evoked excitatory post‐synaptic currents and inhibitory post‐synaptic current is also compromised by mutant Seipin over‐expression. The readily releasable pool and vesicular release probability of synaptic vesicles are both altered in neurons over‐expressing Seipin‐N88S, whereas neither γ‐amino butyric acid (GABA) nor α‐Amino‐3‐hydroxy‐5‐methyl‐4‐ isoxazolepropionic acid (AMPA) induced whole cell currents are affected. Moreover, electron microscopy analysis reveals decreased number of morphologically docked synaptic vesicles in Seipin‐N88S‐expressing neurons. These data demonstrate that Seipin‐N88S mutation impairs synaptic neurotransmission, possibly by regulating the priming and docking of synaptic vesicles at the synapse.

     

Nicotine induces dendritic spine remodeling in cultured hippocampal neurons

Cholinergic neurons in the CNS are involved in synaptic plasticity and cognition. Both muscarinic and nicotinic acetylcholine receptors (nAChRs) influence plasticity and cognitive function. The mechanism underlying nAChR‐induced plasticity, however, has remained elusive. Here, we demonstrate morphological changes in dendritic spines following activation of α4β2* nAChRs, which are expressed on glutamatergic pre‐synaptic termini of cultured hippocampal neurons. Exposure of the neurons to nicotine resulted in a lateral enlargement of spine heads. This was abolished by dihydro‐β‐erythroidine, an antagonist of α4β2* nAChRs, but not by α‐bungarotoxin, an antagonist of α7 nAChRs. Tetanus toxin or a mixture of 2‐amino‐5‐phosphonovaleric acid and 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione, antagonists of NMDA‐ and AMPA‐type glutamate receptors, blocked the nicotine‐induced spine remodeling. In addition, nicotine exerted full spine‐enlarging response in the post‐synaptic neuron whose β2 nAChR expression was knocked down. Finally, pre‐treatment with nicotine enhanced the Ca²⁺‐response of the neurons to glutamate. These data suggest that nicotine influences the activity of glutamatergic neurotransmission through the activation of pre‐synaptic α4β2 nAChRs, resulting in the modulation of spinal architecture and responsiveness. The present findings may represent one of the cellular mechanisms underlying cholinergic tuning of brain function.

     

Functional effects of alcohol withdrawal syndrome on peripheral sympathetic neurotransmission in vas deferens of adult rats

Aims

Alcohol withdrawal syndrome (AWS) is characterized by a set of physiological modifications triggered by abrupt withdrawal and/or decreasing consumption of ethanol (EtOH), which may manifest 16–48 h after ceasing consumption. The relationship between the effects of AWS and central and peripheral sympathetic neurotransmission is unknown. This study investigates the possible mechanisms on the sympathetic system during periods of AWS.

Main methods

Male Wistar rats were treated with EtOH (6–10 g/kg/day/v.o. 5 days). Subsequently, 1 h, 24 h, 48 h and 120 h after administration of the last dose of EtOH, the animals were sacrificed, and their vas deferens (VD) were removed to perform the following evaluations: (a) concentration–effect curves of sympathetic agonist; (b) activity of α₂-adrenoreceptor; (c) function of voltage-dependent calcium channels (Cav); and (d) release of endogenous catecholamines measured in real time coupled to HPLC.

Key findings

The results showed that the maximum effects of contraction were increased by agonists tested in at 24 h and 48 h EtOH withdrawal. The inhibitory affinity (pIC₅₀) of guanfacine was decreased 24 h EtOH withdrawal. The function of Cav was also decreased as pIC₅₀ values dropped 24 h and 48 h EtOH withdrawal. The release of catecholamines increased 48 h after EtOH withdrawal. It is suggested that AWS triggers hyperactivity in peripheral sympathetic neurotransmission.

Significance

The mechanisms underlying hyperactivity are possibly explained by a failure of autoregulation from catecholamines released by α₂-adrenoreceptors and/or an increase of Cav function, which may be potential targets to attenuate the symptoms of AWS at the peripheral level.     

神经系统中的一氧化氮

一氧化氮(NO)是一种广泛存在的独特的生物信使因子和效应因子.NO参与脑内许多生理功能和病理生理过程.NO调节神经递质释放和脑血流,参与神经发育和基因表达调控.NO可能作为一种逆行信使物质参与海马的长时程突触传递增强和小脑的长时程突触传递抑制.过多的NO则具有神经毒性并与许多神经系统疾病有关.     

Sequences in the Amino Termini of GABA ρ and GABAA Subunits Specify Their Selective Interaction In Vitro

Abstract: Molecular cloning has revealed that there are six classes of subunits capable of forming GABA-gated chloride channel receptors. GABA_A receptors are composed of α, β, γ, δ, and ε/χ subunits, whereas GABA_C receptors appear to contain ρ subunits. However, retinal cells exhibiting GABA_C responses express α, β, and ρ subunits, raising the possibility that GABA_C receptors may be a mixture of subunit classes. Using in vitro translated protein, we determined that human GABA_A receptor subunits α1, α5, and β1 did not coimmunoprecipitate with full-length ρ1, ρ2, or the N-terminal domain of ρ1 that contains signals for ρ-subunit interaction. To explore the molecular mechanism underlying these apparently exclusive combinations, chimeric subunits were created and tested for interaction with the wild-type subunits. Transfer of the N terminus of β1 to ρ1 created a β1ρ1 chimera that coimmunoprecipitated with the α1 subunit but not with the ρ2 subunit. Furthermore, exchanging the N terminus of the ρ1 subunit with the corresponding region of β1 produced a ρ1β1 chimera that interfered with ρ1 receptor expression in Xenopus oocytes, whereas the full-length β1 subunit had no effect. Together, these results indicate that sequences in the N termini direct assembly of ρ subunits and GABA_A subunits into GABA_C and GABA_A receptors, respectively.