A machine learning method for the prediction of receptor activation in the simulation of synapses.

Chemical synaptic transmission involves the release of a neurotransmitter that diffuses in the extracellular space and interacts with specific receptors located on the postsynaptic membrane. Computer simulation approaches provide fundamental tools for exploring various aspects of the synaptic transmission under different conditions. In particular, Monte Carlo methods can track the stochastic movements of neurotransmitter molecules and their interactions with other discrete molecules, the receptors. However, these methods are computationally expensive, even when used with simplified models, preventing their use in large-scale and multi-scale simulations of complex neuronal systems that may involve large numbers of synaptic connections. We have developed a machine-learning based method that can accurately predict relevant aspects of the behavior of synapses, such as the percentage of open synaptic receptors as a function of time since the release of the neurotransmitter, with considerably lower computational cost compared with the conventional Monte Carlo alternative. The method is designed to learn patterns and general principles from a corpus of previously generated Monte Carlo simulations of synapses covering a wide range of structural and functional characteristics. These patterns are later used as a predictive model of the behavior of synapses under different conditions without the need for additional computationally expensive Monte Carlo simulations. This is performed in five stages: data sampling, fold creation, machine learning, validation and curve fitting. The resulting procedure is accurate, automatic, and it is general enough to predict synapse behavior under experimental conditions that are different to the ones it has been trained on. Since our method efficiently reproduces the results that can be obtained with Monte Carlo simulations at a considerably lower computational cost, it is suitable for the simulation of high numbers of synapses and it is therefore an excellent tool for multi-scale simulations.

Regulação autonômica das propriedades mecânicas em bioprótese valvar porcina / Autonomic regulation of mechanical properties in porcine mitral valve cusps / Regulación autonómica de las propiedades mecánicas en bioprótesis valvular porcina

FUNDAMENTO: A presença de nervos nas válvulas cardíacas foi demonstrada pela primeira vez há décadas e identificadas em subpopulações: simpáticas e parassimpáticas, e, portanto, é esperado que as válvulas sejam grandemente afetadas pelos nervos autônomos. Entretanto, poucos estudos têm se concentrado na regulação de válvulas cardíacas pelo sistema nervoso autônomo. OBJETIVO: Buscamos identificar o papel do sistema nervoso autônomo na regulação das propriedades mecânicas dos tecidos de válvulas mitrais porcinas. MÉTODOS: As propriedades mecânicas dos folhetos de válvulas mitrais porcinas foram avaliados em resposta à norepinefrina (NE) e acetilcolina (ACH), os principais neurotransmissores. Ao mesmo tempo, fentolamina (FENT), metoprolol (Metop), atropina (Atrop) e desnudamento endotelial foram adicionados ao sistema reativo. RESULTADOS: Sob condições fisiológicas, a rigidez não foi afetada pelo desnudamento endotelial (p > 0,05). A NE significantemente aumentou a rigidez valvar por aumento de 10 vezes na concentração (10-6 vs 10-7, p < 0,05; 10-5 vs 10-6, p < 0,05). Essa resposta foi amenizada por FENT, Metop ou desnudamento endotelial (p < 0,05); entretanto, manteve-se aumentada de maneira significante quando comparada aos Controles (p < 0,05). A ACH causou uma diminuição na rigidez acompanhada por um aumento em sua concentração (alteração significante na rigidez por aumento de 10 vezes na concentração de ACH, 10-6 vs Controle, p < 0,05; 10-5 vs 10-6, p < 0,05), que foi revertida pelo desnudamento endotelial e Atrop (p > 0,05 vs Controle). CONCLUSÃO: Esses achados ressaltam o papel do sistema nervoso autônomo na regulação das propriedades mecânicas das cúspides de válvula mitral porcina, o que reforça a importância do estado nervoso autônomo no funcionamento ideal da válvula.

BACKGROUND: The presence of nerves in heart valves was first depicted decades ago and identified into subpopulations: sympathetic, parasympathetic. So valves are expected to be greatly affected by the autonomic nerves. However, few studies have focused on the regulation of heart valves by the autonomic nervous system. OBJECTIVE: We sought to identify the role of the autonomic nervous system in the regulation of the mechanical properties of porcine mitral valve tissues. METHODS: Mechanical properties of porcine mitral valve leaflets were evaluated in response to norepinephrine (NE) and acetylcholine (ACH), the main neurotransmitters. At the same time, phentolamine (Phent), metoprolol (Metop), atropine (Atrop) and endothelial denudation were added to the reactive system. RESULTS: Under physiological conditions, the stiffness was not affected by endothelial denudation (p > 0.05). NE elevated the valve stiffness significantly per 10-fold increase in concentration (10-6 vs 10-7, p < 0.05; 10-5 vs 10-6, p < 0.05). This response was mitigated by Phent, Metop or endothelial denudation (p < 0.05), however, it was still increased significantly when compared to Controls (p < 0.05). ACH caused a decrease in stiffness accompanied by an increase in its concentration (significant change in stiffness per 10-fold increase in ACH concentration, 10-6 vs Control, p < 0.05; 10-5 vs 10-6, p < 0.05), which were reversed by endothelial denudation and Atrop (p > 0.05 vs Control). CONCLUSION: These findings highlight the role of the autonomic nervous system in the regulation of the mechanical properties of porcine mitral valve cusps, which underline the importance of autonomic nervous status for optimal valve function.

FUNDAMENTO: La presencia de nervios en las válvulas cardíacas quedó demostrada por primera vez hace algunas décadas e identificadas en sub-poblaciones: simpáticas y parasimpáticas y por lo tanto, lo que se espera es que las válvulas reciban una gran afectación de los nervios autónomos. Sin embargo, pocos estudios se han concentrado en la regulación de válvulas cardíacas a través del sistema nervioso autónomo. OBJETIVO: Buscamos identificar el papel del sistema nervioso autónomo en la regulación de las propiedades mecánicas de los tejidos de las válvulas mitrales porcinas. MÉTODOS: Las propiedades mecánicas de las capas de válvulas mitrales porcinas fueron evaluadas en respuesta a la norepinefrina (NE) y a la acetilcolina (ACH), los principales neurotransmisores. Igualmente, la fentolamina (FENT), el metoprolol (Metop), la atropina (Atrop) y la denudación endotelial también se añadieron al sistema reactivo. RESULTADOS: Bajo condiciones fisiológicas, la rigidez no se afectó por el denudación endotelial (p > 0,05). La NE aumentó significativamente la rigidez valvular con un aumento de 10 veces en la concentración (10-6 vs 10-7, p < 0,05; 10-5 vs 10-6, p < 0,05). Esa respuesta fue amenizada por FENT, Metop o denudación endotelial (p < 0,05); pero se mantuvo aumentada de manera significativa cuando se le comparó con los Controles (p < 0,05). La ACH causó una disminución en la rigidez acompañada por un aumento en su concentración (alteración significativa en la rigidez por el aumento en 10 veces de la concentración de ACH, 10-6 vs Control, p < 0,05; 10-5 vs 10-6, p < 0,05), que fue revertida por la denudación endotelial y Atrop (p > 0,05 vs Control). CONCLUSIÓN: Esos hallazgos destacan el rol del sistema nervioso autónomo en la regulación de las propiedades mecánicas de las cúspides de la válvula mitral porcina, lo que refuerza la importancia del estado nervioso autónomo en el funcionamiento ideal de la válvula.

Modeling synaptic dynamics driven by receptor lateral diffusion.

The synaptic weight between a pre- and a postsynaptic neuron depends in part on the number of postsynaptic receptors. On the surface of neurons, receptors traffic by random motion in and out from a microstructure called the postsynaptic density (PSD). In the PSD, receptors can be stabilized at the membrane when they bind to scaffolding proteins. We propose a mathematical model to compute the postsynaptic counterpart of the synaptic weight based on receptor trafficking. We take into account the receptor fluxes at the PSD, which can be regulated by neuronal activity, and the interactions of receptors with the scaffolding molecules. Using a Markovian approach, we estimate the mean and the fluctuations of the number of bound receptors. When the number of receptors is large, a deterministic system is also derived. Moreover, these equations can be used, for example, to fit fluorescence-recovery-after-photobleaching experiments to determine, in living neurons, the chemical binding constants for the receptors/scaffolding molecules interaction at synapses.

Why do we need new and better antidepressants?

The range of available antidepressants is reviewed in relation to mechanisms of action and the evidence of efficacy in general and efficacy in severe depression in particular. In studies investigating efficacy in major depressive disorder, not all antidepressants have been shown to have clear-cut efficacy in severe depression. Here, the minimum standards for the necessary methodology to investigate efficacy in severe depression are reviewed and the methods that are needed to establish efficacy as a superior antidepressant or as an antidepressant with a faster than expected response are suggested. A review of the mechanisms of action of different antidepressants is accompanied by a critical review of the properties of an antidepressant likely to achieve either efficacy in severe depression or the status of a superior antidepressant.

[Basic research of applying extracorporeal shockwaves on the musculoskeletal system. An assessment of current status]. / Grundlagenforschung zur Applikation extrakorporaler Stosswellen am Stütz- und Bewegungsapparat. Eine Standortbestimmung.

Detailed knowledge concerning the action of extracorporeal shock waves on the locomotor system as well as concerning possible side effects of extracorporeal shock wave therapy (ESWT) are crucial to optimize the clinical use of ESWT for the treatment of illnesses such as calcific tendinitis of the shoulder, tennis elbow, plantar fasciitis, aseptic pseudarthrosis, and aseptic hip necrosis. This study presents the current knowledge gained from animal experiments, which have yielded important findings, in particular concerning possible side effects of ESWT. Very recent studies have also provided valuable insights into the molecular actions of extracorporeal shock waves on the locomotor system. Further intensified experimental animal research will greatly improve the scientific basis for the clinical use of ESWT in the near future.

Structure of ligand-gated ion channels: critical assessment of biochemical data supports novel topology.

Rapid signaling across the synaptic junction is partially mediated by the ligand-gated ion channel superfamily (LGICS), which includes inhibitory glycine and GABA receptors and excitatory acetylcholine and serotonin receptors. The glycine receptor (GlyR) can assemble as homopentamers of alpha subunits, and baculovirus expression systems are capable of overexpressing large quantities of active receptors. Limited proteolysis coupled to mass spectrometry on reconstituted alpha1 GlyR homopentamers identified proteolytic cleavages within proposed transmembrane domains postulated to fold as bilayer-spanning alpha helices in the "classical" model and identified unexpected membrane-associated regions in the N-terminal domain (J. F. Leite et al., 2000, J. Biol. Chem. 275, 13683-13689). In this review, optimized sequence alignments were used to integrate these proteolysis data with biochemical information determined in studies of all the LGICS members in order to construct a novel topological model.

Stochastic model of central synapses: slow diffusion of transmitter interacting with spatially distributed receptors and transporters.

A detailed mathematical analysis of the diffusion process of neurotransmitter inside the synaptic cleft is presented and the spatio-temporal concentration profile is calculated. Using information about the experimentally observed time course of glutamate in the cleft the effective diffusion coefficient Dnet is estimated as Dnet approximately 20-50 nm(2) microseconds(-1), implying a strong reduction compared with free diffusion in aqueous solution. The tortuosity of the cleft and interactions with transporter molecules are assumed to affect the transmitter motion. We estimate the transporter density to be 5170 to 8900 micrometer(-2) in the synaptic cleft and its vicinity, using the experimentally observed time constant of glutamate. Furthermore a theoretical model of synaptic transmission is presented, taking the spatial distribution of post-synaptic (AMPA-) receptors into account. The transmitter diffusion and receptor dynamics are modeled by Monte Carlo simulations preserving the typically observed noisy character of post-synaptic responses. Distributions of amplitudes, rise and decay times are calculated and shown to agree well with experiments. Average open probabilities are computed from a novel kinetic model and are shown to agree with averages over many Monte Carlo runs. Our results suggest that post-synaptic currents are only weakly potentiated by clustering of post-synaptic receptors, but increase linearly with the total number of receptors. Distributions of amplitudes and rise times are used to discriminate between different morphologies, e.g. simple and perforated synapses. A skew in the miniature amplitude distribution can be caused by multiple release of pre-synaptic vesicles at perforated synapses.

On the origin of skewed distributions of spontaneous synaptic potentials in autonomic ganglia.

The histograms of spontaneous synaptic potentials at synapses in autonomic ganglia are described by distributions consisting of mixtures of Gaussians, rather than by single Gaussian distributions. The possible origin of these mixed distributions is investigated, using Monte-Carlo simulations of the action of spontaneously released units of transmitter. A single unit of acetylcholine of fixed size, released from an active zone with receptor patches both beneath and adjacent to the zone, does not give rise to the observed histograms. But if the unit is of variable size, consisting of integer multiples of smaller units, and release is from an active zone onto either the receptor patch beneath, or in addition onto adjacent patches, then the histogram is well described by a mixture of Gaussians. However, this explanation is unlikely to be correct as present evidence suggests that in most cases the released unit of transmitter saturates the postsynaptic receptor patch beneath the active zone. The final case considered is where a unit of transmitter is spontaneously released from an active zone, simultaneously with a unit in an adjacent zone less than one micron away. The histogram of potentials then conforms to those observed even when there are differences in the sizes of the receptor patches. It is suggested that this kind of release could provide an explanation for distributions of spontaneous potentials that are mixtures of Gaussians.

The effects of geometrical parameters on synaptic transmission: a Monte Carlo simulation study.

Monte Carlo simulations of transmitter diffusion and its interactions with postsynaptic receptors have been used to study properties of quantal responses at central synapses. Fast synaptic responses characteristic of those recorded at glycinergic junctions on the teleost Mauthner cell (time to peak approximately 0.3-0.4 ms and decay time constant approximately 3-6 ms) served as the initial reference, and smaller contacts with fewer postsynaptic receptors were also modeled. Consistent with experimental findings, diffusion, simulated using a random walk algorithm and assuming a diffusion coefficient of 0.5-1.0 x 10(-5) cm2 s(-1), was sufficiently fast to account for transmitter removal from the synaptic cleft. Transmitter-receptor interactions were modeled as a two-step binding process, with the double-bound state having opened and closed conformations. Addition of a third binding step only slightly decreased response amplitude but significantly slowed both its rising and decay phases. The model allowed us to assess the sources of response variability and the likelihood of postsynaptic saturation as functions of multiple kinetic and spatial parameters. The method of nonstationary fluctuation analysis, typically used to estimate the number of functional channels at a synapse and single channel current, proved unreliable, presumably because the receptors in the postsynaptic matrix are not uniformly exposed to the same profile of transmitter concentration. Thus, the time course of the probability of channel opening most likely varies among receptors. Finally, possible substrates for phenomena of synaptic plasticity, such as long-term potentiation, were explored, including the diameter of the contact zone, defined by the region of pre- and postsynaptic apposition, the number and distribution of the receptors, and the degree of vesicle filling. Surprisingly, response amplitude is quite sensitive to the size of the receptor-free annulus surrounding the receptor cluster, such that expansion of the contact zone could produce an appreciable increase in quantal size, normally attributed to either the presence of more receptors or the release of more transmitter molecules.

In vivo assessment of neurotransmitter system in cardiovascular diseases. Clinical issues.

Cardiac neurotransmitter systems, especially the adrenergic receptor pathway, are impaired in heart diseases. In patients with heart failure, these abnormalities contribute to arrhythmogenesis and to progression of cardiac dysfunction. The use of MIBG with single photon imaging has provided useful information on the mechanisms of ventricular arrhythmias, and on the causes of death in patients with heart failure or hypertrophic cardiomyopathy. It has been suggested as a prognostic indicator in patients with heart failure. Positron Emission Tomography (PET) now allows us to obtain noninvasively the quantitative determination of regional receptor density and affinity in humans as well as innervation integrity and functioning. These measurements are based upon the synthesis of a radioligand, usually either a selective receptor antagonist or a false neurotransmitter labeled with a positron-emitting radioisotope. Mathematical compartmental models are fitted to activity-versus-time curves obtained during saturation or displacement experiments in order to calculate the rate constants and the receptor density in the myocardium. PET has only recently begun to be applied to the study of cardiac physiology and disease. PET and SPECT cardiac neuroimaging techniques are able to demonstrate the physiological regulation of receptors, and to provide the possibility of studying regional abnormalities of cardiac neurotransmission, especially in arrhythmogenic cardiomyopathy. Furthermore these non invasive techniques could be useful in exploring the alteration of neurotransmission in the early stage of heart disease and could allow repeated scintigraphic examinations in order to evaluate the effects of cardiac medications.

Intrinsic quantal variability due to stochastic properties of receptor-transmitter interactions.

Synaptic events at the neuromuscular junction are integer multiples of a quantum, the postsynaptic response to transmitter released from one presynaptic vesicle. At central synapses where quanta are small, it has been suggested they are invariant due to occupation of all postsynaptic receptors, a concept neglecting inherent fluctuations in channel behavior. If this did occur, the quantal release model would not apply there and could not be used to localize sites of synaptic modification. Monte Carlo simulations of quanta include transmitter diffusion and interactions with postsynaptic receptors that are treated probabilistically. These models suggest that when there are few postsynaptic channels available at a synapse, their stochastic behavior produces significant intrinsic variance in response amplitude and kinetics, and saturation does not occur. These results were confirmed by analysis of inhibitory quanta in embryonic and adult Mauthner cells involving a small and large number of channels, respectively. The findings apply to excitatory synapses as well.

[A statistical assessment of the membrane area and of the number of active zones in presynaptic terminals]. / Statisticheskaia otsenka ploshchadi membrany i kolichestva aktivnykh zon v presinapticheskikh okonchaniiakh.

Geometrical features of electron microgram profiles of presynaptic terminals (PTs) in the dorsal horn of the cat spinal cord were studied. The statistical approach which has permitted estimating the surface area of the closed membrane on the basis of parameters of its plane random sections is described. Mean surface area of the plasma membrane of observed PTs is estimated. The probability estimate of the mean number of active zones per each presynaptic bouton is considered. The obtained quantitative data are discussed for the possibility to describe physical processes occurring in presynapses.

Markov, fractal, diffusion, and related models of ion channel gating. A comparison with experimental data from two ion channels.

The gating kinetics of single-ion channels are generally modeled in terms of Markov processes with relatively small numbers of channel states. More recently, fractal (Liebovitch et al. 1987. Math. Biosci. 84:37-68) and diffusion (Millhauser et al. 1988. Proc. Natl. Acad. Sci. USA. 85:1502-1507) models of channel gating have been proposed. These models propose the existence of many similar conformational substrates of the channel protein, all of which contribute to the observed gating kinetics. It is important to determine whether or not Markov models provide the most accurate description of channel kinetics if progress is to be made in understanding the molecular events of channel gating. In this study six alternative classes of gating model are tested against experimental single-channel data. The single-channel data employed are from (a) delayed rectifier K+ channels of NG 108-15 cells and (b) locust muscle glutamate receptor channels. The models tested are (a) Markov, (b) fractal, (c) one-dimensional diffusion, (d) three-dimensional diffusion, (e) stretched exponential, and (f) expo-exponential. The models are compared by fitting the predicted distributions of channel open and closed times to those observed experimentally. The models are ranked in order of goodness-of-fit using a boot-strap resampling procedure. The results suggest that Markov models provide a markedly better description of the observed open and closed time distributions for both types of channel. This provides justification for the continued use of Markov models to explore channel gating mechanisms.