Enhanced Intrinsic Excitability in Basket Cells Maintains Excitatory-Inhibitory Balance in Hippocampal Circuits

SummaryThe dynamics of inhibitory circuits in the cortex is thought to rely mainly on synaptic modifications. We challenge this view by showing that hippocampal parvalbumin-positive basket cells (PV-BCs) of the CA1 region express long-term (>30 min) potentiation of intrinsic neuronal excitability (LTP-IEPV-BC) upon brief repetitive stimulation of the Schaffer collaterals. LTP-IEPV-BC is induced by synaptic activation of metabotropic glutamate receptor subtype 5 (mGluR5) and mediated by the downregulation of Kv1 channel activity. LTP-IEPV-BC promotes spiking activity at the gamma frequency (∼35 Hz) and facilitates recruitment of PV-BCs to balance synaptic and intrinsic excitation in pyramidal neurons. In conclusion, activity-dependent modulation of intrinsic neuronal excitability in PV-BCs maintains excitatory-inhibitory balance and thus plays a major role in the dynamics of hippocampal circuits

Plasticity of intrinsic excitability during LTD is mediated by bidirectional changes in h-channel activity

International audienceThe polarity of excitability changes associated with induction of Long-Term synaptic Depression (LTD) in CA1 pyramidal neurons is a contentious issue. Postsynaptic neuronal excitability after LTD induction is found to be reduced in certain cases (i.e. synergistic changes) but enhanced in others (i.e. compensatory or homeostatic). We examined here whether these divergent findings could result from the activation of two separate mechanisms converging onto a single learning rule linking synergistic and homeostatic plasticity. We show that the magnitude of LTD induced with low frequency stimulation (LFS) of the Schaffer collaterals determines the polarity of intrinsic changes in CA1 pyramidal neurons. Apparent input resistance (R in) is reduced following induction of moderate LTD (40%) induced by repetitive episodes of LFS. The up-regulation of I h observed after moderate LTD results from the activation of NMDA receptors whereas the down-regulation of I h is due to activation of mGluR1 receptors. These changes in R in were associated with changes in intrinsic excitability. In conclusion, our study indicates that changes in excitability after LTD induction follow a learning rule describing a continuum linking synergistic and compensatory changes in excitability

Des égalités fondamentales de Marx à la résolution du problème de la transformation - Cohérence du modèle

Recently, V. Laure van Bambeke used an original approach to solve the famous problem of transformation of values into production prices by considering that capital reallocation to each department (branch) is part of the problem and is required to comply with both Marx’s fundamental equalities: between the total sum of values and the total sum of prices, and between the total surplus value and the total profit (in price). Here, we confirm the validity of this consideration in relation with the satisfaction of demand (social need which is able to pay for the given product). However, V. Laure van Bambeke’s method of solving an overdetermined system of equations implies that compliance with Marx’s fundamental equalities can only be approached, which suggests that Marx’s conception would apply approximatively, rather than as a strict law. Here, on the contrary, we show that the transformation problem is solvable from a determined (two-branch models) or an underdetermined system of equations enabling to obtain exact solutions through an algorithm we provide, with no approximation needed. For systems with three branches or more, the solution of the transformation problem belongs to an infinite ensemble and may be determined by an array of factors (e.g., taste of consumers, advertisement, competition between capitalists, politics), accounting for the observed high competition-driven market fluidity. Furthermore, we show that the transformation problem is solvable in the absence of fixed capital, supporting that dealing with the latter is not essential and cannot be seen as a potential flaw of the approach. In these particular cases, the rate of profit can be determined from the eigenvalue of the sociotechnical coefficient matrix, without any information required about the capital allocation to the various branches. Our algorithm enables simulations illustrating how the transient rise in rate of profit predicted by the Okishio theorem is consistent with the tendency of the rate of profit to fall (TRPF) subsequent to capital reallocation, and how the TRPF is governed by the increase of organic composition, in value. We establish that the long-standing transformation problem is not such a problem, since it is easily solved through our algorithm, whatever the number of branches considered. This emphasizes the high coherence of Marx’s conception, and its impressive relevance regarding issues such as the TRPF, which have remained intensely debated

Les deux principes invariants qu’implique la loi de la valeur de Marx sont nécessaires et suffisants pour résoudre le problème de la transformation en partant du formalisme de Morishima

The unit value of a commodity that Michio Morishima's method and its variations enable to determine correctly, is the sum of the value of the commodities it contains (inputs) and the quantity of labor required for its production. However, goods are sold at their market production price only when they meet a solvent social need that involves the entire economy with its interconnections between the different industrial sectors. This condition gives full meaning to Marx's fundamental equalities, which derive from the law of value and constitute invariants that apply to the economy as a whole. These equalities are necessary to determine market production prices. We demonstrate that they also enable to solve the transformation problem for a simple reproduction system without fixed capital by starting from Morishima's formalism and returning to a formalism closer to that used by Marx.La valeur unitaire d’une marchandise que la méthode de Michio Morishima et ses variantes permettent de déterminer correctement, est la somme de la valeur des marchandises qu’elle contient (intrants) et de la quantité de travail nécessaire pour sa fabrication. Cependant les marchandises ne sont vendues à leur prix de production de marché que si elles répondent à un besoin social solvable qui fait intervenir la totalité de l’économie avec ses interconnections entre les différents secteurs industriels. Cette condition donne tout son sens aux égalités fondamentales de Marx qui découlent de la loi de la valeur et constituent des invariants s’appliquant à l’économie dans sa totalité. Ces égalités sont nécessaires pour déterminer les prix de production de marché. Nous démontrons qu’elles permettent également de résoudre le problème de la transformation pour un système de reproduction simple sans capital fixe en partant du formalisme de Morishima et en revenant à un formalisme plus proche de celui utilisé par Marx

Des égalités fondamentales de Marx à la résolution du problème de la transformation - Cohérence du modèle

Recently, V. Laure van Bambeke used an original approach to solve the famous problem of transformation of values into production prices by considering that capital reallocation to each department (branch) is part of the problem and is required to comply with both Marx’s fundamental equalities: between the total sum of values and the total sum of prices, and between the total surplus value and the total profit (in price). Here, we confirm the validity of this consideration in relation with the satisfaction of demand (social need which is able to pay for the given product). However, V. Laure van Bambeke’s method of solving an overdetermined system of equations implies that compliance with Marx’s fundamental equalities can only be approached, which suggests that Marx’s conception would apply approximatively, rather than as a strict law. Here, on the contrary, we show that the transformation problem is solvable from a determined (two-branch models) or an underdetermined system of equations enabling to obtain exact solutions through an algorithm we provide, with no approximation needed. For systems with three branches or more, the solution of the transformation problem belongs to an infinite ensemble and may be determined by an array of factors (e.g., taste of consumers, advertisement, competition between capitalists, politics), accounting for the observed high competition-driven market fluidity. Furthermore, we show that the transformation problem is solvable in the absence of fixed capital, supporting that dealing with the latter is not essential and cannot be seen as a potential flaw of the approach. In these particular cases, the rate of profit can be determined from the eigenvalue of the sociotechnical coefficient matrix, without any information required about the capital allocation to the various branches. Our algorithm enables simulations illustrating how the transient rise in rate of profit predicted by the Okishio theorem is consistent with the tendency of the rate of profit to fall (TRPF) subsequent to capital reallocation, and how the TRPF is governed by the increase of organic composition, in value. We establish that the long-standing transformation problem is not such a problem, since it is easily solved through our algorithm, whatever the number of branches considered. This emphasizes the high coherence of Marx’s conception, and its impressive relevance regarding issues such as the TRPF, which have remained intensely debated

A fast Markovian method for modeling channel noise in neurons

Channel noise results from rapid transitions of protein channels from closed to open state and is generally considered as the most dominant source of electrical noise causing membrane-potential fluctuations even in the absence of synaptic inputs. The simulation of a realistic channel noise remains a source of possible error. Although the Markovian method is considered as the golden standard for appropriate description of channel noise, its computation time increasing exponentially with the number of channels, it is poorly suitable to simulate realistic features. We describe here a novel algorithm at discrete time unit for simulating ion channel noise based on Markov chains (MC). Although this new algorithm refers to a Monte-Carlo process, it only needs few random numbers whatever the number of channels involved. Our fast MC (FMC) model does not exhibit the drawbacks due to approximations based on stochastic differential equations and the values of spike jitter are comparable to those obtained with the true Markovian method. In fact, we show here, that these drawbacks can be highlighted in the approximation based on stochastic differential equation methods even for a high number of channels (standard deviation of the 5th spike is about two-fold larger than that of MCF or true Markovian method for 5000 sodium channels). The FMC model appears therefore as the most accurate method to simulate channel noise with a fast execution time that does not depend on the channel number

Downregulation of Dendritic Ih in CA1 Pyramidal Neurons after LTP

International audienceHyperpolarization-activated (h)-channels occupy a central position in dendritic function. Although it has been demonstrated that these channels are upregulated after large depolarizations to reduce dendritic excitation, it is not clear whether they also support other forms of long-term plasticity. We show here that nearly maximal long-term potentiation (LTP) induced by theta-burst pairing produced upregulation in h-channel activity in CA1 pyramidal neurons. In contrast, moderate LTP induced by spike-timing-dependent plasticity or high-frequency stimulation (HFS) downregulated the h-current (Ih) in the dendrites. After HFS-induced LTP, the h-conductance (Gh) was reduced without changing its activation. Pharmacological blockade of Ih had no effect on LTP induction, but occluded EPSP-to-spike potentiation, an input-specific facilitation of dendritic integration. Dynamic-clamp reduction of Gh locally in the dendrite mimicked the effects of HFS and enhanced synaptic integration in an input-selective way. We conclude that dendritic Ih is locally downregulated after induction of nonmaximal LTP, thus facilitating integration of the potentiated input