[Physiological Aspects of the Application of Hodgkin-Huxley Model of Action Potential Initiation for Invertebrate and Vertebrate Neurons].

Recent studies have revealed.that in contrast to invertebrate systems, the initiation of action potentials in vertebrate neurons significantly differ from the relatively slow exponential dynamics predicted by Hodgkin-Huxley equations, but rather is characterized by a sharp onset with a kink. These data provided new insights into the link between action potential initiation and abilities of neurons and neuronal networks to encode. high frequency signals. Here, we review recent models describing sharp onset dynamics of action potential initiation, including an alternative model of cooperative activation of sodium channels, as well as the influence of the dynamics of action potential initiation on computational abili- ties of neuronal networks. The importance this topic is due to the fact that, despite the rapid development of neuronal modeling during last decades, the well established models are unable to capture experimentally observed details of the onset dynamics of action potentials in mammalian neurons and the abilities of neurons to reliably encode code high frequency signals Recent advances of experimental and theoretical analysis of generation of action potentials and neuronal encoding, presented in this review, are ofgreat importance for better understanding of neuronal processing and development of a more precise and realistic neuronal model.

Modulation of synaptic transmission by adenosine in layer 2/3 of the rat visual cortex in vitro.

Adenosine is a wide-spread endogenous neuromodulator. In the central nervous system it activates A1 and A2A receptors (A1Rs and A2ARs) which have differential distributions, different affinities to adenosine, are coupled to different G-proteins, and have opposite effects on synaptic transmission. Although effects of adenosine are studied in detail in several brain areas, such as the hippocampus and striatum, the heterogeneity of the effects of A1R and A2AR activation and their differential distribution preclude generalization over brain areas and cell types. Here we study adenosine's effects on excitatory synaptic transmission to layer 2/3 pyramidal neurons in slices of the rat visual cortex. We measured effects of bath application of adenosine receptor ligands on evoked excitatory postsynaptic potentials (EPSPs), miniature excitatory postsynaptic potentials (mEPSPs), and membrane properties. Adenosine reduced the amplitude of evoked EPSPs and excitatory postsynaptic currents (EPSCs), and reduced frequency of mEPSPs in a concentration-dependent and reversible manner. Concurrent with EPSP/C amplitude reduction was an increase in the paired-pulse ratio. These effects were blocked by application of the selective A1R antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine), suggesting that activation of presynaptic A1Rs suppresses excitatory transmission by reducing release probability. Adenosine (20µM) hyperpolarized the cell membrane from -65.3±1.5 to -67.7±1.8mV, and reduced input resistance from 396.5±44.4 to 314.0±36.3MOhm (∼20%). These effects were also abolished by DPCPX, suggesting postsynaptic A1Rs. Application of the selective A2AR antagonist SCH-58261 (2-(2-furanyl)-7-(2-phenylethyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-a-mine) on the background of high adenosine concentrations revealed an additional decrease in EPSP amplitude. Moreover, application of the A2AR agonist CGS-21680 (4-[2-[[6-amino-9-(N-ethyl-ß-d-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride) led to an A1R-dependent increase in mEPSP frequency. Dependence of the A2AR effects on the A1R availability suggests interaction between these receptors, whereby A2ARs exert their facilitatory effect on synaptic transmission by inhibiting the A1R-mediated suppression. Our results demonstrate functional pre and postsynaptic A1Rs and presynaptic A2ARs in layer 2/3 of the visual cortex, and suggest interaction between presynaptic A2ARs and A1Rs.

The determinants of the onset dynamics of action potentials in a computational model.

Action potentials (APs) in the soma of central neurons exhibit a sharp, step-like onset dynamics, which facilitates the encoding of weak but rapidly changing input signals into trains of action potentials. One possibility to explain the rapid AP onset dynamics is to assume cooperative activation of sodium channels. However, there is no direct evidence for cooperativity of voltage gated sodium channels in central mammalian neurons. The fact that APs in cortical neurons are initiated in the axon and backpropagate into the soma, prompted an alternative explanation of the sharp onset of somatic APs. In the invasion scenario, the AP onset is smooth at the initiation site in the axon initial segment, but the current invading the soma before somatic sodium channels are activated produces a sharp onset of somatic APs. Here we used multicompartment neuron models to identify ranges of active and passive cell properties that are necessary to reproduce the sharp AP onset in the invasion scenario. Results of our simulations show that AP initiation in the axon is a necessary but not a sufficient condition for the sharp onset of somatic AP: for a broad range of parameters, models could reproduce distal AP initiation and backpropagation but failed to quantitatively reproduce the onset dynamics of somatic APs observed in cortical neurons. To reproduce sharp onset of somatic APs, the invasion scenario required specific combinations of active and passive cell properties. The required properties of the axon initial segment differ significantly from the currently accepted and experimentally estimated values. We conclude that factors additional to the invasion contribute to the sharp AP onset and further experiments are needed to explain the AP onset dynamics in cortical neurons.

Retrograde signalling with nitric oxide at neocortical synapses.

Long-term changes of synaptic transmission in slices of rat visual cortex were induced by intracellular tetanization: bursts of short depolarizing pulses applied through the intracellular electrode without concomitant presynaptic stimulation. Long-term synaptic changes after this purely postsynaptic induction were associated with alterations of release indices, thus providing a case for retrograde signalling at neocortical synapses. Both long-term potentiation and long-term depression were accompanied by presynaptic changes, indicating that retrograde signalling can achieve both up- and down-regulation of transmitter release. The direction and the magnitude of the amplitude changes induced by a prolonged intracellular tetanization depended on the initial properties of the input. The inputs with initially high paired-pulse facilitation (PPF) ratio, indicative of low release probability, were most often potentiated. The inputs with initially low PPF ratio, indicative of high release probability, were usually depressed or did not change. Thus, prolonged postsynaptic activity can lead to normalization of the weights of nonactivated synapses. The dependence of polarity of synaptic modifications on initial PPF disappeared when plastic changes were induced with a shorter intracellular tetanization, or when the NO signalling pathway was interrupted by inhibition of NO synthase activity or by application of NO scavengers. This indicates that the NO-dependent retrograde signalling system has a relatively high activation threshold. Long-term synaptic modifications, induced by a weak postsynaptic challenge or under blockade of NO signalling, were nevertheless associated with presynaptic changes. This suggests the existence of another retrograde signalling system, additional to the high threshold, NO-dependent system. Therefore, our data provide a clear case for retrograde signalling at neocortical synapses and indicate that multiple retrograde signalling systems, part of which are NO-dependent, are involved.

Synaptic transmission in the neocortex during reversible cooling.

We studied the effects of reversible cooling on synaptic transmission in slices of rat visual cortex. Cooling had marked monotonic effects on the temporal properties of synaptic transmission. It increased the latency of excitatory postsynaptic potentials and prolonged their time-course. Effects were non-monotonic on other properties, such as amplitude of excitatory postsynaptic potentials and generation of spikes. The amplitude of excitatory postsynaptic potentials increased, decreased, or remain unchanged while cooling down to about 20 degrees C, but thereafter it declined gradually in all cells studied. The effect of moderate cooling on spike generation was increased excitability, most probably due to the ease with which a depolarized membrane potential could be brought to spike threshold by a sufficiently strong excitatory postsynaptic potential. Stimuli that were subthreshold above 30 degrees C could readily generate spikes at room temperature. Only at well below 10 degrees C could action potentials be completely suppressed. Paired-pulse facilitation was less at lower temperatures, indicating that synaptic dynamics are different at room temperature as compared with physiological temperatures. These results have important implications for extrapolating in vitro data obtained at room temperatures to higher temperatures. The data also emphasize that inactivation by cooling might be a useful tool for studying interactions between brain regions, but the data recorded within the cooled area do not allow reliable conclusions to be drawn about neural operations at normal temperatures.

Membrane properties and spike generation in rat visual cortical cells during reversible cooling.

We studied the effects of reversible cooling between 35 and 7 C on membrane properties and spike generation of cells in slices of rat visual cortex. Cooling led to a depolarization of the neurones and an increase of the input resistance, thus bringing the cells closer to spiking threshold. Excitability, measured with intracellular current steps, increased with cooling. Synaptic stimuli were most efficient in producing spikes at room temperature, but strong stimulation could evoke spikes even below 10 C. Spike width and total area increased with cooling, and spike amplitude was maximal between 12 and 20 C. Repetitive firing was enhanced in some cells by cooling to 20-25 C, but was always suppressed at lower temperatures. With cooling, passive potassium conductance decreased and the voltage-gated potassium current had a higher activation threshold and lower amplitude. At the same time, neither passive sodium conductance nor the activation threshold of voltage-dependent sodium channels changed. Therefore changing the temperature modifies the ratio between potassium and sodium conductances, and thus alters basic membrane properties. Data from two cells recorded in slices of cat visual cortex suggest a similar temperature dependence of the membrane properties of neocortical neurones to that described above in the rat. These results provide a framework for comparison of the data recorded at different temperatures, but also show the limitations of extending the conclusions drawn from in vitro data obtained at room temperature to physiological temperatures. Further, when cooling is used as an inactivation tool in vivo, it should be taken into account that the mechanism of inactivation is a depolarization block. Only a region cooled below 10 C is reliably silenced, but it is always surrounded by a domain of hyperexcitable cells.

Comparison of the selectivity of postsynaptic potentials and spike responses in cat visual cortex.

Intracellular recordings were made from neurons in the cat visual cortex (area 17) to compare the orientation and direction selectivities of the output of a cell with those of the input the cell receives. The input to a cell was estimated from the PSPs (postsynaptic potentials) evoked by visual stimulation, and the output estimated from the number of spikes generated during the same responses. For the whole sample, selectivity of the output of cells was significantly higher than selectivity of their input. Upon PSP to spike transformation, the selectivity index was, on average, doubled. However, the degree of the selectivity improvement in individual cells was very different, varying from cases in which highly selective output was created from a poorly selective input and thus selectivity was greatly improved, to little or no improvement in other neurons. The improvement of selectivity was not correlated with resting membrane potential, threshold for action potential generation, background discharge rate or amplitude of optimal PSP response. Further, no systematic difference was found between simple and complex cells in the input-output relations, indicating that the 'tip of the iceberg' effect on shaping the response selectivity was cell specific, but not cell type specific. This supports the notion that multiple mechanisms are responsible for generation of the response selectivity, and that the contribution of any particular mechanism may vary from one cell to the other. The heterogeneity of the input-output relations in visual cortical cells could indicate different functions of cells in the cortical network; some cells are creating selectivity de novo, the function of other neurons probably being repetition and amplification of the selected signal and arrangement of the output of a whole column.

Neuroscience. Noise makes sense in neuronal computing.

The united efforts of assemblies of neurons in the brain's primary visual cortex translate incoming visual signals into action potentials. These action potentials encode, for example, the contrast and orientation of different parts of the image. Some neurons are sensitive to one particular orientation, other are sensitive to other orientations, but all neurons respond equally well to the image contrast. In a Perspective, Volgushev and Eysel explain the finding (Anderson et al.) that neurons are able to maintain this sensitivity to the orientation of a stimulus regardless of the contrast by adding noise to the membrane potential, such that action potentials can also be generated in response to weak signals at low contrast.

Interaction between intracellular tetanization and pairing-induced long-term synaptic plasticity in the rat visual cortex.

Long-term changes in synaptic transmission in slices of rat visual cortex were induced either by pairing the excitatory postsynaptic potentials with postsynaptic depolarization or by intracellular tetanization without synaptic stimulation. Changes in the excitatory postsynaptic potential amplitude induced by any of the protocols applied in isolation persisted for longer than 1 h. Pairing-induced long-term potentiation was input specific. We studied the interaction between intracellular tetanization and pairing-induced plasticity by applying the two protocols one after the other at 10-min intervals. The pairing procedure applied after intracellular tetanization did not lead to any further potentiation, but to a depotentiation of the potentiated inputs. A second pairing protocol applied 10 min later led to further depotentiation, while previously unaffected inputs became weakly depressed. If intracellular tetanization was applied after the pairing procedure, the synaptic responses did not change immediately, but a slow return of the excitatory postsynaptic potential amplitude to the control level could be observed. Therefore, intracellular tetanization is not capable of inducing further potentiation after pairing, and pairing cannot further potentiate the inputs which have already been potentiated by intracellular tetanization. The maintenance of long-term potentiation induced by any of the protocols was impaired by successive application of another procedure. These results suggest a similarity of the mechanisms of synaptic changes induced by the two protocols and demonstrate that the direction of synaptic gain change depends on the history of the synapse.

Evidence for an ephaptic feedback in cortical synapses: postsynaptic hyperpolarization alters the number of response failures and quantal content.

The amplitude of excitatory postsynaptic potentials and currents increases with membrane potential hyperpolarization. This has been attributed to an increase in the driving force when the membrane potential deviates from the equilibrium potential of the respective ions. Here we report that in a subset of neocortical and hippocampal synapses, postsynaptic hyperpolarization affects traditional measures of transmitter release: the number of failures, coefficient of variation of response amplitudes, and quantal content, suggesting increased presynaptic release. The result is compatible with the hypothesis of Byzov on the existence of electrical (or "ephaptic") linking in purely chemical synapses. The linking, although negligible at neuromuscular junctions, could be functionally significant in influencing transmitter release at synapses with high resistance along the synaptic cleft. Our findings necessitate reconsideration of classical amplitude-voltage relations for such synapses. Thus, synaptic strength may be enhanced by hyperpolarization of the postsynaptic membrane potential. The positive ephaptic feedback could account for "all-or-none" excitatory postsynaptic potentials at some cortical synapses, large evoked and spontaneous multiquantal events and a high efficacy of large "perforated" synapses whose number increases following behavioural learning or the induction of long-term potentiation.

NMDA receptor blockade prevents LTD, but not LTP induction by intracellular tetanization.

Intracellular tetanization, the activation of a postsynaptic cell without concomitant presynaptic stimulation, was applied to layer II/III pyramidal cells in slices of rat visual cortex. In standard extracellular medium, intracellular tetanization led to LTP (21 of 43 inputs) or LTD (14 of 43 inputs), the direction of the amplitude change depending on initial paired-pulse facilitation (PPF) ratio: inputs with high initial PPF ratio were usually potentiated, and inputs with initially low PPF were most often depressed or did not change. When applied during blockade of NMDA receptors (50 microM APV), intracellular tetanization failed to induce LTD, but was still capable of inducing LTP (14 of 26 inputs). Although LTP could occur in inputs with both, low and high initial PPF ratio, the correlation between the amplitude change and initial PPF ratio remained: potentiation was stronger in inputs with initially higher PPF. These data suggest that intracellular tetanization activated simultaneously NMDA receptor-dependent LTD mechanisms and NMDA receptor-independent LTP mechanisms, the final change of synaptic gain depending on their balance.

Modification of discharge patterns of neocortical neurons by induced oscillations of the membrane potential.

We investigated, with whole-cell recordings from rat visual cortex slices, how sinusoidal modulation of the membrane potential affects signal transmission. Subthreshold oscillations activate tetrodotoxin sensitive, transient inward currents whose threshold, phase lag and duration change with modulation frequency. These periodically recurring phases of enhanced excitability affect synaptic transmission in two ways. Weak and short lasting excitatory postsynaptic potentials evoke discharges only if they are coincident within a few milliseconds with these active membrane responses. Long-lasting, N-methyl-D-aspartate-mediated or polysynaptic excitatory postsynaptic potentials, by contrast, evoke trains of spikes, that are precisely time-locked to the oscillations and may last for more than 100 ms. Thus, oscillations impose a precise temporal window for the integration of synaptic inputs, favouring coincidence detection and they generate temporally-structured responses whose timing and amplitude are largely independent of the input. These properties are ideally suited for the synchronization of neuronal activity and the encoding of information in the precise timing of discharges. A preliminary account of these data has appeared in an abstract form [Volgushev M. et al. (1995) Eur. J Neurosci. 8, 77].

Relations between long-term synaptic modifications and paired-pulse interactions in the rat neocortex.

The phenomenon of paired-pulse facilitation (PPF) was exploited to investigate the role of presynaptic mechanisms in the induction and maintenance of long-term synaptic plasticity in the neocortex. Long-term potentiation (LTP) and depression (LTD) were induced without afferent activation by applying tetani of intracellular pulses. Our results show that synaptic modifications closely resembling LTP and LTD can be induced by postsynaptic activation alone. The polarity of these synaptic modifications depends on initial properties of the input, as indicated by a correlation between initial PPF ratio and post-tetanic amplitude changes: inputs exhibiting strong PPF, which might be associated with low release probability tend to be potentiated, while inputs with small PPF are more likely to show depression. Maintenance of both LTP and LTD involve presynaptic mechanisms, as indicated by changes in PPF ratios and in failure rate after LTP or LTD induction. Presynaptic mechanisms could include changes in release probability and/or in the number of active release sites. Because induction was postsynaptic, this supports the notion of a retrograde signal. The relative contribution of pre- and postsynaptic mechanisms in the maintenance of long-term synaptic modifications depends on the initial state of the synaptic input and on LTP magnitude. PPF changes were especially pronounced in inputs which had initially high PPF and underwent strong potentiation. Since LTP and LTD are associated with changes of PPF ratios these synaptic modifications do not only alter the gain but also the temporal properties of synaptic transmission. Because of the LTP associated reduction of PPF, potentiated inputs profit less from temporal summation, favouring transmission of synchronized, low frequency activity.

A linear model fails to predict orientation selectivity of cells in the cat visual cortex.

1. Postsynaptic potentials (PSPs) evoked by visual stimulation in simple cells in the cat visual cortex were recorded using in vivo whole-cell technique. Responses to small spots of light presented at different positions over the receptive field and responses to elongated bars of different orientations centred on the receptive field were recorded. 2. To test whether a linear model can account for orientation selectivity of cortical neurones, responses to elongated bars were compared with responses predicted by a linear model from the receptive field map obtained from flashing spots. 3. The linear model faithfully predicted the preferred orientation, but not the degree of orientation selectivity or the sharpness of orientation tuning. The ratio of optimal to non-optimal responses was always underestimated by the model. 4. Thus non-linear mechanisms, which can include suppression of non-optimal responses and/or amplification of optimal responses, are involved in the generation of orientation selectivity in the primary visual cortex.

Involvement of silent synapses in the induction of long-term potentiation and long-term depression in neocortical and hippocampal neurons.

Changes in the latency of small excitatory postsynaptic potentials were observed in association with induction of long-term modifications of synaptic transmission in slices of rat neocortex and guinea-pig hippocampus. After potentiation response latency decreased in 3/10 cases in the neocortex and in 6/24 cases in the hippocampus, and increased after depression in 4/8 cases in the neocortex. These latency changes could not be attributed to changes in presynaptic fibre excitability, monosynaptic inhibition, release kinetics or activation kinetics of postsynaptic ion channels. We conclude therefore that potentiation led to the activation of previously silent synapses of fast-conducting afferents and depression to the inactivation of previously functional synapses. Thus, neocortical and hippocampal synapses can be in a non-functional state, and regimes that induce long-term potentiation and depression not only change the efficacy of synapses but also alter their functional state.

Multiple mechanisms underlying the orientation selectivity of visual cortical neurones.

For over three decades, the mechanism of orientation selectivity of visual cortical neurones has been hotly debated. While intracortical inhibition has been implicated as playing a vital role, it has been difficult to observe it clearly. On the basis of recent findings, we propose a model in which the visual cortex brings together a number of different mechanisms for generating orientation-selective responses. Orientation biases in the thalamo-cortical input fibres provide an initial weak selectivity either directly in the excitatory input or by acting via cortical interneurones. This weak selectivity of postsynaptic potentials is then amplified by voltage-sensitive conductances of the cell membrane and excitatory and inhibitory intracortical circuitry, resulting in the sharp tuning seen in the spike discharges of visual cortical cells.

Precise timing of neuronal discharges within and across cortical areas: implications for synaptic transmission.

Multielectrode recordings were performed in a variety of structures of structures of the mammalian brain in order to examine temporal relations among simultaneously measured neuronal responses. Data indicate close correlations between perceptual phenomena and zero-time lag synchronization of distributed neuronal discharges.

All-or-none excitatory postsynaptic potentials in the rat visual cortex.

Intracellular recordings were obtained from supragranular neurons in slices of the rat visual cortex. In approximately 25% of the cells large (0.5-1.6 mV) excitatory postsynaptic potentials (EPSPs) of constant amplitude were observed after minimal, presumably single-fibre stimulation. The amplitude variance of these large EPSPs was surprisingly small and within the range of the variance of the noise. These EPSPs could be reduced in amplitude by paired-pulse and low-frequency stimulation or by raising extracellular Mg2+ concentration. Reduced EPSPs could either continue to behave as all-or-none responses, or they could fluctuate between several amplitude levels. Conversely, responses where the amplitude fluctuated from trial to trial under control conditions could be converted into large all-or-none responses by paired-pulse facilitation. This indicates that the large all-or-none EPSPs were composed of several subunits, probably reflecting the action of several different release sites. It is concluded that these release sites are either independent and operate with a probability close to 1 or, if operating with a lower probability, are coordinated by a mechanism which synchronizes release. Several observations suggest that release probabilities can switch from values close to 1 to 0 with repetitive stimulation or high Mg2+ concentration. Thus, a substantial fraction of single-fibre inputs to supragranular cells possess synapses which operate with high synaptic efficiency and extremely low variance under control conditions but can undergo drastic changes in efficacy when release probabilities are interfered with. Such modifications of release probability could serve as an effective mechanism to regulate the gain of synaptic transmission.

Dynamics of the orientation tuning of postsynaptic potentials in the cat visual cortex.

We evaluated the dynamic aspects of the orientation tuning of the input to cat visual cortical neurons by analyzing the postsynaptic potentials (PSPs) evoked by flashing bars of light. The PSPs were recorded using in vivo whole-cell technique, and we analyzed the orientation tuning during subsequent temporal windows after stimulus onset and offset. Our results show that the amplitudes of the postsynaptic potential are reliably tuned to orientation and matching that of the spike responses only during certain temporal windows. During the first 100 ms after stimulus presentation, orientation tuning of the membrane potential underwent regular changes. Within particular intervals, orientation tuning of the input was much sharper than that estimated according to the whole response. In most cells, optimal orientation was usually stable over the whole period. In several cells which had a second hump of EPSPs in the response, this second hump was tuned to the same orientation as the first one, but always showed sharper tuning. Estimation of the integration time revealed sufficient delay between the appearance of EPSPs and spikes, to let inhibition influence spike generation. These results show that orientation selectivity of the input to cortical cells is a dynamic function, and also indicate the possibility of temporal coding in the visual system.