Biomedical subjects

V Crunelli

Publications and source records attributed to V Crunelli.

At least 37 records · Page 2Linked to original sources

PubMed1998

Sodium current in rat and cat thalamocortical neurons: role of a non-inactivating component in tonic and burst firing.

The properties of the Na+ current present in thalamocortical neurons of the dorsal lateral geniculate nucleus were investigated in dissociated neonate rat and cat neurons and in neurons from slices of neonate and adult rats using patch and sharp electrode recordings. The steady-state activation and inactivation of the transient Na+ current (INa) was well fitted with a Boltzmann curve (voltage of half-maximal activation and inactivation, V1/2, -29.84 mV and -70.04 mV, respectively). Steady-state activation and inactivation curves showed a small region of overlap, indicating the occurrence of a INa window current. INa decay could be fitted with a single exponential function, consistent with the presence of only one channel type. Voltage ramp and step protocols showed the presence of a noninactivating component of the Na+ current (INaP) that activated at potentials more negative (V1/2 = -56.93 mV) than those of INa. The maximal amplitude of INaP was approximately 2.5% of INa, thus significantly greater than the calculated contribution (0.2%) of the INa window component. Comparison of results from dissociated neurons and neurons in slices suggested a dendritic as well as a somatic localization of INaP. Inclusion of papain in the patch electrode removed the fast inactivation of INa and induced a current with voltage-dependence (V1/2 = -56.92) and activation parameters similar to those of INaP. Current-clamp recordings with sharp electrodes showed that INaP contributed to depolarizations evoked from potentials of approximately -60 mV and unexpectedly to the amplitude and latency of low-threshold Ca2+ potentials, suggesting that this noninactivating component of the Na+ channel population plays an important role in the integrative properties of thalamocortical neurons during both tonic and burst-firing patterns.

Action Potentials↗

PubMed1998

Effects of tapering geometry and inhomogeneous ion channel distribution in a neuron model.

Recent experiments have produced direct evidence on the existence of various dendritic voltage-gated ion channels, indicating that these neuronal components are not just a passive medium for the propagation of synaptic excitation but a putative source of neuronal excitability that is reflected in the activity patterns occurring on the soma. In order to study possible changes in neuronal excitability when the distribution of dendritic voltage-activated channels is non-uniform, and the dendritic geometry is not necessarily cylindric, we have developed a neuron model that incorporates two voltage-activated currents [I(Na) and I(K)], and in which space-dependent distributions of the system parameters can be treated in a mathematically simple and efficient way. Simulation results with the model showed that both linearly and exponentially tapering geometries led to marked anisotropy of the propagation of excitation, favouring the soma-to-dendrite direction. Exponentially decaying densities of dendritic voltage-activated channels, with appropriate choice of the parameters, induced bistable behaviour between the normal resting state and an intrinsic, sustained oscillation with cylindric as well as linear and exponential tapering dendritic geometry. Bistability could not be evoked when the model was reduced to a space-independent one (point-like soma). These results suggest that both tapering dendritic geometry and inhomogeneous distribution of ion channels may crucially affect the propagation and integration of synaptic potentials, and that changes in dendritic channel densities might underlie pathological electrophysiological activities.

Action Potentials↗

PubMed1998

Analysis and biophysical interpretation of bistable behaviour in thalamocortical neurons.

Thalamocortical neurons display a wide spectrum of activity patterns that are the expressions of the non-linear interactions between the various voltage-gated ion channels. Here, we show how bistable behaviour can emerge in these neurons, and how it is brought about by the steady-state residual ("window") component of IT, the low-threshold Ca2+ current. In particular, we present results that describe the dependence of bistability on two system parameters: the injected current and the leakage conductance. In addition, we provide a biophysical interpretation of these results by means of the properties of the electrical circuit representing the neuron membrane.

Algorithms↗

PubMed1998

Dynamic clamp study of Ih modulation of burst firing and delta oscillations in thalamocortical neurons in vitro.

The dynamic clamp technique was used in thalamocortical neurons of the rat and cat dorsal lateral geniculate nucleus in vitro to investigate the effects of the hyperpolarization-activated cation current, Ih, and of its neuromodulation on burst firing and delta oscillations. Specific block of endogenous Ih using 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)pyridinium chloride (ZD7288) (300 microM) abolished the depolarizing "sag" response to negative current steps, markedly increased the latency and shortened the duration of the low-threshold Ca2+ potentials, and decreased the number of action potentials in the burst evoked by the low-threshold Ca2+ potential. Subsequent introduction of artificial Ih using the dynamic clamp re-instated the "sag" and all the original properties of the low-threshold Ca2+ potential. In the absence of ZD7288, introduction of artificial outward Ih with the intention of abolishing endogenous Ih removed the depolarizing "sag" and produced similar effects on the low-threshold Ca2+ potentials as those observed during the pharmacological block of Ih. Application of ZD7288 to thalamocortical neurons displaying delta oscillations led to a reduction in the voltage range of their existence or to a complete cessation of this behaviour. A subsequent introduction of artificial Ih re-enabled the generation of delta oscillations. In the presence of ZD7288, physiologically relevant positive shifts in the voltage-dependence of artificial Ih increased the amplitude and duration of the low-threshold Ca2+ potential and increased the likelihood of delta oscillations while negative shifts had opposite effects. These results highlight the important difference between the dependence of burst firing and oscillations on membrane potential and their dependence on the properties of Ih, and demonstrate that the modulation by Ih of low-threshold Ca2+ potentials and burst firing in thalamocortical neurons, as well as the ability of these neurons to generate delta oscillations, is more elaborate than previously described.

2-Amino-5-phosphonovalerate↗

PubMed1997

The 'window' component of the low threshold Ca2+ current produces input signal amplification and bistability in cat and rat thalamocortical neurones.

1. The mechanism underlying a novel form of input signal amplification and bistability was investigated by intracellular recording in rat and cat thalamocortical (TC) neurones maintained in slices and by computer simulation with a biophysical model of these neurones. 2. In a narrow membrane potential range centred around -60 mV, TC neurones challenged with small (10-50 pA), short (50-200 ms) current steps produced a stereotyped, large amplitude hyperpolarization (> 20 mV) terminated by the burst firing of action potentials, leading to amplification of the duration and amplitude of the input signal, that is hereafter referred to as input signal amplification. 3. In the same voltage range centred around -60 mV, single evoked EPSPs and IPSPs also produced input signal amplification, indicating that this behaviour can be triggered by physiologically relevant stimuli. In addition, a novel, intrinsic, low frequency oscillation, characterized by a peculiar voltage dependence of its frequency and by the presence of plateau potentials on the falling phase of low threshold Ca2+ potentials, was recorded. 4. Blockade of pure Na+ and K+ currents by tetrodotoxin (1 microM) and Ba2+ (0.1-2.0 mM), respectively, did not affect input signal amplification, neither did the presence of excitatory or inhibitory amino acid receptor antagonists in the perfusion medium. 5. A decrease in [Ca2+]o (from 2 to 1 mM) and an increase in [Mg2+]o (from 2 to 10 mM), or the addition of Ni2+ (2-3 mM), abolished input signal amplification, while an increase in [Ca2+]o (from 2 to 8 mM) generated this behaviour in neurones where it was absent in control conditions. These results indicate the involvement of the low threshold Ca2+ current (IT) in input signal amplification, since the other Ca2+ currents of TC neurones are activated at potentials more positive than -40 mV. 6. Blockade of the slow inward mixed cationic current (Ih) by 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)-pyrimidinium++ + chloride (ZD 7288)(100-300 microM) did not affect the expression of the large amplitude hyperpolarization, but abolished the subsequent repolarization to the original membrane potential. In this condition, therefore, input signal amplification was replaced by bistable membrane behaviour, where two stable membrane potentials separated by 15-30 mV could be switched between by small current steps. 7. Computer simulation with a model of a TC neurone, which contained only IT, Ih, K+ leak current (ILeak) and those currents responsible for action potentials, accurately reproduced the qualitative and quantitative properties of input signal amplification, bistability and low frequency oscillation, and indicated that these phenomena will occur at some value of the injected DC if, and only if, the 'window' component of IT (IT,Window) and the leak conductance (gLeak) satisfy the relation (dIT,Window/dV)max > gLeak. 8. The physiological implications of these findings for the electroresponsiveness of TC neurones are discussed, and, as IT is widely expressed in the central nervous system, we suggest that 'window' IT will markedly affect the integrative properties of many neurones.

Action Potentials↗

PubMed1997

Morphology and membrane properties of neurones in the cat ventrobasal thalamus in vitro.

1. The morphological (n = 66) and electrophysiological (n = 41) properties of eighty-six thalamocortical (TC) neurones and those of one interneurone in the cat ventrobasal (VB) thalamus were examined using an in vitro slice preparation. The resting membrane potential for thirty-seven TC neurones was -61.9 +/- 0.7 mV, with thirteen neurones exhibiting delta oscillation with and without DC injection. 2. The voltage-current relationships of TC neurones were highly non-linear, with a mean peak input resistance of 254.4 M omega and a mean steady-state input resistance of 80.6 M omega between -60 and -75 mV. At potentials more positive than -60 mV, outward rectification led to a mean steady-state input resistance of 13.3 M omega. At potentials more negative than -75 mV, there was inward rectification, consisting of a fast component leading to a mean peak input resistance of 14.5 M omega, and a slow time-dependent component leading to a mean steady-state input resistance of 10.6 M omega. 3. Above -60 mV, three types of firing were exhibited by TC neurones. The first was an accelerating pattern associated with little spike broadening and a late component in the spike after-hyperpolarization. The second was an accommodating or intermittent pattern associated with spike broadening, while the third was a burst-suppressed pattern of firing also associated with spike broadening, but with broader spikes of a smaller amplitude. All TC neurones evoked high frequency (310-520 Hz) burst firing mediated by a low threshold Ca2+ potential. 4. Morphologically TC neurones were divided into two groups: Type I (n = 31 neurones) which had larger soma, dendritic arbors that occupied more space, thicker primary dendrites and daughter dendrites that followed a more direct course than Type II (n = 35). The only electrophysiological differences were that Type I neurones (n = 16) had smaller peak input and outward rectification resistance and spike after-hyperpolarization, but greater peak inward rectification resistance, and exhibited delta oscillation less often than Type II (n = 13). 5. The morphologically identified interneurone exhibited no outward rectification, only moderate inward rectification, and no high frequency firing associated with the offset of negative current steps below -55 mV. This interneurone had a regular accommodating firing pattern, but the spike after-hyperpolarization had a late component, unlike the accommodating firing in TC neurones. 6. Therefore, the differentiation of TC neuronal types in the cat VB thalamus based on their morphology was reflected by differences in peak input resistance, outward rectification and spike after-hyperpolarization, which could be accounted for by their difference in soma size. More importantly, the firing pattern of the majority of TC neurones in the cat VB thalamus were different from those of TC neurones in other sensory thalamic nuclei. 7. Thalamocortical neurones in the cat VB thalamus were also clearly distinguishable from the interneurone based on the presence of their prominent outward rectification, peak inward rectification and robust low threshold Ca2+ potentials.

Action Potentials↗

PubMed1997

On the nature of anomalous rectification in thalamocortical neurones of the cat ventrobasal thalamus in vitro.

1. Intracellular sharp electrode current clamp and discontinuous single electrode voltage clamp recordings were made from thalamocortical neurones (n = 57) of the cat ventrobasal thalamus in order to investigate the mechanism underlying anomalous rectification. 2. Under current clamp conditions, voltage-current (V-I) relationships in a potential range of -55 to -110 mV demonstrated anomalous rectification with two components: fast rectification, which controlled the peak of negative voltage deviations, and time-dependent rectification. Time-dependent rectification was apparent as a depolarizing sag generated during the course of negative voltage deviations, was first formed at potentials in the range -60 to -70 mV, and was sensitive to 3 mM Cs+ (n = 6). Similarly, under voltage clamp conditions, instantaneous and steady-state I-V relationships demonstrated anomalous rectification. A slowly activating inward current with an activation threshold in the range of -65 to -70 mV formed time-dependent rectification. This current was sensitive to Cs+ (3 mM) (n = 3) and had properties similar to the slow inward mixed cationic current (Ih). 3. 4-(N-Ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)-pyrimidinium++ + chloride (ZD 7288) (100-300 microM) irreversibly blocked time-dependent rectification mediated by Ih (n = 23 of 25 neurones), and led to a hyperpolarization of the resting membrane potential (6.8 +/- 0.5 mV). In the presence of ZD 7288, V-I and I-V relationships, exhibited fast anomalous rectification, first activated from potential more negative than -80 mV. 4. Ba2+ (100 microM) (n = 8), in the continuous presence of ZD 7288, reversibly linearized peak V-I and instantaneous I-V relationships over a potential range of -70 to -120 mV, and led to a membrane depolarization (13.3 +/- 4.2 mV) or tonic inward current (192 +/- 36 pA). 5. The co-application of ZD 7288 and Ba2+ revealed a depolarizing sag in negative voltage deviations under current clamp conditions, or a large inward current with kinetics two to three times slower than those of Ih under voltage clamp conditions. This novel form of time-dependent rectification was first apparent at potentials more negative than about -85 mV, was sensitive to 5 mM Cs+ (n = 4), and is termed Ih,slow. Ih,slow tail currents reversed between -65.3 and -56.6 mV (with potassium acetate electrodes, n = 3) or -57.6 and -50.3 mV (with KCl electrodes, n = 3). 6. Computer simulations confirmed that the pattern of anomalous rectification in thalamocortical neurones of the cat ventrobasal thalamus is mediated by the concerted action of Ih and a Ba(2+)-sensitive current with properties similar to an inwardly rectifying K+ current (IKIR).

Animals↗

PubMed1997

Simulation of intermittent action potential firing in thalamocortical neurons.

Thalamocortical neurons are capable of displaying complex electrophysiological behaviour, such as low- and high-frequency oscillations, because of their large number of different membrane channels. Recent experimental results have provided direct evidence on the involvement of high-threshold Ca2+ currents (I(HVA)) in the genesis of the high-frequency oscillations that underlie intermittent action potential firing. Complementing these findings, we now show by means of a biophysical model of a thalamocortical neuron how the interaction of I(HVA) with a Ca2+-activated K+ current which is also present in these neurons may generate this activity via intracellular Ca2+ processing.

Action Potentials↗

PubMed1997

Postnatal development of membrane properties and delta oscillations in thalamocortical neurons of the cat dorsal lateral geniculate nucleus.

The development of membrane properties, firing patterns, and delta oscillations in neurons of the cat dorsal lateral geniculate nucleus (dLGN) was investigated in vitro during the first 7 postnatal weeks. Compared with adult neurons, the resting membrane potential was more depolarized at postnatal days 1-9 (P1-P9), the input resistance was higher at P1-P7, and action potentials had a higher threshold and a smaller amplitude at P1-P3 and a longer duration at P1-P9. At P1-P3 trains longer than 200 msec were rarely observed, and trains with more than three action potentials were only present in 41% of the neurons, whereas at P1-P7 the normalized slope of the instantaneous frequencies at the first five interspike intervals was smaller than in the adult. A long-lasting (up to 6 sec) afterhyperpolarization followed a short train of action potentials in 88 and 30% of neurons at P1-P3 and P30-P32, respectively, but it was rarely observed in the adult. The low-threshold Ca2+ potential could evoke a burst of action potentials since P1. However, at P1-P7 the number of action potentials per burst was smaller (range, one to five), and at P1-P9 their maximum instantaneous frequency was lower (<190 Hz) than in the adult (range, six to eight, and 344 Hz, respectively). No delta oscillations were observed until P17, and their frequency (0.36 Hz) was lower than that in the adult (1.8 Hz). The percentage of neurons displaying delta oscillations and their frequency reached adult values by the end of the seventh postnatal week, i.e., well after the maturation of the membrane properties and firing patterns (second postnatal week). In conclusion, the maturation of the electrophysiological properties of thalamocortical neurons in the cat dLGN is completed later than the retinogeniculate axon segregation (Shatz CJ, 1983), and the immaturity of the oscillatory, and not of the burst-firing, activity is a limiting factor in the development of delta waves.

Action Potentials↗

PubMed1997

Model of a thalamocortical neurone with dendritic voltage-gated ion channels.

To investigate the functional role of dendrites of thalamocortical neurones, we have used our one-compartmental model to construct a multi-compartmental model with dendritic regions chosen according to a representative soma-to-dendritic terminal path of an X cell of the cat dorsal lateral geniculate nucleus. The multi-compartmental model with dendritic low-threshold CA2+ and delayed rectifier K+ channels yields more accurate results than the one-compartmental model when simulating tonic firing and oscillatory activities, and provides a useful means for the study of propagation of excitation on the dendrites.

Action Potentials↗

PubMed1996

In vivo blockade of thalamic GABA(B) receptors increases excitatory amino-acid levels.

The effect of intrathalamic application of GABA(B) receptor antagonists on the basal excitatory amino-acid levels was studied using microdialysis probes implanted in the dorsal lateral geniculate nucleus and in the ventrobasal complex. In both nuclei, continuous perfusion of the GABA(B) receptor antagonist 3-aminopropyl-(diethoxymethyl)-phosphinic acid (CGP 35348) produced an increase in the extracellular concentration of aspartate and (to a lesser extent) glutamate, but no change was observed in the level of taurine, the main amino acid involved in the regulation of brain osmolarity processes. In contrast, 3-amino-2-hydroxy-2-(4-chlorophenyl)-propanesulphonic acid (2-hydroxy-saclofen), another GABA(B) receptor antagonist, failed to affect the extracellular concentration of aspartate, glutamate and taurine. Thus, the basal level of excitatory amino acids in the thalamus in vivo is under the control of CGP 35348-sensitive GABA(B) receptors.

Animals↗

PubMed1996

Model of a thalamocortical neurone with dendritic voltage-gated ion channels.

To investigate the functional role of dendrites of thalamocortical neurones, we have used our one-compartmental model to construct a multi-compartmental model with dendritic regions chosen according to a representative soma-to-dendritic terminal path of an X cell of the cat dorsal lateral geniculate nucleus. The multi-compartmental model with dendritic low-threshold Ca2+ and delayed rectifier K+ channels yields more accurate results than the one-compartmental model when simulating tonic firing and oscillatory activities, and provides a useful means for the study of propagation of excitation on the dendrites.

Animals↗

PubMed1996

Gamma-hydroxybutyric acid decreases thalamic sensory excitatory postsynaptic potentials by an action on presynaptic GABAB receptors.

The effect of gamma-hydroxybutyric acid (GHB) on the excitatory postsynaptic potential (EPSP) evoked in thalamocortical neurones of the rat dorsal lateral geniculate nucleus and ventrobasal thalamus was investigated in vitro. GHB (0.1-5 mM) dose-dependently and reversibly decreased (36-78%) the amplitude of the sensory EPSP. This effect of GHB was blocked by the GABAB receptor antagonist CGP 35348 (1 mM). NCS 382 (1-3 mM), a putative GHB receptor antagonist, did not antagonise but weakly potentiated both the GHB- and baclofen-mediated decrease of the EPSP amplitude.

Adjuvants, Anesthesia↗

PubMed1996

Electrophysiological and morphological properties of interneurones in the rat dorsal lateral geniculate nucleus in vitro.

1. Intracellular recordings were made from putative interneurones (n = 24) and thalamocortical (TC) projection neurones (n = 45) in slice preparations of the rat dorsal lateral geniculate nucleus (dLGN) in order to compare the electrophysiological properties of these neuronal types. 2. Intracellular injection of biocytin to electrophysiologically identified neurones (n = 34) revealed the morphology of putative interneurones (n = 4) to be similar to class B and that of TC neurones (n = 30) to be similar to class A Golgi-impregnated neurones. 3. Interneurones had resting membrane potentials (-52 mV) relatively positive to those of TC neurones (-63 mV), shorter time constants (36.8 and 58.2 ms, respectively), but similar steady-state input resistances (164 and 180 M omega, respectively). Steady-state voltage-current relationships were nearly linear in interneurones, but highly non-linear in TC neurones. 4. The structure of action potential firing evoked at the break of hyperpolarizing voltage transients was dependent upon neuronal type. Interneurones fired a single action potential or a burst of action potentials with a maximum frequency of < 130 Hz, whilst TC neurones fired a high frequency burst with a minimum frequency of > 250 Hz. In addition, well-defined burst firing of action potentials in response to depolarizing voltage excursions, from membrane potentials negative to -65 mV, could be evoked in TC neurones, but not in interneurones. 5. The directly evoked action potentials of interneurones were characterized by an initial slow pre-potential preceding the fast upstroke of the action potential. The amplitude and width of interneurones' action potentials were smaller than those of TC neurones and the amplitude and duration of the single action potential after-hyperpolarization were greater in interneurones. Both interneurones and TC neurones fired action potentials repetitively in response to suprathreshold voltage excursions, with interneurones demonstrating a greater degree of spike-frequency adaptation. Following a train of action potentials, interneurones and TC neurones generated a slow after-hyperpolarizing potential: in interneurones but not TC neurones this potential was followed by a slow depolarizing potential. 6. An intrinsic, subthreshold membrane potential oscillatory activity with a mean frequency of approximately 8 Hz was observed in interneurones. 7. Electrical stimulation of the optic tract evoked in interneurones apparently pure EPSPs, pure IPSPs or a mixture of EPSPs and IPSPs. EPSPs were found to be biphasic and mediated by the activation of non-N-methyl-D-aspartate (NMDA) and NMDA excitatory amino acid receptors. IPSPs and the response to the iontophoretic application of GABA were found to reverse between -65 and -70 mV. The application of GABAB receptor agonists failed to affect the membrane properties of six of seven interneurones tested. In addition spontaneous EPSPs and IPSPs were recorded in interneurones. 8. These results demonstrate that the electrophysiological properties of putative interneurones are distinct from those of TC neurones of the rat dLGN. The implications of these findings for the control of visual responsiveness of TC neurones are discussed.

Animals↗

PubMed1996

The theoretical basis for non-exponential time-course of the recovery from inactivation of Hodgkin-Huxley-type ionic currents.

In this paper, we have carried out a theoretical analysis of the recovery process of inactivating currents whose voltage-dependent conductances obey the Hodgkin-Huxley equations. We demonstrate that the recovery process is complex, and, in particular, is non-exponential. Consequently, it cannot be characterized by a single-time constant. Nevertheless, its time-course is completely determined by the properties of the activation and inactivation kinetics at the membrane potential at which the deinactivation of the current takes place. Moreover, we show that the recovery asymptotically approaches an exponential time-course whose time-constant, in turn, is found to be identical to that of the inactivation at the membrane potential of deinactivation. The method commonly used to reconstruct the recovery process can, therefore, provide a way of estimating the inactivation time-constant at membrane potentials where a measurement with the usual voltage-clamp protocol would not be possible. The conclusions of our analysis are discussed with regard to recent theoretical and experimental results.

Electrophysiology↗

PubMed1996

Tonic activation of presynaptic GABA(B) receptors on thalamic sensory afferents.

The presence and role of presynaptic GABA(B) receptors in the control of excitatory amino acid-medicated transmission were investigated (using sharp electrode recordings) in the rat dorsal lateral geniculate nucleus and ventrobasal thalamus in vitro by comparing the effects of the selective GABA(B) receptor agonist, (+ or -)-baclofen, and of two antagonists, CGP 35348 and 2-hydroxy-saclofen, on the excitatory postsynaptic potentials evoked in thalamocortical neurons by stimulation of the sensory afferents. Application of CGP 35348 alone blocked the GABA(B) receptor-mediated inhibitory postsynaptic potential evoked in the dorsal lateral geniculate nucleus by stimulation of the optic tract (n = 5), but had no effect on the resting membrane potential and input resistance of thalamocortical cells (n = 6). In contrast, 2-hydroxy-saclofen caused a hyperpolarization (6.9 + or - 0.5 mV, n = 10) and a decrease in the apparent input resistance (26.3 + or - 2.6%, n = 10). This effect of 2-hydroxy-saclofen was antagonized by CGP 35348. When bicuculline was present in the perfusion medium and following intracellular injection of QX 314, GABA(A) and GABA(B) receptors in the recorded neurons were blocked. Under this condition, application of baclofen decreased the amplitude of the medial lemniscus- and optic tract-evoked excitatory postsynaptic potentials in the two thalamic nuclei investigated. This effect was fully antagonized by CGP 35348 and only partially by 2-hydroxy-saclofen. CGP 35348 alone increased (19.3 + or - 4.3%, n = 5) and 2-hydroxy-saclofen alone decreased (29.9 + or - 8.6%, n = 5) the amplitude of the excitatory postsynaptic potential. This effect of 2-hydroxy-saclofen was not blocked by CGP 35348. These results indicate that presynaptic GABA(B) receptors are present on the terminals of the sensory afferents in the rat dorsal lateral geniculate nucleus and in the ventrobasal thalamus. These receptors are tonically activated by endogenous GABA, at least in vitro, and provide a negative control mechanism by which the excitatory amino acid-mediated transmission within these nuclei can be regulated. In contrast, the endogenous GABA level is not sufficient for a tonic activation of postsynaptic GABA(B) receptors. Furthermore, these results indicate that 2-hydroxy-saclofen acts as a partial agonist on postsynaptic CGP 35348-sensitive GABA(B) receptors, and that, in addition to its antagonist action on presynaptic CGP 35348-sensitive GABA(B) receptors, it also has an effect on either presynaptic, CGP 35348-insensitive GABA(B) receptors and/or another presynaptic receptor type.

Animals↗

PubMed1995

A numerical procedure to estimate kinetic and steady-state characteristics of inactivating ionic currents.

The voltage-clamp technique is widely employed to obtain data suitable for the reliable estimation of the steady-state and kinetic parameters of inactivating ionic currents in neurones and other excitable cells. Yet, the estimation procedure itself remains a difficult numerical problem, because of the strong non-linear nature of the currents involved. The majority of the numerical methods of parameter estimation makes use of one or another type of non-linear optimization algorithms, and hence is, by nature, iterative. The optimization criterion is based on the maximum likelihood or the least-square error principle and the search for the optimal values takes place in a multi-dimensional parameter space. It is, therefore, prone to be trapped at some local extremum of the parameter space. Moreover, a large number of iterations may be needed to find the optimum using up large amount of computing time. In this paper, we introduce a method that avoids these shortcomings in that it splits up the multi-parameter non-linear fitting problems into a sequence of linear regressions. Furthermore, it uses the value of tp, the time at which the current trace reaches its peak value, to estimate the activation kinetics of the current. Our approach also guarantees that the estimates will be sufficiently close to the 'real' values, provided the quality of the experimental records is satisfactory. In order to test our method, we used kinetic and steady-state properties of the following three currents as identified in earlier experiments: the low-threshold Ca2+ current, IT, and the K+ currents, IA and IK2. Gaussian noise of constant variance was added to the simulated current traces. The method was also tested on experimental traces of IT.

Ion Channels↗

PubMed1995

Gamma-hydroxybutyrate promotes oscillatory activity of rat and cat thalamocortical neurons by a tonic GABAB, receptor-mediated hyperpolarization.

The actions of gamma-hydroxybutyrate, a drug known to lead to an increase in nocturnal slow wave sleep and induce epileptic states following systemic application, on the membrane properties of thalamocortical neurons from brain slices of the rat and cat dorsal lateral geniculate nucleus were studied using sharp electrode intracellular recordings. Gamma-hydroxybutyrate applied by addition to the perfusion medium led to a concentration-dependent and reversible hyperpolarization of the membrane potential accompanied by a decrease in apparent input resistance (0.1 mM: 2.3 +/- 0.3 mV, 9.5 +/- 1.0%; 10 mM: 11.3 +/- 1.3 mV, 37.5 +/- 10.8%, respectively). In six of seven neurons the iontophoretic or bath (0.1-0.2 mM) application of low concentrations of gamma-hydroxybutyrate led to a hyperpolarization accompanied by the appearance of low-frequency (< 4 Hz) membrane potential oscillations crowned by bursts of action potentials, when the membrane potential of these neurons was initially set depolarized to the range where ongoing oscillatory activity occurred. The gamma-hydroxybutyrate-elicited hyperpolarization was reversibly antagonized by the co-application of the GABAB receptor antagonist CGP 35348 (0.4-1 mM), but was not affected by the putative gamma-hydroxybutyrate receptor antagonist NCS 382 (0.1-5 mM) or tetrodotoxin (1 microM), suggesting that gamma-hydroxybutyrate tonically activates postsynaptic GABAB receptors. The gamma-hydroxybutyrate-induced promotion of oscillatory activity and action potential burst firing of thalamocortical neurons may be one mechanism by which gamma-hydroxybutyrate leads to an increase in the deep stages of sleep and the generation of electroencephalogram and behavioural patterns typical of absence epilepsy.

Action Potentials↗