Biomedical subjects

M Mantegazza

Publications and source records attributed to M Mantegazza.

12 recordsLinked to original sources

PubMed2006

Layer-specific properties of the persistent sodium current in sensorimotor cortex.

We evaluated the characteristics of the persistent sodium current (I(NaP)) in pyramidal neurons of layers II/III and V in slices of rat sensorimotor cortex using whole cell patch-clamp recordings. In both layers, I(NaP) began activating around -60 mV and was half-activated at -43 mV. The I(NaP) peak amplitude and density were significantly higher in layer V. The voltage-dependent I(NaP) steady-state inactivation occurred at potentials that were significantly more positive in layer V (V(1/2): -42.3 +/- 1.1 mV) than in layer II/III (V(1/2): -46.8 +/- 1.6 mV). In both layers, a current fraction corresponding to about 25% of the maximal peak amplitude did not inactivate. The time course of I(NaP) inactivation and recovery from inactivation could be fitted with a biexponential function. In layer V pyramidal neurons the faster time constant of development of inactivation had variable values, ranging from 158.0 to 1,133.8 ms, but it was on average significantly slower than that in layer II/III (425.9 +/- 80.5 vs. 145.8 +/- 18.2 ms). In both layers, I(NaP) did not completely inactivate even with very long conditioning depolarizations (40 s at -10 mV). Recovery from inactivation was similar in the two layers. Layer V intrinsically bursting and regular spiking nonadapting neurons showed particularly prolonged depolarized plateau potentials when Ca2+ and K+ currents were blocked and slower early phase of I(NaP) development of inactivation. The biexponential kinetics characterizing the time-dependent inactivation of I(NaP) in layers II/III and V indicates a complex inactivating process that is incomplete, allowing a residual "persistent" current fraction that does not inactivate. Moreover, our data indicate that I(NaP) has uneven inactivation properties in pyramidal neurons of different layers of rat sensorimotor cortex. The higher current density, the rightward shifted voltage dependency of inactivation as well the slower kinetics of inactivation characterizing I(NaP) in layer V with respect to layer II/III pyramidal neurons may play a significant role in their ability to fire recurrent action potential bursts, as well in the high susceptibility to generate epileptic events.

Action Potentials↗

PubMed2004

Protein-kinase C-dependent phosphorylation inhibits the effect of the antiepileptic drug topiramate on the persistent fraction of sodium currents.

We investigated the interference of protein-kinase C (PKC)-dependent Na(+) channel phosphorylation on the inhibitory effect that the antiepileptic drug topiramate (TPM) has on persistent Na(+) currents (I(NaP)) by making whole cell patch-clamp and intracellular recordings of rat sensorimotor cortex neurons. The voltage-dependent activation of I(NaP) was significantly shifted in the hyperpolarizing direction when PKC was activated by 1-oleoyl-2-acetyl-sn-glycerol (OAG). TPM reduced the peak amplitude of I(NaP), but it did not counteract the OAG-induced shift in I(NaP) activation. Firing property experiments showed that the firing threshold was lowered by OAG. TPM was unable to counteract this effect, which may be due to OAG-dependent enhancement of the contribution of subthreshold I(NaP). These data suggest that PKC activation may limit the effect of the anticonvulsant TPM on the persistent fraction of Na(+) currents. The channel phosphorylation that may occur in cortical neurons as a result of physiological or pathological (e.g. epileptic) events can modulate the action of TPM on Na(+) currents.

Action Potentials↗

PubMed2001

Role of the C-terminal domain in inactivation of brain and cardiac sodium channels.

Inactivation is a fundamental characteristic of Na(+) channels, and small changes cause skeletal muscle paralysis and myotonia, epilepsy, and cardiac arrhythmia. Brain Na(v)1.2a channels have faster inactivation than cardiac Na(v)1.5 channels, but minor differences in inactivation gate structure are not responsible. We constructed chimeras in which the C termini beyond the fourth homologous domains of Na(v)1.2a and Na(v)1.5 were exchanged. Replacing the C-terminal domain (CT) of Na(v)1.2a with that of Na(v)1.5 (Na(v)1.2/1.5CT) slowed inactivation at +40 mV approximately 2-fold, making it similar to Na(v)1.5. Conversely, replacing the CT of Na(v)1.5 with that of Na(v)1.2a (Nav1.5/1.2CT) accelerated inactivation, making it similar to Na(v)1.2a. Activation properties were unaffected. The voltage dependence of steady-state inactivation of Na(v)1.5 is 16 mV more negative than that of Na(v)1.2a. The steady-state inactivation curve of Na(v)1.2a was shifted +12 mV in Na(v)1.2/1.5CT, consistent with destabilization of the inactivated state. Conversely, Na(v)1.5/1.2CT was shifted -14 mV relative to Na(v)1.5, consistent with stabilization of the inactivated state. Although these effects of exchanging C termini were consistent with their effects on inactivation kinetics, they magnified the differences in the voltage dependence of inactivation between brain and cardiac channels rather than transferring them. Thus, other parts of these channels determine the basal difference in steady-state inactivation. Deletion of the distal half of either the Na(v)1.2 or Na(v)1.5 CTs accelerated open-state inactivation and negatively shifted steady-state inactivation. Thus, the C terminus has a strong influence on kinetics and voltage dependence of inactivation in brain Na(v)1.2 and cardiac Na(v)1.5 channels and is primarily responsible for their differing rates of channel inactivation.

Brain↗

PubMed2001

Neutralization of gating charges in domain II of the sodium channel alpha subunit enhances voltage-sensor trapping by a beta-scorpion toxin.

beta-Scorpion toxins shift the voltage dependence of activation of sodium channels to more negative membrane potentials, but only after a strong depolarizing prepulse to fully activate the channels. Their receptor site includes the S3-S4 loop at the extracellular end of the S4 voltage sensor in domain II of the alpha subunit. Here, we probe the role of gating charges in the IIS4 segment in beta-scorpion toxin action by mutagenesis and functional analysis of the resulting mutant sodium channels. Neutralization of the positively charged amino acid residues in the IIS4 segment by mutation to glutamine shifts the voltage dependence of channel activation to more positive membrane potentials and reduces the steepness of voltage-dependent gating, which is consistent with the presumed role of these residues as gating charges. Surprisingly, neutralization of the gating charges at the outer end of the IIS4 segment by the mutations R850Q, R850C, R853Q, and R853C markedly enhances beta-scorpion toxin action, whereas mutations R856Q, K859Q, and K862Q have no effect. In contrast to wild-type, the beta-scorpion toxin Css IV causes a negative shift of the voltage dependence of activation of mutants R853Q and R853C without a depolarizing prepulse at holding potentials from -80 to -140 mV. Reaction of mutant R853C with 2-aminoethyl methanethiosulfonate causes a positive shift of the voltage dependence of activation and restores the requirement for a depolarizing prepulse for Css IV action. Enhancement of sodium channel activation by Css IV causes large tail currents upon repolarization, indicating slowed deactivation of the IIS4 voltage sensor by the bound toxin. Our results are consistent with a voltage-sensor-trapping model in which the beta-scorpion toxin traps the IIS4 voltage sensor in its activated position as it moves outward in response to depolarization and holds it there, slowing its inward movement on deactivation and enhancing subsequent channel activation. Evidently, neutralization of R850 and R853 removes kinetic barriers to binding of the IIS4 segment by Css IV, and thereby enhances toxin-induced channel activation.

Arginine↗

PubMed1999

Inhibition of transient and persistent Na+ current fractions by the new anticonvulsant topiramate.

The actions of the antiepileptic drug topiramate (TPM) on Na+ currents were assessed using whole-cell patch-clamp recordings in dissociated neocortical neurons and intracellular recordings in neocortical slices. Relatively low TPM concentrations (25-30 microM) slightly inhibited the persistent fraction of Na+ current in dissociated neurons and reduced the Na+-dependent long-lasting action potential shoulders, which can be evoked in layer V pyramidal neurons after Ca++ and K+ current blockade. Conversely, the same drug concentrations were ineffective in reducing the amplitude of the fast Na+-dependent action potentials evoked in slices or the peak of transient Na+ (INaf) current evoked in isolated neurons from a physiological holding potential. Consistent INaf inhibition became, however, evident only when the neuronal membrane was kept depolarized to enhance resting Na+ channel inactivation. TPM (100 microM) was ineffective on the voltage dependence of activation but induced a leftward shift of the steady-state INaf inactivation curve. The drug-induced inhibitory effect increased with the duration of membrane depolarization, and the recovery of INaf after long membrane depolarizations was slightly delayed in comparison with that observed under control conditions. The obtained evidence suggests that the anticonvulsant action of TPM may operate by stabilizing channel inactivation, which can be induced by depolarizing events similar to those occurring in chronic epileptic conditions. Concurrently, the slight but significant inhibition of the persistent fraction of the Na+ current, obtained with the application of relatively low TPM concentrations, may contribute toward its anticonvulsant effectiveness by modulating the near-threshold depolarizing events that are sustained by this small current fraction.

Animals↗

PubMed1998

Anemone toxin (ATX II)-induced increase in persistent sodium current: effects on the firing properties of rat neocortical pyramidal neurones.

1. The experiments were performed on sensorimotor cortex using current-clamp intracellular recordings in layer V pyramidal neurones and whole-cell voltage-clamp recordings in dissociated pyramidal neurones. The intracellularly recorded neurones were classified on the basis of their firing characteristics as intrinsically bursting (IB) and regular spiking (RS). The RS neurones were further subdivided into adapting (RSAD) or non-adapting (RSNA), depending on the presence or absence of spike frequency adaptation. Since burst firing in neocortical pyramidal neurones has previously been suggested to depend on the persistent fraction of Na+ current (INa, p), pharmacological manipulations with drugs affecting INa inactivation have been employed. 2. ATX II, a toxin derived from Anemonia sulcata, selectively inhibited INa fast inactivation in dissociated neurones. In current-clamp experiments on neocortical slices, ATX II enhanced the naturally occurring burst firing in IB neurones and revealed the ability of RSNA neurones to discharge in bursts, whereas in RSAD neurones it increased firing frequency, without inducing burst discharges. During the ATX II effect, in all the three neuronal subclasses, episodes of a metastable condition occurred, characterized by long-lasting depolarizing shifts, triggered by action potentials, which were attributed to a peak in the toxin-induced inhibition of INa inactivation. The ATX II effect on IB and RSNA neurones was compared with that induced by veratridine and iodoacetamide. Veratridine induced a small increase in the INa and a large shift to the left in the voltage dependence of INa activation. Accordingly, its major effect on firing characteristics was the induction of prolonged tonic discharges, associated with burst facilitation less pronounced than that induced by ATX II. The alkylating agent iodoacetamide was able to induce a selective small increase in the INa,p, with a similar but less pronounced effect than ATX II on firing behaviour. 3. The present results show that pharmacological manipulations capable of slowing down INa inactivation significantly enhance burst behaviour in IB neurones and promote burst firing in otherwise non-bursting RSNA neurones. We suggest that IB and, to a lesser extent, RSNA neurones are endowed with a relatively large fraction of INa,p which, in physiological conditions, is sufficient to sustain bursting in IB but not in RSNA neurones.

Alkylating Agents↗

PubMed1998

Modalities of distortion of physiological voltage signals by patch-clamp amplifiers: a modeling study.

An extensive evaluation of the possible alterations affecting physiological voltage signals recorded with patch-clamp amplifiers (PCAs) working in the current-clamp (CC) mode was carried out by following a modeling approach. The PCA output voltage and current signals obtained during CC recordings performed under simplified experimental conditions were exploited to determine the equations describing the generation of error currents and voltage distortions by PCAs. The functions thus obtained were used to construct models of PCAs working in the CC mode, which were coupled to numerical simulations of neuronal bioelectrical behavior; this allowed us to evaluate the effects of the same PCAs on different physiological membrane-voltage events. The models revealed that rapid signals such as fast action potentials are preferentially affected, whereas slower events, such as low-threshold spikes, are less altered. Prominent effects of model PCAs on fast action potentials were alterations of their amplitude, duration, depolarization and repolarization speeds, and, most notably, the generation of spurious afterhyperpolarizations. Processes like regular firing and burst firing could also be altered, under particular conditions, by the model PCAs. When a cell consisting of more than one single intracellular compartment was considered, the model PCAs distorted fast equalization transients. Furthermore, the effects of different experimental and cellular parameters (series resistance, cell capacitance, temperature) on PCA-generated artifacts were analyzed. Finally, the simulations indicated that no off-line correction based on manipulations of the error-current signals returned by the PCAs can be successfully performed in the attempt to recover unperturbed voltage signals, because of alterations of the overall current flowing through the cell-PCA system.

Action Potentials↗

PubMed1998

Valproate selectively reduces the persistent fraction of Na+ current in neocortical neurons.

The effect of valproate (VPA) on Na+ currents (INa), was studied by means of voltage clamp recordings using whole-cell patch clamp configuration in 21 acutely dissociated neocortical neurons. Concentrations of VPA up to 200 microM failed to induce any detectable decrease in fast INa (I(Naf)), but the persistent fraction (I(NaP)) was significantly reduced by low VPA concentrations (10-30 microM), corresponding to the lower values of the 'therapeutic' range in epileptic patients. Since it is known that I(NaP) critically regulates the firing properties of pyramidal neurons, it is suggested that the anticonvulsant effectiveness of VPA is mainly due to its effect on I(NaP).

Animals↗

PubMed1996

Action potentials recorded with patch-clamp amplifiers: are they genuine?

A growing number of experimental studies have used patch-clamp amplifiers (PCAs) in the current-clamp (CC) mode to investigate classical excitability. In this paper we show that the measurements obtained in this way are affected by errors due to the electronic design of the PCA input section. We present experimental evidence of such errors, and demonstrate that they derive from PCA current absorption. Moreover, we propose a new PCA input-circuit configuration for the CC mode, which is suitable for accurately recording physiological voltage signals and is perfectly compatible with the standard voltage-clamp mode.

Action Potentials↗

PubMed1995

Stimulation of single L-type calcium channels in rat pituitary GH3 cells by thyrotropin-releasing hormone.

Hormonal stimulation of voltage-dependent Ca2+ channels in pituitary cells is thought to contribute to the sustained phase of Ca2+ entry and secretion induced by secretion stimulating hormones and has been suggested as a mechanism for refilling the Ca2+ stores. Using the cell-attached patch-clamp technique, we studied the stimulation of single Ca2+ channels by thyrotropin-releasing hormone (TRH) in rat GH3 cells. We show that TRH applied from the bath switched the activity of single L-type Ca2+ channels from a gating mode with very low open probability (po) to a gating mode with slightly smaller conductance but 10 times higher po. Interconversions between these two gating modes were also observed under basal conditions, where the equilibrium was shifted towards the low po mode. TRH applied from the pipette had no effect, indicating the involvement of a cytosolic compound in the stimulatory pathway. We show that TRH does not potentiate all the L-type Ca2+ channels in a given membrane patch and report evidence for co-expression of two functionally different L-type Ca2+ channels. Our results uncover the biophysical mechanism of hormonal stimulation of voltage-dependent Ca2+ channels in GH3 cells and are consistent with differential modulation of different subtypes of dihydropyridine-sensitive Ca2+ channels.

Animals↗

PubMed1994

Muscarinic regulation of Ca2+ currents in rat sensory neurons: channel and receptor types, dose-response relationships and cross-talk pathways.

We studied, in rat sensory neurons, the modulation of high voltage-activated Ca2+ currents (ICa) mediated by the pertussis toxin-sensitive activation of muscarinic receptors, which were found to be of subtypes M2 or M4. Muscarine reversibly blocked somatic Ca2+ spikes but strong predepolarizations only partially relieved the inhibited Ca2+ current. On the other hand, the putative coupling messenger could not rapidly diffuse towards channels whose activity was recorded from a macro-patch. The perforated patch technique virtually prevented the response rundown present during whole-cell experiments. Both omega-conotoxin GVIA (omega-CgTx)-sensitive channels and omega-CgTx- and dihydropyridine-resistant channels are coupled to the muscarinic receptor, but not the L-channel. When measured in the same neuron, dose-response relationships for the first and subsequent agonist applications differed; maximal inhibition, the reciprocal of half-maximal concentration and the Hill coefficient were always highest in the first trial. Muscarine and oxotremorine exhibited monotone dose-response curves, but oxotremorine-M showed non-linear relationships which became monotonic when cells were intracellularly perfused with inhibitors of protein kinase A (PKA) and C (PKC), suggesting that either PKA or receptor-induced PKC could phosphorylate and thus inactive G-proteins or other unknown proteins involved in inhibitory muscarinic actions on ICa. In summary, these data provide a preliminary pharmacological characterization of the muscarinic inhibition of the Ca2+ channels in sensory neurons, with implications about agonist specificity and the interplay between signalling pathways.

Action Potentials↗

PubMedDate not supplied

[Clinical aspects and diagnosis of neonatal infections caused by group B beta-hemolytic Streptococcus].

Early-onset infection findings caused by Group B Streptococcus occur within 24 hours of birth (60 per cent of cases) but they may appear anytime during the first 5 days of life. In our experience early-onset infection affects both preterm and term neonates. The Authors report the usual clinical signs described for bacterial infections. Unusual findings are also reported: among 34 infants with early-onset infection, the congenital diaphragmatic hernia was associated with GBS septicemia in two neonates; beads of perspiration were the first only clinical finding in one neonate too. Two cases of late-onset infection are also reported.

Age Factors↗