Targeting Mycobacterium tuberculosis Antigens to Dendritic Cells via the DC-Specific-ICAM3-Grabbing-Nonintegrin Receptor Induces Strong T-Helper 1 Immune Responses

Tuberculosis remains a major global health problem and efforts to develop a more effective vaccine have been unsuccessful so far. Targeting antigens (Ags) to dendritic cells (DCs) in vivo has emerged as a new promising vaccine strategy. In this approach, Ags are delivered directly to DCs via antibodies that bind to endocytic cell-surface receptors. Here, we explored DC-specific-ICAM3-grabbing-nonintegrin (DC-SIGN) targeting as a potential vaccine against tuberculosis. For this, we made use of the hSIGN mouse model that expresses human DC-SIGN under the control of the murine CD11c promoter. We show that in vitro and in vivo delivery of anti-DC-SIGN antibodies conjugated to Ag85B and peptide 25 of Ag85B in combination with anti-CD40, the fungal cell wall component zymosan, and the cholera toxin-derived fusion protein CTA1-DD induces strong Ag-specific CD4⁺ T-cell responses. Improved anti-mycobacterial immunity was accompanied by increased frequencies of Ag-specific IFN-γ⁺ IL-2⁺ TNF-α⁺ polyfunctional CD4⁺ T cells in vaccinated mice compared with controls. Taken together, in this study we provide the proof of concept that the human DC-SIGN receptor can be efficiently exploited for vaccine purposes to promote immunity against mycobacterial infections.

Role of the glycine site of the N-methyl-D-aspartate receptor in synaptic plasticity induced by pairing

In the hippocampal CA1 region of the rat, activity-dependent plasticity requires substantial postsynaptic depolarization and activation of the N-methyl-D-aspartate glutamate receptor subtype (NMDAR). Exogenous and endogenous compounds selectively modulate NMDAR function by acting at the glycine coagonist site. Here we investigate the modulatory role of the glycine site in the induction of bidirectional synaptic plasticity. Plasticity was induced by pairing low-frequency afferent pulses with different levels of postsynaptic depolarization in the absence and presence of glycine site compounds. We found strong dependence of glycine site agonist modulation on membrane voltage during induction. Thus, D-serine and glycine were more effective in enhancing long-term potentiation (LTP) during pairing of small depolarization (-60 or -50 mV) with subthreshold EPSCs than during pairing of stronger depolarization (-40 mV) with suprathreshold synaptic responses. The glycine site role in bidirectional synaptic plasticity was studied with the selective antagonist 7-chlorokynurenic acid. Blockade of the glycine site during the pairing reversed the direction of plasticity from LTP towards long-term depression. The magnitude of depression was dependent on antagonist concentration and the level of depolarization during the pairing. Thus, these experiments demonstrate the role of the glycine site in the induction of bidirectional synaptic plasticity.

Subthreshold contribution of N-methyl-d-aspartate receptors to long-term potentiation induced by low-frequency pairing in rat hippocampal CA1 pyramidal cells

Long-term potentiation (LTP) is a use-dependent and persistent enhancement of synaptic strength. In the CA1 region of the hippocampus, LTP has Hebbian characteristics and requires precisely timed interaction between presynaptic firing and postsynaptic depolarization. Although depolarization is an absolute requirement for plasticity, it is still not clear whether the postsynaptic response during LTP induction should be subthreshold or suprathreshold for the generation of somatic action potential. Here, we use the whole-cell patch-clamp technique and different pairing protocols to examine systematically the postsynaptic induction requirements for LTP. We induce LTP by changes only in membrane potential while keeping the afferent stimulation constant and at minimal levels. This approach permits differentiation of two types of LTP: LTP induced with suprathreshold synaptic responses (LTP(AP)) and LTP induced with subthreshold excitatory postsynaptic current (EPSCs; LTP(EPSC)). We found that LTP(AP) (>40%) required pairing of depolarization (V(m)>or=-40 mV, for 40-60 s) with four to six (0.1 Hz) single synaptically initiated action potentials. LTP(EPSC) was of smaller magnitude (<30%) and required pairing of depolarization to -50 mV (60 s) with six subthreshold EPSCs. The N-methyl-d-aspartate receptor (NMDAR) antagonists aminophosphonovaleric acid and 7-chlorokynurenic acid consistently blocked LTP(EPSC) but were ineffective in preventing LTP(AP). Robust, NMDAR-independent LTP is obtained by stronger postsynaptic depolarization that converts the EPSCs to suprathreshold somatic action potentials. Purely NMDAR-dependent LTP is obtained by pairing mild somatic depolarization with subthreshold afferent pulses to the postsynaptic cell. Our results indicate that the degree of postsynaptic depolarization in the presence of single afferent pulses determines the type and magnitude of LTP.

Cell-type-specific GABA responses and chloride homeostasis in the cortex and amygdala

The GABA responses of fast-spiking (FS) interneurons and regular-spiking (RS) principal cells were studied using whole cell and perforated-patch recordings in slices of the basolateral amygdala, neo-, and perirhinal cortex. In these three areas, responses to exogenous and synaptically released GABA were abolished by GABA(A) receptor antagonists in FS cells but also included a GABA(B) component in RS cells. Moreover, E(GABA(A)) of FS and RS cells differed from the calculated E(Cl) (-61 mV), but in opposite direction (FS, -54 mV; RS, -72 mV). This was not due to a differential dialysis of FS and RS cells by the pipette solution because the discrepancy persisted when recordings were obtained with the perforated-patch-clamp technique, using the cation-selective ionophore gramicidin. Moreover, pharmacological inhibition of cation-chloride cotransporters revealed that the differing E(GABA(A)) of FS and RS neurons arises from cell-type-specific chloride homeostatic mechanisms. Indeed, the prevalent regulators of the intracellular chloride concentration are cotransporters that accumulate chloride in FS cells and extrude chloride in RS neurons. Thus, our results suggest that in the basolateral amygdala as well as in the parietal and perirhinal cortices, FS interneurons are more excitable than principal cells not only by virtue of their dissimilar electroresponsive properties but also because they express a different complement of GABA receptors and chloride homeostatic mechanisms.

Changes in extracellular Ca2+ can affect the pattern of discharge in rat thalamic neurons

1. The aim of this study was to investigate some of the cellular mechanisms involved in the effects caused by changes in extracellular Ca2+ concentration ([Ca2+](o)). 2. Current- and voltage-clamp experiments were carried out on acutely isolated thalamic neurons of rats. 3. Increasing [Ca2+](o) alone induced a transition of the discharge from single spike to burst mode in isolated current-clamped neurons. 4. Increasing [Ca(2+)](o) caused the voltage-dependent characteristics of the low voltage-activated (LVA) transient Ca2+ currents to shift towards positive values on the voltage axis. Changing [Ca2+](o) from 0.5 to 5 mM caused the inactivation curve to shift by 21 mV. 5. Extracellular Ca2+ blocked a steady cationic current. This current reversed at -35 mV, was scarcely affected by Mg2+ and was completely blocked by the non-selective cation channel inhibitor gadolinium (10 microM). The effect of [Ca2+](o) was mimicked by 500 microM spermine, a polyamine which acts as an agonist for the Ca(2+)-sensing receptor, and was modulated by intracellular GTP-gamma-S. 6. At the resting potential, both the voltage shift and the block of the inward current removed the inactivation of LVA calcium channels and, together with the increase in the Ca2+ driving force, favoured a rise in the low threshold Ca2+ spikes, causing the thalamic firing to change to the oscillatory mode. 7. Our data indicate that [Ca2+](o) is involved in multiple mechanisms of control of the thalamic relay and pacemaker activity. These findings shed light on the correlation between hypercalcaemia, low frequency EEG activity and symptoms such as sleepiness and lethargy described in many clinical papers.

Propagation of neocortical inputs in the perirhinal cortex

The perirhinal area is a rostrocaudally oriented strip of cortex in which lesions produce memory and perceptual impairments. It receives topographically organized transverse projections from associative neocortical areas and is endowed with intrinsic longitudinal connections that could distribute neocortical inputs in the rostrocaudal axis. In search of distinguishing network properties that might support perirhinal involvement in memory, we have performed whole-cell recordings in horizontal perirhinal slices with preserved transverse neocortical links and intrinsic longitudinal connections. Neocortical stimulation sites in rostrocaudal register with regular spiking perirhinal neurons elicited a sequence of excitatory and inhibitory synaptic potentials. In contrast, apparently pure excitatory responses were observed when the stimulating and recording sites were separated by >/=1 mm in the rostrocaudal axis. This suggested that adjacent and distant neocortical stimuli influence regular spiking perirhinal neurons by pathways that respectively form and do not form synapses with inhibitory interneurons. In keeping with this, presumed interneurons did not respond to distant neocortical stimuli. These results suggest that neocortical inputs recruit perirhinal inhibitory interneurons located at the same transverse level, limiting the depolarization of principal perirhinal cells. In contrast, distant neocortical inputs only evoke excitation because longitudinal perirhinal pathways do not engage inhibitory interneurons. This leads us to suggest that the perirhinal network is biased to favor Hebbian-like associative interactions between coincident and spatially distributed inputs.

Bistable behavior of inhibitory neurons controlling impulse traffic through the amygdala: role of a slowly deinactivating K+ current

The intercalated cell masses of the amygdala are clusters of GABAergic neurons located strategically to influence behavioral responsiveness. Indeed, they receive glutamatergic sensory inputs from the basolateral amygdaloid complex and generate feedforward inhibition in neurons of the central amygdala that mediate important components of fear responses. In the present study, using whole-cell recording methods in coronal slices of the guinea pig amygdala, we show that the activity of intercalated neurons is a function of their recent firing history because they express an unusual voltage-dependent K(+) conductance (termed I(SD) for slowly deinactivating). This conductance activates in the subthreshold regime, inactivates in response to suprathreshold depolarizations, and deinactivates very slowly upon return to rest. As a result, after bouts of suprathreshold activity, these cells enter a self-sustaining state of heightened excitability associated with an increased input resistance and a membrane depolarization. In turn, these changes increase the likelihood that ongoing synaptic activity will trigger orthodromic action potentials. However, because each orthodromic spike "renews" the inactivation of I(SD), intercalated cells can remain hyperexcitable for a long time and, via the central amygdaloid nucleus, exert a lasting influence on behavior.

Polarized synaptic interactions between intercalated neurons of the amygdala

The intercalated (ITC) cell masses are small GABAergic cell clusters interposed between the basolateral (BL) complex and central (CE) nucleus of the amygdala. ITC cells receive excitatory afferents from the BL complex and generate feed-forward inhibition in the CE nucleus. Recently it was shown that ITC cells could gate impulse traffic between the BL complex and CE nucleus in a spatiotemporally differentiated manner. In addition, it was hypothesized that lateromedial inhibitory interactions between different ITC cell clusters played a critical role in this respect. Given the potential importance of such conditional computations, the present study aimed to characterize the connectivity existing among ITC cells. To this end, whole cell recordings of ITC neurons were obtained under visual guidance in coronal slices of the guinea pig amygdala. Electrical stimuli applied in the BL complex primarily elicited excitatory responses when they were applied at the same lateromedial level or more medially than the recorded ITC cells. As the stimulation site was moved laterally, the character of the response shifted toward inhibition. Both bicuculline and non-N-methyl-D-aspartate receptor antagonists abolished this BL-evoked inhibition, suggesting that it was not mediated by BL inhibitory cells projecting to ITC neurons. In keeping with this, local glutamate injections in and around the ITC clusters revealed that the most effective site to inhibit ITC cells were ITC clusters located laterally with respect to the recorded one. The activation of more medial ITC clusters evoked much smaller responses. Thus, connections between ITC clusters tend to run in a lateromedial direction. To identify the source of these directionally polarized synaptic interactions, the morphological features of ITC cells were analyzed by intracellular injection of Neurobiotin. This analysis revealed that the dendritic tree and axonal arbor of ITC cells are asymmetric in the lateromedial plane. In particular, their laterally directed dendrites were longer than the medial ones, whereas their laterally directed axon collaterals were shorter than the medial ones. It is concluded that the morphological asymmetry of ITC cells accounts for the directional polarization of inter-ITC connections. The significance of these findings for the gating of information transfer from the BL complex to the CE nucleus is discussed.

Distal initiation and active propagation of action potentials in interneuron dendrites

Fast and reliable activation of inhibitory interneurons is critical for the stability of cortical neuronal networks. Active conductances in dendrites may facilitate interneuron activation, but direct experimental evidence was unavailable. Patch-clamp recordings from dendrites of hippocampal oriens-alveus interneurons revealed high densities of voltage-gated sodium and potassium ion channels. Simultaneous recordings from dendrites and somata suggested that action potential initiation occurs preferentially in the axon with long threshold stimuli, but can be shifted to somatodendritic sites when brief stimuli are applied. After initiation, action potentials propagate over the somatodendritic domain with constant amplitude, high velocity, and reliability, even during high-frequency trains.

An inhibitory interface gates impulse traffic between the input and output stations of the amygdala

The central amygdaloid nucleus projects to brainstem and hypothalamic nuclei mediating fear responses and receives convergent sensory inputs from the basolateral amygdaloid complex. However, interposed between the basolateral complex and central nucleus is a string of interconnected GABAergic cell clusters, the intercalated cell masses. Here, we analyzed how intercalated neurons influence impulse traffic between the basolateral complex and central nucleus using whole-cell recordings, microstimulation, and local application of glutamate receptor antagonists in brain slices. Our results suggest that intercalated neurons receive glutamatergic inputs from the basolateral complex and generate feedforward inhibition in neurons of the central nucleus. As the position of the recording site was shifted medially, intercalated cells projected to gradually more medial sectors of the central nucleus and were maximally responsive to progressively more medial stimulation sites in the basolateral complex. Thus, there is a lateromedial correspondence between the position of intercalated cells, their projection site in the central nucleus, and the source of their excitatory afferents in the basolateral complex. In addition, basolateral stimulation sites eliciting maximal excitatory responses in intercalated neurons were flanked laterally by sites eliciting prevalently inhibitory responses via the activation of intercalated cells located more laterally. As a result, the feedforward inhibition generated by intercalated neurons and, indirectly, the amplitude of the responses of central neurons could be increased or decreased depending on which combination of amygdala nuclei are activated and in what sequence. Thus, the output of the central nucleus depends not only on the nature and intensity of sensory inputs but also on their timing and origin.

Physiological properties of central medial and central lateral amygdala neurons

Mounting evidence implicates the central (CE) nucleus of the amygdala in the mediation of classically conditioned fear responses. However, little data are available regarding the intrinsic membrane properties of CE amygdala neurons. Here, we characterized the physiological properties of CE medial (CE(M)) and CE lateral (CE(L)) amygdala neurons using whole cell recordings in brain slices maintained in vitro. Several classes of CE neurons were distinguished on the basis of their physiological properties. Most CE(M) cells (95%), here termed "late-firing neurons," displayed a marked voltage- and time-dependent outward rectification in the depolarizing direction. This phenomenon was associated with a conspicuous delay between the onset of depolarizing current pulses and the first action potential. During this delay, the membrane potential (V(m)) depolarized slowly, the steepness of this depolarizing ramp increasing as the prepulse V(m) was hyperpolarized from -60 to -90 mV. Low extracellular concentrations of 4-aminopyridine (30 microM) reversibly abolished the outward rectification and the delay to firing. Late-firing CE(M) neurons displayed a continuum of repetitive firing properties with cells generating single spikes at one pole and high-frequency (> or =90 Hz) spike bursts at the other. In contrast, only 56% of CE(L) cells displayed the late-firing behavior prevalent among CE(M) neurons. Moreover, these CE(L) neurons only generated single spikes in response to membrane depolarization. A second major class of CE(L) cells (38%) lacked the characteristic delay to firing observed in CE(M) cells, generated single spikes in response to membrane depolarization, and displayed various degrees of inward rectification in the hyperpolarizing direction. In both regions of the CE nucleus, two additional cell types were encountered infrequently (< or =6% of our samples). One type of neurons, termed "low-threshold bursting cells" had a behavior reminiscent of thalamocortical neurons. The second type of cells, called "fast-spiking cells," generated brief action potentials at high rates with little spike frequency adaptation in response to depolarizing current pulses. These findings indicate that the CE nucleus contains several types of neurons endowed with distinct physiological properties. Moreover, these various cell types are not distributed uniformly in the medial and lateral sector of the CE nucleus. This heterogeneity parallels anatomic data indicating that these subnuclei are part of different circuits.

GABA excites immature CA3 pyramidal cells through bicuculline-sensitive and -insensitive chloride-dependent receptors

Intracellular and patch clamp recording techniques were used to investigate the role of GABA in immature CA3 hippocampal neurons. During the first postnatal week spontaneous GABA release was detected as spontaneous ongoing synaptic potentials (SPSPs) or giant depolarizing potentials (GDPs). GDPs were generated at regular intervals and regulated by ionotropic glutamate receptors (GluRs), whereas SPSPs occurred randomly and were unaffected by ionotropic GluRs. Both GDPs and SPSPs were positively modulated by metabotropic GluRs through cyclic AMP-dependent protein kinase. Moreover GABA controlled its own release through GABAA and GABAB receptors, probably localized on GABAergic nerve terminals. At this developmental stage, GABA depolarized CA3 pyramidal cells through two distinct classes of chloride-permeable receptors: bicuculline sensitive and insensitive, respectively. The bicuculline-insensitive responses were blocked by picrotoxin in a noncompetitive way. Whole-cell GABA currents, recorded in the presence of bicuculline, had a slower desensitization rate and faster recovery from desensitization. In excised outside-out patches, in the presence of bicuculline, GABA activated single-channel currents with conductances of 14, 22, and 31 pS. These values were similar to those obtained when GABA was applied in the absence of bicuculline. Interestingly, GABA responses obtained in the absence of bicuculline, were sensitive to the blocking effect of zinc, whereas bicuculline-resistant responses were almost unaffected by this divalent cation. Expression of different subunits in native receptors (particularly of the alpha and rho type) may account for the functional differences observed in the present experiments. Activation of bicuculline-insensitive receptors would strengthen and prolong the depolarizing action of GABA, thus favoring the entry of calcium through voltage-dependent calcium channels. This calcium signal may be essential in promoting stabilization of synaptic contacts during a critical period of postnatal development.

Functional and molecular differences between voltage-gated K+ channels of fast-spiking interneurons and pyramidal neurons of rat hippocampus

We have examined gating and pharmacological characteristics of somatic K+ channels in fast-spiking interneurons and regularly spiking principal neurons of hippocampal slices. In nucleated patches isolated from basket cells of the dentate gyrus, a fast delayed rectifier K+ current component that was highly sensitive to tetraethylammonium (TEA) and 4-aminopyridine (4-AP) (half-maximal inhibitory concentrations <0.1 mM) predominated, contributing an average of 58% to the total K+ current in these cells. By contrast, in pyramidal neurons of the CA1 region a rapidly inactivating A-type K+ current component that was TEA-resistant prevailed, contributing 61% to the total K+ current. Both types of neurons also showed small amounts of the K+ current component mainly found in the other type of neuron and, in addition, a slow delayed rectifier K+ current component with intermediate properties (slow inactivation, intermediate sensitivity to TEA). Single-cell RT-PCR analysis of mRNA revealed that Kv3 (Kv3.1, Kv3.2) subunit transcripts were expressed in almost all (89%) of the interneurons but only in 17% of the pyramidal neurons. In contrast, Kv4 (Kv4.2, Kv4.3) subunit mRNAs were present in 87% of pyramidal neurons but only in 55% of interneurons. Selective block of fast delayed rectifier K+ channels, presumably assembled from Kv3 subunits, by 4-AP reduced substantially the action potential frequency in interneurons. These results indicate that the differential expression of Kv3 and Kv4 subunits shapes the action potential phenotypes of principal neurons and interneurons in the cortex.

Multiple modulatory effects of dopamine on calcium channel kinetics in adult rat sensory neurons

1. The aim of this research was to study the modulatory effects induced on high-voltage-activated (HVA) calcium channels and pharmacologically isolated subtypes through dopamine receptor activation. 2. The experiments were carried out on acutely isolated adult rat sensory neurons, recorded by means of the whole-cell patch-clamp technique. 3. At saturating concentrations dopamine was effective in inducing: (a) a voltage-dependent prolongation of activation kinetics, (b) a voltage-independent scaling down of the currents without any changes in activation and inactivation kinetics, and (c) an acceleration of inactivation kinetics, not affected by a positive conditioning prepulse. 4. These three inhibitory effects were observed on N- and P/Q-type currents, whereas only a voltage-independent scaling up and/or scaling down was observed on L-type current. 5. The inhibitory effects were sometimes observed in isolation in different neurons, but more frequently they were variously combined in the same cell. A correlation analysis of these effects shows no relationship between them, corroborating the conclusion that they are mechanistically distinct. 6. The existence of an inactivating effect accounts for the occurrence of a voltage-dependent inhibitory effect in some cells without an apparent slowing down of activation kinetics, since the increased inactivation may mask the slow component of the activation. 7. The multiple modulatory effects on calcium channels, even on pharmacologically separated N-, L- and P/Q-currents, suggest that pharmacological and functional classifications do not necessarily match completely. 8. The multiple modulatory effects on HVA calcium currents may play a prominent role both in controlling the integrative properties of neurons and in regulating output at a presynaptic level.

Functional differences in Na+ channel gating between fast-spiking interneurones and principal neurones of rat hippocampus

1. GABAergic interneurones differ from glutamatergic principal neurones in their ability to discharge high-frequency trains of action potentials without adaptation. To examine whether Na+ channel gating contributed to these differences, Na+ currents were recorded in nucleated patches from interneurones (dentate gyrus basket cells, BCs) and principal neurones (CA1 pyramidal cells, PCs) of rat hippocampal slices. 2. The voltage dependence of Na+ channel activation in BCs and PCs was similar. The slope factors of the activation curves, fitted with Boltzmann functions raised to the third power, were 11.5 and 11.8 mV, and the mid-point potentials were -25.1 and -23.9 mV, respectively. 3. Whereas the time course of Na+ channel activation (-30 to +40 mV) was similar, the deactivation kinetics (-100 to -40 mV) were faster in BCs than in PCs (tail current decay time constants, 0.13 and 0.20 ms, respectively, at -40 mV). 4. Na+ channels in BCs and PCs differed in the voltage dependence of inactivation. The slope factors of the steady-state inactivation curves fitted with Boltzmann functions were 6.7 and 10.7 mV, and the mid-point potentials were -58.3 and -62.9 mV, respectively. 5. The onset of Na+ channel inactivation at -55 mV was slower in BCs than in PCs; the inactivation time constants were 18.6 and 9.3 ms, respectively. At more positive potentials the differences in inactivation onset were smaller. 6. The time course of recovery of Na+ channels from inactivation induced by a 30 ms pulse was fast and mono-exponential (tau = 2.0 ms at -120 mV) in BCs, whereas it was slower and bi-exponential in PCs (tau 1 = 2.0 ms and tau 2 = 133 ms; amplitude contribution of the slow component, 15%). 7. We conclude that Na+ channels of BCs and PCs differ in gating properties that contribute to the characteristic action potential patterns of the two types of neurones.

Membrane stretch activates a potassium channel in pig articular chondrocytes

Activity of stretch-activated potassium channels has been recorded in articular chondrocytes using patch-clamp technique. Pressure dependence is described by a sigmoidal function with a half-maximum effect at -20.5 mbar. Selectivity for potassium is demonstrated by agreement between the reversal potential measured at different [K+]o and the prediction of Nernst equation and by block of these channels by caesium.

A large-conductance voltage-dependent potassium channel in cultured pig articular chondrocytes

The patch-clamp techniques were used to study voltage-dependent potassium channels in cultured pig articular chondrocytes. A predominant single-channel conductance of 125 pS was found. These channels were reversibly blocked by tetraethylammonium. In cell-attached patches, transient increases in the channel activity were observed, and defined as a switching between low and high activity modes (LAM and HAM). Open-time distributions could be described with two kinetics components (in LAM and HAM) having similar time constants (fast tau1 and slow tau2). In HAM, the area of the slow component was larger. The mean burst length was significantly longer in HAM than in LAM. In both modes, the burst-length distributions were fitted with a sum of three exponentials. In LAM and HAM, the time constants tau1 and tau2 were indistinguishable from those of the open-time distributions. The slowest time constant, tau3, was strongly voltage dependent, and was significantly longer in HAM than in LAM. In both LAM and HAM, the ensemble currents were characterised by a rapid rising phase followed by fast and profound inactivation. The activation kinetics were similar, but the inactivation was faster in HAM. In the outside-out configuration no evidence for mode switching was found. The kinetics of the rising phase of the ensemble currents were also similar to those observed using the cell-attached configuration, but the channels did not inactivate. In the whole-cell configuration, the mode switching was not present. The inactivation time constant showed a large scattering, and was much slower than that measured in the cell-attached patch mode. These currents were blocked by tetraethylammonium and 4-aminopyridine. Our results indicate that intracellular factors are involved in controlling the mode switching and the kinetics of the inactivation of potassium channels in pig articular chondrocytes.

Functionally distinct chloride-mediated GABA responses in rat cerebellar granule cells cultured in a low-potassium medium

The patch-clamp technique was used to study whole cell currents evoked by gamma-aminobutyric acid (GABA) in rat cerebellar granule cells cultured in 5 mM potassium, a condition that favors the development of functional GABAergic synapses. GABA activated both high- and low-sensitivity receptors. The high-sensitivity receptor had an effective concentration producing half the maximum response (EC50) of 13 microM, whereas the low-sensitivity one had an EC50 of 255 microM. The GABAA receptor agonist isoguvacine activated only the high-sensitivity receptor with an EC50 of 16 microM. When GABA was applied during the desensitized phase of the response elicited by a saturating concentration of isoguvacine, it was still able to induce a small response, whereas when isoguvacine was applied during the desensitizing phase of GABA-evoked current no response was detected. GABA responses were highly heterogeneous regarding their sensitivity to bicuculline. In a small number of cells (3 of 25), bicuculline (10 microM) completely abolished GABA-evoked currents. In the majority of the neurons (22 of 25) the blocking effect of bicuculline (100 microM) was 64 +/- 4% (mean +/- SE). The bicuculline-resistant component was abolished by picrotoxin (100 microM). In bicuculline, the dose-response curve for GABA was fitted with a sigmoidal curve with an EC50 value of 209 microM. These data indicate that functional new GABA receptor types with unusual pharmacology could be switched on by conditions that maintain cells in their undifferentiated state.

Zinc modulation of bicuculline-sensitive and -insensitive GABA receptors in the developing rat hippocampus

Intracellular recordings were used to study the effects of zinc on the bicuculline-sensitive and -insensitive responses evoked by GABA in CA3 rat hippocampal neurons in slices obtained from postnatal day (P) 0 to P8. In the absence of bicuculline, zinc inhibited GABA-induced responses in a concentration-dependent manner. This effect was developmentally regulated, being maximal (50%) between P0 and P5 and then declining to 30% after P5. In the presence of bicuculline, GABA-resistant responses were potentiated in 49% of cases, depressed in 38% and not affected in 13%. The period of maximum potentiation between P0 and P2 coincided with that of maximum expression of the bicuculline-resistant receptors. The effects of zinc were also studied using the whole-cell and outside-out configuration of the patch-clamp technique on bicuculline-sensitive and -insensitive GABA-induced currents elicited in isolated cells acutely dissociated from the same slices as those used for intracellular recordings. At a holding potential of -50 mV in symmetrical chloride solutions, GABA (50 and 100 microM) activated whole-cell inward currents which were reversibly blocked by zinc. The EC50 values for the blocking effect of zinc on currents evoked by 50 and 100 microM GABA were 6.6 nM and 5.8 microM respectively. In the presence of bicuculline (100 microM), zinc potentiated the residual responses to GABA; the response curve was bell-shaped with a peak at 1 microM. When the response to GABA was completely abolished by bicuculline, zinc (1 microM) was often able to restore it. In the presence of bicuculline, however, zinc was not able to restore the response to isoguvacine. In two excised outside-out patches, zinc (1 microM) increased the activity of opening of bicuculline-resistant GABA-evoked single channel currents (Np) from 1 to 1.87 and from 0.25 to 0.42 respectively, without changing single-channel conductance. These data suggest that down- or up-regulation of bicuculline-sensitive or -insensitive GABA receptors may be functionally important in regulating synaptic activity during development.

The calcineurin inhibitor cyclosporin A-cyclophilin A complex reduces desensitization of GABAA-mediated responses in acutely dissociated rat hippocampal neurons

The effects of the selective inhibitor of calcineurin, cyclosporin A-cyclophilin A (CC) complex on the desensitization kinetics of GABAA receptors was studied in acutely dissociated hippocampal neurons, using the patch clamp technique in the whole cell configuration. In control conditions, the decay of GABA-evoked current could be fitted by a biexponential function having time constants of 0.65 +/- 0.24 s and 3.75 +/- 2 s. The plateau to peak ratio was 0.087 +/- 0.034. Recovery from desensitization was obtained in more than 2 min. In cells dialyzed with the CC complex, the decay of the currents could be fitted with the sum of two exponentials having time constants similar to controls (0.81 +/- 0.47 s and 3.62 +/- 2.1 s), but the percentage of the fast component was smaller. The plateau to peak ratio was significantly larger than control (0.185 +/- 0.07). With CC complex, recovery from desensitization was completed in almost 30 s. The cyclosporin A derivative PSC 833, which does not inhibit calcineurin, did not affect desensitization kinetics. These results suggest that phosphatase 2B regulates desensitization of GABAA receptors.

Calcium influx in rat thalamic relay neurons through voltage-dependent calcium channels is inhibited by enkephalin

High and low voltage-activated, transient (HVA and LVA,T) Ca2+ currents are crucial in determining the characteristic thalamic firing pattern, during the oscillatory mode. The modulatory effects induced by D-ala2-D-leu5-enkephalin (DADLE) on voltage-dependent Ca2+ channels have been investigated on acutely dissociated neurons from rat ventro-basal (VB) thalamus, by means of whole cell patch-clamp technique. DADLE (400 nM) reduced HVA Ca2+ channel currents in 37 out of 44 cells tested (-53 +/- 5.3% to 0 mV test potential, n = 24,). In 50% of the cases DADLE induced an effect which was persistent at all the potentials tested, i.e. a voltage-independent one. In the remaining neurons, the inhibition partially or totally disappeared on the currents evoked at the highest potentials. DADLE was also able to inhibit LVA Ca2+ channels (-40% in five out of 12 cells). In conclusion, thalamic relay neurons present opioid receptors negatively coupled to both HVA and LVA Ca2+ channels. The presence of two inhibitory effects of DADLE on the total HVA Ca2+ channels has been observed, and they are distinguishable on the basis of their sensitivity to voltage. It is suggested that Ca2+ current modulation may play a role in the production and tuning of the rhythmic burst discharge in these neurons.

Spontaneous GABA-mediated synaptic currents in cerebellar granule cells in culture

The whole-cell configuration of the patch clamp technique was used to characterize the electrophysiological properties of spontaneous GABA-mediated synaptic currents in cerebellar granule cells grown in a low (5 mM) potassium medium. In the presence of kynurenic acid (1 mM), to block the excitatory drive, bicuculline-sensitive synaptic events were recorded. Their amplitude distribution could be fitted by several Gaussians having the same interpeak distance. In tetrodotoxin (TTX, 1 microM) spontaneous miniature events occurred at a lower frequency. Spontaneous currents reversed polarity at 8.17 +/- 0.63 mV, a potential close to ECl; the decay phase could be fitted with a single exponential having at -60 mV a time constant of 48.7 +/- 1.5 ms. In low noise recordings, channel closing could be resolved during the decay phase of miniature events. It appeared that a single quantum of GABA opened few channels on the postsynaptic membrane.

Whole cell and single channel properties of a new GABA receptor transiently expressed in the Hippocampus

1. The patch-clamp technique was used to characterize, in acutely dissociated CA3 rat hippocampal neurons, the whole cell and single channel properties of a novel response to gamma-aminobutyric acid (GABA) present only during a restricted period of postnatal development. 2. At postnatal days 0-10 (P0-P10), both GABA (100 microM) and isoguvacine (50 microM) evoked at a holding potential of -50 mV, in symmetrical chloride solution, whole cell inward currents. Bicuculline blocked the response to isoguvacine but only reduced the response to GABA (from 512 +/- 137 pA to 60 +/- 13 pA, mean +/- SE). After P12, bicuculline abolished the response to GABA. 3. The bicuculline-insensitive GABA currents were Cl- mediated and antagonized by picrotoxin. The desensitization rate was slower than the conventional bicuculline-sensitive response. The peak to plateau ratio induced by 0.1 or 1 mM of GABA shifted from 4.6 +/- 0.4 and 17.7 +/- 2.6 to 1.5 +/- 0.1 and 3.1 +/- 0.5 in the absence or in the presence of bicuculline, respectively. The recovery from desensitization was significantly faster for the bicuculline-insensitive responses. 4. In excised outside-out patches, GABA (20 microM) activated, in the presence of bicuculline (100 microM), single channel currents having conductances of 14, 22, and 31 pS. These values were similar to those obtained in the same preparation, in the absence of bicuculline. 5. These findings suggest that this new receptor type, which mediates bicuculline-insensitive responses with slow kinetics, may potentiate the depolarizing action of GABA during a critical period of postnatal development and therefore play a crucial role in synaptogenesis.

The effect of intracellular Ca2+ on GABA-activated currents in cerebellar granule cells in culture

The patch clamp technique was used to study the effects of intracellular free calcium ([Ca2+]i) on GABAA-evoked whole-cell and single channel currents of cultured cerebellar granule cells. Changes in [Ca2+]i were obtained by adding to the extracellular solution the calcium ionophore A23187 (2 microM). The relationship between [Ca2+]i and [Ca2+]o in the presence or absence of A23187 was assessed using fluorimetric measurements from Fura-2 loaded cells. In 2 mM [Ca2+]o and A23187, [Ca2+]i was about 1.5 microM, whereas in the absence of A23187 it was about 250 nM. In whole-cell experiments (symmetrical chloride concentrations) at -50 mV, GABA (0.5 microM) evoked inward currents that did not desensitize. Bath application of A23187 significantly reduced the steady-state amplitude of GABA currents by 37 +/- 6%. Single channel currents activated by GABA (0.5 microM) were also recorded in the outside-out configuration of the patch clamp technique. Kinetic analysis of single channel events revealed that A23187 significantly increased the long closed time constant (tau c3) without affecting the open time constants (tau o1 and tau o2) or the short and medium closed time constants (tau c1 and tau c2). Moreover, application of A23187 induced a significant reduction of burst duration (tau b). We conclude that a rise in [Ca2+]i by A23187 may decrease the binding affinity of GABA for the GABAA receptor.

Developmental changes in spontaneous GABAA-mediated synaptic events in rat hippocampal CA3 neurons

Ongoing spontaneous postsynaptic potentials (SPSPs) were intracellularly recorded at 34-36 degrees C from hippocampal CA3 neurons in slices obtained from postnatal days (P) 0-6 and 7-31. SPSPs occurred randomly, and their frequency distribution was fitted by a single exponential function. They were little affected by kynurenic acid, but were reversibly blocked by bicuculline, implying that they were mediated by GABAA receptors. The mean amplitude was 4.53 +/- 0.89 mV in control conditions and 4.07 +/- 0.79 mV in kynurenic acid. In kynurenic acid (with CsCl-filled microelectrodes), SPSPs reversed polarity at 2.4 +/- 2 mV. When tetrodotoxin (1 microM) was added to kynurenic acid solution, GABAA-mediated miniature postsynaptic potentials (MPSPs) were recorded. Under these conditions large events disappeared. The mean amplitude of MPSPs was 2.51 +/- 0.43 mV. The mean frequency decreased from 2.96 +/- 1.04 Hz in kynurenic acid to 0.4 +/- 0.15 Hz in kynurenic acid plus tetrodotoxin. In contrast with P0-P6, at P7-P31 SPSPs were significantly affected by kynurenic acid. The mean amplitude of SPSPs shifted from 4.71 +/- 0.82 mV in control conditions to 3.79 +/- 0.76 mV in kynurenic acid. At this developmental stage, the reversal potential of GABAA-mediated SPSPs shifted towards more negative values (-23.7 +/- 1.3 mV). Addition of tetrodotoxin to kynurenic acid solution abolished larger events and revealed GABAergic MPSPs. The mean amplitude of MPSPs was 2.72 +/- 0.5 mV, a value very close to that observed at P0-P6. Synaptic currents were recorded at 22-24 degrees C from voltage-clamped CA3 pyramidal neurons (at P6) using the tight-seal whole-cell recording technique.(ABSTRACT TRUNCATED AT 250 WORDS)

Culture and differentiation of chondrocytes entrapped in alginate gels

We studied the response to culture conditions and the differentiative ability in suspension culture in alginate gels of resting chondrocytes from the preosseous cartilage of adult pig scapula. It was found that the maximum rate of chondrocyte duplication is reached at the fourth day in culture whereas the rate of proteoglycan synthesis and alkaline phosphatase expression do not gain a maximum value before the seventh day. During the culture time, the chondrocytes undergo differentiation as it is demonstrated by the alkaline phosphatase specific activity increase and by morphological criteria (hypertrophy, increase of the number of mitochondria per cell, increased endoplasmic reticulum, matrix vesicle production). The alginate gels can be easily dissolved to obtain cell populations in which the variation of cytosolic calcium concentration following a proliferative stimulus can be conveniently observed using the conventional procedure of Fura 2.

Calcium-activated potassium channels in chondrocytes

The presence of calcium-activated potassium channels in chondrocytes of growing cartilage was tested. Results obtained with fura-2 on cultured resting chondrocytes indicate that the cells respond to an elevation of extracellular calcium concentration ([Ca2+]o) from 0.1 to 2 mM increasing the intracellular concentration of the ion ([Ca2+]i) from 117 to 187 nM. This increment may be blocked by 3 microM La3+. Patch clamp experiments in cell-attached configuration showed that, when [Ca2+]i rises, the open probability (Po) of the K+ channels increases. Increments in both Po and unitary currents of the K+ channels can be obtained after applying 2.5 microM A23187 with 2 mM [Ca2+]o. Hence, the results demonstrate that, in chondrocytes, a class of Ca(2+)-activated K+ channels is present and their activity is related to an increase of [Ca2+]i.

A potassium channel in cultured chondrocytes

Chondrocytes, obtained from preosseous cartilage, were studied by patch clamp technique in cell-attached recording configuration, and single potassium channels were characterized at different stages of culture. After 3 days, outward currents were present, with an open probability increasing with depolarization, and the K+ channels showing a mean slope conductance of 82 pS in asymmetric and 168 pS in symmetric potassium solution. Tetraethylammonium (TEA) and quinidine blocked the channels. Cells at confluence showed similar channel activity, with conductances of 121 and 252 pS, respectively. We suggest that culture time and/or conditions may modify K+ channels or induce the expression of a new type of channels.

Modification of plasma membrane of differentiating preosseous chondrocytes: evidence for a degradative process in the mechanism of matrix vesicle formation

Chondrocytes of the growth plate are differentiating cells. Their evolution leads to matrix vesicle formation and to cartilage mineralization. This is an in vitro study of the plasma membrane of chondrocytes at two differentiation stages. Differences in protein and glycoprotein components, increased membrane fluidity, and responsiveness to PTH indicate that hypertrophic ("ossifying") chondrocytes possess a plasma membrane widely different from that of resting chondrocytes. Their plasma membrane is particularly enriched in alkaline phosphatase (Mr 70K). Purified matrix vesicles contain the 70K form of alkaline phosphatase, but a 50K species is also detectable, a signal of degradative process. In fact, proteins and glycoproteins of matrix vesicles are less numerous than those of cell plasma membranes. It is suggested that, in vivo, matrix vesicle formation may be mediated by Ca2(+)-activated neutral proteases.

[Solid-phase conglutinin tests for circulating IgA immunocomplexes]

In some renal, gastroenterological and dermatological diseases the pathogenic role of circulating immune complexes (CIC) of IgA type has been defined. The methods so far proposed to investigate CIC are for the most part able to identify IgGIC, while only few can do the same for IgAIC. The Authors discuss in details the technique and the sensibility and specificity of a new test for IgAIC based on the linkage with the conglutinin in solid phase and propose this test for the clinical study of IgA diseases.