Calcium currents in rat cerebellar Purkinje cells maintained in culture

Physiological and morphological development of the rat cerebellar Purkinje cell

Cerebellar Purkinje cells integrate multimodal afferent inputs and, as the only projection neurones of the cerebellar cortex, are key to the coordination of a variety of motor- and learning-related behaviours. In the neonatal rat the cerebellum is undeveloped, but over the first few postnatal weeks both the structure of the cerebellum and cerebellar-dependent behaviours mature rapidly. Maturation of Purkinje cell physiology is expected to contribute significantly to the development of cerebellar output. However, the ontogeny of the electrophysiological properties of the Purkinje cell and its relationship to maturation of cell morphology is incompletely understood. To address this problem we performed a detailed in vitro electrophysiological analysis of the spontaneous and intracellularly evoked intrinsic properties of Purkinje cells obtained from postnatal rats (P0 to P90) using whole-cell patch clamp recordings. Cells were filled with neurobiotin to enable subsequent morphological comparisons. Three stages of physiological and structural development were identified. During the early postnatal period (P0 to approximately P9) Purkinje cells were characterized by an immature pattern of Na(+)-spike discharge, and possessed only short multipolar dendrites. This was followed by a period of rapid maturation (from approximately P12 to approximately P18), consisting of changes in Na(+)-spike discharge, emergence of repetitive bursts of Na(+) spikes terminated by Ca(2+) spikes (Ca(2+)-Na(+) bursts), generation of the trimodal pattern, and a significant expansion of the dendritic tree. During the final stage (> P18 to P90) there were minor refinements of cell output and a plateau in dendritic area. Our results reveal a rapid transition of the Purkinje cell from morphological and physiological immaturity to adult characteristics over a short developmental window, with a close correspondence between changes in cell output and dendritic growth. The development of Purkinje cell intrinsic electrophysiological properties further matches the time course of other measures of cerebellar structural and functional maturation.

Dendritic low-threshold Ca2+ channels in rat cerebellar Purkinje cells: possible physiological implications

Low-voltage activated (LVA) Ca2+ currents have been characterized in a large variety of neurons including cerebellar Purkinje cells (PCs). This review summarizes and discusses the biophysical, pharmacological properties, as well as the molecular identity of LVA Ca2+ channels described in PCs in various experimental conditions. Putative functional roles for LVA Ca2+ currents include generation of low-threshold Ca2+ spikes (LTS) that underlie burst firing, promotion of intrinsic oscillatory behaviour, Ca2+ entry close to the resting membrane potential and synaptic potentiation. Based on our recent findings on cerebellar rat PCs in slice cultures, this review presents the major evidence demonstrating that LVA Ca2+ channels produce a dendritic initiated LTS with a regulated propagation to the soma. This new role for LVA Ca2+ channels is particularly important in determining firing patterns in PCs.

Cerebellar slice cultures from mice lacking the P/Q calcium channel: electroresponsiveness of Purkinje cells

To investigate the role of P/Q type Ca(2+) channels in determining the firing pattern of Purkinje cells (PCs) we compared the somatically evoked discharge of action potentials (APs) in PCs from 3 to 4 week old cerebellar slice cultures obtained with ataxic mice lacking alpha(1A)-subunit (alpha(-/-)) and with normal mice (non-ataxic alpha(+/-) or alpha(+/+)) using the whole-cell configuration of the patch-clamp recording method. Whereas evoked responses of PCs in normal mice were mainly fast APs, those of PCs from ataxic mice were mainly low-threshold Ca(2+) spikes (LTS). Furthermore, a sustained plateau potential due to the activation of cadmium sensitive Ca(2+) conductances was not observed in PCs from ataxic mice by blocking K(+) channels. These results confirm that P/Q Ca(2+) channels elicit Ca(2+)-dependent plateau potentials and control the propagation of the dendritic LTS to the soma.

R-Type Ca2+ channels are coupled to the rapid component of secretion in mouse adrenal slice chromaffin cells

Patch-clamp measurements of Ca(2+) currents and membrane capacitance were performed on slices of mouse adrenal glands, using the perforated-patch configuration of the patch-clamp technique. These recording conditions are much closer to the in vivo situation than those used so far in most electrophysiological studies in adrenal chromaffin cells (isolated cells maintained in culture and whole-cell configuration). We observed profound discrepancies in the quantities of Ca(2+) channel subtypes (P-, Q-, N-, and L-type Ca(2+) channels) described for isolated mouse chromaffin cells maintained in culture. Differences with respect to previous studies may be attributable not only to culture conditions, but also to the patch-clamp configuration used. Our experiments revealed the presence of a Ca(2+) channel subtype never before described in chromaffin cells, a toxin and dihydropyridine-resistant Ca(2+) channel with fast inactivation kinetics, similar to the R-type Ca(2+) channel described in neurons. This channel contributes 22% to the total Ca(2+) current and controls 55% of the rapid secretory response evoked by short depolarizing pulses. Our results indicate that R-type Ca(2+) channels are in close proximity with the exocytotic machinery to rapidly regulate the secretory process.

Low-threshold potassium channels and a low-threshold calcium channel regulate Ca2+ spike firing in the dendrites of cerebellar Purkinje neurons: a modeling study

Various types of voltage-gated ion channels are distributed along the dendrites of neurons in the central nervous system. We have recently shown experimentally that the dendrites of cerebellar Purkinje neurons contain low-threshold voltage-gated Ca(2+) channels and low-threshold voltage-gated K+ channels. Although we found that these channels are involved in regulating the onset of Ca(2+)-dependent action potentials in the dendrites, we were unable to identify which of the known types of low-threshold Ca2+ channels and K+ channels were responsible, since there was no reliable method of discriminating between them. Here, we have built a detailed compartmental model of a Purkinje neuron by incorporating two types of low-threshold Ca2+ channel (T-type and class-E, or R-type) and two types of low-threshold K+ channel (A-type and D-type), in addition to another eight voltage-gated channel types, using a compartmental model neuron simulator. The model reproduces the basic features of the depolarization-induced responses of Purkinje neurons, such as fast Na+ spikes in the soma, Ca2+ spikes in the dendrites, the slow onset of Ca2+ spikes, repetitive Ca2+ spikes in the presence of TTX, the marked shortening of Ca2+ spike onset in the presence of 4-aminopydridine, and the longer Ca2+ spike onset in the presence of Ni2+. Our model shows that the D-type K+ channel and the class-E Ca2+ channel regulate the onset of depolarization-induced Ca2+ spikes in Purkinje neurons. These channels might be involved in integrating synaptic inputs in Purkinje neurons.

Dendro-somatic distribution of calcium-mediated electrogenesis in purkinje cells from rat cerebellar slice cultures

The role of Ca2+ entry in determining the electrical properties of cerebellar Purkinje cell (PC) dendrites and somata was investigated in cerebellar slice cultures. Immunohistofluorescence demonstrated the presence of at least three distinct types of Ca2+ channel proteins in PCs: the alpha1A subunit (P/Q type Ca2+ channel), the alpha1G subunit (T type) and the alpha1E subunit (R type). In PC dendrites, the response started in 66 % of cases with a slow depolarization (50 +/- 15 ms) triggering one or two fast (approximately 1 ms) action potentials (APs). The slow depolarization was identified as a low-threshold non-P/Q Ca2+ AP initiated, most probably, in the dendrites. In 16 % of cases, this response propagated to the soma to elicit an initial burst of fast APs. Somatic recordings revealed three modes of discharge. In mode 1, PCs display a single or a short burst of fast APs. In contrast, PCs fire repetitively in mode 2 and 3, with a sustained discharge of APs in mode 2, and bursts of APs in mode 3. Removal of external Ca2+ or bath applications of a membrane-permeable Ca2+ chelator abolished repetitive firing. Tetraethylammonium (TEA) prolonged dendritic and somatic fast APs by a depolarizing plateau sensitive to Cd2+ and to omega-conotoxin MVII C or omega-agatoxin TK. Therefore, the role of Ca2+ channels in determining somatic PC firing has been investigated. Cd2+ or P/Q type Ca2+ channel-specific toxins reduced the duration of the discharge and occasionallyinduced the appearance of oscillations in the membrane potential associated with bursts of APs. In summary, we demonstrate that Ca2+ entry through low-voltage gated Ca2+ channels, not yet identified, underlies a dendritic AP rarelyeliciting a somatic burst of APs whereas Ca2+ entry through P/Q type Ca2+ channels allowed a repetitive firing mainly by inducing a Ca2+-dependent hyperpolarization.

Differential roles of two types of voltage-gated Ca2+ channels in the dendrites of rat cerebellar Purkinje neurons

The distribution and function of voltage-gated Ca2+ channels in Purkinje neurons in rat cerebellar slices were studied using simultaneous Ca2+ imaging and whole-cell patch clamp recording techniques. Voltage-gated Ca2+ channels were activated by applying depolarizing voltage steps through the pipette attached at the soma in a voltage-clamp mode in the presence of tetrodotoxin. Poor space clamp due to extensive arborization of the dendrites allowed the dendrites to fire Ca2+ spikes. Ca2+ imaging with Fura-2 injected through the pipette, showed a steady [Ca2+]i increase at the soma and transient, spike-linked [Ca2+]i jumps in the dendrites. omega-Agatoxin-IVA (200 nM) abolished the depolarization-induced Ca2+ spikes, the spike-linked [Ca2+]i increase in the dendrites, and the steady [Ca2+]i increase at the soma. omega-Conotoxin-GVIA (5 microM) and nifedipine (3 microM) had no significant effect on the depolarization-induced responses. In the presence of 4-aminopyridine(2 mM) and omega-Agatoxin-IVA, transient [Ca2+]i increases remained in the dendrites. Low concentrations of Ni2+(100 microM) reversibly suppressed this [Ca2+]i increase. The voltage for half-maximal activation and inactivation of this component were lower than -50 mV and -31 mV, respectively. In normal conditions, low concentration of Ni2+ slowed the onset of the Ca2+ spike without changing the time course of the spikes or the amplitude of the accompanying [Ca2+]i increase. These results show that omega-Agatoxin-IVA-sensitive Ca2+ channels are distributed both in the soma and the dendrites, and are responsible for dendritic Ca2+ spikes, whereas low-voltage activated, Ni2+-sensitive Ca2+ channels are distributed in the whole dendrites including both thick and fine branches, and provide boosting current for spike generation.

Low-threshold Ca2+ currents in dendritic recordings from Purkinje cells in rat cerebellar slice cultures

Voltage-dependent Ca2+ conductances were investigated in Purkinje cells in rat cerebellar slice cultures using the whole-cell and cell-attached configurations of the patch-clamp technique. In the presence of 0.5 mM Ca2+ in the extracellular solution, the inward current activated with a threshold of -55 +/- 1.5 mV and reached a maximal amplitude of 2.3 +/- 0.4 nA at -31 +/- 2 mV. Decay kinetics revealed three distinct components: a fast (24.6 +/- 2 msec time constant), a slow (304 +/- 46 msec time constant), and a nondecaying component. Rundown of the slow and sustained components of the current, or application of antagonists for the P/Q-type Ca2+ channels, allowed isolation of the fast-inactivating Ca2+ current, which had a threshold for activation of -60 mV and reached a maximal amplitude of 0.7 nA at a membrane potential of -33 mV. Both activation and steady-state inactivation of this fast-inactivating Ca2+ current were described with Boltzmann equations, with half-activation and inactivation at -51 mV and -86 mV, respectively. This Ca2+ current was nifedipine-insensitive, but its amplitude was reduced reversibly by bath-application of NiCl2 and amiloride, thus allowing its identification as a T-type Ca2+ current. Channels with a conductance of 7 pS giving rise to a fast T-type ensemble current (insensitive to omega-Aga-IVA) were localized with a high density on the dendritic membrane. Channel activity responsible for the ensemble current sensitive to omega-Aga-IVA was detected with 10 mM Ba2+ as the charge carrier. These channels were distributed with a high density on dendritic membranes and in rare cases were also seen in somatic membrane patches.

Depolarization-induced calcium signals in the somata of cerebellar Purkinje neurons

Cerebellar Purkinje neurons express voltage-gated Ca2+ channels that are located on their somata and dendrites. Previous reports, based on microelectrode recordings and fura-2 Ca2+ imaging, suggested that depolarization-mediated intracellular Ca2+ signaling is confined almost completely to the dendrites. We investigated the spatial distribution of depolarization-induced Ca2+ signals in Purkinje neurons by applying whole-cell patch-clamp recordings combined with fluorometric Ca2+ imaging to cerebellar slices. Under our recording conditions, depolarizing pulses produced the dendritic but also large somatic Ca2+ signals. By selective perfusion of the slice with a Ca(2+)-free EGTA-containing solution, we could isolate experimentally Ca2+ signals in somata and dendrites, respectively. Moreover, experiments performed on cerebellar slices from young rats (up to postnatal day 6), in which Purkinje neurons are almost completely devoid of dendrites, showed that Ca2+ currents produced by the activation of somatic Ca2+ channels are associated with Ca2+ transients similar to those seen in the somata of adult Purkinje neurons. Our results strongly indicate that the depolarization-induced somatic Ca2+ signals are caused by Ca2+ entry through voltage-gated channels located on the somatic membrane of Purkinje neurons.

Characteristics of voltage sensitive calcium channels in dendrites of cultured rat cerebellar neurons

We examined the effects of various pharmacological agents on the Ca2+ channels existing in the dendrites of cultured rat cerebellar neurons. The cultures consisted of Purkinje cells and non-Purkinje cells (deep cerebellar nuclear neurons and other non-granule neurons). Changes in intracellular free calcium concentration, [Ca2+]i, were monitored by digital imaging microfluorimetry using fura-2 as the indicator dye. In the Purkinje cell population increases in dendritic [Ca2+]i evoked by brief pulses of high K+ were very effectively blocked by (> 80%) by omega-Aga-IVA at low concentrations. Nimodipine and omega-conotoxins GVIA and MVIIC only blocked small components of the [Ca2+]i rise. In the non-Purkinje cells, omega-Aga-IVA was much less effective. omega-Conotoxin-GVIA blocked somewhat more and nimodipine blocked a similar percentage of the [Ca2+]i rise. omega-Conotoxin-MVIIC was quite ineffective in these cells. The results are discussed in terms of the types of voltage sensitive Ca2+ channels existing in the dendrites of these cells.

P-type calcium channels in the somata and dendrites of adult cerebellar Purkinje cells

The pharmacological and single-channel properties of Ca2+ channels were studied in the somata and dendrites of adult cerebellar Purkinje cells. The Ca2+ channels were exclusively of the high threshold type: low threshold Ca2+ channels were not found. These high threshold channels were not blocked by omega-conotoxin GVIA and were inhibited rather than activated by BAY K 8644. They were therefore pharmacologically distinct from high threshold N- and L-type channels. Funnel web spider toxin was an effective blocker. The channels opened to conductance levels of 9, 14, and 19 pS (in 110 mM Ba2+). These slope conductances were in the range of those reported for N- and L-type channels. Our results are in agreement with previous reports suggesting that Ca2+ channels in Purkinje cells can be classified as P-type channels according to their pharmacology. The results also suggest that distinctions among Ca2+ channel types based on the single-channel conductance are not definitive.

Voltage-dependent calcium currents in dissociated granule cells from rat cerebellum

Voltage-dependent calcium currents were investigated by the patch-clamp technique in whole-cell recording configuration in cultures from 8-day-old rat cerebella, which contained greater than or equal to 90% granule cells. In solutions designed to minimize the sodium and potassium conductances and in 20 mM barium, an inward current activated around -25 mV, reached a peak amplitude at +20 mV and reversed around +80 mV. In 20 mM calcium, this current was approximately 50% of that in barium. From one to three days in vitro only 16% of the cells tested (n = 20) had a current exceeding 50 pA in maximum amplitude, while after four days in vitro the current reached 100 pA in all neurons tested (n greater than 70). Verapamil (50-100 microM) reversibly depressed this current. The dihydropyridine agonist Bay K 8644 (1 microM) enhanced the maximum conductance by 25 +/- 8% (n = 4), caused a negative shift in the activation of 21 +/- 5 mV and a prolongation of the deactivation time course as the voltage was stepped back from +20 to -80 mV. The GABAB agonist baclofen (50 microM) reversibly depressed the current by 27 +/- 8% in 80% of the cells. The effect was similar to that of GABA (10 microM), when the GABAA response (chloride current) was partially blocked by bicucculine. This current can be classified as a dihydropyridine-sensitive high-voltage-activated calcium current. The modulation by GABAB agonists is likely to be significant for presynaptic inhibition.

Ionic control of intracellular pH in rat cerebellar Purkinje cells maintained in culture

1. Intracellular pH (pHi) was measured in single rat cerebellar Purkinje cells maintained in primary culture using microspectrofluorescence analysis of the intracellularly trapped pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5 (and -6)-carboxyfluorescein (BCECF). 2. The ratio of the fluorescence signals measured at 530 nm in response to an alternating excitation at 450 and 490 nm was calibrated using the K(+)-H+ ionophore nigericin. This calibration gave a steady-state pHi of 7.06 +/- 0.02 (S.E.M., n = 17) when cells were perfused by a 5% CO2-25 mM-HCO3(-)-buffered solution at an external pH of 7.40 at 37 degrees C. 3. Replacement of external chloride with gluconate in the presence of bicarbonate induced a cytoplasmic alkalinization of about 0.3 pH unit. This alkalinization was independent of external sodium and was greatly reduced by 0.5 mM-DIDS, indicating the presence of a chloride-bicarbonate exchange. 4. In bicarbonate-free (HEPES-buffered) solution the steady-state pHi was 7.37 +/- 0.02 (n = 19), significantly higher than in bicarbonate-buffered solution. Recovery from an intracellular acid load brought about by the ammonium chloride pre-pulse technique was blocked by the removal of external sodium or the addition of 1.5 mM-amiloride, indicating the presence of a sodium-hydrogen exchange. 5. In bicarbonate-buffered solution pHi recovery after an acid load was also completely blocked by addition of 1.5 mM-amiloride indicating the absence of a bicarbonate-dependent acid extrusion mechanism. 6. Addition of 12-O-tetradecanoylphorbol-13-acetate (TPA, 100 nM) induced an amiloride-sensitive alkalinization of about 0.3 pH unit in bicarbonate-buffered solution but had no effect in HEPES-buffered solution. This observation suggests that in cultured Purkinje cells the sodium-hydrogen exchanger could be activated through a protein kinase C pathway only when pHi is maintained at a low physiological value by the activity of the chloride-bicarbonate exchange.

Voltage-activated calcium channels in rat Purkinje cells maintained in culture

Cell-attached patch recordings were used to study calcium channels on the dendritic membrane of rat cerebellar Purkinje cells maintained in culture. Experiments were performed with isotonic BaCl2 (110 mM) in the pipette and isotonic potassium gluconate in the bath to zero the cell membrane potential. Two distinct types of voltage-activated calcium channels were identified. The first one had a small conductance (9 pS), was activated at a low threshold (congruent to -50 mV) and could be inactivated by holding the membrane potential at -30 mV. This channel had the same characteristics as the T channel described in other neuronal preparations. The second type of Ca channel activated at a high threshold (-30 or +10 mV depending on whether BAY K 8644 was added or not to the pipette solution) and was still activatable even when the membrane was held at -40 mV. In the presence of BAY K 8644 this channel had a conductance of 21 pS with long openings. All these characteristics are similar to those of the S (L) Ca channel described in many preparations. The present study is in agreement with our previous experiments on Purkinje dendrites, where we identified low and high threshold Ca currents using the whole-cell configuration. Up to now, no channel corresponding to the N current has been observed but we cannot exclude its presence.

Potassium currents in rat cerebellar Purkinje neurones maintained in culture in L15 (Leibovitz) medium

Cerebellar Purkinje cells (PC) can be maintained in culture for one to two weeks in L15, a rich medium known to allow expression of a normal excitability in peripheral neurones. When examined using whole cell recordings, PC proved to be inexcitable in these conditions, and this inexcitability could be related to the presence of large outward K currents. Depolarizing steps of -100 mV revealed a voltage-dependent biphasic K current with a large early transient phase followed by a small plateau phase. The early transient phase could be selectively eliminated by holding the cell at -40 mV or by extracellularly applying 5 mM 4-aminopyridine (4-AP), whereas the plateau was abolished by 15 mM tetraethylammonium (TEA). Hereafter, these currents will be identified as the IA and the delayed current respectively, IA being the predominant current. IA activated between -25 and +65 mV with a midpoint at +3 mV; inactivation occurred between -70 and -20 mV with a midpoint at -57 mV. Current decay followed an exponential time course with a time constant of about 30 ms between -20 and +10 mV. In the cell-attached recording configuration, depolarization elicited openings of two types of K channels, one inactivating and one non-inactivating. The non-inactivating K channel probably corresponded to the delayed K current and had a conductance of 22 pS in a physiological K gradient.