Inward rectification and low threshold calcium conductance in rat cerebellar Purkinje cells. An in vitro study

Influence of functional glia on the electrophysiology of Purkinje cells in organotypic cerebellar cultures

Previous studies have shown that exposure of organotypic cerebellar explants to cytosine arabinoside (Sigma) for the first five days in vitro drastically reduced the granule cell population and severely affected glial function. Myelination was absent and astrocytes failed to ensheath Purkinje cells. In the absence of astrocytic ensheathment, Purkinje cell somata became hyperinnervated by Purkinje cell recurrent axon collaterals. Recurrent axon collaterals also projected to Purkinje cell dendritic spines. In later studies, exposure of cerebellar cultures to a different formulation of cytosine arabinoside (Pfanstiehl) also affected granule cells and oligodendrocytes but did not compromise astrocyte function. The different susceptibility of astrocytes to the two preparations of cytosine arabinoside (Sigma and Pfanstiehl) has provided the opportunity to examine the electrophysiological properties of Purkinje cells in the presence and absence of functional glia. Ensheathed Purkinje cells in granuloprival cultures exhibit within two weeks in vitro similar passive membrane properties as Purkinje cells in control cultures. Their input resistance is significantly higher and their spontaneous single-unit discharge is significantly lower than that of unensheathed Purkinje cells. This effect suggests that ensheathed Purkinje cells in cytosine arabinoside (Pfanstiehl)-treated cultures are more responsive to the profuse Purkinje cell recurrent axon collateral inhibitory projection to dendritic spines. These studies also show that the presence of functional glia and/or astrocytic ensheathment can be correlated with the development of complex spike activity by Purkinje cells in vitro. Purkinje cells in cultures treated with cytosine arabinoside (Pfanstiehl), which does not compromise astrocytic ensheathment, display membrane conductances and spike activity similar to mature Purkinje cells in control cultures. By contrast, Purkinje cells in cultures treated with cytosine arabinoside (Sigma), and devoid of astrocytic ensheathment, display mainly simple spike activity reminiscent of the type of activity seen in less mature neurons.

Dopamine modulates inwardly rectifying hyperpolarization-activated current (Ih) in cultured rat olfactory receptor neurons

The presence of dopamine receptors in olfactory receptor neurons (ORNs) suggests that odor sensitivity may be modulated by neurotransmitters at the level of primary sensory neurons. Using standard patch-clamp techniques on rat ORNs, we found that 1 microM dopamine, 500 microM SQ 22536 (SQ, an adenylyl cyclase inhibitor), 20 and 50 microM quinpirole (a selective dopamine D2 receptor agonist), and 1 mM adenosine 3', 5'-cyclic monophosphate (cAMP) modulate the hyperpolarization-activated current Ih. On hyperpolarizing from a holding potential of -58 mV, a small Cs+-sensitive inwardly rectifying current (Ih) was observed. Increases in extracellular K+ increased Ih amplitude without shifting its voltage dependence of activation, whereas increases in temperature produced an increase in Ih amplitude and a hyperpolarizing shift in the activation curve. Application of 1 microM dopamine reversibly shifted Ih activation to more negative potentials and decreased Ih current amplitudes. These effects were blocked by concomitant application of dopamine with sulpiride, a selective dopamine D2 receptor antagonist. The effects of dopamine were mimicked by quinpirole. Quinpirole (20 microM) decreased Ih current amplitude, but was without effect on Ih voltage dependence of activation. However, 50 microM quinpirole produced both a reduction of Ih peak currents and a hyperpolarizing shift in the activation curve for Ih. External application of the adenylyl cyclase inhibitor SQ 22536 produced a reversible decrease in peak currents but had no effect on Ih voltage dependence of activation, whereas internal application of cAMP shifted Ih activation to more depolarized potentials. Because Ih modulates cell excitability and spike frequency adaptation, our findings support a role for dopamine in modulating the sensitivity and output of rat ORNs to odorants.

The weaver mutation causes a loss of inward rectifier current regulation in premigratory granule cells of the mouse cerebellum

Considerable interest has recently focused on the weaver mutation, which causes inward rectifier channel alterations leading to profound impairment of neuronal differentiation and to severe motor dysfunction in mice (Hess, 1996). The principal targets of mutation are cerebellar granule cells, most of which fail to differentiate and degenerate in a premigratory position (Rakic and Sidman, 1973a,b). Two hypotheses have been put forward to explain the pathogenetic role of mutant inward rectifier channels: namely that inward rectifier channel activity is either lacking (Surmeier et al., 1996) or altered (Kofuji et al., 1996; Silverman et al., 1996; Slesinger et al., 1996). We have examined this question by recording inward rectifier currents from cerebellar granule cells in situ at different developmental stages in wild-type and weaver mutant mice. In wild-type mice, the inward rectifier current changed from a G-protein-dependent activation to a constitutive activation as granule cells developed from premigratory to postmigratory stages. In weaver mutant mice, G-protein-dependent inward rectifier currents were absent in premigratory granule cells. A population of putative granule cells in the postmigratory position expressed a constitutive inward rectifier current with properties compatible with mutated GIRK2 channels expressed in heterologous systems. Because granule cells degenerate at the premigratory stage (Smeyne and Goldowitz, 1989), the loss of inward rectifier current and its regulation of membrane potential are likely to play a key role in the pathogenesis of weaver neuronal degeneration.

The weaver mutation causes a loss of inward rectifier current regulation in premigratory granule cells of the mouse cerebellum

Considerable interest has recently focused on the weaver mutation, which causes inward rectifier channel alterations leading to profound impairment of neuronal differentiation and to severe motor dysfunction in mice (Hess, 1996). The principal targets of mutation are cerebellar granule cells, most of which fail to differentiate and degenerate in a premigratory position (Rakic and Sidman, 1973a,b). Two hypotheses have been put forward to explain the pathogenetic role of mutant inward rectifier channels: namely that inward rectifier channel activity is either lacking (Surmeier et al., 1996) or altered (Kofuji et al., 1996; Silverman et al., 1996; Slesinger et al., 1996). We have examined this question by recording inward rectifier currents from cerebellar granule cells in situ at different developmental stages in wild-type and weaver mutant mice. In wild-type mice, the inward rectifier current changed from a G-protein-dependent activation to a constitutive activation as granule cells developed from premigratory to postmigratory stages. In weaver mutant mice, G-protein-dependent inward rectifier currents were absent in premigratory granule cells. A population of putative granule cells in the postmigratory position expressed a constitutive inward rectifier current with properties compatible with mutated GIRK2 channels expressed in heterologous systems. Because granule cells degenerate at the premigratory stage (Smeyne and Goldowitz, 1989), the loss of inward rectifier current and its regulation of membrane potential are likely to play a key role in the pathogenesis of weaver neuronal degeneration.

Hyperpolarization induces a rise in intracellular sodium concentration in dopamine cells of the substantia nigra pars compacta

We investigated the effect of changes in membrane-voltage on intracellular sodium concentration ([Na+]i) of dopamine-sensitive neurons of the substantia nigra pars compacta in a slice preparation of rat mesencephalon. Whole-cell patch-clamp techniques were combined with microfluorometric measurements of [Na+]i using the Na+-sensitive probe, sodium-binding benzofuran isophthalate (SBFI). Hyperpolarization of spontaneously active dopamine neurons (recorded in current-clamp mode) caused the cessation of action potential firing accompanied by an elevation in [Na+]i. In dopamine neurons voltage-clamped at a holding potential of -60 mV elevations of [Na+]i were induced by long-lasting (45-60 s) voltage jumps to more negative membrane potentials (-90 to -120 mV) but not by corresponding voltage jumps to -30 mV. These hyperpolarization-induced elevations of [Na+]i were depressed during inhibition of I(h), a hyperpolarization-activated inward current, by Cs+. Hyperpolarization-induced elevations in [Na+]i might occur also in other cell types which express a powerful I(h) and might signal lack of postsynaptic activity.

Membrane properties of mouse dorsal cochlear nucleus neurons in vitro

Intracellular recordings were made from neurons of the mouse dorsal cochlear nucleus (DCN) in vitro using current clamp techniques in the presence or absence of different ion channel blocking drugs. Four electrophysiologically distinct cell groups were identified in the DCN. The groups were characterized on the basis of their spontaneous firing properties, the shape of the action potential (AP) and the pattern of firing, the shape of the current-voltage (I/V) relationship and the effects of channel blocking agents. By comparison with known histology, three of the four DCN groups were postulated to be cartwheel-like, fusiform-like, or tuberculoventral-like cells. The fourth group was postulated to be a stellate-like as it had similar properties to the spike train (stellate) cell of the AVCN. DCN stellate-like cells were spontaneously active, the action potentials (APs) were always followed by a large, brief hyperpolarization and the cells had linear current voltage relationships. The fusiform-like cells were spontaneously active and spontaneous IPSPs were also observed. The I/V relationship was linear for these cells. Tuberculoventral-like cells were not spontaneously active, but APs could be elicited by inward current injection. The I/V relationships for tuberculoventral-like cells were linear. Cartwheel-like cells were spontaneously active. These cells were characterized by the distinctive shape of their APs which were single, large amplitude, short duration APs sometimes followed by a series of complexes consisting of small, long duration APs. Cartwheel-like cells were the only cell type in the DCN which had non-linear I/V relationships. All cells in the DCN had APs which were abolished by tetrodotoxin. Different calcium dependent channels play a role in the formation of both the fast single AP and the slow complex AP in the cartwheel-like cells since all APs were abolished by the use of high concentrations of verapamil. Verapamil dramatically increased the duration of APs in fusiform-like cells and had no effect on tuberculoventral-like cells. In both tuberculoventral-like cells and cartwheel-like cells, 4-aminopyridine (4AP) depolarized the cells and all APs were abolished. Tetraethylammonium chloride (TEA) had a similar effect in cartwheel-like cells. In stellate-like, tuberculoventral-like and fusiform-like cells, the hyperpolarization which followed the AP was abolished by TEA. The AP duration in these cells was also increased by TEA. 4AP had a similar effect in stellate-like and fusiform-like cells. The data for DCN suggest that electrophysiological properties can be used to distinguish and identify neurons.

Heterogeneous voltage dependence of inward rectifier currents in spiral ganglion neurons

Inward rectification was characterized in neonatal spiral ganglion neurons maintained in tissue culture. Whole cell current and voltage-clamp techniques were used to show that the hyperpolarization-activated cationic (Ih) current underlies most or all of the inward rectification demonstrated in these neurons. The average reversal potential (-41.3 mV) and cesium sensitivity were typical of that found in other neurons and cell types. What was unique about the hyperpolarization-activated currents, however, was that the half-maximal voltages (V1/2) and slope factors (k) that characterized Ih current activation were graded from neuron to neuron. Voltage-clamp recordings made with standard bath and pipette solutions revealed V1/2 values that ranged from -78.1 to -122.1 mV, with slope factors from 7.6 to 13.1. These gradations in the voltage-dependent features of the Ih current did not result from variability in the recording conditions because independently measured Na+ current-to-voltage relationships were found to be uniform (peak current at -20 mV). Moreover, the range and average V1/2 and slope values could be altered with activators [8-(4-chlorophenylthio) adenosine 3',5'-cyclic monophosphate in combination with okadaic acid] or inhibitors {N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide}of protein indicating that Ih current heterogeneity most likely resulted from differential phosphorylation.

Metabotropic glutamate receptor agonists alter neuronal excitability and Ca2+ levels via the phospholipase C transduction pathway in cultured Purkinje neurons

Selective agonists for metabotropic glutamate receptor (mGluR) subtypes were tested on mature, cultured rat cerebellar Purkinje neurons (> or = 21 days in vitro) to identify functionally relevant mGluRs expressed by these neurons and to investigate the transduction pathways associated with mGluR-mediated changes in membrane excitability. Current-clamp recordings (nystatin/perforated-patch method) were used to measure the membrane response of Purkinje neurons to brief microperfusion pulses (1.5 s) of the group I (mGluR1/mGluR5) agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (300 microM), quisqualate (5 microM), and (R,S)-3,5-dihydroxyphenylglycine (50-500 microM). All group I mGluR agonists elicited biphasic membrane responses and burst activity in the Purkinje neurons. In addition, the group I mGluR agonists produced alterations in the active membrane properties of the Purkinje neurons and depressed the OFF response after hyperpolarizing current injection. In parallel microscopic Ca2+ imaging experiments, application of the group I mGluR agonists to fura-2-loaded cells elicited increases in intracellular Ca2+ in both the somatic and dendritic regions. The group II (mGluR2/mGluR3) agonist (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (10 microM) and the group III (mGluR4/mGluR6/mGluR7/mGluR8) agonists L(+)-2-amino-4-phosphonobutyric acid (1 mM) and O-phospho-L-serine (200 microM) had no effect on the membrane potential or intracellular Ca2+ levels of the Purkinje neurons. The cultured Purkinje neurons, but not granule neurons or interneurons, showed immunostaining for mGluR1alpha in both the somatic and dendritic regions. All effects of the group I mGluR agonists were blocked by (+)-alpha-methyl-4-carboxyphenylglycine (1 mM), an mGluR antagonist. Furthermore, the phospholipase C inhibitor 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H -pyrrole-2,5-dione (2 microM) blocked the group I mGluR agonist-mediated electrophysiological response and greatly attenuated the Ca2+ signal elicited by group I mGluR agonists, particularly in the dendrites. The inactive analogue 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-2, 5-pyrrolidine-dione (2 microM) was relatively ineffective against the electrophysiological response and Ca2+ signal. These results indicate that functional group I mGluRs (but not group II or III mGluRs) can be activated on mature Purkinje neurons in culture and result in changes in neuronal excitability and intracellular Ca2+ mediated through phospholipase C. These data obtained from a defined neuronal type, the Purkinje neuron, confirm biochemical and molecular studies on the transduction mechanisms of group I mGluRs and show that this transduction pathway is linked to neuronal excitability and intracellular Ca2+ release in the Purkinje neurons.

Analysis of spontaneous electrical activity in cerebellar Purkinje cells acutely isolated from postnatal rats

Whole-cell patch recording techniques were used to analyze spontaneous electrical activity in cerebellar Purkinje cells acutely isolated from postnatal rats. Spontaneous activity was present in 65% of the cells examined, and it included simple and complex firing patterns which persisted under conditions that eliminated residual or reformed synaptic contacts. Under voltage clamp, both spontaneous and quiescent cells displayed similar voltage-dependent conductances. Inward current was carried by Na+ through tetrodotoxin (TTX)-sensitive channels and by Ca2+ through P-type and T-type Ca channels. P-type current was present in all cells examined. T-type current was found in <50%, and it did not correlate with spontaneous activity. We found no evidence of a transient (A-type) potassium current or hyperpolarization-activated cationic current in either spontaneous or quiescent cells. Spontaneous activity did correlate with a lower activation threshold of the Na current, resulting in substantial overlap of the activation and inactivation curves. TTX reduced the holding current of spontaneous cells clamped between -50 and -30 mV, consistent with the presence of a Na "window" current. We were unable, however, to measure a persistent component of the Na current using voltage steps, a result which may reflect the complex gating properties of Na channels. An Na window current could provide the driving force underlying spontaneous activity, as well as plateau potentials, in Purkinje cells.

Synchronized oscillations in the inferior olive are controlled by the hyperpolarization-activated cation current I(h)

The participation of a hyperpolarization-activated cationic current in the generation of oscillations in single inferior olive neurons and in the generation of ensemble oscillations in the inferior olive nucleus (IO) of the guinea pig and ferret was investigated in slices maintained in vitro. Intracellular recordings in guinea pig or ferret 10 neurons revealed that these cells could generate sustained endogenous oscillations (4-10 Hz) at hyperpolarized membrane potentials (-60 to -67 mV) after the intracellular injection of a brief hyperpolarizing current pulse. These oscillations appeared as the rhythmic generation of a low-threshold Ca2+ spike that typically initiated one or two fast Na+-dependent action potentials. Between low-threshold Ca2+ spikes was an afterhyperpolarization that formed a "pacemaker" potential. Local application of apamin resulted in a large reduction in the amplitude of the afterhyperpolarization, indicating that a Ca2+-activated K+ current makes a strong contribution to its generation. However, even in the presence of apamin, hyperpolarization of IO neurons results in a "depolarizing sag" of the membrane potential that was blocked by local application of Cs+ or partial replacement of extracellular Na+ with choline+ or N-methyl-D-glucamine+, suggesting that I(h) also contributes to the generation of the afterhyperpolarization. Extracellular application of low concentrations of cesium resulted in hyperpolarization of the membrane potential of IO neurons and spontaneous 5- to 6-Hz oscillations in single, as well as networks, of IO neurons. Application of larger concentrations of cesium reduced the frequency of oscillation to 2-3 Hz or blocked the oscillation entirely. On the basis of these results, we propose that I(h) contributes to single and ensemble oscillations in the IO in two ways: 1) I(h) contributes to the determination of the resting membrane potential such that reduction of I(h) results in hyperpolarization of the membrane potential and an increased propensity of oscillation through removal of inactivation of the low-threshold Ca2+ current; and 2) I(h) contributes to the generation of the afterhyperpolarization and the pacemaker potential between low-threshold Ca2+ spikes.

Localization and developmental changes of the expression of two inward rectifying K(+)-channel proteins in the rat brain

We have raised affinity-purified polyclonal antibodies specific for the inward rectifying K+ channel (IRK1/Kir2.1) and the G protein-activated inward rectifying K+ channel (GIRK1/Kir3.1) examined their distributions in the rat brain immunohistochemically. The regional expression pattern of the IRK1 and GIRK1 proteins were similar to those of mRNA of the previous in situ hybridization study. The subcellular distribution was studied in the cerebellum; cerebral cortex and hippocampus. In the cerebellum, the IRK1 protein was clearly detected in the somata and proximal dendrites of Purkinje cells, while the GIRK1 protein was present in the somata and clustered dendrites of granule cells. In the cerebral cortex and hippocampus, both IRK1- and GIRK1-immunoreactivities were detected in the somata and apical dendrites of the pyramidal cells. The presence of IRK1 or GIRK1 proteins in the axons could not proved by the present study. The developmental changes of the expression pattern of the GIRK1 protein were also investigated in the hippocampus and in the cerebellum of postnatal day (P) 7 to P17 rats. The GIRK1 protein was detected neither in the subgranular zone of the dentate gyrus nor in the proliferative zone of the external granule cell layer of the cerebellum, in which granule cell precursors are reported to proliferate, while it was clearly detected in the adjacent layer in which postmitotic but immature cells exist. These results imply that the expression of the GIRK1 protein starts just after the neuronal precursors finished the last mitotic cell division.

In vitro electrophysiology of rat subicular bursting neurons

Intracellular recordings were used to study the electrophysiological properties of rat subicular neurons in a brain slice preparation in vitro. Cells were classified as bursting neurons (n = 102) based on the firing pattern induced by depolarizing current pulses. The bursting response recorded at resting membrane potential (-66.1 +/- 6.2 mV, mean +/- SD n = 94) was made up of a cluster of fast action potentials riding on a slow depolarization and was followed by an afterhyperpolarization. Tonic firing occurred at a membrane potential of approximately -55 mV. A burst also occurred upon termination of a hyperpolarizing current pulse. Tetrodotoxin (TTX, 1 microM) blocked the burst and decreased or abolished the underlying slow depolarization. These effects were not induced by the concomitant application of the Ca2+ channel blockers Co2+ (2 mM) and Cd2+ (1 mM). Subicular bursting neurons displayed voltage- and time-dependent inward rectifications of the membrane during depolarizing and hyperpolarizing current pulses. The inward rectification in the depolarizing direction was abolished by TTX, while that in the hyperpolarizing direction was blocked by extracellular Cs+ (3 mM), but not modified by Ba2+ (0.5-1 mM), TTX, or Co2+ and Cd2+. Tetraethylammonium (10 mM)-sensitive, outward rectification became apparent in the presence of TTX. These results suggest that neurons in the rat subiculum can display voltage-dependent bursts of action potentials as well as membrane rectification in the depolarizing and hyperpolarizing directions. These results also indicate that activation of a voltage-gated Na+ conductance may be instrumental in the initiation of bursting activity.

Spatial distribution of synaptically activated sodium concentration changes in cerebellar Purkinje neurons

The spatial distribution of Na(+)-dependent events in guinea pig Purkinje cells was studied with a combination of high-speed imaging and simultaneous intracellular recording. Individual Purkinje cells in sagittal cerebellar slices were loaded with either fura-2 or the Na+ indicator sodium binding benzofuran isophthalate (SBFI) with sharp electrodes or patch electrodes on the soma or dendrites. [Na+]i changes were detected in response to climbing fiber and parallel fiber stimulation. These changes were located both at the anatomically expected sites of synaptic contact in the dendrites and in the somatic region. The variation in time course of these [Na+]i changes in different locations implies that Na+ enters at the synapse and diffuses rapidly to locations of lower initial [Na+]i. The synaptically activated somatic [Na+]i changes probably reflect Na+ entry through voltage-sensitive Na+ channels because they were detected only when regenerative potentials were recorded in the soma. [Na+]i changes in response to antidromically or intrasomatically evoked Na+ action potentials also were confined to the cell body. These observations are in agreement with other evidence that Na+ spikes are generated in the somatic region of the Purkinje neuron and spread passively into the dendrites. Plateau potentials, evoked by depolarizing pulses to the soma or dendrites, caused [Na+]i changes only in the soma, indicating that the noninactivating Na+ channels contributing to this potential also were concentrated in this region. The climbing fiber-activated [Na+]i changes were blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, indicating that these changes were not due to direct stimulation of the Purkinje neuron or activation of metabotropic receptors. Direct depolarization of the soma or dendrites never caused dendritic [Na+]i increases, suggesting that the climbing fiber-activated [Na+]i changes in the dendrites are due to Na+ entry through ligand-gated channels. A climbing fiber-like regenerative potential could be recorded in the soma after anode break stimulation, parallel fiber activation, or depolarizing pulses to the soma. The [Na+]i changes evoked by all of these potentials were confined to the cell body region of the Purkinje cell. [Ca2+]i changes in the dendrites evoked by the anode break potential were small relative to climbing fiber-activated changes, suggesting that a Ca2+ spike was not evoked by this response. The anode break and directly responses were blocked by tetrodotoxin. These results suggest that the somatically recorded climbing fiber response is predominantly a Na(+)-dependent event, consisting of a few fast action potentials and a slower regenerative response activating the same channels as the Na+ plateau potential.

Sensory characteristics of the P afferent neurone of the crab thoracic-coxal muscle receptor organ

Intracellular recordings were made from the P fibre, the smallest of the three afferent neurones innervating the thoracic-coxal muscle receptor organ of the crab (Carcinus maenas). While the two larger afferents are nonspiking, the response of the P fibre to a trapezoidal change in receptor muscle length consists of a single action potential signalling the onset of stretch superimposed on a graded amplitude receptor potential. The P fibre is sensitive to the velocity of the applied stretch, but is insensitive to static joint position, stretch amplitude and the velocity of the release phase. The presence and amplitude of the action potential depends on the initial length of the receptor muscle, the tension caused by efferent activation of the receptor muscle prior to receptor stretch, and on the velocity of stretch. Length constant (1.9 mm) and specific membrane resistance (76 K omega x cm2) values obtained for the P fibre, together with its small diameter (7 microns) suggest that this neurone is less well adapted to conveying passive signals to the thoracic ganglion than are the S and T fibres. It is likely that the P fibre complements the length sensitivity of the S fibre and the tension and velocity sensitivity of the T fibre by signalling the onset of receptor stretch via single action potentials.

Tyrosine kinase is required for long-term depression in the cerebellum

Long-term depression (LTD) at the parallel fiber-Purkinje cell synapse in the cerebellum is a well-known example of synaptic plasticity. Although LTD is thought to reflect an enduring loss of postsynaptic AMPA receptor sensitivity, the underlying mechanisms are unclear. Protein-tyrosine kinases (PTKs) are able to modulate ionotropic receptor function and are enriched in Purkinje cells. Using intracellular recording from Purkinje cells, it is shown that two structurally and mechanistically distinct PTK inhibitors, lavendustin A and herbimycin A, block LTD induced by pairing parallel fiber stimulation with postsynaptic Ca2+ spiking. Intracellular application of the protein kinase C (PKC) activator, (-)-indolactam V, consistently depressed parallel fiber-Purkinje cells EPSPs and occluded pairing-induced LTD. Herbimycin A nullified the run-down produced by (-)-indolactam V. These data suggest that PTKs are necessary for LTD at the parallel fiber-Purkinje cell synapse and that PKC-induced synaptic depression requires PTK activity.

Ataxia and epileptic seizures in mice lacking type 1 inositol 1,4,5-trisphosphate receptor

The inositol 1,4,5-trisphosphate (InsP3) receptor acts as an InsP3-gated Ca2+ release channel in a variety of cell types. Type 1 InsP3 receptor (IP3R1) is the major neuronal member of the IP3R family in the central nervous system, predominantly enriched in cerebellar Purkinje cells but also concentrated in neurons in the hippocampal CA1 region, caudate-putamen, and cerebral cortex. Here we report that most IP3R1-deficient mice generated by gene targeting die in utero, and born animals have severe ataxia and tonic or tonic-clonic seizures and die by the weaning period. An electroencephalogram showed that they suffer from epilepsy, indicating that IP3R1 is essential for proper brain function. However, observation by light microscope of the haematoxylin-eosin staining of the brain and peripheral tissues of IP3R1-deficient mice showed no abnormality, and the unique electrophysiological properties of the cerebellar Purkinje cells of IP3R1-deficient mice were not severely impaired.

Membrane properties of complex spike firing neurons of the mouse dorsal cochlear nucleus in vitro

Intracellular recordings were made from complex spike firing neurons of the mouse dorsal cochlear nucleus (DCN) in vitro. The whole cochlear nucleus was dissected out and maintained submerged in rapidly flowing artificial cerebrospinal fluid (CSF). Recordings were made with current clamp techniques in the presence or absence of ion channel blocking drugs tetrodotoxin (TTX, 1 microM), tetraethylammonium (TEA, 20 mM), 4-aminopyridine (4-AP, 5 mM) or verapamil (50, 100, 150, 250 microM). The cells showed both spontaneous firing and responses to injections of depolarising current consisting of a mixture of a tall single action potential and complexes of 2 to 3 smaller wider action potentials superimposed on a plateau depolarisation. The membrane properties were: resting membrane potential -68.8 +/- 8.5 mV, cell resistance 54.1 +/- 26.5 M omega, time constant 9.6 +/- 5.4 ms and capacitance 0.25 +/- 0.5 nF; the first three variables had bimodel distributions. The current/voltage (I/V) relationship at membrane below resting was non-linear. Previously published histological evidence from the mouse DCN has shown that both cartwheel cells and Purkinje-like neurons are present. Both DCN cartwheel cells and cerebellar Purkinje cells are known to fire both tall single action potentials and complexes of smaller wider action potentials. It is therefore possible that the recordings shown here were made from these neuron types. TTX (1 microM) abolished both the tall single and the complexes of smaller action potentials, suggesting that the tall single action potentials are sodium dependent and possibly that a TTX sensitive sodium channel is responsible for the plateau as is suggested for Purkinje cells in the cerebellum. Verapamil (100 microM) abolished only the complex action potentials and the plateau leaving the tall narrow action potentials intact, which is consistent with the smaller complexes being calcium dependent. Higher concentrations abolished all spiking activity. TEA and 4-AP used separately both caused marked depolarisation to around -20 mV, suggesting that there is a large potassium current active at and near resting.

Sodium dependency of the inward potassium rectifier in horizontal cells isolated from the white bass retina

The ionic properties underlying the inwardly rectifying potassium current in cultured voltage-clamped white bass horizontal cells were studied. Anomalous rectification was apparent upon membrane hyperpolarization with a reversal potential depolarized from the predicted value of EK. In raised extracellular potassium, the current increased and the reversal potential shifted toward a more depolarized membrane potential. Solutions containing decreased sodium caused a rapid decrease in the inward rectifier current but only slightly affected the reversal potential. Extracellular cesium or barium caused a reversible voltage-dependent reduction of the inward current. We interpret these results to mean that the inward rectifying channel in white bass horizontal cells is mainly permeable to potassium ions, but is sodium dependent. It may shape the photoresponses of the horizontal cells and may contribute to a hyperpolarization activated conductance increase measured in situ.

Electrophysiological differences between Purkinje cells in organotypic and granuloprival cerebellar cultures

Organotypic cerebellar cultures derived from newborn mice were exposed to cytosine arabinoside for the first five days in vitro to destroy granule cells and functionally compromise glia. Such granuloprival cultures undergo a circuit reorganization featured by Purkinje cells sprouting recurrent axon collaterals that hyperinnervate other Purkinje cells. Intracellular recordings were used to compare the electrophysiological properties of Purkinje cells in granuloprival cultures to those of Purkinje cells in standard cultures. Purkinje cells in granuloprival cultures have similar membrane potentials to those of Purkinje cells in standard cultures, but have a lower input resistance. A reduced input resistance could affect the effectiveness of inhibitory synaptic input. Intracellular recordings from Purkinje cells of standard cerebellar cultures between 13 and 21 days in vitro exhibit spike activity consisting of a mixture of complex and simple spikes. The complex spikes contain a fast rising action potential followed by a depolarizing potential on which a plateau and several spike-like components are superimposed. This type of activity has been observed in mature Purkinje cells in vivo and in vitro. By contrast, at resting membrane potential Purkinje cells in granuloprival cultures have simple spike activity reminiscent of the type of activity seen in immature Purkinje cells, while at hyperpolarized potentials they generate complex spikes. These observations indicate differences in the expression of intrinsic electrophysiological properties underlying complex spike generation between Purkinje cells of organotypic and granuloprival cerebellar cultures. Our results illustrate the considerable plasticity of Purkinje cells in the presence of altered neuronal circuitry. In the absence of normal excitatory input, their spontaneous activity is regulated by intrinsic membrane properties.

Properties of the hyperpolarization-activated cation current Ih in rat midbrain dopaminergic neurons

Intracellular electrophysiological recordings in current- and voltage-clamp mode were obtained from dopaminergic neurons of the rat mesencephalon in an in vitro slice preparation. In current-clamp mode, a time-dependent anomalous rectification (TDR) of the membrane was observed in response to hyperpolarizing current pulses. In single-electrode voltage-clamp mode, a slowly developing inward current (Ih) underlying the TDR was studied by hyperpolarizing voltage commands from a holding potential of -50 to -60 mV. Ih started to be activated at approximately -69 mV, was fully activated at -129 to -141 mV, with half-maximal activation at -87 mV, and showed no inactivation with time. The time course of development of Ih followed a single exponential, and its time constant was voltage-dependent. At -81 mV, Ih activated with a time constant of 379 +/- 47.6 ms, whereas at -129 mV Ih activated with a time constant of 65 +/- 2.2 ms. Its estimated reversal potential was -35 +/- 4 mV. Raising the extracellular concentration of K+ from 2.5 to 6.5 and to 12.5 mM increased the amplitude of Ih while reducing the extracellular concentration of Na+ from 153.2 to 27.2 mM caused a reduction in amplitude of Ih. Bath application of caesium (1-5 mM) reversibly reduced or blocked the TDR/Ih. Perfusion of tetrodotoxin (0.5-1 microM), tetraethylammonium (10-20 mM) or barium (0.3-2 mM) did not significantly affect Ih. Ih was also present in cells impaled with CsCl-filled electrodes.(ABSTRACT TRUNCATED AT 250 WORDS)

Motor deficit and impairment of synaptic plasticity in mice lacking mGluR1

Metabotropic glutamate receptor 1 (mGluR1) is a member of a large family of G-protein-coupled glutamate receptors, the physiological functions of which are largely unknown. Mice deficient in mGluR1 have severe motor coordination and spatial learning deficits. They have no gross anatomical or basic electrophysiological abnormalities in either the cerebellum or hippocampus, but they show impaired cerebellar long-term depression and hippocampal mossy fibre long-term potentiation. mGluR1-deficient mice should therefore be valuable models for studying synaptic plasticity.

cAMP-dependent inward rectifier current in neurons of the rat suprachiasmatic nucleus

Electrophysiological properties of the inward rectification of neurons in the rat suprachiasmatic nucleus (SCN) were examined by using the single-electrode voltage-clamp method, in vitro. Inward rectifier current (IH) was produced by hyperpolarizing step command potentials to membrane potentials negative to approximately -60 mV in nominally zero-Ca2+ Krebs solution containing tetrodotoxin (1 microM), tetraethylammonium (40 mM), Cd2+ (500 microM) and 4-aminopyridine (1 mM). IH developed during the hyperpolarizing step command potential with a duration of up to 5 s showing no inactivation with time. IH was selectively blocked by extracellular Cs+ (1 mM). The activation of the H-channel conductance (GH) ranged between -55 and -120 mV. The GH was 80-150 pS (n = 4) at the half-activation voltage of -84 +/- 7 mV (n = 4). The reversal potential of IH obtained by instantaneous current voltage (I/V) relations was -41 +/- 6 mV (n = 4); it shifted to -51 +/- 8 mV (n = 3) in low-Na+ (20 mM) solution and to -24 +/- 4 mV (n = 4) in high-K+ (20 mM) solution. Forskolin (1-10 microM) produced an inward current and increased the amplitude of IH. Forskolin did not change the half-activation voltage of GH. 8-Bromo-adenosine 3',5'-cyclic monophosphate (8-Br-cAMP, 0.1-1 mM) and dibutyryl-cAMP (0.1-1 mM) enhanced IH. 3-Isobutyl-1-methylxanthine (IBMX, 1 mM) also enhanced IH. The results suggest that the inward rectifier cation current is regulated by the basal activity of adenylate cyclase in neurons of the rat SCN.

Physiology and morphology of complex spiking neurons in the guinea pig dorsal cochlear nucleus

Intracellular recordings from the dorsal cochlear nucleus have identified cells with both simple and complex action potential waveforms. We investigated the hypothesis that cartwheel cells are a specific cell type that generates complex action potentials, based on their analogous anatomical, developmental, and biochemical similarities to cerebellar Purkinje cells, which are known to discharge complex action potentials. Intracellular recordings were made from a brain slice preparation of the guinea pig dorsal cochlear nucleus. A subpopulation of cells discharged a series of two or three action potentials riding on a slow depolarization as an all-or-none event; this discharge pattern is called a complex spike or burst. These cells also exhibited anodal break bursts, anomalous rectification, subthreshold inward rectification, and frequent inhibitory postsynaptic potentials (IPSPs). Seven complex-spiking cells were stained with intracellular dyes and subsequently identified as cartwheel neurons. In contrast, six identified simple-spiking cells recorded in concurrent experiments were pyramidal cells. The cartwheel cell bodies reside in the lower part of layer 1 and the upper part of layer 2 of the nucleus. The cells are characterized by spiny dendrites penetrating the molecular layer, a lack of basal dendritic processes, and an axonal plexus invading layers 2 and 3, and the inner regions of layer 1. The cartwheel cell axons made putative synaptic contacts at the light microscopic level with pyramidal cells and small cells, including stellate cells, granule cells, and other cartwheel cells in layers 1 and 2. The axonal plexus of individual cartwheel cells suggests that they can inhibit cells receiving input from either the same or adjacent parallel fibers and that this inhibition is distributed along the isofrequency contours of the nucleus.

Initiation and spread of sodium action potentials in cerebellar Purkinje cells

Simultaneous whole-cell recordings were made from the soma and dendrites of cerebellar Purkinje cells in rat brain slices. Sodium action potentials, evoked by either depolarizing current pulses or synaptic stimulation of parallel or climbing fibers, always occurred first at the soma and decreased in amplitude with increasing distance into the dendrites. Simultaneous somatic and axonal recordings showed that these action potentials were initiated in the axon. Outside-out patches excised from the soma and dendrites revealed that the sodium channel current density decreased with distance from the soma. Consistent with this finding, comparable attenuation was observed for evoked action potentials and simulated action potential waveforms when sodium channels were blocked. These results show that sodium action potentials in cerebellar Purkinje cells are initiated in the axon and then spread passively into the dendritic tree.

Synaptic and intrinsic properties of neurons of origin of the perforant path in layer II of the rat entorhinal cortex in vitro

Layer II of the entorhinal cortex (EC) provides the first step in the hippocampal trisynaptic loop via the perforant path projection to the dentate gyrus. While a great deal is known about this projection and the properties of the dentate granule cells, much less information is available concerning the properties of and synaptic inputs to the cells of origin of the pathway in layer II. The present experiments have employed a slice preparation of the rat EC to study the intrinsic membrane properties and synaptic organization of layer II neurons. Two types of neurons could be identified electrophysiologically. The majority were designated type I and displayed a pronounced time-dependent inward rectification in the hyperpolarizing direction. Type II displayed little evidence of this characteristic. However, morphological examination suggested that both types were spiny stellate neurons projecting via the perforant path. Synaptic responses of both types displayed evidence of excitatory inputs mediated by both N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors. In general, however, at low frequencies the responses were dominated by inhibitory inputs mediated by both GABAA and GABAB receptors. At higher frequencies the bias was shifted much more toward excitation. The contribution of synaptic and intrinsic properties of layer II neurons to the processing capabilities of the EC is discussed.

Specific bradycardic agents block the hyperpolarization-activated cation current in central neurons

A class of pharmacologically active substances, known as "specific bradycardic agents", exerts a negative chronotropic influence on cardiac activity, which heavily relies upon a potent blockade of the hyperpolarization-activated cation current in Purkinje fibers. Since the cation conductance activated by hyperpolarization seems to represent an ubiquitous class of membrane channel in mammals, the present study was undertaken to evaluate the influence of specific bradycardic agents [UL-FS 49 (zatebradine) and its derivative DK-AH 268] on excitable cells of the central nervous system. Thalamocortical relay neurons of the dorsolateral geniculate nucleus, prepared from the guinea-pig thalamus as in vitro slices, were taken as model cells, because the significance of the hyperpolarization-activated cation current (Ih) for electrogenic activity is well documented in these neurons. Local application to relay neurons of the bradycardic agents at concentrations in the range 10(-5) to 10(-3) M resulted in a significant reduction in the amplitude of the Ih current, in the amplitude of the Ih activation curve, and in the slope of the fully activated Ih I/V-relationship. The bradycardic agents did not affect the instantaneous currents with no contribution of Ih, the time course of Ih activation, the voltage range of Ih activation, or the reversal potential of Ih. The inhibitory effect was critically dependent upon Ih activation with open Ih channels probably representing a sufficient condition for blockade. Significant recovery from block did not occur. Under current-clamp conditions, slow anomalous inward rectification of the membrane in the hyperpolarizing direction was blocked, and the resting input resistance increased by 30% associated with a negative shift (average 10 mV) of the membrane potential into a region of Ca(2+)-mediated burst activity. Parameters of electrophysiological activity outside the range of Ih activation were not significantly affected. These data indicate a selective and use-dependent blockade exerted by specific bradycardic substances on the conductance underlying Ih with no alteration in the gating properties. In view of the existence of hyperpolarization-activated cation conductances in neurons from various regions of the mammalian peripheral and central nervous systems, the results of the present study remind us of possible neuronal side-effects of bradycardia-producing agents.

Physiology, morphology and detailed passive models of guinea-pig cerebellar Purkinje cells

1. Purkinje cells (PCs) from guinea-pig cerebellar slices were physiologically characterized using intracellular techniques. Extracellular caesium ions were used to linearize the membrane properties of PCs near the resting potential. Under these conditions the average input resistance, RN, was 29 M omega, the average system time constant, tau 0, was 82 ms and the average cable length, LN, was 0.59. 2. Three PCs were fully reconstructed following physiological measurements and staining with horseradish peroxidase. Assuming that each spine has an area of 1 micron 2 and that the spine density over the spiny dendrites is ten spines per micrometre length, the total membrane area of each PC is approximately 150,000 microns 2, of which approximately 100,000 microns 2 is in the spines. 3. Detailed passive cable and compartmental models were built for each of the three reconstructed PCs. Computational methods were devised to incorporate globally the huge number of spines into these models. In all three cells the models predict that the specific membrane resistivity, Rm, of the soma is much lower than the dendritic Rm (approximately 500 and approximately 100,000 omega cm2 respectively). The specific membrane capacitance, Cm, is estimated to be 1.5-2 muF cm-2 and the specific cytoplasm resistivity, Ri, is 250 omega cm. 4. The average cable length of the dendrites according to the model is 0.13 lambda, suggesting that under caesium conditions PCs are electrically very compact. Brief somatic spikes, however, are expected to attenuate 30-fold when spreading passively into the dendritic terminals. A simulated 200 Hz train of fast, 90 mV somatic spikes produced a smooth 12 mV steady depolarization at the dendritic terminals. 5. A transient synaptic conductance increase, with a 1 nS peak at 0.5 ms and a driving force of 60 mV, is expected to produce approximately 20 mV peak depolarization at the spine head membrane. This EPSP then attenuates between 200- and 900-fold into the soma. Approximately 800 randomly distributed and synchronously activated spiny inputs are required to fire the soma. 6. The passive model of the PC predicts a poor resolution of the spatio-temporal pattern of the parallel fibre input. An equally sized, randomly distributed group of approximately 1% of the parallel fibres, activated within a time window of a few milliseconds, would result in approximately the same composite EPSP at the soma.

Modelling the cerebellar Purkinje cell: experiments in computo

Detailed compartmental models of neurons are useful tools for investigating neuronal properties and mechanisms that are not accessible to experimental procedures. If a rigorous approach is used in building the model, simulation studies can be as valuable as laboratory experimentation. As such, modelling becomes an additional method for exploring the function of neurons and nervous systems. As an example, a complex compartmental model with active dendritic membrane of a Purkinje cell is described. The response properties of the model to parallel fiber inputs were investigated. The model fired simple spikes in patterns comparable with those recorded from Purkinje cells in vivo. Synchronous activation of only 20 granule cell inputs was sufficient to generate a measurable response in simulated peri-stimulus histograms. This sensitivity to small excitatory inputs was caused by P-type Ca2+ channels in the dendritic membrane. Such P channels may also be present in the spine heads. Simulations suggest, however, that Ca2+ channels in spine heads cannot be activated by single parallel fiber inputs.

Carbachol potentiates Q current and activates a calcium-dependent non-specific conductance in rat hippocampus in vitro

Intracellular recordings were made from CA1 neurons in rat hippocampal slices maintained in vitro. When Na+ currents were blocked with tetrodotoxin and K+ conductances known to be sensitive to suppression by muscarinic agonists were blocked by 2 mM Ba2+, CA1 cells were depolarized by carbachol (3-10 microM) with an attendant conductance increase, whereas prior to Ba2+ the agonist produced a decrease or no change in conductance. Under voltage clamp at approximately -60 mV and in the presence of tetrodotoxin and Ba2+, carbachol (3-10 microM) induced a variable-latency biphasic inward current of up to 380 pA associated with a conductance increase of approximately 50%. The first phase was associated with an increase (more than 2-fold) of the Cs(+)-sensitive, hyperpolarization-activated cationic current, IQ. Carbachol also accelerated the kinetics of IQ at -100 mV with an average 24% reduction in its activation time constant. The second phase reflected an additional inward current that was Cs(+)-resistant, displayed little apparent voltage sensitivity and had a mean extrapolated reversal potential, determined in the presence of external Cs+ (< or = 5 mM), of approximately -20 mV. In a small proportion of cells the second phase of inward current was followed (or overlapped) by an outward current, also associated with a conductance increase, which reversed at approximately -70 mV. These carbachol actions were prevented by extracellular 300 microM Cd2+ and 2 mM Mn2+, by high levels (> 5 mM) of extracellular Mg2+ or Ca2+, and by omission of Ca2+ or reduction of extracellular Na+ to 25 mM by substitution of NaCl with Tris or N-methyl-D-glucamine. Carbachol action was not mimicked by oxotremorine (< or = 60 microM), but was irreversibly blocked by this drug. Likewise, atropine (100 nM) irreversibly and gallamine (10 microM) reversibly antagonized carbachol's action. The action of carbachol was blocked shortly after prior exposure of slices to 2-5 mM caffeine. Chronic or acute incubation of slices with 2 mM Li+ potentiated (between 1- and 2-fold) carbachol responses. The data indicate that muscarinic activation increases cationic flux by a calcium-dependent potentiation of IQ and activation of a non-selective conductance. The probability that inositol phospholipid metabolism is involved in triggering these events is discussed.

Serotonin alters an inwardly rectifying current (Ih) in rat cerebellar Purkinje cells under voltage clamp

The effects of serotonin (5-HT) on the hyperpolarization-activated sodium-potassium inward current (H-current: Ih) were examined in cerebellar Purkinje cells (PCs) under current- and voltage-clamp conditions. Quasi-steady state current versus voltage relationships under voltage clamp conditions showed that 5-HT decreased the inward rectification at potentials negative to -70 mV with a corresponding decrease in the slope conductance. In 70% of PCs, 5-HT produced dose-related attenuations of the Ih current with corresponding decreases in slope conductances across a range of hyperpolarized potentials. A negative shift in the Ih activation curve was elicited by 5-HT so that the amplitude of Ih current active near rest and at more hyperpolarized ranges was decreased. Serotonin-induced inhibition of Ih shows some receptor subtype selectivity in that DOI, the 5-HT2/IC receptor agonist, most closely mimicked the actions of 5-HT. This study reveals a novel inhibitory action of 5-HT on Ih in cerebellar Purkinje cells which may contribute to direct inhibitory effects of 5-HT on spontaneous firing and modulatory actions on gamma-aminobutyric acid (GABA)-mediated responses to PCs.

Ionic contributions to the oscillatory firing activity of rat Purkinje cells in vitro

Oscillatory firing activity in cerebellar Purkinje cells (PCs) can be maintained by intrinsic ionic conductances in the apparent absence of excitatory and inhibitory synaptic input as demonstrated by application of TTX or antagonists of amino acid-mediated transmission or both. Bursting activity in these cells is associated with a region of ZSR (zero slope resistance, the beginning part of a negative slope resistance region) of the whole cell quasi-steady-state I-V relationship. Blockade of Na+ current by TTX unmasked the ZSR region in all PCs tested. Based on current and voltage clamp experiments, hyperpolarization-activated cation current (Ih) participates in the rhythmic firing activity by influencing the amplitude and duration of the interburst interval and the resultant pattern of the burst generation. Blockade of Ih with cesium (Cs+) retards the membrane rebound from the after-hyperpolarization and results in longer and more negative hyperpolarizations between bursts. However, Cs+ did not affect the presence and characteristic of the ZSR region of the whole cell quasi-steady-state I-V curve.

Membrane properties and dendritic arborization of the intermediolateral nucleus neurons in the guinea-pig thoracic spinal cord in vitro

The morphological and electrophysiological properties of neurons in the intermediolateral nucleus (IML) were studied in the transverse and longitudinal slice of guinea-pig thoracic spinal cord (T2-T3) using intracellular staining and recording techniques. Two morophologically different types of neurons were observed: fusiform cells with craniocaudally oriented dendrites, and multipolar cells with dendrites diffusely extending in the IML. The ratio of fusiform to multipolar cells was 4:1. The fusiform cells were identified as sympathetic preganglionic neurons (SPNs) by their antidromic responses to stimulation of the ventral root exit zone, while the multipolar cells were not antidromically activated by stimulation of this site. Both cell types showed similar resting membrane potential and input resistance. The tonic responses of these neurons to hyperpolarizing current pulses were characteristically different: the SPNs had a marked hyperpolarizing sag at the break of the pulse, caused by an A current, while the unidentified neurons showed no A current. In addition, the SPNs had much longer duration of spike and afterhyperpolarization, as well as lower frequency of spontaneous or current-evoked firing, than the unidentified neurons. These observations suggest that, in the absence of the criterion of antidromic activation by stimulation of the axon, it is still possible to differentiate SPNs from other IML neurons on the basis of morphological and electrophysiological properties of the neuron.

The central nucleus of the rat amygdala: in vitro intracellular recordings

Membrane properties of neurons from the central nucleus of the rat amygdala (ACe) were analyzed using intracellular current-clamp recordings from in vitro coronal slices of adult rat amygdala. Two types of neurons were identified and classified according to their accommodation characteristics and the nature of their afterhyperpolarizations (AHP). Type A neurons represented 74% of the population and were identified by a lack of accommodation and a medium-AHP (m-AHP) in response to transient (100 ms) depolarizing current injection. The m-AHP was defined by a fast decay time constant with a mean tau AHP = 113.6 ms. In both Type A and Type B ACe cells the m-AHP can be reduced with cadmium and rubidium. Type B neurons represented 26% of the population and were identified by the presence of accommodation and a long duration slow-AHP (s-AHP) following the m-AHP. The s-AHP was defined by a slow decay time constant with a mean tau AHP = 1.7 s. The s-AHP was similar to the AHP mediated by IAHP, a long duration calcium-dependent, noradrenaline-sensitive current present in hippocampal neurons. In Type B cells, the s-AHP was reduced by cadmium and noradrenaline. There was no significant difference between Type A and B ACe neurons in passive electrical properties such as the membrane input resistance (RiA = 113 M omega, RiB = M omega), and the membrane time constant (tau A = 15 ms, tau B = 16 ms). However, there was a statistically significant difference in the resting membrane potentials of Type A and B ACe neurons (RMPA = -67 mV; RMPB = -63 mV). These data suggest that the characteristic active membrane properties displayed by Type A and Type B neurons will determine the ability of each type to integrate and encode neuronal information.

Electrophysiology of guinea-pig supraoptic neurones: role of a hyperpolarization-activated cation current in phasic firing

1. Immunocytochemically identified magnocellular neurosecretory cells (MNCs) in the guinea-pig supraoptic nucleus (SON) were studied using the in vitro intracellular recording technique. Cells were identified as containing arginine vasopressin (AVP) or oxytocin (OT) following recordings made with biocytin-filled electrodes. Both AVP and OT MNCs demonstrated a fusiform or pyramidal shape (15-20 microns by 26-39 microns), with two to three processes. There were no significant differences in the proportion of AVP and OT cells in the retrochiasmatic (caudal) versus the rostral slices. 2. No significant differences in passive membrane properties were observed between AVP and OT cells, except that AVP cells exhibited a significantly broader action potential width (1.51 +/- 0.1 ms, n = 11) than did OT cells (1.01 +/- 0.08 ms, n = 7). 3. Firing patterns were recorded for 100 MNCs, 41% of which fired in a phasic manner (repeated clustering of action potentials into bursts). Of the seventy-seven cells which were immunocytochemically identified, only AVP-containing MNCs displayed phasic firing. Phasic firing occurred only in MNCs demonstrating a depolarizing potential which followed hyperpolarizing after-potentials (HAPs). The presence of the depolarizing potential was not always associated with phasic firing, however, as both OT cells and non-phasic AVP cells sometimes exhibited a depolarizing potential. 4. In 160 MNCs examined for the presence of the time-dependent inward rectification (TDR in current clamp, or Ih in voltage clamp), a significant difference in the proportion of cells expressing the Ih was observed in the two cell types. The Ih was expressed in forty-five of fifty-four AVP MNCs (83%) and in six of fifteen OT MNCs (40%). No significant association was found with firing pattern. 5. The Ih exhibited properties similar to those found in other CNS and peripheral tissues. It appeared on steps to potentials more hyperpolarized than -65 mV. It was augmented by raising the extracellular potassium concentration, blocked by 2 mM CsCl, and insensitive to 100-500 microM BaCl2. Activation followed a single exponential, and the time constant of activation was voltage dependent. 6. The adenylate cyclase activator forskolin increased the Ih and shifted its activation curve to more depolarized levels. In cells recorded for several hours, the Ih varied in amplitude, suggesting intrinsic modulation, possibly by intracellular second messenger systems. The Ih in guinea-pig SON MNCs appears to serve an excitatory role, bringing cells closer to firing threshold.

Membrane properties and synaptic responses of Golgi cells and stellate cells in the turtle cerebellum in vitro

1. Intracellular recordings from anatomically identified Golgi cells and deep stellate cells were obtained in a slice preparation of the turtle cerebellar cortex. 2. Golgi cells and stellate cells had very similar firing patterns, which differed from those of Purkinje cells. In the interneurones, a short time constant and a high input resistance ensured a short response time. A pronounced spike after-hyperpolarization (spike AHP) participated in the rapid repolarization following a depolarizing input. The active and passive membrane properties of the interneurones ensured a very tight temporal coupling between input and output. 3. TTX abolished both the action potentials and a subthreshold depolarizing response. The Na+ excitability was increased by addition of Mn2+ or Co2+ to block calcium channels, or by addition of potassium channel blockers. 4. Ca2+ spikes and a Ca2+ plateau could be evoked following addition of potassium channel blockers. A partly 4-aminopyridine (4-AP)-sensitive transient hyperpolarization was found to control Ca2+ excitability in Golgi cells. It is suggested that this hyperpolarization is due to an A-like conductance. 5. A strong anomalous rectification was activated just below spike threshold, and dominated the subthreshold membrane potential at time scales longer than ca 100 ms. The anomalous rectification was partly blocked by Cs+. 6. Temporal integration over time scales up to ca 25 s was provided by activity-dependent adaptation in firing frequency and a long-lasting after-hyperpolarization (AHPL), which had both TTX-sensitive, Ca(2+)-independent, and Ca(2+)-dependent components. 7. Spontaneous IPSPs and EPSPs were abundant. The IPSPs were abolished by bicuculline. EPSPs were easily evoked by parallel fibre stimulation, had a shorter time course than in Purkinje cells, and were suppressed by the spike AHP. 8. Due to a short response time and a relatively short overall time frame for temporal integration, cerebellar interneurones operate on a faster time scale than the Purkinje cells, the output neurones of the cerebellar cortex. 9. It is suggested that information from shared sources, e.g. the parallel fibres, is distributed onto dynamically different cellular populations based on differences in the intrinsic membrane properties of the postsynaptic neurones.

The Impact of Parallel Fiber Background Activity on the Cable Properties of Cerebellar Purkinje Cells

Neurons in the mammalian CNS receive 104-105 synaptic inputs onto their dendritic tree. Each of these inputs may fire spontaneously at a rate of a few spikes per second. Consequently, the cell is bombarded by several hundred synapses in each and every millisecond. An extreme example is the cerebellar Purkinje cell (PC) receiving approximately 100,000 excitatory synapses from the parallel fibers (p.f.s) onto dendritic spines covering the thin dendritic branchlets. What is the effect of the p.f.s activity on the integrative capabilities of the PC? This question is explored theoretically using analytical cable models as well as compartmental models of a morphologically and physiologically characterized PC from the guinea pig cerebellum. The input of individual p.f.s was modeled as a transient conductance change, peaking at 0.4 nS with a rise time of 0.3 msec and a reversal potential of +60 mV relative to rest. We found that already at a firing frequency of a few spikes per second the membrane conductance is several times larger than the membrane conductance in the absence of synaptic activity. As a result, the cable properties of the PC significantly change; the most sensitive parameters are the system time constant (τ0) and the steady-state attenuation factor from dendritic terminal to soma. The implication is that the cable properties of central neurons in freely behaving animals are different from those measured in slice preparation or in anesthetized animals, where most of the synaptic inputs are inactive. We conclude that, because of the large conductance increase produced by the background activity of the p.f.s, the activity of the PC will be altered from this background level either when the p.f.s change their firing frequency for a period of several tens of milliseconds or when a large population of the p.f.s fires during a narrow time window.

Depolarization of rat locus coeruleus neurons by adenosine 5'-triphosphate

Intracellular recordings were performed in a pontine slice preparation of the rat brain containing the locus coeruleus. The enzymatically stable P2-purinoceptor agonist alpha,beta-methylene ATP increased the firing rate without altering the amplitude or shape of action potentials; the afterhyperpolarization following a spike was not changed either. When locus coeruleus neurons were hyperpolarized by current injection in order to prevent spontaneous firing, alpha,beta-methylene ATP produced depolarization and a slight increase in the apparent input resistance. A combined application of kynurenic acid and bicuculline methiodide failed to alter the alpha,beta-methylene ATP-induced depolarization, and tetrodotoxin only slightly depressed it. A gradual shift of the membrane potential by hyperpolarizing current injection led to a corresponding decrease, but no abolition or reversal of the alpha,beta-methylene ATP effect. In the hyperpolarized region, the current-voltage curve of alpha,beta-methylene ATP came into close approximation with, but did not cross, the control curve. Elevation of the external K+ concentration, or the intracellular application of Cs+ by diffusion from the microelectrode, depressed the response to alpha,beta-methylene ATP; external tetraethylammonium was also inhibitory. External Ba2+ and Cs+ had no effect or only slightly decreased the alpha,beta-methylene ATP-induced depolarization. A low Na+, or a low Ca2+ high Mg2+ medium, as well as the presence of Co2+ in the medium, markedly reduced or even abolished the depolarization by alpha,beta-methylene ATP. ATP itself did not produce consistent changes in the membrane potential or input resistance. However, in the presence of the P1-purinoceptor antagonist 8-cyclopentyl-1,3-dipropylxanthine, ATP consistently increased the firing rate and evoked an inward current. In conclusion, P2-purinoceptor activation appears to depolarize locus coeruleus neurons by inhibiting a persistent potassium current, and at the same time opening calcium-sensitive sodium channels or calcium-sensitive non-selective cationic channels.

Modulation of inwardly rectifying Na(+)-K+ channels by serotonin and cyclic nucleotides in salivary gland cells of the leech, Haementeria

The electrically excitable salivary cells of the giant Amazon leech, Haementeria, display a time-dependent inward rectification. Under voltage clamp, hyperpolarizing steps to membrane potentials negative to about -70 mV were associated with the activation of a slow inward current (Ih) which showed no inactivation with time. The time course of activation of Ih was described by a single-exponential function and was strongly voltage dependent. The activation curve of Ih ranged from -72 to -118 mV, with half-activation occurring at -100 mV. Ion-substitution experiments indicated that Ih is carried by both Na+ and K+ ions. 5-Hydroxytryptamine (5-HT) increased the amplitude of Ih and its rate of activation. It also produced a positive shift of the activation curve of the conductance underlying Ih (Gh) without altering the slope factor, thus indicating that the voltage dependence of Ih was modulated by 5-HT. Cs+ blocked both Ih and the 5-HT-potentiated current in a voltage-independent manner, whereas Ba2+ had little effect. It is concluded that 5-HT increases Ih by modulating the inwardly rectifying Na(+)-K+ channels in the salivary cells. The effect of 5-HT may be mediated by an increase in adenylate cyclase activity since Ih was increased by 8-bromo-cyclic AMP and by the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine. In contrast, Ih was reduced by 8-bromo-cyclic GMP and by zaprinast (an inhibitor of cyclic GMP-sensitive phosphodiesterase). Cyclic GMP itself also reduced Ih, and the effect was specific to the 3',5' form; 2',3'-cyclic GMP was inactive. The results suggest that the inward-rectifier channel may be modulated in opposite directions by cyclic AMP and cyclic GMP.

An inward rectifier is present in presynaptic nerve terminals in the chick ciliary ganglion

Inwardly rectifying voltage-sensitive channels have been detected in the cell bodies and axons of a number of excitable cells. The question of whether similar channels exist at axon terminals has been a matter of speculation for some time. We now report the first direct evidence for the existence of inward rectifiers in vertebrate presynaptic nerve terminals. Following impalement with intracellular electrodes, the large calyciform nerve terminals innervating chick ciliary ganglion neurons exhibit pronounced inward rectification upon hyperpolarization that increases with increasing current strength. The response is blocked by 2 mM Cs+, but is insensitive to Ba2+, tetraethylammonium and tetrodotoxin. The inward rectifier exhibits dependence on both Na+ and K+, but is unaffected by altering extracellular Ca2+. Ciliary neurons innervated by these nerve terminals display inward rectification with similar properties. We conclude that the inward rectifier present in these presynaptic nerve terminals resembles the H-current previously described in sensory ganglion neurons and the Q-current found in hippocampal pyramidal neurons. The presence of channels that are activated by hyperpolarization may serve to enhance the excitability of the calyciform nerve terminals, which are capable of relatively high frequencies (greater than 100 Hz) of discharge.

Activation of beta-adrenergic receptor induces Na(+)-dependent inward currents in acutely dissociated motoneurons of bullfrog spinal cord

Effects of adrenergic drugs on single motoneurons acutely dissociated from the lumbar enlargement of adult bullfrogs were examined. The dissociated large cells were identified as motoneurons by retrograde labeling with a fluorescent dye. Adrenaline caused membrane depolarization with a decrease in input resistance. Under whole-cell voltage clamp conditions at a holding potential of -70 mV, adrenergic drugs induced inward currents in a dose-dependent manner. Adrenaline was more potent than noradrenaline. Under K(+)-free conditions, adrenaline (10(-6)-10(-5) M) induced inward currents which were blocked by propranolol (10(-6) M) but not by phentolamine (10(-5) M). CoCl2 (1 mM) did not affect the currents. Substitution of choline+ in the recording solution for Na+ abolished the currents, but tetrodotoxin (TTX, 10(-6) M) had no effect on them. The adrenaline-induced currents exhibited a characteristic voltage-dependency: the conductance became large at hyperpolarized membrane potential (-150 to -30 mV) and approached zero at the depolarized membrane potential (greater than -30 mV), but was never reversed up to 30 mV, suggesting that the currents are different from non-specific cation currents. Substitution of isethionate- for Cl- in the recording solution had no effect on the voltage-dependency of the adrenaline-induced currents, whereas substitution of choline+ for Na+ apparently attenuated the voltage-dependency of the currents. These results indicate that adrenaline induces Na(+)-dependent inward currents through activation of beta-adrenergic receptors in bullfrog motoneurons.

A transient voltage-dependent outward potassium current in mammalian cerebellar Purkinje cells

Utilizing the single electrode voltage clamp technique applied to rat Purkinje cells (PCs), we have recorded a transient outward voltage-dependent potassium current, Ito. Half maximal values for inactivation and activation were -65 mV and -39 mV, respectively. Ito decays as a single exponential with a time constant of 95 ms and is reduced by 4-aminopyridine. Based on criteria of voltage dependency of activation and inactivation, kinetics of inactivation, ionic selectivity and pharmacologic sensitivity, we have verified a strong resemblance between the typical A current in other neurons, and Ito in PCs. As in other cells, Ito may be important in shaping PC output by modulating intrinsic and extrinsic factors that govern PC firing.

Co-cultures of inferior olive and cerebellum: electrophysiological evidence for multiple innervation of Purkinje cells by olivary axons

Slices of inferior olive (IO) and cerebellum were co-cultured for several weeks by means of the roller tube technique. Recordings were carried out intracellularly from Purkinje cells (PCs) which were identified morphologically by intracellular injection of the fluorescent dye Lucifer yellow, or by immunohistochemical stainings with antibodies raised against the 28 kD Ca(2+)-binding protein calbindin. Following stimulation of olivary tissue, an all-or-none full complex spike response was recorded in some PCs consisting of a fast rising spike followed by a depolarizing potential. In other PCs, graded stimulation of the olivary explant induced synaptic potentials which were characterized by step-wise variation in their amplitude and resembled the ones occurring spontaneously. In contrast, only smoothly graded synaptic potentials were observed in cerebellar mono-cultures. These results indicate that some of the PCs in olivo-cerebellar co-cultures are innervated by several olivary neurons.

Two inward currents and the transformation of low-frequency oscillations of rat and cat thalamocortical cells

1. The contribution of a slow, mixed Na(+)-K+, inward rectifying current (Ih) and the T-type Ca2+ current (IT) (that underlies low-threshold Ca2+ potentials) to the low-frequency oscillations observed in rat and cat thalamocortical (TC) cells in vitro was studied using current clamp and single-electrode voltage clamp recordings. 2. From a holding potential of -50 mV, voltage steps negative to -60 mV showed the presence of a slow, non-inactivating inward current, Ih. This current was unaffected by Ba2+ (1-4 mM), tetrodotoxin (0.5-1 microM) and TEA (20 mM, n = 6), reversibly blocked by Cs+ (1-3 mM), and its reversal potential (-33.0 +/- 1.2 mV) followed changes in the extracellular Na+ and K+, but not Cl-, concentration. 3. Application of Cs+ (1-3 mM) abolished the pacemaker oscillations (n = 9), while in six cells that did not show any oscillatory activity Cs+ first evoked the spindle-like oscillations that, in the continuous presence of these ions, were then transformed into the pacemaker oscillations before all activities were finally blocked: all these effects were accompanied by a hyperpolarization and a progressive decrease and final blockade of Ih. Cs+ had no effect on the 'N-methyl-D-aspartate' (NMDA) oscillations (n = 5) and Ba2+ (2 mM, n = 8) did not block the pacemaker, the spindle-like and the 'NMDA' oscillations. 4. In ten cells that showed the pacemaker oscillations selective activation of beta-adrenoceptors by 10-50 microM-noradrenaline (in the presence of alpha-noradrenergic antagonists) or by 20 microM-isoprenaline first transformed the pacemaker oscillations into the spindle-like oscillations that, in the continuous activation of beta-receptors, were finally abolished: all these effects were accompanied by a depolarization and a progressive increase of Ih. 5. In TC cells that showed the pacemaker oscillations application of 1-octanol (50-100 microM), an antagonist of T-type Ca2+ currents, reversibly blocked this activity but concomitantly decreased (50%) the cell input resistance (n = 5). Application of Ni2+ (0.2-0.5 mM, n = 13), another antagonist of IT reversibly blocked the pacemaker, the spindle-like and the 'NMDA' oscillations. 7. In cells showing the pacemaker oscillations it was found that the current developing from the most hyperpolarized potential of an oscillation cycle was an inward relaxation whose time course differed from that of Ih evoked at the same potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Inward rectification in rat olfactory receptor neurons

Inwardly rectifying currents in enzymically dissociated olfactory receptor neurons of rat were studied by using patch-clamp techniques. Upon hyperpolarization to membrane potentials more negative than -100 mV, small inward-current relaxations were observed. Activation was described by a single exponential with a time constant that decreased e-fold for a 21 mV hyperpolarization. The current was not reduced by the external application of 5 mM Ba2+, but was abolished by the addition of 5 mM Cs+ to the bath solution. Increasing the external K+ concentration ([K+]o) to 25 mM dramatically enhanced the current without affecting the voltage range or the kinetics of activation. In 25 mM [K+]o, tail currents reversed at -26 mV, significantly more positive than the K+ equilibrium potential of -44 mV. These characteristics are consistent with those of a mixed Na+/K+ inward rectification that has been reported in several types of neuronal, cardiac and smooth muscle cells. The current may contribute to controlling cell excitability during the response to some odorants.

Reconstruction of hippocampal granule cell electrophysiology by computer simulation

A model of the hippocampal granule cells was created that closely approximated most of the measured intracellular responses from a neuron under a variety of stimulus conditions. This model suggests that: (1) A simple, four-conductance model can account for most of the intracellular behavior of these neurons. (2) The repolarization mechanism in granule cells may be different from that in squid axons. A weak potassium conductance may be present in hippocampal granule neurons, which simultaneously give rise to a small, passive depolarizing afterpotential. (3) The strength duration properties may assist in identifying the electronic and sodium channel properties with short stimulus pulse widths. (4) Repetitive firing responses are highly dependent on the cell's recent history of activation and the regulation of the slow potassium conductance and calcium dynamics. (5) The anodic break response is probably not a property of typical granule cells. Through thorough and precise comparison of experimental and model responses, computer simulations can help assembling channel information into verifiable models that accurately reproduce intracellular data.

Pairing of pre- and postsynaptic activities in cerebellar Purkinje cells induces long-term changes in synaptic efficacy in vitro

1. An in vitro slice preparation of rat cerebellar cortex was used to analyse long-lasting modifications of synaptic transmission at parallel fibre (PF)-Purkinje cell (PC) synapses. These use-dependent changes were induced by pairing PF-mediated EPSPs evoked at low frequency (1 Hz) with different levels of membrane polarization (or bioelectrical activities) of PCs for 15 min. 2. Experiments were performed on forty-eight PCs recorded intracellularly in a conventional perfused chamber, and in fifty other cells maintained in a static chamber either in the presence (n = 21) or in the absence (n = 29) of 400 nM-phorbol 12,13-dibutyrate (PDBu). 3. In these three experimental conditions, PF-mediated EPSPs were always measured on PCs maintained at a holding potential of -75 mV, and further hyperpolarized by constant hyperpolarizing pulses. This allowed us both to test the input resistance of PCs and to avoid their firing during PF-mediated EPSPs. 4. In all cells retained for the present study, latencies of PF-mediated EPSPs evoked at 0.2 Hz were stable during the pre-pairing period, and the same was true for their amplitude and time course. 5. In the perfused chamber, pairing of PF-mediated EPSPs with the same hyperpolarization of PCs as that used for measurements of synaptic responses had no effect on these EPSPs in 30% of PCs. It induced long-term depression (LTD) and long-term potentiation (LTP) in 23 and 47% of the tested cells respectively (n = 17). 6. In the perfused chamber, pairing of PF-mediated EPSPs with moderate depolarization of PCs (n = 19) giving rise to a sustained firing of sodium spikes significantly favoured the appearance of LTP as compared to the previous pairing protocol. However, there were still 27 and 15% of cells which showed no modification and LTD respectively. 7. In contrast, pairing of PF-mediated EPSPs with calcium (Ca2+) spikes evoked by strong depolarization of PCs (n = 12) led to LTD of synaptic transmission in nearly half of the tested cells, whereas LTP was now observed in less than 20% of them. 8. In the static chamber and in the absence of PDBu, LTD of PF-mediated EPSPs was observed in most cells, whatever the pairing protocol with sodium or Ca2+ spikes. 9. This shift towards LTD was significantly reversed by PDBu in the pairing protocol using firing of sodium spikes, but not in the case of pairings with Ca2+ spikes.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of a hyperpolarization-activated cation current and its role in rhythmic oscillation in thalamic relay neurones

1. The physiological and functional features of time-dependent anomalous rectification activated by hyperpolarization and the current which underlies it, Ih, were examined in guinea-pig and cat thalamocortical relay neurones using in vitro intracellular recording techniques in thalamic slices. 2. Hyperpolarization of the membrane from rest with a constant-current pulse resulted in time-dependent rectification, expressed as a depolarizing sag of the membrane potential back towards rest. Under voltage clamp conditions, hyperpolarizing steps to membrane potentials negative to approximately -60 mV were associated with the activation of a slow inward current, Ih, which showed no inactivation with time. 3. The activation curve of the conductance underlying Ih was obtained through analysis of tail currents and ranged from -60 to -90 mV, with half-activation occurring at -75 mV. The time course of activation of Ih was well fitted by a single-exponential function and was strongly voltage dependent, with time constants ranging from greater than 1-2 s at threshold to an average of 229 ms at -95 mV. The time course of de-activation was also described by a single-exponential function, was voltage dependent, and the time constant ranged from an average of 1000 ms at -80 mV to 347 ms at -55 mV. 4. Raising [K+]o from 2.5 to 7.5 mM enhanced, while decreasing [Na+]o from 153 to 26 mM reduced, the amplitude of Ih. In addition, reduction of [Na+]o slowed the rate of Ih activation. These results indicate that Ih is carried by both Na+ and K+ ions, which is consistent with the extrapolated reversal potential of -43 mV. Replacement of Cl- in the bathing medium with isethionate shifted the chloride equilibrium potential positive by approximately 30-70 mV, evoked an inward shift of the holding current at -50 mV, and resulted in a marked reduction of instantaneous currents as well as Ih, suggesting a non-specific blocking action of impermeable anions. 5. Local (2-10 mM in micropipette) or bath (1-2 mM) applications of Cs+ abolished Ih over the whole voltage range tested (-60 to -110 mV), with no consistent effects on instantaneous currents. Barium (1 mM, local; 0.3-0.5 mM, bath) evoked a steady inward current, reduced the amplitude of instantaneous currents, and had only weak suppressive effects on Ih. 6. Block of Ih with local application of Cs+ resulted in a hyperpolarization of the membrane from the resting level, a decrease in apparent membrane conductance, and a block of the slow after-hyperpolarization that appears upon termination of depolarizing membrane responses, indicating that Ih contributes substantially to the resting and active membrane properties of thalamocortical relay neurones.(ABSTRACT TRUNCATED AT 400 WORDS)