Ionic currents in cultured rat hypothalamic neurones

Chlorpromazine, an Inverse Agonist of D1R-Like, Differentially Targets Voltage-Gated Calcium Channel (CaV) Subtypes in mPFC Neurons

The dopamine receptor type 1 (D1R) and the dopamine receptor type 5 (D5R), which are often grouped as D1R-like due to their sequence and signaling similarities, exhibit high levels of constitutive activity. The molecular basis for this agonist-independent activation has been well characterized through biochemical and mutagenesis in vitro studies. In this regard, it was reported that many antipsychotic drugs act as inverse agonists of D1R-like constitutive activity. On the other hand, D1R is highly expressed in the medial prefrontal cortex (mPFC), a brain area with important functions such as working memory. Here, we studied the impact of D1R-like constitutive activity and chlorpromazine (CPZ), an antipsychotic drug and D1R-like inverse agonist, on various neuronal Ca_V conductances, and we explored its effect on calcium-dependent neuronal functions in the mouse medial mPFC. Using ex vivo brain slices containing the mPFC and transfected HEK293T cells, we found that CPZ reduces Ca_V2.2 currents by occluding D1R-like constitutive activity, in agreement with a mechanism previously reported by our lab, whereas CPZ directly inhibits Ca_V1 currents in a D1R-like activity independent manner. In contrast, CPZ and D1R constitutive activity did not affect Ca_V2.1, Ca_V2.3, or Ca_V3 currents. Finally, we found that CPZ reduces excitatory postsynaptic responses in mPFC neurons. Our results contribute to understanding CPZ molecular targets in neurons and describe a novel physiological consequence of CPZ non-canonical action as a D1R-like inverse agonist in the mouse brain.

Zn2+ sensitivity of high- and low-voltage activated calcium channels

The essential cation zinc (Zn2+) blocks voltage-dependent calcium channels in several cell types, which exhibit different sensitivities to Zn2+. The specificity of the Zn2+ effect on voltage-dependent calcium channel subtypes has not been systematically investigated. In this study, we used a transient protein expression system to determine the Zn2+ effect on low- and high-voltage activated channels. We found that in Ba2+, the IC50 value of Zn2+ was alpha1-subunit-dependent with lowest value for CaV1.2, and highest for CaV3.1; the sensitivity of the channels to Zn2+ was approximately ranked as CaV1.2>CaV3.2>CaV2.3>CaV2.2=CaV 2.1>or=CaV3.3=CaV3.1. Although the CaV2.2 and CaV3.1 channels had similar IC50 for Zn2+ in Ba2+, the CaV2.2, but not CaV3.1 channels, had approximately 10-fold higher IC50 to Zn2+ in Ca2+. The reduced sensitivity of CaV2.2 channels to Zn2+ in Ca2+ was partially reversed by disrupting a putative EF-hand motif located external to the selectivity filter EEEE locus. Thus, our findings support the notion that the Zn2+ block, mediated by multiple mechanisms, may depend on conformational changes surrounding the alpha1 pore regions. These findings provide fundamental insights into the mechanism underlying the inhibitory effect of zinc on various Ca2+ channel subtypes.

Electrophysiological properties and thermosensitivity of mouse preoptic and anterior hypothalamic neurons in culture

Responses of mouse preoptic and anterior hypothalamic neurons to variations of temperature are key elements in regulating the setpoint of homeotherms. The goal of the present work was to assess the relevance of culture preparations for investigating the cellular mechanisms underlying thermosensitivity in hypothalamic cells. Our working hypothesis was that some of the main properties of preoptic/anterior hypothalamic neurons in culture are similar to those reported by other authors in slice preparations. Indeed, cultured preoptic/anterior hypothalamic neurons share many of the physiological and morphological properties of neurons in hypothalamic slices. They display heterogenous dendritic arbors and somatic shapes. Most of them are GABAergic and their activity is synaptically driven by the activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptors. Active membrane properties include a depolarizing "sag" in response to hyperpolarization, and a low threshold spike, which is present in a majority of cells and is generated by T-type Ca2+ channels. In a fraction of the cells, the low threshold spike repeats rhythmically, either spontaneously, or in response to depolarization. The background synaptic noise in cultured neurons is characterized by the presence of numerous postsynaptic potentials which can be easily distinguished from the baseline, thus providing an opportunity for assessing their possible roles in thermosensitivity. An unexpected finding was that GABA-A receptors can generate both hyper- and depolarizing postsynaptic potentials in the same neuron. About 20% of the spontaneously firing preoptic/anterior hypothalamic neurons are warm-sensitive. Warming (32-41 degrees C) depolarizes some cells, a phenomenon which is Na+-dependent and tetrodotoxin-insensitive. The increased firing rate of warm-sensitive cells in response to warming can be prepotential and/or synaptically driven. Overall, our data suggest that a warm-sensitive phenotype is already developed in cultured cells. Therefore, and despite obvious differences in their networks, cultured and slice preparations of hypothalamic neurons can complement each other for further studies of warm-sensitivity at the cellular and molecular level.

Modulation of voltage-gated Ca2+ current by 4-hydroxynonenal in dentate granule cells

Although recent studies have suggested that dentate granule cells play a key role in hippocampal functions, electrophysiological properties in these cells have not been sufficiently explored. In the present study, modification of voltage-gated Ca2+ channels by 4-hydroxynonenal (4HN), a major aldehydic product of membrane lipid peroxidation, in cultured dentate granule cells was examined using the whole-cell patch clamp technique. When whole-cell voltage clamp was applied, the cells exhibited a high-voltage-activated Ca2+ current, which was totally sensitive to 30 microM Cd2+ and partially sensitive to 2 microM nifedipine. 4HN enhanced the Ca2+ current in these cells. When L-type Ca2+ channels were blocked by application of nifedipine, the enhancement was completely canceled, whereas application of omega-conotoxin-GVIA or omega-agatoxin-IVA, blockers of N- and P/Q-type Ca2+ channels, respectively, had no effect. These results suggest that 4HN modulates L-type Ca2+ channels in the dentate granule cells, and thereby plays a role in the physiological and pathophysiological responses of these cells to oxidative stress.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Synchronization of circadian firing rhythms in cultured rat suprachiasmatic neurons

The circadian clock in mammals is located in the suprachiasmatic nucleus (SCN) which consists of multiple oscillating neurons. Integration of the cellular oscillations is essential for the generation of a single circadian period in the SCN. By using a multielectrode dish (MED), we measured circadian firing rhythms in individual SCN neurons for more than 2 weeks continuously, and examined the involvement of synaptic communication in the synchronization of circadian rhythms. Cross-correlation analysis of spontaneous action potentials revealed that a neuron pair was functionally connected by synapses when their circadian rhythms were synchronized. No correlation was found between the paired neurons whose circadian rhythms were not synchronized. Calcium (Ca2+)-dependent synaptic transmission in the cellular communication was indicated by dose-dependent lengthening of an intercellular spike interval and loss of spike correlation with a Ca2+ channel blocker. Approximately 60% of the SCN neurons in culture were immunoreactive to antibodies against gamma-aminobutyric acid (GABA) or glutamic acid decarboxylase (GAD). Spontaneous firing of all the neurons tested was either increased or decreased by bicuculline, the GABAA receptor antagonist. These findings indicate that synaptic communication plays a critical role in the synchronization of circadian rhythms in individual SCN neurons and the GABAergic transmission is involved in the synchronization mechanism.

Distribution of the voltage-dependent calcium channel alpha1G subunit mRNA and protein throughout the mature rat brain

The molecular identity of a gene which encodes the pore-forming subunit (alpha1G) of a member of the family of low-voltage-activated, T-type, voltage-dependent calcium channels has been described recently. Although northern mRNA analyses have shown alpha1G to be expressed predominantly in the brain, the detailed cellular distribution of this protein in the central nervous system (CNS) has not yet been reported. The current study describes the preparation of a subunit specific alpha1G riboprobe and antiserum which have been used in parallel in situ mRNA hybridization and immunohistochemical studies to localize alpha1G in the mature rat brain. Both alpha1G mRNA and protein were widely distributed throughout the brain, but variations were observed in the relative level of expression in discrete nuclei. Immunoreactivity for alpha1G was typically localized in both the soma and dendrites of many neurons. Whilst alpha1G protein and mRNA expression were often observed in cells known to exhibit T-type current activity, some was also noted in regions, e.g. cerebellar granule cells, in which T-type activity has not been described. These observations may reflect differences between the subcellular distribution of channels that can be identified by immunohistochemical methods compared with electrophysiological techniques.

Low-voltage activated calcium channels: achievements and problems

Low-voltage activated Ca2+ channels, which possess unique properties quite different from those of common (high-voltage activated) channels, were discovered 15 years ago but the first alpha1 subunit has only recently been identified which might provide their structural basis. However, simultaneously, extensive data are being accumulated on the functional diversity of low-voltage activated Ca2+ currents with regard to their pharmacological sensitivity, ionic selectivity, activation and inactivation kinetics. Such diversity corresponds to equally prominent heterogeneity in the location and function of the channels. This commentary summarizes the data available in an attempt to predict a possibly wider structural subdivision of low-voltage activated Ca2+ channels into subtypes.

Bidirectional effects of the neuroactive steroid tetrahydrodeoxycorticosterone on GABA-activated Cl- currents in cultured rat hypothalamic neurons

1. The non-genomic effects of tetrahydrodeoxycorticosterone (THDOC; 5-alpha-pregnane-3-alpha, 21-diol-20-one) were studied in cultured hypothalamic neurons of the rat. 2. The effects of THDOC (10 nM - 1 microM) on responses to different concentrations of exogenously applied GABA and on spontaneous inhibitory postsynaptic currents (IPSCs) were measured with whole-cell voltage clamp recordings. 3. Application of GABA induced inward currents with dose-dependently increasing amplitudes (up to 3.9 nA at a holding potential of -20 mV). High doses of THDOC (100 nM-1 microM) induced small inward currents on its own (14+/-3 and 24+/-3 pA, respectively). 4. Simultaneous application of 10 microM GABA with 100 nM or 1 microM THDOC increased current amplitudes by 125 and 128%, respectively. At 10 nM THDOC exerted no consistent effects on GABA currents. 5. Responses to 1 microM of GABA were modulated in a bidirectional manner by different doses of THDOC: 10 nM THDOC reduced the amplitude of GABA responses to 80% (P=0.018, n=15), whereas 100 nM and 1 microM THDOC enhanced the GABA response to 115 and 180% (P=0.0007, n = 15), respectively. 6. The time constant of decay of spontaneous inhibitory postsynaptic currents (IPSCs) was reversibly increased from 91+/-10 to 314+/-34 ms (n=3) by the application of THDOC (1 microM). The amplitudes of the IPSCs were not affected by THDOC. 7. These data indicate that THDOC modulates GABA responses of hypothalamic neurons in a bidirectional manner, resulting in a complex tuning of neuronal excitability in the hypothalamus.

Developmental changes in the expression of low-voltage-activated Ca2+ channels in rat visual cortical neurones

1. The functional properties of low-voltage-activated (LVA) Ca2+ channels were studied in pyramidal neurones from different rat visual cortical layers in order to investigate changes in their properties during early postnatal development. Ca2+ currents were recorded in brain slices using the whole-cell patch-clamp technique in rats from three age groups: 2, 3 and 12 days old (postnatal day (P) 2, P3 and P12). 2. It was demonstrated that LVA Ca2+ currents are present in neurones from superficial (I-II) and deep (V-VI) visual cortex layers of P2 and P3 rats. No LVA Ca2+ currents were observed in neurones from the middle (III-IV) layers of these rats. The LVA Ca2+ currents observed in P2 and P3 neurones from both superficial and deep layers could be completely blocked by nifedipine (100 microM) and were insensitive to Ni2+ (25 microM). 3. The density of LVA Ca2+ currents decreased rapidly during the early stages of postnatal development, while the density of high-voltage-activated (HVA) Ca2+ currents progressively increased up to the twelfth postnatal day. No LVA Ca2+ currents were found in P12 neurones from any of the layers. Only HVA Ca2+ currents with high sensitivity to F- applied through the patch pipette were observed. 4. The kinetics of LVA Ca2+ currents could be well approximated by the m2h Hodgkin-Huxley equation with an inactivation time constant of 24 +/- 6 ms. The steady-state inactivation curve fitted by a Boltzmann function had the following parameters: membrane potential at half-inactivation, -86.9 mV; steepness coefficient,3.4 mV. 5. It is concluded that, in visual cortical neurones, LVA Ca2+ channels are expressed only in the neurones of deep and superficial layers over a short period during the earliest postnatal stages. These channels are nifedipine sensitive and similar in functional properties to those in the laterodorsal (LD) thalamic nucleus. However, the cortical neurones do not express another ('slow') type of LVA Ca2+ channel, which is permanently present in LD thalamic neurones after the second postnatal week, indicating that the developmental time course of cortical and thalamic cells is different.

Synaptic modulation of oscillatory activity of hypothalamic neuronal networks in vitro

1. Rhythmic bursts of action potentials in neurosecretory cells are a key factor in hypothalamic neurosecretion. Rhythmicity and synchronization may be accomplished by pacemaker cells synaptically driving follower cells or by a network oscillator. 2. In this review we describe a hypothalamic cell culture which may serve as a model for a hypothalamic network oscillator. An overview is given of neurochemical phenotypes, synaptic mechanisms and their development, properties of receptors for fast synaptic transmission, and membrane properties of cells in dissociated rat embryonic hypothalamic culture. 3. Rhythmic activity spreads in the cultured network through synapses that release glutamate, activating a heteromultimeric AMPA-type receptor containing a GluR2 subunit which is associated with a high-conductance channel for Na+ and K+. Rhythmic activity is controlled by synapses that release GABA to activate GABAA receptors. The presumed function of the two receptor types is facilitated by their respective location, GABAA receptors predominating near the soma and AMPA receptors being abundant in dendrites. 4. Network oscillators may be more reliable for the presumed function than single-cell oscillators. They are controlled through synaptic modulation, which may prove to represent a process important for the release of hormones.

A-type K+ current in neurons cultured from neonatal rat hypothalamus and brain stem: modulation by angiotensin II

The regulation of A-type K+ current (I(A)) and the single channel underlying I(A) in neonatal rat hypothalamus/brain stem cultured neurons were studied with the use of the patch-clamp technique. I(A) had a threshold of activation between -30 and -25 mV (n = 14). Steady-state inactivation of I(A) occurred between -80 and -70 mV and had a membrane voltage at which I(A) was half-maximum of -52.2 mV (n = 14). The mean values for the activation and inactivation (decay) time constants during a voltage step to +20 mV were 2.1 +/- 0.3 (SE) ms (n = 8) and 13.6 +/- 1.9 ms (n = 8), respectively. Single-channel recordings from outside-out patches revealed A-type K+ channels with voltage-dependent activation, 4-aminopyridine (4-AP) sensitivity, and inactivation kinetics similar to those of I(A). The single-channel conductance obtained from cell-attached patches was 15.8 +/- 1.3 pS (n = 4) in a physiological K+ gradient and 41.2 +/- 3.7 pS (n = 5) in symmetrical 140 mM K+. Angiotensin II (Ang II, 100 nM) reduced peak I(A) by approximately 20% during a voltage step to +20 mV (n = 8). Similarly, Ang II (100 nM) markedly reduced single A-type K+ channel activity by decreasing open probability (n = 4). The actions of Ang II on I(A) and single A-type K+ channels were reversible either by addition of the selective angiotensin type 1 (AT1) receptor antagonist losartan (1 microM) or on washout of the peptide. Thus the activation of AT1 receptors inhibits a tetraethylammonium-chloride-resistant, 4-AP-sensitive I(A) and single A-type K+ channels, and this may underlie some of the actions of Ang II on electrical activity of the brain.

Dopamine inhibition: enhancement of GABA activity and potassium channel activation in hypothalamic and arcuate nucleus neurons

Dopamine (DA) decreases activity in many hypothalamic neurons. To determine the mechanisms of DA's inhibitory effect, whole cell voltage- and current-clamp recordings were made from primary cultures of rat hypothalamic and arcuate nucleus neurons (n = 186; 15-39 days in vitro). In normal buffer, DA (usually 10 microM; n = 23) decreased activity in 56% of current-clamped cells and enhanced activity in 22% of the neurons. In neurons tested in the presence of glutamate receptor antagonists D,L-2-amino-5-phosphonovalerate (AP5; 100 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM), DA application (10 microM) revealed heterogeneous effects on electrical activity of cells, either hyperpolarization and decrease in activity (53% of 125) or depolarization and increase in spontaneous activity (22% of 125). The DA-mediated hyperpolarization of membrane potential was associated with a decrease in the input resistance. The reversal potential for the DA-mediated hyperpolarization was -97 mV, and it shifted in a positive direction when the concentration of K+ in the incubating medium was increased, suggesting DA activation of K+ channels. Because DA did not have a significant effect on the amplitude of voltage-dependent K+ currents, activation of voltage-independent K+ currents may account for most of the hyperpolarizing actions of DA. DA-mediated hyperpolarization and depolarization of neurons were found during application of the Na+ channel blocker tetrodotoxin (1 microM). The hyperpolarization was blocked by the application of DA D2 receptor antagonist eticlopride (1-20 microM; n = 7). In the presence of AP5 and CNQX, DA (10 microM) increased (by 250%) the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) in 11 of 19 neurons and evoked IPSCs in 7 of 9 cells that had not previously shown any IPSCs. DA also increased the regularity and the amplitude (by 240%) of spontaneous IPSCs in 9 and 4 of 19 cells, respectively. Spontaneous and DA-evoked IPSCs and inhibitory postsynaptic potentials were blocked by the gamma-aminobutyrate A (GABA(A)) antagonist bicuculline (50 microM), verifying their GABAergic origin. Pertussis toxin pretreatment (200 ng/ml; n = 15) blocked the DA-mediated hyperpolarizations, but did not prevent depolarizations (n = 3 of 15) or increases in IPSCs (n = 6 of 10) elicited by DA. Intracellular neurobiotin injections (n = 21) revealed no morphological differences between cells that showed depolarizing or hyperpolarizing responses to DA. Immunolabeling neurobiotin-filled neurons that responded to DA (n = 13) showed that GABA immunoreactive neurons (n = 4) showed depolarizing responses to DA, whereas nonimmunoreactive neurons (n = 9) showed both hyperpolarizing (n = 6) and depolarizing (n = 3) responses. DA-mediated hyperpolarization, depolarization, and increases in frequency of postsynaptic activity could be detected in embryonic hypothalamic or arcuate nucleus neurons after only 5 days in vitro, suggesting that DA could play a modulatory role in early development. These findings suggest that DA inhibition in hypothalamic and arcuate nucleus neurons is achieved in part through the direct inhibition of excitatory neurons, probably via DA D2 receptors acting through a Gi/Go protein on K+ channels, and in part through the enhancement of GABAergic neurotransmission.

Synaptic connectivity in cultured hypothalamic neuronal networks

We have developed a novel approach to analyze the synaptic connectivity of spontaneously active networks of hypothalamic neurons in culture. Synaptic connections were identified by recording simultaneously from pairs of neurons using the whole cell configuration of the patch-clamp technique and testing for evoked postsynaptic current responses to electrical stimulation of one of the neurons. Excitatory and inhibitory responses were distinguished on the basis of their voltage and time dependence. The distribution of latencies between presynaptic stimulation and postsynaptic response showed multiple peaks at regular intervals, suggesting that responses via both monosynaptic and polysynaptic paths were recorded. The probability that an excitatory event is transmitted to another excitatory neuron and results in an above-threshold stimulation was found to be only one in three to four. This low value indicates that in addition to evoked synaptic responses other sources of excitatory drive must contribute to the spontaneous activity observed in these networks. The various types of synaptic connections (excitatory and inhibitory, monosynaptic, and polysynaptic) were counted, and the observations analyzed using a probabilistic model of the network structure. This analysis provides estimates for the ratio of inhibitory to excitatory neurons in the network (1:1.5) and for the ratio of postsynaptic cells receiving input from a single GABAergic or glutamatergic neuron (3:1). The total number of inhibitory synaptic connections was twice that of excitatory connections. Cell pairs mutually connected by an excitatory and an inhibitory synapse occurred significantly more often than predicted by a random process. These results suggests that the formation of neuronal networks in vitro is controlled by cellular mechanisms that favor inhibitory connections in general and specifically enhance the formation of reciprocal connections between pairs of excitatory and inhibitory neurons. These mechanisms may contribute to network formation and function in vivo.

Expression and functional analysis of voltage-activated Na+ channels in human prostate cancer cell lines and their contribution to invasion in vitro

Ion channels are important for many cellular functions and disease states including cystic fibrosis and multidrug resistance. Previous work in the Dunning rat model of prostate cancer has suggested a relationship between voltage-activated Na+ channels (VASCs) and the invasive phenotype in vitro. The objectives of this study were to 1) evaluate the expression of VASCs in the LNCaP and PC-3 human prostate cancer cell lines by Western blotting, flow cytometry, and whole-cell patch clamping, 2) determine their role in invasion in vitro using modified Boyden chambers with and without a specific blocker of VASCs (tetrodotoxin). A 260-kd protein representing VASCs was found only in the PC-3 cell line, and these were shown to be membrane expressed on flow cytometry. Patch clamping studies indicated that functional VASCs were present in 10% of PC-3 cells and blocking these by tetrodotoxin (600 nmol/L) reduced their invasiveness by 31% (P = 0.02) without affecting the invasiveness of the LNCaP cells. These results indicate that the reduction of invasion is a direct result of VASC blockade and not a nonspecific action of the drug. This is the first report of VASCs in a human prostatic cell line. VASCs are present in PC-3 but not LNCaP cells as determined by both protein and functional studies. Tetrodotoxin reduced the invasiveness of PC-3 but not LNCaP cells, and these data suggest that ion channels may play an important functional role in tumor invasion.

Two types of low-voltage-activated Ca2+ channels in neurones of rat laterodorsal thalamic nucleus

1. The pharmacological and kinetic properties of two types of low-voltage-activated (LVA) Ca2+ currents were studied in thalamocortical neurones of the laterodorsal (LD) thalamic nucleus during early postnatal development. The whole-cell patch-clamp technique was used on brain slices from rats of three age groups: 12, 14 and 17 days old (postnatal day (P) 12, P14 and P17). 2. In P12 neurones, the population of LVA Ca2+ channels was homogeneous. LVA Ca2+ current elicited by depolarizing voltage steps from a holding potential more negative than -70 mV was sensitive to nifedipine (Kd = 2.6 microM). This current reached a maximum at about -55 mV and had a fast monoexponential decay with a time constant, tau h,f, of 32.3 +/- 4.0 ms. 3. The population of LVA Ca2+ channels in P14 and P17 neurones was found to be heterogeneous. A subpopulation of nifedipine-insensitive LVA Ca2+ channels was observed. The current-voltage curve of the Ca2+ current had a characteristic hump with two peaks at about -65 and -55 mV. As well as the fast component (designated IT,f), the decay of the LVA current also included a slow component (designated IT,s), with inactivation time constants (tau h,s) of 54.2 +/- 4.5 and 68.6 +/- 3.17 ms for P14 and P17 neurones, respectively. 4. The kinetics of both components could be well approximated by the m2h Hodgkin-Huxley equation. No significant difference in activation kinetics was observed. The activation time constants for the fast (tau m,f) and slow (tau m,s) components were 6.3 +/- 1.0 and 7.3 +/- 1.5 ms, respectively. 5. La3+ at a concentration of 1 microM effectively blocked the IT,f component but Ni2+ (25 microM) completely eliminated the IT,s component. 6. Steady-state inactivation curves of both components could be best fitted by a Boltzmann function with membrane potential values at half-maximal inactivation of -85.5 and -98.1 mV for the fast and slow components, respectively. 7. It was concluded that two different subtypes of LVA Ca2+ channel are present in LD neurones. Only the fast type is well expressed at the earliest postnatal stage (P12). The slow type could be found at the end of the second week (P14). The amplitude of the slow current increased progressively up to P17, obviously coinciding with dendritic expansion as judged by progressive increase of the membrane capacitance of the corresponding neurones. This property appears to differentiate neurones of the associative nuclei from neurones of other thalamic nuclei.

Calcium channel types contributing to excitatory and inhibitory synaptic transmission between individual hypothalamic neurons

The contribution of L-, N-, P- and Q-type Ca2+ channels to excitatory and inhibitory synaptic transmission and to whole-cell Ba2+ currents through Ca2+ channels (Ba2+ currents) was investigated in rat hypothalamic neurons grown in dissociated cell culture. Excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) were evoked by stimulating individual neurons under whole-cell patch-clamp conditions. The different types of high-voltage-activated (HVA) Ca2+ channels were identified using nifedipine, omega-Conus geographus toxin VIA (omega-CTx GVIA), omega-Agelenopsis aperta toxin IVA (omega-Aga IVA), and omega-Conus magus toxin VIIC (omega-CTx MVIIC). N-, but not P- or Q-type Ca2+ channels contributed to excitatory as well as inhibitory synaptic transmission together with Ca2+ channels resistant to the aforementioned Ca2+ channel blockers (resistant Ca2+ channels). Reduction of postsynaptic current (PSC) amplitudes by N-type Ca2+ channel blockers was significantly stronger for IPSCs than for EPSCs. In most neurons whole-cell Ba2+ currents were carried by L-type Ca2+ channels and by at least two other Ca2+ channel types, one of which is probably of the Q-type and the others are resistant Ca2+ channels. These results indicate a different contribution of the various Ca2+ channel types to excitatory and inhibitory synaptic transmission and to whole-cell currents in these neurons and suggest different functional roles for the distinct Ca2+ channel types.

Electrophysiological properties of neurones in cultures from postnatal rat dentate gyrus

Electrophysiological properties of neurofilament-positive neurones in dissociated cell cultures were prepared at postnatal days 4-5 from rat dentate gyrus and studied using the whole-cell patch-clamp technique. These cells expressed a fast-inactivating, 0.5 microM tetrodotoxin-sensitive Na+ current; a high-voltage-activated (HVA) Ca2+ current, which was 30 microM Cd(2+)- and partially 2 microM nicardipine-sensitive; and an inward rectifier current, which was sensitive to extracellularly applied 1 mM Cs+. The outward current pattern was composed of a delayed rectifier-like outward current sensitive to 20 mM tetraethylammonium (TEA) and a fast-inactivating, Ca(2+)-dependent outward current. This transient Ca(2+)-dependent K+ outward current was identified by a subtraction procedure. K+ currents recorded under conditions of blocked Ca2+ currents (after rundown of the HVA Ca2+ current or blocked by extracellularly applied Cd2+) were subtracted from control currents. By comparison with the current pattern of identified dentate granule cells, it is concluded that the investigated cell type originated from interneurones or projection neurones of the dentate hilus.

Whole-cell K+ currents in identified olfactory bulb output neurones of rats

1. Voltage-gated whole-cell K+ currents have been investigated in olfactory bulb (OB) output (mitral/tufted) neurones from neonatal rats, which were retrogradely labelled by rhodamine or Fast Blue and identified after enzymatic dissociation. Forty-five per cent of labelled neurones exhibited either phasic or non-phasic spontaneous firing in cell-attached configuration. 2. Four outward K+ currents have been identified in all such identified OB output neurones. They are the transient (IA), the delayed rectifier (IDK), and two Ca(2+)-dependent (IK(Ca)) currents. No inward rectifier was detected. 3. The IA was activated at around -45 mV and reached its peak within 3-10 ms. The decay phase could be described by single exponential distribution with the time constant of 45.2 +/- 3.8 ms at depolarizations 10-60 mV from a holding potential of -70 mV. Its activation and steady-state inactivation processes could be fitted with Boltzmann equations yielding half-maximal activation potentials of 7.6 +/- 0.4 and -47.4 +/- 0.2 mV, respectively. It was sensitive to block by 4-AP (1 mM) and by Zn2+ (1 mM). 4. The IDK was activated at potentials more positive than -30 mV, with half-maximal activation at 21 mV. It was sustained during 1 s test pulses without apparent decay. It was blocked by TEA at a concentration of 20 mM. About 8% of the sustained current, in 11/24 cells tested, was found to resist block by a combination of all pharmacological agents tested. 5. Apamin at 100 nM blocked a TEA-insensitive component which accounted for about 23% of the maximal sustained currents. Iberiotoxin (IbTX), which has been found to block maxi K+ currents more selectively than does charybdotoxin, reversibly blocked Ca(2+)-activated K+ current, with a half-maximal dose of about 100 nM in 8/13 OB output neurones tested. This accounted for 20% of the maximal sustained K+ current. The effect of IbTX was not observed in the presence of 20 mM external TEA. 6. Direct evidence is provided in this study regarding kinetic and pharmacological properties of four types of outward K+ channels in OB output neurones.

Ionic currents in cultured rat suprachiasmatic neurons

Whole-cell voltage-clamp recordings were made from cultured neurons obtained by dissociation of the suprachiasmatic area of rat fetuses. Neurons were held for seven to 14 days in culture. These neurons possessed several voltage-dependent ionic currents. A transient inward Na+ current was present, which could be completely blocked by tetrodotoxin. No inward Ca2+ currents were detected. Three types of outward K+ currents were recorded, which could be separated to a reasonable extent by their differences in voltage sensitivity and pharmacology. These K+ currents corresponded to the transient current IA, the delayed rectifier current IKo and a calcium-dependent current IK(Ca) as described in other neurons. The A current activated at -50 mV, reached half-maximal conductance at about -30 mV and maximum conductance between 0 and 30 mV. During depolarizing steps it inactivated completely within 100 ms and steady-state inactivation was half-maximal at -66 mV. The outward rectifier activated at -30 mV, reached half-maximal conductance close to 0 mV and maximum conductance at about 70 mV. Slow inactivation of IKo occurred with 50% reduction in amplitude at the end of 2 s depolarizations above 0 mV. The K+ channel blocker 4-amino-pyridine (4 mM) reduced the amplitude of IA by 21% and of IKo by 32%, whereas tetraethylammonium (10 mM) decreased IA by 27% and IKo by 83%. The calcium-dependent K+ component was also voltage dependent and was present at voltages more positive than 0 mV. No inward rectifying K+ current was present. Considering its voltage dependence, IA must play a role in determining the excitability of these neurons, through its probable influence on the action potential threshold and interspike interval. Both IA and IKo should take part in membrane repolarization following an action potential. The Ca(2+)-dependent current should also contribute to repolarization following any event which gives rise to an increase in intracellular Ca2+. Apart from IA, which may make a slight contribution, none of these currents appear to be involved in determining the resting membrane potential. All three outward current components will act together in suprachiasmatic neurons to control their spontaneous firing frequency, which is the major feature of the output of these neurons in vivo. Variations in properties of these conductances could contribute to the circadian rhythm in firing frequency described in suprachiasmatic hypothalamic neurons.

A rapidly activating type of outward rectifier K+ current and A-current in rat suprachiasmatic nucleus neurones

1. The properties of calcium-independent (i.e. persisting in the absence of external calcium) depolarization-activated potassium currents in suprachiasmatic nucleus (SCN) neurones (n = 75) were studied under voltage-clamp conditions with whole-cell patch-clamp recordings in rat hypothalamic slices (150-175 microns). 2. Two distinct types of potassium currents were found. One was a rapidly activating and slowly inactivating type of outward rectifier (named IK(FR) for fast rectifier potassium current), similar to a potassium current described in cardiac muscle, and the other was a transient A-current (IA). 3. The rates of activation and deactivation of IK(FR) were voltage dependent. Time constants of activation fitted to n4 kinetics and declined from 3.5 ms (at -20 mV) to 1.1 ms (at 60 mV). Inactivation had a biexponential time course with voltage-independent time constants of 0.3 s (minor component) and 3.0 s (major component) between 10 and 50 mV. IK(FR) was activated above -40 mV with a V1/2 (membrane potential at half-maximal activation) of 14 +/- 2 mV and slope factor of -17 +/- 1 mV reaching a conductance (not maximal) of 10.8 +/- 1.7 nS at 60 mV. Steady-state inactivation had a slope factor of 11 +/- 1 mV. 4. IK(FR) was highly selective for K+ (PNa/PK = 0.002). Tetraethylammonium (TEA) reduced IK(FR) reversibly (IC50 = 20 mM), while 4-aminopyridine (4-AP) at 10 mM had little effect. The remaining current in 30 mM TEA was similar to that in control conditions, indicating that TEA reduced IK(FR) rather than revealed an additional TEA-resistant current. 5. The rate of IA activation was voltage dependent with time to peak declining from 8.5 ms (at -40 mV) to 3.6 ms (at 60 mV). Inactivation had a biexponential time course with voltage-dependent and voltage-independent time constants. The two components were similar in amplitude. IA was activated above -60 mV, reaching a maximal conductance of 3.6 +/- 0.4 nS at above 20 mV. Steady-state inactivation was complete above -10 mV. Rates of onset of inactivation (at -40 mV) and recovery from inactivation (at -100 mV) were similar with time constants of 39 +/- 5 and 41 +/- 9 ms, respectively. 6. IK(FR) and IA were found in every neurone tested in the SCN and in all locations throughout the nucleus. The possible function of these currents is discussed, particularly in relation to the circadian rhythm of firing rate in the SCN.

Transient outward current in adult rat supraoptic neurones with slice patch-clamp technique: inhibition by angiotensin II

1. Outward potassium currents were recorded from microscopically identified supraoptic neurones of adult Wistar male rats using the whole-cell patch-clamp technique in thin-slice preparations. The basic characteristics of transient outward current (IA or A-current) and the effects of angiotensin II (AII) on the currents were studied. 2. IA was isolated by subtracting outward currents elicited by stepping from two different holding potentials to a test potential or by applying 4-aminopyridine (4-AP) at 5 mM. The isolated IA had a threshold for activation between -55 and -65 mV and was characterized by fast activation and inactivation. Values of the time to peak and the inactivation time constants for current decay at different test potentials were voltage dependent. 3. Normalized currents for activation and steady-state inactivation of IA were fitted to the Boltzmann function. The mid-points and the slope factors were, respectively, -35.0 and -14.3 +/- 0.40 mV (n = 5) for the activation curve, and -72.0 and 7.0 +/- 0.68 mV (n = 5) for the inactivation curve. 4. The time course of recovery from inactivation was best fitted to a single exponential function with the time constant of 37.8 +/- 6.6 ms (n = 6). 5. The effects of AII on IA and delayed rectifier current (IK) were investigated. According to their responses to AII, cells were classified into two groups, sensitive and low-sensitive. Bolus injection of AII (10 microM, 100 microliters) decreased the IA amplitude by 25.1 +/- 2.4% in seven (53.8%) of the thirteen neurones tested (sensitive group), whereas the other six neurones (low-sensitive group) changed by only 2.2 +/- 0.8%. Perfusion of AII (0.1 microM) decreased the IA amplitude by 21.3 +/- 3.1% in six (54.5%) of eleven neurones tested (sensitive group), whereas the other five neurones (low-sensitive group) changed only by 1.7 +/- 0.8%. Bolus injection of AII (10 microM, 100 microliters) decreased the IK amplitude 9.6 +/-1.6% mV in five (45.5%) of the eleven neurones tested (sensitive group), whereas the other six neurones (low-sensitive group) changed only by 0.46 +/- 0.27%. In the sensitive groups, the reduction of IA by AII was significantly larger than that of IK (P < 0.05). 6. Application of saralasin at 1 microM, an AII antagonist, blocked the effects of AII on IA. 7. These results suggest that the excitatory action of AII on supraoptic neurosecretory cells is mediated at least in part through suppression of IA.

Synchronous development of spontaneous and evoked calcium-dependent properties in hypothalamic neurons

The development of various related parameters was compared in hypothalamic neurons grown in primary culture. We measured: (i) low- and high-voltage-activated calcium currents; (ii) spontaneous and N-methyl-D-aspartate (NMDA)-induced fluctuations of intracellular calcium concentration; (iii) basal and NMDA- or potassium-evoked somatostatin release. Spontaneous calcium fluctuations appeared after 5 days in culture and increased progressively in amplitude and frequency over the next 8 days studied. Basal release of somatostatin was not detectable in 3 day-old cultures and reached a plateau at day 5. Responses evoked by exogenous stimulations (voltage-activated calcium currents, agonist-induced intracellular calcium rise and somatostatin release) appeared early in culture, increased in amplitude during 7-10 days and then stabilized. We conclude that, in hypothalamic neurons, the main neuronal functions develop in synchrony over a limited period of time.

Activation of specific ATP receptors induces a rapid increase in intracellular calcium ions in rat hypothalamic neurons

We have used real-time dynamic video imaging of Fura-2 fluorescence to study the acute effects of external ATP on [Ca2+]i in cultured rat hypothalamic neurons. The addition of ATP at microM concentrations, but not adenosine, AMP, ADP or GTP, produced a rapid, dose-dependent increase in cytosolic Ca2+. The hydrolysis-resistant ATP analogues 3-thio-ATP and beta,gamma-imido-ATP produced a similar response but alpha,beta-methylene ATP had much lower efficacy. The ATP response was inhibited by 10 microM nifedipine, abolished by 50 microM cadmium and by the absence of extracellular Ca2+, but was unaffected by ryanodine or omega-conotoxin GVIA. The P2-purinoceptor antagonist suramin reversibly and selectively inhibited the ATP response but had no effect on other neurotransmitter-induced Cai2+ responses. Antagonists to muscarinic, nicotinic, NMDA, non-NMDA, GABA, 5-HT and adenosine receptors had no effect on the ATP response. Thus the Ca2+ response of hypothalamic neurons to ATP is mediated by specific suramin-sensitive ATP-receptors, activation of which is independent of ATP hydrolysis and results in an influx of extracellular Ca2+ largely through high voltage-gated Ca2+ channels. These findings support the assertion that ATP acts in the CNS as an excitatory neurotransmitter.

Zinc modulation of a transient potassium current and histochemical localization of the metal in neurons of the suprachiasmatic nucleus

The effect of Zn2+ on a voltage-dependent, transient potassium current (IA) in acutely dissociated neurons from the suprachiasmatic nucleus was studied with the whole-cell patch-clamp technique. At micromolar concentrations, Zn2+ markedly potentiated IA activated from a holding potential of -60 mV, which is the resting potential of these neurons. This potentiation occurred at a Zn2+ concentration as low as 2 microM and increased with higher Zn2+ concentrations. The Zn2+ action appears to arise from a shift in the steady-state inactivation of IA to more positive voltages. At 30 microM, Zn2+ shifted the half-inactivation voltage by +20 mV (from -80 mV to -60 mV), and 200 microM Zn2+ shifted this voltage by +45 mV (from -80 mV to -35 mV). Histochemically, we have also observed Zn2+ staining throughout the suprachiasmatic nucleus; the staining is particularly intense in the ventrolateral region of the nucleus, which receives the major fiber inputs. Our findings suggest that Zn2+, presumably synaptically released, may modulate the electrical activity of suprachiasmatic nucleus neurons through IA. Because vesicular Zn2+ is fairly widespread in the central nervous system, it is conceivable that this kind of Zn2+ modulation on IA, and possibly on other voltage-activated currents, exists elsewhere in the brain.

Characterization of neurochemical phenotypes in cultured hypothalamic neurons with immunohistochemistry and in situ hybridization

The expression of neurochemical phenotypes was studied in long-term cultures of dissociated embryonic neurons from rat hypothalamus. With time in culture, these neurons establish a complex dendritic and axonal network, as indicated by staining with antibodies against microtubulin-associated protein (MAP2) and neurofilaments (SMI32 and SMI33) as well as GABA and glutamate decarboxylase mRNA immunoreactivity. Neurons expressing neuropeptide Y (NPY) mRNA and NPY peptide and opioid-like peptides as well as vasopressin were observed. Further, weakly acetylcholinesterase- and NADPH diaphorase (nitric-oxide synthase)-labelled neurons were present. In conclusion, the neurochemical phenotypes reported for hypothalamic neurons in vivo can be observed in these cultures. This indicates that the culture conditions allow morphological and molecular differentiation. These findings support the view that long-term hypothalamic cultures provide a valuable model for studying mechanisms of neurosecretion in hypothalamic networks.

Inhibition of high voltage-activated calcium currents by L-glutamate receptor-mediated calcium influx

The modulation of high voltage-activated (HVA) Ca2+ currents by L-glutamate and its agonists was investigated in cultured rat hypothalamic neurons. L-Glutamate and agonists selective for NMDA or non-NMDA receptors reversibly inhibited HVA Ca2+ currents. The putative presynaptic glutamate receptor agonist L-2-amino-4-phosphonobutyric acid and the selective metabotropic agonist trans-ACPD were ineffective. Inhibition was dependent on the presence of extracellular Ca2+ and blocked by internal perfusion of the cells with BAPTA. The calmodulin antagonists trifluoperazine and calmidazolium completely prevented the inhibition. Increases in the intracellular Ca2+ concentration due to Ca2+ influx through non-NMDA receptor channels were visualized using fura-2. These results indicate that not only NMDA but also non-NMDA receptor channels in these neurons are permeable for Ca2+ and that Ca2+ influx through these channels activates a calmodulin-dependent mechanism, which leads to HVA Ca2+ current inhibition.