Calcium dependence of serotonin-evoked conductance in C6 glioma cells

Decrease in Ca2+-activated K+ conductance in differentiated C6-glioma cells

Ca2+-activated K+ channels were studied in C6-glioma cells in an attempt to correlate changes in expression with cell proliferation and differentiation. In this study, we treated C6-glioma cells with thapsigargin for 48 h. Cell proliferation was markedly inhibited, and cell morphology changed from round to a spindle differentiated shape. Furthermore, intracellular calcium concentration was initially increased during acute treatment with thapsigargin. The internal [Ca2+]i pool was eventually depleted after a 48-h thapsigargin treatment. We have characterized Ca2+-activated K+ currents in less differentiated C6 cells. After differentiation of C6 cells induced by thapsigargin, Ca2+-activated K+ currents were selectively suppressed. These data lend further support to the notion that the expression of Ca2+-activated K+ channels is intimately associated with the proliferation of C6-glioma cells, and the suppression of Ca2+-activated K+ channels coincides with the inhibition of proliferation and subsequent induction of cell differentiation.

Zolmitriptan stimulates a Ca(2+)-dependent K(+) current in C6 glioma cells stably expressing recombinant human 5-HT(1B) receptors

Stimulation of a Ca(2+)-dependent K(+) current by zolmitriptan, a 5-HT(1B/1D) receptor partial agonist, was investigated in C6 glioma cells stably expressing recombinant human 5-HT(1B) receptors. Outward K(+) currents (I(K)) were examined in non-transfected C6 glioma cells and in cells expressing cloned human 5-HT(1B) receptors using the patch-clamp technique in the whole-cell configuration. In C6 glioma cells expressing recombinant human 5-HT(1B) receptor, zolmitriptan increased I(K) in a concentration-dependent manner (maximum increase 16.3+/-7.8%, n=5, p<0.001) with a pD(2) value (geometric mean with 95% confidence intervals) of 7.03 (7.90-6.10). Zolmitriptan failed to elicit increases in I(K) in non-transfected C6 cells. In the presence of the mixed 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2(-methyl-4(5-methyl-1 ,2,4)-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide 2HCl (GR 127935, 0. 1 mcM), zolmitriptan (1 mcM) failed to significantly increase I(K) in C6 cells expressing human 5-HT(1B) receptors confirming that zolmitriptan-evoked responses were indeed mediated by human 5-HT(1B) receptors. In C6 cells expressing cloned human 5-HT(1B) receptors, zolmitriptan-induced increases in I(K) were prevented by the calcium chelator, EGTA (5 mM) when included in the patch pipette (maximum increase -3.3+/-4.2%, n=4, P=NS). The Ca(2+)-dependent K(+) channel blockers, iberiotoxin (0.1 mcM) and tetraethylammonium (TEA, 1 mM), abolished zolmitriptan-induced increases in I(K) (4.5+/-7.3%, n=4 and -0.8+/-1.7%, n=4, respectively, P=NS in each case) in C6 cells expressing human 5-HT(1B) receptors, confirming the involvement of Ca(2+)-dependent K(+) channels. In conclusion, the 5-HT(1B/1D) receptor partial agonist, zolmitriptan, stimulates I(K/Ca) in C6 glioma cells stably transfected with human 5-HT(1B) receptors suggesting an increase of hyperpolarizing current.

Serotonin induces inward potassium and calcium currents in rat cortical astrocytes

Ca2+ imaging and patch-clamp techniques were used to study the effects of serotonin (5-HT) on ionic conductances in rat cortical astrocytes. 1 and 10 microM serotonin caused a transient increase in intracellular calcium (Ca(i)) levels in fura-2AM-loaded cultured astrocytes and in astrocytes acutely isolated and then cultured in horse serum-containing medium for over 24 h. However, the acutely isolated (less than 6 h from isolation) astrocytes, as well as acutely isolated astrocytes cultured in serum-free media, failed to respond to 5-HT by changes in Ca(i). Coinciding with the changes in Ca(i) levels, inward currents were activated by 10 microM 5-HT in cultured, but not in acutely isolated astrocytes. Two separate types of serotonin-induced, small-conductance inward single-channel currents were found. First, in both Ca2+-containing and Ca2+-free media serotonin transiently activated a small-conductance apamin-sensitive channel. Apamin is a specific blocker of the small-conductance Ca2+-activated K+ channel (sK(Ca)) When cells were pre-treated with phospholipase C inhibitor U73122 no 5-HT-induced sK(Ca) channel openings were seen, indicating that this channel was activated by Ca2+ released from intracellular stores via IP3. A second type of small inward channel activated later, but only in the presence of external Ca2+. It was inhibited by the L-type Ca2+ channel blockers, nimodipine and nifedipine. Both types of channel activity were inhibited by ketanserin, indicating activation of the 5-HT2A receptor.

Chronic treatment with antidepressants, verapamil, or lithium inhibits the serotonin-induced intracellular calcium response in individual C6 rat glioma cells

The effects of chronic treatment with antidepressants, verapamil, or lithium on serotonin (5-HT)-induced Ca2+ increase were investigated in single C6BU-1 glioma cells with digital imaging microscopy. Clomipramine and citalopram, at a concentration of 100 nM, decreased the peak values of 5-HT-induced [Ca2+]i changes. Verapamil (100 nM), a calcium antagonist, and lithium (1 mM) also inhibited the peak amplitudes in the same way. The present findings suggest that chronic treatment with antidepressants, verapamil, or lithium, at therapeutic concentrations, have the common action of inhibiting 5-HT-mediated [Ca2+]i increase.

Expression of serotonin receptors in human fetal astrocytes and glioma cell lines: a possible role in glioma cell proliferation and migration

Expression of seven serotonin or 5-hydroxytryptamine (5-HT) receptors (5-HT1D alpha, 5-HT1E, 5-HT2, 5-HT1A, 5-HT1C, 5-HT1D beta, and 5-HT6) was investigated in human normal fetal astrocytes and eight glioma cell lines by reverse transcription and polymerase chain reaction (RT-PCR). No expression of 5-HT1D beta and 5-HT6 was observed in any of the cell lines studied. The 5-HT1D alpha receptor was found to be expressed in two human glioma cell lines but not in normal astrocytes. In addition, only three glioma cell lines expressed the 5-HT1E receptor. The 5-HT1C receptor was expressed in six glioma cell lines but not in normal astrocytes while the 5-HT1A was found to be expressed in normal astrocytes from the left hemisphere and in six glioma cell lines but not in normal astrocytes from the cerebellum. Interestingly, the 5-HT2 receptor was expressed in all cells studied but very weakly in normal astrocytes. The effect of 5-HT on glioma cell proliferation, migration, and invasion was also investigated. Serotonin was found to positively modulate these three processes in vitro. These results suggest that 5-HT may play an important role in the control of the biological properties of human glioma cells.

Characterization of the 5-hydroxytryptamine2A receptor-activated cascade in rat C6 glioma cells

We have investigated the identity and intracellular cascade of responses resulting from activation of the endogenous 5-hydroxytryptamine receptor in the C6 rat glioma cell line. Sequence analysis of reverse transcription-polymerase chain reaction products derived from C6 glioma cell messenger RNA revealed complete homology with a portion of the rat 5-hydroxytryptamine2A receptor. The binding of [3H]ketanserin to cell membranes demonstrated a significant correlation with the 5-hydroxytryptamine2A receptor in rat frontal cortex. On intact cells, 5-hydroxytryptamine stimulated a concentration-dependent increase in phosphatidyl inositide turnover and intracellular [Ca2+] mediated by 5-hydroxytryptamine2A receptors. In whole-cell patch-clamp recordings, 5-hydroxytryptamine induced an outward current mediated predominantly by K+ ions (reversal potential = -80 mV). Using caged molecules containing Ca2+ or inositol 1,4,5-trisphosphate in the patch electrode solution, we found that rapid photolytic release of Ca2+ and particularly inositol 1,4,5-trisphosphate within the cytosol induced an outward current with characteristics similar to those seen after application of 5-hydroxytryptamine. Comparison between differentiated and undifferentiated cells revealed significantly higher receptor density and maximal phosphoinositide response to 5-hydroxytryptamine in undifferentiated cells but the associated rise in [Ca2+]i and activation of an outward current was observed more frequently in differentiated cells. Prolonged exposure of the cells to 5-hydroxytryptamine led to a decrease in all responses and to the down-regulation of receptor number. We conclude that the rat C6 glioma cell expresses a 5-hydroxytryptamine2A receptor identical to that found in rat brain and that stimulation of the receptor in C6 cells leads to the activation of Ca2+ activated K+ channels via phosphoinositide hydrolysis and subsequent rise in cytosolic Ca2+ ion concentration. However, the contrasting effects of differentiation on receptor number and phosphoinositide response to 5-hydroxytryptamine compared to Ca2+ release and conductance change indicate that a complex relationship exists between the component parts of the receptor-activated cascade.

Physiology of transformed glial cells

Much of our present knowledge of glial cell function stems from studies of glioma cell lines, both rodent (C6, C6 polyploid, and TR33B) and human (1321N1, 138MG, D384, R-111, T67, Tp-276MG, Tp-301MG, Tp-483MG, Tp-387MG, U-118MG, U-251MG, U-373MG, U-787MG, U-1242MG, and UC-11MG). New methods such as patch clamp and Ca2+ imaging have lead to rapid progress the last few years in our knowledge about glial cells, where an unexpected presence and diversity of receptors and ion channels have emerged. Basic mechanisms related to membrane potential and K+ transport and the presence of voltage gated ion channels (Na+, inwardly rectifying K+, Ca(2+)-activated K+, Ca2+, and Cl- channels) have been identified. Receptor function and intracellular signaling for glutamate, acetylcholine, histamine, serotonin, cathecolamines, and a large number of neuropeptides (bradykinin, cholecystokinin, endothelin, opioids, and tachykinins) have been characterized. Such studies are facilitated in cell lines which offer a more homogenous material than primary cultures. Although the expression of ion channels and receptors vary considerably between different cell lines and comparative studies are rare, a few differences (compared to astrocytes in primary culture) have been identified which may turn out to be characteristic for glioma cells. Future identification of specific markers for receptors on glial and glioma cells related to cell type and growth properties may have great potential in clinical diagnosis and therapy.

Interactions among calcium compartments in C6 rat glioma cells: involvement of potassium channels

1. Variations in intracellular free Ca2+ concentration ([Ca2+]i) induced by alteration of the extracellular concentrations of Ca2+ ([Ca2+]o) and K+ ([K+]o) were imaged in single fluo-3-loaded C6 glioma cells. In addition, the effect of membrane potential on [Ca2+]i was investigated in fura-2-loaded, voltage-clamped cells. 2. Step alterations of [Ca2+]o from 0 to 10 nM were followed by proportional variations in [Ca2+]i, with a maximum 7-fold increase and an apparent half-maximum at [Ca2+]o of 1.5 mM. 3. The time to half-maximum change (t1/2) of [Ca2+]o-associated [Ca2+]i variations ranged between 10 and 50 s, and was inversely related to the amplitude of [Ca2+]o steps. 4. Transient, serotonin-induced [Ca2+]i elevations, used as a measure of Ca2+ availability in inositol 1,4,5-trisphosphate-sensitive stores, were diminished within 10 min in 0 mM [Ca2+]o, but were unaffected by [Ca2+]o changes in the 1-5 mM range. 5. Restoration of normal [Ca2+]i following its elevation by serotonin was delayed by removal of external Na+ or Cl- and was enhanced by warming the medium to 37 degrees C. These conditions did not affect [Ca2+]o-associated [Ca2+]i variations. 6. [Ca2+]o-associated [Ca2+]i variations were depressed by La3+ and Ba2+, while blockers of voltage-activated Ca2+ channels were ineffective. 7. Elevated [K+]o depressed the basal level of [Ca2+]i, and in high concentrations (70-140 mM) also diminished the response to serotonin. 8. Depolarizing the membrane potential of voltage-clamped cells reversibly reduced [Ca2+]i. These membrane-potential associated [Ca2+]i variations were blocked by La3+, Ba2+ and TEA, all of which also depolarized membrane resting potential. 9. Apamin (at 1-10 microM), a blocker of [Ca2+]i-activated K+ channel, totally and reversibly prevented [Ca2+]o-associated [Ca2+]i variations. 10. These studies indicate that C6 cells are responsive to variations in [Ca2+]o, and that a K+ channel is a possible path through which Ca2+ penetrates into the cell.