Variation in the pattern of [Ca2+]i change induced by acetylcholine in cultured hippocampal neurons

Ca2+-inhibited non-inactivating K+ channels in cultured rat hippocampal pyramidal neurones

1. Whole-cell perforated-patch recording from cultured CA1-CA3 pyramidal neurones from neonatal rat hippocampus (20-22 C; [K+]o = 2.5 mM) revealed two previously recorded non-inactivating (sustained) K+ outward currents: a voltage-independent 'leak' current (Ileak) operating at all negative potentials, and, at potentials >= -60 mV, a time- and voltage-dependent 'M-current' (IK(M)). Both were inhibited by 1 mM Ba2+ or 10 microM oxotremorine-M (Oxo-M). In ruptured-patch recording using Ca2+-free pipette solution, Ileak was strongly enhanced, and was inhibited by 1 mM Ba2+ but unaffected by 0.5 mM 4-aminopyridine (4-AP), 1 mM tetraethylammonium (TEA) or 1-10 nM margatoxin. 2. Single channels underlying these currents were sought in cell-attached patch recordings. A single class of channels of conductance approximately 7 pS showing sustained activity at resting potential and above was identified. These normally had a very low open probability (Po < 0. 1), which, however, showed a dramatic and reversible increase (to about 0.9 at approximately 0 mV) following the removal of Ca2+ from the bath. Under these (Ca2+-free) conditions, single-channel Po showed both voltage-dependent and voltage-independent components on patch depolarization from resting potential. The mean activation curve was fitted by a modified Boltzmann equation. When tested, all channels were reversibly inhibited by addition of 10 microM Oxo-M to the bath solution. 3. The channels maintained their high Po in patches excised in inside-out mode into a Ca2+-free internal solution and were strongly inhibited by application of Ca2+ to the inner face of the membrane (IC50 = 122 nM); this inhibition was observed in the absence of MgATP, and therefore was direct and unrelated to channel phosphorylation/dephosphorylation. 4. Channels in patches excised in outside-out mode were blocked by 1 mM Ba2+ but were unaffected by 4-AP or TEA. 5. Channels in cell-attached patches were inhibited after single spikes, yielding inward ensemble currents lasting several hundred milliseconds. This was prevented in Ca2+-free solution, implying that it was due to Ca2+ entry. 6. The properties of these channels (block by internal Ca2+ and external Oxo-M and Ba2+, and the presence of both voltage-dependent and voltage-independent components in their Po/V relationship) show points of resemblance to those expected for channels associated with both Ileak and IK(M) components of the sustained macroscopic currents. For this reason we designate them Ksust ('sustained current') channels. Inhibition of these channels by Ca2+ entry during an action potential may account for some forms of Ca2+-dependent after-depolarization. Their high sensitivity to internal Ca2+ may provide a new, positive feedback mechanism for cell excitation operating at low (near-resting) [Ca2+]i.

Carbachol-stimulated Ca2+ increase in single neuroblastoma SH-SY5Y cells: effects of ethanol

The effect of ethanol on the characteristics of carbachol-stimulated release of Ca2+ from intracellular Ca2+ stores was studied in single SH-SY5Y cells. Stimulation with carbachol, in the absence of extracellular Ca2+, elicited a rapid Ca2+ increase in SH-SY5Y cells peaking within seconds after addition of maximal agonist concentration. The Ca2+ response pattern in single cells resembled the population response, and there was no evidence of oscillatory changes in cytosolic [Ca2+] ([Ca2+]i). However, cell-to-cell variability could be detected in the magnitude and the latency time of the response, and in the rate of [Ca2+]i increase. In a carbachol dose-response analysis, the EC50 for the number of responsive cells and for the peak [Ca2+]i response was lower than that for carbachol-induced inositol 1,4,5-trisphosphate formation by a factor of 5 to 50. Ethanol (100 mM) caused a significant suppression of the number of responsive cells, but only when cells were stimulated with nonsaturating carbachol concentrations (1 and 10 microM). The suppression by ethanol was evident primarily in those cells that gave a Ca2+ response after several seconds of stimulation, whereas cells that responded within the initial seconds of receptor stimulation remained relatively unaffected. In responding cells stimulated with 10 microM carbachol, ethanol exposure also suppressed the maximal Ca2+ increase primarily in those cells that responded late. We suggest that ethanol suppression of muscarinic receptor-mediated signal transduction through the phospholipase C pathway may depend on the potentiation of feedback inhibition that requires receptor stimulation.

Nicotine increases intracellular calcium in rat hippocampal neurons via voltage-gated calcium channels

The effect of nicotinic receptor activation on intracellular calcium concentrations ([Ca2+]i) was quantitated in populations of cultured hippocampal neurons loaded with Fura-2. Nicotine (50 microM) and cytisine (50 microM) increased [Ca2+]i by 100%. This response was abolished in the presence of the nicotinic antagonist methyllycaconitine (MLA) whereas KCl-evoked increases in [Ca2+]i were insensitive to MLA. Glial cultures were unaffected by nicotine, although they did respond to glutamate with increased [Ca2+]i. In hippocampal neurons, responses to nicotinic agonists and KCl were dependent on the presence of extracellular Ca2+ and were similarly sensitive (85% inhibition) to CdCl2. These results are consistent with the presence of functional nicotinic receptors on hippocampal neurons. The receptors appear to elevate [Ca2+]i by promoting the influx of extracellular Ca2+ through voltage-gated calcium channels.

Imaging of caffeine-inducible release of intracellular calcium in cultured embryonic mouse telencephalic neurons

To gain a better understanding of Ca(2+)-induced Ca2+ release in central neurons, we have studied the increase in intracellular Ca2+ concentration ([Ca2+]i) induced by application of caffeine to cells cultured from embryonic mouse telencephalon (hippocampus or cortex). The magnitudes and distributions of changes in [Ca2+]i in neuron somata were measured by quantitative video microscopy. We observed that application of caffeine to pyramidally shaped neurons typically initiated an increase in [Ca2+]i in the cytoplasmic region between the nucleus and the base of a major dendrite. [Ca2+] in this region increased over a period of 3 to 6 s and was followed by, with a slight delay, a surge of Ca2+ that moved across the soma and into or over the nucleus. Similar Ca2+ responses to caffeine were observed in Ca(2+)-containing and nominally Ca(2+)-free external solutions, suggesting that caffeine was inducing Ca2+ release from intracellular stores. Ca2+ responses to caffeine were potentiated by inducing a tonic Ca2+ influx through N-methyl-D-aspartate (NMDA)-type glutamate receptors activated by 0.3 microM glutamate and multiple responses to caffeine could be elicited by using this Ca2+ influx to refill the intracellular stores. Ryanodine inhibition of caffeine-induced Ca2+ release was use- and concentration-dependent; the median effective concentration EC50 for ryanodine declined from 22 microM for the first application of caffeine to 20 nM for the fourth. We conclude, based on these responses to caffeine, that ryanodine-sensitive mechanisms of intracellular Ca2+ release are active in hippocampal and cortical neurons and may be involved in generation of directed Ca2+ waves that engulf the nucleus.

A single optical fiber fluorometric device for measurement of intracellular Ca2+ concentration: its application to hippocampal neurons in vitro and in vivo

We developed a new system to measure the intracellular Ca2+ concentration in the deep region of the central nervous system with a single optical fiber (300 microns in diameter), used for both excitation and detection of the fluorescence of previously loaded fura-2. With this system, a brain region loaded with fura-2 was illuminated by a rotating disc bearing three different interference filters of 340, 360 and 380 nm at a rate of 600 rpm. The emitted fluorescence was collected by the same fiber connected to a photomultiplier whose output was fed into a computer which regulates the timing of illumination and detection. The time course of the change in the fluorescence due to 340, 360 or 380 nm excitation was measured simultaneously at the maximum sampling rate of 10 points/s. Ratios of fluorescence intensities were obtained after the experiment. After confirming that this system was sensitive enough to detect the change of intracellular Ca2+ concentration in cultured hippocampal neurons and hippocampal slices during depolarization by high potassium medium (50 mM), we applied this system to anesthetized rats. In the hippocampus preloaded with fura-2, characteristic changes in fluorescence intensities ascribed to an increase in intracellular Ca2+ concentration were detected after asphyxia. The system is potentially useful for investigating the physiological and pathological roles of Ca2+ in the brain.

Involvement of postsynaptic G proteins in hippocampal long-term potentiation

The possible involvement of postsynaptic guanosine 5-triphosphate (GTP)-binding proteins (G proteins) in long-term potentiation (LTP) was studied in rat hippocampal slices, using whole-cell recording techniques. The inclusion of guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) or guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) in the recording pipette significantly reduced or abolished the baclofen-induced hyperpolarization of pyramidal neurons, which indicates uncoupling of the signal transduction from G protein-coupled receptors by these compounds. Both GDP beta S and GTP gamma S significantly attenuated the magnitude of LTP in the fimbria-CA3 synapses, but not in the mossy fiber-CA3 synapses. GTP gamma S did not attenuate LTP in the Schaffer-CA1 synapses. The effects of guanine nucleotide analogs on fimbrial LTP were reversed by postsynaptic depolarization during high frequency stimulation. These results suggest that postsynaptic G proteins may be involved in the generation of LTP in the fimbrial synapses, possibly by affecting membrane depolarization during high frequency afferent activation.