Whole-cell NMDA-evoked current in suprachiasmatic neurones of the rat: modulation by extracellular calcium ions

The mechanisms of energy crisis in human astrocytes after subarachnoid hemorrhage

BACKGROUND

Calcium (Ca2+) is a cofactor of multiple cellular processes. The mechanisms that lead to elevated cytosolic Ca2+ concentration are unclear.

OBJECTIVE

To illuminate how bloody cerebrospinal fluid (bCSF) from patients with intraventricular hemorrhage causes cell death of cultured human astrocytes.

METHODS

Cultured astrocytes were incubated with bCSF. In control experiments, native CSF was used. Cytosolic Ca2+ concentration was measured by fura-2 fluorescence. Apoptosis and necrosis were evaluated by staining with Hoechst-3342 and propidium iodide.

RESULTS

Incubation of astrocytes with bCSF provoked a steep Ca2+ concentration peak that was followed by a slow Ca2+ rise during the observation period of 50 minutes. Necrosis, but not apoptosis, was induced. Blockade of ATP-sensitive P2 receptors with suramin inhibited the bCSF-induced initial Ca2+ peak and necrosis. Blockade of P1 receptors with 8-phenyltheophylline or of N-methyl-D-aspartate receptors with D(-)-2-amino-5-phosphopentanoic acid had no significant effect. Preincubation with xestospongin D, a blocker of inositol 1,4,5-trisphosphate receptors, prevented the initial Ca2+ rise and reduced the rate of necrosis. Preemptying of the endoplasmic reticulum with thapsigargin protected astrocytes from the bCSF-induced Ca2+ peak. Inhibition of mitochondrial permeability transition pores opening with cyclosporin A reduced the rate of astrocytic necrosis significantly, although it did not influence the initial Ca peak.

CONCLUSION

bCSF elicits a steep, transient Ca rise when administered to human astrocytes by activation of ATP-sensitive P2 receptors and subsequent inositol 1,4,5-trisphosphate-dependent Ca release from endoplasmic reticulum. This massive Ca overload leads to subsequent mitochondrial permeability transition pores opening and necrosis of the cells.

Suppression of potassium channels elicits calcium-dependent plateau potentials in suprachiasmatic neurons of the rat

By using whole-cell recordings in acute and organotypic hypothalamic slices, we found that following K+ channel blockade, sustained plateau potentials can be elicited by current injection in suprachiasmatic neurons. In an attempt to determine the ionic basis of these potentials, ion-substitution experiments were carried out. It appeared that to generate plateau potentials, calcium influx was required. Plateau potentials were also present when extracellular calcium was replaced by barium, but were independent upon an increase in the intracellular free calcium concentration. Substitution of extracellular sodium by the impermeant cation N-methyl-D-glucamine indicated that sodium influx could also contribute to plateau potentials. To gain some information on the pharmacological profile of the Ca++ channels responsible for plateau potentials, selective blocker of various types of Ca++ channel were tested. Plateau potentials were unaffected by isradipine, an L-type Ca++ channel blocker. However, they were slightly reduced by omega-conotoxin GVIA and omega-agatoxin TK, blockers of N-type and P/Q-type Ca++ channels, respectively. These data suggest that R-type Ca++ channels probably play a major role in the genesis of plateau potentials. We speculate that neurotransmitters/neuromodulators capable of reducing or suppressing potassium conductance(s) may elicit a Ca++-dependent plateau potential in suprachiasmatic neurons, thus promoting sustained firing activity and neuropeptide release.

Glutamate and aspartate do not exhibit the same changes in their extracellular concentrations in the rat striatum after N-methyl-D-aspartate local administration

To determine whether glutamate (Glu) and aspartate (Asp) undergo a similar regulation of their extracellular levels, Glu and Asp were simultaneously monitored in the striatum of anesthetized rats after local N-methyl-D-aspartate (NMDA) receptor stimulation, using 1-min in vivo microdialysis coupled to capillary electrophoresis with laser-induced fluorescence detection. Application of NMDA (10 min, 10(-3) M) through the dialysis probe induced 1) an increase (+50%) in Asp during the NMDA administration and 2) a surprising biphasic effect on Glu, with a rapid increase (+30%) and a return to baseline before the end of NMDA application, followed by a second increase (+40%) occurring after and linked to the end of NMDA administration. When studied in the presence of 10 microM tetrodotoxin (TTX) or 0.1 mM Ca(2+), the increase in Asp was partially TTX-dependent, and the early increase in Glu appeared to be partially TTX and Ca(2+) dependent, whereas the second increase in Glu was not. The second increase in Glu level was still present when NMDA antagonists (AP5 or MK-801) were administered at the end of NMDA application. Finally, only extracellular Asp was increased through application of lower NMDA concentrations (10(-4) M, 10(-5) M), whereas extracellular Glu was not affected. In conclusion, these results suggest a differential control of Glu and Asp extracellular levels in rat striatum by distinct mechanisms linked to NMDA receptors and involving neuronal or nonneuronal release.

NMDA-evoked calcium transients and currents in the suprachiasmatic nucleus: gating by the circadian system

A variety of evidence suggests that the effects of light on the mammalian circadian system are mediated by glutamatergic mechanisms and that the N-methyl- D-aspartate (NMDA) receptor plays an important role in this regulation. One of the fundamental features of circadian oscillators is that their response to environmental stimulation varies depending on the phase of the daily cycle when the stimuli are applied. For example, the same light treatment, which can produce phase shifts of the oscillator when applied during subjective night, has no effect when applied during the subjective day in animals held in constant darkness (DD). We examined the hypothesis that the effects of NMDA on neurons in the suprachiasmatic nucleus (SCN) also vary from day to night. Optical techniques were utilized to estimate NMDA-induced calcium (Ca2+) changes in SCN cells. The resulting data indicate that there was a daily rhythm in the magnitude and duration of NMDA-induced Ca2+ transients. The phase of this rhythm was determined by the light-dark cycle to which the rats were exposed with the Ca2+ transients peaking during the night. This rhythm continued when animals were held in DD. gamma-Aminobutyric acid (GABA)ergic mechanisms modulated the NMDA response but were not responsible for the rhythm. Finally, there was a rhythm in NMDA-evoked currents in SCN neurons that also peaked during the night. This study provides the first evidence for a circadian oscillation in NMDA-evoked Ca2+ transients in SCN cells. This rhythm may play an important role in determining the periodic sensitivity of the circadian systems response to light.

Circadian phase shifts to neuropeptide Y In vitro: cellular communication and signal transduction

Mammalian circadian rhythms originate in the hypothalamic suprachiasmatic nuclei (SCN), from which rhythmic neural activity can be recorded in vitro. Application of neurochemicals can reset this rhythm. Here we determine cellular correlates of the phase-shifting properties of neuropeptide Y (NPY) on the hamster circadian clock in vitro. Drug or control treatments were applied to hypothalamic slices containing the SCN on the first day in vitro. The firing rates of individual cells were sampled on the second day in vitro. Control slices exhibited a peak in firing rate in the middle of the day. Microdrop application of NPY to the SCN phase advanced the time of peak firing rate. This phase-shifting effect of NPY was not altered by block of sodium channels with tetrodotoxin or block of calcium channels with cadmium and nickel, consistent with a direct postsynaptic site of action. Pretreatment with the glutamate receptor antagonists (DL-2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione disodium) also did not alter phase shifts to NPY. Blocking GABAA receptors with bicuculline (Bic) had effects only at very high (millimolar) doses of Bic, whereas blocking GABAB receptors did not alter effects of NPY. Phase shifts to NPY were blocked by pretreatment with inhibitors of protein kinase C (PKC), suggesting that PKC activation may be necessary for these effects. Bathing the slice in low Ca2+/high Mg2+ can block phase shifts to NPY, possibly via a depolarizing action. A depolarizing high K+ bath can also block NPY phase shifts. The results are consistent with direct action of NPY on pacemaker neurons, mediated through a signal transduction pathway that depends on activation of PKC.