GABAmimetic intravenous anaesthetics inhibit spontaneous Ca2+ -oscillations in cultured hippocampal neurons

The positive allosteric modulation of GABAA receptors mRNA in immature hippocampal rat neurons by midazolam affects receptor expression and induces apoptosis

Background: Numerous experimental studies show that anesthetics are potentially toxic to the immature brain. Even though benzodiazepines are widely used in pediatric anesthesia and intensive care medicine, only a few studies examine the effects of these drugs on immature neurons. Methods: Hippocampal neuronal cell cultures of embryonic Wistar rats (15 days in culture) were incubated with midazolam 100 or 300 nM for either 30 min or 4 h. The time course of the mRNA expression of the glutamate receptors subunits NR1, NR2A and NR2B of the NMDA receptor, the GluA-1 and A-2 subunits of the AMPA receptor as well as the alpha 1 subunit of the GABA_A receptor were examined by PCR. Apoptosis was detected using Western blot analysis for BAX, Bcl-2 and Caspase-3. Results: Midazolam at 100 and 300 nM applied for 30 min and 100 nM for 4 h affected glutamate receptor and GABA_A receptor subunit expression. However, these effects were reversible within 72 h following washout. When 300 nM midazolam was applied for 4 h a significant increase in the NR 1 and NR 2A mRNA subunit expression could be detected. The increase in NR 2B receptor subunit expression as well as the GluA1 subunit expression was not reversible within 72 h following washout. This increase in mRNA glutamate receptor subunit expression was associated with a significant increase in neuronal apoptosis. Conclusion: In immature neurons midazolam altered GABA and glutamate mRNA receptor subunit expression. Prolonged increase in midazolam-induced glutamate receptor expression was associated with apoptosis.

3D nerve cell cultures and complex physiological relevance

The field of tissue engineering has not yet provided knowledge on which a consensus for the complex physiological relevance (CPR) of neuronal cultures could be established. The CPR of 3D neuronal cultures can have a profound impact on the drug discovery process through the validation of in vitro models for the study of neuropsychiatric and degenerative diseases, as well as screening for neurotoxicity during drug development. Herein, we assemble evidence in support of the potential of [Ca²⁺]_i oscillation frequency as a CPR outcome that can demonstrate the in vivo-like behavior of 3D cultures and differentiate them from 2D monolayers. We demonstrate that [Ca²⁺]_i oscillation frequencies in 2D cultures are significantly higher than those found in 3D cultures, and provide a possible molecular explanation.

Toxic effects of midazolam on differentiating neurons in vitro as a consequence of suppressed neuronal Ca2+-oscillations

BACKGROUND

In immature neurons anesthetics induce apoptosis and influence neuronal differentiation. Neuronal Ca(2+)-oscillations regulate differentiation and synaptogenesis. We examined the effects of the long-term blockade of hippocampal Ca(2+)-oscillations with midazolam on neuronal synapsin expression.

MATERIAL AND METHODS

Hippocampal neurons were incubated at day 15 in culture with the specific GABA(A) receptor agonist muscimol (50μM) or with midazolam (100 and 300nM), respectively, for 24h. TUNEL and activated-Caspase-3 staining were used to detect apoptotic neurons. Ca(2+)-oscillations were detected using the Ca(2+)-sensitive dye FURA-2 and dual wavelength excitation fluorescence microscopy. Synapsin was identified with confocal anti-synapsin immunofluorescence microscopy.

RESULTS

Muscimol, when applied for 24h, decreased the amplitude and frequency Ca(2+)-oscillations significantly. Midazolam concentration-dependently suppressed the amplitude and frequency of the Ca(2+)-oscillations. This was associated by a downregulation of the synapsin expression 24h after washout.

CONCLUSION

Neuronal Ca(2+)-oscillations mediate neuronal differentiation and are involved in synaptogenesis. By acting via the GABA(A) receptor, midazolam exerts its toxic effect through the suppression of neuronal Ca(2+)-oscillations, a reduction in synapsin expression and consecutively reduced synaptic integrity.

The toxic effects of s(+)-ketamine on differentiating neurons in vitro as a consequence of suppressed neuronal Ca2+ oscillations

BACKGROUND

In the immature brain, neuronal Ca2+ oscillations are present during a time period of high plasticity and regulate neuronal differentiation and synaptogenesis. In this study we examined the long-term blockade of hippocampal Ca2+ oscillations, the role of the N-methyl-D-aspartate (NMDA) receptors and the effects of S(+)-ketamine on neuronal synapsin expression.

METHODS

Hippocampal neurons were incubated at day 15 in culture with the specific NMDA receptor antagonists dizocilpine (MK 801, 100 μM) or S(+)-ketamine (3 μM to 25 μM) for 24 hours. Terminal-deoxynucleotidyl-transferase (TUNEL) and activated caspase3 were used to detect apoptotic neurons. Ca2+ oscillations were detected after loading the neurons with the Ca2+-sensitive dye fura-2AM, and dual wavelength excitation fluorescence microscopy was performed. Ca2+/calmodulin kinase II (CaMKII) was measured using Western blots. Synapsin was identified with confocal antisynapsin immunofluorescence.

RESULTS

Blocking the NMDA receptor with MK 801 or 25 μM S(+)-ketamine resulted in a significant increase in apoptotic neurons. MK 801 led to a significant increase in cytosolic Ca2+ concentration and reduction of the amplitude and frequency of the Ca2+ oscillations. Similar to MK 801, the long-term application of S(+)-ketamine resulted in a significant increase in cytosolic Ca2+ concentration 24 hours after washout. This was associated with a down-regulation of the CaMKII and a reduction of the synapsin 24 hours after washout.

CONCLUSION

Neuronal Ca2+ oscillations mediate neuronal differentiation and synaptogenesis via activating CaMKII. By acting via the NMDA receptor, S(+)-ketamine exerts its toxic effect through the suppression of neuronal Ca2+ oscillations, down-regulation of the CaMKII, and consecutively reduced synaptic integrity.

Isoflurane enhances spontaneous Ca(2+) oscillations in developing rat hippocampal neurons in vitro

BACKGROUND

During the nervous system development, spontaneous synchronized Ca(2+) oscillations are thought to possess integrative properties because their amplitude and frequency can influence the patterning of neuronal connection, neuronal differentiation, axon outgrowth, and long-distance wiring. Accumulating studies have confirmed that some drugs such as volatile anesthetic isoflurane produced histopathologic changes in the central nervous system in juvenile animal models. Because the hippocampus plays an important role in learning and memory, the present work was designed to characterize the Ca(2+) oscillations regulated by volatile anesthetic isoflurane in primary cultures of developing hippocampal neurons (5-day-cultured).

METHODS

Primary cultures of rat hippocampal neurons (5-day-cultured) were loaded with the Ca(2+) indicator Fluo-4AM (4 microM) and were studied with a confocal laser microscope.

RESULTS

Approximately 22% of 5-day-cultured hippocampal neurons exhibited typical Ca(2+) oscillations. These oscillations were dose-dependently enhanced by isoflurane (EC50 0.5 MAC, minimum alveolar concentration) and this effect could be reverted by bicuculline (50 microM), a specific gamma-aminobutyric acid (GABA(A)) receptor antagonist.

CONCLUSION

Unlike its depressant effect on the Ca(2+) oscillations in adult neurons in previous researches, isoflurane dose-dependently enhanced calcium oscillations in developing hippocampal neurons by activating GABA(A) receptors, a major excitatory receptor in synergy with N-methyl-d-aspartate receptors at the early stages of development. It may be involved in the mechanism of an isoflurane-induced neurotoxic effect in the developing rodent brain.