The molecular mechanism of gliotransmitter

The molecular mechanism of gliotransmitter release is not fully understood until now, and previous studies have emphasized that the elevation of [Ca2+]i triggers vesicular ACPT-I synthesis of glutamate. Parpura et al. reported that the essential role of Ca2+ release from internal stores in glutamate release. The H1 receptor antagonist assays showed that 10 nM levocetirizine neither inhibited Ca2+ signalling nor glutamate release, while at a higher concentration both cellular responses were inhibited, indicating the contribution of the intracellular Ca2+ release caused by H1 receptors to glutamate release. In contrast, H2 receptor antagonist ranitidine increased [Ca2+]i, but not glutamate release, in the presence of 10 μM histamine, suggesting that ranitidine could induce [Ca2+]i elevation independently of internal Ca2+ stores through a yet unknown mechanism. We also revealed that histamine-induced glutamate release was increased after Ca2+ response reached a plateau. This result suggests that Ca2+-independent signalling was also involved in histamine-induced glutamate release. Various reports have described the involvement of other intracellular signalling pathways in glutamate release (reviewed by Malarkey and Parpura). Since our results indicate that PLC plays a crucial role in glutamate release, DAG, which is produced through PLC activation, independently of Ca2+ signalling, becomes a potential facilitator for H1 receptor-dependent glutamate release. This hypothesis was previously proven by Mungenast et al. showing DAG-induced gliotransmitter release from cultured astrocytes. However, in order to fully understand the involved mechanism, additional studies are required. Till this day, it remains unknown whether cAMP regulation by Gs and Gi-proteins contributes to gliotransmitter release. In a previous study it was argued that increased cAMP concentrations could promote astrocytic glutamate release. Our study showed that forskolin positively affected glutamate release, supporting the involvement of cAMP in glutamate release. Interestingly, forskolin further enhanced glutamate release in the presence of histamine, indicating that the contribution of cAMP elevation to histamine-induced glutamate release was negligible. In fact, the level of cAMP after histamine or forskolin treatment was significantly different. Moreover, the H2 receptor agonist dimaprit as well as the two selective H2 receptor antagonists ranitidine and famotidine did not have any impact on glutamate release. Therefore, H1 receptor plays a predominant role in histamine-induced glutamate release, and the H1 and H2 receptor signals are not cross-linked. In contrast, it appears that histamine H2 receptor signalling increases cAMP to levels that were sufficient to initiate further downstream signalling to activate CREB. Moreover, CREB can be activated by different upstream signals, and responds quickly to different stimuli selectively, thereby contributing to a variety of functions, including learning and memory. Future investigations will provide a better understanding of the ways in which histamine-induced CREB activation affects brain function. Summarizing these data, histamine activates astrocyte signalling in the 1321N1 cell line through two distinct receptors, H1 and H2 receptors. We used primary rat astrocytes to validate our results from an astrocytic cell line, supporting our hypothesis that histamine-induced Ca2+ signalling and gliotransmitter release are crucial for brain function, and are therefore, largely conserved across mammals. Our findings emphasize the need for in vivo studies to further investigate the effects of histamine-dependent astrocyte signalling on brain function. Previous studies have shown that histamine binding capacity decreases in various neurological disorders, including Alzheimer's disease, especially in the cortex. Based on our in vitro results, it is likely that H1 receptors are the main regulators of histamine-dependent astrocyte signalling in vivo. Future studies should use novel conditional H1 receptor knock-out mouse models that lack H1 receptors specifically on astrocytes or neurons to better address this possibility.