Effects Of Histamine And Its Receptors On NMDA-induced Neurotoxicity In Cultured Cortical Neurons

It is well known that histamine acts as a neurotransmitter and neuromodulator in the peripheral and central nervous system.The histaminergic system is involved in various physiological and behavioral functions including sleep/wake cycles,emotion, appetite control,locomotor activity,stress behavior,neuroendocrine control,and learning and memory through histamine H₁,H₂ and H₃ receptors.The NMDA receptor has also been implicated in a number of important brain functions and pathologies,including learning,neuronal development and stroke. Recently,the interactive function between histamine and NMDA receptor have attracted considerable attention in view of the effects on synaptic plasticity,learning and ischemic neuronal damage.Histamine can affect long-term potentiation,which may involve NMDA receptors,in a number of ways.Histamine selectively facilitates currents evoked by NMDA in neurons both in the hippocampal slice and in culture.On the other hand,histaminergic neurotransmission plays a protective role in the development of delayed neuronal death caused mainly by NMDA-induced excitotoxicity during cerebral ischemia. However,the mechanisms of histamine action and how its receptors interact with NMDA receptors remain unclear.Our behavioral data from rats indicate that,in radial maze performance,histamine reverses the working memory deficits induced by the NMDA receptor antagonist MK-801 via an interaction with Hi but not H₂ receptors.In cerebral ischemic neuronal injury,which is mediated by excessive activation of glutamate receptors,NMDA receptor antagonists have protective effects,whereas H₁ receptor antagonists increase neuronal damage.On the other hand,the H₂ receptor is a potent stimulator of the cAMP pathway,which is necessary for the late phase of NMDA receptor-dependent long-term potentiation as well as for the induction of mossy fiber long-term potentiation.Pre-ischemic administration of histamine suppresses the increase of glutamate levels and alleviates neuronal damage via H₂ receptor stimulation. H₃ receptors are exclusively pre-synaptic,where they provide negative feedback to restrict histamine synthesis and release,and the release of other neurotransmitters,such as glutamate,acetylcholine and GABA.However,little is known about the effects of the H₃ receptor on NMDA-induced neurotoxicity.The general aim of the present study was to explore the possible role of histamine in cell death in an in vitro model of cerebral ischemia on the basis of NMDA-induced excitotoxicity in cultured cortical neurons.We focused on the different histamine receptors involved in the modulation of NMDA-induced excitotoxicity and the associated second messenger cascades involved.Partâ… Effects of histamine on NMDA-induced neurotoxicity in cultured cortical neuronsUsing histamine,the roles of histamine receptors in NMDA-induced necrosis were investigated in rat cultured cortical neurons.Within 3 h of intense NMDA insult,most neurons died by necrosis.Histamine reversed the neurotoxicity in a concentration-dependent manner and showed peak protection at a concentration of 10^-7 mol/L.This protection was antagonized by the H₂ receptor antagonists cimetidine and zolantidine but not by the H₁ receptor antagonists pyrilamine and diphenhydramine.In addition,the selective H₂ receptor agonist amthamine mimicked the protection by histamine.This action was prevented by cimetidine but not by pyrilamine.8-Br-cAMP also mimicked the effect of histamine.In contrast,both the adenylyl cyclase inhibitor SQ-22536 and the PKA inhibitor H-89 reversed the protection by histamine.In addition, histamine can enhance cell surface NMDAR1 intemaliation,which may also participate in the mechanism of the protection of histamine.Partâ…¡Effects of H₃ receptor antagonists on NMDA-induced neurotoxicity in cultured cortical neuronsUsing the histamine H₃ receptor antagonist thioperamide and clobenpropit,the roles of histamine H₃ receptors on NMDA-induced necrosis were investigated in rat cultured cortical neurons.Thioperamide and clobenpropit attenuated NMDA-induced excitotoxicity,which was reversed by the H₃ receptor agonist(R)-α-methylhistamine but not by pyrilamine and cimetidine.In addition,the protection by thioperamide and clobenpropit was inhibited by the GABAA receptor antagonist picrotoxin and bicuculline.Further study demonstrated that the protection by thioperamide and clobenpropit was due to increased GABA release in NMDA-stimulated samples.The inducible GABA release by clobenpropit was also inhibited by(R)-α-methylhistamine, but not by pyrilamine or cimetidine.Furthermore,both the adenylyl cyclase inhibitor SQ-22536 and the PKA inhibitor H-89 reversed the protection and the GABA release by clobenpropit.In addition,clobenpropit reversed the NMDA-induced increase in intracellular calcium level,which was antagonized by(R)-α-methylhistamine.These results indicate that clobenpropit enhanced GABA release to protect against NMDA-induced excitotoxicity,which was induced through the cAMP/PKA pathway, and reduction of intracellular calcium level may also be involved.Summary1.Within 3 h of intense NMDA insult,most neurons died by necrosis in cultured cortical neuron.2.Histamine attenuates NMDA-induced neurotoxicity which is dependent on the H₂ receptor/cAMP/cAMP-dependent protein kinase pathway.3.Histamine can enhance cell surface NMDAR1 internalation,which is independent of H₁ and H₂ receptror.4.H₃ receptor antagonists enhanced GABA release to protect against NMDA-induced excitotoxicity,which was induced through the cAMP/PKA pathway, and reduction of intracellular calcium level may also be involved.