Histamine-neurotrophin-3 interactions in cultured rat astrocytes

Background

Neurotrophin-3 (NT-3) is produced by astrocytes, in addition to neurons, and monoamine neurotransmitters play a role in controlling NT-3 synthesis [1]. The impact of histamine on the regulation of NT-3 synthesis in cultured astrocytes has not been studied in detail. Therefore, we focused our present study on the active involvement of multiple histaminergic receptor and intracellular mechanisms in the regulation of NT-3 production by histamine.

Results

Histamine (1 μM) significantly and transiently elevates NT-3 mRNA levels by 2.2-fold after 30 min of incubation following by 2.1-fold increase in NT-3 intracellular levels after 6 h. Its stimulation was partly inhibited by the H₁ antagonists triprolidine and mepyramine, the H₂ antagonists famotidin and cimetidin, and by the H₃ antagonist ciproxifan. NT-3 levels in astrocytes were increased by specific and selective H₁, H₂ and H₃ agonists, but none of the tested agonists was able to reach the level of histamine's stimulatory effect. Different activators of basic intracellular histamine receptor second messenger systems (forskolin, dibutyryl cAMP, dBcAMP; as well as calcimycin, i.e. Ca²⁺ ionophore A23187) and phorbol 12-myristate 13-acetate, TPA) markedly increase the cellular level of NT-3 protein. Histamine-induced cellular levels of NT-3 were significantly reduced by H-89 (an inhibitor of protein kinase A, PKA) and by staurosporin (an inhibitor of protein kinase C, PKC). Studying histamine receptor subtype expression in astrocytes using quantitative RT-PCR we confirmed that the expression levels of histamine H₁ and H₂ receptors (already shown in radioligand binding) as well as of H₃ receptors are high.

Conclusion

Our study confirmed that the synthesis of NT-3 in astrocytes is regulated by the histaminergic system and indicate the possible involvement of multiple, complex histamine H₁, H₂ and H₃ receptors and corresponding intracellular mechanisms involving the cAMP/PKA pathway, as well as mobilisation of Ca²⁺ ions and activation of PKC.