Involvement of histaminergic receptor mechanisms in the stimulation of NT-3 synthesis in astrocytes.

Neurotrophin-3 (NT-3) is produced by astrocytes, in addition to neurons, and monoamine neurotransmitters play a role in controlling NT-3 synthesis. The impact of histamine (HA) on the regulation of NT-3 synthesis in cultured astrocytes has not been studied. We evaluated the involvement of histamine receptors and intracellular mechanisms in the regulation of NT-3 production by HA. Real-time PCR was performed to examine the expression of all known histamine receptor subtypes in cultured rat cortical astrocytes. Pharmacological tools, selective for the H₁, H₂ and H₃ receptors and intracellular systems, were utilized to confirm functional properties of HA receptors in histaminergic up-regulation of astrocytic NT-3 synthesis. HA potently and transiently elevated NT-3 expression and protein levels by more than twofold. In addition to H₁ and H₂ receptors, cultured astrocytes also express H₃ receptors, which activate G(i/o) proteins to inhibit adenylyl cyclase and modulate MAP kinase activity. Histaminergic stimulation was partly inhibited by selective H₁, H₂, and H₃ antagonists whereas selective H₁, H₂, and H₃ agonists or mediators of the intracellular histaminergic pathways increased NT-3 levels. Inhibitors of PKA, PKC, and CaMK II significantly reduced the HA-induced increase in NT-3 cellular levels whereas the MAP kinase cascade inhibitor completely blocked the stimulatory action of HA and all selective agonists. In conclusion, the synthesis of astrocytic NT-3 stimulated by HA is a receptor-mediated process, which is fine-tuned via subtle modulation of parallel histaminergic H₁, H₂, and H₃ pathways that converge at the level of MAP kinase activity. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.