Ciproxifan, a histamine H3 receptor antagonist and inverse agonist, presynaptically inhibits glutamate release in rat hippocampus

- Ciproxifan presynaptically reduces glutamate release in the hippocampus in vitro.
- Decrease in voltage-dependent Ca2 + influx is involved.
- A role for the PLA2/PGE2/EP2 pathway in the action of ciproxifan is suggested.
- Decreased ERK and synapsin I activity is also involved.
- This study provides new insight into the mode by which ciproxifan acts in the brain.

Ciproxifan is an H3 receptor antagonist and inverse agonist with antipsychotic effects in several preclinical models; its effect on glutamate release has been investigated in the rat hippocampus. In a synaptosomal preparation, ciproxifan reduced 4-aminopyridine (4-AP)-evoked Ca2 +-dependent glutamate release and cytosolic Ca2 + concentration elevation but did not affect the membrane potential. The inhibitory effect of ciproxifan on 4-AP-evoked glutamate release was prevented by the Gi/Go-protein inhibitor pertussis toxin and Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel blocker ω-conotoxin MVIIC, but was not affected by the intracellular Ca2 +-release inhibitors dantrolene and CGP37157. Furthermore, the phospholipase A2 (PLA2) inhibitor OBAA, prostaglandin E2 (PGE2), PGE2 subtype 2 (EP2) receptor antagonist PF04418948, and extracellular signal-regulated kinase (ERK) inhibitor FR180204 eliminated the inhibitory effect of ciproxifan on glutamate release. Ciproxifan reduced the 4-AP-evoked phosphorylation of ERK and synapsin I, a presynaptic target of ERK. The ciproxifan-mediated inhibition of glutamate release was prevented in synaptosomes from synapsin I-deficient mice. Moreover, ciproxifan reduced the frequency of miniature excitatory postsynaptic currents without affecting their amplitude in hippocampal slices. Our data suggest that ciproxifan, acting through the blockade of Gi/Go protein-coupled H3 receptors present on hippocampal nerve terminals, reduces voltage-dependent Ca2 + entry by diminishing PLA2/PGE2/EP2 receptor pathway, which subsequently suppresses the ERK/synapsin I cascade to decrease the evoked glutamate release.