Imaging Elevated Brain Arachidonic Acid Signaling in Unanesthetized Serotonin Transporter (5-HTT)-Deficient Mice

Abstract

Certain polymorphisms reduce serotonin (5-HT) reuptake transporter (5-HTT) function and increase susceptibility to psychiatric disorders. Heterozygous (5-HTT^+/−)-deficient mice, models for humans with these polymorphisms, have elevated brain 5-HT concentrations and behavioral abnormalities. As postsynaptic 5-HT_2A/2C receptors are coupled to cytosolic phospholipase A₂ (cPLA₂), which releases arachidonic acid (AA) from membrane phospholipid, 5-HTT-deficient mice may have altered brain AA signaling and metabolism. To test this hypothesis, signaling was imaged as an AA incorporation coefficient k^* in unanesthetized homozygous knockout (5-HTT^−/−), 5-HTT^+/− and wild-type (5-HTT^+/+), mice following saline (baseline) or 1.5 mg/kg s.c. DOI, a partial 5-HT_2A/2C receptor agonist. Enzyme activities, metabolite concentrations, and head-twitch responses to DOI were also measured. Baseline k^* was widely elevated by 20–70% in brains of 5-HTT^+/− and 5-HTT^−/− compared to 5-HTT^+/+ mice. DOI increased k^* in 5-HTT^+/+ mice, but decreased k^* in 5-HTT-deficient mice. Brain cPLA₂ activity was elevated in 5-HTT-deficient mice; cyclooxygenase activity and prostaglandin E₂ and F_2α and thromboxane B₂ concentrations were reduced. Head-twitch responses to DOI, although robust in 5-HTT^+/+ and 5-HTT^+/− mice, were markedly fewer in 5-HTT^−/− mice. Pretreatment with para-chlorophenylalanine, a 5-HT synthesis inhibitor, restored head twitches in 5-HTT^−/− mice to levels in 5-HTT^+/+ mice. We propose that increased baseline values of k^* in 5-HTT-deficient mice reflect tonic cPLA₂ stimulation through 5-HT_2A/2C receptors occupied by excess 5-HT, and that reduced k^* and head-twitch responses to DOI reflected displacement of receptor-bound 5-HT by DOI with a lower affinity. Increased baseline AA signaling in humans having polymorphisms with reduced 5-HTT function might be identified using positron emission tomography.