Knocking out the dopamine reuptake transporter (DAT) does not change the baseline brain arachidonic acid signal in the mouse

Abstract

Background: Dopamine transporter (DAT) homozygous knockout (DAT(-/-)) mice have a 10-fold higher extracellular (DA) concentration in the caudate-putamen and nucleus accumbens than do wildtype (DAT(+/+)) mice, but show reduced presynaptic DA synthesis and fewer postsynaptic D(2) receptors. One aspect of neurotransmission involves DA binding to postsynaptic D(2)-like receptors coupled to cytosolic phospholipase A(2) (cPLA(2)), which releases the second messenger, arachidonic acid (AA), from synaptic membrane phospholipid. We hypothesized that tonic overactivation of D(2)-like receptors in DAT(-/-) mice due to the excess DA would not increase brain AA signaling, because of compensatory downregulation of postsynaptic DA signaling mechanisms.

Methods: [1-(14)C]AA was infused intravenously for 3 min in unanesthetized DAT(+/+), heterozygous (DAT(+/-)), and DAT(-/-) mice. AA incorporation coefficients k* and rates J(in), markers of AA metabolism and signaling, were imaged in 83 brain regions using quantitative autoradiography; brain cPLA(2)-IV activity also was measured.

Results: Neither k* nor J(in) for AA in any brain region, or brain cPLA(2)-IV activity, differed significantly among DAT(-/-), DAT(+/-), and DAT(+/+) mice.

Conclusions: These results differ from reported increases in k* and J(in) for AA, and in brain cPLA(2) expression, in serotonin reuptake transporter (5-HTT) knockout mice, and suggest that postsynaptic dopaminergic neurotransmission mechanisms involving AA are downregulated despite elevated DA in DAT(-/-) mice.

Figure 1. Coronal autoradiographs of brain showing regional AA incorporation coefficients k* in mice of different genotypes

Values of k* (ml/s/g brain × 10⁻⁴) are on a color scale from 6 (blue) to 20 (orange). Acg, anterior cingulate cortex; Cb, cerebellar gray matter; CPu, caudate putamen; Hipp, hippocampus; Mot, motor cortex.

Figure 2. Brain cPLA₂-IV activity in relation to DAT genotype

Data are mean ± SD (n = 5–7). No significant genotype effect was observed in the one-way ANOVA.