Chronic fluoxetine selectively upregulates dopamine D₁-like receptors in the hippocampus

Abstract

The dentate gyrus of the hippocampus has been implicated in mechanisms of action of selective serotonin reuptake inhibitors (SSRIs). We have recently demonstrated that the SSRI fluoxetine can reverse the state of maturation of the adult dentate granule cells and enhances serotonin 5-HT₄ receptor-mediated synaptic potentiation at the synapses formed by their mossy fiber axons. Here, we show that fluoxetine can induce long-lasting enhancement of dopaminergic modulation at the mossy fiber synapse. Synaptic responses arising from the mossy fiber-CA3 pyramidal cell synapse were recorded using acute mouse hippocampal slices. Dopamine potentiates mossy fiber synaptic transmission by activating D₁-like receptors. Chronic fluoxetine treatment induced a prominent increase in the magnitude of dopamine-induced synaptic potentiation, and this effect was maintained at least up to 1 month after withdrawal of fluoxetine. Quantitative autoradiography revealed that binding of the D₁-like receptor ligand [³H]SCH23390 was selectively increased in the dentate gyrus and along the mossy fiber in fluoxetine-treated mice. However, binding of the 5-HT₄ receptor ligand [³H]GR113808 was not significantly changed. These results suggest that chronic fluoxetine enhanced the dopaminergic modulation at least in part by upregulating expression of D₁-like receptors, while the enhanced serotonergic modulation may be mediated by modifications of downstream signaling pathways. These enhanced monoaminergic modulations would greatly increase excitatory drive to the hippocampal circuit through the dentate gyrus. The highly localized upregulation of D₁-like receptors further supports the importance of the dentate gyrus in the mechanism of action of SSRIs.

Figure 1

Chronic fluoxetine induces long-lasting enhancement of dopaminergic synaptic modulation. (a) Bath-applied dopamine induced reversible potentiation of mossy fiber synaptic transmission. The magnitude of potentiation was clearly increased in fluoxetine-treated mice (FLX) as compared with control mice (CNT). Sample traces show averages of 15 consecutive field excitatory postsynaptic potentials (fEPSPs) before and during dopamine application. Scale bar: 10 ms, 0.2 mV. (b) Effects of fluoxetine and paroxetine (PAR) on dopaminergic synaptic modulation. Dopamine-induced potentiation was significantly increased after 2 weeks (n=5, p<0.05) and 4 weeks (n=11, p<0.001, Bonferroni's multiple comparison test) of fluoxetine treatments at 22 mg/kg per day and 4 weeks of PAR treatment at 30 mg/kg per day (n=4, p<0.001), but not after 4 weeks of fluoxetine treatment at 14 mg/kg per day (n=8). (c) Dopamine-induced potentiation remained enhanced for at least 1 month after withdrawal of fluoxetine (n=6 each, p=0.0133). ^*p<0.05; ^***p<0.001.

Figure 2

Serotonin dependence of fluoxetine-induced enhancement of dopaminergic modulation. (a) Chronic fluoxetine significantly increased the magnitude of dopamine-induced potentiation in saline-injected mice (control mice (CNT): n=6, fluoxetine-treated mice (FLX): n=3, p<0.001, Bonferroni's multiple comparison test), but not in 5,7-dihydroxytryptamine (DHT)-injected mice (CNT: n=5, FLX: n=4). There was no significant difference between saline- and DHT-injected control groups. (b) Chronic fluoxetine significantly increased the magnitude of dopamine-induced potentiation in both wild-type (5-HT₄+/+, CNT: n=6, FLX: n=8, p<0.01) and mutant mice (5-HT₄-/-, CNT: n=10, FLX: n=9, p<0.05, Dunn's multiple comparison test). ^*p<0.05; ^**p<0.01; ^***p<0.001.

Figure 3

Selective increase in D₁-like ligand binding in dentate gyrus and CA3 in fluoxetine-treated mice (FLX). (a) Representative autoradiograms of [³H]SCH23390 binding at four different section levels. (b) Summary data showing significant increases in [³H]SCH23390 binding after fluoxetine treatment in CA3 (p=0.019) and dentate gyrus (p=0.0121) (control mice (CNT): n=6, FLX: n=7). DG, dentate gyrus; FC, frontal cortex; SN, substantia nigra. ^*p<0.05.

Figure 4

Reduced 5-HT₄ ligand binding in fluoxetine-treated mice (FLX). (a) Representative autoradiograms of [³H]GR113808 binding in the striatum, amygdala and hippocampus (indicated by arrows). (b) Fluoxetine significantly reduced [³H]GR113808 binding in amygdala (p=0.0043), striatum (p=0.0005) and substantia nigra (p=0.0239) (control mice (CNT): n=6, FLX: n=7). ^*p<0.05; ^**p<0.01; ^***p<0.001.

Figure 5

Reduced 5-HT_1A ligand binding in fluoxetine-treated mice (FLX). (a) Representative autoradiograms of [³H]8-OHDPAT binding. The arrow indicates the hippocampal CA1 region. (b) Fluoxetine significantly reduced [³H]8-OHDPAT binding in CA1 (p=0.007) (control mice (CNT): n=6, FLX: n=7). ^**p<0.01.