Effects of chronic antidepressant treatments on serotonin transporter function, density, and mRNA level

Abstract

To investigate functional changes in the brain serotonin transporter (SERT) after chronic antidepressant treatment, several techniques were used to assess SERT activity, density, or its mRNA content. Rats were treated by osmotic minipump for 21 d with the selective serotonin reuptake inhibitors (SSRIs) paroxetine or sertraline, the selective norepinephrine reuptake inhibitor desipramine (DMI), or the monoamine oxidase inhibitor phenelzine. High-speed in vivo electrochemical recordings were used to assess the ability of the SSRI fluvoxamine to modulate the clearance of locally applied serotonin in the CA3 region of hippocampus in drug- or vehicle-treated rats. Fluvoxamine decreased the clearance of serotonin in rats treated with vehicle, DMI, or phenelzine but had no effect on the clearance of serotonin in SSRI-treated rats. SERT density in the CA3 region of the hippocampus of the same rats, assessed by quantitative autoradiography with tritiated cyanoimipramine ([(3)H]CN-IMI), was decreased by 80-90% in SSRI-treated rats but not in those treated with phenelzine or DMI. The serotonin content of the hippocampus was unaffected by paroxetine or sertraline treatment, ruling out neurotoxicity as a possible explanation for the SSRI-induced decrease in SERT binding and alteration in 5-HT clearance. Levels of mRNA for the SERT in the raphe nucleus were also unaltered by chronic paroxetine treatment. Based on these results, it appears that the SERT is downregulated by chronic administration of SSRIs but not other types of antidepressants; furthermore, the downregulation is not caused by decreases in SERT gene expression.

Fig. 1.

Representative electrochemical signals generated by the local application of the same amount of 5-HT (24 pmol) into the CA3 region of a control (solid line) or a paroxetine-treated rat (dashed line). For clarity, only the oxidation current curves are shown.

Fig. 2.

Representative 5-HT electrochemical signals from the CA3 region of dorsal hippocampus in a control (A) and a paroxetine-treated rat (B) before administration of fluvoxamine (solid line). The signal was generated by local application of 5-HT. The amount of 5-HT applied was 26 pmol for the control rat and 8 pmol for the paroxetine rat. The effect of local application of fluvoxamine on 5-HT clearance is illustrated by thedashed line. Fluvoxamine was pressure-ejected 60–90 sec before the next application of 5-HT. For clarity, only the oxidation current curves are shown.

Fig. 3.

Effects of antidepressants on fluvoxamine-induced changes in the T80 parameter of 5-HT clearance. Electrochemical recordings were performed in the CA3 region of dorsal hippocampus of rats treated for 21 d with paroxetine (PRX; 5 or 10 mg), sertraline (SRTL), desipramine (DMI), phenelzine (PHEN), or vehicle (CTR). Fluvoxamine was pressure-ejected 60–90 sec before the second application of 5-HT. Barsand brackets represent mean ± SEM. The number of animals in each group is indicated at the bottom of each bar. *p < 0.01 comparison of each treatment group with control group, ANOVA, Newman–Keuls post hoccomparison.

Fig. 4.

Representative autoradiograms of [³H]CN-IMI (1 nm) binding in coronal rat brain sections taken at the level of plate 33 of the atlas ofPaxinos and Watson (1986) of control and drug-treated rats. Nonspecific binding defined with sertraline amounted to <5% of total binding.

Fig. 5.

Effects of antidepressants on [³H]CN-IMI binding (1 nm) in the CA3 region. Values are expressed as the percentage of the control value (100%), which is 351 ± 42 fmol/mg protein (n= 15). Bars and brackets represent mean ± SEM. The number of animals per drug-treated group is indicated at the bottom of each bar. *p < 0.01 comparison of each treatment group with control group, ANOVA, Newman–Keuls post hoccomparison.

Fig. 6.

Serotonin transporter messenger RNA in the raphe nuclei of rats detected by in situ hybridization in a control and paroxetine-treated rat (10 mg/kg). Coronal brain sections were taken at the level of plate 49 of the atlas of Paxinos and Watson (1986). Different levels of labeling were seen in the dorsomedial, ventromedial, and lateral portions of the dorsal raphe nucleus (DRN) complex. High labeling is also found in the median raphe nucleus (MRN).

Fig. 7.

Effect of chronic paroxetine treatment (10 mg · kg⁻¹ · d⁻¹) on the mRNA levels of the SERT in the raphe nuclei. Values for the dorsal raphe nucleus (DRN) and median raphe nucleus (MRN) represent mean integrated density ± SEM. Number of animals is indicated at the bottom of each bar.