Serotonin clearance in vivo is altered to a greater extent by antidepressant-induced downregulation of the serotonin transporter than by acute blockade of this transporter

Abstract

Serotonin uptake, mediated by the serotonin transporter (SERT), is blocked acutely by antidepressants such as the selective serotonin reuptake inhibitors (SSRIs), but such blockade does not correlate temporally with the onset of therapeutic improvement. Treatment with SSRIs for 21 d induced downregulation of the SERT (Benmansour et al., 1999). The time course of SERT downregulation as well as the time course for its recovery after cessation of treatment with the SSRI sertraline were investigated using tritiated cyanoimipramine to measure SERT binding sites. To determine if there was a temporal correlation between the time when sertraline induced downregulation of the SERT and when marked alteration in SERT function occurred, clearance of locally applied 5-HT into the CA3 region of hippocampus was achieved using in vivo electrochemistry. After 4 or 10 d treatment with sertraline, SERT binding sites decreased very little (15-30%), and the chronoamperometric signals for serotonin in sertraline-treated rats were comparable with ones obtained in control animals. By contrast, after 15 d of treatment, when SERT binding sites were markedly reduced by 80%, there was robust decrease in the clearance of 5-HT. Moreover, the functional consequences of SERT downregulation as measured by chronoamperometry were significantly greater than those seen after acute blockade of the SERT by SSRIs. SERT binding sites decreases are not a consequence of reduced SERT gene expression, as revealed by in situ hybridization measurements. SSRI-induced downregulation of the SERT may be a key component for the clinical response to SSRIs.

Fig. 1.

Time course of the reduction (A) and recovery (B) of the binding of [³H]-CN-IMI (1 nm) after sertraline treatment. To study the onset of effects (A), rats were treated by osmotic minipump with sertraline (7.5 mg · kg⁻¹ · d⁻¹, s.c.) or vehicle (for control rats) for 4, 10, 15, or 21 d, always followed by a 2 d washout. To study the time course of recovery (B), rats were treated for 21 d with sertraline or vehicle followed by 2, 6, 8, 10, or 16 d of washout. Serotonin uptake sites were measured using quantitative autoradiography for [³H]-cyanoimipramine binding, as described in Materials and Methods. Results obtained were similar throughout the brain. Shown for illustration are results obtained in the CA3 region of the hippocampus, the parietal cortex (Par. CTX), and the basolateral amygdaloid nucleus posterior (BLP). The number of animals in each drug-treated group was four; the control group included eight animals. *p < 0.05 comparison of each time point for the treatment group with the corresponding time point of the control group. ANOVA, followed by Newman–Keulspost hoc comparison.

Fig. 2.

Representative 5-HT electrochemical signals from the CA3 region of the dorsal hippocampus in a control rat (dashed lines) and in one treated with sertraline for 15 d (solid lines). In vivochronoamperomeric measurements were performed, as described in Materials and Methods, in rats treated with sertraline by osmotic minipump (7.5 mg · kg⁻¹ · d⁻¹, s.c.) for 15 d followed by a 2 d washout. Once reproducible electrochemical signals from 5-HT were obtained, the response to three different amounts of 5-HT (5.2, 10.4, 15.6 pmol) was tested in each drug-treated or control rat. For clarity, only oxidation current curves are shown. Each amount of serotonin produced a greater response in the sertraline-treated rat than in the control one.

Fig. 3.

Effect of increasing the amount of 5-HT on the serotonin signal of control and sertraline-treated rats. Rats were treated with vehicle or sertraline for 4, 10, or 15 d followed by 2 d of washout, as described in Materials and Methods. Several parameters are obtained from the electrochemical signal in response to local application of equal amounts of 5-HT (5.2, 10.4, or 15.6 pmol) into the CA3 region of the hippocampus of these rats. Parameters analyzed here were signal amplitude, T80, the time it takes for the peak amplitude to be reduced by 80%; total time course (t-course), the total time for the signal to return to baseline from the time of application of 5-HT. Each point represents the means ± SEM of an n of five to eight rats. *p < 0.05 comparison of each time point in the treatment groups with the corresponding time in the control group; ANOVA, followed by Newman–Keuls post hoccomparison.

Fig. 4.

Time course of the effect of sertraline treatment and cessation of treatment on the density and mRNA levels of the SERT in the dorsal raphe nucleus. SERT density (solid line) was measured by quantitative autoradiography of [³H]-CN-IMI (1 nm) binding, and mRNA levels for the SERT (dashed line) were measured byin situ hybridization in the same group of animals treated with sertraline, as described in Materials and Methods. Each point is expressed as a percentage of respective control values ± SEM of an N of 4–11 rats. SEM for percentage of control binding is shown by the dashed horizontal lines, and the one for percentage of control mRNA is shown by the solid horizontal lines. Control values for [³H]-CN-IMI in the DRN were 3097 ± 70 fmol/mg of protein, and the mean integrated density for mRNA levels in control rats was 2.27 ± 0.3 nCi/mg. The x-axis of the graph represents the number of days of treatment plus the washout time. Up to 23 d, this represents a variable number of treatment days plus a fixed 2 d washout time. After day 23, the numbers represent a fixed 21 d treatment plus a variable number of washout days. *p < 0.05 comparison of each time point of treatment with the appropriate time point of the control group; ANOVA followed by Newman–Keuls post hoccomparisons.