Lasting syndrome of depression produced by reduction in serotonin uptake during postnatal development: evidence from sleep, stress, and behavior

Abstract

Dysfunction of the serotonin system is implicated in sleep and emotional disorders. To test whether these impairments could arise during development, we studied the impact of early-life, transient versus genetic, permanent alterations of serotonin reuptake on sleep-wakefulness patterns, depression-related behavior, and associated physiological features. Here, we show that female mice treated neonatally with a highly selective serotonin reuptake inhibitor, escitalopram, exhibited signs of depression in the form of sleep anomalies, anhedonia, increased helplessness reversed by chronic antidepressant treatment, enhanced response to acute stress, and increased serotoninergic autoinhibitory feedback. This syndrome was not reproduced by treatment in naive adults but resembled the phenotype of mutant mice lacking the serotonin transporter, except that these exhibited decreased serotonin autoreceptor sensitivity and additional anxiety-like behavior. Thus, alteration of serotonin reuptake during development, whether induced by external or genetic factors, causes a depressive syndrome lasting into adulthood. Such early-life impairments might predispose individuals to sleep and/or mood disorders.

Figure 1.

Quantitative changes in sleep characteristics in adult mice that had received early-life treatment. WT mice had been treated with saline (WT) or the SSRI escitalopram (WTneoSSRI). Mice with a genetic deletion of the serotonin transporter had been treated with saline (KO). A, Examples of hypnograms from mice from three different groups. B, REMS total duration (left), number of episodes (center), and average latency from sleep onset (right). C, NREMS total duration (left), number (center), and mean duration (right) of episodes for eight WT, nine WTneoSSRI, and seven KO mice. Light phase, 7:00 A.M. to 7:00 P.M.; dark phase, 7:00 P.M. to 7:00 A.M. Values plotted are mean ± SEM *p < 0.05; **p < 0.01; ***p < 0.001, significant difference between groups.

Figure 2.

Alteration in sleep patterns and hormonal response after an acute stress in the mouse. A, Examples of hypnograms 12–14 h after stress (for the time period 7:00–9:00 A.M.) in mice from the same groups as in Figure 1. B, Time course of REMS duration after stress (90 min of immobilization). Results are expressed as a fraction of REMS duration in the sham condition for WT (dashed line; n = 5), WTneoSSRI (thin line; n = 5), and KO (bold line; n = 7) groups. Significance labels are as follows: (a), WT and WTneoSSRI significantly different from sham condition; (b), WT significantly different from sham condition; (c), WT and WTneoSSRI significantly different from KO. C, Serum corticosterone levels immediately after the end of the acute stress are expressed as a fraction of average serum corticosterone levels in sham conditions. Group sizes are as follows: WT: sham, n = 10; stress, n = 10; WTneoSSRI: sham, n = 8; stress, n = 10; KO: sham, n = 7; stress, n = 7. Values plotted are mean ± SEM *p < 0.05, significant difference between groups; ^#p < 0.05; ^##p < 0.01; ^###p < 0.001, significantly different from sham condition.

Figure 3.

Effect of 5-HT_1A receptor activation on core temperature. The time course of temperature (A) and maximal hypothermia (B) after 8-OH-DPAT injection (0.4 mg/kg, s.c.) is shown. The group labeling is as in Figure 1 (WT, n = 5; WTneoSSRI, n = 7; KO, n = 6). Values plotted are mean ± SEM, and significance is tested against the WT group. WTneoSSRI and KO are significantly different (p < 0.001) for the times 15–60 min. *p < 0.05; **p < 0.01; ***p < 0.001.

Figure 4.

Anxiety- and depression-related behavior in adult mice after transient neonatal and permanent genetic inactivation of the serotonin transporter. The group labeling is as in Figure 1 (WT, n = 12–14; WTneoSSRI, n = 24–26; KO, n = 16). A, Anxiety, measured in the dark–light box test, is expressed as time spent in the (anxiogenic) light box (for a test duration of 5 min). The elevated-plus maze graph displays the time spent in the (anxiogenic) open arm (left), for a test duration of 5 min, and the number of rears (reflecting the exploratory behavior) during the entire test (right). B, Depression-like behavior: anhedonia in the sucrose preference test (left). Sucrose solution consumption is reported as a percentage of the total liquid consumption. The immobility total duration (in seconds) in the TST (middle) and the FST (right) is shown. Values plotted are mean ± SEM *p < 0.05; **p < 0.01; ***p < 0.001, significant difference between groups.

Figure 5.

Effect of chronic fluoxetine treatment in adult WTneoSSRI mice. Immobility total duration is measured in WTneoSSRI mice receiving fluoxetine in drinking water (fluox; n = 14) or plain water (water; n = 9) in their bottle; the x-axis represents baseline (Bsl) tests and the days of test sessions (D7–D30) or of withdrawal (W10–W12). Values plotted are mean ± SEM. *p < 0.05, significant difference between groups; ^#p < 0.05, within-group significant difference with Bsl (paired test); °p < 0.05, within-group significant difference from Bsl and D30.

Figure 6.

Absence of long-term effect of a 2 week SSRI treatment with escitalopram performed at adulthood. The tests were started 6 weeks after the end of treatment. veh, Vehicle. A, Sucrose preference expressed as a fraction of total liquid intake (WT adult veh, n = 7; WT adult SSRI, n = 8). B, Immobility time in the TST (WT adult veh, n = 11; WT adult SSRI, n = 12) and FST (WT adult veh, n = 7; WT adult SSRI, n = 8). C, REMS and NREMS duration expressed as a fraction of time spent in this state (WT adult veh, n = 6; WT adult SSRI, n = 6). Light phase, 7:00 A.M. to 7:00 P.M.; dark phase, 7:00 P.M. to 7:00 A.M. D, Increase in serum corticosterone levels immediately after an acute stress expressed as a fraction of basal corticosterone in sham conditions (in each condition: WT adult veh, n = 6; WT adult SSRI, n = 7); ^###p < 0.001, significantly different from sham condition. E, Maximal hypothermia after 8-OH-DPAT injection (0.4 mg/kg, s.c.; WT adult veh, n = 5; WT adult SSRI, n = 6). Values plotted are mean ± SEM.