Inhibition of serotonin but not norepinephrine transport during development produces delayed, persistent perturbations of emotional behaviors in mice

Abstract

Serotonin (5-HT) acts as a neurotransmitter, but also modulates brain maturation during early development. The demonstrated influence of genetic variants on brain function, personality traits, and susceptibility to neuropsychiatric disorders suggests a critical importance of developmental mechanisms. However, little is known about how and when developmentally perturbed 5-HT signaling affects circuitry and resulting behavior. The 5-HT transporter (5-HTT) is a key regulator of extracellular 5-HT levels and we used pharmacologic strategies to manipulate 5-HTT function during development and determine behavioral consequences. Transient exposure to the 5-HTT inhibitors fluoxetine, clomipramine, and citalopram from postnatal day 4 (P4) to P21 produced abnormal emotional behaviors in adult mice. Similar treatment with the norepinephrine transporter (NET) inhibitor, desipramine, did not adversely affect adult behavior, suggesting that 5-HT and norepinephrine (NE) do not share the same effects on brain development. Shifting our period of treatment/testing to P90/P185 failed to mimic the effect of earlier exposure, demonstrating that 5-HT effects on adult behavior are developmentally specific. We have hypothesized that early-life perturbations of 5-HT signaling affect corticolimbic circuits that do not reach maturity until the peri-adolescent period. In support of this idea, we found that abnormal behaviors resulting from postnatal fluoxetine exposure have a post-pubescent onset and persist long after reaching adult age. A better understanding of the underlying 5-HT sensitive circuits and how they are perturbed should lead to new insights into how various genetic polymorphisms confer their risk to carriers. Furthermore, these studies should help determine whether in utero exposure to 5-HTT blocking drugs poses a risk for behavioral abnormalities in later life.

Figure 1.

Fluoxetine and norfluoxetine levels in serum and brain of PN-fluoxetine treated mice. A, Serum fluoxetine levels. B, Serum norfluoxetine levels. C, Brain fluoxetine levels. D, Brain norfluoxetine levels. Solid lines and filled circles indicate that mice received PN-fluoxetine treatment. Dashed lines and open circles indicate cessation/ending of PN-fluoxetine treatment. Mean ± SEM values are shown (n = 4–5 per data point).

Figure 2.

Exploratory behavior in the novel open field and the elevated plus maze of PN-antidepressant- and PN-vehicle-treated mice. A, Total ambulatory time in the novel open field (OF). B, Total vertical activity in the novel open field. C, Total arm entries in the elevated plus maze (EPM). VEH, PN-vehicle; FLX, PN-fluoxetine; CMI, PN-clomipramine; CTM, PN-citalopram; DMI, PN-desipramine. Mean ± SEM values are shown (n = 40–65 per group). *p < 0.05; **p < 0.01; ***p < 0.001 compared with PN-vehicle.

Figure 3.

Behavioral measures of PN-antidepressant and PN-vehicle treated mice in hyponeophagia tests. A–C, NSF. A, Latency to feed; B, home-cage food consumption; C, weight loss. D–F, NIH. D, Latency to drink; E, inhibition; F, home-cage food consumption. VEH, PN-vehicle; FLX, PN-fluoxetine; CMI, PN-clomipramine; CTM, PN-citalopram; DMI, PN-desipramine. Mean ± SEM values are shown (n = 32–58 per group). *p < 0.05; **p < 0.01; ***p < 0.001 compared with PN-vehicle.

Figure 4.

Behavioral stress response of PN-antidepressant- and PN-vehicle-treated mice. Latency to escape in the shock escape test (SE). VEH, PN-vehicle; FLX, PN-fluoxetine; CMI, PN-clomipramine; CTM, PN-citalopram; DMI, PN-desipramine. Mean ± SEM values are shown (n = 33–55 per group). *p < 0.05; **p < 0.01 compared with PN-vehicle.