Neonatal antidepressant exposure has lasting effects on behavior and serotonin circuitry

Abstract

A significant fraction of infants born to mothers taking selective serotonin reuptake inhibitors (SSRIs) during late pregnancy display clear signs of antidepressant withdrawal indicating that these drugs can penetrate fetal brain in utero at biologically significant levels. Previous studies in rodents have demonstrated that early exposure to some antidepressants can result in persistent abnormalities in adult behavior and indices of monoaminergic activity. Here, we show that chronic neonatal (postnatal days 8-21) exposure to citalopram (5 mg/kg, twice daily, s.c.), a potent and highly selective SSRI, results in profound reductions in both the rate-limiting serotonin synthetic enzyme (tryptophan hydroxylase) in dorsal raphe and in serotonin transporter expression in cortex that persist into adulthood. Furthermore, neonatal exposure to citalopram produces selective changes in behavior in adult rats including increased locomotor activity and decreased sexual behavior similar to that previously reported for antidepressants that are nonselective monoamine transport inhibitors. These data indicate that the previously reported neurobehavioral effects of antidepressants are a consequence of their effects on the serotonin transporter. Moreover, these data argue that exposure to SSRIs at an early age can disrupt the normal maturation of the serotonin system and alter serotonin-dependent neuronal processes. It is not known whether this effect of SSRIs is paralleled in humans; however, these data suggest that in utero, exposure to SSRIs may have unforeseen long-term neurobehavioral consequences.

Figure 1

Effect of neonatal exposure to saline (SAL; n=6), citalopram (CTM; n=5), and clomipramine (CMI; n=6) on locomotor activity: (a, b) distance traveled; (c) ambulatory time. Data represent the mean±SEM. Single factor analysis of variance (ANOVA) followed by LSD test (**P<0.01; *P<0.05 vs SAL).

Figure 2

Effect of neonatal exposure to saline (SAL; n=6), citalopram (CTM; n=5), and clomipramine (CMI; n=6) on male sexual behavior: (a) Kaplan–Meier survival analysis of mounting behavior using Log-Rank statistic, SAL vs CTM=10.26, P=0.0014; SAL vs CMI=4.37, P=0.0365. (b) Number of intromissions and ejaculations; ANOVA followed by LSD test (*P<0.05 vs SAL).

Figure 3

Effect of neonatal exposure to saline (SAL), citalopram (CTM), and clomipramine on body weight. Data represent the mean±SEM.

Figure 4

Representative photomicrographs through the DR show the caudal to rostral distribution of TPH-immunoreactive cells in untreated (a–c), clomipramine (CMI; d–f)-, and citalopram (CTM; g–i)-treated subjects at PN22. Note that reduced TPH expression was detected along the entire extent of the DR neuraxis (arrows), but was particularly evident within midline subregions. The median raphe, also depicted here, shows similar reductions in TPH immunoreactivity (asterisk). Scale bar=100 μm.

Figure 5

Representative photomicrographs of TPH immunoreactivity with CY3 in the DR nucleus of saline (a)- and citalopram (CTM; b)-treated (PN8–21) rats at PN130.

Figure 6

SERT expression in medial prefrontal cortex (layer II/III) of PN22 rats following saline (a), clomipramine (CMI; b), and citalopram (CTM; c) treatment from PN8 to PN21. (a–c) Illustrate immunoreactive fiber networks as visualized with CY3, while (d–f) demonstrate changes in the morphology of individual axons. Immediately following neonatal CTM treatment, the innervation density of SERT-positive processes is drastically reduced (c), and fibers exhibit a beaded appearance (f). Discontinuity of the axon is marked by a lack of immunolabeling within intervaricose segments. Arrows in (f) depict a trail of seemingly ‘isolated’ synaptic boutons. Scale bar: a–c=100 μm; d–f=20 μm.

Figure 7

Representative photomicrographs depict SERT terminal expression in the medial prefrontal cortex (layer II/III) of PN130 rats following saline (a) and citalopram (CTM; b) treatment from PN8 to PN21. (a, b) Illustrate immunoreactive fiber networks as visualized with CY3. Scale bar=100 μm.

Figure 8

Photomicrograph taken at PN22 following neonatal citalopram treatment demonstrates TPH-labeled cells in DR that colocalize the neuron-specific marker, NeuN. Taken together with insets (a) and (b) of the same tissue section, which show single-labeled TPH (red, CY3) and viable NeuN-positive (green, CY2) cells, this figure illustrates that citalopram-related reductions in TPH are not due to cell death. As can be seen from surveys of individual cells in (a), treatment produces a nonuniform decrease in the expression of TPH. NeuN staining, however, remains prominent (arrows), despite these reductions.