Reduced Motivation in Perinatal Fluoxetine-Treated Mice: A Hypodopaminergic Phenotype

Abstract

Early life is a sensitive period, in which enhanced neural plasticity allows the developing brain to adapt to its environment. This plasticity can also be a risk factor in which maladaptive development can lead to long-lasting behavioral deficits. Here, we test how early-life exposure to the selective-serotonin-reuptake-inhibitor (SSRI), fluoxetine, affects motivation, and dopaminergic signaling in adulthood. We show for the first time that mice exposed to fluoxetine in the early postnatal period exhibit a reduction in effort-related motivation. These mice also show blunted responses to amphetamine and reduced dopaminergic activation in a sucrose reward task. Interestingly, we find that the reduction in motivation can be rescued in the adult by administering bupropion, a dopamine-norepinephrine reuptake inhibitor used as an antidepressant and a smoke cessation aid but not by fluoxetine. Taken together, our studies highlight the effects of early postnatal exposure of fluoxetine on motivation and demonstrate the involvement of the dopaminergic system in this process.SIGNIFICANCE STATEMENT The developmental period is characterized by enhanced plasticity. During this period, environmental factors have the potential to lead to enduring behavioral changes. Here, we show that exposure to the SSRI fluoxetine during a restricted period in early life leads to a reduction in adult motivation. We further show that this reduction is associated with decreased dopaminergic responsivity. Finally, we show that motivational deficits induced by early-life fluoxetine exposure can be rescued by adult administration of bupropion but not by fluoxetine.

Keywords: dopamine; fluoxetine; gestation; motivation; postnatal; serotonin.

Figure 1.

Novelty induced hypophagia. A, Naive, PN-saline-treated, and PN-fluoxetine-treated mice were food deprived overnight. The next day, they were placed for 30 min in a large arena (a standard rat cage) with a Petri dish containing 20 sucrose pellets. The number of pellets consumed was counted. PN-fluoxetine-treated animals ate fewer pellets than naive/saline control animals. B, Naive and PN-treated mice were food deprived overnight and given a Petri dish containing 20 sucrose pellets in their home cage. All groups consumed approximately the same number of pellets in 30 min (n = 10–19). C, Non-food-deprived mice were placed in a large arena containing a Petri dish with 20 sucrose pellets for 30 min. PN-fluoxetine-treated mice consumed less of this novel food than PN-saline-treated animals. The same group of animals were food deprived and given only sucrose pellets to eat for a day. The mice were then returned to their normal diet. PN-fluoxetine and PN-saline mice, non-food-deprived, consumed the same amount of sucrose pellets when presented again with this food (n = 14–22). Data are expressed as the mean ± SEM; *p < 0.05.

Figure 2.

Dopamine's role in effort-related motivation. A, Experimental design. Mice were trained on a FR1 schedule of reinforcement until reaching criteria for three consecutive days. They were then trained for an additional 3 d on the FR5 schedule. The following day, they were tested on the PR 1 h after receiving an injection of either saline-bupropion vehicle (VB), bupropion (B), haloperidol-vehicle (VH), or haloperidol (H). Treatments were repeated counterbalanced with 3-d intervals. After testing in PR, mice were tested in FR1 in response to the same drug treatments in counterbalanced order. B, Progression on the PR. C, Break point on PR schedule after adult treatment (n = 19 repeated measures). D, Sucrose rewards during FR1. Data are expressed as the mean ± SEM; *p < 0.05, ***p < 0.001.

Figure 3.

PN-fluoxetine treatment impairs effort-related motivation. A, Experimental design. PN-treated mice (FLX, fluoxetine; SAL, saline) were trained on a FR1 schedule of reinforcement until reaching criteria. They were then trained for an additional 3 d on the FR5 schedule. The following day, they were tested on the PR for 3 d. B, Days until criteria. C, Average number of rewards received on the last 3 d of FR1 training (in which they reached criteria). D, Average number of rewards received on 3 d of FR5. E, Average break point on PR schedule by sex. F, Average break point on PR schedule for both sexes. G, Average number of rewards obtained on PR schedule. H, Break point on PR in each individual trial (n_PN-saline = 14, n_{PN-fluoxetine} = 22). Data are expressed as the mean ± SEM; *p < 0.05.

Figure 4.

PN-fluoxetine treatment blunts amphetamine response. A, PN-treated mice were placed in an open-field and their locomotion was recorded in 1-min bins for a total of 60 min; 20 min after the beginning of the experiments, mice were injected with saline or 3 mg/kg of amphetamine. We observed blunted hyperlocomotion in response to an amphetamine challenge in PN-fluoxetine-treated mice (n_{PN-saline/saline} = 8, n_{PN-saline/amphetamine} = 7, n_{PN-fluoxetine/saline} = 8, n_{PN-fluoxetine/amphetamine} = 8). B, Microdialysis analysis of dopamine levels at baseline and after amphetamine challenge (arrow; 3 mg/kg, i.p.). Top right inset, Area under the curve (AUC) for preamphetamine and postamphetamine injection (n_PN-saline = 10, n_{PN-fluoxetine} = 10). C, Placements of microdialysis probes verified histologically. Data are expressed as the mean ± SEM; *p < 0.05, ***p < 0.001.

Figure 5.

PN-fluoxetine treatment blunts dopaminergic response to sucrose. A, Scheme of photometry apparatus with 405-nm control channel and 470-nm GCamp6 activity recording channel. B, Placement of the optic fiber probe tips. C, TH colocalization with GCamp6+ cells. D, There was a significantly higher increase in fluorescence during sucrose consumption in PN-saline animals than in PN-fluoxetine animals (n_PN-saline = 11, n_{PN-fluoxetine} = 7). E, Individual recordings of a PN-saline (top) and PN-fluoxetine (bottom) animal. Time 0 marks the first lick. F, Subject averaged traces. Data are expressed as the mean ± SEM; *p < 0.05.

Figure 6.

PN-fluoxetine impairment in effort-related motivation can be rescued in adulthood by bupropion. A, Experimental design. PN-treated mice were trained on a FR1 schedule of reinforcement until reaching criteria. They were then trained for an additional 3 d on the FR5 schedule. The following day, they were tested on the PR 1 h after receiving an injection of either saline (SAL), bupropion (BUP; dopamine-reuptake-inhibitor), or fluoxetine (FLX; SSRI) in a counterbalanced design. B, Break point on PR schedule after adult treatment (n_PN-saline = 19, n_{PN-fluoxetine} = 24). Data are expressed as the mean ± SEM; *p < 0.05, ***p < 0.001.