Fluoxetine Exerts Age-Dependent Effects on Behavior and Amygdala Neuroplasticity in the Rat

The selective serotonin reuptake inhibitor (SSRI) Prozac® (fluoxetine) is the only registered antidepressant to treat depression in children and adolescents. Yet, while the safety of SSRIs has been well established in adults, serotonin exerts neurotrophic actions in the developing brain and thereby may have harmful effects in adolescents. Here we treated adolescent and adult rats chronically with fluoxetine (12 mg/kg) at postnatal day (PND) 25 to 46 and from PND 67 to 88, respectively, and tested the animals 7–14 days after the last injection when (nor)fluoxetine in blood plasma had been washed out, as determined by HPLC. Plasma (nor)fluoxetine levels were also measured 5 hrs after the last fluoxetine injection, and matched clinical levels. Adolescent rats displayed increased behavioral despair in the forced swim test, which was not seen in adult fluoxetine treated rats. In addition, beneficial effects of fluoxetine on wakefulness as measured by electroencephalography in adults was not seen in adolescent rats, and age-dependent effects on the acoustic startle response and prepulse inhibition were observed. On the other hand, adolescent rats showed resilience to the anorexic effects of fluoxetine. Exploratory behavior in the open field test was not affected by fluoxetine treatment, but anxiety levels in the elevated plus maze test were increased in both adolescent and adult fluoxetine treated rats. Finally, in the amygdala, but not the dorsal raphe nucleus and medial prefrontal cortex, the number of PSA-NCAM (marker for synaptic remodeling) immunoreactive neurons was increased in adolescent rats, and decreased in adult rats, as a consequence of chronic fluoxetine treatment. No fluoxetine-induced changes in 5-HT1A receptor immunoreactivity were observed. In conclusion, we show that fluoxetine exerts both harmful and beneficial age-dependent effects on depressive behavior, body weight and wakefulness, which may relate, in part, to differential fluoxetine-induced neuroplasticity in the amygdala

Effect of fluoxetine on body weight in adolescent and adult rats.

<p>Data are presented as mean ± S.E.M. body weight (g) in adolescent rats (A) and adult (B) rats (n = 10). 21 Days of fluoxetine treatment had no effect on bodyweight of adolescent rats, but reduced bodyweight in adult rats. ^*p<0.05 fluoxetine <i>versus</i> methylcellulose in age group.</p

Effect of fluoxetine treatment on the awake state, non-REM sleep, spindles, and REM sleep in adolescent and adult rats.

<p>Data are presented as mean ± S.E.M. minutes time spent in the awake state (A), non-REM sleep (B), spindles (C), and REM sleep (D) (n = 4–7). These states were measured at four time intervals: 17.30–18.30 p.m., 19.30–20.30 p.m., 5.30–6.30 a.m. and 7.30–8.30 a.m. Rats were housed under a reversed 12 hr day/light cycle, with lights on at 19.00 p.m. 14–17 Days following chronic fluoxetine treatment (12 mg/kg) wakefulness was increased and non-REM sleep was decreased in adult, but not adolescent rats during the 17.30–18.30 p.m. interval. ^∧p<0.05 age x treatment interaction; ^ap<0.05 age effect; ^*p<0.05 fluoxetine effect significantly different from methylcellulose effect.</p

Effect of fluoxetine treatment on anxiety in adolescent and adult rats.

<p>Data are presented as mean ± S.E.M. of time spent in the open of the elevated plus maze (n = 10). 10 Days following chronic fluoxetine treatment (12 mg/kg) anxiety in the elevated plus maze test was increased in both adolescent and adult rats. ^#p<0.05 main treatment effect.</p

Effect of fluoxetine on exploratory behavior in adolescent and adult rats.

<p>Data are presented as mean ± S.E.M. of distance moved within 60 min (n = 10). 7 Days following chronic fluoxetine treatment (12 mg/kg) exploratory behavior in the open field was not affected in adolescent, nor adult, rats. ^ap<0.05 main age effect.</p

Time schedule of experiments.

<p>Each group consisted of adolescent and adult rats treated with either fluoxetine or methylcellulose.</p

Effect of fluoxetine treatment on behavioral despair in adolescent and adult rats.

<p>Data are presented as mean ± S.E.M. of time spent on immobility (n = 10). 10 Days following chronic fluoxetine treatment (12 mg/kg) behavioral despair, expressed as immobility in the forced swim test, was increased in adolescent, but unaffected in adult rats. ^ap<0.05 main age effect; ^∧p<0.05 age x treatment interaction; ^*p<0.05 fluoxetine <i>versus</i> methylcellulose in age group.</p

Fluoxetine (ng/ml) and norfluoxetine (ng/ml) levels in blood plasma.

<p>Fluoxetine (ng/ml) and norfluoxetine (ng/ml) levels in blood plasma.</p

Effect of fluoxetine treatment on the startle reflex and PPI in adolescent and adult rats.

<p>Data are presented as mean ± S.E.M. of the startle reflex (n = 10; A) and PPI (n = 9; B). 7 Days following chronic fluoxetine treatment (12 mg/kg) the acoustic startle response was reduced in adult, but not adolescent rats. Fluoxetine had no significant effects on PPI. ^ap<0.05 main age effect; ^#p<0.05 main treatment effect; ^*p<0.05 fluoxetine <i>versus</i> methylcellulose in age group.</p

PSA-NCAM immunoreactivity in the dorsal raphe nucleus, mPFC, and amygdala of fluoxetine and methylcellulose treated adolescent and adult rats.

<p>Data are presented as mean ± S.E.M. of the numer of immunoreactive neurons in the dorsal raphe nucleus (A), mPFC (B), and amygdala (C) (n = 4–5) per 100×100 µm. 14–17 Days following chronic fluoxetine (12 mg/kg) treatment the number of PSA-NCAM immunoreactivity was lower in adult compared to adolescent rats, but only in the dorsal raphe nucleus. In addition, we obtained a significant age x treatment interaction for PSA-NCAM immunoreactivity in the amygdala, which tended to be increased in adolescent, and decreased in adult rats. ^ap<0.05 main age effect; ^∧p<0.05 age x treatment interaction.</p