Research reportContrasting regional Fos expression in adolescent and young adult rats following acute administration of the antidepressant paroxetine
Introduction
The selective serotonin reuptake inhibitors (SSRIs) have dominated the antidepressant market since the late 1990s due to their efficacy in the treatment of depression (Cipriani et al., 2009) and anxiety disorders (Bandelow et al., 2012), and improved side effect and safety profile compared to the older tricyclic antidepressants and monoamine oxidase inhibitors (Dodd et al., 2011). SSRIs are also widely used in the treatment of child and adolescent depression and anxiety (Karanges et al., 2014), despite questions about their safety and efficacy in these populations. SSRIs have also been associated with a small but significant increase in the risk of self-harm, suicidal ideation and behavior in young people (Hetrick et al., 2012). Such findings led the US Food and Drug Administration to introduce a black box warning on all SSRIs, notifying consumers and practitioners of this risk in individuals up to the age of 25. While these adverse effects occur with many SSRIs (Bridge et al., 2007, Hammad et al., 2006), paroxetine (PRX) is arguably the worst offender (Robertson and Allison, 2009).
The mechanisms underlying the differential response to SSRIs in children and adolescents compared to adults are poorly understood. SSRIs directly affect the serotonin (5-hydroxytryptamine; 5-HT) system, blocking the serotonin transporter and increasing the level of 5-HT in the synapse, yet they also have various other neural effects likely related to their therapeutic actions. In adults, SSRI treatment results in desensitization and/or downregulation of various 5-HT receptor subtypes (Piñeyro and Blier, 1999), alterations in corticosterone concentrations and hypothalamic-pituitary-adrenal (HPA) axis activity (Schüle, 2007), changes in dopaminergic markers (Hamon and Blier, 2013), stimulation of neurogenesis (Drzyzga et al., 2009), and changes in proteins implicated in intracellular signaling pathways (Karanges et al., 2013). However, these effects have been largely determined following SSRI administration to adult animals and human subjects, and therefore may not accurately reflect those occurring in the developing brain.
The brain undergoes extensive reorganization and growth through childhood, adolescence and into early adulthood. Changes are apparent in neurotransmitter synthesis, turnover and receptor density, maturation of higher cortical regions, elevated synaptic plasticity, and remodeling and strengthening of connections between cortical and limbic areas (Crews et al., 2007, Spear, 2007). Maturational changes to the serotonin system occur throughout the juvenile period and into early adulthood. Specifically, serotonin levels and 5‐HT1A and 5-HT2A receptor densities are elevated in the adolescent brain, while serotonin transporter density and turnover is lower and increases toward adult levels as adolescence progresses. There is evidence for altered interaction between neurotransmitter systems during adolescence, particularly with respect to communication between the dopamine and serotonin systems. This has been proposed as a potential contributor to the atypical response of some adolescents to antidepressants (Karanges and McGregor, 2011).
In addition, there are significant developmental shifts in HPA axis reactivity, the release of glucocorticoids and other stress hormones, and the maturational state of regions involved in HPA axis modulation such as the amygdala, hippocampus and prefrontal cortex (Romeo, 2010). Studies using adolescent animals suggest an interaction between these developmental processes and the neural effects of SSRI treatment, particularly within the monoaminergic systems and in the regulation of cellular signaling and neurogenesis (Homberg et al., 2011, Karanges et al., 2013, Karanges and McGregor, 2011).
The adolescent period in rats spans from approximately post-natal day (PND) 28–55 and is characterized by behavioral and neural changes closely resembling those occurring in humans (Spear, 2007). Direct comparison between developmental age in rodents and humans is unclear. The peripubertal period in rats (PND 28–37) corresponds to childhood and the periadolescent period in humans; approximately 10–14 years of age (Andersen, 2003, Spear, 2007). In this study, we chose PND 28 and 56 as comparator age groups which can be viewed as assessing rats during the early juvenile stage of development and the late adolescent/early adult phase of development (age 18–22 years). We describe these two groups as adolescent and young adult rats throughout.
Expression patterns of the protein c-Fos, a marker of neural activation, can provide insight into the brain regions affected by administration of drugs and other stimuli. Previous studies have shown the validity of c-fos expression to identify regions activated by broad classes of psychotropic drugs, including antidepressants such as imipramine, nortriptyline, tranylcypromine, citalopram, fluoxetine, mirtazapine and lithium chloride (Sumner et al., 2004). Although variations exist between different antidepressants, acute treatment of adult rodents with SSRIs reliably induces expression of c-Fos in limbic regions implicated in anxiety and emotion regulation, including the central amygdala (CEA) and bed nucleus of the stria terminalis (BNST) (Sumner et al., 2004, Veening et al., 1998). In addition, activation of cortical, brainstem, noradrenergic and dopaminergic regions have been variously observed (Slattery et al., 2005, Sumner et al., 2004). However, these compelling findings have been limited to adult rodents and to our knowledge only one previous study has compared the acute effects of an SSRI (fluoxetine, 10 mg/kg) on c-Fos expression in adult and adolescent rats (Arrant et al., 2013). However, this study examined c-Fos expression in a limited number of brain regions.
Previously, we showed that chronic PRX had different behavioral and neurochemical effects in adolescents compared to adult rats (Karanges et al., 2011). PRX had greater adverse effects in adolescents with little to no accompanying antidepressant efficacy. Therefore, in order to further investigate the effects of SSRIs on the developing brain, the current study compared the pattern of c-Fos expression in response to acute PRX administration in adolescent and young adult rats, where differences in pharmacokinetics, pharmacodynamics and neural maturation may moderate the drug response. In addition, plasma corticosterone and PRX were measured to assess differences in HPA axis activity and pharmacokinetics, respectively.
Section snippets
Subjects
The subjects were 16 adolescent (PND 28) and 12 young adult (PND 56 on test day) male albino Wistar rats (Rattus norvegicus) (Animal Resource Centre, Perth, Australia). Rats were weaned on PND 21 and delivered to the laboratory the next day. Rats of similar age were housed in groups of two in a temperature- (22 ± 2 °C) and humidity-controlled colony room with 12-hour reverse light cycle (lights on at 19:00 h) until needed. Testing was performed in the dark phase. Food and water were freely
c-Fos expression following PRX
Table 1 shows the number of c-Fos positive cells per group among in the 35 regions counted (Fig. S1). c-Fos expression in VEH-treated rats was low in most regions in accordance with the typical levels in well-habituated control rats. There were no significant differences in c-Fos expression between adolescent and young adult VEH-treated rats in any brain region.
Two-way ANOVAs followed by post hoc tests revealed 5 regions where c-Fos expression was elevated in both adolescent and young adult
Discussion
The present study compared regional c-Fos expression patterns to the SSRI PRX in young adult and adolescent rats. The results here are consistent with the notion that there are developmental differences in the response to SSRIs, although PRX also induced Fos expression in an overlapping set of brain regions in both adolescents and young adults. Commonly activated regions between adolescent and young adult rats were found in limbic (BNST-dorsolateral, CEA), thalamic (paraventricular) and
Acknowledgements
Research is supported by an Australian Research Council (ARC) grant and fellowship to ISM and NHMRC funding to ISM and GEH. ISM is currently a NHMRC Principal Research Fellow. EK is supported by an Australian Postgraduate Award and University of Sydney Vice Chancellor’s scholarship.
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