Chronic effects of pre-adolescent pharmacological and non-pharmacological interventions on depressive-like behaviour in rats

Abstract

Similar to adults, major depressive disorder (MDD) also affects children and adolescents, with comparable treatment outcomes. However, only two antidepressants, namely the serotonin reuptake inhibitors fluoxetine and escitalopram, are currently approved for the treatment of juvenile MDD. Additionally, the serotonin-noradrenaline reuptake inhibitor venlafaxine, is a popular ‘off-label’ treatment option for depressed juveniles. The efficacy of pro-serotonergic, and not pro-noradrenergic antidepressants in juvenile patients have been ascribed to the earlier maturation of the serotonergic, relative to the noradrenergic system. Nevertheless, the pathophysiology of MDD in juvenile patients generally appears to be comparable to that in adults. In this regard, increased central inflammation and oxidative stress damage are also observed in depressed juveniles, resulting in compromised neuroplasticity that ultimately contribute to decreased monoaminergic neurotransmission, the main target of the approved antidepressants. During childhood and adolescence dynamic and adaptable neurodevelopment renders the young brain vulnerable to possible long-term detrimental or beneficial effects of antidepressants. Our current knowledge of such long-term effects are limited, warranting further investigation. Also, non-pharmacological interventions have attracted attention as possible augmentative strategies to currently approved therapies. Based on preliminary evidence, and due to their purported improved safety profiles, omega-3 essential fatty acid (ω-3 EFA) supplementation and exercise have been investigated for their possible antidepressant properties. Both these interventions beneficially affect numerous MDD-associated neurobiological processes, yet the exact mechanisms of action remain unknown. Nevertheless, similar to antidepressant treatment, how and to which extent such early-life interventions, either as mono-treatment or in combination with antidepressants, could affect the developing brain and behaviour later in life, also warrants further investigation. The current project therefore investigated the early-life and lasting bio-behavioural effects of pre-pubertal pharmacological and non-pharmacological treatments in a stress-sensitive animal model of depression (i.e. Flinders Sensitive Line rat; FSL). Pre-pubertal male FSL rats were treated with saline (control; SAL), fluoxetine (FLX; 5 mg/kg/day), escitalopram (ESC; 10 mg/kg/day) or venlafaxine (VEN; 10 mg/kg/day), with or without dietary intervention (standard rat chow [STD] or ω-3 EFA coated rat chow [OM3]) or exercise (sedentary behaviour [SED] or low intensity exercise [EXE]) for fourteen days, starting from postnatal day 21 (PND21) to PND34 (i.e. pre-puberty). Drugs were administered subcutaneously, whereas ω-3 EFAs were administered via diet and low intensity forced exercise was introduced by a custom-built treadmill. Following chronic, pre-pubertal treatment, a sub-set of animals underwent behavioural analyses in the open field test (OFT) and forced swim test (FST) immediately following treatments on PND35 (i.e. puberty) and were euthanized on PND36 for neurochemical analyses. Another sub-set of animals received the same pre-pubertal treatments, followed by no treatment for 26 days until PND60 (i.e. early adulthood) to determine lasting bio-behavioural effects and were euthanized on PND61 for neurochemical analyses. In addition, the main study was foregone by pilot studies to develop a juvenile forced exercise regimen and ω-3 EFA coated rat chow. Only FLX treatment alone reduced early-life depressive-like behaviour and induced lasting antidepressant-like effects in FSL rats on PND60. Pre-pubertal EXE alone, decreased depressive-like behaviour and promoted behaviour associated with enhanced serotonergic neurotransmission, supported by increased cortical serotonin and hippocampal brain-derived neurotrophic factor (BDNF) concentrations in rats on PND35. This antidepressive-like behaviour became more pronounced later in life, with significant increased cortical monoamine concentrations observed on PND60. Contrary, OM3 treatment alone failed to improve depressive-like behaviour, yet putatively induced enhanced coping mechanisms, possibly masking any antidepressive-like behaviour on PND35 and PND60. Nevertheless, both non-pharmacological interventions displayed strong augmentation properties that lasted into early-adulthood. FLX+EXE combination treatment significantly decreased early-life depressive-like behaviour, but did not last into early-adulthood. When combined with EXE, VEN decreased depressive-like behaviour later in life via enhanced noradrenergic and serotonergic neurotransmission. Similarly, pre-pubertal ESC+OM3 treatment appeared ineffective in early-life, yet exerted long-lasting antidepressant-like properties into early-adulthood, associated with increasing serotonin turnover later in life. Finally, the triple combination of ESC+OM3+EXE showed great potential in reducing early-life depressive-like behaviour, with more pronounced antidepressant-like effects later in life. In conclusion, the current project confirmed the use of FLX in pre-pubertal individuals with beneficial lasting effects. Furthermore, that the long-term effects of different classes of antidepressants were beneficially augmented by ω-3 EFA supplementation and low intensity exercise, suggests neurodevelopmental processes to be positively affected, resulting in improved behaviour later in life. Furthermore, the newly developed age-related, intensity-specific exercise regimen for pre-pubertal rodents is a novel improvement to existing regimens, as is the successful formulation of the ω-3 EFA supplementation coating on rat chow. Overall, the current project underlines the value of improved lifestyle and nutritional modifications to augment the pharmacological treatment of juvenile MDD, and in particular its potential value for long-lasting effects into adulthood.