Development and characterisation of a novel preclinical test battery for detecting anxiolytic and antidepressant properties of drugs
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Depression is a chronic, recurring and potentially life-threatening disorder that is highly comorbid with anxiety. The aetiology of depression, anxiety and their comorbidity is still largely unknown. Despite the development of several different neurochemical theories of these disorders, all current first-line treatments work by the same fundamental mechanism – by altering monoamine neurotransmission. However, these drugs have several limitations, including a therapeutic lag, numerous adverse side effects and low remission rates. Therefore, there is a need for novel acting drugs, and thus, a need for improved preclinical screening for detection of these drugs. The present work aimed to develop and characterise a novel preclinical test battery for assessing acute and chronic anxiolytic and antidepressant properties of drugs in rats. Behavioural tests of anxiety (elevated plus maze; EPM) and depression (forced swim test; FST) were first characterised to determine optimal experimental parameters for detecting drug efficacy. Following this, several behavioural tests were incorporated into two battery study designs to provide a more time-efficient and detailed screening of a drug’s pharmacological properties, with subsequent assessment of hippocampal brain derived neurotrophic factor (BDNF) and its receptor, tyrosine receptor kinase B (TrkB) as potential biomarkers of antidepressant response. Briefly, the first battery study design involved the assessment of acute anxiolytic properties in the EPM and open field test or the resident-intruder paradigm. Animals from both subsets received further chronic dosing, after which they were tested in the FST or by assessment of anhedonia in the saccharin preference test (SPT) after acute systemic lipopolysaccharide (LPS) administration. The second battery study involved acute/subacute drug assessment in the FST or novelty-induced hypophagia test (NIH), followed by assessment of chronic drug effects in the SPT, NIH testing and FST. Standard antidepressant and anxiolytic drugs were used to validate the first (desipramine, fluoxetine and diazepam) and second (desipramine, fluoxetine, venlafaxine, ketamine and diazepam) test battery. In the initial characterisation studies, whilst some experimental parameters did not affect behavioural testing (breeding source), the route of drug administration and the subject’s age and housing environment differentially affected baseline and/or drug-induced effects in the EPM and FST. In addition, an improved method for screening false positive drugs prior to the FST was developed, whereby home cage locomotor assessment was successfully incorporated into the FST study design. The first battery study design provided important verification that EPM and open field testing can be reliably incorporated prior to assessment of drug effects in the FST, validating a more efficacious screening of compounds. Whilst the resident-intruder protocol used within subset 2 reliably detected antidepressant and anxiolytic drug effects, the systemic LPS model of depression did not induce an anhedonia effect, and thus, chronic drug effects could not be assessed in this test. In the second battery study, whilst the FST successfully detected subacute drug effects, chronic drug effects in the FST, systemic LPS model and NIH test, as well acute effects in the NIH test were not detected, due to several methodological factors. Nocturnal home cage locomotor activity was reduced in the desipramine group on days 7 and 14, and in the venlafaxine group on day 14 of drug treatment. As expected, due to the lack of chronic drug effects, hippocampal BDNF and TrkB mRNA levels were not altered. This research has provided important methodological information for the improved implementation of behavioural tests for detecting anxiolytic and antidepressant drug efficacy, and emphasised the importance of standardisation between studies. Moreover, it has provided a basis for the future development of an improved battery study design, having provided insight into important methodological complications that can arise when incorporating several behavioural tests into one study design.
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