Differential strain sensitivity to low-dose anxiolytic diazepam in anxiety-related behaviour in male Wistar and Wistar-Kyoto, a rat model of endogenous depression

Abstract

Objectives: Treatment resistant depression (TRD) prevails among individuals with major depressive disorder (MDD) with comorbid anxiety. The Wistar Kyoto (WKY) rat strain, which demonstrates exaggerated vulnerability to anxiety, has recently been suggested as a model for TRD with similar pathophysiology to MDD, non-responsiveness to antidepressants but responsive to deep brain stimulation and ketamine. At the clinical level, TRD is associated with reduced occipital cortical levels of Gamma-AminoButyric Acid (GABA), with a reduction in spontaneous GABAergic synaptic activity reported in WKY. Diazepam (DZP), a GABA agonist, is a widely used anxiolytic, so the present study was carried out to evaluate its efficacy through a low dose, oral administration in male WKY rats, with the progenitor strain Wistar, serving as vehicular control. Materials and Methods: Adult Wistar and WKY rats were treated with 1 mg/kg body weight DZP administered per os (p. o.) for 10 days. From the 6th day, rats were exposed to a comprehensive battery of behavioural paradigms, including novelty-based open field (OPF), anxiogenic elevated plus maze (EPM), light-dark box (LDB) and the stress coping behaviour assessing forced swim test (FST). Results: DZP reversed the EPM-induced anxiety in Wistars by increasing open-arm duration (P < 0.05), entries (P < 0.05) and exploratory behaviour (P < 0.01) while concomitantly decreasing closed-arm duration (P < 0.05) and entries (P < 0.05), with no effect in WKY. DZP also reduced latency to the dark zone (P < 0.05) in LDB and increased swimming behaviour in FST (P < 0.05) in Wistars, with no effect in WKY. Baseline strain differences were observed with reduced exploratory behaviour in OPF (P < 0.01), open arm entries (P < 0.01) and head dips (P < 0.01) in EPM and swimming (P < 0.05) in FST in WKY as compared to Wistars. Strain differences persisted also in the DZP-treated groups where, as compared to matched Wistars, WKY demonstrated reduced open arm duration (P < 0.05), entries (P < 0.001) and head dips (P < 0.001) and increased closed arm duration (P < 0.001) and entries (P < 0.001) in the EPM. WKY also showed reduced time spent (P < 0.05) and entries (P < 0.01) into the light zone and increased time spent in the dark zone (P < 0.05) of LDB. Further, WKY showed increased immobility (P < 0.05) during habituation and reduced swimming behaviour (P < 0.001) during the test. Conclusion: Strain-specific differences and increased baseline anxiety levels in WKY, as compared to Wistars, induced differential effects of DZP with drug-induced effects observed in Wistars but not in WKY, furthering the treatment resistant aspect of this model. DZP efficacy, therefore, varies in different rat strains and manifests in differential strain-specific responses emanating from exaggerated vulnerability to stress. Results also indicated differential sensitivity of tested paradigms to the anxiolytic activity of DZP and stressed the use of a battery of tests that enable a teasing out of anxiety and depression. However, further studies are needed that would unravel GABAergic differences at the receptor level, such as differential receptor binding affinities, underlying gene polymorphisms and the implications thereof for this TRD model.