Extended characterisation and validation of an animal model of post-traumatic stress disorder : behavioural, molecular and pharmacological studies
Posttraumatic Stress Disorder (PTSD) is an anxiety disorder precipitated by exposure to a severe traumatic event. Given the socio-economic impact of the disorder, and the increasing rates of trauma worldwide, PTSD is set to become a major global health problem. There exists a clear need for the development of drug treatments specifically for PTSD, yet the neurobiology of the disorder remains to be completely elucidated. In this regard, animal models are critical tools in the study of the pathophysiological mechanisms of stress, as well as in the testing of potential drug treatments. These animal models should be well-validated, reliable and generalisable (factors that are often overlooked in validation studies) to ensure that findings from the models will be meaningful and that research animals are not used unnecessarily. Earlier behavioural, endocrine and pharmacological studies in our laboratory had established that the time-dependent sensitisation (TDS) model (single prolonged stress + re-stress) presents with noteworthy construct-, face- and predictive validity. However, subsequent studies in our laboratory and elsewhere have yielded contrasting or inconclusive results, especially with regard to behavioural changes. The primary aim of the current study was therefore to re-investigate the TDS model as analogous PTSD model, with regard to cognitive performance, anxiety-like behaviour and endocrine function. The original validation was also extended by examining arousal behaviour and the influence of chronic fluoxetine administration on TDS-induced endocrine changes. Furthermore, the robustness of the model was investigated by subjecting it to more stringent testing, using a greater range of parameters and criteria provided by computerised behavioural monitoring with powerful software. The reliability and generalisability of the TDS model was also studied by comparing results obtained from the current study with those from the original validation study. Finally, with the increasing importance of neuronal plasticity and resilience in stress-related disorders and antidepressant action, the effects of TDS stress on a broad range of cellular plasticity and resilience proteins was studied in selected limbic brain regions, Sprague-Dawley and Wistar rats were left undisturbed (controls) or subjected to the TDS model consisting of a single prolonged stress (SPS) (2 hours restraint, 15 minutes forced swim, halothane exposure) and a re-stress (RS) (20 minutes forced swim) 7 days later. Seven days after the re-stress, animals were tested for spatial learning and memory, anxiety-like behaviour or arousal in the Morris Water maze (MWM), elevated plus maze (EPM) or acoustic startle response (ASR), respectively. The activity of endocrine function as measured by plasma corticosterone was also investigated in control and TDS behavioural test exposed (Sprague-Dawley and Wistar), test naive (Sprague-Dawley and Wistar) and test naive saline or fluoxetine treated (Wistar) rats. Finally, the expression of selected cellular plasticity and resilience proteins was determined by Western blot in the hippocampus and frontal cortex of test naive Wistar rats. In contrast to the findings of the original validation studies, TDS stress failed to have a marked effect on spatial learning and memory and anxiety-like behaviour, suggesting a lack of reliability and generalisability of the TDS model. In the extended characterisation of the model, TDS stress also did not induce any significant changes in arousal. Data from the behavioural studies indicate a lack of robustness of the TDS model, which may be due to habituation to the re-stress procedure. TDS stress was, however, able to significantly (bidirectionally) alter endocrine function, while TDS stress induced suppression of corticosterone was prevented by chronic fluoxetine administration. These data suggest face validity, as well as possible construct- and predictive validity for the TDS model with regard to endocrine function. Finally, while fluoxetine had notable effects on the expression, phosphorylation and/or relative activation of cellular plasticity and resilience proteins tested, TDS stress failed to have a marked effect on these same proteins. However, the latter negative findings may not necessarily be an indication of a lack in validity or robustness of the TDS model. Although the TDS model demonstrated face validity, as well as possible construct- and predictive validity in terms of endocrine function, data from the behavioural studies suggest that the model lacks reliability and generalisibility and hence, relevance. The current data suggest that improvements to the model include omission of the re-stress procedure, or alternatively, replacement of the re-stress with a situational reminder of the SPS. The effects of stress on cellular plasticity and resilience proteins warrant further investigation with such an improved animal model. In conclusion, the current study therefore serves to highlight the importance of thorough validation of any behavioural animal model, especially confirmation by investigators other than those involved in the original studies.
- Health Sciences