Cognitive flexibility in the deer mouse and its response to escitalopram and levetiracetam

MSc (Pharmacology), North-West University, Potchefstroom CampusObsessive-compulsive disorder (OCD)1 causes functional impairment which severely and negatively impacts the lives of affected individuals. As its name suggests, OCD is characterised by both obsessions and compulsions. The former includes intrusive and unwanted thoughts, mental images and urges while the latter consists of overt or covert compulsive mental routines and repetitive behaviours which manifest to attenuate the high degree of obsession-related stress and anxiety. Four main obsession/compulsion symptom dimensions have been described, i.e. contamination/washing, safety/checking, symmetry/ordering, and forbidden or taboo thoughts/covert mental compulsions. On a neurobiological level, OCD is broadly founded upon dysfunctional cortico-striato-thalamo-cortical (CSTC)2 circuit processing. This circuit subdivides into a direct, behaviourally activating and an indirect, behaviourally inactivating, pathway. On a theoretical level, it is often proposed that an overactive direct pathway underlies the expression of compulsive-like behavioural symptoms. This in turn is believed to result from a hyposerotonergic state, which allows the unopposed propagation of dopaminergic signalling by means of limited serotonin heteroreceptor-mediated negative feedback. Following from the above, OCD can effectively be treated with selective serotonin reuptake inhibitors (SSRIs)3. However, approximately one third of patients do not respond to treatment with SSRIs. In these cases, up to 50 % of SSRI-resistant patients respond to cognitive behavioural therapy (CBT)4. Thus, it is likely that some phenotypes of OCD may differentially be associated with uniquely aberrant cognitive processes which may influence their treatment response. As such, valuable insights may be gained from research that explores treatment aimed at addressing deficits in cognitive function. One such potentially useful compound, levetiracetam (LEV)5, has been shown to improve neuropsychological and cognitive performance in patients suffering from Alzheimer’s disease and epilepsy, but it has not yet been trialled in other neuropsychiatric disorders that may present with similar deficits in cognitive function. While the exact mechanism of action of this anti-epileptic drug is still unclear, it is apparent that, as opposed to other neural stabilisers, LEV1 binds to synaptic vesicle glycoprotein 2A (SV2A)2, thereby regulating neurotransmitter release and synaptic function. Therefore, instead of targeting a single neurotransmission system in isolation, it is believed that LEV broadly normalises neurotransmission in the central nervous system (CNS)3, thereby restoring brain function in cases of imbalanced signalling. The deer mouse (Peromyscus maniculatus bairdii) model system of compulsive-like behaviour is one of the foremost naturalistic animal models of compulsive like behaviour. The validity of this model is founded upon the naturalistic presentation of stereotypical behaviour, i.e. repetitive vertical jumping and pattern running. Such behaviours vary within the deer mouse population, with 35 – 45 % of male and female deer mice presenting with high-stereotypical (H)4 behaviour. Since H behaviour waxes and wanes over the course of a dark cycle, serves no apparent realistic function and has shown response to chronic, high-dose exposure to the SSRI5, escitalopram (ESC)6, stereotypy in deer mice has been proposed to mimic clinical compulsive behaviour in humans. Further, H animals, but not their normal (low-stereotypical) (N)7 behavioural counterparts, present with significantly reduced striatal serotonin transporter (SERT)8 density, lending a high degree of construct validity to the model. That said, since motor stereotypy is also a symptom of several neuropsychiatric disorders which are devoid of direct cognitive involvement, it remains difficult to argue that such behaviours are indeed related to an underlying psychobiological construct. Thus, in this work, we aimed to explore whether motor stereotypy associates with deficits in cognitive flexibility as measured by spontaneous alternation in a T-maze. The T-maze assessment is a popular cognitive task used to assess various cognitive constructs in rodents and to detect cognitive dysfunction in neuropsychological diseases. Subsequently, we aimed to investigate whether any potential association between stereotypy and altered spontaneous alternation in H1 compared to N2 animals would respond to ESC3 and the potentially novel cognitive enhancer, LEV4. Ultimately, the broad theme of this work was to highlight potentially useful avenues for future anti-compulsive therapeutic research which may be based on targeting cognitive, rather than motor processes in patients with OCD5. Briefly, 124 mice of both sexes (aged 10-12 weeks at the onset of experimentation) were screened for stereotypical behaviour and divided into two behavioural cohorts, i.e. N and H. Since only 35 – 45 % of animals show H behaviour, an initial pool of 124 animals was necessary to yield 31 N and 34 H animals (total n = 65 used for experimentation). The 59 animals that have not been selected for further study, were euthanised. After quantification of stereotypical behaviour and stereotypical classification, all N and H mice were subjected to T-maze assessment, in which the number of successive alternate arm choices are calculated, i.e. the higher the number of left-right or right-left alternations, the more flexible an animal was proposed to be. Animals were then divided into the following drug exposure groups i.e. normal water control, ESC (50 mg/kg/day), and LEV (75 mg/kg/day) (both obtained from BLD Pharm®, Shanghai, China). Final group numbers were as follows: N control & ESC: 11 each; N LEV: 9; H control and ESC: 11 each; H LEV: 12. Groups were as far as possible constituted by an equal number of male and female animals. On the second last day of water or drug exposure, all N and H animals were again assessed for alternation behaviour in the T-maze, followed by a single night of stereotypy assessment on the last day of drug exposure. All subjects were then euthanised as per the standard protocol of the Vivarium of the North-West University (NWU)6. Prior to the onset of experimentation, the study was approved by the AnimCare Research Ethics Committee of the North-West University (approval number: NWU-00522-20-A5). The main findings of this work were that 1) higher stereotypical intensity in deer mice significantly and negatively predicted spontaneous alternation scores in the T-maze, 2) LEV, but not control or ESC exposure, significantly increased the time that H, but not N animals spent engaging in no stereotypical behaviour, and 3) neither of the interventions employed here, affected spontaneous alternation scores in N1 and H2 deer mice. With respect to our first main finding, we showed that stereotypical motor expression in deer mice negatively predicted spontaneous alternation in a T-maze. That deer mouse stereotypy is associated to some degree with impaired cognitive flexibility as quantified by decreased spontaneous alteration scores in a T-maze, we argue that such behaviour is not simply an artefact of aberrant processes regulating motor control, but that deficits in cognitive ability, e.g. action-outcome-planning, may underlie the expression of such routines. While this notion remains to be explored in future studies, the validity of the deer mouse model is strengthened as a framework in which to investigate the psychobiology of naturalistic compulsive-like behaviours. Secondly, we showed that LEV3 significantly increased the time that H animals engaged in no stereotypical activity compared to LEV-exposed N animals. The fact that neither ESC4 nor LEV influenced the expression of H behaviour per se, was an important result, since such a lack of H adaptation to ESC differs markedly from the results of our previous work. While we provided potential explanations for this unexpected result in this dissertation, future work will be needed to elucidate the exact reasons underlying our present data. It is possible that H behaviour might arise from both anxiety- and compulsive-like processes, for which the appropriate controls, e.g. anxiolytic drug intervention and additional anxiety testing, were not included, since said confounding results were not foreseen from the outset. Last, neither control nor ESC or LEV intervention influenced T-maze alternation. While this result could have been expected for ESC—if it is argued that ESC targets hyperactive cortico-striatal motor circuits instead of cognitive (read executive) processes—the LEV result is somewhat confounding. Nevertheless, it is likely that restoration of the cognitive deficits that associate with H behaviour by LEV were masked by the methods followed here and that a different paradigm, i.e. trained, cue-reward contingency T-maze alternation, might have been more informing. In conclusion, we explored whether high motor stereotypy as observed in deer mice, is associated with cognitive rigidity as measured in a T-Maze, which was shown to be so. Further, the current work provided valuable insights into the potential interactions between anxiety- and compulsive-like processes that may act in concert to give rise to naturalistic stereotypy; this will need careful and well-conceptualised future work to clarify. Nevertheless, naturalistic stereotypy in deer mice is innately associated with perturbations in cognitive flexibility. This paves the way for future cognitive and anti-compulsive therapeutic research in this species.Master