The Effects of Repeated Dopamine D2 Receptor Antagonism on D2 Receptor Expression and Behavioural Inflexibility
Background: Reversal learning has been shown to improve as a function of dopamine D2 receptor expression, and manipulations that downregulated D2 expression also impaired behavioural flexibility. The possibility remains that an upregulation may facilitate reversal learning. Further, D2 receptor expression and reversal learning were compromised following repeated methamphetamine exposure. It stands to reason that restoration of receptor expression following methamphetamine exposure might prevent this impairment from occurring.
Objectives: Develop a procedure for measuring reversal learning and determine whether repeated D2 receptor antagonism increases D2 receptor expression and improves behavioural flexibility in rats. Further, the possibility that this treatment would ameliorate behavioural inflexibility produced by prior methamphetamine exposure was examined.
Methods: A task was developed to measure reversal learning. I systematically replicated an existing procedure, but then developed a novel procedure that focused on identifying a reasonable criterion for visual discrimination. To do this, male Sprague Dawley rats completed 25 days of visual discrimination. Various criteria requiring different levels of accuracy and persistence were retrospectively applied to the data and a single criterion was selected from these. The procedure was then extended to include three reversal tasks. In the next study, male Sprague-Dawley rats received repeated daily pretreatment with the dopamine D2 antagonist, eticlopride (0.0 or 0.3 mg/kg/day, IP, 14 days). Three days after treatment, whole-brain (minus olfactory bulbs and cerebellum) dopamine D2 receptor expression was measured using flow cytometry. Another cohort was exposed to the same regimen between the second and third reversal task to determine whether this treatment improved reversal learning. The final study was then conducted, the only difference from the previous study being that methamphetamine (0.0mg/kg or 2.0mg/kg, SC, 4 injections, 2 hours apart) was administered the day before eticlopride treatment commenced in order to produce behavioural inflexibility.
Results: The criterion developed for the acquisition of visual discrimination was adopted for several reasons. First, the entire sample was able to meet the criterion. Second, the number of days to acquisition was equivalent irrespective of which visual cue served as the discriminative stimulus. Finally, accuracy was persistent both within and between test sessions. Importantly, therefore, these criteria eliminated confounds that might otherwise have been expected to impact reversal learning performance. The adjunct reversal tasks were typically completed within 2-5 test days, which was important since neuroadaptations can revert over time, and individual differences in performance were consistent across tasks. As expected, eticlopride treatment increased D2 receptor expression and improved reversal learning. Further, this improvement was predicted by baseline behavioural flexibility measures, such that, the improvement produced by repeated eticlopride treatment preferentially mpacted rats that were less behaviourally flexible prior to treatment. Methamphetamine treatment impaired reversal learning, but this impairment was prevented by repeated eticlopride treatment.
Conclusions: Choice of criterion impacts behavioural performance in visual discrimination learning. D2 receptor expression is also a critical determinant of behavioural flexibility, irrespective of whether the discriminative stimulus is visual or spatial. Further, pre-existing, or experimentally produced behavioural inflexibility is sensitive to a treatment that upregulated D2 expression. Collectively, these findings support the critical role of D2 expression in reversal learning and suggest it is an effective target for facilitating behavioural flexibility.