Enhanced cognitive flexibility and phasic striatal dopamine dynamics in a mouse model of low striatal tonic dopamine

Abstract

The catecholamine neuromodulators dopamine and norepinephrine are implicated in motor function, motivation, and cognition. Although roles for striatal dopamine in these aspects of behavior are well established, the specific roles for cortical catecholamines in regulating striatal dopamine dynamics and behavior are less clear. We recently showed that elevating cortical dopamine but not norepinephrine suppresses hyperactivity in dopamine transporter knockout (DAT-KO) mice, which have elevated striatal dopamine levels. In contrast, norepinephrine transporter knockout (NET-KO) mice have a phenotype distinct from DAT-KO mice, as they show elevated extracellular cortical catecholamines but reduced baseline striatal dopamine levels. Here we evaluated the consequences of altered catecholamine levels in NET-KO mice on cognitive flexibility and striatal dopamine dynamics. In a probabilistic reversal learning task, NET-KO mice showed enhanced reversal learning, which was consistent with larger phasic dopamine transients (dLight) in the dorsomedial striatum (DMS) during reward delivery and reward omission, compared to WT controls. Selective depletion of dorsal medial prefrontal cortex (mPFC) norepinephrine in WT mice did not alter performance on the reversal learning task but reduced nestlet shredding. Surprisingly, NET-KO mice did not show altered breakpoints in a progressive ratio task, suggesting intact food motivation. Collectively, these studies show novel roles of cortical catecholamines in the regulation of tonic and phasic striatal dopamine dynamics and cognitive flexibility, updating our current views on dopamine regulation and informing future therapeutic strategies to counter multiple psychiatric disorders.

Fig. 1. NET-KO mice have enhanced reversal learning in a Probabilistic Reversal Learning (PRL) task.

NET-KO mice have A significantly greater numbers of reversals per session, *p < 0.05 Mixed-effects model main genotype effect and B fewer trial omissions for each genotype averaged over each session for 10 days, ***p < 0.001 t-test, WT vs NET-KO, C greater number of reversals normalized by number of completed trials, from Day 1 through Day 10. **p < 0.01 Mixed-effects model main genotype effect, NET WT vs KO. D More total reversals averaged over each session for 10 days **p < 0.01 t-test, WT vs NET-KO, E more inactive nosepokes averaged over each session for 10 days, *p < 0.05 t-test, NET WT vs KO but F similar normalized inactive nosepokes averaged over each session for 10 days, and similar G errors to criterion initial discrimination, H Trials to 1^st reversal and I errors to criterion after 1^st reversal, compared to WT controls. n = 11–12 mice per genotype (6/7 WT and 6/4 NET-KO male/female mice).

Fig. 2. NET-KO mice have enhanced phasic dopamine transients in the Probabilistic Reversal Learning (PRL) task.

A Fiber photometry setup and representative images of optical probe implant site and dLight and mCherry AAV expression in the DMS of WT and NET-KO mice. B Dopamine measurements using dLight 1.3b in the 465 nm channel over the course of an entire daily PRL session of 50 min. C–J i -peri-event time histograms (PETH), ii - AUC ΔF/F values (peri-event: 0–2.5 s) and iii – Peak ΔF/F responses (peri-event: 0–2.5 s) of phasic dopamine transients in WT and NET-KO mice for different reward contingencies as mentioned in the figure, at early (C–F) or late (G–J) phases of PRL task. Dashed line denotes onset of reward delivery or omission. *p < 0.05, **p < 0.01 unpaired t-test, WT vs NET-KO; ns not significant. n = 5 mice per genotype (4 male/1 female, WT and NET-KO mice).

Fig. 3. Probabilistic Reversal Learning is intact in NE lesioned mice.

Compared to vehicle (Veh) controls, NE lesioned mice (6-OHDA) showed: A similar numbers of reversals per session from Day 1 through Day 8. p = 0.77 Mixed-effects model Repeated measures, Veh vs 6-OHDA; B similar numbers of trial omissions averaged over each session for 8 days, p = 0.73, t-test, Veh vs 6-OHDA. C similar numbers of inactive nosepokes averaged over each session for 8 days p = 0.82, t-test, Veh vs 6-OHDA. and D lower levels of NE but not DA or 5-HT as measured by HPLC analysis of PFC tissue. **p < 0.01 Two-Way ANOVA, Veh vs 6-OHDA. n = 6 (3 male and 3 females) mice for each treatment group.

Fig. 4. Nestlet shredding and open-field activity in NET-KO mice.

A NET-KO and WT control, n = 8 per genotype or B PFC NE-lesioned or vehicle (Veh) control, n = 8 per group mice were placed in fresh cages with nestlets, and weights of nestlets were measured every hour for 4 h. Data are presented as percent of the nestlet shredded. *P < 0.05 by Mixed-effects model Repeated measures, post hoc Tukey’s test, comparing NET WT and KO (4 male and 4 female mice for KO and 5 male and 3 females for WT mice) or Veh control and PFC NE lesion (4 male and 4 female mice for WT and KO mice). C, D Mice were placed in an open field chamber and distance traveled (locomotor activity) was measured for 90 min. n = 7–8 per genotype **P < 0.01 by t-test comparing NET WT and KO (4/5 male and 3 female mice for KO/WT mice).