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Abstract
Dopamine contributes to the regulation of higher order information processing and executive control. It is important for memory consolidation processes, and for the adaptation of learned responses based on experience. In line with this, under aversive learning conditions, application of dopamine receptor antagonists prior to extinction result in enhanced memory reinstatement. Here, we investigated the contribution of the dopaminergic system to extinction and memory reinstatement (renewal) of an appetitive spatial learning task in rodents. Rats were trained for 3 days in a T-maze (context “A”) to associate a goal arm with a food reward, despite low reward probability (acquisition phase). On day 4, extinction learning (unrewarded) occurred, that was reinforced by a context change (“B”). On day 5, re-exposure to the (unrewarded) “A” context took place (renewal of context “A”, followed by extinction of context “A”). In control animals, significant extinction occurred on day 4, that was followed by an initial memory reinstatement (renewal) on day 5, that was, in turn, succeeded by extinction of renewal. Intracerebral treatment with a D1/D5-receptor antagonist prior to the extinction trials, elicited a potent enhancement of extinction in context “B”. By contrast, a D1/D5-agonist impaired renewal in context “A”. Extinction in the “A” context on day 5 was unaffected by the D1/D5-ligands. Treatment with a D2-receptor antagonist prior to extinction had no overall effect on extinction in context “B” or renewal in context “A”, although extinction of the renewal effect was impaired on day 5, compared to controls. Taken together, these data suggest that dopamine acting on the D1/D5-receptor modulates both acquisition and consolidation of context-dependent extinction. By contrast, the D2-receptor may contribute to context-independent aspects of this kind of extinction learning.
Highlights
During extinction learning, conditioned responses become diminished during exposure to the conditioned stimulus (CS) in the absence of the unconditioned stimulus (US; Bouton, 2004; Myers and Davis, 2007)
A significant increase in correct choices was apparent between day 1 and day 2 (Figure 1; within-subject analysis of variance (ANOVA): for animals subsequently treated with vehicle, F(1,6) = 14.427; p = 0.009, n = 7; for animals subsequently treated with a dopamine D1 agonist, F(1,7) = 9.215; p = 0.019, n = 8)
Our data demonstrate that the dopamine D1/D5-receptor contributes to extinction learning of a context-dependent appetitive task by supporting extinction learning and by suppression of renewal
Summary
During extinction learning, conditioned responses become diminished during exposure to the conditioned stimulus (CS) in the absence of the unconditioned stimulus (US; Bouton, 2004; Myers and Davis, 2007). Reinstatement, or renewal, of the original conditioned response is typically reactivated upon reexposure to the CS in the original context, or to conditions that are sufficiently dissimilar to the extinction context (Rachman, 1989; Bouton, 2004; Craske et al, 2008) Neuromodulators such as dopamine play a crucial role in memory processes and regulate synaptic information storage mechanisms such as synaptic plasticity (Hansen and Manahan-Vaughan, 2014). Dopamine is important for the emotional weighting of experiences, and for memory consolidation (Huang and Kandel, 1995; Bissière et al, 2003; Sajikumar and Frey, 2004; Lisman et al, 2011) It regulates cue-dependent fear conditioning (Fadok et al, 2010), and the consolidation of extinction of fear memory (Holtzman-Assif et al, 2010). Less is known about the role of dopamine in appetitive context-dependent extinction learning processes that are supported by the hippocampus, and the precise role of dopamine receptor subtypes in this phenomenon is unclear
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