The role of dopamine in the prelimbic cortex and the dorsomedial striatum in instrumental conditioning.

The prelimbic (PL) region of the prefrontal cortex and the posterior subregion of the dorsomedial striatum (pDMS) are components of a corticostriatal circuit subserving instrumental learning. Here, we examined whether dopamine (DA) signals conveyed to the PL and pDMS are critical for instrumental learning. Rats with 6-hydroxydopamine or vehicle infusion into the PL and pDMS were trained to press 2 levers, either for food pellets or a sucrose solution. Thereafter, we tested whether the animals were sensitive 1) to a selective degradation of 1 of 2 outcomes using a specific satiety procedure and 2) to a selective degradation of 1 of 2 contingencies controlling instrumental behavior. Rats with PL DA depletion displayed a reduced rate of lever presses but appeared to be sensitive to outcome devaluation and contingency degradation. Thus, PL DA seems to modulate lever pressing but does not support instrumental conditioning. In contrast, rats with pDMS DA depletion had intact response rates and were sensitive to selective outcome devaluation; however, they showed a reduced sensitivity to contingency degradation. Therefore, pDMS DA signaling seems not to be involved in maintaining lever pressing but instead contributes to instrumental conditioning by supporting the detection of causal relationships between an action and its consequences.

The role of nucleus accumbens dopamine in outcome encoding in instrumental and Pavlovian conditioning.

Considerable evidence suggests that dopamine in the core subregion of the nucleus accumbens is not only involved in Pavlovian conditioning but also supports instrumental performance. However, it is largely unknown whether NAc dopamine is required for outcome encoding which plays an important role both in Pavlovian stimulus-outcome learning and instrumental action-outcome learning. Therefore, we tested rats with 6-hydroxydopamine (6-OHDA) induced dopamine depletion of the NAc core for their sensitivity to outcome devaluation in a Pavlovian and an instrumental task. Results indicate that 6-OHDA-lesioned animals were sensitive to outcome devaluation in an instrumental task. This finding provides support to the notion that NAc core dopamine may not be crucial in encoding action-outcome associations. However, during instrumental conditioning lever pressing rates in 6-OHDA-lesioned animals were markedly lower which could reflect an impaired behavioral activation. By contrast, after outcome-specific devaluation in a Pavlovian task, performance in 6-OHDA-lesioned animals was impaired, i.e. their magazine-directed responding was non-selectively reduced. One possibility to explain non-selective responding is that NAc core DA depletion impaired the ability of conditioned stimuli to activate the memory of the current value of the reinforcer.

Disconnection of the entorhinal cortex and dorsomedial striatum impairs the sensitivity to instrumental contingency degradation.

The capacity to detect changes in the causal efficacy of actions is mediated by a number of brain areas, including the entorhinal cortex (EC) and the posterior part of the dorsomedial striatum (pDMS). In this study we examined whether interactions between the EC and pDMS are required to detect changes in the instrumental contingency. Rats that received EC-pDMS disconnection lesions, that is, unilateral cell body lesions of the EC and contralateral dopamine depletions of the pDMS, were trained to press two levers, with one delivering food pellets and the other a sucrose solution. Thereafter, we tested whether rats were sensitive (1) to a selective devaluation of the value of one of two outcomes using a specific satiety procedure, and (2) to a selective degradation of one of two contingencies controlling instrumental choice behavior. Our results reveal that rats with EC-pDMS disconnection lesions were sensitive to outcome devaluation. However, unlike rats with sham lesions or unilateral EC and pDMS lesions, rats with EC-pDMS disconnection lesions showed a reduced sensitivity to contingency degradation. These findings suggest that EC and pDMS may be part of a neural system that supports the detection of changes in the causal relationship between an action and its consequences.