Infralimbic D2 receptors are necessary for fear extinction and extinction-related tone responses

Abstract

Background: Fear extinction is dependent on plasticity in the infralimbic prefrontal cortex, an area heavily innervated by midbrain dopaminergic inputs. Dopamine D2 receptors are concentrated in infralimbic output neurons that are involved in the suppression of conditioned fear after extinction. Here, we examined the specific role of infralimbic D2 receptors in mediating associative learning underlying fear extinction using the selective D2 antagonist raclopride.

Methods: Raclopride was administered systemically or infused into the infralimbic prefrontal cortex before fear extinction, and extinction retention was tested the following day. Rats were also prepared for single-unit recording in the infralimbic prefrontal cortex to assess the effect of raclopride on firing properties.

Results: We found that systemic injection of raclopride given before extinction impaired retrieval of extinction when rats were tested drug-free the next day but also induced catalepsy during extinction training. To determine whether impaired extinction was due to impaired motor function or disruption of extinction consolidation, we infused raclopride directly into the infralimbic prefrontal cortex. Raclopride infused immediately before extinction training did not produce motor deficits but impaired recall of extinction when tested drug-free. Furthermore, in animals that underwent extinction training, systemic raclopride reduced the tone responsiveness of infralimbic prefrontal cortex neurons in layers 5/6, with no changes in average firing rate.

Conclusions: We suggest that D2 receptors facilitate extinction by increasing the signal-to-noise of infralimbic prefrontal cortex neurons that consolidate extinction.

Figure 1. Systemic raclopride induces mild catalepsy and impairs long-term retention of fear extinction

(A) Systemic injections of 0.3 mg/kg raclopride (arrow) before extinction on day 2 led to a significant increase in percent freezing and impaired retention of extinction on day 3. (B) This dose of raclopride (0.3 mg/kg) reduced pre-tone bar-press rates during extinction and reduced locomotor activity as measured by line crosses in an open field. (C) Systemic injections of 0.1 mg/kg raclopride (arrow) before extinction did not affect percent freezing, nor impaired retention of extinction on day 3. (D) This dose of raclopride (0.1 mg/kg) did not affect pre-tone bar-press rates during extinction or locomotor activity in an open field. Data shown in blocks of 2 trials for A and C. *p < 0.05, ***p < 0.001

Figure 2. Infusion of raclopride into IL impairs long-term retention of fear extinction, without inducing catalepsy

(A) Coronal drawings (bregma, 3.20 mm, adapted with permission from (45)) show placements of injector tips for all rats in IL. (B) Before extinction, rats were infused (arrow) with either the D2 receptor antagonist raclopride or saline. Rats infused with raclopride into IL showed normal within-session extinction, but were impaired in their recall of extinction on day 3. Data shown in blocks of 2 trials. (C) Raclopride infusions into IL did not affect pre-tone bar-press rates during extinction. *p < 0.05

Figure 3. Raclopride attenuates extinction-related tone-evoked activity in IL units

(A) Coronal drawing shows location of recording electrodes (bregma + 2.80 mm; adapted with permission from (45)). (B) Systemic raclopride had no effect on firing rate relative to saline. (C) In tone-responsive neurons, systemic raclopride attenuated the magnitude of tone-evoked responses in the first 400 ms after tone onset. (D) The reduction in tone responses are shown in the group peri-event time histogram (bin width 100 ms). (E) Raclopride did not alter firing rate of these tone-responsive neurons, but (F) markedly reduced tone responsiveness in each of these individual neurons (dashed line indicates significant tone response, p < 0.05). **p < 0.01.