Role of the anterior cingulate cortex in the retrieval of novel object recognition memory after a long delay

Abstract

Previous in vivo electrophysiological studies suggest that the anterior cingulate cortex (ACgx) is an important substrate of novel object recognition (NOR) memory. However, intervention studies are needed to confirm this conclusion and permanent lesion studies cannot distinguish effects on encoding and retrieval. The interval between encoding and retrieval tests may also be a critical determinant of the role of the ACgx. The current series of experiments used micro-infusion of the GABA_A receptor agonist, muscimol, into ACgx to reversibly inactivate the area and distinguish its role in encoding and retrieval. ACgx infusions of muscimol, before encoding did not alter NOR assessed after a delay of 20 min or 24 h. However, when infused into the ACgx before retrieval muscimol impaired NOR assessed after a delay of 24 h, but not after a 20-min retention test. Together these findings suggest that the ACgx plays a time-dependent role in the retrieval, but not the encoding, of NOR memory, neuronal activation being required for the retrieval of remote (24 h old), but not recent (20 min old) visual memory.

Figure 1.

Effect of saline (sal) or muscimol (62.5 and 125 ng per side) infused into the anterior cingulate cortex before the sample phase on novel object recognition after a retention delay of 20 min. N = 10 rats in total. (A) Timeline of experiment 1 to illustrate the 20-min retention delay between the sample and the choice phase, the timing of the infusions 10 min before the sample phase and the repeated (×3) testing in the within-subjects design. (B) Effect of muscimol on the first minute exploration time (sec, mean ± SEM) of a novel (Nov) and a familiar (Fam) object during the choice phase. (*) P < 0.05, significant difference when compared with the familiar object. (C) Choice performance presented as the discrimination ratio during the first minute of exploration (mean ± SEM); stippled line indicates chance. (#) P < 0.05, significantly different from chance.

Figure 2.

Effect of saline (sal) or muscimol (62.5 and 125 ng/side) infused into the cingulate cortex before the choice phase on novel object recognition after a retention delay of 20 min. N = 12 rats in total. (A) Timeline of experiment 2 to illustrate the 20-min retention delay between the sample and the choice phase, the timing of the infusions 10 min before the choice phase and the repeated (×3) testing in the within-subjects design. (B) Effect of muscimol on the first minute of exploration time (sec, mean ± SEM) of a novel (Nov) and a familiar (Fam) object during the choice phase. (*) P < 0.05, significant difference when compared with the familiar object. (C) Choice performance presented as the discrimination ratio during the first minute of exploration (mean ± SEM); stippled line indicates chance. (#) P < 0.05, significantly different from chance.

Figure 3.

Effect of saline (sal) or muscimol (62.5 and 125 ng/side) infused into the cingulate cortex before the sample phase on novel object recognition after a retention delay of 24 h. N = 11 rats in total. (A) Timeline of experiment 3 to illustrate the 24-h retention delay between the sample and the choice phase, the timing of the infusions 10 min before the sample phase and the repeated (×3) testing in the within-subjects design. (B) Effect of muscimol on the first minute of exploration time (sec, mean ± SEM) of a novel (Nov) and a familiar (Fam) object during the choice phase. (*) P < 0.05, (**) P < 0.001, significant difference when compared with the familiar object. (C) Choice performance presented as the discrimination ratio during the first minute of exploration (mean ± SEM); stippled line indicates chance. (#) P < 0.05, (##) P < 0.001, significantly different from chance.

Figure 4.

Effect of saline (sal) or muscimol (62.5 and 125 ng/side) infused into the cingulate cortex before the choice phase on novel object recognition after a retention delay of 24 h. N = 10 rats in total. (A) Timeline of experiment 4 to illustrate the 24 h retention delay between the sample and the choice phase, the timing of the infusions 10 min before the choice phase and the repeated (×3) testing in the within-subjects design. (B) Effect of muscimol on the first minute of exploration time (sec, mean ± SEM) of a novel (Nov) and a familiar (Fam) object during the choice phase. (*) P < 0.05, significant difference when compared with the familiar object. (C) Choice performance presented as the discrimination ratio during the first minute of exploration (mean ± SEM); stippled line indicates chance. (*) P < 0.05, significantly different from the saline condition; (#) P < 0.05, significantly different from chance.

Figure 5.

Histological verification of cannula placements for rats used in experiments 1–4. Approximate locations of infusion tips, in the ACgx. The black dots (left panel) represent the location of the injector tips in experiments 1 and 2; the gray dots (right panel) the location of the injector tips in experiments 3 and 4. Placements are shown on coronal plates adapted from Paxinos and Watson (1998) with numbers indicating distance from bregma in millimeters.