Dopaminergic modulation of auditory cortex-dependent memory consolidation through mTOR

Abstract

Previous studies in the auditory cortex of Mongolian gerbils on discrimination learning of the direction of frequency-modulated tones (FMs) revealed that long-term memory formation involves activation of the dopaminergic system, activity of the protein kinase mammalian target of rapamycin (mTOR), and protein synthesis. This led to the hypothesis that the dopaminergic system might modulate memory formation via regulation of mTOR, which is implicated in translational control. Here, we report that the D1/D5 dopamine receptor agonist SKF-38393 substantially improved gerbils' FM discrimination learning when administered systemically or locally into the auditory cortex shortly before, shortly after, or 1 day before conditioning. Although acquisition performance during initial training was normal, the discrimination of FMs was enhanced during retraining performed hours or days after agonist injection compared with vehicle-injected controls. The D1/D5 receptor antagonist SCH-23390, the mTOR inhibitor rapamycin, and the protein synthesis blocker anisomycin suppressed this effect. By immunohistochemistry, D1 dopamine receptors were identified in the gerbil auditory cortex predominantly in the infragranular layers. Together, these findings suggest that in the gerbil auditory cortex dopaminergic inputs regulate mTOR-mediated, protein synthesis-dependent mechanisms, thus controlling for hours or days the consolidation of memory required for the discrimination of complex auditory stimuli.

Figure 1.

Preconditioning ip injection of the D1/D5 agonist SKF-38393 into naïve gerbils improves learning, and presession ip injection of the D1/D5 antagonist SCH-23390 into well-trained gerbils impairs performance of the FM discrimination. Data were collected in Experiment 1. (A) Design of pharmacological treatment. Gerbils were randomly assigned to 1 of 2 groups, A and B, and trained on the FM discrimination task once per day for a total of 18 sessions. Training-free intervals of 2 days were interspersed after sessions 5, 10, and 15. Injections of vehicle (Veh), SKF-38393 (SKF; 5 mg/kg), or SCH-23390 (SCH; 0.1 mg/kg) were performed 30 min prior to the indicated training sessions. (B) Discrimination rates D per training session. Arrows indicate the approximate injection times. All data points represent group means ± SEM; (*) significantly different from the value of group A; (^#) significantly different from the value in session 16.

Figure 2.

Postconditioning local injections of SKF-38393 into the gerbil auditory cortex induce a rapamycin-sensitive improvement in the FM discrimination monitored during retraining 24 h later. Data were collected in Experiment 2. Gerbils were trained on the FM discrimination task every 24 h for 3 days. Vehicle (n = 8), or 0.2 mM SKF-38393 (n = 16), or a combination of 0.2 mM SKF-38393 and 60 nM rapamycin (n = 9) was applied twice, that is, immediately and 2 h after completion of the first training session. Discrimination rates D per training session are shown. Arrows indicate the approximate injection times. All data points represent group means ± SEM; (*) significantly different from the value of vehicle-treated controls.

Figure 3.

Local injections of SKF-38393 into the gerbil auditory cortex 1 day before conditioning improve the FM discrimination reaction monitored during retraining 2 days after injections. Data were collected in Experiment 3. Gerbils were trained on the FM discrimination task every 24 h for 3 days. Vehicle (n = 9) or 0.2 mM SKF-38393 (n = 8) was applied twice, that is, 24 and 22 h prior to the beginning of the first training session. Left: Discrimination rates D per training session. Middle: Numbers of hurdle crossings in response to FMs, that is, the sums of correct conditioned responses and false alarms, expressed as percent of total trial number. Right: Escape latencies, that is, the times required to change the compartment after the onset of foot-shock. Arrows indicate the approximate injection times. All data points represent group means ± SEM; (*) significantly different from the value of vehicle-treated controls.

Figure 4.

The SKF-38393–induced increment in FM discrimination learning requires mTOR activity and protein synthesis. Data were collected in Experiment 4. Gerbils were trained on the FM discrimination task every 24 h for 2 days. Injections were locally applied to the auditory cortex. (A) Experiment 4A: 0.2 mM SKF-38393 was applied either alone (n = 10) or in combination with 60 nM rapamycin (n = 12) twice, 24 and 22 h prior to initial training. (B) Experiment 4B: 0.2 mM SKF-38393 was applied twice, that is, 24 and 22 h prior to initial training; vehicle (n = 7) or 60 nM rapamycin (n = 14) was applied twice, that is, immediately and 2 h after completion of initial training. (C) Experiment 4C: Vehicle (n = 5), or 0.2 mM SKF-38393 (n = 3), or 0.2 mM SKF-38393 in combination with 75 mM anisomycin (n = 8) was applied twice, 24 and 22 h prior to initial training. (D) Experiment 4D: 0.2 mM SKF-38393 was applied twice, that is, 24 and 22 h prior to initial training; vehicle (n = 8) or 75 mM anisomycin (n = 9) was applied twice, that is, immediately and 2 h after completion of initial training. Discrimination rates D per training session are shown. Filled arrows indicate the approximate injection times for SKF-38393, open arrows indicate the approximate injection times for vehicle or inhibitors. All data points represent group means ± SEM; (*) significantly different from the value in SKF-38393-treated gerbils; (^§) significantly different from the value in vehicle-treated gerbils.

Figure 5.

Local injections of SKF-38393 into the gerbil auditory cortex 1 day before differential conditioning to FMs contribute to between-session information processing. Data collected in Experiments 3–5 were pooled for groups that received local injections of either vehicle (n = 35) or SKF-38393 (n = 36) one day before, and no injections or vehicle injections shortly after initial training. Each training session was subdivided into 5 blocks of 12 trials. Discrimination rates D per trial block are shown. All data points represent group means ± SEM; (*) significantly different from the value of vehicle-treated controls; (^#) significantly different from the corresponding value in trial block 5 of session 1.

Figure 6.

Photomicrographs demonstrating the distribution of D1 dopamine receptor immunoreactivity in the gerbil brain. (A) Low-power view of a frontal section showing the D1 receptor staining patterns in several cortical areas, including the auditory (AC), visual (VC), and perirhinal cortex (PRh), in the hippocampal formation (HF), in the thalamus (LG, lateral geniculate body; MGB, medial geniculate body), and in the SN. (B) Higher magnification of the area boxed in (A), showing the laminar staining pattern in the primary auditory cortex (field A1). Note the labeled somata of pyramidal neurons in infragranular layers V and VI. (C) High-power view demonstrating D1 receptor–positive large pyramidal cells of layer V. The staining often appears as little punctae at the perikarya of these cells (arrows). (D) Low-power view of a horizontal section showing the D1 receptor staining patterns in several cortical areas, including the auditory (AC) and somatosensory cortex (SC), in the hippocampal formation (HF), and in the CPu complex. (E) High-power view of the area boxed in (D), showing 2 labeled nonpyramidal cells in layer VI of the AC that have the appearance of a multiform (white arrow) and a horizontal cell (black arrow). Orientation in (A) applies to (A–C), orientation in (D) applies to (D, E). Scale bars: 1 mm (A, D), 100 μm (B), and 50 μm (C, E). Other abbreviations: c, caudal; d, dorsal; ec, external capsule; m, medial; V1, primary visual cortex.