Methylphenidate facilitates learning-induced amygdala plasticity

Abstract

Although methylphenidate (Ritalin) has been used therapeutically for nearly 60 years, the mechanisms by which it acutely modifies behavioral performance are poorly understood. Here we combined intra-lateral amygdala in vivo pharmacology and ex vivo electrophysiology to show that acute administration of methylphenidate, as well as a selective dopamine transporter inhibitor, facilitated learning-induced strengthening of cortico-amygdala synapses through a postsynaptic increase in AMPA receptor-mediated currents, relative to those in saline-treated rats. Furthermore, local administration of methylphenidate in the lateral amygdala enhanced cue-reward learning through dopamine D1 receptor-dependent mechanisms and suppressed task-irrelevant behavior through D2 receptor-dependent mechanisms. These findings reveal critical and distinct roles for dopamine receptor subtypes in mediating methylphenidate-induced enhancements of neural transmission and learning performance.

Figure 1

MPH enhances task performance by altering different aspects of behavior through distinct D1 and D2 receptor–dependent mechanisms. (a) Intra-LA drug infusion alters reward earning (F_6,47 = 5.161, P < 0.001). Relative to saline-treated rats, MPH and GBR groups earned significantly more rewards per minute, whereas SCH-treated rats earned a significantly fewer. MPH+SCH-treated, but not MPH+RAC-treated, rats earned significantly fewer than MPH-treated alone. (b) Task efficiency was altered by intra-LA drug infusion (F_6,47 = 3.886, P = 0.004). Relative to saline, MPH, GBR and MPH+RAC-treated groups all showed significantly higher task efficiency. The MPH+SCH group, but not the MPH+RAC, showed lower task efficiency than the group treated with MPH alone. (c) Relative to saline, SCH-treated rats showed significantly lower task accuracy, and MPH+SCH-treated rats showed an attenuation of the enhancements induced by MPH alone, but MPH+RAC-treated rats did not differ from those treated with MPH alone (F_6,47 = 3.806, P = 0.019). (d) Relative to the saline-treated group, MPH and GBR-treated groups showed significantly less off-task behavior, whereas RAC-treated rats showed significantly more (F_6,47 = 8.024, P < 0.001). In a–d, numbers in bars indicate rats per group. All values are mean ± s.e.m. One-way analysis of variance followed by all-pairwise multiple comparison procedure (Fisher least significant difference method; *P < 0.05, **P < 0.01).

Figure 2

Intra-LA NXT before training enhances memory retention but not acute task performance. Left column, performance on the initial training session; right column, performance during a 20-min memory retrieval test. No significant differences were observed during training, but NXT dose-dependently enhanced task accuracy (F_3,26 = 4.209, P = 0.002) during a test session on the next day, on which no infusions were performed, suggesting that NET blockade may enhance memory retention in this task. Saline vehicle, N = 8; NXT 2 μg per side, N = 6; NXT 4 μg per side, N = 7; NXT 8 μg per side, N = 6. **P < 0.01.

Figure 3

Inhibition of the dopamine transporter gates cortico-amygdala synaptic potentiation. (a) Representative traces of AMPAR/NMDAR ratios evoked from thalamic and cortical afferents for each group. (b,c) AMPAR/NMDAR ratios evoked from thalamic (F_8,62 = 3.471, P = 0.003) or cortical (F_8,59 = 3.557, P = 0.002) afferents for each drug-treatment group after training were significantly altered. Inset shows rats treated with saline, MPH and GBR that were returned to their home cages in lieu of training. Numbers in bars indicate the number of cells per group. (b) SCH-treated rats show a significant decrease in thalamo-amygdala AMPAR/NMDAR. (c) MPH and GBR groups show significant increases in cortico-amygdala AMPAR/NMDAR. *P < 0.05, **P < 0.01.

Figure 4

Dopamine modulates learning-induced increases in mEPSC amplitude but not frequency. (a) Sample mEPSCs from each drug-treatment group. (b) Mean mEPSC amplitude for each group varied (F_8,113 = 10.177, P < 0.001) with treatment. MPH, GBR and MPH+RAC groups had higher, whereas SCH-treated rats had lower, mEPSC amplitude than did saline controls. Inset: saline-, MPH- and GBR-treated home-cage controls. (c) Cumulative probability plot of mEPSC amplitude for representative cells from each group; 1 pA bins. (d) No significant change in mEPSC frequency of any groups relative to saline (F_8,113 = 0.202, P = 0.990). (e) Cumulative probability plot of mEPSC frequency; 20-ms bins. *P < 0.05, **P < 0.01, ***P < 0.001.

Figure 5

D1R antagonism in the lateral amygdala attenuates learning-induced synaptic changes. (a–h) Rats given, before training, bilateral infusions of SCH compared with rats given unilateral infusions of SCH and contralateral infusions of saline. (a) Unilateral infusion of SCH and saline significantly decreased reward earning (F_2,19 = 5.107, P = 0.018) relative to bilateral saline infusion (P = 0.009) and did not differ from bilateral SCH. (b–d) Rats treated with unilateral SCH and saline infusions did not show significant differences from rats treated with bilateral saline in task efficiency, task accuracy or off-task behavior. (f,h) Unilateral infusions of SCH and saline provide a within-subjects (N = 6 rats; n = 11 cells from saline-treated side; n = 9 cells from SCH-treated side) comparison of the effects of D1 receptor antagonism on learning-induced plasticity. (e–h) For both between-subjects and within-subjects comparisons, treatment with SCH significantly attenuated learning-induced increases in mEPSC amplitude (*P < 0.05, **P < 0.01, ***P < 0.001, Student’s t-test) relative to saline (e,f), with no change in mEPSC frequency (g,h).

Figure 6

Dopamine signaling in the amygdala is necessary for mediating enhancements of learning performance induced by systemic administration of MPH. Behavioral measures of four groups of rats treated before training with (1) i.p. saline and intra-LA saline (N = 8 rats), (2) i.p. MPH and intra-LA saline (N = 8 rats), (3) i.p. MPH and intra-LA SCH (N = 7 rats) and (4) i.p. MPH and intra-LA RAC (N = 9 rats). (a–c) Intra-LA infusion of SCH significantly attenuated, whereas RAC spared, enhancements induced by systemic MPH in reward earning (F_3,31 = 8.568, P < 0.001), task efficiency (F_3,31 = 20.194, P < 0.001) and task accuracy (F_3,31 = 6.004, P = 0.003). (d) Intra-LA infusion of RAC or SCH significantly attenuated reductions induced by systemic MPH in off-task behavior (F_3,31 = 4.48, P = 0.011). In a–d, *P < 0.05, **P < 0.01,***P < 0.001.