Infralimbic BDNF/TrkB enhancement of GluN2B currents facilitates extinction of a cocaine-conditioned place preference

Abstract

Brain-derived neurotrophic factor (BDNF) regulates synaptic activity and behavioral flexibility, and reduction of BDNF strongly predicts psychiatric disorders and cognitive dysfunction. Restoration of BDNF-dependent activity could alleviate these impairments, but BDNF has limited clinical utility due to its pharmacokinetics. Here we demonstrate that activation of a primary BDNF target, the tropomyosin-related kinase B (TrkB) receptor, enhances the amplitude and prolongs the decay kinetics of N-methyl-d-aspartate receptor (NMDAR) currents in male rat infralimbic prefrontal pyramidal neurons. Moreover, these effects were prevented and reversed by blockade of NMDARs containing the GluN2B subunit. Our results show that this signaling cascade bidirectionally regulates extinction of a cocaine-induced conditioned place preference (CPP), a task that requires behavioral flexibility. Blockade of infralimbic TrkB receptors or GluN2B-containing NMDARs disrupted consolidation of extinction of the CPP. In contrast, extinction was strengthened by potentiation of TrkB receptor activity with infralimbic infusions of BDNF or systemic injections of 7,8 dihydroxyflavone (7,8DHF), the newly synthesized TrkB receptor agonist. The 7,8DHF-induced enhancement of extinction was prevented by infralimbic infusions of a GluN2B-specific receptor antagonist, demonstrating that TrkB receptor activation enhances extinction of cocaine-CPP via GluN2B-containing NMDARs. Together, infralimbic TrkB receptor activation strengthens GluN2B-containing NMDAR currents to support extinction learning. TrkB receptor agonists would therefore be useful as pharmacological adjuncts for extinction-based therapies for treatment of psychiatric disorders associated with reduced BDNF activity.

Keywords: NR2B-containing NMDA receptor; TrkB receptor; brain-derived neurotrophic factor; extinction learning; medial prefrontal cortex; patch-clamp electrophysiology.

Figure 1.

Characterization of NMDAR EPSCs in IL-mPFC. A, Photomicrographs of a biocytin-filled pyramidal neuron and GABAergic interneuron. Scale bar, 100 μm. B, Example traces of NMDAR EPSCs evoked by 50, 150, and 350 μA stimulation of IL-mPFC neurons. C, NMDAR EPSCs were larger in pyramidal neurons as compared with GABAergic interneurons. D, Example traces of IL-mPFC NMDAR-dependent EPSCs before (gray) and after (black) pyramidal neurons were treated with ifenprodil (n = 7) or APV (n = 3). E, Ifenprodil significantly reduced evoked EPSCs in pyramidal neurons, and APV abolished evoked EPSCs. F, Baseline and final 5 min of recording were individually averaged; **p < 0.01 compared with baseline, ∧p < 0.01 compared with ifenprodil-treated neurons. Scale bars: vertical, 50 pA; horizontal, 50 ms. Error bars indicate SEM.

Figure 2.

TrkB receptor activation enhances IL-mPFC GluN2B-containing NMDAR EPSCs in brain slices taken from naive rats. A, Example traces of IL-mPFC NMDAR-dependent EPSCs before (gray) and after (black) vehicle (n = 8), 7,8DHF (n = 7), or 7,8DHF+ifenprodil (n = 6) treatment. B, C, 7,8DHF enhanced NMDAR currents, and this effect was prevented by ifenprodil. D, Example traces from neurons that were previously treated with vehicle (n = 7) or 7,8DHF (n = 3) before (gray) and after (black) ifenprodil application. E, F, Ifenprodil reversed the 7,8DHF-induced potentiation of NMDAR-dependent EPSCs. G, Example traces before (gray) and after (black) 7,8DHF treatment. H, 7,8DHF (orange bars; n = 10) prolonged the decay half-time compared with control (black bars; n = 8), and this effect was both prevented by ifenprodil (gray bars; n = 10) and reversed by ifenprodil; **p < 0.01 compared with control at the same time point, ∧p < 0.01 compared with 7,8DHF-treated neurons at 40 min. Scale bars: vertical, 50 pA; horizontal, 50 ms. Error bars indicate SEM. Ifen, Ifenprodil.

Figure 3.

TrkB receptor activation enhances IL-mPFC GluN2B-containing NMDAR EPSCs in brain slices taken from cocaine-conditioned rats. A, Example traces of IL-mPFC NMDAR-dependent EPSCs before (gray) and after (black) 7,8DHF (n = 3) or 7,8DHF+ifenprodil (n = 4) treatment. B, C, 7,8DHF enhanced NMDAR currents and this effect was prevented by ifenprodil. The orange line represents EPSC data from 7,8DHF-treated neurons in brain slices taken from naive rats. ***p < 0.001 compared with 7,8DHF+ifenprodil-treated neurons at 40 min. Scale bars: vertical, 50 pA; horizontal, 50 ms. Error bars indicate SEM. Ifen, Ifenprodil.

Figure 4.

IL-mPFC BDNF and GluN2B-containing NMDARs mediate consolidation of extinction of a cocaine-CPP. A, Coronal drawings (bregma, +3.72 mm) showing injector tip placements for vehicle (n = 8) and BDNF (n = 8) IL-mPFC infusions. B, IL-mPFC infusions (arrows) of BDNF before the first CPP extinction trial enhanced extinction of the CPP. C, Coronal drawings (bregma, +3.72 mm) showing injector tip placements for vehicle (n = 11) and ifenprodil (n = 12) IL-mPFC infusions. D, IL-mPFC infusions (arrows) of ifenprodil immediately after each CPP extinction trial prevented extinction of the CPP; ***p < 0.001, **p < 0.01, *p < 0.05. Error bars indicate SEM.

Figure 5.

Extinction of a cocaine-CPP requires and is enhanced by TrkB receptor activation of GluN2B-containing NMDA receptors. A, Coronal drawings (bregma, +3.72 mm) showing injector tip placements for IL-mPFC infusions. B, IL-mPFC infusions (arrows) of ANA-12 (n = 8), but not vehicle (n = 11), before the first CPP trial prevented extinction of the CPP. C, Systemic injections (black arrows) of 7,8DHF (n = 21) before the first CPP trial enhanced extinction compared with vehicle (n = 9), an effect blocked by IL-mPFC infusions (gray arrows) of ifenprodil (n = 10) immediately after the first CPP trial; ***p < 0.001, **p < 0.01. Error bars indicate SEM.