Brain-derived neurotrophic factor controls cannabinoid CB1 receptor function in the striatum

Abstract

The role of brain-derived neurotrophic factor (BDNF) in emotional processes suggests an interaction with the endocannabinoid system. Here, we addressed the functional interplay between BDNF and cannabinoid CB(1) receptors (CB(1)Rs) in the striatum, a brain area in which both BDNF and CB(1)s play a role in the emotional consequences of stress and of rewarding experiences. BDNF potently inhibited CB(1)R function in the striatum, through a mechanism mediated by altered cholesterol metabolism and membrane lipid raft function. The effect of BDNF was restricted to CB(1)Rs controlling GABA-mediated IPSCs (CB(1)R(GABA)), whereas CB(1)Rs modulating glutamate transmission and GABA(B) receptors were not affected. The action of BDNF on CB(1)R(GABA) function was tyrosine kinase dependent and was complete even after receptor sensitization with cocaine or environmental manipulations activating the dopamine (DA)-dependent reward system. In mice lacking one copy of the BDNF gene (BDNF(+/-)), CB(1)R(GABA) responses were potentiated and were preserved from the action of haloperidol, a DA D(2) receptor (D(2)R) antagonist able to fully abolish CB(1)R(GABA) function in rewarded animals. Haloperidol also enhanced BDNF levels in the striatum, suggesting that this neurotrophin may act as a downstream effector of D(2)Rs in the modulation of cannabinoid signaling. Accordingly, 5 d cocaine exposure both reduced striatal BDNF levels and increased CB(1)R(GABA) activity, through a mechanism dependent on D(2)Rs. The present study identifies a novel mechanism of CB(1)R regulation mediated by BDNF and cholesterol metabolism and provides some evidence that DA D(2)R-dependent modulation of striatal CB(1)R activity is mediated by this neurotrophin.

Figures

Figure 1.

BDNF blocks CB1R(GABA) function via tyrosine kinase activation. A, The graph shows that the depressant effect of the CB1 receptor agonist HU210 on sIPSC was completely abolished by incubation of striatal slices with BDNF (control, n = 9 cells; BDNF, n = 11 cells). B, DHPG was ineffective in reducing mIPSC frequency in BDNF-treated slices (control, n = 7 cells; BDNF, n = 9 cells). C, D, Cumulative distribution of sIPSC interevent interval recorded from control slices (C) and from BDNF-treated slices (D) before and during the application of HU210. The effect of HU210 on cumulative probability distribution was blocked in BDNF-treated slices, as reveled by Kolmogorov–Smirnov test. E, F, Amplitude–frequency histograms of sIPSCs before and during the application of HU210 in control (E) and BDNF-treated (F) slices. G, Lavendustin A and K252a, inhibitors of TrkB tyrosine kinase, did not affect HU210 responses per se on sIPSC frequency (n = 7 cells for each group) but were able to rescue the effect of HU210 in BDNF-treated slices (lavendustin A, n = 10 cells; K252a, n = 8 cells). H, The graph shows that the HU210-induced reduction on sIPSC frequency was normal after 10 min of BDNF application (n = 9 cells). The effect of HU210 was fully blocked after 30 and 60 min of BDNF application (n = 11 cells for each group). The electrophysiological traces below are examples of voltage-clamp recordings showing the loss of sIPSC frequency reduction induced by HU210 in a neuron from BDNF-treated slices and the recovery of the effects of HU210 induced by lavendustin A. *p < 0.05; **p < 0.01.

Figure 2.

GABAB receptors and CB1Rs(Glu) are insensitive to BDNF. A, The depressant effect of the GABAB receptor agonist baclofen on sIPSC frequency was similar in control and BDNF-treated slices (control, n = 8 cells; BDNF, n = 10 cells). B, The graph shows that the depressant action of HU210 on sEPSC frequency was similar in control and BDNF-treated slices (control, n = 9 cells; BDNF, n = 9 cells). **p < 0.01.

Figure 3.

In vivo manipulations of BDNF levels alters the sensitivity of CB1Rs(GABA). A, HU210 was ineffective in reducing sIPSC frequency in mice receiving BDNF via a single intracerebroventricular injection (i.c.v. vehicle, n = 10 cells; i.c.v. BDNF, n = 11 cells). B, HU210-mediated inhibition of sIPSC frequency was selectively potentiated in striatal neurons from mice with partial genetic BDNF depletion [wild type (WT), n = 8 cells; BDNF+/−, n = 9 cells]. This effect was completely prevented by preincubation with the CB1 receptor antagonist AM251 (n = 8 cells for each group). The electrophysiological traces below are examples of voltage-clamp recordings before and during the application of HU210 in control and BDNF+/− mice. C, D, Cumulative distribution of sIPSC interevent interval recorded from wild-type and BDNF+/− mice before and during the application of HU210. The effect of HU210 on cumulative probability distribution was enhanced in BDNF+/− mice, as reveled by Kolmogorov–Smirnov test. E, F, Amplitude–frequency histograms of sIPSCs before and during the application of HU210 in slices from wild-type and BDNF+/− mice. *p < 0.05; **p < 0.01.

Figure 4.

Involvement of cholesterol metabolism in BDNF-mediated suppression of CB1R(GABA) sensitivity. Reduction of cholesterol contents with MCD (30 min incubation, n = 10 cells) or mevastatin (60 min incubation, n = 9 cells) did not affect per se HU210 responses on sIPSC frequency. Both treatments were, however, able to rescue the effect of HU210 in BDNF (60 min)-treated slices (BDNF plus MCD, n = 9 cells; BDNF plus mevastatin, n = 9 cells). The electrophysiological traces are examples of voltage-clamp recordings showing the loss of sIPSC frequency reduction induced by HU210 in a neuron from BDNF-treated slices and the recovery of the effects of HU210 induced by MCD. **p < 0.01.

Figure 5.

BDNF and haloperidol block CB1Rs(GABA) in rewarded mice. A, The graph shows that HU210-induced reduction of sIPSC were potentiated after 5 d of cocaine treatment (n = 11 cells), 7 d of sucrose exposure (n = 12 cells), and after 15 d of exposure to running wheel (n = 12 cells). Intracerebroventricular injection of BDNF, at the end of cocaine (n = 10 cells), sucrose (n = 8 cells), and wheel (n = 8 cells) exposure protocols, fully blocked the HU210-mediated inhibition of sIPSC. B–D, The DA D2R antagonist haloperidol fully blocked the HU210-mediated inhibition of striatal sIPSC frequency in cocaine (5 d)-exposed mice (B), in sucrose (7 d)-exposed mice (C), and in wheel (15 d)-exposed mice (D) (n = 9 cells for each group). *p < 0.05; **p < 0.01.

Figure 6.

Effects of cocaine and natural rewards on striatal and hippocampal BDNF levels. A, The graph shows BDNF levels in the striatum and the hippocampus in control mice, in cocaine-treated mice, and in cocaine plus haloperidol-treated mice. BDNF levels were reduced in the hippocampus and the striatum of cocaine-treated mice. Treatment with haloperidol blocked cocaine-induced BDNF reduction (n = 6 mice for each group and brain area). B, The graph shows BDNF levels in the striatum and the hippocampus in control mice, in sucrose (7 d)-exposed mice, and in wheel (15 d)-exposed mice. BDNF levels were increased in the hippocampus of both groups of rewarded mice, whereas striatal contents of BDNF were similar to control mice (n = 8 mice for each group and brain area). *p < 005; **p < 001.

Figure 7.

Haloperidol does not block CB1Rs(GABA) in BDNF+/− mice and increases BDNF levels in the striatum. A, Haloperidol failed to block the effects of HU210 on sIPSC frequency in BDNF+/− mice (n = 11 cells). The electrophysiological traces on the right are examples of voltage-clamp recordings before and during the application of HU210 in control and haloperidol-treated BDNF+/− mice. B, BDNF levels were enhanced in both the striatum and the hippocampus of mice treated with intraperitoneal haloperidol compared with mice treated with intraperitoneal control vehicle (n = 8 mice for each group and brain area). C, The graph shows that HU210-induced reduction of sIPSC was fully blocked in haloperidol-treated mice (vehicle, n = 10 cells; haloperidol, n = 9 cells). *p < 0.05; **p < 0.01.