Presynaptic homeostatic plasticity rescues long-term depression after chronic Delta 9-tetrahydrocannabinol exposure

Abstract

Alterations of long-term synaptic plasticity have been proposed to participate in the development of addiction. To preserve synaptic functions, homeostatic processes must be engaged after exposure to abused drugs. At the mouse cortico-accumbens synapses, a single in vivo injection of Delta9-tetrahydrocannabinol (THC) suppresses endocannabinoid-mediated long-term depression. Using biochemical and electrophysiological approaches, we now report that 1 week of repeated in vivo THC treatment reduces the coupling efficiency of cannabinoid CB1 receptors (CB1Rs) to G(i/o) transduction proteins, as well as CB1R-mediated inhibition of excitatory synaptic transmission at the excitatory synapses between the prefrontal cortex and the nucleus accumbens (NAc). Nonetheless, we found that cortico-accumbens synapses unexpectedly express normal long-term depression because of a reversible switch in its underlying mechanisms. The present data show that, in THC-treated mice, long-term depression is expressed because a presynaptic mGluR2/3 (metabotropic glutamate receptor 2/3)-dependent mechanism replaces the impaired endocannabinoid system. Thus, in the NAc, a novel form of presynaptic homeostasis rescues synaptic plasticity from THC-induced deficits.

Figure 1.

Activity-dependent LTD in the NAc is blocked after a single in vivo exposure to THC but recovers after repeated treatment. A, Representative experiments showing blockade of activity-dependent LTD in the accumbens after a single in vivo exposure to THC (1d THC). Stimulation of PFCx afferents to the NAc (10 min at 13Hz) induced LTD of the fEPSP in vehicle-treated mice (sham) that was absent after a single in vivo exposure to THC (1d THC, 3 mg/kg). Averaged traces taken at the times indicated by numbers are shown in the insets. Calibration: 0.2 mV (y-axis), 10 ms (x-axis). B, Typical experiments and sample traces showing recovery of 13 Hz-induced LTD in the accumbens of mice treated in vivo with THC for 7 d (7d THC). Calibration: 0.2 mV (y-axis), 20 ms (x-axis). C, Summary data of all of the experiments performed: the LTD induced in the NAc by 10 min at 13Hz stimulation of PFCx afferents does not differ between mice treated for 7 d with THC (7d THC, n = 8) or vehicle (sham, n = 7, p > 0.05). D, Compensatory adaptation of LTD is apparent in response to long, but not short, cortical stimulation. The data show the summary of all of the experiments performed: LTD induced by a 5 min at 10 Hz tetanus did not recover in slices prepared from mice repeatedly exposed to THC (7d THC, n = 4) compared with vehicle-treated mice (sham, n = 3).

Figure 2.

Repeated THC exposure desensitizes CB1R in the NAc. A, Dose–response curves showing the inhibition of the fEPSP amplitude by the cannabinoid agonist CP55,940 in the NAc of mice repeatedly exposed to THC (n = 3–5) or vehicle (n = 3–5). Reduced CP55,940-mediated inhibition of accumbens fEPSP was observed in THC-treated mice (*p < 0.05). B, Perfusion of SR141716A (SR-1; 1μm) did not affect baseline synaptic transmission in the NAc of THC-treated (black circles; n = 3) or vehicle-treated (white circles; n = 3) mice, indicating the absence of residual THC prebound to CB1R. C, CB1R autoradiographic labeling in the accumbens after repeated exposure to THC or vehicle. D, Stimulation of [³⁵S]GTPγS binding by the cannabinoid agonist WIN55,212-2 in the NAc of mice treated with THC (n = 9) or vehicle (n = 10) for 7 d. Impaired ability of the cannabinoid agonist to stimulate [³⁵S]GTPγS binding was observed after repeated exposure to THC (*p<0.05).E, Absence of residual THC in brain sections after 7 d exposure. Similar basal [³⁵S]GTPγS binding levels were measured in the NAc of sham (white bar; n = 10) and 7 d THC-treated (black bar; n = 9) mice, and incubation with the CB1R antagonist SR-1 (1 μm) did not increase basal [³⁵S] GTPγS binding in the NAc of THC-injected mice (gray bar; n = 8).

Figure 3.

Repeated exposure to THC alters neither basal synaptic strength and release probability at the PFCx–NAc synapses nor the basic electrophysiological properties of MSNs. A, Similar input–output curves were obtained when stimulating PFCx afferences and recording the EPSCs evoked in accumbens MSNs from vehicle-treated (sham, n=16) and THC-treated (7d THC, n=15) mice.B, The PPR of the EPSCs did not differ between the MSNs from sham (n=14) and 7 d THC-treated (n = 14) mice. C, Representative 3 s sweeps showing the sEPSCs recorded from MSNs in the NAc of sham and 7 d THC-treated mice. Calibration: 20 pA (y-axis), 200 ms (x-axis). D, E, The distribution of the sEPSCs amplitude (D) and interevent interval (E) did not differ between MSNs from sham (n = 20) and 7 d THC-treated (n = 21) mice. F, G, Typical responses to hyperpolarizing and depolarizing somatic current pulses of an MSN in the NAc of a sham (F) and a 7 d THC-treated (G) mouse. Calibration: 20 pA (y-axis), 50 ms (x-axis). Similar average I–V curves were calculated from MSNs recorded in the NAc of sham (n = 8) and THC-treated (n = 11) mice.

Figure 4.

CB1Rs do not mediate activity-dependent LTD in the NAc of 1 week THC-treated mice. A, The CB1R antagonist SR141716A (SR-1; 1 μm) abolished 10 min at 13 Hz-induced LTD in the NAc of mice treated for 7 d with vehicle but not in the THC-treated group (*p < 0.05). Sample fEPSPs taken at the times indicated by numbers are shown in the insets. Calibration: 0.2 mV (y-axis), 20 ms (x-axis). B, Summary bar graph showing the effects of CB1R blockade on the LTD induced by 10 min at 13 Hz-stimulation of PFCx afferences in the NAc of vehicle-treated mice (*p < 0.05) and after repeated exposure to THC. C, Recovery of CB1R-mediated LTD in the NAc 1 week after repeated THC treatment. SR-1 (1 μm) abolished 10 min at 13 Hz-induced LTD in slices from animals taken 1 week after the last THC injection (*p < 0.05).

Figure 5.

Endocannabinoid-mediated LTD and mGluR2/3-mediated LTD occlude each other in naive animals. A, The data show the summary of all of the experiments performed as follows: mGluR2/3-mediated LTD was first induced by perfusion with the mGluR2/3 agonist LY354740 (200 nm, 10 min) (Robbe et al., 2002c), and the 10 min at 13 Hz-tetanus (Robbe et al., 2002b) was given 70 min thereafter. In these conditions, bath application of LY354740 reliably induced LTD of evoked transmission (fEPSP was 73 ± 3% 50–55 min after LY354740; n = 5) but endocannabinoid-mediated (10 min at 13 Hz) LTD was never observed afterward [fEPSP was 105 ± 8.1% 50–55 min after tetanus (n = 5) in experiments in which mGluR2/3-LTD was induced first with LY354740 versus 78.9 ± 5.6% in control (n = 10); p < 0.05], showing the complete occlusion between both forms of plasticity. B, Conversely, mGluR2/3-dependent LTD was also occluded in slices in which endocannabinoid-mediated LTD was induced first. The data show the summary of all of the experiments performed as follows: perfusion of LY354740 (200 nm, 10 min) after saturation of 10 min at 13 Hz-induced LTD (fEPSP was 83 ± 2.3% 50–55 min after tetanus; n = 11) results in acute inhibition but not in LTD (fEPSP was 94.8 ± 3% 50–55 min after LY354740 in experiments in which CB1R–LTD was induced first by tetanus saturation; n = 11 vs 78.9 ± 5.6%; n = 10 in control; p < 0.05), showing the occlusion of mGluR2/3-dependent LTD after saturation of endocannabinoid-mediated LTD.

Figure 6.

Immunolocalization of CB1R and mGluR2 in the core of the nucleus accumbens. Representative confocal images showing labeling for CB1R (green, left) and mGluR2 (red, middle) in the neuropil of the core of the accumbens. As seen in the merged picture (right), both receptors colocalize in axons (arrow) and punctates reminiscent of synaptic terminals (arrowheads). Scale bar, 10 μm.

Figure 7.

mGluR2/3 mediates activity-dependent LTD in the NAc after repeated in vivo THC administration. A, The mGluR2/3 antagonist LY341495 (200 nm) prevented the expression of 10 min at 13 Hz-induced LTD in NAc slices from THC-treated mice (n = 7) but not in the vehicle-treated group (n = 6) (*p < 0.01). B, Summary bar graph of the effects of the mGluR2/3 antagonists LY341495 (200 nm, black bars) and eGlu (200 μm, gray bars) on 10 min at 13 Hz-induced LTD in the NAc of mice treated with THC or vehicle for 7 d(*p < 0.05).