Cocaine evokes projection-specific synaptic plasticity of lateral habenula neurons

Abstract

Addictive drugs share the ability to increase dopamine (DA) levels and trigger synaptic adaptations in the mesocorticolimbic system, two cellular processes engaged in the early stages of drug seeking. Neurons located in the lateral habenula (LHb) modulate the activity of DA neurons and DA release, and adaptively tune goal-directed behaviors. Whether synaptic modifications in LHb neurons occur upon drug exposure remains, however, unknown. Here, we assessed the influence of cocaine experience on excitatory transmission onto subsets of LHb neurons using a combination of retrograde tracing and ex vivo patch-clamp recordings in mice. Recent evidence demonstrates that AMPA receptors lacking the GluA2 subunit mediate glutamatergic transmission in LHb neurons. We find that cocaine selectively potentiates AMPA receptor-mediated EPSCs in LHb neurons that send axons to the rostromedial tegmental nucleus, a GABAergic structure that modulates the activity of midbrain DA neurons. Cocaine induces a postsynaptic accumulation of AMPA receptors without modifying their subunit composition or single-channel conductance. As a consequence, a protocol pairing presynaptic glutamate release with somatic hyperpolarization, to increase the efficiency of GluA2-lacking AMPA receptors, elicited a long-term potentiation in neurons only from cocaine-treated mice. This suggests that cocaine resets the rules for the induction of synaptic long-term plasticity in the LHb. Our study unravels an early, projection-specific, cocaine-evoked synaptic potentiation in the LHb that may represent a permissive step for the functional reorganization of the mesolimbic system after drug exposure.

Figure 1.

Cocaine increases the efficacy of excitatory transmission onto LHb^→RMTg neurons. A, LHb-containing sagittal sections illustrating the injections sites with retrobeads in the RMTg (red) and the VTA (green, left panels). Confocal images (10×/40×) illustrating retrogradely labeled LHb^→RMTg and LHb^→VTA neurons (right panels). sm, Stria medullaris; fr, fasciculus retroflexus. B, Sample traces of mEPSCs obtained in LHb^→RMTg neurons in slices from saline-treated (black) and cocaine-treated (red) mice. C, Cumulative probability distributions (KS test, p < 0.0001) for a representative LHb^→RMTg neuron, bar-graph and scatter plot for mEPSC amplitude in the saline and cocaine groups (n = 10–15; saline 14.1 ± 1.1 pA; cocaine 17.9 ± 0.9 pA; **p < 0.01; t₍₂₂₎ = 2.9). D, Same as C for mEPSC frequency (n = 10–15; saline 3 ± 0.9 Hz; cocaine 4.9 ± 1.3 Hz; p > 0.05; t₍₂₂₎ = 0.9). E, Same as B for LHb^→VTA neurons. F, Same as C for LHb^→VTA neurons (KS test, p > 0.05) (n = 9–14; saline 16.7 ± 2.2 pA; cocaine 14.08 ± 1.3 pA; p > 0.05; t₍₂₁₎ = 1.1). G, Same as F for mEPSC frequency (n = 9–14; saline 4.6 ± 1.8 Hz; cocaine 4.1 ± 0.7 Hz; p > 0.05; t₍₂₁₎ = 0.3). Sal, Saline; Coc, cocaine.

Figure 2.

Synaptic potentiation in LHb^→RMTg neurons requires a larger synaptic pool of AMPARs with no change in subunit composition. A, Averaged traces and bar-graph illustrating the PPR in the saline and cocaine groups (saline 0.82 ± 0.1; cocaine 0.74 ± 0.05; p > 0.05; t₍₁₂₎ = 0.64; n = 6–7). B, Sample traces illustrating AMPA and NMDA-EPSCs recorded at +40 mV and AMPA/NMDA ratios in LHb^→RMTg neurons in slices from saline (black) and cocaine (red) groups (n = 5–7; saline 1.1 ± 0.1; cocaine 2.6 ± 0.3; **p < 0.01; t₍₉₎ = 4.9). C, Sample traces of AMPA-EPSCs recorded at −70, 0, and +40 mV and RIs in LHb^→RMTg neurons in slices from saline (black) and cocaine (red) groups (n = 6; saline 3.85 ± 1.12; cocaine 3.1 ± 0.3; p > 0.05; t₍₁₅₎ = 0.7). D, Example of peak-scaled NSFA of LHb^→RMTg neurons in the saline- and cocaine-treated group. Insets, Overlay of 50 consecutive traces. E, F, Pooled data for conductance (γ) and number of channels (N) open at the peak (n = 5–8; γ: saline 12.9 ± 0.4 pS; cocaine 13.1 ± 0.9 pS; p > 0.05; t₍₁₁₎ = 0.1; N: saline 19.2 ± 4.8; cocaine 42 ± 4.2; **p < 0.01; t₍₁₁₎ = 3.4). Sal, Saline; Coc, cocaine.

Figure 3.

Cocaine-evoked plasticity in LHb^→RMTg neurons is postsynaptic. A, Confocal image of LHb neurons filled with Alexa Fluor Red-594 (left) and magnified dendrite (right). B, Sample mEPSCs and (L)-EPSCs. Note blockade of all events with NBQX. C, Mean values and scatter plot for mEPSC and (L)-EPSC amplitudes and decays (90–37%) in naive animals (n = 20). D, Averaged traces of (L)-EPSC at −60, 0, and +40 mV in slices from saline and cocaine-treated animals. E, Box and scatter plot for the AMPA/NMDA ratios in the saline and cocaine groups (n = 9–13 cells and 24–39 synapses; ***p < 0.001; t₍₆₁₎ = 5.3). F, G, (L)-AMPA-EPSC (L-AMPA) and l-NMDA-EPSC (L-NMDA) vs AMPA/NMDA ratio, scatter plot (empty circles) and mean values (full circles) (L-AMPA-EPSCs: saline 16.5 ± 1.12 pA; cocaine 20.7 ± 1; **p < 0.01; t₍₆₁₎ = 2.68; (L)-NMDA-EPSCs: saline 15.1 ± 1.6 pA; cocaine 12.8 ± 1.7; p > 0.05; t₍₆₁₎ = 0.96). Sal, Saline; Coc, cocaine.

Figure 4.

Cocaine evokes metaplasticity in LHb^→RMTg neurons. A, Averaged traces and amplitudes versus time plot of AMPAR-EPSCs in LHb^→RMTg neurons before and after conditioning protocol (arrow) in slices of saline and cocaine groups (n = 6–8; saline 86.6 ± 3.7%; cocaine 145.2 ± 11.8%; p < 0.01; t₍₁₂₎ = 4.1). B, Same as A but for LHb^→VTA neurons (n = 5–6; saline 77.9 ± 4.1%; cocaine 78.2 ± 2.5%; p > 0.05; t₍₁₀₎ = 0.04). C, Example of peak-scaled NSFA of mEPSCs of an LHb^→RMTg neuron before and after bath application of 0.1 μm NBQX in the cocaine group. Pooled data for amplitudes, conductances (γ), and number of channels (N) open at the peak of the response (n = 13; amplitude: baseline 20.1 ± 1.2 pA; NBQX 16.2 ± 1 pA; **p < 0.01; t₍₁₂₎ = 6.7; γ: baseline 13.3 ± 0.6 pS, NBQX 13.3 ± 0.4 pS; p > 0.5; t₍₁₂₎ = 0.1; N: baseline 36.1 ± 1.7; NBQX 27.2 ± 1.6; ***p < 0.001; t₍₁₂₎ = 6.6). D, Averaged sample traces and amplitude versus time plot of AMPAR-EPSCs from LHb^→RMTg neurons before and after the conditioning protocol in slices from cocaine-exposed animals in the presence or absence of NBQX 0.1 μm (n = 7; control 149.3 ± 15.5%; NBQX 80.42 ± 9.9%; **p < 0.01; t₍₁₂₎ = 3.7).