Acute and chronic modulation of striatal endocannabinoid-mediated plasticity by nicotine

Abstract

The endocannabinoid (eCB) system modulates several phenomena related to addictive behaviors, and drug-induced changes in eCB signaling have been postulated to be important mediators of physiological and pathological reward-related synaptic plasticity. Here, we studied eCB-mediated long-term depression (eCB-LTD) in the dorsolateral striatum, a brain region critical for acquisition of habitual and automatic behavior. We report that nicotine differentially affects ex vivo eCB signaling depending on previous exposure in vivo. In the nicotine-naïve brain, nicotine facilitates eCB-signaling and LTD, whereas tolerance develops to this facilitating effect after subchronic exposure in vivo. In the end, a progressive impairment of eCB-induced LTD is established after protracted withdrawal from nicotine. Endocannabinoid-LTD is reinstated 6 months after the last drug injection, but a brief period of nicotine re-exposure is sufficient to yet again impair eCB-signaling. LTD induced by the cannabinoid 1 receptor agonist WIN55,212-2 is not affected, suggesting that nicotine modulates eCB production or release. Nicotine-induced facilitation of eCB-LTD is occluded by the dopamine D2 receptor agonist quinpirole, and by the muscarinic acetylcholine receptor antagonist scopolamine. In addition, the same compounds restore eCB-LTD during protracted withdrawal. Nicotine may thus modulate eCB-signaling by affecting dopaminergic and cholinergic neurotransmission in a long-lasting manner. Overall, the data presented here suggest that nicotine facilitates eCB-LTD in the initial phase, which putatively could promote neurophysiological and behavioral adaptations to the drug. Protracted withdrawal, however, impairs eCB-LTD, which may influence or affect the ability to maintain cessation.

Keywords: dopamine; endocannabinoid; synaptic plasticity.

Figure 1

Nicotine facilitates endocannabinoid‐mediated long‐term depression (eCB‐LTD). (a) Simplified schematic drawing showing the postulated signaling pathways through which nicotine facilitates eCB‐LTD. Nicotine may activate nicotinic acetylcholine receptors (nAChRs) on dopaminergic terminals, thereby enhancing dopamine release and D2 receptor activation, indirectly reducing the cholinergic tone. This, in turn, decreases the activation of muscarinic acetylcholine receptors (mAChRs), which relieves the break on postsynaptic l‐type calcium channels, thereby facilitating calcium influx and eCB production. (b) Schematic drawing showing the position of recording in the dorsolateral striatum (DLS). (c and d) Ex vivo exposure to nicotine (1 μM) enhanced LTD induced by high‐frequency stimulation (HFS) in a manner that was prevented by pre‐treatment with the nAChR antagonist mecamylamine (10 μM). (e) Upper example traces showing evoked postsynaptic spikes (PSs) at baseline (black) and following HFS (gray) in an aCSF‐treated slice, while lower traces show evoked PSs in a nicotine‐treated slice. Calibrations are 0.2 mV and 2 ms. (f) Pre‐treatment with the dopamine D1 receptor antagonist SCH23390 (0.5 μM) did not prevent nicotine‐induced facilitation of eCB‐LTD. (g) LTD was blocked by the CB1R antagonist AM251 (2 μM) and not restored by nicotine, supporting a role for eCBs in mediating the depression. (h) The fatty acid amide hydrolase (FAAH) inhibitor TC‐2F (5 μM) greatly enhanced HFS‐induced depression and occluded nicotine‐induced facilitation, supporting a role for the eCB anandamide in mediating eCB‐LTD. (i) Synaptic depression induced by the CB1R agonist WIN55,212‐2 (2 μM) was not enhanced by nicotine. (j) Pre‐treatment with the dopamine D2 receptor agonist quinpirole (5 μM) occluded nicotine‐induced facilitation of eCB‐LTD. (k) Nicotine did not promote eCB‐LTD in slices treated with the non‐selective mAChRs antagonist scopolamine (10 μM). Time course figures show mean amplitude as compared with individual baseline ± SEM. The arrows mark time points for HFS stimulation. The number of brain slices (n), obtained from at least three different animals, is indicated in each figure. All experiments were run in parallel to their individual control

Figure 2

Impaired endocannabinoid‐mediated long‐term depression (eCB‐LTD) following protracted abstinence. (a) Time course graph showing locomotor activity during the progress of behavioral sensitization. (b) Bar graph showing behavioral sensitization toward the locomotor‐stimulatory properties of nicotine when comparing the first with the last session. (c) Repeated nicotine‐treatment terminated 1 week prior to recordings did not alter high‐frequency stimulation (HFS)‐induced eCB‐LTD. Facilitation of eCB‐LTD by ex vivo nicotine, however, was not present in brain slices from rats previously exposed to nicotine. (d and e) Endocannabinoid‐LTD was significantly impaired in brain slices from rats treated with nicotine 1 or 3 months earlier. (f) The fatty acid amide hydrolase (FAAH) inhibitor TC‐2F did not restore eCB‐LTD in brain slices from nicotine treated rats. (g) Following 6 months of withdrawal from nicotine eCB‐LTD was reinstated (nicotine) but impaired after brief re‐administration of nicotine (nicotine + 6). (h) The bar graph shows mean PS amplitude ± SEM after 70–75 minutes in brain slices from vehicle, from rats treated with nicotine 6 months earlier (nicotine), and previously treated rats receiving six additional exposures to nicotine (nicotine + 6). Time course figures show mean amplitude as compared with baseline ± SEM. The arrows mark time points for HFS. Significance compared with baseline *P < 0.05. The number of brain slices (n), obtained from at least three different animals, is indicated in each figure

Figure 3

Dopamine D2 receptor activation rescues impaired endocannabinoid‐mediated long‐term depression (eCB‐LTD) after 3 months of nicotine abstinence. (a) Endocannabinoid‐LTD was impaired in brain slices from rats treated 3 months earlier with nicotine. (b) Activation of presynaptic mGluR2/3s with LY354740‐induced LTD in both treatment groups. (c) WIN55,212‐2‐induced LTD independent of treatment suggesting that presynaptic eCB‐signaling remains intact during nicotine withdrawal. (d) Group I mGluR agonist DHPG selectively suppressed postsynaptic spike (PS) amplitude in brain slices from vehicle‐treated rats. (e and f) Neither nicotine nor bicuculline restored eCB‐LTD in brain slices from nicotine‐treated rats. (g) Example traces show evoked PS at baseline (black) and following high‐frequency stimulation (HFS; gray) in a slice from a vehicle‐treated rat. (h) Simplified schematic drawing showing postulated signaling pathways underlying impaired eCB‐LTD. Dopamine D2 receptor tone is reduced following protracted abstinence from nicotine, leading to enhanced cholinergic activity and muscarinic acetylcholine receptor (mAChR) activation. This, in turn, reduces l‐type calcium channel activation and decreases the chance for postsynaptic calcium to reach the level required for eCB production. (i) Activation of dopamine D2 receptors prior to HFS reinstated eCB‐LTD in slices from nicotine‐treated rats, (j) as did the mAChR antagonist scopolamine. (k) Example traces show evoked PS at baseline (black) and following HFS (gray) in a slice from a nicotine‐treated rat. Calibration is 2 ms and 0.1 mV. Time course figures show mean amplitude as compared with baseline ± SEM. The arrows mark time points for HFS. The number of brain slices (n) is indicated in each figure, and taken from at least three different animals