The anandamide transport inhibitor AM404 reduces the rewarding effects of nicotine and nicotine-induced dopamine elevations in the nucleus accumbens shell in rats

Abstract

Background and purpose: The fatty acid amide hydrolase inhibitor URB597 can reverse the abuse-related behavioural and neurochemical effects of nicotine in rats. Fatty acid amide hydrolase inhibitors block the degradation (and thereby magnify and prolong the actions) of the endocannabinoid anandamide (AEA), and also the non-cannabinoid fatty acid ethanolamides oleoylethanolamide (OEA) and palmitoylethanolamide (PEA). OEA and PEA are endogenous ligands for peroxisome proliferator-activated receptors alpha (PPAR-α). Since recent evidence indicates that PPAR-α can modulate nicotine reward, it is unclear whether AEA plays a role in the effects of URB597 on nicotine reward.

Experimental approach: A way to selectively increase endogenous levels of AEA without altering OEA or PEA levels is to inhibit AEA uptake into cells by administering the AEA transport inhibitor N-(4-hydroxyphenyl)-arachidonamide (AM404). To clarify AEA's role in nicotine reward, we investigated the effect of AM404 on conditioned place preference (CPP), reinstatement of abolished CPP, locomotor suppression and anxiolysis in an open field, and dopamine elevations in the nucleus accumbens shell induced by nicotine in Sprague-Dawley rats.

Key results: AM404 prevented the development of nicotine-induced CPP and impeded nicotine-induced reinstatement of the abolished CPP. Furthermore, AM404 reduced nicotine-induced increases in dopamine levels in the nucleus accumbens shell, the terminal area of the brain's mesolimbic reward system. AM404 did not alter the locomotor suppressive or anxiolytic effect of nicotine.

Conclusions and implications: These findings suggest that AEA transport inhibition can counteract the addictive effects of nicotine and that AEA transport may serve as a new target for development of medications for treatment of tobacco dependence.

Linked articles: This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Figure 1

Effects of AM404 and THC on CPP. (A) When different doses of AM404 (1.25 to 10 mg·kg⁻¹, i.p.) were examined in the CPP test, only the higher dose of 10 mg·kg⁻¹ was able to induce a significant CPP (P < 0.05 vs. corresponding pretest). (B) THC alone (0.1 to 3 mg·kg⁻¹) produced no significant CPP or aversion compared with the vehicle group (P= ns). All data are expressed as time spent in s (mean ± SEM, n= 9–10) in the nicotine-paired compartment during a15 min period. Pretest sessions and 15 min Test sessions and were analysed by one-way anova between subjects.

Figure 2

Effects of AM404 and THC on development of nicotine-induced CPP. A nicotine dose of 0.4 mg·kg⁻¹ produced significant CPP (**P < 0.001 vs. corresponding pretest; n= 9–10). When rats were pretreated with AM404 (5 mg·kg⁻¹) before each nicotine conditioning session, CPP did not develop (P= ns; n= 9–10) and AM404 did not potentiate effects of a low 0.05 mg·kg⁻¹ ineffective threshold dose of nicotine. When rats were pretreated with THC (1 mg·kg⁻¹) before each nicotine conditioning session, nicotine-induced CPP was not reduced (**P < 0.01 vs. corresponding pretest; n= 9–10) and THC potentiated the effects of a low 0.05 mg·kg⁻¹ dose of nicotine (*P < 0.05 vs. corresponding pretest; n= 9–10). All data are expressed as time spent in s (mean ± SEM) in the nicotine-paired compartment during a 15-min period. Pretest session and 15 min Test session and were analysed by one-way anova between subjects.

Figure 3

Effects of AM404 on nicotine-induced reinstatement of abolished CPP. (A) A priming injection of nicotine (0.1 mg·kg⁻¹) completely reinstated the attenuated CPP (extinction) developed by a nicotine dose of 0.4 mg·kg⁻¹ (**P < 0.001 Test vs. Pretest; ##P < 0.001 Extinction vs. Test; $$P < 0.001 Reinstatement vs. Extinction; P > 0.05 Reinstatement vs. Test; n= 9). (B) AM404 pretreatment prevented nicotine-induced reinstatement of the CPP (*P < 0.05 Test vs. Pretest; ##P < 0.01 Extinction vs. Test; P > 0.05 Reinstatement vs. Extinction; §P < 0.05 Reinstatement vs. Test; n= 10). (C) AM404 alone failed to reinstate the attenuated CPP (**P < 0.001 Test vs. Pretest; ##P < 0.001 Extinction vs. Test; P > 0.05 Reinstatement vs. Extinction; §§P < 0.001 Reinstatement vs. Test; n= 7). All data are expressed as time spent in s (mean ± SEM) in the nicotine-paired compartment during a 15 min period. Pretest sessions, 15 min Test sessions, 15 min Extinction sessions and 15 min Reinstatement sessions and were analysed by one-way anova between subjects.

Figure 4

Effects of nicotine, AM404 and THC on locomotor activity. (A) Distance travelled, expressed in cm. (B) Amount of time spent in the centre zone during the 20 min session, as a measure of anxiety. (*P < 0.05; **P < 0.01; all comparisons vs. Vehicle-saline condition). Nicotine (0.4 mg·kg⁻¹) decreased general activity (distance travelled) and increased time spent in the centre zone, which may indicate an anxiolytic effect. Pretreatment with AM404 or THC did not alter the effects of 0.4 mg·kg⁻¹ nicotine, but general activity was decreased by the combination of 1 mg·kg⁻¹ THC with 0.05 mg·kg⁻¹ nicotine.

Figure 5

Effects of AM404 on nicotine-induced elevations in dopamine levels in the nucleus accumbens shell. (A) Pretreatment with AM404 (5 mg·kg⁻¹), but not its vehicle, given before nicotine (0.4 mg·kg⁻¹), significantly reduced the increase in extracellular dopamine levels produced by nicotine (two-way anovaP= 0.0019, n= 5). AM404 alone did not alter dopamine levels. Arrows represent time of injection of each drug. Results are means, with vertical bars representing SEM, of dopamine levels in 20 min dialysate samples, expressed as a percentage of basal values. (B) Verification of microdialysis probe location by histology. Vertical lines indicate the placement of microdialysis probes within brain sections (as modified from Paxinos and Watson, 1998) in the shell of the nucleus accumbens. Numbers beside each plate represent distance from bregma.