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Cannabinoid and Heroin Activation of Mesolimbic Dopamine Transmission by a Common µ1 Opioid Receptor Mechanism

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Science  27 Jun 1997:
Vol. 276, Issue 5321, pp. 2048-2050
DOI: 10.1126/science.276.5321.2048

Abstract

The effects of the active ingredient of Cannabis, Δ9-tetrahydrocannabinol (Δ9-THC), and of the highly addictive drug heroin on in vivo dopamine transmission in the nucleus accumbens were compared in Sprague-Dawley rats by brain microdialysis. Δ9-THC and heroin increased extracellular dopamine concentrations selectively in the shell of the nucleus accumbens; these effects were mimicked by the synthetic cannabinoid agonist WIN55212-2. SR141716A, an antagonist of central cannabinoid receptors, prevented the effects of Δ9-THC but not those of heroin. Naloxone, a generic opioid antagonist, administered systemically, or naloxonazine, an antagonist of μ1 opioid receptors, infused into the ventral tegmentum, prevented the action of cannabinoids and heroin on dopamine transmission. Thus, Δ9-THC and heroin exert similar effects on mesolimbic dopamine transmission through a common μ1 opioid receptor mechanism located in the ventral mesencephalic tegmentum.

AlthoughCannabis is the most commonly abused illicit substance, its reinforcing properties are difficult to demonstrate in animals, and efforts to obtain consistent self-administration of the active component of Cannabis, Δ9-THC, in laboratory animals have been unsuccessful (1). Δ9-THC acts in the brain on a G protein–coupled receptor (CB1) whose putative endogenous ligand has been identified as anandamide, an arachidonic acid derivative (2, 3). The reinforcing properties of Δ9-THC may be mediated by its action on the mesolimbic dopamine (DA) system, which projects from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) (4); this neuronal system is also one of the candidate substrates for the reinforcing and addictive actions of other drugs and substances of abuse (5). However, the evidence for an action of Δ9-THC on central DA transmission has been contradictory (6).

The availability of specific cannabinoid receptor agonists (WIN55212-2, CP55940) (7) and antagonists (SR141716A) (8) allowed us to reexamine this issue. We used brain microdialysis with vertical concentric probes (9) to monitor changes in extracellular DA concentration in the two main subdivisions of the NAc, the shell and the core (10), elicited by the administration of the natural cannabinoid Δ9-THC, the synthetic cannabinoid agonist WIN55212-2, and cannabinol, an inactive cannabinoid (11). To reduce the possibility of the occurrence of nonspecific changes resulting from the stress of injection, we administered Δ9-THC and WIN55212-2 intravenously (iv) through chronically implanted catheters (9). The effects of cannabinoids were compared with those of the addictive drug heroin administered iv at doses that are able to maintain iv self-administration in rats (12).

Δ9-THC significantly increased dialysate DA in the NAc shell at doses of 0.15 mg per kilogram of body weight (mg/kg) iv (F 8,27 = 2.502, P < 0.05) and 0.30 mg/kg iv (F 8,4 = 3.522, P< 0.002) (Fig. 1). These effects were dose-dependent (F 1,87 = 6.092, P< 0.02; post hoc, P < 0.05) and time-dependent (F 8,80 = 5.606, P < 0.001). No significant changes in dialysate DA were obtained in the NAc core after Δ9-THC doses of 0.15 and 0.30 mg/kg iv (F 8,45 = 0.650, P = 0.73). Post hoc comparison of the effect of Δ9-THC on dialysate DA in the NAc shell and core showed a preferential effect in the shell at both doses of Δ9-THC (Fig. 1). The increase in dialysate DA in the NAc shell elicited by a Δ9-THC dose of 0.30 mg/kg was prevented by pretreatment with the cannabinoid antagonist SR141716A (13) [1 mg/kg intraperitoneally (ip), 40 min in advance]. SR141716A by itself did not modify dialysate DA in the NAc shell (Fig. 1). Cannabinol, a nonpsychoactive cannabinoid, dissolved in the same vehicle as Δ9-THC, failed to modify dialysate DA in the NAc shell at doses of 0.30 mg/kg iv (F 8,18 = 0.722; P > 0.05) or 1.0 mg/kg iv (F 8,27 = 0.511, P> 0.05).

Figure 1

Effect of intravenous Δ9-THC, WIN55212-2, and heroin on dialysate DA in the shell (upper panels) and core (lower panels) of the NAc. (A and B) Δ9-THC doses of 0.15 and 0.30 mg/kg iv; (Cand D) WIN55212-2 doses of 0.15 and 0.30 mg/kg iv; and (E and F) heroin doses of 0.018 and 0.030 mg/kg iv. Rats were pretreated with saline (circles) or SR141716-A (triangles) (1 mg/kg sc) or with naloxone (diamonds) (0.1 mg/kg ip). Results are means ± SEM of the amount of DA in 10-min dialysate samples, expressed as percent of basal values. Solid symbols:P < 0.05 compared with basal values. Asterisks:P < 0.05 compared with the corresponding value obtained in the shell of saline-pretreated controls.

The effect of Δ9-THC was mimicked by the centrally active cannabinoid agonist WIN55212-2. Thus, WIN55212-2 significantly increased dialysate DA in the NAc shell at doses of 0.15 mg/kg iv (F 8,27 = 6.976, P < 0.001) and 0.30 mg/kg iv (F 8,27 = 9.299, P< 0.001). This effect of WIN55212-2 was dose-dependent (F 1,70 = 6.093, P < 0.02; post hoc, P < 0.05) and time-dependent (F 8,63 = 10.24, P < 0.001). SR141716A (1.0 mg/kg, 40 min in advance) prevented this effect of WIN55212-2 (main effect of group, F 1,61 = 48.208, P < 0.001; time × group interaction,F 1,45 = 2.953, P < 0.01) (Fig.1). As in the case of Δ9-THC, no significant change in dialysate DA was obtained in the core after a WIN55212-2 dose of 0.30 mg/kg iv (F 8,45 = 0.73, P > 0.05). Post hoc comparison of the effect of WIN55212-2 on dialysate DA in the NAc shell and in the core showed a preferential effect in the shell at both doses of WIN55212-2 (Fig. 1).

Heroin increased dialysate DA in the NAc shell at doses of 0.018 mg/kg iv (F 8,27 = 7.141, P < 0.001) and 0.030 mg/kg iv (F 8,27 = 8.9999,P < 0.001) in a dose-related (F 1,70 = 12.991, P < 0.001; post hoc, P < 0.05) and time-related fashion (F 8,63 = 16.008, P < 0.001) (Fig. 1). As in the case of Δ9-THC and WIN55212-2, heroin was ineffective in modifying DA in the NAc core at doses of 0.018 mg/kg iv (F 8,18 = 2.405, P < 0.06) and 0.03 mg/kg iv (F 8,27 = 0.983,P = 0.47). Post hoc comparison of the effect of heroin on dialysate DA in the NAc shell and core showed a preferential effect in the shell at both doses of the drug (Fig. 1).

SR141716A antagonized the effects of Δ9-THC and of WIN5521-2 but failed to modify the effect of heroin (F 1,79 = 0.719, P = 0.45). On the other hand, a low dose of the opiate antagonist naloxone [0.1 mg/kg subcutaneously (sc), 15 min in advance] prevented the effect of heroin (main effect of group, F 1,61 = 62.94,P < 0.001; group × time interaction,F 8,45 = 4.386, P < 0.0001) (Fig. 1).

The same dose of naloxone given 15 min in advance also reduced the effect of Δ9-THC (0.30 mg/kg iv) on dialysate DA in the NAc shell (main effect of group, F 1,61 = 62.940,P < 0.001; post hoc, P < 0.05) as well as that of WIN55212-2 (main effect of group,F 1,61 = 50.107, P < 0.001; post hoc, P < 0.05). Naloxone (0.1 mg/kg sc) by itself failed to modify dialysate DA in the NAc shell.

Naloxonazine, a pseudo-irreversible μ1 antagonist (14), bilaterally infused at the dose of 3 μg per side in the VTA (15) 20 to 24 hours before the microdialysis experiment, prevented the effect of Δ9-THC (0.015 mg/kg iv) and of heroin (0.030 mg/kg iv) on dialysate DA in the NAc shell (vehicle in VTA plus Δ9-THC iv versus naloxonazine in VTA plus Δ9-THC iv: main effect of group,F 1,61 = 67.72, P < 0.001; post hoc, P < 0.05; time × group interaction,F 8,45 = 3.293, P < 0.005; vehicle in VTA plus heroin iv versus naloxonazine in VTA plus heroin iv: main effect of group, F 1,61 = 68.138,P < 0.001; time × group interaction,F 8,45 = 7.540, P < 0.001) (Fig.2). The effect of naloxonazine was specific because intra-VTA naloxonazine failed to modify the increase of dialysate DA in the NAc by the DA receptor antagonist haloperidol (0.025 mg/kg iv) (F 1,58 = 0.231,P = 0.635).

Figure 2

(A) Effect of naloxonazine (squares) or vehicle (circles), bilaterally infused in the VTA, on dialysate DA in the NAc stimulated by Δ9-THC and by heroin. Results are means ± SEM of the amount of DA found in 10-min dialysate samples expressed as percent of basal values uncorrected for probe recovery. Solid symbols: P < 0.05 compared with basal values. Asterisks: P < 0.05 compared with the corresponding value of saline-pretreated controls. (B) Forebrain sections, redrawn from (9), represent the track corresponding to the dialyzing portion of the probes. Although dialysis probes aimed to the NAc shell were implanted on the side of the brain contralateral to that of the NAc core, they are shown here on the same side to save space. The midbrain section, redrawn from (15), represents the location of the tip of the cannulae used for infusing naloxonazine. On each section, the anterior coordinate (measured from bregma) is indicated. Abbreviations: CPu, caudate putamen; LM, medial lemniscus; PAG, periaqueductal gray matter; and SN, substantia nigra.

Our data show that Δ9-THC and its synthetic analog WIN55212-2 increase extracellular DA selectively in the NAc shell but not in the NAc core. This effect was attributable to an action on specific cannabinoid receptors because it was prevented by the CB1 antagonist SR141716A. This marked topographical selectivity of Δ9-THC on DA transmission within the NAc may account for the inconsistencies in the effect of Δ9-THC on DA transmission reported in the literature (6).

Δ9-THC and heroin can now be added to the list of drugs of abuse (morphine, cocaine, amphetamine, and nicotine) that increase DA transmission preferentially in the NAc shell relative to the core (16). Given the extensive connections of the NAc shell with limbic brain areas involved in emotion (10), the activation of DA transmission in the NAc shell may be involved in the affective and motivational properties of Δ9-THC.

Although SR141716-A prevented the action of the cannabinoids but not that of heroin, the opioid antagonists naloxone and naloxonazine prevented the effects of both (17). Therefore, stimulation of specific cannabinoid receptors may activate DA transmission in the NAc by activating an endogenous opioid system impinging on μ1 opioid receptors of the VTA (18). These homologies between Δ9-THC and heroin may be relevant to the issue, raised by epidemiological studies, of the relation between the degree and frequency of Cannabis use and the probability of subsequent heroin self-administration (19). Although our results do not provide direct evidence for a causal relation betweenCannabis and heroin use, they are nonetheless consistent with this possibility.

  • * Present address: Department of Neuroscience, University “La Sapienza,” Viale dell'Università 30, 00185 Roma, Italy.

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