μ-opioid receptors in the stimulation of mesolimbic dopamine activity by ethanol and morphine in Long-Evans rats: a delayed effect of ethanol

Abstract

Rationale: Naltrexone, a non-selective opioid antagonist, decreases the euphoria and positive subjective responses to alcohol in heavy drinkers. It has been proposed that the μ-opioid receptor plays a role in ethanol reinforcement through modulation of ethanol-stimulated mesolimbic dopamine release.

Objectives: To investigate the ability of naltrexone and β-funaltrexamine, an irreversible μ-opioid specific antagonist, to inhibit ethanol-stimulated and morphine-stimulated mesolimbic dopamine release, and to determine whether opioid receptors on mesolimbic neurons contribute to these mechanisms.

Methods: Ethanol-naïve male Long Evans rats were given opioid receptor antagonists either intravenously, subcutaneously, or intracranially into the ventral tegmental area (VTA), followed by intravenous administration of ethanol or morphine. We measured extracellular dopamine in vivo using microdialysis probes inserted into the nucleus accumbens shell (n = 114).

Results: Administration of naltrexone (intravenously) and β-funaltrexamine (subcutaneously), as well as intracranial injection of naltrexone into the VTA did not prevent the initiation of dopamine release by intravenous ethanol administration, but prevented it from being as prolonged. In contrast, morphine-stimulated mesolimbic dopamine release was effectively suppressed.

Conclusions: Our results provide novel evidence that there are two distinct mechanisms that mediate ethanol-stimulated mesolimbic dopamine release (an initial phase and a delayed phase), and that opioid receptor activation is required to maintain the delayed-phase dopamine release. Moreover, μ-opioid receptors account for this delayed-phase dopamine response, and the VTA is potentially the site of action of this mechanism. We conclude that μ-opioid receptors play different roles in the mechanisms of stimulation of mesolimbic dopamine activity by ethanol and morphine.

Fig. 1

Naltrexone or vehicle pretreatment effects. a) Naltrexone (NAL) or saline pretreatment effect on dialysate dopamine concentrations. Numbers after NAL in the figure indicate the dose of naltrexone (mg/kg). Naltrexone infusion is at time 0. b) The effect of saline infusion in naltrexone or saline pretreated animals. Numbers after NAL in the figure indicate the dose of naltrexone (mg/kg). Saline infusion is at time 0. Naltrexone or saline pretreatment is 20 min prior. c) Naltrexone or vehicle microinjections into the ventral tegmental area. Numbers after NAL in the figure indicate the dose of naltrexone (μg). Naltrexone infusion is at time 0. For all figures, asterisks indicate significance from baseline.

Fig. 2

Intravenous naltrexone effect on morphine- or ethanol- stimulated dopamine release. a) Naltrexone (NAL) effect on morphine-stimulated dopamine (1 mg/kg). b) Naltrexone effect on ethanol-stimulated dopamine (1 g/kg). c) Dialysate ethanol concentrations from the shell of the nucleus accumbens after intravenous (i.v.) pretreatment with saline or naltrexone and i.v. infusion with ethanol (1 g/kg). The legend is the same as described for fig 2b above. d) Naltrexone effect on ethanol-stimulated dopamine (1 g/kg) using 3 min dialysate sampling intervals. e) For the 3-minute dialysis experiment, dialysate ethanol concentrations from the shell of the nucleus accumbens after intravenous (i.v.) pretreatment with saline or naltrexone and i.v. infusion with ethanol (1 g/kg). The legend is the same as described for fig 2d above. For all figures, numbers after NAL in the figure indicate the dose of naltrexone (mg/kg). Morphine or ethanol infusion is at time 0, naltrexone (or vehicle) infusion is 20 min prior, mean ± SEM is shown for each point, and asterisks indicate difference between control and experimental conditions at those time points.

Fig. 3

Subcutaneous β-funaltrexamine effect on morphine (1 mg/kg) - or ethanol (1 g/kg) - stimulated dopamine release. a) β-funaltrexamine (BFNA) effect on morphine-stimulated dialysate dopamine. b) β-funaltrexamine effect on ethanol-stimulated dialysate dopamine. c) Dialysate ethanol concentrations for the β-funaltrexamine experiment. For all figures, 20 mg/kg β-funaltrexamine (or saline) was given subcutaneously approximately 24 h before intravenous morphine or ethanol, morphine or ethanol infusion is at time 0, and mean ± SEM is shown for each point

Fig. 4

Microinjection of naltrexone into the ventral tegmental area effect on morphine (1 mg/kg) - or ethanol (1 g/kg) - stimulated dopamine release. a) Naltrexone (NAL) effect on morphine-stimulated dopamine release. b) Naltrexone effect on ethanol-stimulated dopamine release. c) Dialysate ethanol concentrations from the shell of the nucleus accumbens after microinjection of naltrexone. Asterisks indicate significant difference between control and experimental conditions at those time points. For all figures, the dose of naltrexone was 2.4 μg, ethanol or morphine infusion is at time 0, and mean ± SEM is shown for each point

Fig. 5

Microinjection of cresyl violet into the ventral tegmental area (VTA) was used to estimate the extent of diffusion of naltrexone. Diffusion of cresyl violet was limited to less than 1 mm in any direction. Overlay is from Paxinos and Watson (2007) with the VTA highlighted in yellow (−5.76 mm from bregma)