Vol. zy 1 I. zy No. 4 Junc zy IVY7 zy Characterization of the p and 6 Opioid Receptors in the Brain of the C57BL/6 and DBA/2 Mice, Selected for Their Differences in Voluntary Ethanol Consumption Jean-Pascal de Waele and Christina Gianoulakis Genetically determined differences in the activity of the hypotha- lamic P-endorphin system have been demonstrated between the C57BU6 (alcohol-preferring) and DBN2 (alcohol-aversive) inbred strains of mice. The present studies examined the distribution and density of the zyxwvutsrqp p and 6 receptors in specific brain regions that may mediate the rewarding and reinforcing effects of ethanol, using quantitative autoradiographyand the specific p agonist FK 33-824 and 6 agonist DPDPE, in their iodinatedform. lZ51-FK 33-824 recog- nizes a high affinity binding site in brain membrane preparations from both the C57BU6 (Kd = 1.37 zyxwvutsrqpo a 0.22 nM; B,,, = 80 zyxwvut f 12.3 fmol/mg protein) and DBN2 mice (Kd = 1.02 zyxwvuts f 0.16 nM; B,,, = 39.5 zyxwvutsrqp 2 9.6 fmol/mg protein), whereas lZ51-DPDPE binds to a high affinity bindingsite in brain membranes from both the C57BU6 (Kd = 1.08 f 0.34 nM; B,,, = 24.4 f 4.5 fmol/mg protein) and DBN2 mice (Kd = 0.68 f 0.24 nM; B,,, = 15.3 a 3.7 fmol/mg protein). The auto- radiographic studies demonstrated differences in the density of the p opioid receptors between the two strains of mice in brain nuclei that are not directly related to the brain reward system. However, strain-related differences in the density of 6 opioid receptors were observed in regions of the limbic system known to mediatethe pos- itive reinforcing effects of many drugs of abuse. The density of 6 receptors was significantly higher in the ventral tegmental area and nucleus accumbens of the C57BU6 mice. The results of the present study support the hypothesis that geneticallydetermineddifferences exist in the density of opioid receptors in distinct regions of the brain between the C57BU6 and DBN2 inbred strains of mice, which may play a role in controlling their voluntary ethanol consumption. Key Words: Ethanol, Mu Receptors, Delta Receptors, Opioid Pep- tides. T IS WELL recognized that genetic factors may be in- I volved in the predisposition to excessive ethanol con- sumption and alcoholism in humans. 132 This hypothesis is supported by the development of inbred and outbred strains and lines of animals, showing either preference or aversion to ethanol, which can be used to study the bio- chemical basis of alcoholism at the level of the central nervous system, where the rewarding and reinforcing ef- fects of drugs of abuse are mediated. Because ethanol has no specific receptors in the brain, it is believed that it alters the activity of various neurotransmitter3" and neuropep- zyxwvu From the Douglas Hospital Research Centre. McCill Universiy, Verdim, Received for publication October 8, 1996: accepted Fehniary 18, 1997 This work was suppoporred by a grant ffom the Natural Science and Engi- neering Research Cortricil of Canada. Reprints requests: Dr. C. Giunoulakis, Douglas Hospital ReJearcli Centre. 6875 Bortle~~ard LaSalle. Verdirn, Qut%ec. Canuda. H4H 1 R3. Copyright zyxwvutsrqp 0 1997 by The Research Sociep on Alcoholism. QuPhec, Canada. 754 systems in the brain which then mediate a number of its behavioral and pharmacological effects. Similarities be- tween some physiological effects of ethanol and opiates' and the observation that nonspecific opiate antagonists, such as naloxone and naltrexone"" or the selective 6 antagonist ICI-174864'3 and antagonist p-funaltrexam- inel' reduce voluntary ethanol consumption by animals in a number of experimental paradigms, suggest that the endog- enous opioid system may, at least in part, modulate volun- tary ethanol consumption. In addition, naltrexone has been used in clinical studies'"." aiming at reducing craving for ethanol consumption. The endogenous opioid system consists of three distinct families of opioid peptides (the endorphins, the enkepha- lins, and the dynorphins)17 and three major classes of opi- oid receptors (p, 6, and k)." The enkephalins show higher affinity for the 6 binding sites, the dynorphins for the k sites, while P-endorphin (P-EP) binds with equal affinity to both p and 6 opioid receptors.19 The three types of opioid receptors are coupled to inhibitory G proteins, thus ex- plaining the inhibition of adenylate cyclase activity exhib- ited by opioid peptides in brain and cell culture prepara- tions"' and the inhibitory effects of opioid peptides on neuronal activity and neurotransmitter Auto- radiographic studies have demonstrated differences in the anatomical distribution of the three types of opioid recep- tors in the rat brain,183z5 with p binding sites being predom- inant in hypothalamus, thalamus, nucleus accumbens, cau- date, hippocampus, and periaqueductal grey, while the 6 binding sites are found mainly in the nucleus accumbens, amygdala, caudate and pontine nuclei. In the rat brain, the K binding sites have a similar distribution as the p recep- tors, however they are less n u m e r o ~ s . ' ~ * * ~ The three types of opioid receptors may be present in the same nucleus,'7925 but with a different intranucleus localization, as demon- strated in the caudate nucleus,25 suggesting that the three types of opiate receptors may have distinct functional roles. The p and 6 receptors have been shown to enhance dopa- mine release in the nucleus accumbens,22.2" a mechanism involved in the mediation of the positive reinforcing effects of many drugs of abuse.' while the K receptors have been shown to decrease dopamine releasez4 and may mediate aversive states. The endogenous opioid peptide with the most potent Akolrol Ch Etp Res. Vol 21. No 4. 1997: pp 754762