The dopaminergic mesolimbic system plays a key role in the mechanisms of reinforcement elicited by alcohol (ethanol) and other drugs of abuse. Numerous lines of evidence indicate that ethanol reinforcement mechanisms involve, at least partially, the ethanol-induced activation of the endogenous opioid system. Ethanol may alter opioidergic transmission at different levels, including the biosynthesis, release, and degradation of opioid peptides, as well as binding of endogenous ligands to opioid receptors. Several studies suggest that mu and delta opioid receptors play a major role in ethanol reinforcement and dependence. These studies implicate enkephalins and beta-endorphin as physiological mediators of ethanol's actions in the brain. In this review we describe the pharmacological characteristics of opioid receptors and their distribution in brain, as well as the major functions of their endogenous ligands. Thereafter, we present evidence supporting the participation of mu and delta opioid receptors in ethanol reinforcement mechanisms and high alcohol drinking behaviour. The use of opioid receptor agonists and antagonists, as well as ethanol-preferring selected rodents and knockout mice, has contributed to understand the role of mu and delta receptors in these processes. The effects of ethanol on binding of selective ligands to opioid receptors in different experimental models are also reviewed. The relevance of opioid receptors in human alcoholism is further evidenced by the association of mu receptor polymorphisms with ethanol dependence. The clinical implication of these findings is discussed regarding the differential responses observed in some alcoholic patients to treatment with opioid receptor antagonists such as naltrexone.

The role of phospholemman (PLM) in taurine and Cl(-) efflux elicited by 30% hyposmotic solution was studied in cultured cerebellar astrocytes with reduced PLM expression by antisense oligonucleotide (AO) treatment. PLM, a substrate for protein kinases (PK) C and A, is a protein that increases an anion current in Xenopus oocytes and forms taurine-selective channels in lipid bilayers. Taurine contributes as an osmolyte to regulatory volume decrease (RVD) and is highly permeable through PLM channels in bilayers. Two antisense oligonucleotides (AO1 and AO2) effectively decreased the expression of the PLM protein by 40% and 30%, respectively, and markedly reduced [(3)H]taurine efflux by 67% and 62%. AO treatment also decreased the osmosensitive release of Cl(-), followed as (125)I. The inhibition of Cl(-) efflux (23% for AO1 and 13% for AO2) was notably lower than for [(3)H]taurine. The contribution of PKC and PKA in the function of PLM was also evaluated in astrocytes. Pharmacological activation or inhibition of PKC and PKA revealed that the osmosensitive taurine efflux is essentially PKC-independent while (125)I efflux is reduced by the PKC blockers H-7 (21%) and Gö6983 (41%). The PKA activator forskolin and dbcAMP increased taurine efflux by 66-70% and (125)I efflux by 21-45%. Norepinephrine increased the osmosensitive taurine efflux at about the same extent as dbcAMP and forskolin, and this was reduced by PKA blockers. These results suggest that PLM plays a role in RVD in astrocytes by predominantly influencing taurine fluxes, which are modulated by PKA but not PKC.

The role of the phospholemman (PLM) on the efflux of taurine and chloride induced by swelling was studied in HEK293 cells overexpressing stable transfected PLM. PLM, a substrate for protein kinases C and A, is a protein that induces an anion current in Xenopus oocytes and forms taurine-selective channels in lipid bilayers. Taurine contributes as an osmolyte to regulatory volume decrease (RVD) and is highly permeable through PLM channels in bilayers. In PLM-overexpressing cells the process of RVD was more rapid and efficient (75%) than in control cells (44%). Also, [(3)H]taurine and (125)I efflux induced by hyposmolarity were markedly increased (30-100%) in two subclones of cells overexpressing PLM. This increased efflux was sensitive to the Cl channel blockers DDF, NPPB and DIDS. Acute treatment of control cells with isoproterenol and norepinephrine induced a significant potentiation (50-60%) of [(3)H]taurine release induced by hyposmolarity. In PLM-overexpressing cells the potentiation by these drugs was higher (100%). Insulin induced also an increase in [(3)H]taurine release, but only in PLM-overexpressing cells (50%). These results indicate that PLM may play a role in the RVD and that its phosphorylation may have a physiological significance during this process. The mechanisms involved in this process could include the activation of PLM itself as channel or the modulation of other preexisting channels.

Our results indicate that enkephalin expression in regions of the rat mesocorticolimbic system is differentially altered by acute ethanol treatment and suggest that enkephalins may play a key role in ethanol reinforcement mechanisms.

Thyrotropin-releasing hormone (TRH) synthesized in hypothalamic paraventricular nucleus directs hypothalamus-pituitary-thyroid (HPT) axis function, regulating thyrotropin release from adenohypophysis and thyroid hormones serum concentration. Pyroglutamyl aminopeptidase II (PPII), a Zn-dependent metallopeptidase located in adenohypophysis and medial-basal-hypothalamus degrades TRH released from the median eminence and participates in HPT axis function by regulating TRH-induced thyrotropin release from adenohypophysis. It is unknown whether dietary Zn deficiency down-regulates PPII. Our aim was to compare adenohypohyseal and medial-basal-hypothalamic PPII activity and expression of adult rats fed a Zn-deficient diet (2ppm) throughout their lifespan (DD), prenatally (DC) or after weaning (CD) vs. that of animals fed a control diet (20ppm:CC). Female rats consumed a Zn-deficient or control diet from two weeks before gestation and up to the end of lactation. We analyzed adenohypophyseal and medial-basal-hypothalamic PPII activity of dams and male offspring when adults; its relation to median eminence TRH, serum thyrotropin, leptin and thyroid hormones concentration. Offspring ate the same diet as their dams (CC, DD) or were switched from dietary regime after weaning (CD, DC) and until 2.5 months of age. DD males showed decreased adenohypophyseal and medial-basal-hypothalamic PPII activity, along with high thyrotropin serum concentration. Post-weaning Zn-deficiency (CD) decreased PPII activity only in adenohypophysis and increased thyrotropin circulating levels. Zn-replenishment (DC) normalized PPII activity in both regions and serum thyrotropin concentration. Adenohypophyseal PPII activity decreased and prolactin levels increased in Zn-deficient dams. We concluded that long-term changes in dietary Zn down-regulate PPII activity independently of T3, increasing thyrotropin serum concentration, overall resembling sub-clinical hypothyroidism.

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The polyunsaturated fatty acids, arachidonic, linoleic, and linolenic acids, were potent blockers of regulatory volume decrease (RVD) and of the swelling-activated efflux of [3H]taurine, D-[3H]aspartate, [3H]inositol, and 125I (used as marker of Cl) from rat cerebellar astrocytes in culture. The monounsaturated oleic and ricinoleic acids and saturated fatty acids were ineffective. The amino acid and 125I fluxes were similarly inhibited by fatty acids, whereas inositol release was less sensitive. Polyunsaturated fatty acids appear to directly affect RVD in trypsinized astrocytes as the inhibition was immediate and fully reversible. Blockers of the arachidonic acid metabolic pathways, indomethacin (cyclooxygenase), esculetin (lipoxygenases), and metyrapone (P-450 monooxygenases), did not prevent the effect of arachidonic acid, suggesting that further metabolism is not required for displaying the effects of arachidonic acid on RVD and osmolyte fluxes. Some blockers of arachidonic acid metabolic pathways, such as nordihydroguaiaretic acid (lipoxygenases) and naphthoflavone (P-450 monooxygenases), also exhibited marked inhibitory effects on RVD and on osmolyte fluxes. The predominant arachidonic acid metabolite in astrocytes, 12-hydroxyeicosatetraenoic acid, did not affect RVD or osmolyte fluxes. These results suggest that arachidonic acid and other polyunsaturated fatty acids directly inhibit the permeability pathways correcting cell volume after swelling in cultured astrocytes.

Anxiolytic effects of alcohol participate in the reinforcing properties of the drug, in which nucleus accumbens (NAcc) is implicated. The opioidergic system in NAcc is considered a main pathway involved in the emotional responses of animals: rats microinjected with morphine in NAcc and the systemic administration of μ-opioid receptors (MOR) agonists yield low anxiety scores in the elevated plus maze (EPM), a behavioral test of anxiety. However, the specific participation of NAcc MOR in the anxiolytic effect of ethanol has not been studied. AC5, a cAMP-synthezising adenylyl-cyclase, is highly expressed in NAcc; it is negatively coupled to MOR and has been implicated in anxiety levels of animals. We evaluated the anxiolytic effects of an intra-gastric administration of ethanol (2.5 g/kg) in animals subjected to EPM at 1, 4, and 8 h after drug or water exposure. Locomotion was assayed with the open-field test; we also measured accumbal AC5 and MOR mRNA levels by RT-PCR. After 1 h, ethanol-exposed animals showed anxiolytic-like behavior, as well as decreased and increased AC5 and MOR expression in NAcc, respectively. Intra-accumbal injection of β-funaltrexamine (FNA), a MOR antagonist, did not block ethanol-induced anxiolysis, rather it induced a tendency to increase anxiety levels in the water-exposed group. FNA partially decreased accumbal AC5 expression in ethanol-treated rats. We concluded that AC5 in NAcc is participating in the emotional effects of ethanol; that MOR was not mediating the drug-induced AC5 reduction in NAcc nor the ethanol-induced anxiolysis. MOR only might be involved in basal levels of anxiety of animals.