Kappa opioid receptor antagonism and chronic antidepressant treatment have beneficial activities on social interactions and grooming deficits during heroin abstinence.

Addiction is a chronic brain disorder that progressively invades all aspects of personal life. Accordingly, addiction to opiates severely impairs interpersonal relationships, and the resulting social isolation strongly contributes to the severity and chronicity of the disease. Uncovering new therapeutic strategies that address this aspect of addiction is therefore of great clinical relevance. We recently established a mouse model of heroin addiction in which, following chronic heroin exposure, 'abstinent' mice progressively develop a strong and long-lasting social avoidance phenotype. Here, we explored and compared the efficacy of two pharmacological interventions in this mouse model. Because clinical studies indicate some efficacy of antidepressants on emotional dysfunction associated with addiction, we first used a chronic 4-week treatment with the serotonergic antidepressant fluoxetine, as a reference. In addition, considering prodepressant effects recently associated with kappa opioid receptor signaling, we also investigated the kappa opioid receptor antagonist norbinaltorphimine (norBNI). Finally, we assessed whether fluoxetine and norBNI could reverse abstinence-induced social avoidance after it has established. Altogether, our results show that two interspaced norBNI administrations are sufficient both to prevent and to reverse social impairment in heroin abstinent animals. Therefore, kappa opioid receptor antagonism may represent a useful approach to alleviate social dysfunction in addicted individuals.

Dissociation of heroin-induced emotional dysfunction from psychomotor activation and physical dependence among inbred mouse strains.

RATIONALE: Opiate addiction is a brain disorder emerging through repeated intoxication and withdrawal episodes. Epidemiological studies also indicate that chronic exposure to opiates may lead in susceptible individuals to the emergence of depressive symptoms, strongly contributing to the severity and chronicity of addiction. We recently established a mouse model of heroin abstinence, characterized by the development of depressive-like behaviors following chronic heroin exposure. OBJECTIVES: While genetic factors regulating immediate behavioral responses to opiates have been largely investigated, little is known about their contribution to long-term emotional regulation during abstinence. Here, we compared locomotor stimulation and physical dependence induced by heroin exposure, as well as emotional dysfunction following abstinence, across mice strains with distinct genetic backgrounds. METHODS: Mice from three inbred strains (C57BL/6J, Balb/cByJ, and 129S2/SvPas) were exposed to an escalating chronic heroin regimen (10-50 mg/kg). Independent cohorts were used to assess drug-induced locomotor activity during chronic treatment, naloxone-precipitated withdrawal at the end of chronic treatment, and emotional-like responses after a 4-week abstinence period. RESULTS: Distinct behavioral profiles were observed across strains during heroin treatment, with no physical dependence and low locomotor stimulation in 129S2/SvPas. In addition, different behavioral impairments developed during abstinence across the three strains, with increased despair-like behavior in 129S2/SvPas and Balb/cByJ, and low sociability in 129S2/SvPas and C57BL/6J. CONCLUSIONS: Our results indicate that depressive-like behaviors emerge during heroin abstinence, whatever the severity of immediate behavioral responses to the drug. In addition, inbred mouse strains will allow studying several aspects of mood-related deficits associated with addiction.

A single nucleotide polymorphic mutation in the human mu-opioid receptor severely impairs receptor signaling.

Large scale sequencing of the human mu-opioid receptor (hMOR) gene has revealed polymorphic mutations that occur within the coding region. We have investigated whether the mutations N40D in the extracellular N-terminal region, N152D in the third transmembrane domain, and R265H and S268P in the third intracellular loop alter functional properties of the receptor expressed in mammalian cells. The N152D receptor was produced at low densities. Binding affinities of structurally diverse opioids (morphine, diprenorphine, DAMGO and CTOP) and the main endogenous opioid peptides (beta-endorphin, [Met]enkephalin, and dynorphin A) were not markedly changed in mutant receptors (<3-fold). Receptor signaling was strongly impaired in the S268P mutant, with a reduction of efficacy and potency of several agonists (DAMGO, beta-endorphin, and morphine) in two distinct functional assays. Signaling at N40D and R265H mutants was highly similar to wild type, and none of the mutations induced detectable constitutive activity. DAMGO-induced down-regulation of receptor-binding sites, following 20 h of treatment, was identical in wild-type and mutant receptors. Our data show that natural sequence variations in hMOR gene have little influence on ligand binding or receptor down-regulation but could otherwise modify receptor density and signaling. Importantly, the S268P mutation represents a loss-of-function mutation for the human mu-opioid receptor, which may have an incidence on opioid-regulated behaviors or drug addiction in vivo.

The human delta-opioid receptor: genomic organization, cDNA cloning, functional expression, and distribution in human brain.

We have used the mouse delta-opioid receptor (mDOR) cDNA to isolate the mDOR gene and its human homologue. In both species the coding region is interrupted by two introns with conserved exon-intron boundaries located after transmembrane domains 1 and 4. Using the polymerase chain reaction and primers based on the sequence of the cloned human delta-opioid receptor (hDOR) gene, we have obtained a full length cDNA encoding the hDOR from SH-SY5Y neuroblastoma cells. The cDNA sequence is 100% identical to the cloned human genomic sequence and 94% identical to the mouse sequence at the protein level. When expressed in COS cells, hDOR displays nanomolar affinities for delta-selective ligands, whereas the affinities for mu- and kappa-selective ligands are in the micromolar range. The delta agonists [D-Ala2, D-Leu5]enkephalin, cyclic [D-penicillamine2,D-penicillamine5]enkephalin, and BW373U86 efficiently decrease forskolin-induced cAMP levels in hDOR-expressing COS cells, indicating functional coupling of the receptor. The distribution of hDOR mRNA in human brain was investigated using delta-selective reverse transcription-polymerase chain reaction amplification, followed by Southern hybridization with a delta-specific probe. The transcript is found in cortical areas, including olfactory bulb, hippocampus, and amygdala, as well as in basal ganglia and hypothalamus. No expression is detected in internal globus pallidus, thalamus, any investigated brainstem structure, or pituitary gland. Taken together, our results indicate similar structural, pharmacological, functional, and anatomical properties for the hDOR and the mDOR and therefore support the use of rodent models for the study of these receptors in opioid function.