Effect of the kappa opioid agonist R-84760 on cocaine-induced increases in striatal dopamine levels and cocaine-induced place preference in C57BL/6J mice

Effects of the plant-derived hallucinogen salvinorin A on basal dopamine levels in the caudate putamen and in a conditioned place aversion assay in mice: agonist actions at kappa opioid receptors

RATIONALE

Salvinorin A is a naturally occurring hallucinogen derived from the plant Salvia divinorum. Salvinorin A is also a potent and selective kappa opioid receptor agonist in vitro. It has been shown that kappa agonists decrease dopamine levels in the caudate putamen and nucleus accumbens and cause conditioned place aversion in rodents.

OBJECTIVES

To study the effects of salvinorin A on basal dopamine levels in the caudate putamen and nucleus accumbens, and to determine whether salvinorin A induces conditioned place preference or aversion and changes in locomotor activity in the mouse.

METHODS

In the first experiment, changes in dopamine levels in these brain regions after administration of salvinorin A were measured with in vivo microdialysis. In the second experiment, we examined whether salvinorin A led to conditioned place preference or aversion, and changes in locomotor activity during conditioning sessions.

RESULTS

The higher doses of salvinorin A studied (1.0 mg/kg and 3.2 mg/kg, i.p.) significantly decreased dopamine levels in the caudate putamen, but not in the nucleus accumbens, and this effect was completely blocked by pre-injection with 10 mg/kg of the kappa opioid receptor antagonist nor-binaltorphimine. The same doses of salvinorin A caused conditioned place aversion and decreased locomotor activity.

CONCLUSIONS

The inhibitory effect of salvinorin A on striatal dopamine levels may contribute to its induction of conditioned place aversion and decreases in locomotion in mice. These findings are consistent with the in vitro characterization of salvinorin A as a kappa opioid receptor agonist. It is of interest that a compound such as salvinorin A, that lowers striatal dopamine levels and leads to conditioned place aversion in rodents, is self-administered by humans under certain conditions.

Microarray studies of psychostimulant-induced changes in gene expression

Alterations in the expression of multiple genes in many brain regions are likely to contribute to psychostimulant-induced behaviours. Microarray technology provides a powerful tool for the simultaneous interrogation of gene expression levels of a large number of genes. Several recent experimental studies, reviewed here, demonstrate the power, limitations and progress of microarray technology in the field of psychostimulant addiction. These studies vary in the paradigms of cocaine or amphetamine administration, drug doses, route and also mode of administration, duration of treatment, animal species, brain regions studied and time of tissue collection after final drug administration. The studies also utilize different microarray platforms and statistical techniques for analysis of differentially expressed genes. These variables influence substantially the results of these studies. It is clear that current microarray techniques cannot detect small changes reliably in gene expression of genes with low expression levels, including functionally significant changes in components of major neurotransmission systems such as glutamate, dopamine, opioid and GABA receptors, especially those that may occur after chronic drug administration or drug withdrawal. However, the microarray studies reviewed here showed cocaine- or amphetamine-induced alterations in the expression of numerous genes involved in the modulation of neuronal growth, cytoskeletal structures, synaptogenesis, signal transduction, apoptosis and cell metabolism. Application of laser capture microdissection and single-cell cDNA amplification may greatly enhance microarray studies of gene expression profiling. The combination of rapidly evolving microarray technology with established methods of neuroscience, molecular biology and genetics, as well as appropriate behavioural models of drug reinforcement, may provide a productive approach for delineating the neurobiological underpinnings of drug responses that lead to addiction.

Evolving perspectives on neurobiological research on the addictions: celebration of the 30th anniversary of NIDA

The roots of the Laboratory of the Biology of the Addictive Diseases are in the development of methadone maintenance for the treatment of opiate addiction. Methadone maintenance therapy continues to be one of the major effective forms of addiction pharmacotherapy and underscores the importance of biological factors in the physiology and treatment of the addictive diseases. Recent work in the Laboratory has focused on the neurobiological, neurochemical, neuroendocrine and behavioral aspects of addictive diseases (principally cocaine and the opiate addictions), using an interdisciplinary approach. The models we have focused on range from in vitro molecular biology and neuroscience, to in vivo animal models, to experiments in normal human populations and patients with specific addictive diseases, and most recently to the human molecular genetics of different addictive diseases. Two long-term corollary hypotheses have guided the Laboratory's work: (1) That the endogenous opioid peptide/receptor systems play a central role in the addictive states and therefore in their treatment. (2) That atypical responsivity to stressors (e.g., in the hypothalamic-pituitary-adrenal axis) plays a role in vulnerability and relapse to specific addictive diseases. This atypical responsivity may be drug-induced, environmentally acquired, and/or due to genetic variation.

Redefinition of the human kappa opioid receptor gene (OPRK1) structure and association of haplotypes with opiate addiction

The kappa opioid receptor (KOR) plays a role in stress responsivity, opiate withdrawal and responses to cocaine. KOR activation by its endogenous ligand dynorphin A(1-17) decreases basal and drug-induced striatal levels of dopamine. The complete structure of the human KOR gene (hOPRK1) has not been previously determined. This study: (i) characterized the genomic structure of the hOPRK1 gene; (ii) identified single nucleotide polymorphisms (SNPs) in the hOPRK1 gene; and (iii) investigated possible associations of these variants with vulnerability to develop heroin addiction. Analysis of 5'-RACE cDNA clones revealed the presence of a novel exon 1 ranging in length from 167 to 251 nucleotides in the 5' 5'-untranslated region of the hOPRK1 mRNA. We found that the hOPRK1 gene has four major exons and three introns, similar to rodent OPRK1 genes. Direct sequencing of amplified DNA containing all four exons and intron 1 of the hOPRK1 gene were evaluated for polymorphisms in 291 subjects (145 former heroin addicts and 146 controls). Twelve SNPs were identified, nine novel variants and three previously reported SNPs. Using logistic regression with opioid dependence as the dependent variable, the 36G>T SNP exhibited a point-wise significant association (P = 0.016) with disease status. The number of haplotypes seen in the three ethnic groups were nine, six and five for African-Americans, Caucasians, and Hispanics, respectively, with corresponding significance levels for differences in haplotype frequencies between cases and controls of P = 0.0742, 0.1015 and 0.0041. Combining ethnicities by Fisher's method yields an empirical significance level of P = 0.0020.