Haplotype variation and linkage disequilibrium in 313 human genes.

Variation within genes has important implications for all biological traits. We identified 3899 single nucleotide polymorphisms (SNPs) that were present within 313 genes from 82 unrelated individuals of diverse ancestry, and we organized the SNPs into 4304 different haplotypes. Each gene had several variable SNPs and haplotypes that were present in all populations, as well as a number that were population-specific. Pairs of SNPs exhibited variability in the degree of linkage disequilibrium that was a function of their location within a gene, distance from each other, population distribution, and population frequency. Haplotypes generally had more information content (heterozygosity) than did individual SNPs. Our analysis of the pattern of variation strongly supports the recent expansion of the human population.

Role for GDNF in biochemical and behavioral adaptations to drugs of abuse.

The present study examined a role for GDNF in adaptations to drugs of abuse. Infusion of GDNF into the ventral tegmental area (VTA), a dopaminergic brain region important for addiction, blocks certain biochemical adaptations to chronic cocaine or morphine as well as the rewarding effects of cocaine. Conversely, responses to cocaine are enhanced in rats by intra-VTA infusion of an anti-GDNF antibody and in mice heterozygous for a null mutation in the GDNF gene. Chronic morphine or cocaine exposure decreases levels of phosphoRet, the protein kinase that mediates GDNF signaling, in the VTA. Together, these results suggest a feedback loop, whereby drugs of abuse decrease signaling through endogenous GDNF pathways in the VTA, which then increases the behavioral sensitivity to subsequent drug exposure.

Enhancement of locomotor activity and conditioned reward to cocaine by brain-derived neurotrophic factor.

The mesolimbic dopamine (DA) system has been implicated in drug reward, locomotor sensitization, and responding for reward-related stimuli [termed conditioned reinforcers (CR)]. Here, we investigated the effect of brain-derived neurotrophic factor (BDNF), which enhances the survival and function of dopaminergic neurons, on stimulant-induced locomotor sensitization and responding for CR. In experiment 1, BDNF was infused into the nucleus accumbens (NAc) or ventral tegmental area over 2 weeks via chronically implanted minipumps (1-2.5 microgram/d), and the psychomotor stimulant effects of cocaine (5-15 mg/kg, i.p.) were studied. We found that BDNF enhanced the initial stimulant effects of cocaine and seemed to facilitate the development of sensitization to repeated cocaine doses. In experiment 2, we studied the effects of intra-NAc BDNF infusions on responding for CR. BDNF-treated rats showed twice as many CR responses compared with controls when saline was first administered. BDNF enhanced responding on the CR lever more than four times that seen in control animals after a cocaine injection (10 mg/kg, i.p.). The enhanced response to cocaine in BDNF-treated animals persisted for more than a month after the BDNF infusions had stopped, indicating long-lasting changes in the mesolimbic DA system caused by BDNF administration. In experiment 3, we examined locomotor sensitization to cocaine in heterozygous BDNF knock-out mice and found that the development of sensitization was delayed compared with wild-type littermates. These results demonstrate the profound effects of BDNF on the enhancement of both cocaine-induced locomotion and facilitation of CR and suggest a possible role for BDNF in long-term adaptations of the brain to cocaine.

Regulation of tyrosine hydroxylase promoter activity by chronic morphine in TH9.0-LacZ transgenic mice.

Levels of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis, are known to be upregulated in specific brain regions by chronic administration of drugs of abuse. Chronic morphine administration increases TH levels in the locus coeruleus and ventral tegmental area, whereas chronic cocaine administration increases TH levels in the ventral tegmental area only. While such upregulation of TH has been related to behavioral effects of the drugs, the mechanism underlying these adaptations has remained controversial. To study the possibility that upregulation of TH occurs at the transcriptional level, we investigated the effect of chronic morphine or cocaine treatment on the activity of the TH gene promoter (9.0 kb), coupled to the LacZ reporter gene, in transgenic mice. These TH9.0-LacZ mice have been shown to exhibit correct tissue-specific expression and regulation of the reporter gene. We show here that chronic (but not acute) exposure of the TH9.0-LacZ mice to morphine increases the expression of beta-galactosidase (which is encoded by the LacZ gene) in the locus coeruleus by twofold compared with sham-treated mice. In contrast, beta-galactosidase expression in the ventral tegmental area was decreased 20-25% by chronic morphine and unaffected by chronic cocaine administration. Similar results were obtained after analysis of TH mRNA levels in these brain regions by in situ hybridization. These results suggest that chronic morphine upregulates TH expression via transcriptional mechanisms in the locus coeruleus but by post-transcriptional mechanisms in the ventral tegmental area.