Cocaine reward and hyperactivity in the rat: sites of mu opioid receptor modulation.

Opioid receptor agonists and antagonists have profound effects on cocaine-induced hyperactivity and conditioned reward. Recently, the role specifically of the mu opioid receptor has been demonstrated based on the finding that i.c.v. administration of the selective mu opioid receptor antagonist, D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), can attenuate cocaine-induced behaviors. The purpose of the present study was to determine the location of mu opioid receptors that are critical for cocaine-induced reward and hyperactivity. Adult male Sprague-Dawley rats received injections of CTAP into the caudate putamen, the rostral or caudal ventral tegmental area (VTA) or the medial shell or core of the nucleus accumbens prior to cocaine to determine the role of mu opioid receptors in cocaine-induced reward and hyperactivity. Cocaine-induced reward was assessed using an unbiased conditioned place preference procedure. Results demonstrate that animals pre-treated with CTAP into the nucleus accumbens core or rostral VTA, but not the caudal VTA, caudate putamen or medial nucleus accumbens shell, during conditioning with cocaine showed an attenuation of the development of cocaine-induced place preference. In contrast, CTAP injected into the nucleus accumbens shell but not the core attenuated the expression of cocaine place preference. Intra-nucleus accumbens core, caudate putamen or caudal VTA CTAP significantly attenuated cocaine-induced hyperactivity. In addition, the number of cFos positive cells was increased in the motor cortex, medial and ventromedial aspects of the nucleus accumbens shell, basolateral amygdala and caudal VTA during the expression of cocaine place preference, and this increase was attenuated in the animals that received intra-accumbens core CTAP during daily cocaine conditioning. These results demonstrate the importance of mu opioid receptors in the nucleus accumbens and VTA in cocaine-induced reward and hyperactivity and suggest that some aspects of the behavioral effects of cocaine are mediated by endogenous activation of mu opioid receptors in these brain regions.

Isolation and characterization of LRP6, a novel member of the low density lipoprotein receptor gene family.

A novel member of the low density lipoprotein receptor (LDLR) gene family has been identified and characterized. This gene, termed LDL receptor-related protein 6 (LRP6), encodes a transmembrane protein which has 71% identity and is structurally similar to the protein encoded by LRP5, a proposed candidate gene for type 1 diabetes located on human chromosome 11q13. LRP6 maps to human chromosome 12p11-p13. Mouse Lrp6 encodes a protein that has 98% identity to human LRP6 and maps to chromosome 6. Unlike other members of the LDLR family, LRP6 and LRP5 display a unique pattern of four epidermal growth factor (EGF) and three LDLR repeats in the extracellular domain. The cytoplasmic domain of LRP6 is not similar to other members of the LDLR family, while comparison with LRP5 reveals proline-rich motifs that may mediate protein-protein interactions. Thus, it is likely that LRP6 and LRP5 comprise a new class of the LDLR family.