G protein-coupled receptor (GPCR) trafficking in the central nervous system: relevance for drugs of abuse
Introduction
Drugs of abuse, when administered either acutely or chronically, induce alterations in receptor number and/or sensitivity. These alterations in receptor activity are believed to be responsible for some of the biochemical and cellular processes related to tolerance and dependence. Thus, acute or chronic administration of various hallucinogens such as lysergic acid diethylamide (LSD) or 4-iodo-2,5-dimethoxyphenylisopropylamine (DOI) leads to both tolerance (Jonas and Downer, 1964, Rech et al., 1975, Carino and Horita, 1977) and down-regulation of 5-HT2A and 5-HT2C serotonin receptors in vivo (McKenna et al., 1989, Buckholtz et al., 1990). As will be discussed below, these two processes (down-regulation and desensitization) can be distinguished by appropriate experimental design (Roth et al., 1995). Thus, tolerance, the in vivo correlate of desensitization, and down-regulation are not likely to be caused by identical cellular mechanisms. In a similar manner, chronic opiate administration leads to tolerance and a desensitization of opioid receptors. In many but not all instances, a down-regulation of opioid receptors in vivo (previously reviewed by Loh et al., 1988) occurs following chronic opioid administration. Thus most investigators (see for example Loh et al., 1988, Belcheva et al., 1993, Rothman et al., 1993, Yoburn et al., 1993, Belcheva et al., 1994a, Belcheva et al., 1994b, Polastron et al., 1994, Diaz et al., 1995, Belcheva et al., 1996a, Patrini et al., 1996, Ronnekleiv et al., 1996, Petruzzi et al., 1997, Zhao and Bhargava, 1997), have noted a down-regulation of various opioid receptors following opioid administration in vivo.
In addition to these effects of opioids and hallucinogens on receptor levels in vivo, several investigators have found that opioid (Baumhaker et al., 1993, Zadina et al., 1994, Kim et al., 1995, Belcheva et al., 1996b, Cvejic et al., 1996, Capeyrou et al., 1997, Chakrabarti et al., 1997, Chen et al., 1997, Yabaluri and Medzihradsky, 1997) and 5-HT2A (Ivins and Molinoff, 1990, Leysen and Pauwels, 1990, Ivins and Molinoff, 1991, Ferry et al., 1993, van Huizen et al., 1993, Ferry et al., 1994, Rinaldi-Carmona et al., 1994, Ferry and Molinoff, 1996, Saucier and Albert, 1997) receptor agonists may induce down-regulation in vitro in various cellular systems. In addition, several investigators have demonstrated that 5-HT2A (Vouret-Craviari et al., 1995, Berry et al., 1996) and opioid (Law et al., 1984, Arden et al., 1995, Belcheva et al., 1995, Keith et al., 1996, Sternini et al., 1996, Trapaidze et al., 1996, Chu et al., 1997, Cvejic and Devi, 1997, Gaudriault et al., 1997) receptors may be internalized following agonist exposure. Interestingly, for opioid receptors, agonist-selectivity may play a role in internalization (Keith et al., 1996), since morphine does not induce internalization of μ-opioid receptors while other agonists (e.g. etorphine) can. Additionally, my group (Berry et al., 1996, Willins et al., 1997Willins et al., in press) has recently demonstrated that 5-HT2A antagonists can induce internalization of 5-HT2A receptors in vivo and in vitro, although there were clear differences in the abilities of individual antagonists to induce internalization. Thus, ligand-specific effects on intracellular trafficking of GPCRs may occur.
These results make it clear that drugs of abuse induce dynamic changes in neurotransmitter receptors, and that these alterations may be important for various adaptive changes that occur following acute (e.g. desensitization) and chronic (e.g. tolerance and down-regulation) administration. The relationship(s) between desensitization, tolerance, down-regulation and internalization are not clear, and a model will be proposed that can serve as a template for describing the relationships among these processes. Additionally, newly developed molecular probes that may elucidate the relationships between these processes will be described in some detail. This review will not describe in any detail the phenomenology of internalization, desensitization and down-regulation, but will summarize the cellular and biochemical mechanisms involved in these processes, since little attention has been paid to these mechanisms. Additionally, since most of the mechanistic studies have been performed in cultured cells — particularly fibroblasts — it is likely that the processes described will not be entirely recapitulated in neurons or neuronal cultures. Where particular gaps in our understanding of these processes which are relevant for the actions of drugs of abuse are encountered, these will be highlighted.
Section snippets
Clathrin cages and clathrin-coated pits
Cell surface receptors are dynamic entities which, like other membrane proteins, are continually being modified, sorted, synthesized and degraded. Since this review focuses on internalization, these pathways will be described in some detail. The pathways by which cell membrane proteins are internalized have been extensively studied in the past several years by cell and molecular biologists and a general schema of membrane protein recycling has been developed (see Damke, 1996, Riezman et al.,
Evidence for internalization of G protein-coupled receptors in the central nervous system
If GPCRs are regulated in the CNS in a manner similar to that found in vitro in fibroblasts, than the following predictions can be made: (1) subcellular localization studies should show that GPCRs are enriched in both synaptic membrane and intracellular membrane (microsomal, endosomal) fractions; (2) GPCRs should be found in coated vesicles isolated from brain; (3) treatment with agonists should induce internalization in neurons; and (4) if internalization is essential for
Desensitization of 5-HT2A receptors by hallucinogens
It has been known for several decades that LSD and related hallucinogens induce a rapid tolerance when administered in vivo, (Jonas and Downer, 1964, Knoll and Vizi, 1970, Winter, 1971, Carino et al., 1975) typically by desensitizing 5-HT2A- and/or 5-HT2C-receptors (see Krebs and Geyer, 1994, Aulakh et al., 1995, Mazzola-Pomietto et al., 1995, Mazzola-Pomietto et al., 1996a, Mazzola-Pomietto et al., 1996b for examples). Additionally, many studies have demonstrated that 5-HT2A receptors may be
Future prospects
As can be seen from the foregoing, our understanding of the precise role of internalization and other processes of receptor trafficking are poorly understood, especially when receptors for hallucinogens and opioids are considered. In light of the fact that these two classes of drugs continue to be abused and the fact that the processes which regulate the activities of their receptors are likely to be similar, a deeper understanding of how these receptors are regulated and how these processes
Acknowledgements
This work was supported by RO1MH57635, KO2MHO1366 and a grant from the Scottish Rite Schizophrenia Research Foundation to BLR. DLW was a NARSAD Young Investigator.
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