Drug-induced reinstatement of heroin- and cocaine-seeking behaviour following long-term extinction is associated with expression of behavioural sensitization

The present study was designed to evaluate the relationship between reinstatement of drug-seeking behaviour following long-term extinction of intravenous (i.v.) drug self-administration (an animal model for craving) and long-term behavioural sensitization. Rats were allowed to self-administer heroin (50 microg/kg per inj., 14 daily sessions), cocaine (500 microg/kg per inj., 10 daily sessions) or saline. Following a 3-week extinction period, reinstatement tests were performed to evaluate priming effects of amphetamine, cocaine and heroin on nonreinforced drug-seeking behaviour. In addition, the occurrence of long-term behavioural sensitization in rats with a history of heroin or cocaine self-administration was determined. Heroin-seeking behaviour was reinstated by heroin (0.25 mg/kg), amphetamine (1.0 mg/kg) and cocaine (10 mg/kg). In addition, animals with a history of heroin self-administration displayed locomotor sensitization to both heroin and amphetamine. Cocaine-seeking behaviour was reinstated by cocaine and amphetamine, but not by heroin. Interestingly, locomotor sensitization to amphetamine, but not heroin, was observed in animals with a history of cocaine self-administration. In other words, the induction of drug-seeking behaviour following a prolonged drug-free period was found to be associated with the expression of long-term behavioural sensitization. These data provide experimental evidence for a role of behavioural sensitization in the incentive motivation underlying drug-seeking behaviour. If drug hyperresponsiveness would indeed be a crucial factor in drug-induced craving in human addicts, pharmacological readjustment of the neuroadaptations underlying drug sensitization may prevent relapse to drug use long after detoxification.     

Endotoxin and cytokines increase hepatic sphingolipid biosynthesis and produce lipoproteins enriched in ceramides and sphingomyelin

Alterations in triglyceride and cholesterol metabolism often accompany inflammatory diseases and infections. We studied the effects of endotoxin (lipopolysaccharide [LPS]) and cytokines on hepatic sphingolipid synthesis, activity of serine palmitoyltransferase (SPT), the first and rate-limiting enzyme in sphingolipid synthesis, and lipoprotein sphingolipid content in Syrian hamsters. Administration of LPS induced a 2-fold increase in hepatic SPT activity. The increase in activity first occurred at 16 hours, peaked at 24 hours, and was sustained for at least 48 hours. Low doses of LPS produced maximal increases in SPT activity, with half-maximal effect seen at approximately 0.3 microg LPS/100 g body weight. LPS increased hepatic SPT mRNA levels 2-fold, suggesting that the increase in SPT activity was due to an increase in SPT mRNA. LPS treatment also produced 75% and 2.5-fold increases in hepatic sphingomyelin and ceramide synthesis, respectively. Many of the metabolic effects of LPS are mediated by cytokines. Interleukin 1 (IL-1), but not tumor necrosis factor, increased both SPT activity and mRNA levels in the liver of intact animals, whereas both IL-1 and tumor necrosis factor increased SPT mRNA levels in HepG2 cells. IL- produced a 3-fold increase in SPT mRNA in HepG2 cells, and the half-maximal dose was 2 ng/mL. IL-1 also increased the secretion of sphingolipids into the medium. Analysis of serum lipoprotein fractions demonstrated that very low density lipoprotein, intermediate density lipoprotein, and low density lipoprotein isolated from animals treated with LPS contained significantly higher amounts of ceramide, glucosylceramide, and sphingomyelin. Taken together, these results indicate that LPS and cytokines stimulate hepatic sphingolipid synthesis, which results in an altered structure of circulating lipoproteins and may promote atherogenesis.     

Distinct roles for sphingolipids and glycosphingolipids at different stages of neuronal development

Studies on the roles of sphingolipids (SLs) and glycosphingolipids (GSLs) at distinct stages of neuronal development have been performed using primary cultures of hippocampal neurons, which are unique among neuronal cultures inasmuch as they develop by a well-characterized and stereotypic sequence of events that gives rise to fully differentiated axons and dendrites. Our data demonstrate that SLs and GSLs play at least three distinct roles in regulating neuronal development, namely: (i) ceramide enhances the formation of minor neuronal processes from lamellipodia and the subsequent stage of axonogenesis; (ii) glucosylceramide synthesis, but not the synthesis of higher-order GSLs, is required for normal axon growth and for accelerated axonal growth upon stimulation by growth factors; and (iii) at both of these stages, ceramide at high concentrations can induce apoptotic cell death. Together, these observations are consistent with the possibility that minor process formation and apoptosis are regulated by ceramide-dependent signaling pathways, whereas axonal growth requires glucosylceramide synthesis, perhaps to support an intracellular transport pathway.     

Human African trypanosomiasis: an emerging public health crisis

There is a dramatic resurgence of human African trypanosomiasis (HAT) in sub-Saharan Africa. T.b. gambiense is spreading epidemically in large areas of Central Africa, especially the Southern Sudan, Congo-Zaire, Angola, Uganda and the Central African Republic. Devastating epidemics of T.b. rhodesiense have occurred in south-eastern Uganda. The causes of the re-emergence of sleeping sickness as a public health problem include widespread civil disturbance and war, declining economies, reduced health financing and the dismantling of disease control programmes. Despite the inevitably fatal outcome without treatment, HAT is often given low priority by donors and national governments. The advances made in diagnosis, treatment and vector control have not been sufficiently implemented. To limit the human impact in some of the poorest communities in Africa, endemic countries will require external support to implement strategies for disease control. Donor agencies, NGOs and mission organisations could play an important role in supporting control efforts. National authorities will need to control and co-ordinate these efforts with assistance from WHO and the international community.     

Importance of extracellular domains for ligand binding in the thyrotropin-releasing hormone receptor

The role of putative extracellular sequences for ligand binding in the TRH receptor was examined using deletion or substitution mutations. Each mutant receptor was transiently expressed in TRH receptor-minus GH(1)2C(1)b rat pituitary cells, and binding of 4 Nu Mu [3H]pGlu-N(tau)-MeHis-Pro-NH2 ([3H] MeTRH) was measured. When binding was not detected, signal transduction at 10 microM MeTRH was measured to assess receptor expression. Deletion of most of the N-terminal sequences (Glu(2)-Leu(22)), including two potential glycosylation sites, had no effect on the affinity of the receptor for MeTRH. Segmental deletions or simultaneous substitution of multiple amino acid residues in the first, second, or third extracellular loop (EL1, EL2, or EL3) resulted, however, in total loss of [3H]MeTRH binding, suggesting important roles for the loop sequences in either receptor expression or ligand binding. Individual substitutions were made to test further the role of the specific extracellular loop sequences in TRH binding. In EL1, conversion of Tyr93 to Ala resulted in more than 20-fold decrease in affinity for MeTRH. In EL2 and the top portion of the fifth transmembrane helix, conversion of Tyr181 to Phe, Tyr188 to Ala, and Phe199 to Ala resulted in a large ( > 100-fold) decrease in affinity for MeTRH, and conversion of Tyr 188 to Phe and Phe196 to Ala caused an agonist-specific 4- to 5-fold decrease in affinity. In EL3, conversion of Asn289 to Ala and of Ser290 to Ala caused a large ( > 100-fold) decrease in affinity for MeTRH. These results suggest important roles for the extracellular loops in high affinity TRH binding and lead us to propose a model in which TRH binds to the extra-cellular domain of its receptor.     

Regulation of insulin-like growth factors I and II and their binding proteins in human bone marrow stromal cells by dexamethasone

Müller cells are highly permeable to potassium ions and play a major role in maintaining potassium homeostasis in the vertebrate retina during light-evoked neuronal activity. Potassium fluxes across the Müller cell's membrane are believed to underlie the light-evoked responses of these cells. We studied the potassium currents of turtle Müller cells in the retinal slice and in dissociated cell preparations and their role in the genesis of the light-evoked responses of these cells. In either preparation, the I-V curve, measured under voltage-clamp conditions, consisted of inward and outward currents. A mixture of cesium ions, TEA, and 4-AP blocked the inward current but had no effect on the outward current. Extracellular cesium ions alone blocked the inward current but exerted no effect on the photoresponses. Extracellular barium ions blocked both inward and outward currents, induced substantial depolarization, and augmented the light-evoked responses, especially the OFF component. Exposing isolated Müller cells to a high potassium concentration did not cause any current or voltage responses when barium ions were present. In contrast, application of glutamate in the presence of barium ions induced a small inward current that was associated with a substantially augmented depolarizing wave relative to that observed under control conditions. This observation suggests a role for an electrogenic glutamate transporter in generating the OFF component of the turtle Müller cell photoresponse.