Evidence for European presence in the Americas in AD 1021.

Transatlantic exploration took place centuries before the crossing of Columbus. Physical evidence for early European presence in the Americas can be found in Newfoundland, Canada1,2. However, it has thus far not been possible to determine when this activity took place3-5. Here we provide evidence that the Vikings were present in Newfoundland in AD 1021. We overcome the imprecision of previous age estimates by making use of the cosmic-ray-induced upsurge in atmospheric radiocarbon concentrations in AD 993 (ref. 6). Our new date lays down a marker for European cognisance of the Americas, and represents the first known point at which humans encircled the globe. It also provides a definitive tie point for future research into the initial consequences of transatlantic activity, such as the transference of knowledge, and the potential exchange of genetic information, biota and pathologies7,8.

The magnitude and impact of the 431 CE Tierra Blanca Joven eruption of Ilopango, El Salvador.

The Tierra Blanca Joven (TBJ) eruption from Ilopango volcano deposited thick ash over much of El Salvador when it was inhabited by the Maya, and rendered all areas within at least 80 km of the volcano uninhabitable for years to decades after the eruption. Nonetheless, the more widespread environmental and climatic impacts of this large eruption are not well known because the eruption magnitude and date are not well constrained. In this multifaceted study we have resolved the date of the eruption to 431 ± 2 CE by identifying the ash layer in a well-dated, high-resolution Greenland ice-core record that is >7,000 km from Ilopango; and calculated that between 37 and 82 km3 of magma was dispersed from an eruption coignimbrite column that rose to ∼45 km by modeling the deposit thickness using state-of-the-art tephra dispersal methods. Sulfate records from an array of ice cores suggest stratospheric injection of 14 ± 2 Tg S associated with the TBJ eruption, exceeding those of the historic eruption of Pinatubo in 1991. Based on these estimates it is likely that the TBJ eruption produced a cooling of around 0.5 °C for a few years after the eruption. The modeled dispersal and higher sulfate concentrations recorded in Antarctic ice cores imply that the cooling would have been more pronounced in the Southern Hemisphere. The new date confirms the eruption occurred within the Early Classic phase when Maya expanded across Central America.

Radiocarbon-based approach capable of subannual precision resolves the origins of the site of Por-Bajin.

Inadequate resolution is the principal limitation of radiocarbon dating. However, recent work has shown that exact-year precision is attainable if use can be made of past increases in atmospheric radiocarbon concentration or so-called Miyake events. Here, this nascent method is applied to an archaeological site of previously unknown age. We locate the distinctive radiocarbon signal of the year 775 common era (CE) in wood from the base of the Uyghur monument of Por-Bajin in Russia. Our analysis shows that the construction of Por-Bajin started in the summer of 777 CE, a foundation date that resolves decades of debate and allows the origin and purpose of the building to be established.

Adolescent cannabis exposure increases heroin reinforcement in rats genetically vulnerable to addiction.

On the basis of epidemiological studies it has been proposed that cannabis use plays a causal role in the abuse of highly addictive drugs (Gateway Hypothesis). However, epidemiological studies are intrinsically unable to provide evidence of causality. Experimental studies can provide this evidence but they are feasible only in animal models and to date such evidence is lacking. In view of the importance of genetic factors in drug abuse, we investigated the influence of adolescent cannabis exposure on adult heroin reinforcement in two inbred rat strains differentially vulnerable to drugs of abuse, addiction prone Lewis (LEW) and addiction resistant Fischer 344 (F344) strains. Male LEW and F344 rats aged six weeks were exposed to increasing Δ9-tetrahydrocannabinol (THC) doses, twice a day for 3 days (2, 4, 8 mg/kg, i.p.). At adulthood they were allowed to self-administer heroin (0.025 mg/kg) under both Fixed- (FR) and Progressive- (PR) ratio schedules of responding. Following extinction, responding was reinstated by drug-cues and/or by heroin priming. THC pre-exposure increased responding for heroin and heroin intake under FR-3 and FR-5 as well as PR protocols and increased breaking point in PR schedules in LEW but not F344 rats. Drug cues and heroin priming reinstated responding in LEW and F344, but THC pre-exposure increased reinstatement by priming in LEW rats and by cues in F344 rats. These observations show that in genetically predisposed individuals, adolescent cannabis exposure increases heroin reinforcing properties, thus providing a mechanism for a causal role of adolescent cannabis use in heroin abuse.

Impairment of acquisition of intravenous cocaine self-administration by RNA-interference of dopamine D1-receptors in the nucleus accumbens shell.

Microdialysis during i.v. drug self-administration (SA) have implicated nucleus accumbens (NAc) shell DA in cocaine and heroin reinforcement. However, this correlative evidence has not been yet substantiated by experimental evidence obtained by studying the effect of selective manipulation of NAc shell DA transmission on cocaine and heroin SA. In order to investigate this issue, DA D1a receptor (D1aR) expression was impaired in the NAc shell and core by locally infusing lentiviral vectors (LV) expressing specific D1aR-siRNAs (LV-siRNAs). Control rats were infused in the same areas with LV expressing GFP. Fifteen days later, rats were trained to acquire i.v. cocaine or heroin self-administration (SA). At the end of behavioral experiments, in order to evaluate the effect of LV-siRNA on D1aR expression, rats were challenged with amphetamine and the brains were processed for immunohistochemical detection of c-Fos and D1aR. Control rats acquired i.v. cocaine and heroin SA. Infusion of LV-siRNAs in the medial NAc shell reduced D1aR density and the number of c-Fos positive nuclei in the NAc shell, while sparing the core, and prevented the acquisition of cocaine, but not heroin SA. In turn, LV-siRNAs infusion in the core reduced D1aR density and the number of c-Fos positive nuclei in the same area, while sparing the shell, and failed to affect acquisition of cocaine. The differential effect of LV impairment of NAc shell D1aR on cocaine and heroin SA indicates that NAc shell DA acting on D1aR specifically mediates cocaine reinforcement.