Cocaine seeking by rats is a goal-directed action.

In two experiments rats were trained to self-administer intravenous cocaine on chained schedules using different responses in the initial (drug-seeking) and terminal (drug-taking) links. In both between- (Experiment 1) and within-subject designs (Experiment 2), the drug-taking response was then either extinguished or reinforced in the absence of the opportunity to perform the seeking response. In a subsequent extinction test with the seeking manipulanda alone, the rate of drug seeking was reduced after the prior extinction of the associated taking response. An additional group trained with a sucrose reinforcer showed a comparable devaluation effect. These findings demonstrate that seeking responses for cocaine and food rewards are mediated by a representation of the contingency between seeking responses and the opportunity to take the reward.

Structural and functional characterization of tnpI, a recombinase locus in Tn21 and related beta-lactamase transposons.

A novel discrete mobile DNA element from Tn21 from the plasmid R100.1 is described, and its mobilization function was confirmed experimentally. In addition, the element behaves as a recombinase-active locus (tnpI) which facilitates insertions of antibiotic resistance genes as modules or cassettes at defined hot spots or integration sites. A similar tnpI sequence was detected by DNA hybridization in a series of beta-lactamase transposons and plasmids and localized on their physical maps. The genetic function of the locus cloned from Tn21 into pACYC184 was tested for conduction and integration into the plasmids R388 and pOX38Km, and the results suggested recombinase-integrase activity and recA independence. DNA sequence analysis of the tnpI locus revealed no inverted or direct terminal repeats or transposition features of class I and class II transposons. The coding capacity revealed three putative open reading frames encoding 131, 134, and 337 amino acids. Orf3 encoded a putative polypeptide product of 337 amino acids that shared highly significant identity with the carboxyl region of integrase proteins. A comparison and an alignment of the tnpI locus from Tn21 and its flanking sequences identified similar sequences in plasmids and in transposons. The alignment revealed discrete nucleotide changes in these tnpI-like loci and a conserved 3' and 5' GTTA/G hot spot as a duplicated target site. Our data confirm the remarkable ubiquity of tnpI associated with antibiotic resistance genes. We present a model of transposon modular evolution into more complex multiresistant units via tnpI and site-specific insertions, deletions, and DNA rearrangements at this locus.

Evolutionary perspectives on multiresistance beta-lactamase transposons.

A series of intragenic DNA probes, encoding the major part of the transposase resolvase and inverted repeats of transposons Tn3, Tn21, and Tn2501, were used in hybridization assays for homologous DNA sequences in 18 transposons studied. The tnpA and tnpR probes detected extensive homology with Tn3-like and Tn21-like elements for 11 transposons. This high degree of homology was confirmed with the 38- and 48-base-pair inverted-repeat oligonucleotide probes of Tn3, Tn21, and Tn2501. The Southern-type gel hybridization experiments localized the tnpA-homologous sequences on the physical DNA maps constructed. The genetic and physical maps of the transposons were compared, as were their nucleic acid sequence homologies. These comparisons suggested a subfamily of mobile elements distinct from but related to the Tn21 group. Based on these results, an evolutionary model is proposed and a pedigree is presented for the genesis of multiresistance beta-lactamase transposons.

Defense reactions of mosquitoes to filarial worms: effect of host age on the immune response to Dirofilaria immitis microfilariae.

The melanization response of Aedes aegypti black-eyed Liverpool strain (LVP) and Aedes trivittatus against intrathoracically inoculated Dirofilaria immitis microfilariae (mff) was assessed in mosquitoes less than 1, 14, 21, and 28 days after adult ecdysis. There was a significant decrease in the melanization response of A. aegypti 14 days of age and older at 1, 3, and 5 days postinoculation (PI) compared to less than 1-day-old mosquitoes. The response also was reduced significantly in 14- to 28-day-old A. trivittatus on days 1 and 3 PI. Although essentially 100% of recovered mff were melanized by day 5 PI in A. trivittatus, the amount of melanin deposited was much less than that seen in 0-day-old mosquitoes. Potential mechanisms responsible for a reduced immune competence in older mosquitoes and the possible relationship to vector potential are discussed.

Parasite-induced suppression of the immune response in Aedes aegypti by Brugia pahangi.

The melanization response against intrathoracically inoculated Brugia pahangi and Dirofilaria immitis microfilariae (mff) isolated from vertebrate host blood was evaluated in both uninfected Aedes aegypti black-eyed Liverpool strain and in mosquitoes harboring a developing B. pahangi infection. The immune response against inoculated mff of either species was significantly reduced by 28-47% in infected as compared with uninfected mosquitoes. Attempts to passively transfer this suppression factor(s) by inoculating naive mosquitoes with 0.1-0.2 microliter of hemolymph from B. pahangi-infected mosquitoes produced equivocal results. The role this parasite-induced immune suppression might play in aiding parasite survival in compatible vectors is discussed.

Is the selenium drinking water standard justified?

Four cases are presented which suggest that the present U.S.E.P.A. drinking water standard for selenium of 10 micrograms/L in inappropriate. The rationale upon which this standard is based is that selenium is carcinogenic, induces dental caries formation, and is highly toxic to animals. However, a critical assessment of this literature can not support these claims. Case no. 1 demonstrates that there is insufficient evidence to classify selenium as a carcinogen. Data derived from the three respective groups of researchers claiming a carcinogenic effect induced by selenium are obscure due to 1) the inability to accurately identify malignancies, 2) the apparent opposite effects of different selenium compounds, and 3) the lack of proper controls. Case no. 2 reviews recent evidence that selenium reduces the incidence of cancer in laboratory animals and in man, an effect which can probably be attributed to the antioxidant properties of selenium compounds. Case no. 3 provides evidence which does not permit the classification of selenium as a cariogenic element. Epidemiological studies supporting such a claim are inadequate since they lack properly matched control groups. Animal data do not support this link as well. Case no. 4 is a review of studies which clearly demonstrate the essentiality of selenium, an aspect of selenium metabolism that was not considered when the 10 micrograms/L standard was promulgated. In light of the four cases presented and an assessment of selenium toxicity in man, it is concluded that the 10 micrograms/L standard can not be justified. Instead, it is suggested that 50 micrograms/L selenium should provide sufficient protection from the toxic effects of this element. This is consistent with the current state of knowledge with respect to the potential adverse health effects associated with selenium.