Feedback Inhibition and Throughput Properties of an Integrate-and-Fire-or-Burst Network Model of Retinogeniculate Transmission

Computational modeling has played an important role in the dissection of the biophysical basis of rhythmic oscillations in thalamus that are associated with sleep and certain forms of epilepsy. In contrast, the dynamic filter properties of thalamic relay nuclei during states of arousal are not well understood. Here we present a modeling and simulation study of the throughput properties of the visually driven dorsal lateral geniculate nucleus (dLGN) in the presence of feedback inhibition from the perigeniculate nucleus (PGN). We employ thalamocortical (TC) and thalamic reticular (RE) versions of a minimal integrate-and-fire-or-burst type model and a one-dimensional, two-layered network architecture. Potassium leakage conductances control the neuromodulatory state of the network and eliminate rhythmic bursting in the presence of spontaneous input (i.e., wake up the network). The aroused dLGN/PGN network model is subsequently stimulated by spatially homogeneous spontaneous retinal input or spatio-temporally patterned input consistent with the activity of X-type retinal ganglion cells during full-field or drifting grating visual stimulation. The throughput properties of this visually-driven dLGN/PGN network model are characterized and quantified as a function of stimulus parameters such as contrast, temporal frequency, and spatial frequency. During low-frequency oscillatory full-field stimulation, feedback inhibition from RE neurons often leads to TC neuron burst responses, while at high frequency tonic responses dominate. Depending on the average rate of stimulation, contrast level, and temporal frequency of modulation, the TC and RE cell bursts may or may not be phase-locked to the visual stimulus. During drifting-grating stimulation, phase-locked bursts often occur for sufficiently high contrast so long as the spatial period of the grating is not small compared to the synaptic footprint length, i.e., the spatial scale of the network connectivity.     

Towards a phylogeny and evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia)

The Acochlidia are unique among opisthobranch gastropods in combining extremely high morphological and ecological diversity with modest species diversity. The phylogeny of acochlidians has never been addressed by cladistic means, as their evolution has remained unknown. This study gives a first overview on more than 150 biological and morphological characters that are potentially useful for phylogenetic analysis. Based on 107 characters, a parsimony analysis (PAUP) was performed for all 27 valid acochlidian species together with 11 (plus two) outgroup taxa. The resulting strict consensus tree shows a moderate overall resolution, with at least some bootstrap support for most resolved nodes. The Acochlidia are clearly monophyletic, and originate from an unresolved basal opisthobranch level. The Acochlidia split into the Hedylopsacea (Tantulum (Hedylopsis (Pseudunela (Strubellia (‘Acochlidium’, ‘Palliohedyle’))))) and Microhedylacea (Asperspina (Pontohedyle, ‘Parhedyle’, ‘Microhedyle’, (Ganitus, Paraganitus))). The formerly enigmatic Ganitidae, resembling sacoglossan opisthobranchs by having dagger‐like rachidian radular teeth, are likely to be highly derived microhedylids. The paraphyly of some of the traditionally recognized family level taxa induced a preliminary reclassification. From the phylogenetic hypothesis obtained, we conclude that the acochlidian ancestor was marine mesopsammic. The colonization of limnic systems occurred twice, independently: first in the Caribbean (with the development of the small interstitial Tantulum elegans), and second in the Indo‐Pacific, with a radiation of large‐sized benthic acochlidian species. The evolution of extraordinary reproductive features, such as hypodermic impregnation by a complex copulative aparatus in hedylopsaceans, cutaneous insemination via spermatophores in microhedylaceans, and gonochorism in Microhedylidae s.l. (including Ganitidae), is discussed. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158 , 124–154.     

Dietary Exposure to Organochlorine Compounds in Tarragona Province (Catalonia,Spain): Health Risks

The human health risks due to the dietary exposure to organochlorine compounds (OC) were assessed in the Catalan stretch of the Ebro River (Spain). The concentrations of various persistent organic pollutants (POPs): polychlorinated biphenyls (PCBs), hexachlorobenzene (HCB), pentachlorobenzene (PeCB), various hexachlorocyclohexanes (HCHs), as well as dichloro-diphenyl-trichloroethane (DDT) and derivatives, were determined in samples of fish and seafood, vegetables, fruits, and rice, acquired in localities of the zone under evaluation. In general terms, pollutant concentrations were similar to recent levels reported in the literature. With the exception of the consumption of fish and seafood by children, and due to the presence of PCBs (Aroclor 1254), food intake from local sources does not pose non-carcinogenic risks for all population groups examined. The presence of OC in foods of local origin in the Catalan basin of the Ebro River would not increase the health risks for the area's consumers.     

The D2 dopamine receptor gene variant C957T affects human fear conditioning and aversive priming

Polymorphisms of DRD2 and A NKK1 have been associated with psychiatric syndromes where there is believed to be an underlying learning process deficit such as addiction, post-traumatic stress disorder and psychopathy. We investigated the effects of the DRD2 C957T and ANKK1 Taq IA single nucleotide polymorphism (SNP), which have been associated with psychopathic traits in alcoholic patients, on fear conditioning and aversive priming in healthy volunteers. We found that the DRD2 C957T SNP, but not the ANKK1 Taq IA SNP, was associated with both differential conditioning of the skin conductance response and the aversive priming effect. There were no differences between the genotype groups with respect to the extinction of the skin-conductance conditioned response. These results suggest that the C957T SNP could be related to learning differences associated with the risk of developing psychiatric disorders in individuals that are carriers of the C homozygous genotype. Our genetic data raise the possibility that the dopaminergic system functional variations determined by this SNP could affect fear learning.     

Stocking density at early developmental stages affects growth and sex ratio in the European eel (Anguilla anguilla)

To investigate the effect of stocking density on growth and sex ratio in European eel, four constant density conditions were tested during the transition from the glass to the elver stage for 90 days (Period 1). The test conditions combined the weight of fish per unit surface or volume (surface density or volume density) resulting in four experimental groups: low surface density (0.5 kg/m(2)) and low volume density (5 kg/m(3)) (group S(0.5)V(5)); low surface density (0.5 kg/m(2)) and high volume density (10 kg/m(3)) (group S(0.5)V(10)); high surface density (2 kg/m(2)) and low volume density (5 kg/m(3)) (group S(2)V(5)); and high surface density (2 kg/m(2)) and high volume density (10 Kg/m(3)) (group S(2)V(10)). Subsequently, fish from the S(0.5)V(5), S(2)V(5), and S(2)V(10) groups were transferred to low density conditions (0.1-0.4 kg/m(2) or 0.1-0.3 kg/m(3)) for another 21 months (630 days; Period 2). After Period 1, fish maintained at high surface density, regardless of the volume density, showed higher standard growth rates (SGRs) and RNA/DNA ratio in muscle than those cultured at low surface density. The percentage of mortality was similar in three of the groups (34.2%-41.8%), but not in the S(2)V(10) group (83.3%). At the end of Period 2, most fish (about 95%) exhibited fully differentiated gonads, but different sex ratios were observed in each group. Thus, the S(2)V(5) group showed a higher proportion of females (36.1%) and a lower proportion of males (56.8%) than the S(0.5)V(5) group (11.4% and 72.5%, respectively), while all survivor fish from the S(2)V(10) group developed into females. The gonadosomatic index and SGR were higher in females than in males. These results suggest that glass eels maintained at high surface density during the first months of growth tend to develop into females. The data also indicate that growth and sex ratio are linked processes during eel development, with growth seeming to be sex dependant rather than being influenced by the density conditions in which glass eels are maintained.     

The multi-KH domain protein of Saccharomyces cerevisiae Scp160p contributes to the regulation of telomeric silencing

Multi-KH domain proteins are highly evolutionarily conserved proteins that associate to polyribosomes and participate in RNA metabolism. Recent evidence indicates that multi-KH domain proteins also contribute to the structural organization of heterochromatin both in mammals and Drosophila. Here, we show that the multi-KH domain protein of Saccharomyces cerevisiae, Scp160p, contributes to silencing at telomeres and at the mating-type locus, but not to ribosomal silencing. The contribution of Scp160p to silencing is independent of its binding to the ribosome as deletion of the last two KH domains, which mediate ribosomal binding, has no effect on silencing. Disruption of SCP160 increases cell ploidy but this effect is also independent of the contribution of Scp160p to telomeric silencing as strong relief of silencing is observed in Deltascp160 cells with normal ploidy and, vice versa, Deltascp160 cells with highly increased ploidy show no significant silencing defects. The TPE phenotype of Deltascp160 cells associates to a decreased Sir3p deposition at telomeres and, in good agreement, silencing is rescued by SIR3 overexpression and in a Deltarif1Deltarif2 mutant. Scp160p shows a distinct perinuclear localization that is independent of its ability to bind ribosomes. Moreover, telomere clustering at the nuclear envelope is perturbed in Deltascp160 cells and disruption of the histone deacetylase RPD3, which is known to improve telomere clustering, rescues telomeric silencing in Deltascp160 cells. These results are discussed in the context of a model in which Scp160p contributes to silencing by helping telomere clustering.     

Introns and reading frames: correlation between splicing sites and their codon positions

Computer analyses of the entire GenBank database were conducted to examine correlation between splicing sites and codon positions in reading frames. Intron insertion patterns (i.e., splicing site locations with respect to codon positions) have been analyzed for all of the 74 codons of all the eukaryote taxonomic groups: primates, rodents mammals, vertebrates, invertebrates, and plants. We found that reading frames are interrupted by an intron at a codon boundary (as opposed to the middle of a codon) significantly more often than expected. This observation is consistent with the exon shuffling hypothesis, because exons that end at codon boundaries can be concatenated without causing a frame shift and thus are evolutionarily advantageous. On the other hand, when introns interrupt at the middles of codons, they exist in between the first and second bases much more frequently than between the second and third bases, despite the fact that boundaries between the first and second bases of codons are generally far more important than those between the second and third bases. The reason for this is not clear and yet to be explained. We also show that the length of an exon is a multiple of 3 more frequently than expected. Furthermore, the total length of two consecutive exons is also more frequently a multiple of 3. All the observations above are consistent with results recently published by Long, Rosenberg, and Gilbert (1995).