Spike-based synaptic plasticity and the emergence of direction selective simple cells: mathematical analysis

In the companion paper we presented extended simulations showing that the recently observed spike-timing dependent synaptic plasticity can explain the development of simple cell direction selectivity (DS) when simultaneously modifying the synaptic strength and the degree of synaptic depression. Here we estimate the spatial shift of the simple cell receptive field (RF) induced by the long-term synaptic plasticity, and the temporal phase advance caused by the short-term synaptic depression in response to drifting grating stimuli. The analytical expressions for this spatial shift and temporal phase advance lead to a qualitative reproduction of the frequency tuning curves of non-directional and directional simple cells. In agreement with in vivo recordings, the acquired DS is strongest for test gratings with a temporal frequency around 1–4 Hz. In our model this best frequency is determined by the width of the learning function and the time course of depression, but not by the temporal frequency of the training stimuli. The analysis further reveals the instability of the initially symmetric RF, and formally explains why direction selectivity develops from a non-directional cell in a natural, directionally unbiased stimulation scenario.     

Presynaptic Frequency- and Pattern-Dependent Filtering

Dual intracellular recordings from pairs of synaptically connected neurones have demonstrated that the frequency-dependent pattern of transmitter release varies dramatically between different classes of connections. Somewhat surprisingly, these patterns are not determined by the class of neurone supplying the axon alone, but to a large degree by the class of postsynaptic neurone. A wide range of presynaptic mechanisms, some that depress the release of transmitter and others that enhance release have been identified. It is the selective expression of these different mechanisms that determines the unique frequency- and pattern-dependent properties of each class of connection. Although the molecular interactions underlying these several mechanisms have yet to be fully identified, the wealth and complexity of the protein-protein and protein-lipid interactions that have been shown to control the release of transmitter suggest many ways in which the properties of a synapse may be tuned to respond to particular patterns and frequencies.     

A review of clinical and experimental observations about antidepressant actions and side effects produced by Hypericum perforatum extracts

Hypericum perforatum is an herbaceous perennial plant, also known as "St. John's wort", used popularly as a natural antidepressant. Although some clinical and experimental studies suggest it has some properties similar to conventional antidepressants, the proposed mechanism of action seems to be multiple: a non-selective blockade of the reuptake of serotonin, noradrenaline and dopamine; an increase in density of serotonergic and dopaminergic receptors and an increased affinity for GABAergic receptors; moreover, the inhibition of monoaminoxidase enzyme activity has been involved. In any case, the increase of monoamine concentrations in the synaptic cleft resembles several actions exerted by clinically effective antidepressants. In the present article, we review some of the controversial evidence derived from clinical and experimental studies suggesting that H. perforatum exerts antidepressant-like actions, and we also review some of its side effects, such as nausea, rash, fatigue, restlessness, photosensitivity, acute neuropathy, and even episodes of mania and serotonergic syndrome when administered simultaneously with other antidepressant drugs. All of the foregoing suggests that H. perforatum extracts appear to exert potentially significant pharmacological activity involving several neurotransmission systems supposed to be involved in the pathophysiology of depression. However, little information regarding the safety of H. perforatum is available, including potential herb-drug interactions. There is a need for additional research on the pharmacological and biochemical activity of H. perforatum, as well as its side-effects and its several bioactive constituents to further elucidate the mechanisms of antidepressant actions.     

Characterization of ATP-dependent proteolysis in embryos of the brine shrimp, Artemia franciscana

Under anoxia, embryos of Artemia franciscana enter a state of quiescence. During this time protein synthesis is depressed, and continued degradation of proteins could jeopardize the ability to recover from quiescence upon return to favorable conditions. In this study, we developed an assay for monitoring ATP/ubiquitin-dependent proteolysis in order to establish the presence of this degradation mechanism in A. franciscana embryos, and to describe some characteristics that may regulate its function during anoxia-induced quiescence. For lysates experimentally depleted of adenylates, supplementation with ATP and ubiquitin stimulated protein degradation rates by 92 ± 17% (mean ± SE) compared to control rates. The stimulation by ATP was maximal at concentrations ≥11 μmol · l⁻¹. In the presence of ATP and ubiquitin, ubiquitin-conjugated proteins were produced by lysates during the course of the 4-h assays, as detected by Western blotting. Acute acidification of lysates to values approximating the intracellular pH observed under anoxia completely inhibited ATP/ubiquitin-dependent proteolysis. Depressed degradation was also observed under conditions where net ATP hydrolysis occurred. These results suggest that ATP/ubiquitin-dependent proteolysis is markedly inhibited under cellular conditions promoted by anoxia. Inhibition of proteolysis during quiescence may be one critical factor that increases macromolecular stability, which may ultimately govern the duration of embryo survival under anoxia. Accepted: 2 November 1999     

Outcrossing rate and inbreeding depression in the perennial yellow bush lupine, Lupinus arboreus (Fabaceae)

Little is known about the breeding systems of perennial Lupinus species. We provide information about the breeding system of the perennial yellow bush lupine, Lupinus arboreus, specifically determining self-compatibility, outcrossing rate, and level of inbreeding depression. Flowers are self-compatible, but autonomous self-fertilization rarely occurs; thus selfed seed are a product of facilitated selfing. Based on four isozyme loci from 34 maternal progeny arrays of seeds we estimated an outcrossing rate of 0.78. However, when we accounted for differential maturation of selfed seeds, the outcrossing rate at fertilization was lower, ～0.64. Fitness and inbreeding depression of 11 selfed and outcrossed families were measured at four stages: seed maturation, seedling emergence, seedling survivorship, and growth at 12 wk. Cumulative inbreeding depression across all four life stages averaged 0.59, although variation existed between families for the magnitude of inbreeding depression. Inbreeding depression was not manifest uniformly across all four life stages. Outcrossed flowers produced twice as many seeds as selfed flowers, but the mean performance of selfed and outcrossed progeny was not different for emergence, seedling survivorship, and size at 12 wk. Counter to assumptions about this species, L. arboreus is both self-compatible and outcrosses ～78% of the time.     

Inbreeding depression and genetic load in laboratory metapopulations of the butterfly Bicyclus anynana

Abstract.— We investigated the effects of inbreeding on various fitness components and their genetic load in laboratory metapopulations of the butterfly Bicyclus anynana . Six metapopulations each consisted of four subpopulations with breeding population sizes of N = 6 or N = 12 and migration rate of m = 0 or m = 0.33. Metapopulations were maintained for seven generations during which coancestries and pedigrees were established. Individual inbreeding coefficients at the F₇ were calculated and ranged between 0.01 and 0.51. Even though considerable purging had occurred during inbreeding, the genetic load remained higher than that of many outbreeding species: approximately two lethal equivalents were detected for egg sterility, one for zygote survival, one for juvenile survival, and one for longevity. Severe inbreeding depression occurred after seven generations of inbreeding, which jeopardized the metapopulation survival. This finding suggests that the purging of genetic load by intentional inbreeding cannot be recommended for the genetic conservation of species with a high number of lethal.     

Evaluation of major genetic loci contributing to inbreeding depression for survival and early growth in a selfed family of Pinus taeda

The magnitude of fitness effects at genetic loci causing inbreeding depression at various life stages has been an important question in plant evolution. We used genetic mapping in a selfed family of loblolly pine (Pinus taeda L.) to gain insights on inbreeding depression for early growth and viability. Two quantitative trait loci (QTLs) were identified that explain much of the phenotypic variation in height growth through age 3 and may account for more than 13% inbreeding depression in this family. One of these QTLs maps to the location of cad-nl, a lignin biosynthesis mutation. Both QTLs show evidence of overdominance, although evidence for true versus pseudo-overdominance is inconclusive. Evidence of directional dominance for height growth was noted throughout the genome, suggesting that additional loci may contribute to inbreeding depression. A chlorophyll-deficiency mutation, spf did not appear to be associated with growth effects, but had significant effects on survival through age 3. Previously identified embryonic viability loci had little or no overall effect on germination, survival, or growth. Our results challenge, at least in part, the prevailing hypothesis that inbreeding depression for growth is due to alleles of small effect. However, our data support predictions that loci affecting inbreeding depression are largely stage specific.     

INBREEDING DEPRESSION IN GYNODIOECIOUS LOBELIA SIPHILITICA: AMONG-FAMILY DIFFERENCES OVERRIDE BETWEEN-MORPH DIFFERENCES

If inbreeding depression is caused by deleterious recessive alleles, as suggested by the partial dominance hypothesis, a negative correlation between inbreeding and inbreeding depression is predicted. This hypothesis has been tested several times by comparisons of closely related species or comparisons of populations of the same species with different histories of inbreeding. However, if one is interested in whether this relationship contributes to mating-system evolution, which occurs within populations, comparisons among families within a population are needed; that is, inbreeding depression among individuals with genetically based differences in their rate of selfing should be compared. In gynodioecious species with self-compatible hermaphrodites, hermaphrodites will have a greater history of potential inbreeding via both selfing and biparental inbreeding as compared to females and may therefore express a lower level of inbreeding depression. We estimated the inbreeding depression of female and hermaphrodite lineages in gynodioecious Lobelia siphilitica in a greenhouse experiment by comparing the performance of selfed and outcrossed progeny, as well as sibling crosses and crosses among subpopulations. We did not find support for lower inbreeding depression in hermaphrodite lineages. Multiplicative inbreeding depression (based on seed germination, juvenile survival, survival to flowering, and flower production in the first growing season) was not significantly different between hermaphrodite lineages (δ = 0.30 ± 0.08) and female lineages (δ = 0.15 ± 0.18), although the trend was for higher inbreeding depression in the hermaphrodite lineages. The population-level estimate of inbreeding depression was relatively low for a gynodioecious species (δ = 0.25) and there was no significant inbreeding depression following biparental inbreeding (δ = 0.01). All measured traits showed significant variation among families, and there was a significant interaction between family and pollination treatment for four traits (germination date, date of first flowering, number of flowers, and aboveground biomass). Our results suggest that the families responded differently to selfing and outcrossing: Some families exhibited lower fitness following selfing whereas others seemed to benefit from selfing as compared to outcrossing. Our results support recent simulation results in that prior inbreeding of the lineages did not determine the level of inbreeding depression. These results also emphasize the importance of determining family-level estimates of inbreeding depression, relative to population-level estimates, for studies of mating-system evolution.     

Factors determining the midday depression of photosynthesis in trees under monsoon climate

 Field studies of gas exchange of Populus deltoides, Prosopis juliflora and Acacia auriculiformis showed large diurnal changes in net photosynthesis (A) and stomatal conductance (g_s) during autumn. P. deltoides and P. juliflora undergo pronounced midday depression in A and g_s while A. auriculiformis showed a one-peak response. Several factors indicative of photosynthetic performance were found to be reversibly affected during afternoon decline. These include (i) decrease in initial slope of the CO₂ response curve (carboxylation efficiency), (ii) substantial increase in CO₂ compensation point and (iii) decrease in overall quantum yield of photosystem II. The phenomenon can be duplicated in potted plants by simulating a typical daily pattern of PPFD and VPD. It is found that high VPD induces significant decline in A and g_s at moderate temperature and saturating PPFD (800 μmol m^–2 s^–1) whereas these parameters are only marginally affected at high PPFD and low VPD. Fluorescence data show that the tree species under study have a high capacity for safe dissipation of excessive excitation energy. The activation of photorespiration, as evident from an increase in CO₂ compensation point, maintains constant internal CO₂ concentration (C_i) which may aid in minimizing photoinhibition during stomatal closure at midday. In case of P. deltoides and P. juliflora the stomata seem to be quite sensitive to the changes in humidity whereas this does not appear to be essential in case of A. auriculiformis because of its phyllode structure that endows it with mechanisms for conserving water without undergoing large-scale stomatal changes. Received: 16 October 1997 / Accepted: 5 March 1998     

Are inbred offspring less fit? Survival in a natural population of Mexican jays

The importance of inbreeding avoidance for the fitness of individualsis currently undergoing reevahation. Especially when the frequencyof inbreeding is low, it may be difficult to accumulate sufficientinformation. In a long-term study of the Mexican jay (Aphelocomaultamarina) in Arizona, we found inbreeding, as determined bypedigree, to be rare even though this species exhibits strongnatal philopatry and commonly lives in social groups of closerelatives. Brood sizes of inbred pairs were significantly lowerthan those of outbred pairs, suggesting hatching failure; butthere was no difference in die probability that one or moreyoung would fledge from a nest. Survival of inbred nestlingsto die next year was significantly lower than that of outbrednestlings.