Mechanisms underlying nutrient-induced segmentation in isolated guinea pig small intestine

Mechanisms underlying nutrient-induced segmentation within the gut are not well understood. We have shown that decanoic acid and some amino acids induce neurally dependent segmentation in guinea pig small intestine in vitro. This study examined the neural mechanisms underlying segmentation in the circular muscle and whether the timing of segmentation contractions also depends on slow waves. Decanoic acid (1 mM) was infused into the lumen of guinea pig duodenum and jejunum. Video imaging was used to monitor intestinal diameter as a function of both longitudinal position and time. Circular muscle electrical activity was recorded by using suction electrodes. Recordings from sites of segmenting contractions showed they are always associated with excitatory junction potentials leading to action potentials. Recordings from sites oral and anal to segmenting contractions revealed inhibitory junction potentials that were time locked to those contractions. Slow waves were never observed underlying segmenting contractions. In paralyzed preparations, intracellular recording revealed that slow-wave frequency was highly consistent at 19.5 (SD 1.4) cycles per minute (c/min) in duodenum and 16.6 (SD 1.1) c/min in jejunum. By contrast, the frequencies of segmenting contractions varied widely (duodenum: 3.6-28.8 c/min, median 10.8 c/min; jejunum: 3.0-27.0 c/min, median 7.8 c/min) and sometimes exceeded slow-wave frequencies for that region. Thus nutrient-induced segmentation contractions in guinea pig small intestine do not depend on slow-wave activity. Rather they result from a neural circuit producing rhythmic localized activity in excitatory motor neurons, while simultaneously activating surrounding inhibitory motor neurons.     

Local inhibitory reflexes excited by mucosal application of nutrient amino acids in guinea pig jejunum

The motility of the gut depends on the chemicals contained in the lumen, but the stimuli that modify motility and their relationship to enteric neural pathways are unclear. This study examined local inhibitory reflexes activated by various chemical stimulants applied to the mucosa to characterize effective physiological stimuli and the pathways they excite. Segments of the jejunum were dissected to allow access to the circular muscle on one-half of the preparation while leaving the mucosa intact on the circumferentially adjacent half. Chemicals were transiently applied to the mucosa, and responses were recorded intracellularly in nearby circular muscle cells. The amino acids l-phenylalanine, l-alanine, or l-tryptophan (all 1 mM) evoked inhibitory junction potentials (IJPs; latency 150-300 ms, amplitude 3-8 mV, each n > 6) that were blocked by TTX and partially blocked by antagonists of P2X receptors and/or a combination of antagonists at 5-HT(3) and 5-HT(4) receptors. The putative mediators 5-HT (10 microM), ATP (1 mM), and CCK-8 (1-10 microM) elicited IJPs mediated via 5-HT(3), P2X, and CCK-B receptors, respectively. Responses were only partially reduced by the effective antagonists. IJPs evoked by electrically stimulating the mucosa were unaffected by antagonists that reduced chemically evoked responses. Both chemically and electrically evoked IJPs were resistant to nicotinic, NK(1), NK(3), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, N-methyl-d-aspartate, or CGRP receptor blockade. We conclude that mucosal stimulation by amino acids activates local neural pathways whose pharmacology depends on the nature of the stimulus. Transmitters involved at some synapses in these pathways remain to be identified.     

Condition dependence of female choosiness in a field cricket

Females generally choose mates that produce the loudest, brightest or most elaborate sexual displays, and these costly male displays are predicted to be condition dependent. However, mate choice itself is a costly behaviour also expected to be condition dependent. Male fall field crickets, Gryllus pennsylvanicus, produce a conspicuous long‐distance calling song that attracts females and is condition dependent. In this study, we tested the condition dependence of female preferences (preference function and choosiness) for male calling effort in G. pennsylvanicus. We manipulated female condition by raising crickets from hatching on either a low‐ or high‐quality diet. In a series of two‐speaker phonotaxis trials, both low‐ and high‐condition females preferred playbacks reflecting greater calling effort. However, relative to low‐condition females, high‐condition females took significantly longer to make a choice, were more likely to fail to choose within the time allotted for a phonotaxis trial and significantly increased their latency to choose over the course of multiple trials. We discuss these results with respect to the possibility that female G. pennsylvanicus may be foraging for direct benefits when they choose their mates.     

Contrasting sexual selection on males and females in a role-reversed swarming dance fly, Rhamphomyia longicauda Loew (Diptera: Empididae)

Although there are several hypotheses for sex-specific ornamentation, few studies have measured selection in both sexes. We compare sexual selection in male and female dance flies, Rhamphomyia longicauda (Diptera: Empididae). Swarming females display size-enhancing abdominal sacs, enlarged wings and decorated tibiae, and compete for nuptial gifts provided by males. Males preferentially approach large females, but the nature of selection and whether it is sex-specific are unknown. We found contrasting sexual selection for mating success on structures shared by males and females. In females, long wings and short tibiae were favoured, whereas males with short wings and long tibiae had a mating advantage. There was no assortative mating. Females occupying potentially advantageous swarm positions were large and, in contrast to selection for mating success, tended to have larger tibiae than those of rivals. We discuss our findings in the context of both the mating biology of dance flies, and the evolution of sexual dimorphism in general.     

Toward a dynamic model of deposition and utilization of yolk steroids

The discovery by Schwabl that maternal steroid hormones aretransferred to the egg yolk and have effects on the phenotypeof offspring revealed a new pathway for non-genetic maternaleffects. The initial model relied on passive transfer. The thinkingwas that steroids passively entered the lipophillic yolk duringyolk deposition and then were deposited in the yolk until theywere passively delivered to the embryo as the yolk was used.Subsequent studies revealed that the system is much more dynamicthan that. Here, we explore questions about how dynamic thesystem really is and look at questions like: Is transfer ofmaternal steroids to the yolk passive or is it actively regulated?At what stages of the maternal reproductive cycle are steroidstransferred? During reproduction, how dynamic are the levelsof yolk steroids? Especially in the case of potentially deleterioussteroids (e.g., androgens in female offspring; glucocorticoids),once deposited can they come out of the yolk over time? Canthey be metabolized by the yolk or by the embryo? During incubation,how much do steroid levels in the yolk change? Can steroidsdiffuse from the yolk to the embryo prior to yolk utilization?Does the embryo contribute to yolk steroid levels as it develops?We believe that comprehensive answers to questions like thesewill eventually allow us to generate a much more accurate andcomplete model of the transfer and utilization of yolk steroidsand that this model will be much more dynamic and active thanthe one initially proposed.     

Age-related changes in the haematology of female F344 rats

As little comprehensive baseline data are available on age-related haematological changes in genetically-defined rat strains, the haematology of female F344 rats is described in animals sampled at 2, 4, 8, 20, 66 and 121 weeks of age. Values for Hb, RBC and PCV increased from 2 weeks of age to reach adult levels at 8 weeks, whereas MCV, MCH and reticulocyte counts were high initially but decreased to reach the adult range at 8 weeks. Between 66 and 121 weeks, reticulocyte counts were significantly increased and values for MCHC significantly decreased. Lymphocytes were the predominant white cell type in each age group. The absolute numbers of neutrophils and lymphocytes showed slight variations between 2 and 66 weeks and both cell types increased significantly between 66 and 121 weeks. Platelet counts showed no overall age-related trends. Fibrinogen values increased from 2 weeks of age to reach the adult level at 8 weeks. One animal of the 14 sampled at 121 weeks showed changes in the blood, liver and spleen consistent with a diagnosis of lymphoid leukaemia.     

Mate selection by female katydids (Orthoptera: Tettigoniidae, Conocephalus nigropleurum)

Female katydids receive a large spermatophore at mating which they subsequently eat. Available evidence indicates that spermatophore nutrients are important to female reproduction. Heavier males produce larger spermatophores. When given the choice between two singing males of different weights, females always mated with the larger individual.     

Are solitary and gregarious Mormon crickets (Anabrus simplex, Orthoptera, Tettigoniidae) genetically distinct?

Phase polyphenisms are usually thought to reflect plastic responses of species, independent of genetic differences; however, phase differences could correlate with genetic differentiation for various reasons. Mormon crickets appear to occur in two phases that differ in morphology and behaviour. Solitary individuals are cryptic and sedentary whereas gregarious individuals form bands, migrate, and are aposematically coloured. These traits have been thought to be phenotypically plastic and induced by environmental conditions. However, there has been no previous investigation of the extent of genetic differences between solitary and gregarious populations of this widespread North American species. We sequenced two mitochondrial genes, COII and COIII, in samples of Mormon crickets from gregarious populations west of the continental divide and solitary mountain populations primarily east of the divide. Sequencing revealed two genetically distinct clades that broadly correspond with the solitary eastern populations and the mainly gregarious western populations. We used coalescent modelling to test the hypothesis that the species consists of two deep genetic clades, as opposed to a series of equally distinct populations. Results allowed us to reject the null hypothesis that a radiation independent of phase produced these clades, and molecular clock estimates indicate the time of divergence to be approximately 2 million years ago. This work establishes that the solitary populations found in the mountains on the eastern slope are part of a clade that is genetically distinct from the western populations, which are primarily gregarious, and the implications of this apparent correlation between phase and genetic differentiation are discussed.     

Esterification of cholesterol in high density lipoprotein decreases its ability to support ACTH-stimulated steroidogenesis by rat adrenocortical cells

Addition of high density lipoprotein 3 (HDL3) isolated from human plasma of d greater than 1.125 g/ml which had been preincubated for 24 h at 37 degrees C enhanced steroidogenesis by cultured rat adrenal cells only 38% as well as HDL3 isolated from unincubated plasma. Loss of steroidogenic activity due to preincubation was associated with a decrease in the percent HDL3 cholesterol remaining unesterified. Inhibition of lecithin-cholesterol acyltransferase activity by heating (60 degrees C, 1 h) or addition of dithionitrobenzoic acid (1.4 mM) prevented esterification of cholesterol in HDL and also prevented loss of steroidogenic activity. Although incubation of plasma of d greater than 1.125 g/ml prior to isolation caused cholesterol esterification, there was no change in the ratio of total cholesterol to protein in HDL, size and shape of the HDL particle as assayed by measurement of sedimentation velocity, nor affinity for the putative HDL receptor. Addition of unesterified cholesterol to preincubated HDL restored steroidogenic activity. These results indicate that unesterified cholesterol in HDL is preferentially used as substrate for rat adrenal steroidogenesis. The effects of nonlipoprotein serum proteins on HDL action in the adrenal were also examined. The ability of HDL3 to enhance rat adrenal steroidogenesis was not significantly less in serum-free media than in media supplemented with lipoprotein-poor fetal calf serum or human plasma of d greater than 1.21 g/ml, suggesting that rat adrenal uptake of HDL cholesterol does not depend on participation of plasma enzymes or transport proteins.