Arginine Vasotocin: Structure-Activity Relationships and Influence on Gonadal Growth and Function

When AVT (arginine vasotocin) was given neonatally during theperiod when the brain is undergoing sexual differentiation,increased growth of the reproductive organs was observed inadulthood. Injection of AVT after this neonatal period in immatureanimals led to diminished growth of the accessory organs andin some cases the gonads themselves. The hypertrophic responseof the in situ ovary in adult mice following unilateral ovariectomy(UO) was inhibited in a dose-related manner by a single intraperitonealinjection of freshly prepared AVT. Much less AVT was requiredfor this response when injected into the third ventricle. Afterintraperitoneal injection, arginine vasopressin (AVP), lysinevasopressin (LVP), and 4-leucine vasotocin (4-leu-AVT) alsoinhibited compensatory ovarian hypertrophy whereas oxytocindid not. The commonality in die structure of these antigonadotrophicpeptides include a closed ring and a basic amino acid in position8. After opening the disulfide bond of these nonapeptides withmercaptoethanol, a single injection of the reduced AVT, AVP,LVP, or 4-leu-AVT into UO mice causes exaggerated hypertrophyof the remaining ovary. When added with leuteinizing hormone-releasinghormone (LRH) to culture medium containing hemipituitaries fromcastrated estrogen-progesterone primed female rats, AVT significantlyincreased the release of radioimmunoassayable LH above thatdue to LRH alone. AVT might interact at all levels of the hypothalamo-hypophysealgonadalaxis.     

The Relationship between Stimulus Intensity and Oriented Phototactic Response (Topotaxis) in Chlamydomonas

Two techniques have been used to study the quantitative relationship between stimulus intensity and oriented phototactic response (topotaxis) in Chlamydomonas. The net response of a cell population was monitored photometrically and was recorded continuously against time. The responses of individual cells were observed through a microscope and their swimming tracks were recorded on film. The net response of the population is positive at low stimulus intensity and negative at high intensity. The direction of response can be reversed within two seconds by raising or lowering the intensity. The intensity-response curve for phototaxis is similar to the dose-response curve for phototropism. The net response has no distinct threshold; it increases linearly with log intensity; then it decreases and finally becomes negative. The individual-cell studies reveal that the intensity-dependent increase in net topotactic response is due primarily to an increase in the number of cells responding and in the directness of their swimming path. As stimulus intensity is raised, the swimming path becomes increasingly well-aligned with the stimulus beam, whether net response is positive throughout the intensity range tested, negative throughout that range, or changing from positive to negative. Changes in swimming rate do not contribute significantly to the intensity-dependent changes in net response. Swimming rate shows virtually no change throughout the intensity range of positive topotaxis and shows only a small increase in the negative range. However, a transient decrease in swimming rate (stop response) is often observed at the onset of stimulation. The implications of these results for the orientation mechanism are discussed.     

Changes in Enzyme Activities and Lipid Content of Echinoderm Egg Membranes, at Maturation and Fertilization

Membrane enriched fractions of eggs show changes in relativelipid and acylglyceride fatty acid content at maturation andfertilization, when many critical membrane changes occur. Manyof the changes in lipids seen at fertilization in sea urchinsare reversed from those seen at maturation in starfish. Maybea special permissive lipid content appears at maturation anddisappears at fertilization. Mycostatin alters the lipid contentof unfertilized eggs, prevents fertilization, blastula cellaggregation and normal development, with decreasing susceptibilitywith age, suggesting altered cell surfaces during development.Membrane enriched fractions also show changes in enzyme specificactivity at fertilization. Attempts were made to localize theseenzymes in various surface components (membrane or associatedcoats or adherent cortex) and several fractions were separated.Egg surfaces are complex and therefore difficult to study. DTTor pronase treatment, for example, changes the activity of G6PDin the surface of whole eggs, with further changes occurringin such eggs following fertilization. Increases in G6PD activityin ghosts and supernate of fertilized eggs appear due to isoenzymechanges instead of one enzyme changing compartment. Isozymepatterns of G6PD extracted from pellets differ from those ofsupernates. Patterns also differ following fertilization orpartial activation by ammonia or calcium ionophore. Some isozymesseem to be breakdown products of larger forms, and can be producedby treatment with papain, DTT, or urea. Isozyme studies supporta dynamic model of the egg surface. Surface changes may occurenzymatically, by insertion of vesicles, or by release of materialfrom vesicles.     

Periods of Nutritional Stress in the Annual Cycles of Endotherms: Fact or Fiction?

An animal's nutritional status depends on (1) nutrient accessibility,(2) nutrient demand, and (3) physiological, metabolic, morphological,and behavioral compensations that avert or minimize discrepancies,if any, between the first two. The main thesis of this essayis that the diversity and potency of such compensatory mechanismshave often been underrated, and hence the frequency and intensityof nutritional stress in free-living animals have often beenexaggerated. This theme is explored in conjunction with an inventoryof the modes of compensation for actual or potential dietarydeficiencies.     

Donor Plant Growth Factors Affecting Anther Culture of Maize and Sweetcorn (Zea mays)

Anthers from seven unselected commercial sweetcorn lines andten experimental maize lines were cultured on a liquid/solidbi-layer culture medium, containing 13 % maltose as the carbonsource. Mean anther efficiencies (number of embryos per 100anthers plated) of 0 to 27.6 % were recorded, with the maximumefficiency of 57.1 % from one plant. The anther efficiency wasfound to be dependent on genotype, microspore developmentalstage and the growth temperatures of donor plants. Immaturemicrospores were found to continue their development duringthe cold pretreatment of the spikes, and this in turn influencedthe level of response to culture. Direct regeneration of embryoidsto plants occurred most frequently when well formed uni- orbi-polar embryos were produced. The quality of embryo producedwas apparently inversely correlated with the number of embryosproduced. Zea mays, haploid culture, embryos, microspores