Neuroendocrine control of gonadotropin secretion

Luteinizing hormone releasing hormone (LHRH), a hypothalmic peptide that is concentrated in granules of neurons, has the capacity to release gonadotropins (luteinizing hormone (LH) and follicle stimulating hormone) from the pituitary gland. LHRH has been found in hypophysial portal blood of rats, monkeys, and rabbits. Antibodies to LHRH depress plasma LH concentrations in castrated animals and evoke testicular atrophy, but passive immunization against LHRH does not block the LH surge induced by estrogen in monkeys. Estrogens, progestin, prolactin, and dopamine have marked effects on LH secretion, yet an association between these effects and altered hypophysial portal blood concentrations of LHRH is not established. In view of the paucity of evidence demonstrating such a cause and effect relationship, two alternative proposals have become tenable. One, hormones and neurotransmitters may not alter the levels of portal blood LHRH, but rather alter the frequency of pulsatile LHRH secretion. Two, hormones, such as estrogens, progesterone, and prolactin, may alter the responsiveness of the gonadotropin-secreting cells to LHRH by affecting the secretion of dopamine.     

Mode of action of N,N′-diphenylurea: The isolation and identification of the cytokinins in the transfer RNA from tobacco callus grown in the presence of N,N′-diphenylurea

Summary The tRNA from cytokinin-dependent tobacco callus (Nicotiana tabacum) grown on mineral medium containing N,N-diphenylurea as the source of cytokinin was found to contain 3 cytokinin-active ribonucleosides. The 2 ribonucleosides present in the largest amounts were identified conclusively by their chromatographic properties, ultra-violet and low-resolution mass spectra as the naturally-occurring cytokinins 6-(4-hydroxy-3-methyl-cis-2-butenylamino)-9--D-ribofuranosylpurine and 6-(3-methyl-2-butenylamino)-9--ribofuranosylpurine. A third ribonucleoside, present in smaller amounts, was identified as another naturally-occurring cytokinin 6-(4-hydroxy-3-methyl-2-butenylamino)-2-methylthio-9--D-ribofuranosylpurine on the basis of its chromatographic behaviour. No evidence was found to associate the mode of action of the non-purine cytokinin, N,N-diphenylurea, with tRNA.Abbreviation DPU N,N-diphenylurea     

Co-ordinating interneurones of the locust which convey two patterns of motor commands: their connexions with flight motoneurones.

1. Some flight motoneurones receive two superimposed rhythms of depolarizing synaptic potentials when the locust is not flying; a slow rhythm which is invariably linked to the expiratory phase of ventilation, and a fast rhythm with a period of about 50 ms which is similar to the wingbeat period in flight. 2. By recording simultaneously from groups of motoneurones, the synaptic potentials which underly these rhythms have been revealed in 30 flight motoneurones in the three thoracic ganglia. The inputs occur in elevator motoneurones and some depressors but are of lower amplitude in the latter. The inputs have not been found in leg motoneurones. 3. The rhythmic depolarizations are usually subthreshold but sum with sensory inputs to evoke spikes in flight motoneurones at intervals equal to or multiples of the wingbeat period in flight. 4. Both rhythms originate in the metathoracic ganglion and are mediated by the same interneurones. They can be adequately explained by supposing that there are two symmetrical interneurones which each make widespread connexions with left and right flight motoneurones in the three ganglia. 5. The slow rhythm is coded in the overall burst of interneurone spikes during expiration and the fast rhythm in the interval between the spikes of a burst.     

Monosynaptic connexions between wing stretch receptors and flight motoneurones of the locust.

1. The connexions between stretch receptors of the wings and motoneurones innervating flight muscles have been studied anatomically and physiologically. 2. Filling with cobaltous chloride shows that the single neurone of a forewing stretch receptor has a complex pattern of branches within the mesothoracic ganglion and branches which extend into the pro- and meta-thoracic ganglia. The single neurone of a hindwing stretch receptor has extensive branches in the metathoracic ganglion and branches in themesothoracic ganglion. The branches of both receptors are confined to the ipsilateral halves of the ganglia. 3. A stretch receptor gives information about the velocity and extent of elevation of a wing. 4. Each spike of a forewing stretch receptor casuses an EPSP in ipsilateral mesothoracic depressor motoneurones and an IPSP in elevators. The connexions are thought to be monosynaptic for the following reasons. The EPSPs in the first basalar (depressor) motoneurone follow each spike of the stretch receptor at a frequency of 125 Hz and with a constant latency of about 1 msec. In a Ringer solution containing 20 mM-Mg2+ the amplitude EPSP declines gradually. The IPSP'S upon elevators have similar properties but occur with a latency of 4-6 msec. 5. The connexions therefore comprise a monosynaptic negative feed-back loop; elevation of the wing excites the stretch receptor which then inhibits the elevator motoneurones and excites the depressors. 6. A hindwing stretch receptor synapses upon metathoracic flight motoneurones in the same way, causing EPSPs in depressor and IPSPs in elevator motoneurones. 7. No connexions of either fore- or hindwing stretch receptors have been found with contralateral flight motoneurones. 8. Interganglionic connexions are made by both receptors. For example, both fore- and hindwing stretch receptors cause EPSPs upon the meso- and metathoracic first basalar motoneurones. 9. Stimulation of the axon of a stretch receptor with groups of three stimuli repeated every 50-100 msec thus simulating the pattern which it shows during flight, causes subthreshold waves of depolarization in depressor motoneurones. When summed with an unpatterned input, the stretch receptor is able to influence the production of spikes in motoneurones on each cycle. During flight, it is expected that the stretch receptor will influence the time at which a motoneurone will spike and hence have an effect on the amplitude of the upstroke and upon the phase relationship between spikes of motoneurones.     

Skp1-Cul1-F-box Ubiquitin Ligase (SCFβTrCP)-mediated Destruction of the Ubiquitin-specific Protease USP37 during G2-phase Promotes Mitotic Entry

Ubiquitin-mediated proteolysis is a key regulatory process in cell cycle progression. The Skp1-Cul1-F-box (SCF) and anaphase-promoting complex (APC) ubiquitin ligases target numerous components of the cell cycle machinery for destruction. Throughout the cell cycle, these ligases cooperate to maintain precise levels of key regulatory proteins, and indirectly, each other. Recently, we have identified the deubiquitinase USP37 as a regulator of the cell cycle. USP37 expression is cell cycle-regulated, being expressed in late G₁ and ubiquitinated by APC^Cdh1 in early G₁. Here we report that in addition to destruction at G₁, a major fraction of USP37 is degraded at the G₂/M transition, prior to APC substrates and similar to SCF^βTrCP substrates. Consistent with this hypothesis, USP37 interacts with components of the SCF in a βTrCP-dependent manner. Interaction with βTrCP and subsequent degradation is phosphorylation-dependent and is mediated by the Polo-like kinase (Plk1). USP37 is stabilized in G₂ by depletion of βTrCP as well as chemical or genetic manipulation of Plk1. Similarly, mutation of the phospho-sites abolishes βTrCP binding and renders USP37 resistant to Plk1 activity. Expression of this mutant hinders the G₂/M transition. Our data demonstrate that tight regulation of USP37 levels is required for proper cell cycle progression.     

Hysteresis in poly‐2′‐deoxycytidine i‐motif folding is impacted by the method of analysis as well as loop and stem lengths

In DNA, i‐motif (iM) folds occur under slightly acidic conditions when sequences rich in 2′‐deoxycytidine (dC) nucleotides adopt consecutive dC self base pairs. The pH stability of an iM is defined by the midpoint in the pH transition (pH_T) between the folded and unfolded states. Two different experiments to determine pH_T values via circular dichroism (CD) spectroscopy were performed on poly‐dC iMs of length 15, 19, or 23 nucleotides. These experiments demonstrate two points: (1) pH_T values were dependent on the titration experiment performed, and (2) pH‐induced denaturing or annealing processes produced isothermal hysteresis in the pH_T values. These results in tandem with model iMs with judicious mutations of dC to thymidine to favor particular folds found the hysteresis was maximal for the shorter poly‐dC iMs and those with an even number of base pairs, while the hysteresis was minimal for longer poly‐dC iMs and those with an odd number of base pairs. Experiments to follow the iM folding via thermal changes identified thermal hysteresis between the denaturing and annealing cycles. Similar trends were found to those observed in the CD experiments. The results demonstrate that the method of iM analysis can impact the pH_T parameter measured, and hysteresis was observed in the pH_T and T_m values.     

Paedomorphic Facial Expressions Give Dogs a Selective Advantage

How wolves were first domesticated is unknown. One hypothesis suggests that wolves underwent a process of self-domestication by tolerating human presence and taking advantage of scavenging possibilities. The puppy-like physical and behavioural traits seen in dogs are thought to have evolved later, as a byproduct of selection against aggression. Using speed of selection from rehoming shelters as a proxy for artificial selection, we tested whether paedomorphic features give dogs a selective advantage in their current environment. Dogs who exhibited facial expressions that enhance their neonatal appearance were preferentially selected by humans. Thus, early domestication of wolves may have occurred not only as wolf populations became tamer, but also as they exploited human preferences for paedomorphic characteristics. These findings, therefore, add to our understanding of early dog domestication as a complex co-evolutionary process.     

Soil drainage and phosphorus depletion contribute to retrogressive succession along a New Zealand chronosequence

Background and aims

Models of retrogressive succession have emphasised the role of phosphorus (P) depletion in driving biomass loss on surfaces of increasing geologic age, but the influence of impeded drainage on old surfaces has received much less attention. We tested whether poor drainage contributed to changes in ecosystem properties along a 291,000-year chronosequence in New Zealand (the Waitutu chronosequence).

Methods

Soil and ecosystem properties were measured at 24 evenly distributed points within each of eight 1.5 ha plots located on young, intermediate and old surfaces. Regression analyses tested whether drainage, in addition to P, affected ecosystem functioning. A complementary fertilization experiment tested whether P was indeed limiting on the most nutrient-depleted sites.

Results

Most phosphorus depletion occurred in the early stages of pedogenesis (within 24,000 years), and the older surfaces were similar in soil-P contents, whereas drainage was initially good but became increasingly impeded with surface age. In the fertilizer experiment, species showed positive responses to both nitrogen (N) and P addition on the oldest surfaces, supporting Walker and Syer’s model. However, water table depth was also found to be strongly correlated with plant species composition, forest basal area, light transmission, and litter decomposition when comparisons were made across sites, emphasising that it too has strong influences on ecosystem processes.

Conclusions

Poor drainage influences the process of retrogressive succession along the Waitutu chronosequence. We discuss the implications of our work with regard to other chronosequences, suggesting that topography is likely to have strong influences on retrogressive processes.