The neurotrophic factors brain-derived neurotrophic factor and neurotrophin-3 are ligands for the trkB tyrosine kinase receptor.

Neurotrophic factors are essential for neuronal survival and function. Recent data have demonstrated that the product of the tyrosine kinase trk proto-oncogene binds NGF and is a component of the high affinity NGF receptor. Analysis of the trkB gene product, gp145trkB, in NIH 3T3 cells indicates that this tyrosine kinase receptor is rapidly phosphorylated on tyrosine residues upon exposure to the NGF-related neurotrophic factors BDNF and NT-3. Furthermore, gp145trkB specifically binds BDNF and NT-3 in NIH 3T3 cells and in hippocampal cells, but does not bind NGF. Thus, the trk family of receptors are likely to be important signal transducers of NGF-related trophic signals in the formation and maintenance of neuronal circuits.     

Retention of ingested latex particles in Peyer's patches of germfree and conventional mice

Conventional and germfree mice ingested a suspension of 2-micron latex particles in drinking water for a 15-day period. Number and distribution of intestinal Peyer's patches did not differ significantly in the two types of mice. Cleared Peyer's patches were compared with regard to size and particle content. The location of particles within Peyer's patch follicles of germfree mice was similar to that of conventional mice, but the latter had significantly larger follicles and greater accumulations of latex particles. Latex concentration varied with patch location. Proximal patches contained the majority of particles in germfree mice, whereas particles were most abundant in distal patches of conventional mice. The results show that particle uptake into Peyer's patches takes place even in the complete absence of bacteria in the gut.     

Diaphragmatic plate electrode stimulation of the hamster diaphragm

We developed a new technique of diaphragmatic stimulation by apposing plate electrodes directly against the diaphragm (DPS) in adult Golden Syrian hamsters. The electrophysiological and the mechanical responses to DPS were compared with those with phrenic nerve stimulation. In four animals, evaluation of the electromyogram before and after curare demonstrated that plate electrode stimulation occurred via the phrenic nerve filaments. In four animals, similar transdiaphragmatic pressure was produced at maximal current with DPS and phrenic nerve stimulation. Using DPS increasing current beyond a certain level resulted in recruitment of muscles besides the diaphragm. In six animals, an external abdominal pressure of 15 cmH2O produced maximal transdiaphragmatic pressure, suggesting that the diaphragm was contracting near optimal position with this external abdominal pressure. In another four animals the twitch and pressure-frequency characteristics with the use of DPS were found to be reproducible over a 2-h period. We conclude that DPS is an effective method of diaphragmatic stimulation and should prove to be a valuable technique to study the diaphragm in long-term studies of small rodents.     

Seed development in Pisum

The Ih and lh _i alleles have been shown previously to reduce the level of endogenous gibberellin A₁ (GA₁) in shoots of pea (Pisum sativum L.), resulting in a dwarf phenotype compared with the wild type, cv. Torsdag (Lh). In addition, plants homozygous for the lh ⁱ allele have reduced seed yield compared with Lh (tall, wild type) and lh (dwarf) plants. In this paper we show that the lh ⁱ mutation is expressed in developing seeds and pods. Comparison of GA levels in young shoots and developing seeds of genotypes lh and lh ⁱ demonstrates that the relative severity of the two mutations varies in different tissues. Homozygous h ⁱ seeds have reduced GA levels, weigh less, and are less likely to develop to maturity when compared with Lh seeds. However, fertilization of lh ⁱ plants with Lh pollen increases seed GA levels, seed weight and seed survival, indicating that an increase in seed GA levels due to the presence of the Lh allele can restore normal seed growth. Pods developing on self-pollinated lh ⁱ plants are shorter than pods on Lh (wild type) plants, although this may be an indirect effect of the increased seed abortion of lh _i plants. Based on these results we suggest that endogenous GAs play an important role in the development of seeds of P. sativum L.Abbreviations GA_(n) gibberellin A_n We wish to thank Katherine McPherson, Peter Newman, Leigh Johnson and Peter Bobbi for technical assistance, Professor L. Mander (ANU, Canberra) and Professor B.O. Phinney (UCLA, USA) for labelled GA standards, and the Australian Research Council for financial support.     

HYBRIDIZATION AS A DISPERSAL MECHANISM

An example from the genus Eucalyptus is used to argue that hybridization may be of evolutionary significance as a means of gene dispersal where seed dispersal is limited. A previous study of regeneration of E. risdonii and E. amygdalina indicated that the current selective regime was favoring E. risaonii. However, the dispersal of E. risdonii by seeds is shown to be limited (s, = 4.6 m). By comparison, the flow of E. risdonii genes into the range of E. amygdalina by pollen dispersal and F₁ hybridization is widespread (s_p = 82 m). While the actual level of hybridization is low, interspecific hybridization effectively doubles the dispersal of E. risdonii genes into the range of E. amygdalina. This pollen flow can have a significant genetic impact, since isolated hybrids or patches of abnormal phenotypes have been found 200–300 m from the species boundary. Based on lignotuber size, some of these patches appear to have been founded by F₁ hybrids. The frequency of E. risdonii types in the patches appears to increase with patch size suggesting that there is selection for this phenotype in subsequent generations. E. risdonii-like individuals were recovered in the progeny from both intermediate and E. risdonii backcross phenotypes. These results suggest that E. risdonii may invade suitable habitat islands within the E. amygdalina forest, independently of seed migration, by long-distance pollen migration followed by selection for the gene combinations of the pollen parent.