New views on primate origins

Most primates live in trees, and many of them have strikingly human-like hands and faces. Scientists who study primate evolution agree that these two facts must be connected in some way. The details, however, are a matter of debate. Early theories explained the human-like peculiarities of primates simply as arboreal adaptations. More recent accounts have traced the origins of these peculiarities to more specific ways of arboreal life, involving leaping locomotion, shrub-layer foraging, visually guided predation on insects, or fruit-eating.     

Neurochemical and behavioral correlates of antidepressant drug action

Chronic (4 days or more) administration of imipramine or mianserin, but not atropine, leads to an extinction in muricidal behavior in the rat. Moreover, receptor binding assays revealed that there is a significant decline in the number of beta-adrenergic, but not serotonin2, receptors in the frontal cortex at the onset of the behavioral modification. While the antidepressants also induced receptor binding changes in nonmuricidal control animals, the pattern of these changes differed from that observed in the muricidal subjects suggesting that the receptor modification was, to some extent, trait-dependent. These findings indicate that, with the muricidal model, chronic rather than acute drug treatment may be a more selective test for antidepressant efficacy. In addition, the data suggest that a decline in brain beta-adrenergic receptors may be causily related to the behavioral modification.     

New views of S-RNase-based self-incompatibility

S-RNase-based self-incompatibility (SI) is the most widespread form of genetically controlled mate selection in plants. S-RNase controls pollination specificity in the pistil, while the newly discovered SLF/SFB controls pollination specificity in the pollen. A widely discussed model suggests that compatibility is explained by ubiquitylation and degradation of nonself-S-RNase and that, conversely, incompatibility is caused by failure to degrade self-S-RNase. This model is consistent with the long-standing view that S-RNase inhibition is central to SI. Recent results show, however, that S-RNase is compartmentalized in pollen tubes and, significantly, that compatibility might not require SLF/SFB. S-RNase compartmentalization and dislocation into the pollen tube cytoplasm might be similar to the trafficking of other cytotoxins such as ricin.     

G protein signaling and the molecular basis of antidepressant action

Over the past four decades, a variety of interventions have been used for the treatment of clinical depression and other affective disorders. Several distinct pharmacological compounds show therapeutic efficacy. There are three major classes of antidepressant drugs: monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), and tricyclic compounds. There are also a variety of atypical antidepressant drugs, which defy ready classification. Finally, there is electroconvulsive therapy, ECT. All require chronic (2-3 weeks) treatment to achieve a clinical response. To date, no truly inclusive hypothesis concerning a mechanism of action for these diverse therapies has been formed. This review is intended to give an overview of research concerning G protein signaling and the molecular basis of antidepressant action. In it, the authors attempt to discuss progress that has been made in this arena as well as the possibility that some point (or points) along a G protein signaling cascade represent a molecular target for antidepressant therapy that might lead toward a unifying hypothesis for depression. This review is not designed to address the clinical studies. Furthermore, as it is a relatively short paper, citations to the literature are necessarily selective. The authors apologize in advance to authors whose work we have failed to cite.     

Cross-bridge action: present views, prospects, and unknowns

When the sliding filament hypothesis was proposed in 1953-1954, existing evidence showed that (1) contributions to tension were given by active sites uniformly distributed within each zone of filament overlap and (2) each site functioned cyclically. These sites were identified by electron microscopy as cross-bridges between the two filaments, formed of the heads of myosin molecules projecting from a thick filament and attaching to a thin filament. The angle of these cross-bridges was found to be different at rest and in rigor, suggesting that the event causing relative motion of the filaments was a change of the angle of the cross-bridges. At first, it seemed likely that the whole cross-bridge rotated about its attachment to actin, but when the atomic structures of actin and myosin were obtained by X-ray crystallography, a possible hinge was found between the "catalytic domain" which attaches to the actin filament and the "light-chain domain" which appears to act as a lever arm. Two attitudes of the lever arm are now well established, the transition between them being driven by a conformational change coupled to some step in the hydrolysis of ATP, but several recent observations suggest that this is not the whole story: a third attitude has been shown by X-ray crystallography; a non-muscle myosin has been shown to produce its working stroke in two steps; and there are suggestions that an additional displacement of the filaments is produced by a change in the attitude of the catalytic domain on the thin filament.