Reinforcement and learning

Evidence has been accumulating to support the process of reinforcement as a potential mechanism in speciation. In many species, mate choice decisions are influenced by cultural factors, including learned mating preferences (sexual imprinting) or learned mate attraction signals (e.g., bird song). It has been postulated that learning can have a strong impact on the likelihood of speciation and perhaps on the process of reinforcement, but no models have explicitly considered learning in a reinforcement context. We review the evidence that suggests that learning may be involved in speciation and reinforcement, and present a model of reinforcement via learned preferences. We show that not only can reinforcement occur when preferences are learned by imprinting, but that such preferences can maintain species differences easily in comparison with both autosomal and sex-linked genetically inherited preferences. We highlight the need for more explicit study of the connection between the behavioral process of learning and the evolutionary process of reinforcement in natural systems.     

Characterization of immunomodulatory properties and accessory cell function of small intestinal epithelial cells.

We have studied the immunomodulatory properties of epithelial cells from the small intestine on T cell immune function in vitro. Proliferation of lymph node cells stimulated either with antigen or with mitogen was inhibited by epithelial cells in a dose-dependent fashion. The epithelial cell-mediated suppression of lymphocyte proliferation was blocked by indomethacin, a cyclooxygenase pathway inhibitor, demonstrating that the suppressive effect of epithelial cells was related to prostaglandin secretion. Furthermore, the action of epithelial cell-secreted prostaglandin on lymphocytes was related to its effect on IL-2 as the suppressive effect of epithelial cells was abrogated by the addition of exogenous IL-2. As previously reported, epithelial cells constitutively express MHC class II and we found them able to present antigen in a class II-restricted fashion when their suppressive effects were blocked by indomethacin. Furthermore, epithelial cells activated by LPS secrete an IL-1 like molecule in a fashion analogous to other antigen-presenting cells. These results demonstrate that epithelial cells can both enhance and suppress in vitro T cell immune responses and further characterize the mechanisms by which intestinal epithelial cells may function in gut-associated immune responses.     

Characteristics of the range of mutations in nystatin resistance induced in yeasts by 6-N-hydroxylaminopurine

A simple procedure for induction of nistatin-resistant mutants in yeasts under the action of 6-N-hydroxylaminopurine (HAP) is proposed. Some differences between the spectra of mutations induced by HAP and UV-light are observed: HAP induces dominant and double recessive mutants more frequently.     

Measurement of positional isotope exchange rates in enzyme-catalyzed reactions by fast atom bombardment mass spectrometry: application to argininosuccinate synthetase

Fast atom bombardment mass spectrometry (FAB-MS) has been used to measure positional isotope exchange rates in enzyme-catalyzed reactions. The technique has been applied to the reactions catalyzed by acetyl-CoA synthetase and argininosuccinate synthetase. The FAB technique is also able to quantitatively determine the oxygen-18 or oxygen-17 content of nucleotides on as little as 10 nmol of material with no prior derivatization. Acetyl-CoA synthetase has been shown by FAB-MS to catalyze the positional exchange of an oxygen-18 of ATP from the beta-nonbridge position to the alpha beta-bridge position in the presence of acetate. These results are consistent with acetyl adenylate as a reactive intermediate in this reaction. Argininosuccinate synthetase was shown not to catalyze a positional isotope exchange reaction designed to test for the formation of citrulline adenylate as a reactive intermediate. Argininosuccinate synthetase was also found not to catalyze the transfer of oxygen-18 from [ureido-18O]citrulline to the alpha-phosphorus of ATP in the absence of added aspartate. This experiment was designed to test for the transient formation of carbodiimide as a reactive intermediate. These results suggest that either argininosuccinate synthetase does not catalyze the formation of citrulline adenylate or the enzyme is able to completely suppress the rotation of the phosphoryl groups of PPi.