Extensive neuroanatomical, neurophysiological, and behavioral evidence demonstrates that GABAergic neurons inhibit endogenous dopamine release in the mammalian corpus striatum. Positron emission tomography (PET) studies in adult female baboons, using the dopamine D2-specific radiotracer 11C-raclopride, were undertaken to assess the utility of this imaging technique for measuring these dynamic interactions in vivo. 11C-raclopride binding was imaged prior to and following the administration of either gamma-vinyl-GABA (GVG), a specific suicide inhibitor of the GABA-catabolizing enzyme GABA transaminase, or lorazepam, a clinically prescribed benzodiazepine agonist. Striatal 11C-raclopride binding increased following both GVG and lorazepam administration. This increase exceeded the test/retest variability of 11C-raclopride binding observed in the same animals. These findings confirm that changes in endogenous dopamine concentrations resulting from drug-induced potentiation of GABAergic transmission can be measured with PET and 11C-raclopride. Finally, this new strategy for noninvasively evaluating the functional integrity of neurophysiologically linked transmitter systems with PET supports its use as an approach for assessing the multiple mechanisms of drug action and their consequences in the human brain.

Aging of the human brain is associated with a decline in dopamine (DA) function, generally interpreted as reflecting DA cell loss. Positron emission tomography studies revealed that in healthy individuals, the age-related losses in DA transporters (presynaptic marker) were associated with losses in D2 receptors (postsynaptic marker) rather than with increases as is known to occur with DA cell loss. This association was specific for DA synaptic markers, because they were not correlated with striatal metabolism. Furthermore, the association was independent of age, suggesting that a common mechanism regulates the expression of receptors and transporters irrespective of age.

SUMMARYA comparison of eight different nutrient solutions showed large differences in ability to support growth of maize, ryegrass and white clover in silica sand. Plants grown with the Ruakura solution, a solution especially designed for pasture species, consistently outyielded those grown with the other nutrient solutions. Insufficient iron was a major cause of poor growth with many of the solutions, particularly for maize. However, where deficiencies of iron were corrected, growth differences, although smaller, still persisted among the nutrient solutions. Results of tissue analysis and measurements of the solution surrounding the roots showed a considerable number of individual factors which were likely to be yield limiting.

From 1992 to 1994, 24 soybean cultivars, ranging in maturity from early group III to late group IV, were assessed in the field for resistance and susceptibility to Macrophomina phaseolina. Cultivars were placed into four relative maturity classes-3.2 to 3.4; 3.5 to 3.8; 3.9 to 4.0; and 4.3 to 4.9—with six cultivars per class. Twelve cultivars were resistant and 12 were susceptible to Heterodera glycines, the soybean cyst nematode. Cultivars were planted on two dates (early May and early June) to assess the effect of environment on yield and lower stem and taproot colonization by the fungus at growth stage R7. Based on seed yields and the levels of lower stem and taproot colonization by M. phaseolina, four cultivars—Asgrow 4715, DeltaPineland 3478, Hamilton, and Jackson II—were rated moderately resistant to M. phaseolina. Seed yields and M. phaseolina levels in host tissues were affected by planting date, relative maturity class, H. glycines response, and the environmental conditions that prevailed over the 3 years of the study. However, in general, the environmental, agronomic, and host genetic factors present in these experiments did not affect the individual cultivar/pathogen interaction. Quantification of microsclerotial propagules in lower stem and taproot tissues at growth stage R7 was a reliable, but time-consuming, method to measure the degree of host compatibility between soybean cultivars and M. phaseolina.

SUMMARYCritical leaf concentrations for deficiency of nitrogen, potassium, phosphorus, sulphur, and magnesium were estimated for perennial ryegrass {Lolium perenne L.) grown in sand culture. The values associated with a 10% reduction in dry matter yield were as follows (all results expressed as g kg~^ DM except for the nitrate-nitrogen value which appears as /ig g~^ DM): Kjeldahl-nitrogen 32; nitrate-nitrogen 500; potassium 28; phosphorus 21; sulphur 18; and magnesium 0-7. A major difference between the critical leaf concentrations estimated in this study and the tentative values published elsewhere was for potassium. Concentrations required in the shoot for near maximum growth were higher than previously reported. Differences in the nitrogen status of the plants probably account for this result. The depressing effect of potassium on the absorption of magnesium and calcium is discussed in relation to plant and animal nutrition. Sulphur was found to be inefficiently absorbed by phosphorus deficient plants despite non-limiting amounts of sulphur applied in the nutrient solution. A possible explanation for this effect may be linked to the observation that the transport of sulphate into the chloroplast was coupled with that of phosphate. The concentration of macroelements required in the nutrient solution to produce maximum shoot growth of perennial ryegrass was very much greater than that generally applied to plants grown in sand culture.