THE RELEASE IN VIVO OF DOPAMINE SYNTHESIZED FROM LABELLED PRECURSORS IN THE CAUDATE NUCLEUS OF THE CAT

Abstract— To study the release of dopamine (DA) evoked in vivo from the caudate nucleus, a push-pull cannula was inserted into the head of the caudate nucleus of cats anaesthetised with pentobarbitone sodium (Nembutal), and the tissue in the vicinity of the cannula tip was continuously irrigated with either l -[¹⁴C]tyrosine or DL-[¹⁴C]3,4-dihydroxyphenylala-nine (DOPA). The contents of [¹⁴C]DA and of the [¹⁴C]acidic metabolites in the perfusates were determined after separation from the labelled precursors by column chromatography, TLC and solvent partition. During perfusion with radioactive tyrosine, only small quantities of [¹⁴C]DA appeared in the effluent while the concentrations of the [¹⁴C]acidic metabolites gradually increased during the course of the experiment. When [¹⁴C]DOPA was substituted for [¹⁴C]tyrosine, the proportion of precursor that was converted to DA and released into the effluent as the amine or as its acidic metabolites was increased ten-fold. In an attempt to increase the resting release of [¹⁴C]DA, D-amphetamine, tropolone or pheniprazine were individually added to the perfusion fluid. Each drug increased the content of [¹⁴C]DA in the perfusate, but the enhanced release was maintained only when pheniprazine was added during perfusion with [¹⁴C]DOPA. Stimulation of the rostral substantia nigra (A5-5) and the medial forebrain bundle caused, in a majority of experiments, a two-to five-fold increase in the concentration of labelled DA in the effluent. Stimulation of the substantia nigra at A4-0 did not enhance the release of [¹⁴C]DA from the caudate nucleus but did enhance the release from the putamen. Since the increase in the output of [¹⁴C]DA was independent of changes in the output of labelled acidic metabolites, the evoked release was apparently not attributable to changes in extracellular fluid dynamics.     

The monensin-mediated transport of sodium ions through phospholipid bilayers studied by 23Na-NMR spectroscopy

The monensin-mediated transport of sodium ions through the walls of large unilamellar vesicles of egg phosphatidylcholine was studied using 23Na-NMR and aqueous shift reagents. The transport is dynamic on the NMR time-scale and is strictly first order in monensin over the concentration ranges studied indicating that transport occurs by a 1:1 Na+-ionophore complex. Transport appears to be inhibited by increasing concentrations of Na+.     

Fat oxidation rate and the exercise intensity that elicits maximal fat oxidation decreases with pubertal status in young male subjects

The range of exercise intensities that elicit high fat oxidation rates (FOR) in youth and the influence of pubertal status on peak FOR are unknown. In a longitudinal design, we compared FOR over a range of exercise intensities in a small cohort of developing prepubertal male subjects. Five boys all at Tanner stage 1 (ages 11-12 yr) and nine men (ages 20-26 yr) underwent an incremental cycle ergometry test to volitional exhaustion. FOR curves were determined from indirect calorimetry during the final 30 s of each increment. The same protocol was duplicated annually in the boys as they progressed through puberty. The peak FOR was considerably higher (P<0.05) in boys at Tanner 1 (8.6+/-1.5 mg.kg lean body mass(-1).min(-1)) (mean+/-SD) compared with men (4.2+/-1.1 mg.kg lean body mass(-1).min(-1)). FOR dropped as boys developed through puberty (Tanner 2/3 peak rate=7.6+/-0.6 mg.kg lean body mass(-1).min(-1); Tanner 4 peak rate=5.4+/-1.8 mg.kg lean body mass(-1).min(-1), main effect of Tanner stage; P<0.05) to the levels found in men (not significant). The exercise intensity that elicited peak FOR was higher in the boys at Tanner 1 [56+/-6% peak aerobic power (VO2 peak)] than in men (31+/-4% VO2 peak) (P<0.001). This value tended to decrease by Tanner stage 4 (45+/-10% VO2 peak, main effect of Tanner stage; P=0.06). We conclude that, compared with men, prepubertal boys have higher relative FOR throughout a wide range of exercise intensities and that FOR drops as boys develop through puberty.