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Insulin stimulates in a dose-dependent manner (concentration range of 0.1 – 10μM) the synaptosomal uptake of amino acids characterized by high-affinity, Na+-dependent, veratridine-sensitive transport systems. This stimulation is observed in synaptosomes prepared from each of several regions of the adult rat brain. Both the initial rate of amino acid uptake and the overall capacity for amino acid accumulation are increased. Since these transport systems have been associated with the neurotransmitter role of the amino acids, we postulate that insulin can modulate neurotransmission in the rat central nervous system by increasing the efficiency of neuroactive amino acid reuptake.

The high affinity, sodium-dependent uptake of proline by rat brain synaptosomes was inhibited by the opioid pentapeptides, Leu-enkephalin and Met-enkephalin. The synaptosomal uptake of other putative neurotransmitter amino acids including glutamic acid, aspartic acid, gamma-aminobutyric acid, and taurine was not altered in the presence of enkephalins. The uptake of a neuroinactive amino acid, leucine, was also unaffected by enkephalins. The extent of proline uptake was half-maximal at a Leu-enkephalin concentration of 1 microM. Both the initial rate of transport and the overall capacity for proline accumulation were reduced. The effect of the enkephalins was vectorial since carrier-mediated efflux of proline was not altered in the presence of enkephalins. Morphine and the opioid peptides, dynorphin and beta-endorphin, were without effect on proline uptake. The inhibition of proline uptake by enkephalins was not diminished by prior incubation of the synaptosomal preparation with naloxone; however, the inhibition was attenuated by 1-butanol. The des-tyrosyl fragments of the enkephalins were as inhibitory as the intact pentapeptides. A modified enkephalin ([D-Ser2]Leu-enkephalin-Thr) with selective affinity for the delta subclass of enkephalin receptor was effective in inhibiting proline uptake. On the basis of the selectivity of these effects, we propose that there is a specific population of nerve endings in the cerebral cortex that contains both a proline-transport system and binding sites for Leu- and Met-enkephalin and furthermore, that these binding sites may be related to the putative delta receptor.

The major excretory metabolite of thyroxine in the insect Corcyra cephalonicaSt. is iodide, though monoiodotyrosine and di-iodotyrosine are present in small amounts. In vitroexperiments with homogenates of larval tissue have shown that an enzyme system capable of forming iodide from thyroxine is present only in tissues of larvae fed thyroxine-supplemented diets and not in those of larvae fed control diet alone. Activity of the enzyme is destroyed by boiling for 10-15 minutes.

The inclusion of arachidoside and anacardioside, the pigments isolated from the outer skin-covering of groundnuts and cashewnuts respectively, in the diet of rats, at a level of 20 mg per rat per day for 7 weeks results in goitre. Incorporation of potassium iodide in the diet counteracts partially the goitrogenic action of the anthocyanins. Results of studies on the distribution of radioactive iodine in the thyroid glands of the rats suggest that the pigments inhibit the organic binding of radioactive iodine.