Robert L. Patrick

Regulation of catecholamine synthesis in rat brain synaptosomes.

Abstract— Catecholamine synthesis in synaptosomal preparations of rat striatum, cortex and brain stem was investigated. The striatum had much higher activity than either the cortex or brain stem. Equilibration of labelled tyrosine between tissue and incubation medium was completed within 2 min. The apparent Km of tyrosine hydroxylase (EC 1.14.3a) and of the overall catecholamine synthetic pathway were both approximately 5 ± 10−6m for tyrosine. The following amines were found to inhibit striatal dopamine synthesis: dopamine, 25% inhibition at 5 ± 10−7m; noradrenaline, 25% inhibition at 5 ± 10−6m;and serotonin, 30% inhibition at 10−5m. The catecholamine‐induced inhibition of synthesis was antagonized by pre‐incubation with cocaine. Increasing the potassium concentration from 5 to 55 mm caused a release of amines into the medium which was accompanied by a 40% increase in dopamine synthesis, when synthesis was measured during the first 5 min of exposure to elevated potassium. These results indicate that synaptosomal catecholamine synthesis is inhibited by increases in intra‐synaptosomal amine levels, and that short‐term exposure to depolarizing concentrations of potassium can increase synthesis.

Phospholipid-induced activation of tyrosine hydroxylase from rat brain striatal synaptosomes☆

Abstract Lysolecithin and phosphatidylserine stimulate rat striatal tyrosine hydroxylase, partially purified from the crude synaptosomal fraction. The stimulatory effect is associated with a 3- to 4-fold decrease in K m for 6-methyl-tetrahydropterin or tetrahydrobiopterin without an alteration in the K m for l -tyrosine or in the V max . In addition, the K i for dopamine inhibition is increased approximately 3-fold. Centrifugation of the enzyme on linear sucrose gradients gave a sedimentation coefficient ( S 20, w ) of 8.6, either in the absence or presence of lysolecithin, indicating that no significant changes in the molecular weight are caused by the phospholipid. The enzyme obtained after high speed centrifugation of whole striatal tissue was activated by a combination of lysolecithin plus a cyclic AMP-ATP mixture to a greater extent than that obtained by either activating condition alone. The data presented suggest a potential regulatory function of phospholipids in the control of striatal neurotransmitter synthesis.

Potentiation by cocaine of the stimulus-induced increase in dopamine synthesis in rat brain striatal synaptosomes

Abstract Rat brain striatal synaptosomes respond to short-term exposure to depolarizing concentrations of veratridine with an increase in the rate of dopamine synthesis. This increase is potentiated by concentrations of cocaine (1–100 μM) or benztropine (10 μM) which significantly inhibit dopamine uptake. In addition, the subsequent inhibition of dopamine synthesis that occurs during prolonged exposure to veratridine is markedly antagonized by cocaine. At a higher concentration (1 mM), cocaine caused a 45% inhibition of synthesis, and completely blocked the veratridine-induced synthesis stimulation. These results suggest that following depolarization newly released dopamine may play a role in regulating its own synthesis rate by a process involving neuronal re-uptake and that this regulation can be significantly altered by agents, such as cocaine, which can inhibit dopamine re-uptake.