Satendra Singh

Inherited, selective hypoanalgesic response to cytisine in the tail-flick test in CF-1 mice.

THIS study extends the pharmacological characterization of the genotype-dependent difference in analgesic responsiveness to neuronal nicotinic agonists between CD-1 and CF-1 strains of mice. Acute analgesic potency of cytisine measured by the tail-flick assay differed by > 3200-fold between CD-1 and CF-1 outbred strains of mice. Analgesic non-responsiveness of the CF-1 strain was pharmacologically selective. Morphine produced a dose-dependent analgesic response of similar magnitude in both strains. Other pharmacological actions of cytisine, including inhibition of locomotor activity, induction of seizures and lethality, did not differ between these strains. Hyporesponsiveness to the analgesic action of both nicotine and cytisine was observed in two different CF-1 sublines. Biodistribution of [3H]cytisine in blood did not differ between the CF-1 and CD-1 strains. These pharmacological characteristics indicate that the CD-1–CF-1 strain pair provides a useful pharmacogenetic tool for investigating the mechanistic bases of analgesia induced by nicotinic cholinergic agonists.

Synthesis and reactivity of the putative neurotoxin tryptamine-4,5-dione

Abstract In dilute aqueous solution at physiological pH tryptamine-4,5-dione ( 1 ; ≤200 μ m ) slowly reacts to give 3-(2-aminoethyl)-6-[3′-(2-aminoethyl)-indol-4′,5′-dione-7-yl]-5-hydroxyindole-4,7-dione ( 7 ) as the major product. Attempts to isolate pure solid samples of 1 by freeze-drying a chromatographically pure solution of the dione always results in the formation of significant (15–35%) amounts of 7,7′-bi-(5-hydroxytryptamine-4-one) ( 10 ). Similarly, relatively high concentrations of 1 (e.g., 20 m m ) in vehicles such as 0.5 m NH 4 Cl, pH 7.0, or isotonic saline containing ascorbic acid (1 mg/ml) at pH 3 rapidly transform into dimer 10 , which subsequently polymerizes. Dione 1 has been claimed to be a neurotoxin which when administered into the brains of laboratory animals causes nerve terminal damage similar to that observed in human Alzheimers disease (AD). However, the present study suggests that under the conditions employed to prepare and centrally adminster solutions of 1 it would be almost completely transformed to 10 . Procedures are given to prepare the solutions of dione 1 and to assess the purity of such solutions.

Influence of glutathione on the electrochemical and enzymatic oxidation of serotonin

Abstract Aberrant oxidative transformations of the indolic neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) might occur in serotonergic neurons in the brains of Alzheimers disease (AD) patients. In the event that such reactions occur in serotonergic nerve terminals or axons, they would do so in the presence of glutathione (GSH). In this investigation the electrochemically driven and tyrosinase + O 2 -mediated oxidation of 5-HT in the presence of GSH has been studied. In the absence of GSH, 5-HT is oxidized to a C(4)-centered carbocation intermediate ( 9a ) which reacts either with 5-HT, to give 5,5′-dihydroxy-4,4′-bitryptamine ( 2 ) as the major product along with a number of other carbon-carbon and ether-linked dimers and oligomers, or with water to give tryptamine-4,5-dione ( 11 ). In the presence of GSH this reaction is partially diverted as a consequence of nucleophilic addition of the tripeptide to 9a to give 4- S -glutathionyl-5-hydroxytryptamine ( 1 ). Other products formed include isomers of the 6- S -glutathionyl conjugates of dimer 2 , 7- S -glutathionyltryptamine-4,5-dione ( 5 ) and glutathionyl conjugates of an indolic trimer and of two tetramers. Compound 1 might be a useful analytical marker molecule for unusual oxidation reactions of 5-HT in the Alzheimer brain.

Interactions between 5,6-dihydroxytryptamine and cysteine

Abstract Autoxidation and enzyme-mediated oxidations of the serotonergic neurotoxin 5,6-dihydroxytryptamine (5,6-DHT) generate the corresponding ortho -quinone ( 1 ) as the initial reactive intermediate. Autoxidation of 5,6-DHT in the presence of an excess of the cellular reductant/nucleophile cysteine results in a reaction where, initially, quinone 1 oxidizes cysteine to cystine with concomitant formation of 5,6-DHT. When the concentration of cysteine is sufficiently reduced 1 attacks, 5,6-DHT to give 2,7′-bi(5,6-dihydroxytryptamine) and, ultimately, an insoluble indolic melanin. When the oxidation of 5,6-DHT is catalyzed by tyrosinase, o -quinone 1 is attacked by cysteine to give 4- S -cysteinyl-5,6-dihydroxytryptamine ( A ). Methods have been devised to isolate A and evidence for its structure is presented. Conjugate A should be useful as a marker of enzyme-catalyzed oxidation of 5,6-DHT in the central nervous system.

2′-Substitution of cocaine selectively enhances dopamine and norepinephrine transporter binding

FEW studies have characterized the effect of substituents at the 2′-position of cocaine on transporter binding potency and selectivity. We synthesized 2′-OH-, 2′-Fand 2′-acetoxy-cocaines and compared their binding potencies for rat dopamine, norepinephrine and 5hydroxytryptamine transporters to cocaine, 3′-OH-, 4′-OH-, 2′-OH,4′-I-cocaine derivatives, and to the transporter selective ligands WIN 35,428, nisoxetine and paroxetine. Unlike most substitutions, 2′-OHand 2′-acetoxy-groups increased cocaines binding potency for the dopamine transporter (10and 4-fold, respectively). These substituents also enhanced binding to the norepinephrine transporter (52and 35-fold, respectively) but had less effect on 5-hydroxytryptamine transporter binding. 2′-Hydroxylation also enhanced binding of 4′-I cocaine, an analog with low DA binding potency. The ability of 2′-substituents to substantially increase cocaine binding potency and to alter selectivity for brain transporters indicates the potential importance of the 2′-position in transporter binding.

A CONVENIENT SYNTHESIS OF 2'- OR 4'-HYDROXYCOCAINE

Abstract: A short, convenient and efficient synthesis of 2′- or 4′-hydroxycocaine is described. The key step involved selective hydrolysis/transesterification of the acetoxy group of 2′- or 4′-acetoxycocaine in methanol saturated with dry HCl gas.