Nicotinic acetylcholine receptor of mammalian brain: naja toxin binding to subcellular fractions of rat brain

Snake postsynaptic neurotoxins: gene structure, phylogeny and applications in research and therapy

Snake venoms are complex mixtures of biologically active polypeptides that target a variety of vital physiological functions in mammals. alpha-Neurotoxins, toxins that cause paralysis by binding to the nicotinic receptors at the postsynaptic region of the neuromuscular junction have been widely studied in terms of their structure-function relationships as well as gene structure, organization and expression. In this review, we describe the structure of alpha-neurotoxin genes and discuss their evolutionary relationships. Almost all members of neurotoxins have been found to exhibit a common evolutionary origin. The importance of alpha-neurotoxins in therapy and research has also been discussed to highlight their potential applications especially in the area of drug discovery.

Differential effects of prenatal and postnatal ACTH or nicotine exposure on 5-HT high affinity uptake in the neonatal rat brain

These studies were designed to examine the differential effects of prenatal or postnatal administration of ACTH 1-39 and nicotine, on 5-HT high affinity uptake in brainstem and hippocampal synaptosomes. ACTH was administered prenatally (to pregnant dams) and postnatally to the neonates. Postnatal administration of ACTH significantly increased high-affinity 5-HT uptake in the hippocampus and especially the brainstem at both 7 and 21 days after birth. Prenatal ACTH, on the other hand, transiently increased 5-HT uptake in only the brainstem at 7 days, a change that was reversed at 21 days. While the effects of postnatal nicotine administration were essentially the same as those of postnatal ACTH treatment, prenatal nicotine, unlike ACTH, did not alter 5-HT uptake in 7-day-old rats but did reduce uptake in both tissues at 21 days. The observation that postnatal nicotine mimics the effects of postnatal ACTH and that nicotine stimulates ACTH release, suggests that the postnatal effects of nicotine may be exerted through ACTH.

Behavioral and neurochemical effects of intraperitoneally injected dendrotoxin

Intraperitoneal administration of dendrotoxin, a polypeptide isolated from Dendroaspis angusticeps venom, provoked in mice the appearance of a complex stereotyped behavior including biting, head nodding, 'wet-dog' shakes and rearing. Signs of autonomic hyperactivity as well as hyperreactivity to sound and touch were prominent. Neurochemical analyses of monoamines and monoamine metabolites showed no change 90 min after dendrotoxin, with a decrease in dopamine concentrations and an increase in their metabolites in the striatum starting 3 hr later. Moreover, at this time, dendrotoxin also produced a significant increase of 5-hydroxytryptamine metabolites. These data are interpreted as indicating that dendrotoxin crosses the blood-brain barrier and provokes an increase of the activity at monoaminergic terminals.

The effects of nicotine administration on 5-HT uptake and biosynthesis in rat brain

The effects of acute and chronic nicotine administration and its withdrawal on the rates of uptake and formation of 5-HT and the uptake of L-tryptophan, by synaptosomes prepared from discrete brain regions, have been studied in the rat. Both the acute and chronic (40 days) administration of nicotine (0.4 mg/kg s.c.) caused a decrease in the rate of formation of 5-HT by hippocampal synaptosomes (P less than 0.05) while 24 h withdrawal of the drug from chronically treated rats resulted in a partial recovery of this effect. Chronic nicotine treatment also reduced the rate of L-tryptophan uptake by hippocampal synaptosomes (P less than 0.01) an effect which appeared to be the result of a reduction in the number of L-tryptophan carrier molecules in the synaptosomal membrane. This effect, which was not reversed by withdrawal of the drug for 24 h, was not seen in synaptosomes prepared from other regions of the rat brain.

Nicotine binding sites and their localization in the central nervous system

The resolution of racemic nicotine to provide optically pure (+)-nicotine and the synthesis of radiolabeled nicotine with high specific activity have facilitated the study of nicotine binding in brain. The actions of the stereoisomers of nicotine on the central nervous system are qualitatively similar in most tests but (-)-nicotine is more potent than the unnatural (+)-isomer by 10-fold or greater. Binding of radiolabeled nicotine to brain has both saturable and nonsaturable components. Only saturable binding is affected by incubation conditions such as time, temperature, pH and ion concentration. Excess concentrations of the stereoisomers are equally effective in displacing (-)-[3H]-nicotine from brain homogenates. Nevertheless, a direct comparison of (+)-[3H]-nicotine and (-)-[3H]-nicotine binding shows that the latter has a KD three times lower than the former. (-)-[3H]-Nicotine is bound to the greatest degree in hypothalamus and hippocampus, areas that also exhibited the most stereoselectivity for nicotine. However, differences in the binding affinities of the two isomers were far less than the pharmacological stereospecificity observed.