[3H]-tetracaine binding on rat synaptosomes and sodium channels

Targeting voltage-gated sodium channels for pain therapy

Drugs inhibiting voltage-gated sodium channels have long been used as analgesics, beginning with the use of local anaesthetics for sensory blockade and then with the discovery that Nav-blocking anticonvulsants also have benefit for pain therapy. These drugs were discovered without knowledge of their molecular target, using traditional pharmacological methods, and their clinical utility is limited by relatively narrow therapeutic windows. Until recently, attempts to develop improved inhibitors using modern molecular-targeted screening approaches have met with limited success. However, in the last few years there has been renewed activity following the discovery of human Nav1.7 mutations that cause striking insensitivity to pain. Together with recent advances in the technologies required to prosecute ion channels as drug targets, this has led to significant progress being made. This article reviews these developments and summarises current findings with these emerging new Nav inhibitors, highlighting some of the unanswered questions and the challenges that remain before they can be developed for clinical use.

Effects of amiloride and its analogues on [3H]batrachotoxinin-A 20-alpha benzoate binding, [3H]tetracaine binding and 22Na influx

The ability of amiloride and its analogues to inhibit [3H]batrachotoxinin-A 20-alpha benzoate [( 3H]BTX-B) and [3H]tetracaine binding to rat synaptosomes and to a rat heart membrane preparation was tested. Their ability to inhibit 22Na influx was determined with rat synaptosomes. 5-N-substituted analogues were generally more potent in inhibiting [3H]BTX-B and [3H]tetracaine binding than compounds substituted on the guanidine group. However, the inhibition was not competitive. Amiloride and some of its analogues were as active or more active in inhibiting [3H]tetracaine binding than they were in inhibiting [3H]BTX-B binding. 22Na influx was inhibited with the same relative potencies as [3H]BTX-B binding and a good correlation was found between the two inhibitions. These results show an effect of amiloride and its analogues on the voltage-sensitive Na+ channels, which could partly explain the inotropic effects of these drugs.

Binding sites for [3H]tetracaine in synaptosomal sodium channel preparations from mouse brain

The present study was an attempt to answer the question whether the local anesthetic [3H]tetracaine labels sodium channels in mouse brain synaptosomes. Binding of [3H]tetracaine had a Kd of 0.19 microM and a Bmax ranging from 3.7 to 5.2 pmol/mg of protein. Local anesthetics other than tetracaine and cocaine-related compounds inhibited [3H]tetracaine binding with Hill numbers between 0.3 and 0.6. Initiation of the dissociation of [3H]tetracaine binding by an excess of unlabeled tetracaine resulted in non-linear curves. These results are consonant with site heterogeneity, negative cooperativity, or complexities arising from the use of synaptosomal preparations instead of broken membranes. There were important differences between the absolute values of the potency of various local anesthetic drugs and cocaine-related compounds in inhibiting binding of [3H]tetracaine and those in inhibiting binding of [3H]batrachotoxinin A 20-alpha-benzoate. Although there were some effects of manipulating the state of the channel with activating toxins on the apparent rate of association and dissociation of [3H]tetracaine binding, the equilibrium binding was not much affected by the toxins. The results indicate that further characterization is necessary before accepting [3H]tetracaine binding as a valid tool for studying sodium channels.

Effects of some antianginal and vasodilating drugs on sodium influx and on the binding of 3H-batrachotoxinin-A 20-alpha-benzoate and 3H-tetracaine

The effects of antianginal drugs, especially arylalkylamines and structurally related derivatives, on 3H-batrachotoxinin-A 20-alpha-benzoate (3H-BTX-B) binding and on 3H-tetracaine binding were studied on rat synaptosomal and heart membrane preparations. The effect of the same drugs on the Na+ influx induced by protoveratrine B was studied on the rat synaptosomal preparation. Antianginal drugs tested inhibited 3H-BTX-B binding in rat synaptosomes, arylalkylamine derivatives being the most potent: IC50 values were 27 nM for flunarizine, 32 nM for prenylamine, 79 nM for cinnarizine. Similarly, these drugs were the most potent when tested in cardiac membrane preparations. All the drugs tested were very weak inhibitors of 3H-tetracaine binding (IC50 ranging from 0.01 mM to more than 1 mM) except for guanabenz, which was more potent (IC50:0.3 microM on the synaptosomal preparation). The various drugs tested inhibited the 22Na+ influx induced by protoveratrine B, with IC50 values ranging from 15 microM (prenylamine) to 110 microM (verapamil), with the exception of nifedipine which had an IC50 of more than 0.1 mM. The inhibition of 22Na+ influx correlated well with the inhibition of 3H-BTX-B binding. These findings suggest that some antianginal drugs, especially the arylalkylamines may have, in addition to their calcium antagonist activity, direct effects on sodium channels.

Anti-anginal arylalkylamines and sodium channels: [3H]-batrachotoxinin-A 20-alpha-benzoate and [3H]-tetracaine binding

[3H]-batrachotoxinin-A 20-alpha-benzoate ([3H]-BTX-B) and [3H]-tetracaine are useful ligands for the study of sodium channels. Inhibition of their binding by various anti-anginal drugs was tested on a rat synaptosomal preparation and on a heart membrane preparation. Diphenylalkylamines and structurally related drugs inhibited [3H]-BTX-B binding in both the synaptosomal preparation and heart membrane preparation. They were almost inactive on [3H]-tetracaine binding. These results suggest that activity of arylalkylamines could be mediated by an interaction on the sodium channel.