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Amphetamine mechanism: More of a rant than a review

BilZ0r

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I've reviewed the theories on the mechanism of amphetamine mediated release before. I just thought I'd point out a few more things.

A fact that amazed people when I gave a seminar on the theories at my department, was that all substrates of the monoamine transporter cause release, even the native monoamines.

Now I'm not really sure if I buy the PKC mediated reversal of transporter which I have outlined before, but I'm really quite confident that the mere act of uptake my the transporter gaurantes release. The substrate does not have to do anything more. Now whether the transporter cycle causes Ca2+ build up and this activates PKC which casues the transporter to reverse, or whether its because transporters act as dimers, which transport in opposing directions (which I don't buy, outlined here), or whether this is some transporter-SNARE/syntaxin 1a interaction (Quick et al., unpublished, as cited by 5-HT2), I don't know.

I don't like the first option (PKC) because PKC inhibitors infused into the Striatum (DAT rich) did not reduce amphetamine mediated Dopamine release (1).

I just don't trust the dimer option, but I haven't quite figured out why.

The third one doesn't fit, because I feel amphetamine mediated release should be the same throughout all of the transporters, and NET mediated release still happens when SNAREs are cleved by bottulinum toxin.

Another reason why I don't trust the third is because of this figure
amphcorrelation.gif
This shows that over about 15 mixed transporter substrates, over the three monoamine transporters, there is a consistant relationship between uptake inhibition (and hence affinity for the transporter, and presumabley transport as a substrate*) and between release... hence uptake correaltes with release, consistantly.


* There is this paper which seems to show that uptake affinity and actual transport are unrelated, that is SERT Ki does not correlate even slightly with uptake Vmax... but the same paper shows a nice correlation between SERT Ki and SERT mediated currents, now I though the currents were dependent on transport, so I don't really know
 
Are you saying that the normal substrates also cause a reversal of the transporter ala amphetmine? Is this just to a much lower extent?
 
, or whether this is some transporter-SNARE/syntaxin 1a interaction (Quick et al., unpublished, as cited by 5-HT2), I don't know.

I don't know whether this mechanism would apply to DAT, and it has not yet been shown definitively that it does apply to SERT (though what Quick told me was tantalizing…)
 
^ Well I think that the mechanism of release is going to be the same across all the transporters, because of data like that graph I made above

Are you saying that the normal substrates also cause a reversal of the transporter ala amphetmine?

They definately do, to the exact same extent as amphetamines (per their affinity for the transporter)
 
^^^ Wow. The more I learn about neuron physiology the more complicated it becomes (naturally). So do you feel that other drugs with affinity for the DAT, like cocaine or methylphenidate, are actually causing a reasonable amount of DA release?
 
Cocaine isn't a substrate for the transporter, it's just a blocker. So it doesn't cause release.

I can never remember whether methylphenidate is a blocker or a substrate though.
 
methylphenidate is a blocker. But wouldn't amphetamine be considered a blocker by your definition since it isn't transported by the DAT?
 
Do amphetamines also bind to postsynaptic dopamine receptors ? Their striking structural similarity leads to me to think this. Why does oral amphetamine last so much longer than oral cocaine?
 
The affinity at which (meth)amphetamine binds to classical receptors is very low in comparison to there transporter affinities. I think amphetamine binds alpha1 ~= 10µM[1], alpha2 at like ~1µM[2] and displaces dopamine at over 10µM [3], meanwhile its NET/DAT affinity is like ~10nM.

I'd geuss oral cocaine gets hit by CYPs, while amphetamine generally goes via kidneys...


1. U'PRICHARD, D.C., GREENBERG, D.A., and SNYDER, S.H.: Binding characteristics of a radiolabeled agonist and antagonist at central nervous system alpha noradrenergic receptors. Mol. Pharmacol. 13: 454-473 (1977).

2. BOYAJIAN, C.L., and LESLIE, F.M.: Pharmacological evidence for alpha-2 adrenoceptor heterogeneity: differential binding properties of [3H]rauwolscine and [3H]idazoxan in rat brain. J Pharmacol Exp Ther. 1987 Jun;241(3):1092-8.


3. BURT, D.R., CREESE, I., and SNYDER, S.H.: Properties of [3H]haloperidol and [3H]dopamine binding associated with dopamine receptors in calf brain membranes. Mol. Pharmacol. 12: 800-812 (1976).
 
Cocaine: Pharmacology, Effects, and Treatment of Abuse

ABSORTION AND METABOLISM

........... Enzymes called esterases play an important role in the metabolism of cocaine. In humans and other mammals, plasma and liver have high levels of esterase activity. Moderate activity is present in other organs including brain (Foldes 1978). Cholinesterases, also referred to as plasma cholinesterase, serum cholinesterase, pseudocholinesterase, nonspecific cholinesterase, play an important role in the metabolism of cocaine (Stewart et al. 1979; Inaba et al. 1978).

Individuals with low cholinesterase activity may have slower metabolism of cocaine and some of its metabolites (Jatlow et al. 1979). In vitro activity of these enzymes becomes an important consideration in cocaine assay procedures (and can make for considerable in vitro losses of cocaine unless inhibited by the addition of fluoride, physostigmine, or other esterase inhibitors) (Jatlow and Bailey 1975). Plasma cholinesterase activity can vary greatly between individuals and between species.

The genetics of plasma esterase inheritance are relatively well understood (Neitlich 1966; Foldes 1978). The percent inhibition of the esterase activity by dibucaine, known as the dibucaine number, is one common clinical measure used to screen for unusual sensitiity to the muscle relaxant succinylcholine. People with low dibucaine numbers seem to be not only slow metabolizers of succinylcholine but may also be slow metabolizers of cocaine, at least as judged by in vitro tests (Jatlow et al. 1979; Stewart et al. 1979). On the other hand, some individuals may have genetically determined increased cholinesterase activity and would be expected to metabolize cocaine more rapidly. A number of disorders including liver disease, the presence of carcinoma, and exposure to anticholinesterase drugs will lower cholinesterase activity (Foldes 1978). Some inconsistencies in the cocaine literature, particularly in issues of tolerance and dependence, may rsult from species variations in cholinesterase activity and resulting cocaine metabolism. Cholinesterase activity is relatively high in humans, horses, and certain species of monkeys (for example, chimpanzees) but not others (for example, macaques).

Cholinesterase activity is much lower in other mammals (for example, dogs, cats, sheep, and rats) and very low in cows. Fourfold differences in cholinesterase activity can occur in various strains of mice. Cholinesterase activity is much lower in the fetus, infants, and aged males, and decreases to a lesser degree during pregnancy.

The pharmacologic significance of such variations in determining cocaine metabolism or toxicity is not entirely clear. Most cocaine studies in animals, particularly behavioral studies, have rarely even measured cocaine or cocaine metabolite levels, let alone determined kinetics. However, there is enough evidence from in vitro assays to indicate that variations in cholinesterase activity in the range commonly encountered clinically can have significant effect on the in vitro metabolism of cocaine and cocaine metabolites. Since there is both hepatic and nonhepatic metabolism of cocaine, the functional impact of very low or much higher than normal cholinesterase activity might depend on route of administration -- for example, it might be more important when the cocaine is given intravenously or perhaps smoked than with oral, intraperitoneal, or subcutaneous admiistration Rate and dose could interact as well.
 
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