Cocaine drug tolerance

In neurochemical terms, amphetamine and cocaine boost monoamine activity. Amphetamine has a threefold mode of action first, it causes dopamine and noradrenaline to leak into the synaptic cleft second, it boosts the amount of transmitter released during an action potential and third, it inhibits the reuptake of neurotransmitter back into presynaptic vesicles. These three modes all result in more neurotransmitter being available at the synapse, thus generating an increase in postsynaptic stimulation. Cocaine exerts a similar overall effect, but mainly by reuptake inhibition. The main neurotransmitters affected are dopamine and noradrenaline, although serotonin is boosted to a lesser extent. These modes of action are outlined in Chapter 3, and the neurochemical rationale for drug tolerance is covered more fully in Chapter 10. The main differences between amphetamine and cocaine are their administration routes (summarised above) and the more rapid onset and shorter duration of action for cocaine. 

Dependence and withdrawal can occur with all of the stimulants. Cocaine is one of the most strongly reinforcing drugs in self-administration paradigms in animals and also has a psychological withdrawal syndrome. A typical pattern of withdrawal includes a ravenous appetite, exhaustion, and mental depression, which may last for several days after the drug is withdrawn. Because tolerance develops quickly, abusers may take large doses, compared with those used medically, for example, as anorexiants. 

A formal diagnosis of substance dependence requires a maladaptive pattern of abuse that leads to clinically significant impairment or distress. More detailed criteria revolve around the development of tolerance, the experience of withdrawal when abstinence is required, the inability to stop using the drug, and continued use over a protracted period of time. The question is whether or not these criteria, clearly applicable to cocaine, heroin, and other drugs, are met by caffeine. 

Other knockout models that could be used to validate candidate genes include mice that lack monoamine oxidase A (MAO-A), which have demonstrated altered behavior and alcohol tolerance [54]. Transgenic mice in which the dopamine transporter gene has been deleted show striking hyperactivity via enhanced persistence of dopamine which is not altered by cocaine or amphetamine administration [55]. Knockouts of the serotonin IB receptor are also available and are best used as models of vulnerability to drug abuse [56]. 

Upregulation of the cAMP pathway may be a common mechanism by which a number of neuronal cell types respond to chronic opiates and develop tolerance and dependence (see Ch. 56). There is also evidence that similar mechanisms involving alterations in the cAMP second-messenger and protein phosphorylation pathway may mediate aspects of addiction to other types of drugs of abuse, for example, cocaine and alcohol [66],... 

LSD produces a rapid and complete tolerance, but it is not powerfully reinforcing the way drugs of abuse like cocaine and heroin are. LSD does not produce any known withdrawal syndrome (Abraham et al. 1996). It does not produce positive reinforcement in animal self-administration models. Concurrently in humans, it does not lead to patterns of repeti-... 

Cronan T, Conrad J, et al (1985) Effects of chronically administered nicotine and sahne on motor activity in rats. Pharmacol Biochem Behav 22(5) 897-899 Curtis L, Buisson B, et al (2002) Potentiation of human alpha4beta2 neuronal nicotinic acetylcholine receptor by estradiol. Mol Pharmacol 61(1) 127-135 Dalton JC, Vickers GJ, et al (1986) Increased self-administration of cocaine following haloperidol sex-dependent effects of the antiestrogen tamoxifen. Pharmacol Biochem Behav 25(3) 497-501 Damsma G, Day J, et al (1989) Lack of tolerance to nicotine-induced dopamine release in the nucleus accumbens. Eur J Pharmacol 168(3) 363-368 Di Chiara G, Imperato A (1988) Drugs abused by humans preferentially increase synaptic dopamine concentrations in the mesolimbic system of freely moving rats. Proc Natl Acad Sci USA 85(14) 5274-5278... 

In contrast, some drugs of abnse prodnce intense craving and are highly addictive bnt do not prodnce physical dependence. The absence of physical dependence indicates the relative lack of physiological withdrawal. This is not synonymous with meaning that discontinuation of these componnds may not be psychologically nncomfortable. Two examples are marijnana and cocaine. One need only look to the recent crack epidemic to see evidence of the way these substances can destroy lives, bnt they do not produce tolerance or risk of withdrawal to the same extent as alcohol or heroin. As a result, we would say that the daily crack or marijuana user meets the dehnition of substance dependence bnt does not exhibit true physical (or physiological) dependence. 

It is beheved that phenomena such as sensitization, tolerance and drug-dependence might also involve synaptic plasticity. In fact, numerous studies indicate that NMDA receptor antagonists block sensitization to amphetamine and cocaine as well as tolerance and dependence to ethanol and opioids in animal models (Trujillo and Akil 1991 Pasternak and Inturrisi 1995 Trujillo and Akil 1995 Mao 1999). Recent studies indicate that the uncompetitive NMDA receptor antagonists dextromethorphan, memantine and neramexane not only prevent the development of morphine tolerance, but also reverse estabhshed tolerance in the continuing presence of this opioid, prevent the expression of withdrawal symptoms in rats (Popik and Skolnick 1996 Popik and Danysz 1997 Popik and Kozela 1999 Houghton et al. 2001) and attenuate the expres-... 

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