Caffeine and coffee tolerance

Drug Tolerance, Drug Addiction, and Drug Anticipation

Environmental cues associated with drugs often elicit withdrawal symptoms and relapse to drug use. Such cues also modulate drug tolerance. The contribution of drug-associated stimuli to withdrawal and tolerance is emphasized in a Pavlovian-conditioning analysis of drug administration. Conditional responses occur in the presence of cues that have been associated with the drug in the past, such as the setting in which the drug was taken. These conditional responses mediate the expression of tolerance and withdrawal symptoms. Recently, it has become apparent that internal predrug cues, as well as environmental cues, elicit pharmacological conditional responses that contribute to tolerance and withdrawal. Such internal cues include cognitive or proprioceptive cues incidental to self-administration, drug-onset cues that are experienced shortly after administration, and emotional cues. According to the conditioning analysis, addiction treatment should incorporate learning principles to extinguish the association between stimuli (environmental and internal) present at the time of drug administration and the effects of the addictive drug.

The Four-Loko Effect

There have been recent reports of mass hospitalizations for alcohol intoxication following consumption of fruit-flavored, caffeinated, alcoholic drinks-especially concerning one brand in particular: Four Loko. Caffeine was quickly determined to be the culprit. In accordance with a directive by the Food and Drug Administration, caffeine was removed from Four Loko and similar beverages. However, the evidence that caffeine played a prominent role in widespread displays of intoxication is far from clear. Rather, it is likely that Four Loko-type drinks are especially effective as intoxicants because they provide alcohol in an unusual context. It has been known for many years that drug tolerance partially results from an association between drug-paired stimuli and the drug effect. When these stimuli are altered, the drug-experienced individual does not display the expected tolerant response to the drug-rather, an enhanced (i.e., nontolerant) response is seen. Four Loko and similar beverages may be especially effective intoxicants because they provide a very novel flavor context for alcohol. A recent announcement by the manufacturer of Four Loko suggests (either by design or happenstance) appreciation of the contribution of alcohol-associated cues to alcohol tolerance.

The cardiovascular effects of methylxanthines

In the concentration range that is normally achieved in humans, e.g., after the drinking of coffee or in patients treated with theophylline, the cardiovascular effects of methylxanthines are primarily due to antagonism of adenosine A(1) and A(2) receptors. Inhibition of phosphodiesterases or mobilization of intracellular calcium requires much higher concentrations. In conscious humans, acute exposure to caffeine results in an increase in blood pressure by an increased total peripheral resistance, and a slight decrease in heart rate. This overall hemodynamic response is composed of direct effects of caffeine on vascular tone, on myocardial contractility and conduction, and on the sympathetic nervous system. Caffeine is the most widely consumed methylxanthine, mainly derived from coffee intake. Regular coffee consumption can affect various traditional cardiovascular risk factors, including a slight increase in blood pressure, an increase in plasma cholesterol and homocysteine levels, and a reduced incidence of type 2 diabetes mellitus. Although most prospective studies have not reported an association between coffee consumption and coronary heart disease, these findings do not exclude that the acute hemodynamic and neurohumoral effects of coffee consumption could have an adverse effect in selected patient groups who are more vulnerable for these effects, based on their genetic profile or medication use.

Learning and the wisdom of the body

According to Spence, the learning researcher's task is to explain the relationship between experimental variables and behavior changes occurring with practice. Spence eschewed biological speculation. In contrast, for a biologist, "explanation" consists of ascertaining how the observed behavior increases reproductive success. Fundamental to achieving reproductive success is survival to sexual maturity, and such survival depends on homeostatic mechanisms attenuating the effects of physiological disturbances that threaten existence. Drugs are one way of disrupting homeostatic functioning, and studies of drug effects indicate that homeostatic mechanisms are engaged not just by pharmacological perturbations, but also by stimuli that signal such perturbations. Similarly, we attenuate the effects of a variety of nonpharmacological stimuli by such anticipatory homeostatic adjustments. The learning researcher is a homeostasis researcher.