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Individual and species survival demand that organisms find and obtain needed resources (e.g., food and shelter) and opportunities for mating despite costs and risks. Such survival-relevant natural goals act as “rewards,” i.e., they are pursued with the anticipation that their consumption (or consummation) will produce desired outcomes (i.e., will “make things better”). Behaviors with rewarding goals tend to persist strongly to a conclusion and increase over time (i.e., they are positively reinforcing) (21). Internal motivational states, such as hunger, thirst, and sexual arousal, increase the incentive value of goal-related cues and of the goal objects themselves and also increase the pleasure of consumption (e.g., food tastes better when one is hungry) (22). External cues related to rewards (incentive stimuli), such as the sight or odor of food or the odor of an estrous female, can initiate or strengthen motivational states, increasing the likelihood that complex and often difficult behavioral sequences, such as foraging or hunting for food, will be brought to a successful conclusion, even in the face of obstacles.

The evidence discussed to date suggests that opioid  peptides play a specific role in some aspect of the neural  system underlying the expression of palatability, the opioidpalatability  hypothesis. In order to evaluate fully this idea in  humans, an alternative measure of orosensory reward is  needed in order to get around the potential problems of  interpreting hedonic rating data. One approach has been to  examine in detail changes both in eating rate and the pattern  of changes in subjective appetite during meals following  manipulations of palatability alone [79]. Increasing the  rated palatability of a food increased intake and also resulted  in increases in rated hunger during the early stages of the  test meal [80].

Until recently in our evolutionary history,  addictive agents have been ingested in foods.  Many are secondary plant metabolites that  evolved because they discourage ingestion by  animals1. The hungers that arise from bodily  needs are non-directive; they merely encourage  us to put things in our mouth. The more acute  the hunger, the greater the range of substances  we will ingest2. We learn to return to the yellow  banana, the purple fig, the pink peach. We also  learn to chew the tobacco leaf and drink the  nectar of fermented fruits and grains. Because  of the need for the nutrients in plants containing  addictive substances, many species  have learned to accept mildly intoxicating  amounts of these compounds. 0

Considerable recent research suggests yet another, quite orthogonal mechanism for tolerance. The concept is simple: When an animal has become accustomed to receiving a drug in a certain situation and is again presented with that same situation, it anticipates the drug administration and makes compensatory responses that serve to reduce the impact of the impending presence of the drug. Given this mobilization of the body's homeostatic processes before the actual drug presentation, the drug is effectively compromised, and the result is less total drug effect or increased drug tolerance.

Drug tolerance has been described and studied for years, but only recently have experiments begun to reveal its complexity. In secondary texts, until the past few years, tolerance has been partitioned into dispositional and pharmacodynamic subgroups (e.g., Jaffe, 1985; see Overstreet & Yamamura, 1979). Dispositional tolerance exists when the body's ability to dispose of the drug becomes enhanced over repeated trials.