Environmental influences affecting the voluntary intake of drugs: an overview.

Drug self-administration studies in animals have generally used drugs that are abused by man. The oral route by adding drug to the drinking water is simple, but the bitter taste of many drugs requires that the animals first be forced to consume treated water. The intravenous route, wherein relatively unrestrained rats or monkeys press a lever to obtain intravenous drugs, permits precise control over the dose and can be readily adapted to schedules and manipulations customarily used in the behavioral sciences. Environmental factors affecting drug intake include the dose its schedule of administration, and conditioning of drug administration to secondary cues. There are differences in drug self-administration of stimulant drugs (as amphetamines) and depressants (as morphine and barbiturates). There is an inverse relation between the size of the dose and number of injections taken, but for stimulants daily intake will remain constant whereas for depressants smaller doses are only partially compensated for by increased numbers of injections. Likewise, drug intake of stimulants is better maintained on ratio schedules. Neutral stimuli, as lights or buzzers, paired with drug injections can be used to elicit conditioned responses. Such responses have been used to evaluate the reinforcing properties of drugs.