Psychoactive substances act in the brain on mechanisms that exist normally to regulate the functions of mood, thoughts, and motivations. In most cases, people use psychoactive substances because they expect benefit from their use, either through the experience of pleasure or either the avoidance of pain, anxiety or depression. The benefit is not necessarily gained directly from the psychoactive action of the substance. Someone drinking beers may be more motivated by the feeling of fellowship or have cultural cofactors, in pro or counter, reasons and desinhibition it gives. However, the psychoactive effect is nevertheless present in alcohol.
Relevant basic investigation of addiction must look at: 1) Form & patterns of substance use; 2) Amount of substance use; 3) Dependence; 4) Psychoactive effects (intoxication); 5) Toxic and other biochemical effects (especially neurobiologic); 6) Chronic social problems; 7) Acute social problems; 8) Accidents/injuries (acute disease); 9) Chronic disease.
Categories are important in relation to alcohol and many illicit drugs, but are poorly measured and mostly excluded from measurements of health effects such as in the Global Burden of Disease (GBD).
Criteria for substance dependence in ICD-10 (international classification of diseases) set: 3 or more of the following must have been experienced or exhibited together at some time during the previous year: 1. a strong desire or sense of compulsion to take the substance; 2. difficulties in controlling substance-taking behaviour in terms of its onset, termination, or levels of use; 3. a physiological withdrawal state when substance use has ceased or been reduced, as evidenced by: the characteristic withdrawal syndrome for the substance; or use of the same (or a closely related) substance with the intention of relieving or avoiding withdrawal symptoms; 4. evidence of tolerance, such that increased doses of the psychoactive substance are required in order to achieve effects originally produced by lower doses; 5. progressive neglect of alternative pleasures or interests because of psychoactive substance use, increased amount of time necessary to obtain or take the substance or to recover from its effects; 6. persisting with substance use despite clear evidence of overtly harmful consequences, such as harm to the liver through excessive drinking, depressive mood states consequent to heavy substance use, or drug-related impairment of cognitive functioning. Efforts should be made to determine that the user was actually, or could be expected to be, aware of the nature and extent of the harm.
Definitions of terms related to use of psychoactive substances are: 1) Harmful use: A pattern of psychoactive substance use that is causing damage to health. The damage may be physical or mental. 2) Hazardous use: A pattern of psychoactive substance use that increases the risk of harmful consequences for the user. 3) Intoxication: A condition that follows the administration of a psychoactive substance and results in disturbances in the level of consciousness, cognition, perception, affect, or behaviour, or other psychophysiological functions and responses. The disturbances are related to the acute pharmacological effects of, and learned responses to, the substance and resolve with time, with complete recovery, except where tissue damage or other complications have arisen. Complications may include trauma, inhalation of vomitus, delirium, coma and convulsions, and other medical complications. The nature of these complications depends on the pharmacological class of substance and mode of administration. 4) Substance abuse: Persistent or sporadic drug use inconsistent with or unrelated to acceptable medical practice. A maladaptive pattern of substance use leading to clinically significant impairment or distress, as manifested by one (or more) of the following: failure to fulfil major role obligations at home, school or work; substance use in situations in which it is physically hazardous; recurrent substance-related legal problems; continued substance use despite having persistent or recurrent social or interpersonal problems exacerbated by the effects of the substance. WHO (1994).
It is by the way that psychoactive drugs interfere with normal physiological processes that they can represent a medicine or a noxious drugs. As a result addictions understanding and pharmacological treatment deeply involve neurophysiological knowledge, its lack and complexity. Brain mechanisms consider first neuroanatomy or the structure of neural tissues but must care the dynamic of functionning networks. Making harder to point just at specific brain places, chemical substance liberated by an electrophysiological depolarisation, or even networks and designed expressions of moods and analytic definitions.
Neuron is the basic cell but surrounding others (glial: which are not so neutral, as previously imagined). Analysis of neuron details with cell body, dendrites (up-influx, often in enormous number, communicating expensions), axon (down-outflux lone expansion of neuron), terminal buttons (synapse in an enormous number only for each one neuron) at the interface of, mostly neural next cells or muscular cells ones; where neural impulse delivers transducing molecules: into a complicated interspace device (called the synapse). One axon is characterized by the liberation of a specific neurotransmitter, a biochemical molecule (scaling from simple ions up to long peptides), often related to the label of a given sensation, neural mechanism or mood effect. Meanwhile a specified synapse also see the close intervention of some secondary other neurotransmitters.
So neurotransmission study: action potential (bioelectrical), receptors (structural biochemical device), neurotransmitters (various kinds and types of biochemical molecules as, like for some main ones: acetylcholine, gamma-aminobutyric acid, glutamate, dopamine, norepinephrine, serotonin and peptides).
Now, despite the still recent start of neurofunctional brain studies (functional neuroimaging), there is already a huge compendium of phenomenologic explainations of addictions since neuroanatomy and neurophysiological laboratory experiments as: classical or Pavlovien conditioning (concepts of reinforcer, reward, incentive, motivation and incentive-motivational responding), reward or pain-suffering brain circuits involvements. Drug reward alone does not explain drug dependence (so the options of drug dependence as a response to incentive-motivation and/or to drug withdrawal).
Dopamine neurotransmitter because of its core structuring role in coordinating and in a qualified concept of reward is still given a core importance (in psychomotor sensitization, sensitization and drug reward or tolerance), but other molecules are of interest like serotonine or glutamate. Thereafter all kind of psychoactive drugs play for most as well as according half a dozen patterns that drugs classification intent to catch according legal, psychologic or biochemical effects or classification of drugs, as well as with specific effects in details which may make clinical sense meanwhile in the "wholistic" approach of addiction problems are constrained by the more or less non specific effects of therapeutic means or the diversity required by human beings needs and problems.