The defining features of drug intoxication and addiction can be traced to disruptions in cell-to-cell signaling.

Drugs of abuse alter the way people think, feel, and behave by disrupting neurotransmission, the process of communication between brain cells. Over the past few decades, studies have established that drug dependence and addiction are features of an organic brain disease caused by drugs' cumulative impacts on neurotransmission. Scientists continue to build on this essential understanding with experiments to further elucidate the physiological bases for drug abuse vulnerability as well as the full dimensions and progression of the disease. The findings provide powerful leads to new medications and behavioral treatments.
This second article in our NIDA Notes Reference Series discusses the central importance of studying drugs' effects on neurotransmission and describes some of the most common experimental methods used in this research. As with other articles in the series, we provide illustrative references from articles published in NIDA Notes.

What is Neurotransmission?

A person reads. The words on the page enter the brain through the eyes and are transformed into information that is relayed, from cell to cell, to regions that process visual input and attach meaning and memory. When inside cells, the information takes the form of an electrical signal. To cross the tiny intercellular gap that separates one cell from the next, the information takes the form of a chemical signal. The specialized chemicals that carry the signals across the intercellular gaps, or synapses, are called neurotransmitters.
The ebb and flow of neurotransmitters—neurotransmission—is thus an essential feature of the brain's response to experience and the environment. To grasp the basic idea of neurotransmission, compare the brain to a computer. A computer consists of basic units (semiconductors) that are organized into circuits; it processes information by relaying electric current from unit to unit; the amount of current and its route through the circuitry determine the final output. The brain's corresponding basic units are the neurons—100 billion of them; the brain relays information from neuron to neuron using electricity and neurotransmitters; the volume of these signals and their routes through the organ determine what we perceive, think, feel, and do.
Of course, the brain, a living organ, is much more complex and capable than any machine. Brain cells respond with greater versatility to more types of input than any semiconductor; they also can change, grow, and reconfigure their own circuits.

Neuroscientists seeking to understand why a drug is abused and the consequences of that abuse focus on two issues: