Chapter 2
Heredity, Environment, Psychoactive Drugs

Overview

This chapter examines the routes that drugs take to the brain and the ways in which they affect brain chemistry. Psychoactive drugs affect both the old (primitive) brain and the new brain, particularly the reward/reinforcement center. Drugs cause their effects by mimicking or modifying neurotransmitters and other brain chemicals. An individual’s drug tolerance, tissue dependence, withdrawal, and metabolism determine additional effects.

Besides the desired effects of drugs, undesirable side effects occur. The level of drug use - abstinence, experimentation, social/recreational use, habituation, abuse, and addiction - depends not only on the amount, frequency, and duration of drug use but on a person’s susceptibility to addiction as determined by heredity and environment. All these factors cause alterations in brain chemistry that can affect a person for a few hours, a few days, or even a lifetime. Many of these alterations can be seen with the assistance of new imaging techniques such as SPECT, CAT, MRI, FMRI and PET brain scans.