Next week when I have my hip replacement surgery, I will be given drugs such as a narcotic and a benzodiazepine to relieve pain and discomfort. Similarly, if you have a glass of wine and feel relaxed, or a cup of coffee and feel alert, then you have altered your mental state by changing your brain chemistry. Alcohol affects activity of such neurotransmitters as glutamate, GABA, and dopamine; caffeine blocks the action of the neurotransmitter adenosine. Such changes are studied in the field of psychopharamacology, which concerns the effects of drugs on the mind and brain. This field investigates both recreational drugs such as alcohol, caffeine, marijuana, cocaine, and LSD, and therapeutic drugs such as antidepressants and antipsychotics.
Studies of the neural and psychological effects of drugs support the mind-brain identity theory over its chief rival, dualism, which claims that minds are distinct from bodies. Identifying minds with brains requires specification of the nature of brain processes as neuroelectrochemical, drawing on the rich biochemistry that underlies the firing patterns commonly taken as the basic units of neural computation.
To explain a mental process, we first identify a sequence of mental events such as thoughts, perceptions, and feeling, and then describe a mechanism whose interacting components produce that sequence. Most current scientific explanations of mental processes operate either at the cognitive level, where the components are mental representations such as rules or concepts, or at the neural level where the components are neurons. Researchers at both of these levels have had substantial explanatory successes, but psychopharmacology shows the need to consider mechanisms at the molecular level as well.
To explain the mental effects of recreational and therapeutic drugs, it is crucial to consider the mechanisms by which drugs can alter how one neuron influences another. Here are some of the more important:
1. A drug can inhibit the synthesis of a neurotransmitter.
2. A drug can stimulate or inhibit the release of a neurotransmitter.
3. A drug can stimulate or block postsynaptic receptors.
4. A drug can inhibit degradation or reuptake of a neurotransmitter.
Even this short list makes it apparent that neurotransmitters and receptors are central components in understanding how drugs affect mental processes. More than 100 different neurotransmitters have been identified, and many of these have multiple kinds of corresponding receptors.
Many people are familiar with the effects that alcohol can have on their mental processes, but not with the neurochemical mechanisms that produce these changes. Alcohol has major effects on glutamate, GABA, dopamine, and opioid systems. Glutamate has receptors on many cells in the central nervous system, and alcohol has its greatest effect on the NMDA receptor. Alcohol reduces the effectiveness of glutamate at the NMDA receptor, which is crucial for learning and memory via long-term potentiation. This inhibition of glutamate transmission explains the memory loss sometimes associated with intoxication. In addition, alcohol binds to the GABAA receptor complex, just like sedatives and anti-anxiety drugs such as Valium. This binding allows chloride ions to enter the postsynaptic neuron, facilitating the inhibitory response that GABA normally produces and thereby decreasing anxiety. The pleasurable effects of alcohol are the result of it increasing the firing rate of dopaminergic neurons in the ventral tegmental area, elevating the amounts of dopamine released in the nucleus accumbens, the brain’s major pleasure center.
Psychopharmacology is relevant to assessing the plausibility of the main current theories of the relation between mind and matter: the identity theory and dualism. When the identity theory was developed in the 1950s, its proponents were mainly defensive, responding to arguments that dualists used to attack the claim that mental processes are brain processes. Very little was known then about the neural mechanisms responsible for sensation, cognition, and emotion. Since then, there have been dramatic advances in both the experimental and theoretical investigation of brain processes. New experimental techniques range from brain scanning technologies such as fMRI machines to using genetically modified mice to discover the chemical activity of different neurotransmitters and receptors. In vivid contrast to the dramatic progress in neuroscientific understanding over the past 50 years, dualism has taught us nothing about the nature of mental processes.
From the perspective of the mind-brain identity theory, it is straightforward to explain the psychological effects of drugs that affect neurochemistry. If mental states just are brain states, then it is obvious why drugs that change brain states should also change mental states. On the other hand, the dualist position that maintains that mental states are different from brain states has little to say about why changing the neurochemistry of the brain has mental effects. As long as mental states are viewed as non-material, it remains a mystery why substances like alcohol, caffeine, antidepressants, and narcotics change mental states.