How do drugs affect the brain?

Alcohol depresses the central nervous system

Question: How do drugs affect the brain?
Position‹2 of 3›: Depressants slow brain and nervous system function
Argument‹3 of 4›: Alcohol depresses the central nervous system

Alcoholic beverages are psychoactive due to the effects of ethanol on the brain and nervous system.

The argument

Ethanol, the active compound in alcoholic drinks, does not act on the brain indiscriminately. It works on two of the brain's most important neurotransmitters, the molecules which transmit signals between networks of neurons, and it pushes each of them in the same direction. Ethanol increases the effects of gamma aminobutyric acid, the primary neurotransmitter which inhibits neuronal signalling, and suppresses the effects of glutamate, the major excitatory neurotransmitter. These are the brain's two opposing controls, one damping signals down and the other driving them up. Strengthening the first while weakening the second shifts the balance decisively towards inhibition, which is what makes the drug a depressant in a pharmacological sense rather than merely a colloquial one. The combined impact of these interactions on the brain is to slow neural function, and the familiar effects of drinking follow from that single change. Signals that travel more slowly produce lowered reaction times and worsened cognitive abilities. The same suppression also enhances sociability by lowering anxiety and social inhibitions, which is why a substance that impairs performance can feel enlivening: inhibition is being reduced across the board, including the inhibitions that ordinarily restrain behaviour. Because the mechanism is dose-dependent, the effects scale with the quantity consumed rather than changing in kind. Consumption of large quantities of alcohol can eventually depress brain activity to the point of unconsciousness or even death, when the same inhibitory shift reaches the systems regulating functions the body cannot suspend. Over a longer period of time, alcohol consumption is associated with a decrease in brain volume, indicating that the consequences are not confined to the hours during which the drug is present. The pattern across all these timescales is consistent with one description: alcohol depresses the central nervous system.

Premises

[P1]Ethanol, the active compound in alcohol, increases the inhibitory neurotransmitter GABA and suppresses the excitatory neurotransmitter glutamate. [P2] This combination slows neural function, lowering reaction times and impairing cognition, and in large quantities can depress brain activity to unconsciousness or death. [C] Therefore, alcohol acts as a depressant that slows the brain and central nervous system.

Counter-arguments

Some qualify this picture rather than reject it. Classifying alcohol as a pure 'depressant' is a simplification: at low doses it can be behaviourally stimulating — increasing talkativeness, confidence and activity — because its early disinhibiting effect on the prefrontal cortex produces excitation before the broader sedative effect dominates, which is why pharmacologists describe it as biphasic. Its action is also not confined to GABA and glutamate; ethanol modulates dopamine, serotonin, endorphin and other systems, which is what drives its rewarding and addictive properties. So while alcohol does ultimately slow the central nervous system, describing it solely as a depressant that 'slows brain function' omits the stimulant-like and reward effects that also characterise how it acts on the brain.

Rejecting the premises

[Rejecting P1] Ethanol acts on many neurotransmitter systems beyond GABA and glutamate — including dopamine, serotonin and endorphins — so the two-neurotransmitter account is an incomplete picture of its brain effects. [Rejecting C] Alcohol is biphasic: at low doses its disinhibiting effect is behaviourally stimulating, so classifying it purely as a function-slowing depressant omits its excitatory and rewarding phase.