By inhibiting the firing of electrical impulses in neurons, alcohol can impair judgment, coordination, alertness, memory, and visual perception, among other things. Exactly, how does alcohol achieve all of these unrelated effects?
Alcohol affects the function of specific proteins or receptors embedded in the membranes of neurons. Alcohol can interact with a variety of neurotransmitter receptors, but at non-fatal concentrations of alcohol in the brain, alcohol interacts primarily with receptors for the amino acid neurotransmitters γ-aminobutyric acid (or GABA) and glutamate (the same amino acid found in “Chinese food” seasoning—MSG or mono-sodium glutamate). When alcohol binds to GABA and glutamate receptors, it causes many of the intoxicating symptoms that develop when one drinks too much.
How does this happen? To answer this, it’s helpful understand how these neurotransmitter receptors function in a neuron. A closer look at these neurotransmitter receptors reveals that they consist of several smaller proteins (called subunits) arranged to form a pore or channel in the middle. Normally the channel is closed. But when the neurotransmitter binds to the receptors, the channels will open briefly, allowing small cations such as sodium (Na+) or calcium (Ca2+) or anions such as chloride (Cl-) to pass into or out of the cell, along the concentration gradient (Figure 2.5). The type of ion that moves through the channel depends on the whether it’s a GABA or a glutamate receptor.
As ions move through the receptor channels, an electrical current is spread over the cell membrane. When positive ions (current) enter the cell, neurons fire electrical impulses. When negative charges (current) enter the cell, neuron firing is suppressed.
Figure 2.5 Watch how alcohol (ethanol) reduces the firing rate of neurons. When GABA binds to its receptor, the ion channel opens and chloride ions (Cl-) flow into the cell with the concentration gradient. In the presence of ethanol, the channel remains open longer so more Cl- goes into the cell. Thus, the neuron can’t fire an electrical impulse as quickly.
Learn more about basic neuron structure and function and view a 3D animation
Alcohol works in a “double-duty” fashion. It can bind to GABA receptors to hold the ion channel open longer. This increases the amount of chloride ions (negative charges) entering the neuron (Figure 2.5). Also, alcohol can bind to glutamate receptors, where it decreases the amount of sodium and calcium (positive charges) entering the neuron. In both cases, the result is that the environment inside the cell becomes more “negative” and this suppresses the electrical activity (i.e., the firing rate) of the neuron. Thus, communication at the GABA synapses is slowed.
The suppression of neural communication causes most of the symptoms of intoxication. The particular symptom of intoxication will depend on where in the brain the suppression of neuron activity occurs. As the blood alcohol concentration increases, new symptoms of intoxication emerge (Figure 2.1).
Interestingly, repeated use of alcohol can making it harder to get intoxicated, so a person will drink more alcohol to achieve intoxication. This is called tolerance. In the next section, you can learn why this happens.