Common Misbelief: Alcohol is mostly used and abused for its psychostimulant actions.
Mechanism of Alcohol Actions
As recently as the last century, alcohol was considered to be a âpsychostimulantâ that was commonly used for such desired effects as to âhave funâ and to âwarm-up after coming in from the cold.â It also was medically used for a variety of indications, including reviving someone who has fainted and treating the fatigue and weakness associated with cachexia. Although this belief was supported by lay and medical observations that drinking too much often resulted in such behaviors as taking off oneâs clothes and dancing naked on a table in a favorite bar or getting into a violent bar fight, alcohol possesses no psychostimulant actions (see the following subsection, âBlood Alcohol Concentrationsâ).
Empirical findings in support of the psychodepressant actions of alcohol include the observation that as more alcohol is consumed during a drinking episode, drinkers do not become more wakeful, they become drowsier and, inevitably, uniformly fall asleep.
The exact mechanism of action by which alcohol depresses the entire CNS remains a mystery. However, research over the last several decades has resulted in two major plausible theories:
- Alcohol readily crosses the blood-brain barrier disrupting the function of brain membrane lipids and glucose metabolism;
- Alcohol acts at two specific receptor sites in the brainâgamma aminobutyric acid (GABAA) receptor sites and N-methyl-D-aspartate (NMDA) receptor sites (e.g., Brower, 2001; Gonzales & Jaworski, 1997; Pagliaro & Pagliaro, 2009; Peoples, Li, & Weight, 1996).
While either theory may contribute to alcoholâs actual mechanism of action, research in glucose metabolism supports the first theory, which also explains alcoholâs effects on learning and memory. Research in support of the second theory suggests that alcohol acts to enhance GABAergic inhibition primarily by modifying the membrane environment of the GABAA receptor complex. This action significantly increases the affinity of the GABAA receptor complex for both endogenous (e.g., GABA) and exogenous sedative-hypnotics (e.g., barbiturates and benzodiazepines), thus, enhancing GABAergic inhibition.7
In addition, alcohol appears to inhibit the activity of glutamate, which is a naturally occurring excitatory neurotransmitter that functions largely at the NMDA receptorsâas well as AMPA receptors and metabotropic glutamate receptors (mGluRs) (Gonzales & Jaworski, 1997). The regular, long-term use of alcohol results in âup-regulation of the NMDA receptors.â The NMDA receptor has been posited, based on data obtained from laboratory animal studies, to play a significant role in both: (1) alcohol-induced neurotoxicity; and (2) the alcohol withdrawal syndrome, particularly the occurrence of withdrawal-related seizures (Chang-Mu, Jen-Kun, & Shing-Hwa, 2010; Charlton, Sweetnan, & Fitzgerald, 2002; Hoffman, 1995; Hoffman & Tabakoff, 1994). (For further related discussion of the GABAA receptor complex, see Chapter 6, âPrescription Sedative-Hypnotics.â)
Additionally, other factors have been identified as contributing to both the pharmacological and toxicological effects related to alcohol use and withdrawal (Vezali Costardi, Teruaki Nampo, & Lourenco Silva, 2015). For exampleâregarding the serotonergic system:8 (1) serotonin release and transmission are increased in the CNS by the acute administration of alcohol; (2) regular long-term, or chronic, use of alcohol leads to adaptive changes to the serotonin 5-HT2 receptor (e.g., âup-regulationâ or an increase in the number of 5-HT2 receptors); and (3) serotonin levels are reduced during alcohol withdrawal (Lovinger, 1997; Tollefson, 1989).