David A.N. Siegel, M.D. 23 W. 73rd St. Ste. 101 New York, NY 10023 (646) 418-7077

NEUROBIOLOGY  (MECHANISMS OF ALCOHOL ADDICTION)

All substances of abuse including alcohol produce reward by enhancing the release of dopamine within the circuits that regulate motivated behavior. These dopamine neurons’ rate of firing is enhanced by alcohol. Chronic intake of alcohol leads to alterations in the excitability of these neurons that can persist for significant periods of time. Genetic differences in the responsiveness of these neurons and their connections may contribute to the factors that drive greater alcohol-seeking behavior in some individuals.

Alcohol interacts with a wide variety of targets. The current consensus is that alcohol's behavioral actions result from interactions with a diverse set of cell membrane channels that regulate neuronal excitability.

Alcohol increases the release of opioid peptides, and selective opioid antagonists (naltrexone) can reduce alcohol consumption in both animals and man. Mice genetically modified to lack the mu opiate receptor do not voluntarily drink alcohol and do not respond to the rewarding effects of opiates, nicotine, or cannabinoids. Opioid antagonists have become a mainstream treatment of alcohol addiction.

Distinct families of subunits make up GABA-A and glycine receptors, and different combinations of these give rise to channels with variable sensitivity to alcohol. Alcohol increases the inhibitory effects of the neurotransmitters GABA and glycine. It both enhances GABA-A and glycine receptor function and increases release of GABA. Stimulation of the GABA-A receptor is responsible for the calming and anti-anxiety effects of alcohol. With chronic alcohol use, GABA-A receptors become less sensitive to GABA. When alcohol is no longer consumed, these down-regulated GABA-A receptors have become so insensitive that the typical amount of GABA produced has little effect. This loss of inhibition from GABA results in unopposed excitatory neurotransmission, leading to the neuronal over-excitation which causes the most disturbing withdrawal symptoms. Benzodiazepines also stimulate the GABA-A receptor, which is why a benzodiazepine taper is used to treat acute withdrawal.

Glutamate is the major excitatory neurotransmitter in the brain and activates three major subtypes of channels. Activation of these channels cause the neuron to fire and are implicated in processes that underlie thought, learning, and memory. One of these channels is readily blocked by alcohol at concentrations associated with intoxication and sedation. Alcohol’s blockade of excitatory NMDA signaling may underlie its rewarding effects since NMDA blockers increase levels of dopamine in reward areas of the brain. Chronic exposure to alcohol increases the density and clustering of NMDA receptors leading to increased neuronal excitability and enhanced susceptibility to seizures that can develop during withdrawal from alcohol.

5-HT3 receptors are channels activated by serotonin. Alcohol increases the effects of the 5-HT3 receptor, and 5-HT3 receptor blockers reduce the probability of drinking alcohol in animal models. Human studies with the 5-HT3 blocker ondansetron (Zofran) showed that the drug significantly reduced drinking in some individuals.

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