copyright alcohol medical scholars program1 alcohol: pharmacology and neurobiology vijay a....

Copyright Alcohol Medical Scholars Program

1

Alcohol: Pharmacology and Neurobiology

Vijay A. Ramchandani, Ph.D.

Indiana University School of Medicine

Copyright Alcohol Medical Scholars Program 2

Outline

• Pharmacokinetics– Absorption

– Distribution

– Metabolism

• Pharmacodynamics– CNS effects

– Tolerance

– Alcohol as a reinforcer

– Neuropharmacological effects


Pharmacokinetics: Absorption

• Rapidly absorbed primarily from duodenum• Rate of absorption is extremely variable• Peak blood alcohol concentration (BAC) depends on:

– Amount and alcohol concentration of beverage– Rate of drinking– Food consumption and composition– Gastric emptying and gastric metabolism– Hepatic first pass


Distribution

• Volume of distribution = Total Body Water• Gender Differences in body composition


Metabolism

• Metabolism– 90-98% metabolized in liver

Alcohol Acetaldehyde Acetate

– Alcohol dehydrogenase saturates at low to moderate BACs (Michaelis-Menten kinetics)

– Apparent zero-order kinetics at moderate BACs• Alcohol Elimination Rate = 7 g per hr

• Disappearance Rate = 0.015% per hr

Alcoholdehydrogenase

Aldehydedehydrogenase


Metabolism

• Metabolism

Alcohol Acetaldehyde Acetate

– Aldehyde dehydrogenase usually not rate-limiting

– Accumulation of acetaldehyde associated with headache, gastritis, nausea, dizziness (hangover)

– Aldehyde dehydrogenase inhibition (disulfiram)

Alcoholdehydrogenase

Aldehydedehydrogenase


Metabolism: Genetic Variation

Genetic variation in alcohol metabolizing enzymes• Alcohol Dehydrogenase (ADH)

– Polymorphism occurs at ADH2 and ADH3 loci

ADH2*1 ADH2*2 ADH2*3 ADH3*1 ADH3*2

White American 95% <5% <5% 50% 50%

Black American 85% <5% 15% 85% 15%

Asian 15% 85% <5% 95% 5%

– 15% of Black Americans have ADH2*3 allele increased alcohol metabolic rate


Metabolism: Genetic Variation

Genetic variation in alcohol metabolizing enzymes• Aldehyde Dehydrogenase (ALDH)

– Polymorphism at the ALDH2 gene

• 50% of Asians have ALDH2*2 allele

decreased elimination of acetaldehyde (and alcohol)

flushing response


Pharmacokinetics: Gender Differences

• Gender Differences– in absorption

• Differences in gastric ADH activity

– in volume of distribution• Differences in body composition and total body water (TBW)

– in metabolism• Differences in liver volume, ADH activity?

• Effect of menstrual cycle on alcohol pharmacokinetics• Effect of sex hormones (OC) on alcohol PK


Pharmacodynamics: CNS Effects

• Alcohol is a CNS depressant• Apparent stimulatory effects result from depression of

inhibitory control mechanisms in the brain• Characteristic response: euphoria, impaired thought

processes, decreased mechanical efficiency


Concentration-Effect Relationship

BAC [%] Effects

0.02-0.03 Mood elevation. Slight muscle relaxation.

0.05-0.06 Relaxation and Warmth. Increased reaction time. Decreased fine muscle coordination.

0.08-0.09 Impaired balance, speech, vision, hearing, muscle coordination. Euphoria.

0.14-0.15 Gross impairment of physical and mental control.

0.20-0.30 Severely intoxicated. Very little control of mind or body.

0.40-0.50 Unconscious. Deep coma. Death from respiratory depression


Tolerance: Definitions

• Tolerance: The phenomenon of decreased effect with prolonged exposure to a drug

• Acute tolerance: during the time-course of a single exposure to drug

• Chronic tolerance: over repeated use of drug


Tolerance: Significance

• Why is tolerance to alcohol important?– One of the determinants of increased alcohol consumption

• maintains or aggravates alcohol dependence

• increases risk of organic complications of alcoholism

– Diagnostic criteria for alcoholism by DSM-IV

– Cross-tolerance to other depressant drugs

– Genetic determinants exist

– Low Response predicts alcoholism


Alcohol as a Reinforcer

• Reinforcer: a substance whose pharmacological effects drive the user to continue to use it.

• Positive reinforcing effects:– Gain pleasure

– Altered consciousness

– Conform to behavior of peers

• Negative reinforcing effects:– Relief of stress and negative emotions

– Relief of withdrawal symptoms


Alcohol as a Reinforcer: Neural Systems

Activation of mesocorticolimbic system


Alcohol as a Reinforcer: Evidence

• Animal models of alcohol preference– Selectively bred animal lines show innate differences in limbic

structures and neurotransmitter function

• Animal models of self-administration– Animals trained to chronically self-administer alcohol show

differences in neurotransmitter levels in the mesolimbic system

– Animals will bar-press repeatedly for intra-cranial injections of alcohol into the VTA


Reinforcement: Neurochemical systems

EnkephalinInhibitoryNeuron

REWARD

Glutamate Excitatory InputEnkephalin or

DynorphinInhibitory Neuron

GABAInhibitory

Neuron

GABA Inhibitory Feedback

Dopamine Neuron GABANeuron

Ventral Tegmental Area(VTA)

Nucleus Accumbens(NAc)

Dopamine Receptors

GABA-A Receptors

PresynapticOpioid

Receptors(, ?)

OpioidReceptors

OpioidReceptors


Neuropharmacology: GABA

• Effects on GABA system– Interaction with GABA-A receptor and facilitation of GABA

transmission• Sedative and anxiolytic effects• Withdrawal


Neuropharmacology: DA, Opioids

• Effects on Dopamine system– Increase dopamine in mesocorticolimbic system

• Reinforcing, rewarding effects

• Effects on Opioid peptide system– Activation of opioid peptide system

• Reinforcing and rewarding effects (Mu)

• Aversion (Kappa)

• Craving


Neuropharmacology: NMDA, 5HT

• Effects on NMDA Glutamate system– Blockage of NMDA receptor (allosteric effect)

• Sedative/hypnotic effects

• Neuroadaptation

• Withdrawal

• Effects on Serotonin system• Neuroadaptation, aversion

• Effects on stress hormones• Stress response


Neuropharmacology: Summary

Experience Transmitter/Receptor

euphoria/pleasure Dopamine, Opioids

anxiolysis/ataxia GABA

sedation/amnesia GABA + NMDA

nausea 5HT3

neuroadaptation NMDA, 5HT

stress CRF

withdrawal GABA, NMDA ( Ca, Mg)


Implications for Pharmacotherapy

• Disulfiram• Naltrexone• Acamprosate• Benzodiazepines• SSRIs

copyright alcohol medical scholars program1 alcohol: pharmacology and neurobiology vijay a....

Documents

pass slide

body composition slide

duodenum rate of absorption

gastric metabolism hepatic

hr disappearance rate

adh3 loci adh2

accumulation of acetaldehyde

composition gastric