neurobiology of alcohol addiction

1. Pablo Picasso (1910)
Neurobiological
basis of
Alcohol
Dependence
Chairperson: Dr. Raka Jain
Presenter: Dr. Ravikant
Kumar

2. Introduction
• Alcohol
– the king of liquids
– excites the taste to the highest degree
– various preparation have opened up to mankind
new sources of enjoyment
– a well known social lubricant
– excessive use produce the most harm to society
of all drugs of abuse (NIAA 2005)

3. Alcohol – the molecule
• Alcohol represents a wide series of compounds
• The alcohol suitable for drinking is ethanol
• Ethanol is a small molecule
– With low binding energy
& compared to other drugs
– A high Hill coefficient,
– require much higher concentrations for intoxication

4. Alcohol - Pharmacokinetics
• Universal solvent
• Readily miscible in water &
• Low lipid solubility
• Readily crosses the cell membrane
• Absorption
– 20% in stomach
– 80% in small intestine
• Elimination
– 5~10% excreted unchanged in urine & from lung
– Rest : metabolized in liver

5. Alcohol - Pharmacokinetics
ethanol
Aldehyde
oxidase
Non-enzymatic
oxidation
MAOS
Fatty acid ehhyl-ester synthase
MEOS
Catalase
Fatty acid
Ethyl esters
acetateacetaldehyde
Alcohol dehydrogenase Aldehyde dehydrogenase
Metabolism

6. Pharmacokinetics & Dependence
Genetic variation in alcohol metabolizing enzymes
 Allelic variation in ADH/ALDH system is responsible for
differential alcohol elimination
 Inactivation of ALDH2: protective factor against alcohol
dependence
 Mutant ALDH2 gene:
 Mongoloids: 40~50%
 Native Americans: 2~5%
 Europeans: 0%
(Goedde 1992)

7. Pharmacokinetics & Dependence
• Metabolic tolerance
– Increased MEOS activity
– P4502E1 is increased 4 times in liver biopsies
of alcoholics
(Tsutsumi 1989)
– increased alcohol dehydrogenase activity

8. Alcohol-Pharmacodynamics
• Old theory:
Alcohol increases membrane fluidity leading to altered
function of macromolecules in the cell membrane
• Recent Evidence :
Alcohol binds to hydrophobic pockets of proteins,
changing their three-dimensional structure and their
function
– Ion channels
– Neurotransmitter receptors
– Enzymes involved in signal transduction

9. Alcohol : CNS Effects
• Alcohol is a CNS depressant
• Apparent stimulatory effects result from
depression of inhibitory control mechanisms in the
brain
• Characteristic response:
– euphoria,
– Anxiolytic like effect,
– impaired thought processes,
– decreased mechanical efficiency

10. Alcohol & CNS
Neurotransmitter systems
• GABAergic system
• Glutaminergic system
• Serotonergic system
• Dopaminergic system
• Nicotinic acetylcholine system
• Glycine neurotransmitter
system
• Nor epinephrine systems
• Endogenous opioid system
• Endocannabinoid system
• Neurosteroids
• neuropeptides
Voltage gated ion channels
• Calcium channel
• Sodium channel
• Potassium channel
Second messenger systems
• Adenosine
Protein kinases
Neurotrophic factors

11. GABA acts on two receptor subtypes GABAa & GABAb
GABAa receptors are pentameric protein connected to a chloride
channel
GABAa receptors have a magnitude of subunits : α β γ δ
GABA
system

12. GABA system
• Renders acute central effects
– Anxiolytic, sedative, anticonvulsant, motor
incoordination
• Enhances GABAergic neurotransmission
– Presynaptic : increase GABA release
– Postsynaptic : agonist to GABAa receptor
• > direct action
• > facilitate GABA binding
(Roberto 2003)
Net effect: increased Cl influx > hyperpolarisation

13. GABA system
• Alcohol potentates GABAa mediated inhibition
– Selective
– depend on the heterogeneity of GABAa receptor
subunit composition (Criswell 2005)
– No single composition implicated
– Presence of α2 subunit increases sensitivity to alcohol
• Also act on GABAb receptor
– Counter the action of GABAa ( Weiner 2004)

14. GABA system
• Alcohol does not increase GABAa mediated inhibition in
all brain regions, all cell types in the same region, nor at all
GABAa receptors on the same neuron
• Molecular basis for the selectivity of the action of alcohol
on the GABAa receptor has been proposed to depend on
the heterogeneity of GABAa receptor subunit composition
(Criswell 2005)
• Various composition have been proposed by different
researchers
• Controversery still present in this regard

15. GABA system
• Effect of chronic alcohol exposure
– Down regulation of GABAa receptor
• Reduced no of GABA-benzodiazepine receptor complex,
especially in frontal lobes
• Reduction of receptors is related to amount of alcohol &
severity of addiction
• Decrease in GABA agonist induced Cl flux
• Reduced gene expression ( mRNA & protein) for alpha1
subunit of GABAa receptor (Malcom 2003)

16. Glutamate system
• Main excitatory neurotransmitter in brain
• 2 categories of receptors
– Ionotropic: NMDA, AMPA, kainate
– Metabotropic: G-protein coupled
• Alcohol : inhibitory effect on NMDA recptors
EPSP
blocks Ca influx inhibits
LTP
• Also inhibits other receptors (Carta 2002)

17. Glutamate system
• Play important role in memory & learning
• Play important role in
– Reinforcing effect in alcohol intake
– Repeating alcohol through learning of environmental
cues
• Glutamate antagonists claimed to suppress conditioning
behavior caused by alcohol
(Shaham 2002)
• Clinical study: reduced cue-induced relapse
(Backstrom 2004)

18. Glutamate system
• Adaptation to chronic alcohol exposure
– Up-regulation of NMDA system
• Increase in receptor subunits
• Increase interaction of NMDA with intracellular messengers :
increase NO synthesis
– Up-regulation of non NMDA pathways
– Increase synaptic glutamate release
• Net result: increased glutamate activity
– In locus ceruleus: withdrawal symptoms
– In neocortex: amnesia

19. Glutamate system
• Protracted withdrawal
– Increased NMDA receptors in VTA
– Hyperexciation in mesolimbic DA pathway
– Depolarization blockade
– Decrease in dopamine release (Rossetti 1998)
• Time course for dissipation of the change in NMDA receptor
properties paralleled the time course for disappearance of protracted
withdrawal symptoms (Snell 1996)
• NMDA antagonists effective in protracted withdrawal

20. Dopaminergic system
• Alcohol increases synthesis & release of dopamine
• Positive reinforcing property of alcohol because of activatation of
mesolimbic dopaminergic pathway
– Dopamine neuron with cell bodies in VTA
– Projections to the NA, amygdala, frontal cortex & other limbic areas
• Proved in animal studies & electrophysiological studies
• Greater increase in alcohol preferring rats
• Administration of dopamine agonists decreased alcohol intake in P
rats (Weiss 1993)

21. Dopaminergic system
• How alcohol increases dopaminergic activity?
– Direct excitatory cellular activation (Brodie 2000)
– Inhibit NMDA receptors (glutamatergic system exerts tonic
inhibitory control on mesolimbic dopaminergic neurons)
(Hoffman 1995)
– Promote binding of opioid agonists to μ receptors present on cell
bodies of dopamine neuron in VTA (Di Chiara 1988)
– Glycine recept in NA also act as a target for alcohol in its
mesolimbic DA activating effect (Molander 2005)

22. Dopaminergic system
• Result of chronic alcohol intake
– Decrease mesostriatal dopamine activity in animal
– Decrease level of dopamine & its metabolites in alcoholic patients
(Diana 1996)
– Adaptive change (increased density) of dopamine receptors
(Rommelspacher 1992)
• Consequences: gradual increase in reward threshold

23. Serotonergic system
• Alcohol stimulate serotonin release in NA
(Yoshimoto 1991)
• 5-HT3 receptor system is implicated in mediating alcohol
sensitivity
– Microinjection of serotonin in VTA enhance dopamine release in
nucleus accumbens
– This dopamine release can be blocked by 5-HT3 antagonist
(Campbell 1995)

24. Serotonergic system
• Some of the pleasurable effects of alcohol are mediated by binding to
5-HT3 receptors
– Shown by co-administration of alcohol & 5-HT3 antagonist
(Johnson 1993)
• Majority of the studies support the hypothesis that increase in
Serotonergic function leads to decrease in alcohol consumption
• Conversely majority of the studies don’t support that decrease in
Serotonergic function leads to increase in alcohol consumption
(LeMarquand 1994)
• Serotonin transporter density was lower in cortex of alcoholic subjects
(Manterew 2002)

25. Endogenous Opioids
• Endogenous opioid systems modulates reinforcing & other
actions of alcohol.
• Reduced alcohol consumption & reduced alcohol-
reinforced operant responding in μ receptor null mutant
mice (Gianoulakis 2001)
• Exact mechanism still not clear
• Increases exrta-cellular β-endorphin levels in nucleus
accumbens (Olive 2001)

26. Endocannabinoid system
• Endocannabinoid & CB1 receptor involved in brain reward
mechanism
• CB1 receptor activation associated with increased DA release in NA
(Gessa 1998)
• CB1 antagonist shown to decrease voluntary alcohol intake in
Sardinian alcohol- preferring rat
(Colombo 1998)
• In mice lacking CB1 receptor gene
– Reduced voluntary intake of alcohol
– Completely lacked alcohol induced DA release as compared to wild mice
(Basalingappa et al 2003)

27. Endocannabinoid system
• Chronic alcohol intake
– increases synthesis of endogenous cannabinoids
– down regulation of CB1 receptors & its signal transduction
(Basavarajappa 2002)
• Above findings
– Provides a novel basis for development of drug targeting CB1
receptor function as a potential treatment for alcohol dependence.

28. Nicotinic acetylcholine receptor system
• Ethanol potentiates central n-Ach receptor function
• Ethanol induced activation of mesolimbic dopamine
system is mediated by stimulation of n-Ach receptor in
ventral tagmentum area
• Imply a role in modulation of ethanol’s
reinforcing/motivational effect
(Soderpalm 2000)

29. Neurosteroids
• Ethanol increases de novo synthesis of neurosteroids in
brain by local action independent of HPA axis
– Increase allopregnanolone content in brain without any increase in
plasma corticosteroids
– Modulate GABAa receptor function : increase amplitude of
GABA mediated IPSP
– May contribute to disturbances in reproductive functions &
associated psychiatric disorder Chronic alcohol exposure
decreases the sensitivity of GABA recptors to neurosteroids
(Paolo 2005)

30. Neuropeptides
• Two neuropeptides appear to be involved in
alcohol related stress
– Corticotropin releasing factor : increased stress
response & negative affect
– Neuropeptide Y : anxiolytic properties

31. Neuropeptides
• Corticotropin releasing factor (CRF)
– Behavioral response to stress mediated by CRF occurs
independently of the HPA axis
– Two receptors : CRF1 & CRF 2
– Anti anxiety property of alcohol involve a suppression of brain
CRF system
– Chronic ethanol exposure
• up regulation of CRF receptors
• hypersensitivity to stress
– Abstinence
• Heightened CRF activity
• A key mediator of the anxious state
• Increased susceptibility to relapse (Valdez 2004)

32. Neuropeptides
• Neuropeptide-Y (NPY)
– Alcohol potentiate the activity of NPY
– mediate anti anxiety & sedative action of alcohol
– Chronic alcohol exposure : blunted NPY activity
(Badia 2003)
– Negative reinforcing effects of alcohol
• Alcohol self administration to alleviate anxiety during
withdrawal (Glenn 2004)

33. Norepinephrine
• During initial stage of alcohol withdrawal
– Excess norepinephrine activity in the region of LC (kovacs 2002)
– Increased level norepinephrine in plasma & CSF (Patkar 2003)
• These findings are indirect indicators of alcohol’s
suppressive action on norepinephrine system
(Ertugrul 2006)

34. Calcium channel
• Effect not only NMDA-dependent Ca channel, but also voltage-gated
calcium channel
• Acute alcohol intake inhibit passage through voltage-gated CA
channel
• chronic alcohol intake causes up-regulation of these channel
(Morad 2003)
• Hyper excitation in alcohol withdrawal is partly due to hyperactivity of
these channel
• CA channel inhibitors is considered useful in treatment of alcohol
withdrawal & in prevention of withdrawal sensitization
(Uzbay 2004)

35. Second Messenger system
• Alcohol inhibits adenosine reuptake
increases extra cellular adenosine
promotes activation of adenosine A2 receptor
activation of cAMP-PKA second messenger system
stimulation of cAMP response-element binding protein (CREB)
gene expression changes via stimulation of CREB transcription
• Changes in gene expression is an important reason of down-regulation
or sensitization of different neurotransmitter system

36. Neurotrophic Factors
• Growth factors important for the development &
maintenance of nervous system
• Ethanol increases NGF (nerve-derived growth factor) &
FGF( fibroblast growth factor) induced signal conduction
to nucleus (Roivainen 1996)
– Produce modulatory effects on neuronal signaling & synaptic
plasticity
– Protect neuronal cells against alcohol’s neurotoxic effects
– Helps to maintain tolerance to alcohol’s effects (Valenzuela 1996)

37. Why do Some Become Addicted?
Occasional /controlled/social use
Chronic/compulsive/uncontrollable use
Genetic variables
Environmental factors
Psychosocial factors
Drug related factors
All
?

38. Drug-related phenomena
contributory to dependence
• Reinforcing effects
• Tolerance to intoxicating effects
• Withdrawal symptoms
• Allostasis

39. Brain Reward Circuitry
Reinforcement :Neuro-anatomical substrates

40. Reinforcement : Neurochemical circuits
Enkephalin
Inhibitory
Neuron
REWARD
Glutamate
Excitatory InputEnkephalin or
Dynorphin
Inhibitory Neuron
GABA
Inhibitory
Neuron
GABA Inhibitory Feedback
Dopamine Neuron GABA
Neuron
Ventral Tegmental Area
(VTA)
Nucleus Accumbens
(NAc)
Dopamine Receptors
GABA-A Receptors
Presynaptic
Opioid
Receptors
(m, d?)
m Opioid
Receptors
k Opioid
Receptors

41. Reinforcing effect of alcohol
• Through wide, but selective action on neurotransmitter
systems in the brain reward system
• GABA system
– Systemic injection of GABAa antagonists decrease operant
alcohol self- administration (Rassnick 1993)
– Selective GABA-b receptor agonist baclofen also decrease alcohol
self administration (Janak 2003)
• Endogenous opioid peptide system (Herz 1997)
– Naltrexone decreases alcohol drinking & self administration in a
variety of animal models (O’Malley 1992)
– clinical use of naltrexone in preventing relapse

42. Reinforcing effect of alcohol
• Mesolimbic dopamine system
– Alcohol self administration increase extra cellular levels of dopamine in
nucleus accumbens in non dependent rats
(Weiss 1993)
– Micro- injections of dopamine receptors antagonists in basal fore-brain
decrease responding for alcohol (Koob 1995)
– Lesion in the mesolimbic dopamine system fail to block operant self
administration of alcohol (Lyness 1992)
• Serotonergic transmission
– increases in the synaptic availability of serotonin with precursor loading
& blockade of serotonin reuptake , decrease alcohol intake
(Sellers 1992)
– Microinjection of 5-HT3 antagonists into amygdala of rats significantly
attenuated alcohol drinking (Kostowski 1995)

43. Reinforcing effect of alcohol
• Other systems attributing to positive
reinforcement
– NMDA glutamate system
– Endocannabinoid system

44. Tolerance
• Progressive shift to right in the concentration-
response function
• gradual decrease in positive reinforcing effects of
alcohol
– Depositional
– Functional : Neuro-adaptive responses to chronic
alcohol exposure
– Down-regulation of GABA, dopamine, endocannabinoids,
neurosteroids, NPY
– Up- regulation of glutamate, calcium channel, CRF

45. Withdrawal & dependence
• A latent state of hyper excitability, representing a
rebound phenomenon from the previously
chronically depressed CNS
• The classic neurotransmitters associated with
regulating the positive reinforcing properties of
alcohol, are compromised during alcohol
withdrawal

46. Withdrawal & dependence
• Negative emotional aspects of withdrawal appear to be more involved
in continued alcohol craving (Glenn 2004)
• Physical withdrawal symptoms are not highly correlated with relapse
in alcoholics
• <25% of the alcoholics attributed physical withdrawal symptoms for
continued drinking whereas >80% of these same patients reported
drinking due to feeling of anxiety, irritability or depressed mood
(Hershon 1997)
• Follow-up study: human alcoholics showed strong correlation between
negative affective symptoms & relapse up to 2 years following
withdrawal (De Soto 1999)
• Rats self-administer alcohol up to 8 weeks post withdrawal in absence
of any physical withdrawal signs (Roberts 2002)

47. Withdrawal & dependence
• protracted withdrawals underscore the chronic nature of
alcohol dependence
• negative affective states are a diving factor in relapse
following long term abstention from alcohol
• The learned association between alcohol & alleviation of
negative affect appears to be critical to excessive drinking
during dependence

48. Withdrawal & dependence
Kindling
• Repeated withdrawals from chronic alcohol intake lead to a
progressive intensification of the withdrawal syndrome
• Increases the severity of physical signs of withdrawals
• Influence the affective & motivational component of withdrawal
• Proved in multiple animal models (Rimodini 2003)
• Results from progressive sensitization of ongoing neuroadaptive
changes with each withdrawal episode
• Stress is capable of facilitating the neuroadaptive processes related to
withdrawal (George 2004)

49. Withdrawal & dependence
Neurochemical basis of physical withdrawal
• Decreased GABA receptor function
• Increased glutamate receptor function
• Increased norepinephrine activity
• Hyperactivity of voltage gated calcium channel

50. Withdrawal & dependence
• Neurobiological basis of the motivational effect of alcohol
withdrawal
– counteradaptive neurochemical events within the brain emotional
system, normally used to maintain emotional homeostasis
(Koob 2001)
– compromised brain reward system as reflected in increase in brain
reward threshold (Schulteis 1996)
• Neurotransmitters involved
– CRF, NPY, dopamine, serotonin, GABA, glutamate

51. Withdrawal & dependence
• Corticotropin-releasing factor
– Increase in extra cellular level of CRF in amygdala during alcohol
withdrawal (Olive 2002)
– Anxiogenic like responses during acute & protracted withdrawal
reversed by intra-cerebral & systemic administration of CRF
antagonist (Breese 2005)
– CRF antagonists that have no effect on alcohol self administration
in non-dependent rats effectively eliminates excessive drinking in
dependent rats
(Valdez 2002)

52. Withdrawal & dependence
• Neuropeptide Y
– Acute withdrawal : decrease in the level of NPY in amygdala & piriform
cortex (Roy 2002)
– NPY administered intra-cerebroventricularly decreases alcohol intake
(Thorsell 2002)
• Dopamine
– Mesolimbic dopamine function is also compromised during alcohol
withdrawal
– Animals self-administered just enough alcohol to return the dopamine
level in the nucleus accumbens back to pre-dependence baseline level
(Weiss 1996
– Reduced dopaminergic neurotransmission is prolonged, outlasting the
physical signs of withdrawal (Bailey 2000)

53. Withdrawal & dependence
• Glutamate receptor system
– Competitive glutamate NMDA antagonists partially
reversed the anxiogenic-like effects of alcohol
withdrawal (Gatch 1999)
• Other systems potentially involved in the
anxiogenic-like effects of alcohol withdrawal
include serotonergic & GABA systems

54. Allostasis
• Allostasis is a concept that was first used to described fluctuation in
blood pressure & immune system function that were inexplicable in
terms of homeostasis.
• Whereas homeostasis refers to the consistency of internal parameters
within a normal range, allostasis describes maintaining a state of
stability outside the normal range in which body varies the parameters
of its physiological systems to match environmental demands
• Allostasis takes into account the concept of tolerance & sensitization
& involves two separate systems : brain reward system & brain stress
regulating system

55. Allostasis
• When alcohol is taken, a positive mood state is experienced followed
by an equally powerful negative affective state.
• Following the negative affect the mood of the alcoholic would return
to a normal homeostatic baseline level
• On repeated administration of alcohol, the positive mood state
diminishes due to tolerance, whereas the subsequent negative affective
state becomes greater due to sensitization
• With progression of time, alcohol use shifts from an state of positive
reinforcement to negative reinforcement state

56. Allostasis
• Chronic elevation in reward threshold
– decrease in the function of GABA, dopamine, serotonin & opioid peptides
in brain reward pathway due to tolerance
• Dysregulation of brain stress system
– involves central nucleus of amygdala, locus ceruleus, & the
hypothalamus
– maintains a balance between CRF & NPY & this balance is crucial in the
regulation of stress, anxiety & depression
– Chronic ethanol intake alter the long term function of CRF & NPY system
• NPY system becomes blunted
• CRF system gets sensitized

57. Allostasis
• Enduring changes in these 2 systems leads to the
development of allostatic state that is represented by
– Chronic deviation of reward set point
– A new set point for mood regulation
• Allostatic state is clinically manifested as loss of control &
compulsive drinking in a failed attempt to regulate their
mood in a homeostatic range
• The neurocircuitry pathology persists into protracted
abstinence, thereby providing a strong motivational basis
for relapse

58. Allostatic Change in Mood State associated
with Transition to Drug Addiction

59. Neuro
biological
risk
factor

60. Neurochemical markers Joelle et al 2002
• Whether vulnerability to alcoholism is associated with pre-
existing abnormality within neurotransmitter system?
• Indices of activity of 5 neurotransmitter systems assessed
in Alcoholics & their children
– GABA, 5-HT, DA, NE, B-endorphine
• 3 criteria to be met for identification of a trait marker
– Heritability
– State independent
– Associated with alcoholism in general population

61. Biological markers Joelle et al 2002
• Studies included
– Baseline studies
– Challenge test studies
– Bio-chemical & neuroimaging studies
– Postmortem studies
– Studies compared neurochemical activities between alcoholics &
unrelated non-alcoholic controls
– Twin & adoption studies
– Studies compared neurochemical activities between individuals at
high risk for alcoholism & low-risk controls

62. Biological markers Joelle et al 2002
• Outcomes
– Increased baseline activity of serotonin transporter
– Increased responsiveness of the pituitary B-
endorphin system to challenges
– Decreased responsiveness in GABA neurotransmission
& reduced baseline GABA level in plasma
– Decreased dopamine receptor reactivity & low plasma
dopamine level

63. Biological markers
• Limitations
– Not assessed in family co-segregation studies
• COGA: collaborative study of the genetics of alcoholism
– Candidate genes on chromosome 4 & 11 reported for genetic
linkage to alcohol dependence ( Long et al 1998)
– GABA receptor gene on chromosome 4 & dopamine receptor
gene on chromosome 11

64. Pharmacotherapy
• Currently approved medications
– Disulfiram
– Acamprosate
– Naltrexone
• Ongoing Clinical Trials
– Nalmefene (opiate receptor antagonist)
– SSRI ( fluxetine, sertraline, citalopram)
– Ondansetron (5HT-3 antagonist)
– Carbamazepine, Topiramate (glutamate? GABA? Na+
channel? Ca++
channel?)
– Rimonabant (CB1 antagonist)
– Baclofen (GABAB antagonist)
– Memantine ( a glutamate antagonist)

65. Conclusion
• Simplest molecule among all substance of abuse
• Wide & complex interaction with neurobiological systems
• No neurotransmitter system spared
• Despite of extensive research & better understanding of
neurobiological basis of its action over last few decades
– only two molecules with limited efficacy for long term pharmacological
intervention
– still to look forward for a better medication
• Warrants more research : large scale-long term clinical trials with
newer molecules which showed efficacy in preclinical studies & small
scale clinical trials

66. hank youT