1. Synaptic Transmission
Effects of Alcohol on
Synaptic Transmission
Background
Although alcohol has been used and abused for thousands of years, the neurobiological mechanisms responsible for its acute behavioral and cognitive effects are, surprisingly, still not fully understood. We also lack a clear understanding of the neurophysiological alterations associated with chronic alcohol exposure and withdrawal that contribute to the etiology of alcoholism. This lecture will present a focused overview of some of the recent advances in our understanding of how acute and chronic alcohol modulate synaptic communication in the mammalian central nervous system, with a focus on how these recent advances may contribute to the development of more effective treatment strategies for alcoholism. For a more complete overview of this topic, please consult the following reviews:
References
Carpenter-Hyland EP, Chandler LJ (2007) Adaptive plasticity of NMDA receptors and dendritic spines: implications for enhanced vulnerability of the adolescent brain to alcohol addiction. Pharmacol Biochem Behav 86:
Criswell HE, Breese GR (2005) A Conceptualization of Integrated Actions of Ethanol Contributing to its GABAmimetic Profile: A Commentary. Neuropsychopharmacology.Gonzales RA, Job MO, Doyon WM (2004) The role of mesolimbic dopamine in the development and maintenance of ethanol reinforcement. Pharmacol Ther 103:
Siggins GR, Roberto M, Nie Z (2005) The tipsy terminal: Presynaptic effects of ethanol. Pharmacol Ther 107:80-98.
Weiner JL, Valenzuela CF (2006) Ethanol modulation of GABAergic transmission: the view from the slice. Pharmacol Ther 111:
Zhang TA, Maldve RE, Morrisett RA (2006) Coincident signaling in mesolimbic structures underlying alcohol reinforcement. Biochem Pharmacol 72:
Ilustration
"The Synapse Revealed.  Created by Graham Johnson of for the Howard Hughes Medical Institute Bulletin ©2004.“

2. Why is Alcohol Used and Abused?
Background
Although it may seem obvious, alcohol is a drug of abuse because of its effects on the brain. These actions must include acute reinforcing effects that increase the likelihood that individuals will continue to consume this drug as well as long term changes that may contribute to the escalation of alcohol intake associated with addictive drinking behavior. What is less obvious is that a growing literature suggests that alcohol is used and abused because of both positive and negative reinforcing effects on the brain. Alcohol drinking is clearly associated with pleasurable effects, typically referred to as euphoric feelings, that can promote further consumption. These euphoric or rewarding feelings are called positive reinforcing effects. There are also effects of alcohol that relieve negative feelings (e.g. stress, anxiety) and these are referred to as negative reinforcing effects. It is likely that distinct neurobiological mechanisms underlie alcohol’s positive and negative reinforcing effects and there is now considerable evidence that both play an important role in the development of alcohol use disorders.
References
Heilig M, Egli M (2006) Pharmacological treatment of alcohol dependence: target symptoms and target mechanisms. Pharmacol Ther 111:
Koob GF, Le Moal M (2008) Addiction and the Brain Antireward System. Annu Rev Psychol 59:29-53.
Le Moal M, Koob GF (2007) Drug addiction: pathways to the disease and pathophysiological perspectives. Eur Neuropsychopharmacol 17:

3. Chemical Synaptic Transmission
Background
Before we discuss the basic mechanisms that may contribute to alcohol’s positive and negative reinforcing effects, it is important to briefly review the primary way that brain cells communicate with each other. The vast majority of interneuronal communication in the CNS is mediated by fast chemical synaptic transmission. In this ultra-rapid process, electrical signals in a presynaptic cell are transiently converted to a chemical signal at a specialized point of apposition between two cells called a synapse. This chemical signal is then rapidly converted back into an electrical signal on the postsynaptic cell. Most neurons in the mammalian CNS receive thousands of excitatory and inhibitory synaptic inputs and the integration of these myriad signals gives rise to the complex processing power of the mammalian CNS.
References
Kandel et al (2000). Principles of Neural Science. New York, NY: McGraw Hill.
Squire et al (2003). Fundamental Neuroscience. San Diego, CA: Elsevier Science.

4. Key Steps in Synaptic Transmission
Ca2+
channel
POSTSYNAPTIC
POTENTIAL
PRESYNAPTIC
TERMINAL
Ligand-Gated
Ion Channels
Background
The key steps in fast synaptic transmission. An action potential, initiated at the axon hillock of the presynaptic cell, propagates to, and depolarizes the presynaptic terminal. Voltage-gated calcium channels in the presynaptic terminal are activated by this depolarizing wave, allowing a rapid and localized increase in calcium at the active zone. This increase in calcium results in the rapid fusion of neurotransmitter-filled vesicles to the presynaptic membrane which then release their contents via exocytosis. The neurotransmitter molecules diffuse across the synaptic cleft where they bind to ligand-gated ion channels which gate the influx of ions into the postsynaptic dendritic bouton. This influx of ions generates an excitatory or inhibitory postsynaptic potential depending on whether the channels are excitatory (glutamatergic) or inhibitory (GABAergic). The neurotransmitter molecules are then taken back up into the presynaptic terminal by active mechanisms. This entire process, from the initiation of the action potential in the presynaptic terminal to the generation of a postsynaptic potential, takes only a couple of milliseconds.
References
Kandel et al (2000). Principles of Neural Science. New York, NY: McGraw Hill.
Squire et al (2003). Fundamental Neuroscience. San Diego, CA: Elsevier Science.
Lisman et al (2007). The sequence of events that underlie quantal transmission at central glutamatergic synapses. Nat Rev: Neurosci. 8,
+++
Neurotransmitter
uptake
POSTSYNAPTIC
DENDRITE

5. EPSP reaches action potential threshold - 45 mV EPSP - 65 mV IPSP
Background
Glutamate is the primary excitatory neurotransmitter and mediates fast excitatory transmission by activating three classes of ion channels (AMPA, Kainate, NMDA) that are primarily permeable to Na+ (and, in some cases, Ca2+). GABA is the primary inhibitory neurotransmitter and mediates fast inhibitory transmission by activating a class of ion channels (GABAA receptors) that are primarily permeable to Cl-. Each of these receptors is comprised of a large and diverse array of subunits and the subunit composition of these ligand-gated ion channels can have profound effects on the physiological and pharmacological properties of these receptors.
Activation of glutamate receptors generates an excitatory postsynaptic potential (EPSP) which depolarizes the membrane potential toward the action potential threshold. If this depolarization is large enough to reach this threshold, it will result in the generation of an “all-or-none” action potential in the postsynaptic cell. Activation of GABAA receptors generates an inhibitory postsynaptic potential (IPSP) which hyperpolarizes the membrane potential, drawing it further away from the action potential threshold. It is important to remember that most neurons receive a continuous barrage of EPSPs and IPSPs and the complex and dynamic process of integrating these signals ultimately gives rise to perception, cognition and other higher brain functions.
References
Mayer ML (2005) Glutamate receptor ion channels. Curr Opin Neurobiol 15:
Schousboe A, Waagepetersen HS (2007) GABA: homeostatic and pharmacological aspects. Prog Brain Res 160:9-19.
Sieghart W (2006) Structure, pharmacology, and function of GABAA receptor subtypes. Adv Pharmacol 54:
- 65 mV
IPSP

6. ETHANOL??? CAFFEINE NICOTINE COCAINE Background
Synapses are major targets for many common drugs that we are familiar with:
1)     Caffeine acts at synapses to block the effects of a neuromodulator called adenosine, which normally inhibits the release of other neurotransmitters, like glutamate
2)     Nicotine mimics some of the actions of the classical neurotransmitter acetylcholine at its postsynaptic receptors
3)     Cocaine acts to inhibit the reuptake of several neurotransmitters, including dopamine, serotonin, and norepinephrine
4)     What about alcohol?
References
Daly JW, Fredholm BB (1998) Caffeine--an atypical drug of dependence. Drug Alcohol Depend 51:
Dani JA, De Biasi M (2001) Cellular mechanisms of nicotine addiction. Pharmacol Biochem Behav 70:
Nestler EJ (2005) Is there a common molecular pathway for addiction? Nat Neurosci 8:
COCAINE

7. Alcohol - + GABA GLUTAMATE Background
Alcohol is classified as a CNS depressant and, although this illustration is clearly an oversimplification, acute alcohol exposure essentially results in a shift in the delicate balance between excitatory glutamatergic synaptic transmission and inhibitory GABAergic transmission in favor of inhibition. This overall increase in inhibitory tone arises, in part, by direct effects of alcohol on glutamatergic and GABAergic synapses. However, this shift also results from many additional interactions between alcohol and other neuronal elements that mediate and regulate synaptic communication (e.g. other neurotransmitter systems, voltage-gated channels, second messenger systems, etc…).
References
Brunton L, Parker K, Lazo J, Buxton I, Blumenthal D (2005) Goodman and Gilman's Pharmacological Basis of Therapeutics, 11 Edition: McGraw-Hill.

8. Chronic Alcohol - + GABA GLUTAMATE Background
Chronic alcohol exposure also has profound effects on synaptic communication. Again, although oversimplified by this figure, repeated alcohol exposure and withdrawal typically result in compensatory changes that oppose the acute effects of this drug. The end result is that withdrawal from chronic alcohol leads to a profound increase in excitability in many brain regions. In the most severe situations, this can result in an acute withdrawal syndrome which is characterized by seizures and delerium tremens. However, a milder form of this imbalance likely persists for a much longer time frame (weeks to months) and may contribute to the negative affective state associated with protracted withdrawal. It is likely that these persistent neuroplastic changes contribute to the increase in the saliency of alcohol’s negative reinforcing effects and may well represent important targets for the development of more effective treatments for AUDs.

9. Electrophysiological Methods
in Alcohol Research
Background
Recent methodological advances in the field of electrophysiology have greatly facilitated the study of how alcohol, and many other drugs, influence the activity of the ion channels that underlie fast synaptic transmission. These methods can be employed to study the function of recombinant channels in single cell expression systems (e.g. Xenopus oocytes, HEK cells) or native channels in acutely prepared brain slices. When used in brain slice preparations, these methods can be used to distinguish between pre- and postsynaptic mechanisms of drug action and are also well suited to study neuroadaptive changes in synaptic function associated with chronic drug exposure.
References
Hamill OP, Marty A, Neher E, Sakmann B, Sigworth FJ (1981) Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches. Pflugers Arch 391:
Kandel et al (2000). Principles of Neural Science. New York, NY: McGraw Hill.
Weiner JL (2000) Electrophysiological assessment of synaptic transmission in brain slices. In: Methods in Alcohol-Related Neuroscience Research (Liu Y, Lovinger DM, eds), pp Boca Raton, Fl.: CRC Press.

10. GABAA Receptor Blockers
GLU
GABA
Brain
Slice +
Glutamate Receptor
Blockers +
GABAA Receptor Blockers
aCSF
CA1
Pyramidal
Cell
Background
The basic steps involved in “whole-cell patch” recording of synaptic currents. Brain slices (appr. 0.5 mm thick) are acutely prepared from a region of interest and incubated in an oxygenated artificial cerebrospinal fluid solution. After a recovery period of min., slices are transferred to a recording chamber for electrophysiological recording. A glass microelectrode with a tip diameter of around 1 m is filled with a solution designed to mimic key elements of the intracellular milieu. A metal electrode in contact with the electrolyte solution in the patch pipette connects the recording electrode to a sensitive electrical circuit that can measure the current flowing across the electrode tip. The glass electrode is then slowly lowered onto the outer surface of a neuron, usually under visual guidance using a microscope equipped with specialized optics. Once the tip of the electrode touches the neuronal membrane, a small amount of pressure is applied and this typically results in the rapid formation of a tight seal between the neuronal membrane and the recording electrode. After seal formation, additional pressure is applied to rupture the seal, resulting in low resistance electrical access to the inside of the cell. Because much of the local neurocircuitry is preserved in brain slices, synaptic currents can be readily evoked by electrical stimulation of afferent inputs to the cell being recorded. Since excitatory and inhibitory inputs usually overlap, pharmacological approaches are usually used to “isolate” a synaptic current of interest. Since the direction of a given synaptic current can be profoundly influenced by experimental parameters (e.g. ion gradients across the cell membrane), it is also important to use pharmacological approaches to confirm the identity of a synaptic current under study.
References
Hamill OP, Marty A, Neher E, Sakmann B, Sigworth FJ (1981) Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches. Pflugers Arch 391:
Kandel et al (2000). Principles of Neural Science. New York, NY: McGraw Hill.
Weiner JL (2000) Electrophysiological assessment of synaptic transmission in brain slices. In: Methods in Alcohol-Related Neuroscience Research (Liu Y, Lovinger DM, eds), pp Boca Raton, Fl.: CRC Press.

11. Alcohol and GABAergic Inhibition
Model of the
GABAA Receptor
Background
GABAergic interneurons are ubiquitously distributed throughout the central nervous system and play an integral role in regulating neuronal excitability and synchronous activity. There is now compelling evidence from behavioral studies that acute and chronic alcohol exposure exert a profound effect on GABAergic synaptic inhibition. For example, alcohol shares many behavioral effects with drugs that are known to bind to, and allosterically modulate, GABAA receptor function (e.g. benzodiazepines, barbiturates) such as sedation, hypnosis, ataxia, and anxiolysis. Cross tolerance also develops between alcohol and classical GABAA receptor modulators and GABAA receptor agonists can substitute for alcohol in drug discrimination studies.
References
Freund TF, Buzsaki G (1996) Interneurons of the hippocampus. Hippocampus 6:
Freund TF, Katona I (2007) Perisomatic inhibition. Neuron 56:33-42.
McBain CJ, Fisahn A (2001) Interneurons unbound. Nat Rev Neurosci 2:11-23.
Trudell JR, Bertaccini E (2004) Comparative modeling of a GABAA alpha1 receptor using three crystal structures as templates. J Mol Graph Model 23:39-49.
Freund and Buzsaki, 1996
Trudell and Bertaccini, 2004

12. Acute Alcohol Exposure Potentiates GABAergic Synaptic Transmission
RAT AMYGDALA
MONKEY DENTATE GYRUS
Figure Deleted – Awaiting Copyright Permissions
Background
Many studies have employed the whole-cell patch recording technique to characterize the acute effects of alcohol on GABAergic synaptic responses. These kinds of studies have been carried out in a variety of species, from mice to non-human primates, and in many different brain regions. The majority of these studies have found that pharmacologically relevant concentrations of alcohol significantly enhance GABAA IPSPs or IPSCs. The figures above are from representative studies illustrating that alcohol enhances GABAA IPSCs in the rodent central nucleus of the amygdala and the monkey dentate gyrus.
References
Ariwodola OJ, Crowder TL, Grant KA, Daunais JB, Friedman DP, Weiner JL (2003) Ethanol modulation of excitatory and inhibitory synaptic transmission in rat and monkey dentate granule neurons. Alcoholism: Clinical & Experimental Research 27.
Roberto M MS, Moore SD, Tallent MK, Siggins GR. (2003) Ethanol increases GABAergic transmission at both pre- and postsynaptic sites in rat central amygdala neurons. Proc Natl Acad Sci U S A 100:
Criswell HE, Breese GR (2005) A Conceptualization of Integrated Actions of Ethanol Contributing to its GABAmimetic Profile: A Commentary. Neuropsychopharmacology.Gonzales RA, Job MO, Doyon WM (2004) The role of mesolimbic dopamine in the development and maintenance of ethanol reinforcement. Pharmacol Ther 103:
Siggins GR, Roberto M, Nie Z (2005) The tipsy terminal: Presynaptic effects of ethanol. Pharmacol Ther 107:80-98.
Weiner JL, Valenzuela CF (2006) Ethanol modulation of GABAergic transmission: the view from the slice. Pharmacol Ther 111:
Roberto et al, 2003
Ariwodola et al, 2003

13. RAT HIPPOCAMPAL CA1 PYRAMIDAL CELLS
Presynaptic Mechanisms Contribute to Alcohol Potentiation of GABAergic Inhibition
RAT CEREBELLAR
GRANULE CELLS
RAT HIPPOCAMPAL CA1 PYRAMIDAL CELLS
Figure Deleted – Awaiting Copyright Permissions
Background
Most of the early studies investigating alcohol potentiation of GABAergic synapses attributed this effect to a postsynaptic mechanism, similar to that of benzodiazepines and barbiturates, which allosterically enhance GABAA receptor function. Surprisingly, a number of recent studies have revealed that, in many brain regions, alcohol actually enhances GABAergic synaptic inhibition through a presynaptic mechanism, by increasing GABA release. This presynaptic effect is most often detected as an increase in the frequency (but not the shape) of spontaneous GABAA IPSCs and may represent the primary mechanism through which alcohol enhances GABAergic synapses in a number of brain areas, including the hippocampus and cerebellum (illustrated above).
References
Carta M, Mameli M, Valenzuela CF (2004) Alcohol enhances GABAergic transmission to cerebellar granule cells via an increase in Golgi cell excitability. J Neurosci 24:
Li Q, Wilson WA, Swartzwelder HS (2006) Developmental differences in the sensitivity of spontaneous and miniature IPSCs to ethanol. Alcohol Clin Exp Res 30:
Siggins GR, Roberto M, Nie Z (2005) The tipsy terminal: Presynaptic effects of ethanol. Pharmacol Ther 107:80-98.
Weiner JL, Valenzuela CF (2006) Ethanol modulation of GABAergic transmission: the view from the slice. Pharmacol Ther 111:
Li et al, 2006
Carta et al, 2004

14. Tonic vs. Phasic GABAergic Inhibition
Background
In addition to fast evoked or phasic GABAergic inhibition, there is also a tonic or background GABAergic conductance in many brain regions. This tonic form of inhibition is mediated by ambient levels of GABA activating specialized classes of GABAA receptors that reside outside of the synaptic cleft. These extrasynaptic receptors are sensitive to low concentrations of GABA and exhibit minimal desensitization and thus provide a continuous or steady-state inhibitory tone. Although the physiological role of tonic GABA currents are not fully understood, this form of inhibition has profound effects on excitability and network activity and extrasynaptic GABAA receptors may represent important targets for some drugs and endogenous neuromodulators (see supplementary comments)
References
Farrant M, Nusser Z (2005) Variations on an inhibitory theme: phasic and tonic activation of GABA(A) receptors. Nat Rev Neurosci 6:
Glykys J, Mody I (2007) Activation of GABAA receptors: views from outside the synaptic cleft. Neuron 56:
mIPSC
Evoked IPSC
Farrant and Nusser, 2005

15. Alcohol Enhances Tonic GABAergic Inhibition
DENTATE GRANULE CELLS
Wei et al, 2004
THALAMIC NEURONS
CEREBELLAR GRANULE CELLS
Background
A large number of studies have demonstrated that acute alcohol exposure significantly potentiates tonic GABAergic inhibition. This effect is usually detected as an increase in the GABAA receptor-dependent component of the resting conductance, as illustrated in the data from recent studies in the dentate gyrus, thalamus, and cerebellum. Alcohol potentiation of tonic GABAergic inhibition is quite potent, with significant effects being reported at concentrations as low as mM ethanol, well within the pharmacologically relevant concentration range.
References
Carta M, Mameli M, Valenzuela CF (2004) Alcohol enhances GABAergic transmission to cerebellar granule cells via an increase in Golgi cell excitability. J Neurosci 24:
Jia F, Pignataro L, Harrison NL (2007) GABAA receptors in the thalamus: alpha4 subunit expression and alcohol sensitivity. Alcohol 41:
Wei W, Faria LC, Mody I (2004) Low ethanol concentrations selectively augment the tonic inhibition mediated by delta subunit-containing GABAA receptors in hippocampal neurons. J Neurosci 24:
Wallner M, Hanchar HJ, Olsen RW (2006) Low dose acute alcohol effects on GABA A receptor subtypes. Pharmacol Ther 112:
Carta et al, 2004
Jia et al, 2007

16. Chronic Alcohol Increases Anxiety and Decreases GABAA Receptor Function
Background
Chronic alcohol exposure and withdrawal are associated with a wide spectrum of behaviors, ranging from increases in anxiety to seizures and delirium tremens. Notably, chronic alcohol also results in profound alterations in GABAergic synaptic inhibition, such as changes in the expression of certain GABAA receptor subunits, that are generally associated with a decrease in GABAergic inhibition. The data illustrated in this slide are from a study that employed a rodent model of chronic alcohol exposure to show that alcohol withdrawal is associated with increases in anxiety-like behavior (assessed on the elevated plus-maze) as well as a shortening of the duration of GABAA IPSCs, likely due, in part, to a decrease in the expression of the 1 subunit of the GABAA receptor. Given that chronic alcohol exposure is generally associated with a decrease in GABAergic inhibition, it is not surprising that positive modulators of synaptic GABAA receptors, such as benzodiazepines, are highly efficacious in the treatment of acute alcohol withdrawal symptoms.
References
Cagetti E, Liang J, Spigelman I, Olsen RW (2003) Withdrawal from chronic intermittent ethanol treatment changes subunit composition, reduces synaptic function, and decreases behavioral responses to positive allosteric modulators of GABAA receptors. Mol Pharmacol 63:53-64.
Myrick H, Anton RF (1998) Treatment of alcohol withdrawal. Alcohol Health Res World 22:38-43.
Rathlev NK, Ulrich AS, Delanty N, D'Onofrio G (2006) Alcohol-related seizures. J Emerg Med 31:
Cagetti et al, 2003

17. Alcohol Potentiation of GABAergic Synapses is not Reduced After Chronic Alcohol Treatments
Figure Deleted – Awaiting Copyright Permissions
Background
Several recent studies have also revealed that chronic alcohol exposure does not result in the development of tolerance to the acute potentiating effects of alcohol on GABAergic synapses. In fact, some studies have reported that alcohol facilitation of GABAergic synapses may actually sensitize after repeated alcohol exposure. The persistence (or sensitization) of alcohol’s facilitatory effects on GABAergic inhibition suggest that this effect may contribute to the increased saliency of alcohol’s negative reinforcing effects that are thought to develop following repeated alcohol exposure. If true, then drugs that could attenuate these effects of alcohol may prove particularly effective in the treatment of alcohol dependence.
References
Kang MH, Spigelman I, Olsen RW (1998) Alteration in the sensitivity of GABA(A) receptors to allosteric modulatory drugs in rat hippocampus after chronic intermittent ethanol treatment. Alcoholism: Clinical & Experimental Research 22:
Liang J, Zhang N, Cagetti E, Houser CR, Olsen RW, Spigelman I (2006) Chronic intermittent ethanol-induced switch of ethanol actions from extrasynaptic to synaptic hippocampal GABAA receptors. J Neurosci 26:
Roberto M, Madamba SG, Stouffer DG, Parsons LH, Siggins GR (2004) Increased GABA release in the central amygdala of ethanol-dependent rats. Journal of Neuroscience 24:
Roberto et al, 2004
Kang et al, 1998

18. Local Amygdalar Infusion of a GABAA Agonist Reduces Alcohol Self-Administrationin Dependent Rats
Background
Taken together, acute alcohol exposure enhances both tonic and phasic GABAergic inhibition in many brain regions. Withdrawal from chronic alcohol exposure results in decreased GABAergic transmission as well as behavioral changes consistent with decreased GABAergic tone (e.g. increased anxiety). Interestingly, several models have been developed to show that chronic alcohol exposure engenders significant increases in alcohol intake. The data in this slide illustrate that local administration of a GABA agonist into the amygdala selectively decreases alcohol self-administration in dependent, but not control, rats, possibly by reducing the negative affective symptoms associated with alcohol withdrawal. Since those early observations, a growing number of preclinical and clinical studies have demonstrated that a range of drugs that either boost GABAergic inhibition and/or alleviate negative symptoms associated with alcohol withdrawal (e.g. anxiety) are particularly efficacious in reducing alcohol drinking and craving in dependent subjects (e.g. baclofen, tiagabine, gabapentin, CRF1 antagonists, neurokinin 1 antagonists)
References
Koob GF (2004) A role for GABA mechanisms in the motivational effects of alcohol. Biochem Pharmacol 68:
Heilig M, Egli M (2006) Pharmacological treatment of alcohol dependence: target symptoms and target mechanisms. Pharmacol Ther 111:
Koob, 2004

19. Alcohol and Glutamatergic Excitation
Background
Glutamate mediates the vast majority of fast excitatory synaptic transmission in the mammalian CNS and glutamatergic synapses make up more than half of all synaptic contacts in the brain. As such, glutamatergic synaptic communication plays an essential role in virtually all aspects of brain function, both under normal and pathophysiological conditions. As with the GABAergic synapse, there is abundant evidence that alcohol acts, at least in part, by modulating the function of glutamatergic synapses. For example, alcohol shares some of the memory impairing and sedative effects of drugs like MK-801, which block the NMDA subtype of ionotropic glutamate receptors and a number of glutamatergic modulators have been shown to reduce alcohol self-administration as well as alcohol’s rewarding effects.
References
Andersen P, Soleng AF (1999) A thorny question: how does activity maintain dendritic spines? Nat Neurosci 2:5-7.
Chandler LJ (2003) Ethanol and brain plasticity: receptors and molecular networks of the postsynaptic density as targets of ethanol. Pharmacol Ther 99:
Gass JT, Olive MF (2008) Glutamatergic substrates of drug addiction and alcoholism. Biochem Pharmacol 75:
Lendvai B, Stern EA, Chen B, Svoboda K (2000) Experience-dependent plasticity of dendritic spines in the developing rat barrel cortex in vivo. Nature 404:
Anderen and Soleng, 1999
Lendvai et al, 2000

20. Acute Alcohol Exposure Inhibits NMDA Receptor Function
Alcohol inhibition of NMDA-evoked currents in cultured rat hippocampal cells
Lovinger et al, 1990
Background
In contrast to the complex pre- and postsynaptic mechanisms that have been shown to mediate the acute effects of alcohol at GABAergic synapses, alcohol appears to exert a more straightforward modulatory effect on glutamatergic synaptic excitation. Many studies, from the early 1990s to the present, have consistently shown that pharmacologically relevant concentrations of alcohol interact directly with the NMDA subtype of ionotropic glutamate receptor to inhibit the activity of these receptors. This inhibitory effect has been observed in the hippocampus and bed nucleus of the terminalis (illustrated above) as well as many other brain regions. Alcohol has also been shown to significantly inhibit NMDA-evoked currents in isolated or cultured neurons or in cells expressing recombinant NMDA receptor subunits, thus confirming the postsynaptic nature of this inhibitory effect. In fact, recent studies have begun to identify specific amino acids on NMDA receptor subunits that may represent a putative alcohol interaction site. For the most part, alcohol has relatively minimal effects on the other subtypes of ionotropic glutamate receptors although, in a few specific brain regions, alcohol has been shown to have relatively potent inhibitory effects on AMPA and kainate receptor-gated synapses.
References
Gass JT, Olive MF (2008) Glutamatergic substrates of drug addiction and alcoholism. Biochem Pharmacol 75:
Kash TL, Matthews RT, Winder DG (2008) Alcohol Inhibits NR2B-Containing NMDA Receptors in the Ventral Bed Nucleus of the Stria Terminalis. Neuropsychopharmacology. 33:
Lovinger DM, White G, Weight FF (1990) NMDA receptor-mediated synaptic excitation selectively inhibited by ethanol in hippocampal slice from adult rat. Journal of Neuroscience 10:
Ren H, Salous AK, Paul JM, Lamb KA, Dwyer DS, Peoples RW (2008) Functional Interactions of Alcohol-sensitive Sites in the N-Methyl-D-aspartate Receptor M3 and M4 Domains. J Biol Chem 283:
Smothers CT, Woodward JJ (2006) Effects of amino acid substitutions in transmembrane domains of the NR1 subunit on the ethanol inhibition of recombinant N-methyl-D-aspartate receptors. Alcohol Clin Exp Res 30:
Alcohol inhibition of NMDA EPSCs in the rat ventral bed nucleus of the stria terminalis
Kash et al, 2008

21. Chronic Alcohol Increases Glutamatergic Excitability
Background
Again, as observed at GABAergic synapses, the effects of repeated alcohol exposure on glutamatergic synapses are generally the opposite of the acute effects of this drug. Thus, chronic alcohol exposure and withdrawal have been shown to lead to increases in the expression and function of NMDA receptors in many brain regions. A recent study by Lack et al (2007) found that exposing rats to chronic alcohol vapor for 10 days (12 hrs/day) results in a significant increase in the NMDA component of glutamatergic synaptic transmission in the basolateral amygdala. This increase in NMDA receptor function is apparent immediately after the chronic alcohol treatment (CIE) and after a 24 hour withdrawal period, a time when anxiety-like behavior is also significantly increased. Another recent report by Hendricson and colleagues (2007), has shown that a similar increase in NMDA receptor function in the hippocampus contributes to epileptiform discharges observed after alcohol withdrawal. Alcohol withdrawal (EtOH WD) results in the appearance of epileptiform spike patterns that are effectively blocked by treatment with APV, an NMDA receptor antagonist.
References
Hendricson AW, Maldve RE, Salinas AG, Theile JW, Zhang TA, Diaz LM, Morrisett RA (2007) Aberrant synaptic activation of N-methyl-D-aspartate receptors underlies ethanol withdrawal hyperexcitability. J Pharmacol Exp Ther 321:60-72.
Krystal JH, Petrakis IL, Mason G, Trevisan L, D'Souza DC (2003) N-methyl-D-aspartate glutamate receptors and alcoholism: reward, dependence, treatment, and vulnerability. Pharmacol Ther 99:79-94.
Lack AK, Diaz MR, Chappell A, DuBois DW, McCool BA (2007) Chronic ethanol and withdrawal differentially modulate pre- and postsynaptic function at glutamatergic synapses in rat basolateral amygdala. J Neurophysiol 98:
Tsai G, Coyle JT (1998) The role of glutamatergic neurotransmission in the pathophysiology of alcoholism. Annual Review of Medicine 49:
Lack et al, 2007
Hendricson et al, 2007

22. Acamprosate Decreases Alcohol Drinking and the Neurotoxicity Associated with Alcohol Withdrawal
Background
Numerous preclinical and clinical studies have demonstrated that a drug called acamprosate is particularly effective at reducing alcohol drinking and relapse in dependent individuals. For example, when rats with a history of alcohol drinking are reintroduced to alcohol after a period of forced abstinence, they often exhibit a significant increase in alcohol drinking, termed the alcohol deprivation effect. Heyser et al (1998) demonstrated that acamprosate blocks this alcohol deprivation effect while having no significant effect on baseline alcohol drinking or water consumption. Although the mechanism of acamprosate action is not fully known, recent studies suggest that it may act, in part by dampening the increase in NMDA receptor activity associated with chronic alcohol exposure. For example, John Littleton and colleagues (DeWitte et al, 2005) have shown that alcohol withdrawal leads to an increase in cell death in hippocampal slice cultures. This necrosis can be significantly attenuated by acamprosate, an effect similar to treatment of slice cultures with an NMDA receptor antagonist (MK-801).
References
De Witte P, Littleton J, Parot P, Koob G (2005) Neuroprotective and abstinence-promoting effects of acamprosate: elucidating the mechanism of action. CNS Drugs 19:
Heyser CJ, Schulteis G, Durbin P, Koob GF (1998) Chronic acamprosate eliminates the alcohol deprivation effect while having limited effects on baseline responding for ethanol in rats. Neuropsychopharmacology 18:
De Witte et al, 2005
Heyser et al, 1998

23. Alcohol and Dopaminergic Signaling
Figure Deleted – Awaiting Copyright Permissions
Background
Drug and alcohol addiction are thought to result, in part, from the acute positive reinforcing (pleasurable) effects associated with drug taking. Interestingly, despite their diverse mechanisms of action, all drugs of abuse seem to converge on a common “reward” circuit in the limbic system that includes dopamine cells in the ventral tegmental area and some of the brain regions innervated by these dopaminergic cells, such as the nucleus accumbens and prefrontal cortex. Acute exposure to virtually all drugs of abuse, including alcohol, results in a significant increase in dopaminergic signaling in these pathways and this increase in mesolimbic dopamine levels is thought to mediate the pleasurable or euphoric feelings associated with drug self-administration. For example, Boileau and colleagues (2003) used a combination of PET imaging and MRI in a group of nonalcoholic moderate drinkers to demonstrate that consumption of an intoxicating dose of alcohol results in a significant increase in dopamine levels in the ventral striatum.
References
Nestler EJ (2005) Is there a common molecular pathway for addiction? Nat Neurosci 8:
Boileau I, Assaad JM, Pihl RO, Benkelfat C, Leyton M, Diksic M, Tremblay RE, Dagher A (2003) Alcohol promotes dopamine release in the human nucleus accumbens. Synapse 49:
Boileau et al, 2003

24. DA NEURONS IN RAT VTA DA NEURONS IN MOUSE MID BRAIN SLICES
Acute Alcohol Exposure Enhances Dopamine Release in the Mesolimbic “Reward” Circuit
DA NEURONS IN RAT VTA
DA NEURONS IN MOUSE
MID BRAIN SLICES
Figure Deleted – Awaiting Copyright Permissions
Background
A number of neurobiological studies have examined the mechanisms through which acute alcohol exposure increases dopamine levels in the mesolimbic system. Results of these studies suggest that alcohol enhances dopamine release by directly increasing the firing rate of dopamine cells in the VTA.
References
Brodie MS, Appel SB (1998) The effects of ethanol on dopaminergic neurons of the ventral tegmental area studied with intracellular recording in brain slices. Alcoholism: Clinical & Experimental Research 22:
Okamoto T, Harnett MT, Morikawa H (2006) Hyperpolarization-activated cation current (Ih) is an ethanol target in midbrain dopamine neurons of mice. J Neurophysiol 95:
Okamoto et al, 2005
Brodie et al, 1998

25. Chronic Alcohol Results in Significant Perturbations in Dopamine Signaling
Background
As with the GABAergic and glutamatergic systems, chronic alcohol exposure and withdrawal result in profound neuroadaptive changes in dopaminergic transmission that, in general, oppose the acute stimulatory effects of alcohol on this system. For example, Volkow and colleagues (2007) used PET imaging techniques to study the dopamine circuitry in detoxified alcoholics and control subjects. They noted a significant decrease in baseline dopamine receptor levels (D2/D3), as revealed by a lower binding potential for the D2/D3 selective ligand [11C]raclopride under placebo conditions. They also observed a significant decrease in dopamine release in alcoholic subjects after a challenge dose of the dopamine re-uptake blocker, methylphenidate (MP), inferred by the smaller change in raclopride binding after the MP treatment in alcoholic subjects. Collectively, these decreases in dopaminergic signaling are thought to contribute to the negative affective state associated with alcohol dependence and withdrawal as well as the decrease in the rewarding properties of alcohol often reported in alcohol dependent individuals.
References
Nestler EJ (2005) Is there a common molecular pathway for addiction? Nat Neurosci 8:
Volkow ND, Wang GJ, Telang F, Fowler JS, Logan J, Jayne M, Ma Y, Pradhan K, Wong C (2007) Profound decreases in dopamine release in striatum in detoxified alcoholics: possible orbitofrontal involvement. J Neurosci 27:
Volkow et al, 2007

26. Naltrexone Reduces Alcohol Stimulation of Dopamine Release and Decreases Alcohol Intake
Background
Given the importance of alcohol’s acute effects on mesolimbic dopamine signaling in promoting abusive alcohol drinking behavior, it would seem that drugs that could attenuate this effect might be effective treatments for alcohol dependence. Indeed, naltrexone, a relatively selective -opioid receptor antagonist, appears to support this hypothesis. Naltrexone has been FDA-approved for the treatment of alcoholism for over a decade and the majority of clinical studies indicate that it is moderately effective in reducing alcohol intake and relapse in recovering alcoholics. Although the specific mechanism responsible for these therapeutic effects in humans are not fully understood, some preclinical rodent studies have demonstrated that naltrexone can significantly attenuate the increase in nucleus accumbens dopamine associated with alcohol consumption and also decrease voluntary alcohol intake. Collectively, these and other ongoing studies suggest that drugs that can blunt the acute stimulatory effects of alcohol on dopamine signaling and/or normalize the dopaminergic hypofunction that results from a history of alcohol abuse may be effective pharmacotherapies for the treatment of alcoholism.
References
Gonzales RA, Weiss F (1998) Suppression of ethanol-reinforced behavior by naltrexone is associated with attenuation of the ethanol-induced increase in dialysate dopamine levels in the nucleus accumbens. Journal of Neuroscience 18:
Modesto-Lowe V, Fritz EM (2005) The opioidergic-alcohol link : implications for treatment. CNS Drugs 19:
Heilig M, Egli M (2006) Pharmacological treatment of alcohol dependence: target symptoms and target mechanisms. Pharmacol Ther 111:
Gonzales and Weiss, 1998

27. SUMMARY
Acute alcohol exposure has profound effects on chemical synaptic transmission
Increase in synaptic and extrasynaptic GABAergic inhibition
Decrease in NMDA receptor function
Increase in the firing rate of VTA dopamine cells
Chronic alcohol exposure and withdrawal lead to persistent changes in synaptic transmission that often oppose the acute effects of this drug
Decrease in GABAergic inhibition
Increase in glutamatergic excitation
Dopaminergic hypofunction
Background
In summary, over the past couple of decades many important advances have been made in unraveling the complex mechanisms through which acute and chronic alcohol modulate synaptic communication in the brain. For example, acute alcohol exposure has been shown to enhance synaptic and extrasynaptic GABAergic inhibition through a dizzying array of pre- and postsynaptic mechanisms. In addition, a wide range of neuroadaptive changes in chemical synapses have been shown to develop following repeated alcohol exposure and withdrawal and some of these changes likely contribute to the development of alcoholism. Indeed, many of the new generation of drugs that are being developed for the treatment of alcohol addiction seem to target the very synapses that are most sensitive to the acute and chronic effects of this drug.
Despite the many advances that have been made, there is still much work to be done. Although numerous synaptic targets of alcohol have been identified, we still lack direct evidence of the behavioral relevance of many of alcohol’s synaptic effects. New experimental strategies are needed to shed light on the synaptic mechanisms that are most important in contributing to the behavioral changes associated with the development of alcohol dependence. It is also apparent that, like other neurological diseases, there are likely a wide variety of inter-related neurobiological pathways that can lead to the development of alcohol dependence. For example, positive and negative reinforcing effects of alcohol may play differential roles in maintaining addictive drinking behavior in certain subsets of alcoholics. The integration of basic neurobiological approaches along with good animal models of alcohol dependence represents a promising approach to gain new insight into the complex etiology alcoholism.

28. SUMMARY (con’t)
Although recent advances have shed considerable light on the mechanisms through which acute and chronic alcohol modulate synaptic transmission, new experimental strategies are needed to begin to address the behavioral relevance of alcohol’s many synaptic effects
Recent preclinical and clinical studies suggest that a better understanding of the neurobiological mechanisms responsible for the acute and long term behavioral consequences of alcohol drinking will greatly facilitate the development of more effective treatment strategies for alcoholism