NEUROTRANSMITTER REVIEW THE PRINCIPLES OF NERVE CELL COMMUNICATION Storage vesicle he nerve cell, or neuron, is the key player in the Cell activity of the nervous system. It conveys informa- membrane Ttion both electrically and chemically. Within the neuron itself, information is passed along through the movement of an electrical charge (i.e., impulse). The neu- ron has three main components: (1) the dendrites, thin fibers that extend from the cell in branched tendrils to Neurotransmitter release receive information from other neurons; (2) the cell body, Presynaptic nerve cell which carries out most of the neuron’s basic cellular func- tioning; and (3) the axon, a long, thin fiber that carries Receptor binding nerve impulses to other neurons. Nerve signals often travel over long distances in the body. For example, if you step barefooted on a sharp object, Cell G membrane the sensory information is relayed from your foot all the way protein to the brain; from there, nerve signals travel back to the leg Receptor Ligand-gated muscles and cause them to contract, drawing back the foot. Postsynaptic ion channel Dozens of neurons can be involved in such a circuit, necessi- nerve cell tating a sophisticated communication system to rapidly convey signals between cells. Also, because individual neu- Signal transmission across the synaptic cleft. The binding rons can be up to 3 feet long, a rapid-relay mechanism with- of neurotransmitters (shown as triangles) to receptors that in the neurons themselves is required to transmit each signal act as ligand-gated ion channels causes these channels to from the site where it is received to the site where it is open, leading in some cases to a depolarization of the part of the membrane closest to the channel. Depolarization passed on to a neighboring cell. Two mechanisms have results in the opening of other ion channels, which in turn evolved to transmit nerve signals. First, within cells, electri- may generate an action potential. Neurotransmitters cal signals are conveyed along the cell membrane. Second, (shown as circles) that bind to second messenger-linked for communication between cells, the electrical signals gen- receptors initiate a complex cascade of chemical events erally are converted into chemical signals conveyed by small that can produce changes in cell function. In this schematic, messenger molecules called neurotransmitters. the first component of such a signaling cascade is a G protein. SIGNAL TRANSMISSION WITHIN NERVE CELLS Signal Transmission Between Cells The mechanism underlying signal transmission within Communication among neurons typically occurs across neurons is based on voltage differences (i.e., potentials) microscopic gaps called synaptic clefts. Each neuron may communicate with hundreds of thousands of other neurons. that exist between the inside and the outside of the cell. A neuron sending a signal (i.e., a presynaptic neuron) re- This membrane potential is created by the uneven distribu- leases a chemical called a neurotransmitter, which binds to tion of electrically charged particles, or ions, the most a receptor on the surface of the receiving (i.e., postsynaptic) important of which are sodium (Na+), potassium (K+), - 2+ neuron. Neurotransmitters are released from presynaptic chloride (Cl ), and calcium (Ca ). Ions enter and exit the terminals, which may branch to communicate with several cell through specific protein channels in the cell’s mem- postsynaptic neurons. Dendrites are specialized to receive brane. The channels “open” or “close” in response to neuronal signals, although receptors may be located else- neurotransmitters or to changes in the cell’s membrane where on the cell. Approximately 100 different neurotrans- potential. The resulting redistribution of electric charge mitters exist. Each neuron produces and releases only one may alter the voltage difference across the membrane. A or a few types of neurotransmitters, but can carry receptors decrease in the voltage difference is called depolarization. on its surface for several types of neurotransmitters. If depolarization exceeds a certain threshold, an impulse To cross the synaptic cleft, the cell’s electrical message (i.e., action potential) will travel along the neuron. Various must be converted into a chemical one. This conversion mechanisms ensure that the action potential propagates in takes place when an action potential arrives at the axon tip, only one direction, toward the axon tip. The generation of resulting in depolarization. The depolarization causes Ca2+ an action potential is sometimes referred to as “firing.” to enter the cell. The increase in intracellular Ca2+ concen- VOL. 21, NO. 2, 1997 107 NEUROTRANSMITTER REVIEW tration triggers the release of neurotransmitter molecules complex network of pathways and feedback loops. The into the synaptic cleft. integrated activity of these circuits regulates mood, activi- Two large groups of receptors exist that elicit specific ty, and the behaviors that may underlie disorders such as responses in the receptor cell: Receptors that act as ligand- alcoholism. I gated ion channels result in rapid but short-lived responses, whereas receptors coupled to second-messenger systems induce slower but more prolonged responses. ALCOHOL AND DOPAMINE Ligand-Gated Channel Receptors. When a neurotransmit- ter molecule binds to a receptor that acts as a ligand-gated Gaetano Di Chiara, M.D. ion channel, a channel opens, allowing ions to flow across the membrane (see figure). The flow of positively charged ions into the cell depolarizes the portion of the membrane Dopamine is a neuromodulator that is used by neurons nearest the channel. Because this situation is favorable to in several brain regions involved in motivation and re- the subsequent generation of an action potential, ligand- inforcement, most importantly the nucleus accumbens gated channel receptors that are permeable to positive ions (NAc). Dopamine alters the sensitivity of its target neu- are called excitatory. rons to other neurotransmitters, particularly glutamate. Other ligand-gated channels are permeable to negatively In addition, dopamine can affect the neurotransmitter charged ions. An increase of negative charge within the cell release by the target neurons. Dopamine-containing makes it more difficult to excite the cell and induce an action neurons in the NAc are activated by motivational stim- potential. Such channels accordingly are called inhibitory. uli, which encourage a person to perform or repeat a Second Messenger-Linked Receptors. Second messengers behavior. Even low alcohol doses can increase (e.g., G proteins) are molecules that help relay signals from dopamine release in part of the NAc. This dopamine re- the cell’s surface to its interior. Neurotransmitters that bind lease may contribute to the rewarding effects of alcohol to second messenger-linked receptors, such as dopamine, and may thereby play a role in promoting alcohol con- initiate a complex cascade of chemical events that can either excite or inhibit further electrical signals (see fig- sumption. In contrast to other stimuli, alcohol-related ure). The neurotransmitters also may attach to receptors on stimuli maintain their motivational significance even af- the transmitting cell’s own presynaptic sites, beginning a ter repeated alcohol administration, which may con- feedback process that can affect future communication tribute to the craving for alcohol observed in alcoholics. through that synaptic cleft. KEY WORDS: dopamine; dopaminergic receptors; cell sig- With so many different receptors on its cell surface, some naling; neurotransmission; reinforcement; motivation; of the signals the neuron receives will have excitatory ef- neurotransmitters; nucleus accumbens; brain; neuron; fects, whereas others will be inhibitory. In addition, some of sensory stimuli; AOD craving; AOD dependence; neu- the signals (e.g., those transmitted through ligand-gated robiological theory; literature review channels) will induce fast responses, whereas others (e.g., those transmitted through second messenger-linked proteins) will trigger slow responses. The integration by the neuron of any substances that relay signals among neurons these often conflicting signals determines whether the neu- (i.e., neurotransmitters) are affected by alcohol. ron will generate an action potential, release neurotransmit- MAmong these, dopamine has received special ters, and thereby exert an influence on other neurons. attention, because several studies have found that alcohol stimulates the activity of a subset of dopamine-releasing neurons and thus enhances dopamine-mediated (i.e., NEUROTRANSMITTERS AND ALCOHOL dopaminergic1) signal transmission in a discrete brain area Among the neurotransmitters of most interest to alcohol called the nucleus accumbens (NAc) (Di Chiara and researchers are dopamine, serotonin, glutamate, gamma- Imperato 1985; Imperato and Di Chiara 1986; Gessa et al. aminobutyric acid (GABA), opioid peptides, and adeno- 1985). Alcohol shares this property with most substances sine, all of which are featured in this special section. These of abuse (Di Chiara and Imperato 1988), including nico- molecules generally fall into three categories: (1) excita- tine, marijuana, heroin, and cocaine (Pontieri et al. 1995, tory neurotransmitters (e.g., glutamate), which activate the 1996; Tanda et al. 1997). These observations have stimu- postsynaptic cell; (2) inhibitory neurotransmitters (e.g., lated many studies on dopamine’s role in alcohol abuse GABA), which depress the activity of the postsynaptic cell; and dependence, also with the intent of finding new phar- and (3) neuromodulators (e.g., adenosine), which modify macological approaches to alcoholism treatment. This the postsynaptic cell’s response to other neurotransmitters. review summarizes some of the characteristics of dopamin- Neurons that release these substances form the basis of ergic signal transmission as well as dopamine’s potential neural circuits that link different areas of the brain in a role in alcohol reinforcement. 108 ALCOHOL HEALTH & RESEARCH WORLD.