Glossary

γ­Aminobutyric acid (GABA): A neurotransmitter that in the brain acts to reduce the activity of the signal­ receiving neuron. Action potential: A rapid change in cell membrane potential (the electrical potential difference across the cell membrane) followed by a return to the resting potential. An action potential is the basis of the signal­carrying ability of nerve cells. Adrenal glands: Small structures located on top of the kidneys that produce numerous hormones (e.g., glucocorticoids, which are important factors in the stress response). Affinity: A measure of how easily and tightly a substance binds to a receptor. Agonist: An agent that mimics the actions or effects of another agent (e.g., a drug that mimics the effects of a neurotransmitter). Alcohol deprivation effect: Animal model of relapse in which animals are first chronically exposed to alcohol, followed by a period during which alcohol is withheld; if alcohol is then reintroduced, the animals consume more alcohol than before the enforced abstinence period. Allele: One of two or more variants of a gene or other DNA sequences. Alternative splicing: The RNA­splicing variation mechanism in which the exons of the primary gene transcript, the pre­messenger RNA (mRNA), are separated and recon­ nected so as to produce alternative ribonucleotide arrangements. Alternative splicing uses genetic expression to facilitate the synthesis of a greater variety of proteins. Amygdala: Almond­shaped groups of nerve cells (i.e., neurons) located deep in the brain that are involved in the processing and memory of emotional reactions. Anisotropic: Not having properties that are all in the same direction; showing differences in a property or effect in different directions. Antagonist: An agent that blocks or reverses the actions or effects of another agent (e.g., a drug that blocks the effects of a neurotransmitter). Anxiogenic: Anxiety inducing. Anxiolytic: Anxiety reducing. Astrocytes: Characteristic star­shaped glial cells in the brain and spinal cord that, among other functions, provide nutrients to the nervous tissue, and play a principal role in repair and scarring process of the brain and spinal cord. Axon: The long, thin fiber protruding from a nerve cell that carries integrated nerve signals in the form of electrical signals to other nerve cells. Basal ganglia: A group of nuclei in the brain interconnected with the cerebral cortex, thalamus, and brainstem. Mammalian basal ganglia are associated with a variety of functions: motor control, cognition, emotions, and learning. Cation: Atom or molecule carrying a positive electrical charge. Caudate nucleus: A nucleus located within the basal ganglia of the brains of many animal species. It is involved with control of voluntary movement, and is an important part of the brain’s learning and memory system. Cerebellum: A region of the brain involved in the coordina­ tion of movement. Cerebrospinal fluid: Fluid that surrounds the brain and spinal cord. Cholinergic: Related to the neurotransmitter acetylcholine. Chromatin: The combination of DNA, RNA, and protein that makes up chromosomes. It is formed by the DNA wrapped around histones. Chromatin remodeling: An epigenetic mechanism, in which the chromatin structure is dynamically regulated either locally (gene expression regulation) or globally (chro­ mosome segregation). Cortico­basal ganglia networks: See basal ganglia. Cortico­mesolimbic dopaminergic pathway: See mesolimbic dopamine system. Corpus callosum: The largest connective pathway in the human brain. It is made of nerve fibers that connect the left and right sides (hemispheres) of the brain. Corticotrophin­releasing factor (CRF): A hormone produced in response to stress that helps mediate the stress response; is produced mainly in the hypothalamus but also can be produced in other brain regions. Cyclic AMP (cAMP): Second messenger molecule whose production by enzymes called adenylyl cyclases is regulated through receptor activation of G­proteins, and which itself activates certain protein kinases. Dentate granule cells: Tiny neurons (i.e., granule cells) that are around 10 micrometers in diameter found in the dentate gyrus. Dentate gyrus: Part of the hippocampus that contributes to new memories as well as other functional roles; one of a few brain structures with high rates of neurogenesis in adult humans. DNA methylation: Epigenetic mechanism of regulation of gene expression, in which a strand of DNA is modified by addition of a methyl group (CH3) to any cytosine located directly before a guanine. Dopamine: Neurotransmitter that is involved, among other functions, in controlling behavior and cognition, motor activity, motivation and reward, mood, attention, and learning. Dopamine­producing nerve cells are located primarily in the ventral tegmental area (VTA) and in regions of the hypothalamus. Dopaminergic: Referring to nerve cells that use dopamine as a neurotransmitter. Endogenous opioids: A group of small molecules, such as endorphins and enkephalins, that are naturally produced in the body and which have similar effects as the opiates (e.g., morphine and heroin); endogenous opioids mod­ ulate the actions of various neurotransmitters. Entorhinal cortex: An important memory center in the brain that generates the main input to the hippocampus and is responsible for the preprocessing (familiarity) of the input signals.