A slight change in the balance of brain chemistry can cause large changes in behavior.
— David Eagleman, Incognito

Neurochemistry: the branch of biochemistry concerned with the processes occurring with nerve tissue and the “nervous system.” (Oxford) The study of the composition, chemical structures, and “chemical reactions” of the nervous system or its components. (MeSH) Study of molecular and cellular aspects of the nervous system, (pathology) of "neurological disorders," and the development of disease specific “biomarkers.” (JNC, Mission Statement)

Agonist: a chemical which combines with a “receptor” and initiates a physiological response. (Oxford) A drug or substance that binds to a receptor inside a cell or on its surface and causes the same action as the substance that normally binds to the receptor. (NCI1)

Antagonist: a substance (or organism) which interferes with or inhibits the action of another. (Oxford) In medicine, a substance that stops the action or effect of another substance. For example, a drug that blocks the stimulating effect of “estrogen” on a “tumor” cell is called an estrogen receptor antagonist. (NCIt)

Endorphins: natural "opiates" made by the body. These powerful "pain” relievers regulate pain naturally, in effect 'closing the valve' on the flow of painful signals traveling up the "spinal cord" to the brain. (Fields, 191) Triggered by pain and "stress," "bind" to their special receptors, altering pain sensations and “mood.” Regulate pain perceptions. ‘Runner’s high’ associated with “aerobic” exercise has been associated. In marathon runners, endorphin levels have been found to increase up to four times over their normal levels.  Primary roles: pain perception, positive emotions. Associated disorder includes opiate addiction. (Hockenbury, 48) Editor's note - opiates bind to the same receptors as endorphins.

Enkephalins: “peptides” occurring naturally in the brain and having effects resembling those of "morphine" or other opiates. (OxfordMed) One of the three major families of “endogenous” opioid peptides. The enkephalins are widespread in the “central” and “peripheral nervous systems” and in the “adrenal medulla.” (MeSH) Editor’s note - opiates also bind to the same receptors as enkephalins. Also referred to as ‘encephalin.'

Messenger Molecules: (found) in a chemical “synapse,” housed in “synaptic vesicles” and released into the “synaptic cleft” (during “synaptic transmission”). (Patestas, 31) Also referred to as ‘ligands’ and 'signaling molecules.’

First Messengers: "neurotransmitters" like "glutamate" are considered first messengers. They are responsible for "signaling" between neurons. (LeDoux, 147)

Second Messengers: chemicals produced inside the cell when a neurotransmitter binds to a particular class of receptor on the cell surface. “Cyclic AMP” is a common second messenger in neurons. (Kandel, 448) (Part of) systems in which an “intracellular” signal is generated in response to an “intercellular” (first) messenger such as a hormone or neurotransmitter. (GHR) Pick up where first messengers leave off. Their job is to initiate chemical reactions within the cell on the basis of information provided from outside the cell during (synaptic) transmission by first messengers. "Calcium" is one of the major second messengers. When glutamate binds to its receptor, calcium takes over and directs the chemical reactions that strengthen "synaptic" connections, both in the short run and the long run. (LeDoux, 147) Calcium “protein channels” located in the “membrane” and “endoplasmic reticulum” allow calcium to act as a second messenger and initiate a cellular response. (Norman, 6/10/09)

Modulation: variation in the activity or form of a cell in response to a changing environment. (Oxford) To adjust, or change. (NCIt) Verb - 'modulate.' 

Neuromodulators: enhance or diminish the overall effectiveness of the synaptic connections and bring about enduring change. Examples include “oxytocin” and “vasopressin.” (Doidge, 118) Have slower and longer-lasting effects (than neurotransmitters). (LeDoux, 57) (Neuromodulatory) hormones are typically released from bodily organs into the bloodstream where they travel to the brain. There they can alter the efficacy of glutamate or "GABA" transmission by binding to specific receptors on cells. (LeDoux, 59)