Drugs and the BrainPart 1

Neurotransmitter Review

Spectrum of Neurotransmitters

• There are at least 50 distinct neurotransmitters that carry messages in the brain

• Only a few have been well studied• Virtually all drugs used in

psychopharmacology today target one of the 5 well studied neurotransmitters

• Neuropharmacology is in its infancy; future developments are likely to be extensive

A Review of Major Neurotransmitters• Norepinephrine -

– One of two major catecholamines – transmitter of the sympathetic nerves of the autonomic nervous system which controls emergency responses (fight or flight)

• Dopamine –– A catecholamine transmitter. A major transmitter

in the corpus striatum, a part of the brain regulating motor behavior. Destruction of these neurons causes Parkinson’s. Blockade of actions of dopamine in other brain areas is mode of action of some antischizophrenic drugs.

• Serotonin – – Transmitter of a group of neurons in the raphe

nuclei of the brain stem. Actions of psychedelic drugs are due to changes in activity of serotonin neurons

A Review of Major Neurotransmitters

(continued)• Acetylcholine –– Neurotransmitter at neuromuscular

junctions for all voluntary muscles, as well as many involuntary synapses. Exact role in brain still unclear

• GABA –– Occurs almost exclusively in the brain.

Inhibitory transmitter that reduces firing of neurons

• Glycine – – Serves as an inhibitory neurotransmitter in

small neurons in the spinal cord and brain stem

A Review of Major Neurotransmitters

(continued)• Glutamic Acid – – An amino acid that also functions as a

neurotransmitter. Probably the major excitatory neurotransmitter in the brain. Acts at several receptor subtypes. May be responsible for damage associated with stroke.

• Nitric Oxide – – Transmitter of nerves to the

intestines and other organs. May regulate emotional behavior in brain.

A Review of Major Neurotransmitters

• Endorphins – – Endogenous morphine-like substances. Any

substance that produces opiate-like effects. Includes (& is sometimes a synonym for) enkephalins.

• Enkephalins – – 2 peptides, each containing 5 amino acids,

that act at opiate receptors in the brain. Analgesics.

• Substance P – – A peptide containing 11 amino acids. A major

transmitter of sensory neurons which convey pain from the periphery to the spinal cord. Opiates block release. Also found in brain.

How Drugs Effect Neurotransmitters

• Effects can occur at many stages in the process

• May block the action of an enzyme required to synthesize a neurotransmitter from a precursor molecule– Many drugs to lower blood pressure work by

decreasing synthesis of norepinephrine

• May cause neurotransmitters to leak from synaptic vesicles, where they are degraded by enzyme– Resperine interferes with storage of norepinephrine

this way (tranquilizer, lowers blood pressure)

How Drugs Effect Neurotransmitters (Cont.)

• May effect the release of transmitters from nerve endings– Slip into vesicles, pushing neurotransmitters out

into the synaptic cleft– Amphetamines release norepinephrine &

dopamine this way• Other drugs block the release process itself• Some drugs inhibit enzymes that degrade

neurotransmitters, increasing the level of transmitter present in the synapse and facilitating transmission– Most antidepressants work this way– MAOI – monoamine oxidase inhibitors inhibit

enzyme which degrades transmitters responsible for mood stabilization

How Drugs Effect Neurotransmitters (Cont.)

• Others block re-uptake– Tricyclics– SSRIs & SNRIs

• May influence the neurotransmitter receptor– May resemble transmitter and mimic effect at

the receptor– May occupy the receptor without causing any

2nd messenger response in the nerve– Blocks access of neurotransmitter to receptor

Location of Drug Action• Where in the brain a drug acts is critical• Each transmitter can bind many locations, has

multiple effects; side effects can be considerable– Example: Acetylcholine– Many insecticides target acetylcholinesterase, the

enzyme that breaks down acetylcholine; can cause death in children by stopping breathing

– Belladona – Plant used since ancient times for stomach upset, slows intestinal contractions. Overdose can cause memory loss, death. Active ingredient, atropine, blocks acetylcholine.

– Acetylcholine deficiency is implicated in memory loss. Acetylcholine-rich neurons of the basal nucleus destroyed in Alzheimer’s patients. Drugs that inhibit acetylcholinesterase improve memory & are used in Alzheimer’s patients.