Psych 255 Unit 5

Neurotransmitters

chemical released by a neuron onto a target that has either an excitatory or inhibitory effect (or other more complex effects)

outside the CNS, many of these chemicals circulate in the bloodstream as hormones

Presynaptic Membrane

- output side of a synapse (sends out neurotransmitters)

Postsynaptic Membrane

- input side of a synapse (receives neurotransmitters)

Synaptic Vesicle

a membrane structure that contains a neurotransmitter

Synaptic Cleft/Synaptic Gap

- Gap that separates the presynaptic membrane from the postsynaptic membrane
- where neurotransmitters are released when stimulated by an action potential

Steps of Neurotransmission

1. Synthesis
2. Packaging & storage
3. release of NT
4. receptor activation
5. deactivation of NT

Steps 1 & 2: Synthesis, Packaging, and Storage of NT

- synthesized in axon terminal with chemical precursors from food pumped into the cell via transporter proteins
- synthesized in the soma using the DNA code and transported on microtubules

Step 3: Release of the NT- EXOCYTOSIS

-at terminal, the AP opens voltagesensitive calcium (Ca++) channels
- incoming Ca++ binds to calmodulin, forming a complex -complex causes some vesicles to empty or get ready

Step 4: Activation of Receptor Site pt.1

-after release, the NT diffuses across the synapse and activates postsynaptic receptors
- These transmitter activated receptors are proteins embedded in the membrane that have a binding site for a specific NT

Step 4: Activation of Receptor Site pt.2

- depolarize the postsynaptic membrane causing an EPSP (e.g., open Na+ ion channels)
Or
- hyperpolarize the postsynaptic membrane causing an IPSP (e.g., open K+ or Cl- ion channels)
Or
- initiate other chemical reactions that modulate either the excitatory or inhibitory effect, or influence the receiving neuron in other ways

Step 4: Activation of Receptor Site pt.3

• Autoreceptors - "Self-receptor" in a neural membrane that responds to the transmitter that the neuron releases (on the presynaptic membrane)

Step 5: Deactivation of NT

- diffuse away from synaptic gap degraded by enzymes in gap
- reuptake into the presynaptic neuron for subsequent re-use
-taken up by nearby glial cells

Type I Synapse

- excitatory
- typically on dendrites
- large active zone
- wide cleft
- round vesicles
- dense material on membranes

Type II Synapse

- inhibitory
- typically on cell body
- small active zone narrow cleft
- flat vesicles
- sparse material on membranes

Types of Neurotransmitters

• ~ 100+ different kinds
• Some are inhibitory at one location and excitatory at another
• >1 neurotransmitter may be active at one synapse
• No simple one-to-one relationship between a single neurotransmitter and a single behaviour

Identifying Neurotransmitters

4 Criteria
• must be synthesized and/or packaged in the neuron • the chemical must produce a response in a target cell when released
• The same response must be obtained when the chemical is experimentally placed on the target
• must be a mechanism for removal after the chemical's work is done

Varieties of Neurotransmitters

Small-Molecule transmitters
• Peptide transmitters
• Lipid transmitters
• Transmitter gases
• Ion transmitters

Small Molecule transmitters

- quick acting neurotransmitters
- synthesized from precursor chemicals found in the diet - Packaged in the axon terminal

Peptide transmitters

- Peptide transmitters are chains of amino acids synthesized from mRNA based on DNA code
- synthesized in soma and shipped
- act slower & replaced slower than small molecule neurotransmitters
- often act as hormones

Lipid transmitters

- not stored in vesicles Endocannabinoids
- synthesized and released from postsynaptic membrane
- Binds to receptor on presynaptic membrane, which decreases NT release (GABA, glutamate) from the presynaptic membrane
- "retrograde transmitter"

Receptors (2 types)

- Ionotropic
Direct
fast

- Metabotropic
Indirect
slow

Direct: Ionotropic Receptors

- embedded membrane protein with a binding site for a NT and pore (similar to a gated channel)
- regulates ion flow to directly & rapidly change the membrane voltage

Indirect: Metabotropic Receptors

- Causes a metabolic reaction that is slower and longer lasting
- Receptors have binding site for a neurotransmitter but no pore/channel
- Receptors are coupled with G-proteins
- G-protein releases a subunit that may affect ion channels

Noradrenergic System

- projections from locus coeruleus
- Related to attention and arousal
- decreases related to depression
- increases related to mania

Serotonergic System

- projections from raphe nuclei
- Related to arousal
- Increases related to schizophrenia
- decreases related to depression

Cholinergic System

- projects from midbrain & basal forebrain nuclei
- Related to arousal
- role in memory
- decrease in ACh related to Alzheimer's

Mesolimbic Dopaminergic System

- projects from ventral tegmental area
- role in pleasure and reward
- system mediating drug addiction
- related to schizophrenia

Nigrostriatal Dopaminergic System

- projects from substantia nigra
- role in normal motor behavior
- decrease in DA related to Parkinson's disease

tripartite synapse

Synapse between a presynaptic neuron, a postsynaptic neuron, and an astrocyte.

chemical synapse

junction at which messenger molecules are released when stimulated by an action potential

presynaptic membrane

the specialized membrane of the axon terminal of the neuron that transmits information by releasing neurotransmitter

postsynaptic membrane

the cell membrane opposite the terminal button in a synapse; the membrane of the cell that receives the message

anterograde synaptic transmission

process that occurs when a neurotransmitter is released from a presynaptic neuron and binds to a receptor on the postsynaptic neuron

Transporters

proteins that move neurotransmitter molecules from the synapse across the cell membrane and back into the axon terminal as part of the reuptake process

ionotropic receptors

Embedded membrane protein that acts as (1) a binding site for a neurotransmitter and (2) a pore that regulates ion flow to directly and rapidly change membrane voltage.

metabotropic receptors

Embedded membrane protein, with a binding site for a neurotransmitter but no pore, linked to a G protein that can affect other receptors or act with second messengers to affect other cellular processes.

autoreceptors

"Self-receptor" on the presynaptic membrane that responds to the transmitter that the neuron releases

quantum (pl. quanta)

number of neurotransmitter molecules, equivalent to the content of a single synaptic vesicle, that produces a just-observable change in postsynaptic electric potential

gap junctions

Area of contact between adjacent cells in which connexin proteins in each cell form connecting hemichannels; when open, the hemichannels allow ions to pass between the two cells; also called an electrical synapse

small molecules transmitter

Quick-acting neurotransmitter synthesized in the axon terminal from products derived from the diet.

histamine (H)

Neurotransmitter that controls arousal and waking; can cause the constriction of smooth muscles and so, when activated in allergic reactions, contributes to asthma, a constriction of the airways.

Serotonin (5-HT)

amine neurotransmitter; helps to regulate mood and aggression, appetite and arousal, perception of pain, and respiration

glutmate (GLU)

Amino acid neurotransmitter; typically opens the Na+ and Ca2+ channels and therefore excites neurons.

Gamma-aminobutyric acid (GABA)

amino acid neurotransmitter; typically inhibits neurons

Neuropeptides

short chains of amino acids

Endocannabinoids

natural, marijuana-like substances produced by the body

Cholinergic neuron

neuron that uses acetylcholine as its main neurotransmitter

Activating systems in the CNS

neural pathways that coordinate brain activity through a single neurotransmitter

noradrenergic neurons

neurons that release norepinephrine

Neuromodulators

Neurotransmitters that alter the firing and synaptic characteristics of other neurons, typically over an extended period