1. Neuronal Transmission
BN Fall 2011
Julia Sobesky

2. Outline Types of synapses Neurotransmitters Receptor types
Electrical
Chemical
Neurotransmitters
Criteria
Types
Release
Inactivation
Receptor types
Ionotropic
Metabotropic
Ligand binding
Plasticity
Outline

3. Electrical synapse: gap junctions
~3nm apart
Very fast communication
Direct pore between cells, allows bidirectional flow of ions
6 connexins= 1 connexon
Allows rapid and synchronous firing of interconnected cells
Examples are contractions during childbirth or cardiac contractions

5. Why would we need anything more?
Why don’t our brains just use electrical transmission?

6. Benefits of Chemical signaling
60+ different NTs and neuromodulators
Each NT can have up to 15 different receptors
Co-localization of several NTs in one synapse
One neuron can have TONS of different synapses
Simple or complex post-synaptic responses
Post synaptic responses: differ in temporal action, more than just alterations in membrane potential
When you think about how complex our thoughts and actions are, and that this is basically a series of synaptic connections, it makes more sense that we would need to have this much complexity to make it happen

7. The chemical synapse ~20-50 nm apart
NTs released by pre- synaptic cell bind receptors on post-synaptic membrane
EPSP, IPSP or complex responses
*** The RECEPTOR determines the response, not the NT ***
Binding of NT is transitory, the more NT in the synapse, the greater the receptor responses

8. Criteria for NTs Synthesized in pre- synaptic cell
Activity dependent release
Mechanism for deactivation
Predictable pharmacological activity

9. Major classes of neurotransmitters
Small neurotransmitter molecules
Synthesized near axon terminals
Acetylcholine, monoamines, indolamines, amino acids
Large neurotransmitter molecules- Neuropeptides
Synthesized in soma
hormones
enkephalin/ endorphin
Soluble gasses
nitric oxide
carbon monoxide
WHY would small NTs be synthesized at terminals while large NTs in soma?
-bc small NTs have dietary precursor components, Large NTs must be translated and transcribed
Enk/end- work as endogenous morphine like molecules/NTs that decrease pain.
Runners high
Soluble gasses…. Don’t want to talk about, because not classical ---- but I have read estimations up to 2% of neurons in the brain use nitric oxide signaling.
Soluble gasses.

10. Small Neurotransmitters
1. Amino Acids
Glutamate/ Gamma-aminobutyric acid (GABA)
MAJOR NTs in the CNS/ All over
2. The Monoamines
Catecholamines
Dopamine- DA- reward/movement
Norepinephrine- NE –sympathetic
Epinephrine-released from adrenals
Indolamines
Serotonin -5-HT
3. Acetylcholine (ACh)
Glut is major excitatory NT in CNS/ GABA is major inhibitory. Glut- long axon neurons. GABA is primarily in interneuron (in one nucleus--- in between the input/out put neurons in one nucleus.)
In a way, you can can anything else a neuromodulator… because these are the major NTs in the CNS, all the rest regulate whats going along the GABA or glutamatergic connections.
2. acetylcholine: contracts skeletal muscles/ ANS. Important in Alzheimer's disease in basal forebrain (Ach decreased)./ interneurons
3. Biogenic amines. Catechol/indol refers to their molecular structure. Localized in small cell groups/ clusters within the brain. Not widespread/ dispersed like the amino acids, but are equally important. NO SMALL EFFECT. Axon collaterals to cortex/ high order functions……
Da----- for example---- involved in addiction. VTA-- NAc. This pathway which utilizes DA transmission is vital to almost all kinds of addiction.

11. L-Dihydroxyphenylalanin (L-Dopa)
Amino Acids
Glutamate
GABA
Catecholamines
Tyrosine
L-Dihydroxyphenylalanin (L-Dopa)
Dopamine
Norepinephrine
Epinephrine
Tyrosine
Hydroxylase
Glutamic
Acid
Decarboxylase
(GAD)
Important in understanding some of the examples of degradation. Just interesting to see that some NT that serve very different purposes in the CNS are precursors for each other. DA vs. NE. Stress response….. Fight of flight response.
TYROSINE HYDROX IS RATE LIMITING FACTOR
Parkinson’s- Da deficiency in pathways bt SN-striatum/ SN-GP. One treatment for PD is L-dopa.
Glut is a precursor for GABA

12. Nuclei are the home of the cell bodies of the neurons that make and release the respective NTs

13. Then what? NTs are synthesized at terminal and packaged Or
Neuropeptides are transcribed, translated, packaged and trafficked down to the terminal
How does an Action Potential initiate their release?

14. Exocytosis Ca++ facilitated SNARE Proteins
Voltage-gates Ca++ channels in the axon terminal open from AP and allow CA++ to enter. SNARE proteins allow vesicle to dock to the internal membrane, Ca++ enables SNARE proteins to fuse the vesicular and cell membranes together, allowing NT release into the synapse
BOTOX
SNARE Proteins

15. What happens to NTs after release?
Diffusion through synapse to post-synaptic cell
NT binding to receptors is TRANSITORY, more NT around to bind, the greater the receptor effects
…….
Uptake and recycled
Enymatically broken down
Taken up by glia cells, released later, like a sponge to soak up excess
Passive diffusion away (can have distal effects)

16. 2 Main Types of Receptors
Ionotropic
Ligand-gated ion channels
Directly alters the membrane potential
Metabotropic
Slower, but can have greater effects
2 types:
G-protein coupled
Tyrosine Kinase receptors
Ionotropic receptors are ion-specific, when the NT binds and produces a conformational change to open the pore, the pore is wide enough to allow specific ions to pass through

17. Ionotropic Receptors Excitatory (EPSP) or Inhibitory (IPSP) responses
K+, Na+, Ca++
CL-
Some can be ligand and voltage-gated (NMDA)
Slow ionotropic: closes K+

18. Complex effects of metabotropic receptors
NO PORE, but binding can initiate:
2nd messenger system
Other products could open ion channels
Modulate enzyme activity
Regulate ion channels in membrane
Initiate gene transcription/translation
Regulation of ion channels, both ligand gated (to alter the sensitivity of a cell to NT) and voltage gated, to alter electrical excitability and firing properties

19. What happens to NTs after release?
Diffusion through synapse to post-synaptic cell
NT binding to receptors is TRANSITORY, more NT around to bind, the greater the receptor effects
NT must be cleared
removal just as important as release
Multiple things can happen….
Uptake and recycled
Enymatically broken down
Taken up by glia cells, released later, like a sponge to soak up excess
Passive diffusion away (can have distal effects)

20. Uptake and degradation

21. Glial Sponge Glial cells can act as buffers for excess NTs
Can process and release NTs
Passive diffusion away from the synapse
Why?
Distal effects, some cells have very distal receptors, if the receptor is able to identify an NT that has passively diffused away, there is TOO much, so the receptor initiates a negative feedback mechanism to release less NT

22. NT binding to receptor shape-specific
Lock and key arrangement
Endogenous vs. exogenous
Drugs work because we already have the receptors in place to receive them
Drug actions are so intense because they cause actions so far above and beyond what endogenous compounds do
Agonists
Antagonists
Full vs. partial
Competitive vs. non- competitive
Allosteric
Allosteric- modifies the receptor to be more or less effected by the natural NT
Competitive: based on affinity of the ligand to the receptor

23. Receptor agonists and antagonists
Full, partial, competative, non-competative, allosteric

24. Organization dictated by experience
Synapses can grow and retract, continually being altered by use
Plasticity can occur in a variety of ways:
Create new synapses
Strengthen or weaken existing synapses
Break old connections
LTP
Tolerance to drugs: can up or down regulate receptor numbers

25. Synaptic connections change over time
These are microglia, but I want you to see how cells move and interact, neurons do this too!

26. Putting it all together: Neuropharmacology
Tolerance develops due to cellular and receptor alterations in response to chronic drug use
The changes also mediate withdrawal symptoms
Withdrawal= opposite of drug effects
Depression is most likely not due to a lack of serotonin (i.e. SSRIs)
…Serotonin receptor is metabotropic
Severe alcohol withdrawal can kill you:
Seizures
Glutamate excitotoxicity
Alcohol= agonizes GABA, GABA receptors are down regulated because there was too much inhibition, cold-turkey= not enough GABA to keep inhibited= overly excited
In an addicted state= drug + altered brain morphology = normal

27. Organization dictated by experience

28. Thanks! Questions?