Synaptic Transmission II Flashcards
Glutamate receptors are…
- ion channels
- They are permeable to cations (positively charged ions) including Na+ and K+
The point at which there is no current…
- is the reversal potential Erev
- The reversal potential for glutamate receptors is 0mV. When the cell’s membrane potential is 0mV, no current will flow through these channels.
GABA
- a neurotransmitter that binds to GABA receptors which are ion channerls permeable to only chloride.
- Erev is -65mV
By voltage-clamping the membrane and measuring how much current flows into or
out of the neuron in the presence of a neurotransmitter…
- you can determine which ions flow through the postsynaptic receptor.
Each EPSP or IPSP produced by binding of a neurotransmitter to areceptor may be only a fraction of a millivolt. How do synaptic responses ever reach threshold?
- Neurons typically have thousands of synapses. The EPSPs and IPSPs produced at each synapse sum together.
Two types of summation:
- spatial and temporal.
Spatial summation:
- adding together of PSPs generated simultaneously in different parts of the dendrite.
Temporal summation:
- adding together of PSPs generated at the same synapse if they occur in rapid succession (within about 1-15ms of one another)
To reach the axon hillock, the current…
- passively propagates through the cell.
- the current decays
Passive current flow:
- Conduction by neurons in the absence of action potentials.
- It is like conduction of electricity in a wire.
Synaptic potentials close to the cell body
will still be large when they reach the cell
body. (Proximal)
But,
- synaptic potentials generated further
away will be much smaller because they
have to passively propagate over a long
distance. (Distal) - the closer potential has a greater chance of producing an action potential.
Neurotransmitters and their receptors
- Glutamate: AMPA receptor
- GABA: GABA receptor
- Acetylcholine: nicotinic acetylcholine receptor
Glutamate:
- the major excitatory neurotransmitter in the brain nearly all excitatory cells in the CNS are glutamatergic over ½ of all brain synapses are glutamatergic
Glutamine, Glutamate, VGLUT, EAATs
- presursor to glutamate
- x
- transports glutamate into the vesicles
- After NT is released and bound to receptors, EAAT transports the recycled glutamate away from synapse.
There are three types of ionotropic glutamate receptors:
- AMPA, NMDA and kainate
- All are non-selective cation channels, permeable to Na+ and K+.
Structure of the AMPA receptor:
- Composed of 4 subunits
- Glutamate binds to the ligand-binding domain
- C-terminal domain can interact with proteins in the cytoplasm
- Binding of glutamate to 2 of the 4 subunits closes the “clamshell” structure and
opens the pore. - Current increases as more binding sites are occupied
The AMPA receptor:
- Permeable to Na+ and K+
- Reversal potential = 0 mV
- Binding of glutamate to the AMPA receptor causes a NET inward positive
current and a depolarization of the membrane. (EPSP)
GABA is a neurotransmitter that binds to…
- ## GABA receptors which are ion channels permeable only to chloride
The GABAA receptor: an ionotropic receptor
Benzodiazepines
- Benzodiazepines: anxiety-reducing drugs
Example: diazepam (Valium) - They enhance GABA transmission by
binding to the alpha and delta subunits of the GABAA receptor. - When GABA is present, benzodiazepines
increase the frequency of channel openings.
End result? - more chloride conductance
The GABAA receptor: an ionotropic receptor
Barbituates
- Barbituates: sedatives and anticonvulsants
Example: phenobarbital - They enhance GABA transmission by
binding to the and subunits of the
GABAA receptor. - When GABA is present, barbituates
increase the duration of channel
openings.
End result? - More chloride conductance
Excitatory actions of GABA in the developing brain
- in an immature neuron, GABA causes an excitatory effect instead of an inhibitory effect. (intracellular region becomes more +)
