chapter 10 - electrical synapses Flashcards
10.1. [electrical synapses] - explain the differences between electrical and chemical synapses
electrical and chemical synapses may co-exist
electrical synapses are faster and mostly bidirectional. Some molecules can be found in both chemical and electrical synapses (e.g. caMKII) and others only in chemical (e.g. PSD-95) or electrical (e.g. ZO-1).
gap junction diameter is larger than in voltage ion channels so it allows the exchange of non-ionic materials (i.e. second messengers, atp, metabolites)
electrical (ionic) exchange between pre and post synapses
passive current flow across the gap junction is fast, virtually instantaneous and results in synchronized electrical activity among neural populations
10.2. [electrical synapses] - describe and schematically draw the structure of gap junctions
see picture on slides
- gap junctions have pairs of hemichannels (each consists of a hexameric complex of connexions)
- in mammals there are around 20 different connexins. At least five are in the ns (Cx36,45,50,57,30.2)
- Two hemichannels with the same (homotypic) or different (heterotypic) connexin composition can form gap junctions
NB. possible heteromeric combinations are limited by the restricted temporal and spacial expression pattern of connexins.
2. While some connexins appear to be promiscuous in their
interaction partners, others are quite restrictive in those with which they can form partners.
3. connexins have 4 transmembrane domains
10.3. [electrical synapses] - summarize how electrical synapses are complex molecular assemblies, describe the functional classes of molecules involved and provide examples of each class
discuss the diversity of electrical synapses and their distribution in the mammalian CNS
electrical synapses are complex. They’re composed of
-channels: mediate transmission of ions and small molecules
-adhesion complexes: mediate apposition of membranes and
clustering of channels
-scaffolds: mediate clustering of channels
-regulatory proteins: involved in modulation of coupling strength
-trafficking proteins: regulate insertion and removal of channels
On EM images of electrical synapses: electron-dense
material at
each side of the gap junction = electrical synaptic density (ESD)
Example of Cx36 interacting scaffolding protein: zona occludens 1 (ZO-1): interacts with ‘multi-PDZ domain protein 1”
(MUPP1), which in turn interacts with CaMKII (= regulatory protein)
Note: establishment of new gap junction
channels: new connexins are trafficked in
vesicles from the Golgi as undocked
hemichannels. Hemichannels are then
inserted in the plasma membrane at the
periphery of existing gap junction plaques,
rapidly docking with hemichannels
inserted in the apposed membrane
they’re in multiple brain areas, including: retina, olfactory bulb, cortex, hippo, thalamus, hypothalamus, olives, brainstem, spinal cord, gastrointestinal system.
They are also diverse: 1. various neuronal types are coupled with electrical synapses, 2. distinct subcellular sites at which es occur, 3. connexin proteins that form neuronal gap junctions induce diversity, 4. gap junctions occur in a wide variety of ultrastructural configurations
examples:
1. Disparate neuronal types are coupled by electrical synapses
a. inhibitory GABA-ergic interneurons
b. excitatory glutamatergic neurons
c. excitatory cholinergic neurons
d. excitatory peptidergic neurons
e. excitaton) noradrenergic neurons
2. Distinct subcellular sites at which electrical synapses occur
a. purely electrical dendro-dendritic synapses
b. purely electrical somato-somatic synapses
c. axo-axonic electrical synapses
d. excitatory mixed synapses: combined chemical plus electrical synapses; axo-somatic or axo-dendritic
e. “reciprocal” (mirror) dendro-dendritic mixed synapses
3. the connexin proteins that form neuronal gap junctions induce diversity
4. gap junctions occur in a wide variety of ultrastructural configurations
10.4. [electrical synapses] - paraphrase the mechanisms that underlie plasticity of electrical synapses, as well as their time scales and signaling pathways involved; predict the effect of a given manipulation on coupling strength.
see figures!!
Mechanisms that alter coupling on three time scales:
1. Short-term change in coupling (B): resulting from activation of R2 postsynaptic receptors on one of the two coupled cells » drop in R2 + drop in coupling coefficient.
time scale: milliseconds, may last a few seconds
2. Intermediate term changes in Rz coupling (C): resulting from modification of existing gap junction channels, such as phosphorylation (reduces Ri) or dephosphorylation (increases Rj).
time scale: seconds to minutes; may last for hours.
3. long-term changes in coupling (D): resulting from decrease or increase in the number of gap junction channels due to expression or turnoverchanges.
time scale: minutes to days.
Notes: 1. the type of connexin that constitutes the gap junction has dramatic effects on channel conductance 2. it is thought that only a small fraction (<20%) of the channels in an electrical synapse are functional and efficient to support electrical transmission, suggesting that a heterogeneous population of channels can coexist in a gap junction plaque. Obviously, if a higher fraction of channels is active, junctional resistance will go down and coupling efficiency will increase.
mechanisms controlling plasticity of photoreceptor electrical synapses
Coupling between mammalian photoreceptors: high at night (darkness), low in the daytime (light).
‘Mechanism 1: daylight 3 increased dopamine secretion » D4 receptor activation on photorecpetors 9 inhibition of adenylate cyclase through Gi signaling - reduced cAMP production
3 reduced activity of cAMP-dependent protein kinase (PKA) - reduced Cx36 phosphorylation - reduced photoreceptor coupling.
note: intermediate term plasticity is controlled by a balance btw protein kinase and phosphatase activities that control phosphorylation state of the connexions
Mechanism 2: adenosine controls photoreceptor coupling through both adenosine A2a and A1 receptors. Extracellular adenosine levels: highest at night (darkness) - adenosine A2a receptor activation - adenylate cyclase activation through
Gs signaling - increased PKA activity - increased
Cx36 phosphorylation.
The A1 receptor has higher affinity for adenosine than A2a - A1 is active during daytime » inhibition of adenylate cyclase through Gi signaling »
reduced PKA activity reduced Cx36 phosphorylation
Note: a second phosphatase, PP1, appears
to target PP2A to put the brakes on the
pathway
amacrine cells: inhibitory interneurons in
the retina; AII amacrine cells use glycine
as neurotransmitter; they capture cellular
input from rod bipolar cells and
redistribute it to cone bipolar cells
10.5. [electrical synapses] - explain the methods used to study electrical synapses and be able to select the right method to tackle a given scientific problem
- Freeze-fracture replica immunogold labeling (FRIL):
-rapidly frozen biological samples are fractured, ‘replicated’, immunostained with gold particle-labeled antibodies and analyzed by transmission electron microscopy (TEM).
-allows for high-resolution, semi-quantitative analysis of symmetric versus asymmetric connexin labeling in matching apposed hemiplaques of individual ultrastructurally visualized gap junctions.
Notes: ’Matched-double-replica FRIL; larger dots in image: Cx36 labeling; smaller dots: Cx45 labeling
*fracturing (= cracking) of frozen tissue is done using a microtome, a knife-like instrument for cutting thin tissue sections. This fracture occurs along weak portions
of the tissue such as membranes or surfaces of organelles. Following fracturing, the sample undergoes a vacuum procedure, called “freeze etching.” The surface of the fractured sample is shadowed with carbon and platinum vapor to make a stable replica, which follows the contours of
the fracture plane. - Dual cell recording under direct visualization:
-electrophysiological analysis involving simultaneous recordings of neighboring cells using patch-clamp electrodes, with the aim to detect electrical coupling.
-this can be combined with the use of transgenic mice expressing EGFP in select neuronal populations, e.g. parvalbumin expressing cells or Cx36 expressing cells - Intracellular dye- or tracer injection and subsequent visualization of tracer transferred
between cells via gap juctions:
-allows for analysis of neuronal gap junction coupling
-examples of tracers: neurobiotin, biocytin - Gap junction blockers:
e.g. mefloquine has high specificity for Cx36-containing gap junctions - Electrophysiological analysis of ‘spikelets’ or ‘fast pre-potentials’:
Transmission of spikes (action potentials) through electrical synapses evokes corresponding coupling potentials in a postsynaptic cells, roughly resembling the tire course of the presynaptic action potential.
These so called ‘spikelets’ or ‘fast pre-potentials’ can be spontaneously observed in electrically-coupled networks
10.6. [electrical synapses] _ explain the potential functions of electrical synapses and their links to disease, and deduce the functional consequences of electrical synapses in a given neuronal network
functions of electrical synapses
1. Promotion of synchronous activity:
Transmission at electrical synapses is usually bidirectional, and therefore changes in membrane potential of one cell within an electrically-coupled network are presumably shared with all the partners in the network.
This allows for high-speed neuronal network oscillatory activities, which are thought to be involved in e.g. learning and memory and consciousness.
- Signal amplification:
e.g. ‘lateral amplification’ increases the sensitivity of sensory neuronal networks.
Figure panel: mechanosensory activation of the crayfish lateral giant neurons that command the tailflip escape response
mechansensory - Noise reduction
e.g. photoreceptors in the vertebrate retina: their electrical coupling promotes a decrease of uncorrelated noise and a relative amplification of correlated visual signals
link to disease: A mutation in a non-coding (regulatory) region of the C×36 gene has been linked to juvenile epilepsy
