Electrical Synapse Communication & Ephaptic Communication

Prepared byMILTAN CHOWDHURY

EDO15003 1

Outline

• Introduction to synapse,types…..•Electrical Synaptic Communication structure,properties,Connexins and its types ,future

challenges •Ephaptic Coupling communication Introduction,why?It’s role,some examples.

2

Synapse Definition

The synapse has been defined as a specialized structure that mediates a functional interaction between two neurons or between a neuron and another cell type.

3

Two type of Synapse

ELectrical synapseChemical Synapse

An electrical synapse is a mechanical and electrically conductive link between two neighboring neurons that is formed at a narrow gap between the pre- and postsynaptic neurons known as a gap junction.

4

• The gap is about 2-4 nm

5

6

Difference Between Chemical and Electrical Synapse

At chemical synapses, an action potential arriving at the pre-synaptic terminal triggers the exocytosis of vesicles filled with neurotransmitters , which are then released in the synaptic cleft. Transmitters diffuse and bind to specific receptors on the post-synaptic cell, where they gate (viz., open or close) ion channels either directly or indirectly, thereby affecting its membrane conductance. In this example, the opening of a ligand-gated channel triggers ionic influx in the post-synaptic cell.

At electrical synapses, gap junction channels allow a direct communication between the cytoplasm of the two coupled cells.

7

8

Connexin

9

The connexins are a family of integral membrane proteins that oligomerise to form intercellular channels that are clustered at gap junctions. These channels are specialised sites of cell-cell contact that allow the passage of ions, intracellular metabolites and messenger molecules, Gap junction channels have relatively large pores (16–20 A ˚ of diameter) (with molecular weight less than 1-2kDa) from the cytoplasm of one cell to its opposing neighbours.

Connexins(Cx)

10

Their conductances, permeability to different molecules, phosphorylation and voltage-dependence of their gating, have been found to vary.Possible communication diversity is increased further by the fact that gap junctions may be formed by the association of different connexin isoforms from apposing cells. However, in vitro studies have shown that not all possible combinations of connexins produce active channels

Connexins(Cx) Contd...

The family of connexin genes comprises 21 members in the human and 20 in the mouse genome, 19 of which can be considered as orthologue pairs on the basis of their sequence.

They are found in almost all vertebrate cell types, and somewhat similar proteins have been cloned from plant species. Invertebrates utilise a different family of molecules, innexins, that share a similar predicted secondary structure to the vertebrate connexins, but have no sequence identity to them

11

Connexin Types• Two sets of nomenclature have been used to identify the connexins. • The first, and most commonly used, classifies the connexin molecules according to

molecular weight, such as connexin43 (abbreviated to Cx43), indicating a connexin of molecular weight close to 43kDa.

• However, studies have revealed cases where clear functional homologues exist across species that have quite different molecular masses; therefore, an alternative nomenclature was proposed based on evolutionary considerations, which divides the family into two major subclasses, alpha and beta, each with a number of members .

12

Property Electrical Synapse(Gap Junctions)• Symmetrical morphology.

• Bidirectional transfer of information(Pre- and postsynaptic elements physically indistinguishable

Bi-directional!), but can be unidirectional.• Pre- and postsynaptic cell membranes are in close Ions can flow through these

gap junctions, providing low-resistance pathway for cells without leakage to the extracellular space signal

• transmission = electrotonic transmission.- Instantaneous, fast transfer from 1 cell to the next (< 0.3 msec), unlike the delay seen with chemical(sometime may be slow)

13

Electrical coupling between mouse hippocampal interneurons. Dual whole-cell patch-clamp recordings performed in brain slices on pairs of fast-spiking interneurons from the dentate gyrus region of the hippocampus have demonstrated that the vast majority of cell pairs are electrically coupled

(A) Fast-spiking interneurons in the dentate gyrus areidentified on the basis of their morphology, location and action potentialfiring patterns (illustrated here by a representative trace).(B) Electrical coupling between fast-spiking interneurons is reciprocal. (C) Electrical coupling is likely to promote the generation of action potentials. When cell 2 of a pair is injected with subthreshold current pulses, no action potentials are recorded in either cell (left traces). In pairs of electrically coupled interneurons a sub-threshold depolarizing current, however, facilitates the generation of action potentials when concomitant firing is evoked in the second interneuron

14

Some facts• Electrical communication is not only happens

in invertebrate brain where faster transmission is needed to accomplish simple, reactive tasks but also present in vertebrates

• It is more abundant in vertebrate CNS. Ex...synchronous oscillatory activity in cortical brain regions and in the dynamic control of retinal circuits.

15

Gap junctions and connexins between neurons

Studies show that the presence of gap junctions and the expression of distinct connexins in different regions (as well as in functions) of the adult mammalian brain, such as the hippocampus, inferior olive,coeruleus, hypothalamus, spinal cord, and olfactory bulb.In situ hybridization, immunocytochemistry, electron microscopy, freeze-fracture immunolabeling,electrophysiology, dye coupling(Are their gap junctions here?), which are ultimately providing a morphological, functional and molecular description of neuronal coupling both in vitro and in vivo. A systematic analysis of freeze-fracture replicas of the rat spinal cord has demonstrated that mixed synapses are relatively abundant between several classes of neurons.

16

Gap junctions in the developing CNSThe electrical coupling actually precedes the establishment of chemical transmission between nerve and muscle cells in culture, hence providing a route for the exchange of signals involved in synapse formation .Thus, at early stages of development it has been proposed that gap junction channels are not only important for electrical synchronization, but are chiefly used as used as a biochemical means that allows neuronal ensembles to exchange small second messenger molecules that shape their activity.It has been noted that the prevalence of gap junctional coupling is well correlated with specific developmental events (includingneurulation, cellular and regional differentiation, migration,axon guidance, and the formation of neuronal circuits) andthat the basic properties of these channels are well suited to mediate the transfer of developmental signals.

17

Connexin’s distribution

18

Mouse Brain

19

Connexins are differentially distributed in the mouse brain. Theprofile of mRNA expression was determined by radioactive in situ hybridization in horizontal brain sections obtained from rats at postnatal day 90. X-ray autoradiograms illustrate the differences between the localization of Cx26, whose transcript is detected only in the meningeal layer (Me) , and Cx43, which is highly expressed in astrocytes . Cx36 mRNA is high in the olfactory bulb (Ob) and also present in other regions, including cortex (Co), hippocampus (Hi) and cerebellum (Cb). Note that the signal for the neuronal Cx36 is absent from white matter structures (arrowhead), such as corpus callosum (Cc), wherelabeling is evident for Cx32, which is expressed by oligodendrocytes . Scale bar is 2 mm

•

Synchronization of the electrical activity of large populations of neurons;(for example cx36)

- e.g., the large populations of neurosecretory neurons that synthesize and release biologically active peptide neurotransmitters and hormones are extensively connected by electrical synapses.

- e.g., Synchronization may be required for neuronal development, including the development of chemical synapses.

- e.g., Synchronization may be important in functions that require instantaneous responses, such as reflexes and pacemakers.

20

So In which kind of situation it is good

21

Electrical signaling and synchronous Oscillatory activity

22

Experiments in mice lacking Cx36 have demonstratedthat the disruption of oscillogenesis is observed only inmodels of gamma frequency, whereas electrical communicationwithin the excitatory neuronal network, as measuredusing ultrafast population activity

Cx36 is involved in gamma frequency (30– 80 Hz), but not ultrafast (150– 200 Hz) oscillations.

Extracellular field recordings were obtained in brainslices prepared from wild-type (black traces) and Cx36 knockout (KO) animals (red traces).

Cellular assemblies communicating through gap junctions mature during development

23

Future Challenges Electrical Synapse Study

24

• All Connexins roles in the Central nervous system are ambiguous or unknown.

• The mechanism by which electrical synapse are regulated are also unknown.

• What rule electrical synapse play in brain function and neural basis behaviour.

Ephaptic Coupling

Neurons communicate via specialized molecular machines, the synapses. Such discrete, point-to-point synaptic connectivity, whether chemical or electric, is important for information processing in the brain1–3. However, such integrative processes do not occur in a vacuum

Instead, neurons are located in a conductive medium, the extracellular space

Neurons Talk Without Synapses

25

Ephaptic coupling to endogenous electric field activity: why bother?

• The idea that the electrical activity generated by nervous tissue may influence the activity of surrounding nervous tissue is one that dates back to the late 19th century.Early experiments, like those by du Bois-Reymond,demonstrated that the firing of a primary nerve may induce the firing of an adjacent secondary nerve (termed "secondary excitation"). This effect was not quantitatively explored, however, until experiments by Katz and Schmitt in 1940, when the two explored the electric interaction of two adjacent limb nerves of the crab Carcinus maenas.

26

In a fruit fly’s antennae, two neighboring neurons can stop each other from firing even if they do not share any direct connections, and help the insect to process smells.This is also ephaptic communication.

Mechanism and Effects

Role in Excitation and InhibitionThe early work performed by Katz and Schmitt demonstrated that ephaptic coupling between the two adjacent nerves was insufficient to stimulate an action potential in the resting nerve. Under ideal conditions the maximum depolarization observed was approximately 20% of the threshold stimulus.This effect is the result of the exchange of ions between the two fibers. As the action potential wave propagates along the active axon it draws from ion stores in the resting axon and the characteristic influx and efflux of ions in the action potential are experienced in reverse by the resting axon. The resting axon experiences hyperpolarization, depolarization, and then a smaller hyperpolarization in contrast to the active axon's progression from depolarization to repolarization, and return to resting potential.

27

Role in Synchronization and TimingIn the simpler case of adjacent fibers that experience simultaneous stimulation the impulse is slowed because both fibers are limited to exchange ions solely with the interstitial fluid (increasing the resistance of the nerve). Slightly offset impulses (conduction velocities differing by less than 10%) are able to exchange ions constructively and the action potentials propagate slightly out of phase at the same velocity.

28

Example

Ephaptic coupling in rat Purkinje cells of the cerebellumOne of the few known cases of a functional system in which ephaptic coupling is responsible for an observable physiological event is in the Purkinje cells of the rat cerebellum.

29

how endogenous and externally imposedelectric fields impact brain function at different spatial (from

synapses to single neurons and neural networks) and temporalscales (from milliseconds to seconds).

30

Rate Hippocampal sliced positioned between parallel plates imposing an extracellular fields.

Outlook on Ephaptic coupling

Advantages 1. it is very fast and has no synaptic delay, and 2. it has very low noise levels. In principle, such

communication will occur in any synapse, but its influence will only become significant if the synapse has certain properties.

The elements that are crucial for ephaptic neuronal communication are:

1. an ephapse with a large resistance to the extrasynaptic space,

2. a current sink and 3. voltage sensitive channels on the opposing membrane

31

Conclusion

•We cant go for Directly go for In Vivo tests to find out Ephaptic coupling communication details about functional role, does it contribute to neural functions.

•There are very less number of evidence to find out which occur very rarely.

32

References• Hormuzdi a , Mikhail A. Filippova , Georgia Mitropouloub , Hannah Monyer a , Roberto Bruzzonea

Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks Sheriar G. 2003

• http://enhancedwiki.altervista.org/en.php?title=Ephaptic_coupling

• https://www.ebi.ac.uk/interpro/entry/IPR002260• http://www.the-scientist.com/?articles.view/articleNo/33350/title/Neurons-Talk-Without-Synapses/

33

Questions

?

34

ThankYou

35