The origin of …

Q1

When did the first (chemical) synapses form?

Q2

In which species and under which ecological constraints?

Q3

Was the ursynapse bidirectional?

Q4

Where the first synapses plastic?

Synaptic polarity

Wiring prior to firing

Did chemical synapses evolve from electrical synapses and paracrine signalling systems?

• Gap-junction in development
• ! Post-synaptic proteins found in Porifera
• SNARE in Fungi
• Continuum from unicellular life, just like with channels?

^{(Ovsepian and Vesselkin 2014)}

Scaffolding-part is old

^{(Sakarya et al. 2007)}

Scaffolding-part is old

^{(Burkhardt 2015)}

Synaptic components

MASC is implicated in plasticity

^{(Ryan and Grant 2009)}

Static or plastic synapses

• MASC is implicated in plasticity

• Quintessential players that mediate changes dependant on neuronal activity during synaptic plasticity are also essen­ tial components of the yeast’s response to environmental changes (CaMKII). ^{(Ryan and Grant 2009)}

• NMDA, AMPA and Kainate receptors involved in plasticity are present in Cnidaria

• No clear ephys/behaviour on plasticity in Cnidaria, lots of work in Lophotrochozoa (Aplyisa).

• Do Protostomes and Deuterostomes learn in the same way? Different genome dublications.

  • Mouse: 186 MASC proteins (20% yeast orthologs)
  • Drosophila: 220 MASC proteins (70% yeast orthologs)

Gapjunction vs Synapses

• Paracrin signalling is older than cytoplasmic bridges

^{(Ovsepian and Vesselkin 2014)}

Symmetric synapse

in Ctenophora/Cnidaria

^{(Hernandez-Nicaise 1973)}

Cytosceleton

• Trafficking

^{(Rolls and Jegla 2015)}

Porifera (Sponges)

• No neurons
• No chemical synapse, but homologs of many genes that play roles in synaptic function
• Gapjunction connected ephitela cells
• Ca²⁺ waves mediate whole-body contraction
• Sessile / benthic
• Filter-feeder
• Coordinated reproduction
• sneezing response
• no specialised sensory cells

Ctenophore neurons are isopolar

They can form synapses everywhere: perikaryon (soma)

Absent	Present
Serontonin	Glutamate
Acetylcholine (?)	nonhomologous neuropeptides

Sophisticated sensory cells for

• photo-
• mechano-
• gravi-reception
• photocytes involved in bioluminestence and light sensing (express photoprotein and opsin)

^{(Horridge 1964; Tamm 2014)}

Questions

• Everything except fossils is indirect inference
• Modeling early nerv nets
  • CPG (Reiss 1962)
  • Input discrimination (Fehmi and Bullock 1967)
  • Rectilinear vs diffusive propagation (Smith and Bullock 1990)
  • Spread in Corals (Chen et al. 2008)
  • Cnidarian CPG (Stoeckel et al., 2011)
• Theoretical questions
  • Symmetry breaking of neuronal polarity
  • Synapse first

References

Thanks for your attention!