Associative Learning by Single Cells Dr. Chrisantha Fernando Systems Biology Centre University of Birmingham

Questions How can associative learning be implemented in single cells? How can we go about trying to find if these implementations exist? How can we make associative learning devices and what are they good for?

Pre-Synaptic (Eccles)Post-Synaptic (Hebb) Coincidence detectors

Coincidence Detection and Memory Pre-synaptic AC coincidence detection –5-HT (G-protein) + Ca 2+ /Calmodulin (Eccles) Post-synaptic NMDA coincidence detection –Ca 2+ + Glutamate (Hebbian) Short and Long Term State Storage –AC --> cAMP [15mins] --> PKA --> Decreased K + conductance –MAPK, Prion like CREB --> CRE gene expression –Increased NMDA localization to membrane,PKC --> AMPA

A Model of Pre-Synaptic AC based Learning Gingrich and Byrne (J. Neurophys. 1987)

Paramecia Exhibit Classical Conditioning Todd Hennessey et al Shock (UCS) + Vibration (CS) classical conditioning of avoidance response in paramecia. UCS = Shock CS = Vibration R = Avoiding Response

Sensory Mechanisms in Paramecia Mechano: Eckert, Naitoh and Friedman. J. Exp. Biol. (1972) K + current Ca 2+ current

MACHEMER & ECKERT 1973 Applying depolarization produces reversal

Ca 2+ channels are on the membrane surrounding the cilia Shaving cilia abolishes Ca 2+ current, until they grow back. Voltage gated Ca 2+ channels are essential

Behaviour of voltage gated Ca 2+ channels can be modulated Vibration??

Ciliary Ca 2+ -Calmodulin activates ciliary AC. Is AC acting as a coincidence detector in classical conditioning in paramecia? How is AC activity influenced by vibration? Is spatial distribution of membrane de/hyper-polorization relevant?

Possible Associative Learning Mechanisms in Paramecia Is learning occurring by a mechanism analogous to pre-synaptic facilitation in Aplysia, i.e. using an AC coincidence detector, and cAMP dependent state changes mediating memory?

AC Depolorization Ca 2+ channel Ca 2+ ATP cAMP PKA Gene mediated memory Reduced CDI Vibration Ciliary beat reversal

An Intra-cellular Hebbian Learning Mechanism I propose an abstract organization for an intra-cellular Hebbian mechanism, i.e. that depends on the extent of ciliary activity (post- synaptic effect) and not just on the coincidence between shock and vibration. This can be implemented for example using a PK, PKK cascade with positive feedback.

u1u1 u2u2 U2*U2* U1*U1* oPKK mPK* 2 mPK mPK* 1 mPK PKK + u 1 PKKu 1 PKK + u 2 PKKu Cilia feedback signal PKK V S

Existing Components oPKK activated along with the effecter by at least two iPKs Two iPKs themselves activated by another mPK only when they are bound to signal molecules or signal molecules themselves are phosp. directly. The oPKK should bind to signal molecules and specifically activate the appropriate mPK The mPK should have a very slow equilibrium compared to the other PKs.

Kinase Cascades with Positive Feedback

u1u1 u2u2 U2*U2* U1*U1* PKK PK* 2 PK PK* 1 PK PKK + u 1 PKKu 1 PKK + u 2 PKKu 2 Promotor Gene NS Phosphatase A more general mechanism

Constructing an Associative Learning Circuit Are such components known? How to go about finding networks in existence? How to go about making them and seeing if the idea works?

Acknowledgements T. Hennessey D. Stekel E. Szathmary J. Rowe