9:45 AM - EP05.03.04
Multiple Mechanisms of Short-Term Presynaptic Plasticity Realized in Peptide-Doped Biomembranes
Joseph Najem1,2,Patrick Collier1,Stephen Sarles2
Oak Ridge National Laboratory1,The University of Tennessee, Knoxville2
It is through modifications of synaptic transmission (i.e., synaptic plasticity) between neurons that the brain is capable of adapting, learning, and modifying its functions in response to stimuli. Synaptic transmission can be either enhanced or depressed by brain activity, and these changes can last from milliseconds to a lifetime. They can occur in the pre-synaptic terminal (e.g., facilitation, PPF and PTP, depression, PPD, and augmentation) or in the post-synaptic terminal (e.g., long-term potentiation, LTP, and depression, LTD). Undoubtedly, each of these mechanisms is essential to the brain and plays major roles such as spatio-temporal filtering, spatial learning, emotional memory, and habituation. Therefore, neuromorphic computing systems that aim to emulate the brain’s parallel architecture and co-location of memory and processing, will require hardware that closely mimics synapses, including possibly multiple mechanisms of plasticity. To date, two-terminal metal-oxide-metal memory resistors (i.e., memristors) have been built, and, while they succeed at emulating mostly long-term memory, they bypass various synaptic functionalities and ignore the actual mechanisms governing synaptic plasticity. Moreover, they remain highly power-hungry, noisy, and rigid—unlike actual synapses. Conversely, neuromorphic hardware needs to be energy efficient, fault tolerant, and preferably soft. To this end, we have previously built a biomolecular memristive device that consisted of an insulating biomembrane doped with voltage-driven, pore-forming peptide, alamethicin. We demonstrated memristance through pinched hysteresis in the current-voltage plane.
Here, we discuss how multiple mechanisms of presynaptic plasticity can be achieved using a different peptide, known as Monazomycin. In response to supra-threshold potentials (>100 mV), monazomycin inserts into the insulating membrane and aggregates to form memristive ionic conductive pathways. When the voltage bias is dropped below the threshold, the peptides exit the membrane—which reclaims its insulating state. Our current-voltage results displayed diode-like pinched-hysteresis, confirming its memristive nature. Compared to alamethicin, monazomycin exhibited a significantly larger hysteresis and much slower formation and decay rates. In response to a stepwise increase in voltage from 10 mV to 150 mV the current exhibited an S-shape response with a time constant of around 20 s. Dropping from the voltage to 10 mV showed that the pores remained open for around 3 minutes before fully exiting the membrane. We demonstrate that in response to a train of pulses the system exhibited facilitation (PPF and PTP), depression, and augmentation. We discuss how the key mechanisms responsible of plasticity in our device depend on channel kinetics, which are similar to those found in biological synapses.