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Physics Faculty Present their Research, Part III

Firouzeh Sabri and Mark Ospeck

Physics Department, University of Memphis, TN 38152

September 17, 2008, 4:00pm, Manning Hall 201

Refreshments served at 3:30pm, Manning Hall 222

Dr. Firouzeh Sabri:         Biocompatible Materials and Aerogels

Dr. Mark Ospeck:                Light-Dark Cycle Memory in the Mammalian SCN

Abstract: The mammalian circadian oscillator, or superchiasmatic, nucleus (SCN) contains several thousand “clock” neurons in its ventrolateral (VL) part, many of which are spontaneous biochemical oscillators with periods that range from 22 to 28 hours.  In complete darkness this clock neuron network synchronizes through the exchange of action potentials that release a neuropeptide called vasoactive intestinal polypeptide (VIP).  Neuropeptides are transcribed neurotransmitters, thus they offer a natural mechanism to couple between RNA transcription and firing rate on a neural network. When clock neurons synchronize together in constant darkness they strike a compromise, free-running period that is close to 24 hours long.  We lock Siberian hamsters to various light-dark cycles, such as 10 hours of light followed by 14 hours of darkness (10-14 LD), and then track their activity into constant darkness in order to show that they retain a memory for the particular light-dark cycle to which they were entrained, before then returning to their own free-running period.  Next, using Leloup-Goldbeter mammalian clock neurons (1) we model the clock neuron network and show that strong rythmicity of the VIP oscillation can account for its synchronization in constant darkness and also explain the light-dark cycle memory that we experimentally observe.   Additionally, light is known to initiate a MAP kinase signaling cascade that induces transcription of the messenger RNAs for the clock protein per and also for an mkp1, a phosphatase that turns off MAP kinase. Our simulations show that the strongly nonlinear phosphatase-kinase interaction can account for the dead zone in the mammalian phase response curve, the time of day when our circadian clock is insensitive to phase-shifting by light.  We hypothesize that the mammalian SCN acts like a lock-in amplifier, in that it displays memory and rejects noise while it entrains the light edges of the circadian day.

1. Leloup and Goldbeter, Toward a detailed computational model for the mammalian circadian clock, PNAS 100:7051-7056 (2003)

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