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Modeling synchronization in darkness and entrainment to light

Modeling synchronization in darkness and entrainment to light
of the mammalian SCN

Given by: Dr. Mark Ospeck
September 19, 2007
4:00pm, Manning Hall 201

ABSTRACT

The mammalian circadian oscillator, or superchiasmatic nucleus (SCN) is known to contain several thousand "clock" neurons in its ventrolateral (VL) part, each of which is a spontaneous biochemical oscillator with a period that ranges from between 22 to 28 hours (1). In complete darkness this network of neurons is able to strike a compromise period close to 24 hours, called its free running period (FRP).

The network's ability to synchronize to a common FRP requires finding the middle ground between a large population of nonlinear oscillators that has a substantial variation in the natural period of each neuron's oscillation. This synchronization problem is interesting because slow biochemical limit-cycles, approximately 24 hours long are somehow being phase-locked together by the exchange of very fast ~10 Hz action potentials. In our model we show that clock neurons are able to synchronize because they exchange a combination of neuropeptides and neurotransmitters that include VIP and GABA.

These phase-shift neuron biochemistry while imposing a circadian oscillation in the firing rate so that when the neuropeptide and neurotransmitter feedbacks are optimally phased the clock neuron network obtains a high degree of synchronization in complete darkness. The synched network's phase response curve (PRC) to skeleton photoperiods is consistent with experiments on mammals (1,2). During acquisition of an external light:dark cycle there is a transient increase in firing rate as the synchronized network entrains the circadian day in a manner similar to that of a lock-in amplifier.

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