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
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.