GABAA-mediated local synaptic pathways connect neurons in the rat
suprachiasmatic nucleus
George J. Strecker, Jean-Pierre Wuarin, and F. Edward Dudek
Department of Anatomy and Neurobiology, Colorado State University, Fort Collins, CO 80523 1670, USA
APStracts 4:120N, 1997.
ABSTRACT
The suprachiasmatic nucleus (SCN) in mammals functions as the biological clock
controlling circadian rhythms, but the synaptic circuitry of the SCN is
largely unexplored. Most SCN neurons use the neurotransmitter GABA, and
anatomical studies indicate many GABAergic synapses and local axon
collaterals; however, physiological evidence for synaptic communication among
SCN neurons is indirect. We have used three approaches to investigate local
circuitry in the SCN in acute hypothalamic slices from rat. First,
tetrodotoxin was used to block action- potential-dependent synaptic release,
which resulted in a decrease in the frequency of spontaneous synaptic currents
in SCN neurons, suggesting that spontaneously active neurons in the slice
connect synaptically to SCN neurons. Postsynaptic currents in SCN neurons were
also evoked by the selective stimulation of other SCN neurons with glutamate,
which avoids direct activation of axons that might originate outside the SCN.
Two different methods of glutamate microapplication (i.e., pressure ejection
and UV photolysis of caged glutamate) indicated that SCN neurons receive
GABAA-receptor-mediated synaptic input from other SCN neurons. In contrast,
glutamate-receptor-mediated synaptic connections between SCN neurons were not
detected. The GABAergic synapses that comprise the network described here
could conceivably be a substrate for the synchronization and amplification of
the circadian rhythm of SCN firing. Alternatively, this circuitry might
mediate other aspects of clock function such as the integration of
environmental and physiological information.
Received 12 May 1997; accepted in final form 30 June 1997.
APS Manuscript Number J387-7
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 24 July 1997