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Volume 16, Number 10,
Issue of May 15, 1996
pp. 3521-3533
Copyright ©1996 Society for Neuroscience
Neuropeptide Y Depresses GABA-Mediated Calcium Transients in
Developing Suprachiasmatic Nucleus Neurons: A Novel Form of Calcium
Long-Term Depression
Received Oct. 24, 1995; revised Feb. 29, 1996; accepted March 4, 1996.
Karl Obrietan1 and
Anthony N. van den Pol1, 2
1 Department of Biological Science, Stanford
University, Stanford, California 94305, and 2 Section of
Neurosurgery, Yale University, School of Medicine, New Haven,
Connecticut 06520
In contrast to its inhibitory role in mature neurons, GABA can
exert excitatory actions in developing neurons, including mediation of
increases in cytosolic Ca2+. Modulation of this
excitatory activity has not been studied previously. We used
Ca2+ digital imaging with Fura-2 to test the
hypothesis that neuropeptide Y (NPY) would depress GABA-mediated
Ca2+ rises in neurons cultured from the
developing suprachiasmatic nucleus (SCN). SCN neurons were chosen as a
model system for this study because SCN neurons are primarily
GABAergic, they express high levels of NPY and GABA receptors, and
functionally, NPY causes profound phase-shifts in SCN-generated
circadian rhythms.
Vigorous GABA-mediated Ca2+ activity was found in
young SCN neurons that were maintained in vitro for 4-14 d.
NPY showed a dose-dependent rapid depression of the amplitude of
Ca2+ rises generated by GABA released from
presynaptic SCN axons. NPY exerted a long-term depression of cytosolic
Ca2+ in the majority of neurons tested, which
lasted more than 1 hr after NPY washout. The magnitude of the NPY
depression was dose-dependent. NPY did not affect
Ca2+ levels when GABAA
receptor activity was blocked by bicuculline; however, when bicuculline
and NPY were withdrawn from the perfusion solution, the subsequent
Ca2+ rise was either significantly reduced or
completely absent, suggesting that the NPY receptor was activated in
the absence of elevated intracellular Ca2+ and
GABAA receptor activity, and that the latent
effect of NPY was revealed only after depolarizing GABA stimulation was
renewed. Pretreating neurons with pertussis toxin greatly reduced the
ability of NPY to depress GABAergic Ca2+ rises,
suggesting that the NPY modulation of the GABA activity was based
largely on a mechanism involving pertussis toxin-sensitive
Gi/Go proteins.
NPY receptor stimulation depressed (<30%) postsynaptic
Ca2+ rises evoked by GABA (20 µM) application in the presence of tetrodotoxin
(TTX). The effects of NPY were mimicked by the NPY Y1 receptor agonist
[Pro34,Leu31] NPY and the
Y2 receptor agonist NPY 13-36 and by peptide YY (PYY). Together, our
data suggest that the Y1 and Y2 type NPY receptors act both
presynaptically and postsynaptically to depress GABA-mediated
Ca2+ rises. If related mechanisms exist in
peptide modulation of inhibitory GABA activity in mature neurons, this
could underlie long-term changes in the behavior of neurons of the SCN
necessary for phase-shifting the circadian clock by NPY. NPY also
modulated GABA responses in neuroendocrine neurons from the
hypothalamic arcuate nucleus. NPY thus can play an important role in
evoking long-term depression of GABA-mediated
Ca2+ activity in these developing neurons,
allowing NPY-secreting cells to modulate the effects of GABA on neurite
outgrowth, gene expression, and physiological stimulation. This is the
first example of such a cellular memory: that is, long-term
Ca2+ depression based on modulation of
depolarizing GABA activity.
Key words:
NPY;
GABAA receptor;
suprachiasmatic nucleus;
arcuate nucleus;
calcium;
neuroendocrine;
modulation
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