MULTIPLE EFFECTS OF SEROTONIN ON MEMBRANE PROPERTIES OF TRIGEMINAL
MOTONEURONS IN VITROMULTIPLE EFFECTS OF SEROTONIN ON MEMBRANE PROPERTIES OF
TRIGEMINAL MOTONEURONS IN VITRO.
C.F. Hsiao, P.R. Trueblood, M.S. Levine and S.H. Chandler .
Department of Physiological Science and the Mental Retardation Research
Center, University of California at Los Angeles, California 90095.
APStracts 4:0066N, 1997.
ABSTRACT
Intracellular recordings from guinea pig trigeminal motoneurons (TMNs) in
brainstem slices were used to determine the underlying ionic mechanisms
responsible for our previously demonstrated enhancement of TMN excitability
during jaw movements by serotonin (5-HT). 5-HT (0.5-100 æM) depolarized
motoneurons, and increased input resistance in the majority of neurons tested.
Additionally, 5-HT reduced the amplitude of the post-spike medium duration AHP
(mAHP), decreased the current threshold for maintained spike discharge, and
increased the maximum slope of the steady-state spike frequency-current
relationship. Under voltage clamp, from holding potentials close to resting
potential, 5-HT produced an inward current and a decrease in instantaneous
slope conductance, suggesting a reduction in a resting K+ conductance. The
instantaneous I-V relationship for the inward 5-HT current (I5-HT) was linear
throughout most of the voltage range tested. However, the steady-state I-V
relationship showed some degree of inward rectification at potentials starting
around -70 mV. The mean reversal potential for the instantaneous I5-HT was -
86.2 + 4.5 mV (n=9), a value slightly negative to the predicted EK+ of -82 mV
in these neurons. In the presence of 2 mM Ba2+, 5-HT application did not
produce a further reduction in input conductance, but did expose a Ba2+-
insensitive residual inward current which was resistant to Cs+ application.
The instantaneous I-V relationship during 5-HT application in the presence of
Ba2+ was shifted downward, and parallel to control, suggesting that Ba2+ and
5-HT block the same resting leak K+ conductance. The residual Ba2+- and Cs+-
insensitive component of the total inward I5-HT was voltage-independent and
was blocked when the extracellular Na+ was replaced by choline, suggesting
that the predominant charge carrier for this residual current is Na+. 5-HT
enhanced a hyperpolarization-activated cationic current, (Ih). In the presence
of Ba2+, the time course of I5-HT resembled Ih and showed a similar voltage-
dependence which was blocked by extracellular Cs+ (1-3 mM). The effects of 5-
HT on membrane potential, input resistance and Ih were mimicked, partially, by
5-HT2 agonists and suppressed by 5-HT2 antagonists. It is concluded that 5-HT
enhances trigeminal motoneuron membrane excitability through modulation of
multiple intrinsic membrane conductances. This provides for a mechanism(s) to
fine tune the input-output discharge properties of these neurons, thus
providing them with greater flexibility in output in response to time- varying
synaptic inputs during various movements of the jaw.
Received 26 July 1996; accepted in final form 4 February 1997.
APS Manuscript Number J598-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 20 February 1997