Tetanic Stimulation Induces Short-term Potentiation of Inhibitory Synaptic
Activity in the Rostral Nucleus of the Solitary Tract.
Gintautas Grabauskas and Robert M. Bradley.
Department of Biologic and Materials Sciences, School of Dentistry,
University of Michigan, Ann Arbor, MI 48109-1078, Department of Physiology,
Medical School, University of Michigan, Ann Arbor, MI 48109-0622.
APStracts 4:275N, 1997.
ABSTRACT
Whole cell recordings from neurons in the rostral nucleus of the solitary
tract (rNST) were made to explore the effect of high frequency tetanic
stimulation on inhibitory postsynaptic potentials (IPSPs). IPSPs were elicited
in the rNST by local electrical stimulation after pharmacological blockade of
excitatory synaptic transmission. Tetanic stimulation at frequencies of 10 -
30 Hz resulted in sustained hyperpolarizing IPSPs which had a mean amplitude
of -68 mV. The hyperpolarization resulted in a decrease in neuronal input
resistance and was blocked by the GABAA antagonist bicuculline. For most of
the neurons (n = 87/102) tetanic stimulation resulted in a maximum
hyperpolarization immediately after initiation of the tetanic stimulation, but
for some neurons the maximum was achieved after 3 or more consecutive shock
stimuli in the tetanic train of stimuli. When the extracellular Ca2+
concentration was reduced the maximum IPSP amplitude was reached after several
consecutive shock stimuli in the tetanic train for all neurons. Tetanic
stimulation at frequencies of 30Hz and higher resulted in IPSPs that were not
sustained but decayed to a more positive level of hyperpolarization. In some
neurons the decay was sufficient to become depolarizing and resulted in a
biphasic IPSP. It was possible to evoke this biphasic IPSP in all the neurons
tested if the cells were hyperpolarized to -75 to -85 mV. The ionic mechanism
of the depolarizing IPSPs was examined and was found to be due to an elevation
of the extracellular K+ concentration and accumulation of intracellular Cl-.
Tetanic stimulation increased the mean 80ms decay time constant of a single
shock evoked IPSP up to 8s. The length of the IPSP decay time constant was
dependent on the duration and frequency of the tetanic stimulation as well as
the extracellular Ca2+ concentration. Afferent sensory input to the rNST
consists of trains of relatively high frequency spike discharges similar to
the tetanic stimulation frequencies used to elicit the IPSPs in the brain
slices. Thus, the short-term changes in inhibitory synaptic activity in the
slice preparation probably occur in vivo and may play a key role in taste
processing by facilitating synaptic integration.
Received 3 April 1997; accepted in final form 26 September 1997.
APS Manuscript Number J271-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 October 1997