NMDA Receptors Contribute to Primary Visceral Afferent Transmission in the
Nucleus of the Solitary Tract.
Maria Luz Aylwin, John M. Horowitz, and Ann C. Bonham.
Department of Internal Medicine, Division of Cardiovascular Medicine and
the Department of Neurobiology, Physiology and Behavior, University of
California, Davis, Davis, CA 95616.
APStracts 4:0036N, 1997.
ABSTRACT
The nucleus of the solitary tract (NTS) is a principal site for coordinating
the reflex control of autonomic function. The nucleus receives and organizes
primary visceral (sensory) afferent inputs from the great vessels, heart, lung
and gastrointestinal organs. Glutamate, the excitatory neurotransmitter
released by the primary afferent fibers, activates non-NMDA receptors on
second-order neurons in the NTS. Still in question is whether NMDA receptors
on the second-order neurons are also activated. Accordingly, the purpose of
this study was to directly determine whether NMDA receptors contribute to
synaptic transmission of primary visceral afferent input to second-order
neurons in the NTS. Whole-cell patch clamp recordings were obtained from
intermediate and caudal NTS neurons in rat coronal medullary slices.
Excitatory postsynaptic currents (EPSCs) were evoked by stimulation of the
solitary tract (1 - 25 V, 0.1 ms, 0.2 or 0.5 Hz) at membrane potentials
ranging from - 90 to + 60 mV. In 28 of 32 neurons in which current-voltage
relationships were obtained for solitary tract-evoked EPSCs, the currents had
short onset latencies (3.42 ñ 1.03 ms) indicating that they were the result of
monosynaptic activation of second-order neurons. Solitary-tract evoked EPSCs
had both a fast and a slow component. The amplitude of the slow component was
non-linearly related to voltage (being revealed only at membrane potentials
positive to - 45 mV); blocked by the NMDA receptor antagonist, APV (50 mM; n =
12; P = 0.0001); and enhanced in nominally Mg}2+} free }perfusate at membrane
potentials negative to - 45 mV (n = 5; P = 0.016), demonstrating that the slow
component was mediated by NMDA receptors. The amplitude of the fast component
was linearly-related to voltage and blocked by the non-NMDA receptor
antagonist, NBQX (3 mM; n = 9; P = 0.0014), demonstrating that the fast
component was mediated by non-NMDA receptors. The slow component of the EPSCs
was not blocked by NBQX (n = 6; P = 0.134), nor was the fast component blocked
by APV (n = 12; P = 0.124). These results show that both NMDA and non-NMDA
receptors coexist on the same second-order NTS neurons and mediate primary
visceral afferent transmission in the NTS. The participation of NMDA receptors
suggests that second-order neurons in the NTS may have previously unrecognized
integrative capabilities in the reflex control of autonomic function.
Received 10 September 1996; accepted in final form 9 January 1997.
APS Manuscript Number J722-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 February 1997