Department of Anesthesiology, Medical College of Wisconsin, and the Zablocki VA Medical Center, Milwaukee, Wisconsin 53295, USA
Address for correspondence: Jeanne L. Seagard, Ph.D., Research Service 151, VA Medical Center, 5000 W. National Ave., Milwaukee, WI 53295. Voice: 414-384-2000, ext. 41589; fax: 414-645-6550.
jseagard{at}mcw.edu
Afferent input from barosensitive receptors, including carotid
baroreceptors and cardiac mechanoreceptors, has been found to
produce different types of discharge patterns in neurons in
the nucleus tractus solitarius (NTS). The discharge patterns
of the neurons may be dependent on many factors, including input
from the different barosensitive receptor subtypes, the contribution
of different ionotropic glutamate receptors [NMDA (N-methyl-D-aspartate)
versus nonNMDA receptors] in transmission of the input, effects
of different neuropeptide neurotransmitters/neuromodulators
on afferent transmission, or the order of the neuron within
the barosensitive reflex arc. It is not clear if the roles of
the glutamate receptor subtypes are the same for neurons activated
by the different barosensitive inputs. In addition, the amount
of afferent input from the barosensitive receptors, due to increases
or decreases in stimulating pressures, may result in altering
the roles of the ionotropic glutamate receptor subtypes. While
most evidence suggests that nonNMDA receptors play the greatest
role in the transmission of afferent activity to second-order
NTS neurons, it is possible that increases in afferent input
may lead to an enhanced role for NMDA receptors in the transmission
of the barosensitive input, since increased depolarization of
the NTS neurons may lead to removal of a Mg
2+ block of the NMDA
channel. Transmission of baroreceptor input at third- and higher-order
neurons has been found to involve both nonNMDA and NMDA receptors,
suggesting a possible functional role for the distribution of
these receptor types. The roles of these different factors in
the initiation of NTS neuronal discharge will be discussed.
