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J Neurophysiol (November 1, 2002). 10.1152/jn.00224.2002
Submitted on 27 March 2002
Accepted on 1 August 2002
1Rammelkamp Center for Education and Research, MetroHealth Medical System and 2Department of Neuroscience, Case Western Reserve University, Cleveland, Ohio 44109-1998
Kline, David D.,
Kristin N. Takacs,
Eckhard Ficker, and
Diana L. Kunze.
Dopamine Modulates Synaptic Transmission in the Nucleus of the
Solitary Tract. J. Neurophysiol. 88: 2736-2744, 2002. 10.1152/jn.00224.2002. Dopamine (DA) modulates the
cardiorespiratory reflex by peripheral and central mechanisms. The aim of this study was to examine the role of DA in synaptic transmission of
the nucleus tractus solitarius (NTS), the major integration site for
cardiopulmonary reflexes. To examine DA's role, we used whole cell,
voltage-clamp recordings in a rat horizontal brain stem slice. Solitary
tract stimulation evoked excitatory postsynaptic currents (EPSCs) that
were reduced to 70 ± 5% of control by DA (100 µM). The
reduction in EPSCs by DA was accompanied by a decrease in the paired
pulse depression ratio with little or no change in input resistance or
EPSC decay, suggesting a presynaptic mechanism. The D1-like agonist SKF
38393 Br (30 µM) did not alter EPSC amplitude, whereas the D2-like
agonist, quinpirole HCl (30 µM), depressed EPSCs to 73 ± 4% of
control. The D2-like receptor antagonist, sulpiride (20 µM),
abolished DA modulation of EPSCs. Most importantly, sulpiride alone
increased EPSCs to 131 ± 10% of control, suggesting a tonic
D2-like modulation of synaptic transmission in the NTS. Examination of
spontaneous EPSCs revealed DA reversibly decreased the frequency of
events from 9.4 ± 2.2 to 6.2 ± 1.4 Hz. Sulpiride, however,
did not alter spontaneous events. Immunohistochemistry of NTS slices
demonstrated that D2 receptors colocalized with synaptophysin and
substance P, confirming a presynaptic distribution. D2 receptors also
localized to cultured petrosal neurons, the soma of presynaptic
afferent fibers. In the petrosal neurons, D2 was found in cells that
were TH-immunopositive, suggesting they were chemoreceptor afferent
fibers. These results demonstrate that DA tonically modulates synaptic
activity between afferent sensory fibers and secondary relay neurons in
the NTS via a presynaptic D2-like mechanism.
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