Dynamics of sensory afferent synaptic transmission in aortic baroreceptor
regions of nucleus tractus solitarius.
Andresen, Michael C., and Mingyong Yang.
Department of Physiology, Oregon Health Sciences University, Portland,
Oregon 97201-3098.
APStracts 2:0093N, 1995.
SUMMARY AND CONCLUSIONS
1. Synaptic responses of medial nucleus tractus solitarius (mNTS) neurons to
solitary tract (ST) activation were studied in a horizontal brainslice
preparation of the rat medulla. Slices included sections of ST sufficiently
long that the ST could be electrically activated several mm's from the
recording site of cell bodies in mNTS. 2. Three types of synaptic events were
evoked in response to tract stimulation: simple excitatory postsynaptic
potentials (EPSPs), simple inhibitory PSP (IPSPs) and complex EPSP-IPSP
sequences. Simple EPSPs had substantially shorter latencies than IPSPs (3.39
msec +/- 0.65, n=42 vs. 5.86 +/- 0.71 msec (n=6), respectively). 3. EPSP
amplitude increased linearly with increasing hyperpolarization with an
extrapolated reversal potential near 0 mV. 4. EPSPs were maximal at <0.5 Hz
of sustained, constant frequency ST stimulation (n=14). EPSP amplitude
declined to an average of 57.5% of control at 10 Hz after 2 seconds of
sustained stimulation. With one minute of sustained, 100 Hz stimulation, EPSP
amplitude declined to near zero. 5. With stimuli intermittently delivered as
bursts of 100 msec each 300 msec, generally comparable average EPSPs were
evoked during constant and burst patterns of ST stimulation. The amplitude of
the initial EPSP in each burst was very well maintained even at intraburst
stimulation rates of 100 Hz. 6. At resting membrane potentials, low constant
frequencies of ST stimulation (<5 Hz) reliably elicited action potentials
and suppressed spontaneous spiking, but higher frequencies led to spike
failures (at 100 Hz >85%). Between 5 and 10 Hz, this periodic
stimulation/suppression cycle clearly entrained action potential activity to
the ST stimuli. Similar patterns of current pulses (5msec) reliably evoked
action potentials with each pulse to higher frequencies (50 Hz) without
failures and entrainment was similar to ST stimulation. 7. In a subset of NTS
neurons (3 of 9 studied), bursts of ST stimuli were as much as 50% more
effective at transmitting high frequencies (>10 Hz) of ST stimulation than
the equivalent constant frequencies (p<0.0001). 8. The long latency, simple
IPSPs with no preceding EPSP reversed to become depolarizing at potentials
more negative than -62.9 mV +/- 7.0 (n=5) and were blocked by the non-NMDA
antagonist CNQX (n=3). The ST stimulation frequency-response relation of these
IPSPs was similar to that for short latency EPSP response excited by ST
synapses. Thus, these IPSPs appear to be activated polysynaptically via a
glutamatergic-GABAergic sequence in response to ST activation. 9. The results
suggest that sensory afferent synapses in mNTS have limited transmission of
high frequency inputs. Both synaptic transmission and the characteristics of
the postsynaptic neuron importantly contribute to the action potential
transmission from afferent to NTS neuron and beyond. This overall frequency
response limitation may contribute to the accommodation of reflex responses
from sensory afferent inputs such as arterial baroreceptors within their
physiological discharge frequency range.
Received 22 March 1994; accepted in final form 24 March 1995.
APS Manuscript Number J-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 25 April 1995.