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Journal of Neuroscience, Vol 12, 1-20, Copyright © 1992 by Society for Neuroscience
Intracellular analysis in vivo of different barosensitive bulbospinal neurons in the rat rostral ventrolateral medulla
AR Granata and ST Kitai
Collge of Medicine, University of Tennessee.
Neurons located in the rostral ventrolateral medulla (RVLM) with
projections to the intermediolateral column (IML) in the spinal cord were
electrophysiologically characterized and anatomically identified using an
intracellular recording technique in vivo. A group of spontaneously active
neurons was antidromically activated by electrical stimulation of the IML
in the thoracic spinal cord (T2-T3 level). The axonal conduction velocities
ranged from 1.5 m/sec to 11.0 m/sec; mean value, 5.5 +/- 2.6 m/sec (+/-
SD). The firing pattern and changes in membrane potential in relation to
the cardiac cycle were investigated in these bulbospinal neurons. A first
group discharged action potentials with higher frequency at the end of the
diastolic/beginning of the systolic period. The average of the neuronal
membrane potentials demonstrated depolarizing potentials at the end of the
diastolic/beginning of the systolic period. These depolarizing potentials
increased in magnitude when the neurons were hyperpolarized. Therefore,
they were characterized as EPSPs. The baroreceptor reflex activation
produced by the increase in systemic arterial pressure following
intravenous injection of phenylephrine elicited hyperpolarization, a
decrease in the rate of discharge, and an increase in the membrane input
resistance, suggesting that a disfacilitatory effect was produced by the
activation of baroreceptor inputs on these bulbospinal neurons. Conversely,
the inactivation of the baroreceptor reflex by intravenous injection of
sodium nitroprusside produced depolarization and an increase in the firing
rate. These neurons were characterized as baroreceptor-sensitive type I
neurons. A second group of bulbospinal neuron in the RVLM was
differentiated from the first group because it demonstrated a decrease in
the frequency of discharge at the end of the diastolic/beginning of the
systolic period. The average of the membrane potentials showed
hyperpolarizing potentials that decreased in magnitude when the neuron was
hyperpolarized. These hyperpolarizing potentials occurred at the end of the
diastolic/beginning of the systolic period and were reversed in polarity
after intracellular injections of chloride ions for several minutes.
Therefore, these potentials were characterized as chloride- dependent IPSPs
locked to the cardiac cycle. In some of these neurons, the electrical
stimulation of the IML produced, in addition to the antidromic action
potential, a monosynaptic EPSP with a shorter latency. Based on these
unique characteristics, these neurons were defined as barosensitive type II
neurons. During constant baroreceptor inactivation achieved by the
hypotension produced by intravenous infusions of sodium nitroprusside, the
pattern of discharge of barosensitive type II neurons became very regular,
and the IPSPs locked to the cardiac cycle were absent.(ABSTRACT TRUNCATED
AT 400 WORDS)
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