Baroreceptor induced inhibition of sympathetic neurones by gaba
acting at a spinal site.
Lewis, D. I., & J. H. Coote.
Department of Physiology, The Medical School, University of
Birmingham, Birmingham B15 2TT UK
APStracts 2:0515H, 1995.
GABA synapses are an important feature of sympathetic circuits in the
spinal cord. The possibility that these spinal synapses participate
in the reduction of sympathetic activity resulting from arterial
baroreceptor activation was tested. For this purpose baroreceptors
were stimulated by a rapid rise in blood pressure induced by i.v.
phenylephrine and the effect of this on a spinally evoked excitatory
response in a renal sympathetic nerve examined before, during and
after removal of tonic excitatory drive from the rostral
ventrolateral medulla (RVLM). Stimulation of descending excitatory
axons in the spinal cord at the C4 spinal level evoked a biphasic
excitatory response in a renal sympathetic nerve, with a latency of
56 +/- 6 ms and duration of 230 +/- 24 ms. The magnitude of this
spinally evoked response was significantly reduced [p < 0.001]
during baroreceptor activation, thus confirming that there is a
spinal component of the baroreceptor inhibitory reflex. Intrathecal
administration of bicuculline reduced baroreceptor mediated
inhibition of the spinally evoked response to 48 +/- 18 % of control
inhibition (p < 0.05). Subsequent removal of tonic excitatory
drive by microinjection of glycine into the RVLM produced no
significant further change in the baroreceptor mediated inhibition.
This suggests that a GABAA mediated effect at a spinal site is
involved in the baroreceptor induced inhibition. Reversing this
procedure by first placing injections of glycine into the RVLM
resulted in a reduction of baroreceptor inhibition to 60 +/- 16 % (p
< 0.05) of control baroreceptor inhibition. Subsequent
intrathecal administration of bicuculline produced no significant
further change in the baroreceptor mediated inhibition of the
spinally evoked response. This suggests that the GABAA mediated
effect of the baroreceptors at a spinal site is dependent on the
integrity of neurones in the RVLM. We conclude that GABA in
sympathetic circuits in the spinal cord plays a significant role in
the modification of sympathetic vasomotor activity by the
baroreceptor reflex.
Received 21 February 1995; accepted in final form 25 October
1995.
APS Manuscript Number H163-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 8 December 95