Lc. Schenberg et Ta. Lovick, NEURONS IN THE MEDULLARY RAPHE NUCLEI ATTENUATE THE CARDIOVASCULAR-RESPONSES EVOKED FROM THE DORSOLATERAL PERIAQUEDUCTAL GREY-MATTER, Brain research, 651(1-2), 1994, pp. 236-240
In rats anaesthetised with alphaxalone/alphadolone, electrical stimula
tion in the dorsolateral part of the periaqueductal grey matter (FAG;
10 s trains of 1 ms pulses at 80 Hz, 40-80 mu A) evoked a presser resp
onse accompanied by tachycardia. Both components of the response were
attenuated following microinjection of 200 nl 0.1 M D,L-homocysteic ac
id into the caudal pole of the nucleus raphe magnus (NRM; n = 12) and
into the nucleus raphe obscurus (NRO; n = 22) to selectively activate
neuronal perikarya. Microinjection of 200 nl 165 mM NaCl into the same
region (n = 15) had no effect. The attenuation of the midbrain-evoked
cardiovascular responses lasted for 10-20 min and was independent of
changes in resting blood pressure and heart rate. The maximum reductio
n in the presser component of the midbrain-evoked responses was simila
r following stimulation in NRM (-35.4%) and NRO (-36.7%). However, the
reduction in the midbrain-evoked tachycardia was greater following st
imulation in NRM (-62.8%) compared to NRO (-27.2%). These results indi
cate that neurones in NRM and NRO may be involved in modulating the le
vel of excitability of the midbrain defence area in the PAG and/or its
efferent pathway.