Neurotransmission of the Bezold-Jarisch reflex in the nucleus tractus solitarii of sino-aortic deafferentated rats.

Abstract

The Bezold-Jarisch _B-J. reflex was activated by serotonin _5-HT, i.v.. before and 10 min after bilateral microinjection of increasing doses of kynurenic acid, a non-selective antagonist of excitatory amino acid _EAA. receptors, into the commissural nucleus tractus solitarii _NTS. of sino-aortic deafferentated _SAD. and sham-operated _SO. unanesthetized rats. Increasing doses of kynurenic acid produced a dose-dependent blockade of the bradycardic and hypotensive responses to B-J reflex activation in both SO _from 0.1 to 10.0 nmolr100 nl. and SAD _from 0.1 to 2.0 nmolr100 nl.. Comparison of the effect of kynurenic acid on the hypotension and bradycardic dose–response curves showed a significant difference between SO and SAD rats, indicating that smaller doses of kynurenic acid are required in SAD rats than in SO rats to block the neurotransmission of the B-J reflex in the NTS. The data also showed that bilateral microinjection of kynurenic acid into the NTS at doses of 0.5 _131"7 vs. 115"8 mmHg. and 2.0 nmolr100 nl _140"11 vs. 116"9 mmHg. produced an acute and significant increase in the basal mean arterial pressure of SAD rats similar to that observed with the same doses in SO rats, which was back to control values 5–10 min later. The increase in basal mean arterial pressure immediately after kynurenic acid microinjection into the NTS of SAD rats suggests that in the absence of the arterial baroreceptors, the B-J reflex plays an important role in the autonomic regulation of the circulation. The data also show different dose–response curves for hypotension and bradycardia in response to B-J reflex activation in SAD than in SO rats in the presence of increasing doses of kynurenic acid into the NTS, indicating that the neurotransmission of the B-J reflex in the NTS of SAD rats is more sensitive to the blockade of the EAA receptors than in SO rats.