NEUROTRANSMISSION OF THE BEZOLD-JARISCH REFLEX IN THE NUCLEUS-TRACTUS-SOLITARII OF SINOAORTIC DEAFFERENTATED RATS

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 aci d (EAA) receptors, into the commissural nucleus tractus solitarii (NTS ) of sine-aortic deafferentated (SAD) and sham-operated (SO) unanesthe tized rats. Increasing doses of kynurenic acid produced a dose-depende nt blockade of the bradycardic and hypotensive responses to B-J reflex activation in both SO (from 0.1 to 10.0 nmol/100 nl) and SAD (from 0. 1 to 2.0 nmo1/100 nl). Comparison of the effect of kynurenic acid on t he hypotension and bradycardic dose-response curves showed a significa nt difference between SO and SAD rats, indicating that smaller doses o f 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 nmo1/100 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 i n the absence of the arterial baroreceptors, the B-J reflex plays an i mportant role in the autonomic regulation of the circulation. The data also show different dose-response curves for hypotension and bradycar dia 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, indicati ng 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 rat s.