Brain sites involved in the antinociceptive effect of bradykinin in rats

1 The localization of brain sites where bradykinin (BK) induces its antinoc iceptive effect in rats, was studied using as index the threshold for the j aw-opening reflex elicited by the dental pulp electrical stimulation test ( DPEST). 2 The microinjection of BK into the lateral or fourth cerebral ventricles i nduced an antinociceptive effect, with Index of Antinociception (IA) of 0.5 1+/-0.03 and 0.68+/-0.05, respectively. However, microinjections of the pep tide into the third ventricle induced a less marked antinociception (IA = 0 .28 +/- 0.08). 3 The brain sites where the microinjection of BK caused an antinociceptive effect were: locus coeruleus, principal nucleus, oral part of the spinal se nsorial trigeminal nucleus, and the sensory root of the trigeminal nerve. 4 The antinociceptive effect was more intense when BK (4-16 nmol) was injec ted into the locus coeruleus. Microinjection of BK (4 nmol) into the fourth ventricle, but not into the locus coeruleus, induced an increase in blood pressure. The microinjection of the peptide into the nucleus tractus solita rius, a site that is also involved in the presser effect of BK, did not ind uce an antinociceptive effect. These results indicate that the antinocicept ive effect of BK is not related to blood pressure changes. 5 The microinjection of BK into some of the sites involved in the mechanism s of analgaesia, including the periaquenductal gray matter (dorsal, lateral and ventrolateral) and the dorsal raphe nucleus did not induce an antinoci ceptive effect. 6 The results suggest that the most likely brain sites involved in the anti nociceptive effect of BK are the locus coeruleus and the principal sensory trigeminal nucleus. The present results did not exclude the involvement of other brain sites surrounding the lateral and the third ventricles.