Temporal modulation of antinociception by reciprocal connections between the dorsomedial medulla and parabrachial region.

Microinjection of carbachol into the dorsal parabrachial regio (PBRd) of guinea pigs induces analgesia from the 5th to the 15th min postinjection, as evaluated by the reduction of the vocalization in response to an electric shock applied to one paw. When reversible blockade of the dorsomedial medulla or specifically of the nucleus tractus solitarius (NTS) is performed with xylocaine 5 min after microinjection of carbachol into the PBRd, the analgesic effect continues up to the 45th and to the 60th min, respectively. Blockade of the dorsomedial medulla is achieved by topical application of xylocaine to the area postrema (AP) or microinjection of the drug into the NTS. A prolongation of the duration of the analgesic effect also occurs after the inverse procedure, i.e., after reversible blockade of the PBRd 5 min after topical application of carbachol (1 microgram/microliter)to the AP or microinjection of carbachol into the NTS. In this case, the analgesic action, which lasted up to 30 min when carbachol was applied to the AP and 60 min when microinjected into the NTS, was prolonged up to 60 min and to 80 min, respectively, after reversible blockade of PBR. The present data suggest that the reciprocal connections between the different regions of the dorsomedial medulla and the PBR play an important role in the modulation of the duration of the analgesic effect, and that this fact may be of adaptive importance in the defensive analgesia that occurs in the confrontation between prey and predator.