The role of the catecholaminergic mechanism in foot shock (FS) stress- and immobilized-water immersion (IW) stress-induced analgesia in mice.

Involvement of the catecholaminergic mechanism in foot shock (FS)- and immobilized-water immersion (IW)-stress-induced analgesia (SIA) and in the development of tolerance to the effect were investigated in mice. With daily treatment with clonidine or daily exposure to stresses, tolerance developed rapidly to the analgesic effect. Clonidine-induced analgesia, which could not be antagonized by naloxone, was potentiated in the animals rendered tolerant to FS-stress, and it was attenuated in the animals tolerant to IW-SIA. On the other hand, animals tolerant to clonidine failed to show the attenuation of FS- and IW-SIA. The analgesic effect of clonidine and the development of tolerance to the effect were not influenced by reserpine. However, reserpine pretreatment completely suppressed the analgesic effect induced by FS- and IW-stresses on the 1st day; but with daily exposure to the stress, the analgesic effect gradually appeared and returned to the control level on the 5th day. These results indicate not only the differences between clonidine analgesia and SIAs but also those between each SIA. Thus, the central catecholaminergic mechanisms play an important role in these SIAs and also in the development of tolerance to the effect, although the degree of participation of these mechanisms seems to be somewhat different between FS- and IW-SIA, as indicated by the cross-tolerance between clonidine analgesia and each SIA.