Administration of myr(+)-G(i2)alpha subunits prevents acute tolerance (tachyphylaxis) to mu-opioid effects in mice.

The administration of efficacious doses of morphine or beta-endorphin causes acute tolerance (tachyphylaxis) to the effects of additional administrations of these opioids. Mice intracerebroventricularly (icv)-injected with biologically active myristoylated (myr(+))-G(i2)alpha subunits developed no tachyphylaxis to morphine antinociception in the tail-flick test. This treatment increased the potency of opioid-induced analgesia during the declining phase. Moreover, animals showing tachyphylaxis to opioid effects exhibited normal responses to the agonists after icv-administration of myr(+)-G(i2)alpha subunits. In morphine tolerant/dependent mice, an icv dose of 12 pmol/mouse myr(+)-G(i2)alpha subunits facilitated complete restoration of morphine antinociception in only 4 or 5 days instead of the 10 to 11 days required for post-dependent mice. This was observed when myr(+)-G alpha subunits were injected within the first 24 h of chronic morphine administration -- but not later when long-term tolerance takes place. These results suggest that during the course of an opioid effect a progressive reduction of receptor-regulated G-proteins occurs, and hence tachyphylaxis develops. Exogenous administration of myr(+)-G alpha subunits may be of therapeutic potential in improving agonist activity and accelerating the recovery of post-dependent receptors.