Induction of chronic Fos-related antigens in rat brain by chronic morphine administration

Previous studies have shown that repeated exposure to cocaine or to several other stimuli induces novel 35-37 kDa Fos-related antigens (chronic Fras) in specific brain regions. Induction of these proteins is associated with prolonged increases in AP-1 DNA binding activity that parallel the long half-life of the chronic Fras in brain. In the current study, we characterized regulation of the chronic Fras in response to prolonged exposure to morphine. After 5 days of morphine treatment, we observed increased levels of the chronic Fras and of AP-1 binding activity in rat striatum and nucleus accumbens, effects that were not seen in most other brain regions that we studied. Concomitant administration of naltrexone, an opioid receptor antagonist, prevented the induction of these proteins. Two-dimensional gel analysis showed that the chronic Fras induced by chronic morphine administration are identical to those induced after chronic cocaine and other treatments. A time course study indicated that chronic Fra induction was first apparent after 3 days of morphine treatment and peaked between 5 and 7 days of treatment in both the striatum and nucleus accumbens. Withdrawal studies demonstrated robust induction of several known acute Fras, including c-Fos, FosB, Fra-1, Fra-2, and delta FosB, at 6 hr after naltrexone precipitation of withdrawal in the striatum, nucleus accumbens, and several other brain regions. Levels of these proteins returned to basal values by 72 hr. In contrast, no further induction of the chronic Fras was evident after 6 hr of withdrawal in the striatum and nucleus accumbens, but levels of the proteins increased beyond their already elevated chronic morphine values after longer periods (72 hr) of withdrawal, even though physical withdrawal symptoms had resolved at this time point. Chronic Fras were also induced after these prolonged withdrawal periods in several other brain regions, where the proteins were not induced by chronic morphine alone. We discuss the possible roles played by the chronic Fras in the adaptive responses of the brain to chronic opiate exposure and opiate withdrawal.