Effects of morphine withdrawal on food competition hierarchies and fighting behavior in rats.

Abstract

The effects of chronic morphine administration and morphine withdrawal on competitive and fighting behaviors in rats were studied in a novel paradigm. Fixed-ratio 20 (FR-20) lever-pressing for food presentation was stabilized before placing pairs of rats, matched by response rate and body weight, in the behavioral testing chambers. Initially, overt fighting occurred, but in 60% of the pairs it dissipated within several sessions, accompanied by the development of stable dominant-submissive responding hierarchies. Invariably, the dominant rat in the initial fighting bouts became the dominant responding rat. Chronic morphine administration and subsequent withdrawal in one or both members of the pairs led to disruptions or reversals in the distribution of responding and to small increases in fighting. These effects were seen as early as 12 h after the second dose of morphine and, in some pairs, lasted at least 28 days after morphine was withdrawn. The results from experiments in which only one rat was made dependent show that both the withdrawn dominant rats and the withdrawn submissive rats contributed to the behavioral changes observed in the pairs during withdrawal. Differential food deprivation in pairs not exposed to morphine did not change the dominant-submissive hierarchies. Significant changes in the distribution of FR-20 responding in the paired condition were evident after FR-20 responding in the unpaired condition returned to control rates. It is therefore concluded that change in so-called appetite for food is not an important determinant of the hierarchical changes observed during protracted withdrawal.