Odum, A. L., Haworth, S. C., & Schaal, D. W. (1998).
Food-deprivation level alters the effects of morphine on pigeons' key pecking.
Journal of the Experimental Analysis of Behavior,
69, 295-310.
Four pigeons pecked response keys under a multiple fixed-ratio 30
fixed-interval 5-min schedule of food presentation. Components
alternated separated by 15-s timeouts; each was presented six
times. Pigeons were maintained at 70%, 85%, and greater than 90%
of their free-feeding weights across experimental conditions.
When response rates were stable, the effects of morphine (0.56 to
10.0 mg/kg) and saline were investigated. Morphine reduced
response rates in a dose-dependent manner under the fixed-ratio
schedule and at high doses under the fixed-interval schedule. In
some cases, low doses of morphine increased rates under the
fixed-interval schedule. When pigeons were less food deprived,
reductions in pecking rates occurred at lower doses under both
schedules for 3 of 4 birds compared to when they were more food
deprived. When pigeons were more food deprived, low doses of
morphine increased rates of pecking in the initial portions of
fixed intervals by a greater magnitude. Thus, food-deprivation
levels altered both the rate-decreasing and rate- increasing
effects of morphine. These effects may share a common mechanism
with increased locomotor activity produced by drugs and with
increased drug self-administration under conditions of more
severe food deprivation.
Key words: morphine, fixed-interval schedules, fixed-ratio
schedules, food deprivation, key peck, pigeon