Some factors controlling oral morphine intake in rats.

Rats were given the opportunity to drink morphine solution following stabilization at three levels of 'passive premedication'. Compared to saline treated controls, premedicated rats consumed more morphine solution, but medication level did not significantly affect morphine intake. Premedicated rats adjusted to a reduction in morphine solution concentration by increasing fluid intake substantially, but nonpremedicated rats did not. When morphine was offered in a vehicle of isotonic saline oral consumption rose sharply in premedicated rats but not in their nonpremedicated counterparts. 'Drinker' and 'nondrinker' rats were identified on the basis of initial response to oral morphine. Premedication eliminated resistance to morphine drinking, but even at the expense of severe fluid deprivation, nonpremedicated nondrinkers refused morphine throughout the entire experiment. [ABSTRACT FROM AUTHOR]