Effects of naloxone and pimozide on initiation and maintenance measures of free feeding.

Latency to initiate and duration of eating of 18 daily 5-pellet meal segments was measured in 22-h food-deprived rats under conditions of pimozide (1 mg/kg, i.p.) and naloxone (2.5, 5.0 and 10.0 mg/kg, i.p.) pretreatment. Neither drug increased latency on the first or second day of testing; pimozide increased mean latency slightly on the third test. Both drugs slowed duration scores dramatically; the slowing was progressive both within and across test sessions in the case of pimozide; it was dose-dependent and progressive within (but not across) sessions in the case of naloxone. In each case, shifts in mean duration scores. In each case, shifts in mean duration score reflected an increase in score variance caused by an increase in the number and extremity of long duration scores. In each case best scores under the drug condition were equal to best scores under control conditions; indeed, very fast latency scores were slightly more frequent under each drug on the first day of testing. Thus each drug decreased the probability of moderate scores but neither drug caused a simple change in the ability to make occasional fast responses. While the effects of the two drugs were similar in some ways, naloxone, unlike pimozide, did not completely suppress feeding.