Interactions of clonidine and naloxone on schedule-controlled behavior in opioid-naive mice

Schedule-controlled responding was maintained under a fixed-ratio schedule in mice. Administered alone, clonidine, morphine and naloxone produced dose-related decreases in rates of responding, with clonidine about 100 times more potent than morphine which was about ten times more potent than naloxone. Decreases in response rates produced by high doses of naloxone were antagonized by clonidine (0.003-0.1 mg/kg) in a dose-dependent manner; however, decreases in response rates produced by clonidine (0.3 mg/kg) were not antagonized by naloxone (1.0-100 mg/kg). Effects of high doses of naloxone (100 mg/kg) were not antagonized by morphine (1.0-100 mg/kg) whereas effects of morphine (17.0 mg/kg) were antagonized by naloxone (0.01-1.0 mg/kg). Thus, clonidine can reverse behavior-disrupting effects of naloxone in non-dependent subjects, indicating that at least some of the interactions of these two drugs are not specific to the opioid-dependent state.

Effects of methsuximide and mephenytoin on the behavior of pigeons under a repeated acquisition procedure

Learning impairment is a potentially serious side effect of antiepilepsy medications. The present study investigated the effects of methsuximide and mephenytoin on learning in pigeons performing under a repeated acquisition procedure. Given acutely, methsuximide (25, 50, 75, and 100 mg/kg) and mephenytoin (60, 120, and 240 mg/kg) produced generally dose-dependent decreases in rates of responding. High doses of each drug increased overall errors; a within-session analysis of error distributions also indicated learning impairment.

Suppression of drinking by naloxone in the rat: a further characterization

The effects of naloxone, an opiate antagonist, were examined on drinking induced by various dipsogenic stimuli. In rats deprived of water for 24 h, naloxone (0.1-10 mg/kg) produced a dose-related suppression of drinking immediately following water presentation but did not alter the latency to begin drinking. Naloxone also produced a dose-related suppression of water consumption induced by isoproterenol and angiotensin II, agents simulating conditions of extracellular dehydration. Naltrexone, a congener of naloxone, was more potent than naloxone in reducing isoproterenol-induced water intake. Schedule-induced polydipsia, which occurs in the absence of body fluid deficits, was not altered by either naloxone or naltrexone at doses attenuating drinking induced by the other methods. These data suggest that the suppressant effects of naloxone on water consumption are not a manifestation of an increased latency to drink or an impairment in the motor components of drinking activity. Furthermore, narcotic antagonists appear to attenuate regulatory, but not adjunctive drinking.