Metergoline antagonizes fluoxetine-induced suppression of food intake but not changes in the behavioural satiety sequence

In this study continuous monitoring was used to yield a true behavioural record. This allows a bidimensional account of drug effects on every unit of behaviour. Behavioural dimensions of duration (dur) and frequency (frq) measures were utilized to monitor the effects of an ED(50) anorectic dose of fluoxetine (10 mg/kg IP) on the behavioural satiety sequence and the effect of a metergoline (1 mg/kg IP) challenge. Fluoxetine reduced food intake by 45% (p < 0.005). The local eating rate was also reduced (p < 0.001), demonstrating a marked slowing of eating behaviour. Eating behaviour was reduced (frq p < 0.005) as was grooming (frq p < 0.05) and activity. Resting was increased (dur p < 0.05) and temporally advanced. There was no gross disruption of behaviour and the profile was adjusted in a way consistent with the expression of satiety. Fluoxetine induced changes were very similar to those produced by prefeeding. Metergoline antagonised fluoxetine's effect on intake and eating duration (dur p < 0.05). However, metergoline did not antagonise the effect of fluoxetine on the frequency of eating (frq p < 0.005), thus increasing the amount consumed per eating episode. Grooming (frq p < 0.005) and activity also remained reduced. At this dose fluoxetine-induced suppression of eating is serotonin dependent as it is reversed by metergoline. Fluoxetine-induced suppression of eating at this dose is consistent with the normal operation of satiety. Fluoxetine-induced slowing of behaviour appears to be mediated by a separate mechanism.