Peripheral serotonin is an incomplete signal for eliciting satiety in sham-feeding rats.

Peripheral administration of serotonin [5-hydroxytryptamine (5-HT)] to rats equipped with gastric cannulae reduced their 30-min consumption of sweetened milk after overnight deprivation whether the cannulae were closed (real feeding) or open (sham feeding). The anorectic action of 5-HT (1.6, 4.0, and 10.0 mumol/kg, IP) in sham feeding was dose-related, rapid in onset, and persisted during the 30-min testing session. However, 5-HT failed to elicit resting--the terminal behavioral phase of satiety--in sham-feeding rats. Direct comparison of the effects of 4.0 mumol/kg 5-HT under both feeding conditions established that this dose promoted resting only when rats fed with their cannulae closed. The actions of 5-HT on feeding and resting were behaviorally selective because serotonergic treatment did not retard the beginning of feeding, alter sham drinking of water, or reduce investigation by food-deprived rats of a novel object in an open field. Together, the results suggest that 5-HT exerts separate actions to inhibit feeding and accelerate the process of satiation as marked by resting. However, peripheral 5-HT is inadequate as a signal for modulating satiety in the absence of postingestive stimuli.

Pentylenetetrazol-induced seizures in pigeons and the effects of ethosuximide thereon.

Previous research has shown that ethosuximide in high enough doses disrupts operant responding in pigeons. Whether or not these same doses protect against seizure activity in this species has not been determined. In the present study a system for scoring pentylenetetrazol-induced seizures in pigeons was developed and the effects of ethosuximide on such seizures were evaluated. Pentylenetetrazol at 15, 27 and 47 mg/kg reliably induced seizures in Experiment 1. In Experiment 2 six doses of ethosuximide were tested for their seizure-controlling effectiveness. Doses of 20, 40, 80, 160 and 320 mg/kg ethosuximide had little effect on seizures induced by 27 mg/kg pentylenetetrazol; 640 mg/kg significantly reduced but did not completely eliminate seizures. This dose (640 mg/kg) is several times higher than the doses found to disrupt operant behavior in our previous studies.

Effects of phenytoin and phenobarbital on schedule-controlled responding and seizure activity in the amygdala-kindled rat.

The purpose of the present study was to examine whether doses of phenytoin and phenobarbital that blocked kindled seizures in rats also disrupted operant behavior. Subjects initially were exposed to a kindling procedure in which repeated electrical stimulation of the amygdala evoked generalized seizures. After kindling, they were trained under a multiple fixed-ratio 30 interresponse-time-greater than 10-sec schedule of food delivery. Once each day, 6 days per week, 30-min exposures to this schedule were followed immediately by amygdaloid stimulation which occurred at no other time. Response rate, reinforcement rate and duration of forelimb clonus were recorded. When response rates were stable, 5 rats were tested with phenytoin (25, 50, 75 and 100 mg/kg) and 5 others were tested with phenobarbital (10, 25 and 40 mg/kg). Results indicated that doses of phenytoin that controlled kindled seizures also affected operant behavior. For this drug, the ED50 for forelimb clonus was 62 mg/kg. For response rate under the fixed-ratio and interresponse-time components, it was 48 and 58.2 mg/kg, respectively. Dose-response curves for the behavioral and antiseizure effects of phenobarbital were similar. However, for this drug the ED50 for forelimb clonus (18.9 mg/kg) was significantly lower than for response rate under the fixed ratio component (37.4 mg/kg).