Effects of selective central muscarinic blockade on schedule-controlled behavior and on the rate-decreasing effects of physostigmine

Behavioral pharmacology and timing

Drug effects on temporally patterned behavior are often described under the rubric of rate dependency: the effect of a drug on behavior is related to the rate of behavior in the absence of the drug. Specifically, drugs increase low rate behavior and decrease high rate behavior. These same types of effects are interpreted in the timing literature, however, as selective changes in temporal discrimination. The present series of experiments arrange situations that allow divergent predictions based on the two interpretations. In one component of a multiple schedule, when the response key is lit blue, food is available after the houselight is presented for a short duration (5 s). In the other component of the multiple schedule, when the response key is lit green, food is available after the houselight is presented for a long duration (30 s). No food is available after intermediate durations. Specific focus is given to a neuropharmacological information-processing model of timing. Predictions were compared for drugs that are thought to affect the clock and memory stages in the model. The results do not generally lend support for the neuropharmacological interpretation of the scalar expectancy theory, but emphasize the need for an explanatory mechanism that is consistent with the empirical generalization of rate dependency.

Effects of physostigmine and scopolamine on rats' performances in object-recognition and radial-maze tests

The effects of physostigmine and scopolamine were evaluated on working memory of rats in object recognition and radial-maze tests. Three doses of physostigmine hemi-sulfate (Phys: 0.05, 0.10 and 0.20 mg/kg), five doses of scopolamine hydrobromide (Scop: 0.125, 0.25, 0.5, 1.0 and 2.0 mg/kg), and one dose of scopolamine methylbromide (Mscop: 2.0 mg/kg) were used. In object recognition test, rats were submitted to three or four intertrial delay conditions (1-min, 15-min and either 60-min or 24-h). The higher doses of Scop (1.0 and 2.0 mg/kg) in 1-min and 15-min delay and of Phys (0.20 mg/kg) in 1-min delay impaired discrimination between new and familiar objects. Mscop impaired discrimination between objects in 60-min but not in 1-min and 15-min delay. This effect may be state dependent. Radial-maze learning was impaired by the lower doses of scopolamine (0.25 and 0.50 mg/kg) which had no effect in object recognition test. These results show that in our conditions, object recognition is less sensitive than radial-maze test to cholinergic drugs.

Effects of repeated administration of soman on schedule-controlled behavior and brain in the rat

The effects of repeated SC administration of soman on schedule-controlled performance and brain pathology were studied in the rat. Soman suppressed response rates in both components of a multiple fixed interval 50-sec fixed-ratio 25 (mult. FI 50-sec FR 25) schedule of reinforcement, although all animals revealed marked tolerance to repeated drug administration. Response rates generally recovered to baseline levels within 1-3 sessions. Three of the six animals studied, however, demonstrated marked deterioration of steady state schedule performance, particularly during the FI 50-sec component of the multiple schedule. Compared to untreated controls, all soman-treated animals exhibited pathological changes in brain. The most salient finding was glial cell proliferation in layer 4 and deep parts of layer 3 of the cerebral cortex. Glial cell proliferation was most marked in animals that exhibited deterioration of steady state schedule performance.

Environmental influences on the development of tolerance to the effects of physostigmine on schedule-controlled behavior

The influence of environmental variables on the development of tolerance to physostigmine's effects in rats was examined using multiple fixed-ratio, extinction schedules of food presentation. Initial administration of physostigmine (0.4 mg/kg) produced nearly maximal decreases in the number of food pellets delivered, running response rate, and overall response rate, under multiple FR 10, EXT and multiple FR 50, EXT schedules. With repeated administration, tolerance to physostigmine's effects was observed when 10 responses were required to produce reinforcement but was not observed when 50 responses were required to produce reinforcement. Tolerance under the multiple FR 10, EXT schedule of reinforcement was also observed when physostigmine was administered post-session. When tolerance was acquired, it was retained for up to 25 drug-free days. These results suggest that tolerance to physostigmine's effects on schedule-controlled behavior is strongly influenced by response requirement, independently of physostigmine-induced reinforcement loss. Additionally, tolerance is not dependent on experience with the schedule while under the effects of physostigmine, and is retained for a substantial period of time in the absence of continued physostigmine administration.