Ricardo Pellón

Adjunctive behaviors are operants

Adjunctive behaviors such as schedule-induced polydipsia are said to be induced by periodic delivery of incentives, but not reinforced by them. That standard treatment assumes that contingency is necessary for conditioning and that delay of reinforcement gradients are very steep. The arguments and evidence for this position are reviewed and rejected. In their place, data are presented that imply different gradients for different classes of responses. Proximity between response and reinforcer, rather than contingency or contiguity, is offered as a key principle of association. These conceptions organize a wide variety of observations and provide the rudiments for a more general theory of conditioning.

Rate-dependency hypothesis and the rate-decreasing effects of d-amphetamine on schedule-induced drinking.

The high levels of drinking induced by intermittent food-reinforcement schedules are dose-dependently reduced by acute doses of d-amphetamine. The present study evaluated whether the effects of d-amphetamine on this schedule-induced drinking reflect the reduction of high rates of responding. Twenty four rats were divided into six groups (n = 4) according to the interval and time durations of a multiple fixed-time (FT) fixed-interval (FI) schedule (15 s, 30 s, 60 s, 120 s, 240 s and 480 s). FT components were signalled by a tone and by lever withdrawal. Doses of 0.25 to 4.0 mg/kg of d-amphetamine were administered i.p. 10 min before test sessions, d-amphetamine produced similar dose-dependent reductions in rate of licking induced by FT and FI schedules. Rate-decreasing effects on operant lever pressing were also found after administrations of d-amphetamine. The dose-dependent decrements produced by d-amphetamine were a function of the infer-food interval length in both schedule-induced and operant behaviours. These rate-decreasing effects were rate-dependent, but d-amphetamine interacted differentially with control rates of adjunctive and operant behaviours, causing a greater suppression of the lower rates of adjunctive licking and the higher rates of operant lever pressing.

Effects of drugs on the temporal distribution of schedule-induced polydipsia in rats

Drinking was induced in food-deprived rats by a fixed-time 1-min schedule of food presentation. With three rats, d-amphetamine (0.25, 0.5, 1.0, and 2.0 mg/kg) led to a dose-related increase in licking early in the interfood intervals, the peak of the temporal distribution of licking being shifted to earlier values. These effects were seen even when d-amphetamine had no effect on overall rates of licking and drinking. With another three rats, however, diazepam (0.5, 1.0, 2.0, and 4.0 mg/kg) did not shift the peak of the temporal distribution of licks in interfood intervals, even at doses that produced small increases in overall rates of licking and drinking. However, diazepam did reduce the peak of the distributions of licks at doses that did not decrease water intake and licks per minute.

Antipunishment effects of diazepam on two levels of suppression of schedule-induced drinking in rats

Food-deprived Wistar rats were exposed to a fixed-time (FT) 60-s food delivery schedule until they developed schedule-induced drinking. Rats were matched in pairs according to their licking rates and were designated master or yoked at random. Every fifth lick by master rats was followed by an electric shock during two signalled 5-min periods, which ran concurrently with the food delivery schedule. For the master rats, shock intensities were adjusted to reduce licking to 5-30% (low suppression) or 50-75% (high suppression) of the unpunished licking rates. Yoked rats received the same shocks as master rats, but independently of their own licking. The drinking by yoked animals was not decreased by the presentation of these lick-independent shocks. Diazepam (0.3-10.0 mg/kg) was studied for its effects on punished and nonpunished schedule-induced drinking. Intermediate doses of the drug increased the punished behavior of master rats, but only when schedule-induced drinking was highly suppressed. Diazepam dose dependently decreased licking rates in all other conditions. The antipunishment effects of benzodiazepines may depend on the level of suppression of schedule-induced drinking, and this is in keeping with the results of other experimental preparations where behavior was under aversive control.

Individual differences in schedule-induced polydipsia: Neuroanatomical dopamine divergences

Autoradiography analysis of D1 and D2 dopamine receptors and c-Fos activity were performed in brain of rats classified as low drinkers (LD) and high drinkers (HD) according to schedule-induced polydipsia (SIP) performance. Previous studies have shown that groups selected according to their rate of drinking in SIP differ in behavioral response to dopaminergic drugs. This study reports differences between LD and HD rats in dopamine D1 and D2 receptor binding through different mesocorticolimbic brain areas. LD and HD rats showed opposite patterns of binding in dopamine D1 and D2 receptors in the nucleus accumbens, medial prefrontal cortex, amygdala, ventral tegmental area and substantia nigra. Whereas LD rats showed higher binding than HD rats for D1 receptors, HD rats showed higher binding than LD rats for D2 receptors (except in substantia nigra that were roughly similar). These neuroanatomical differences in dopamine receptor binding were also associated with an elevated c-Fos count in the medial prefrontal cortex of HD rats. In tandem with previous evidence, our results suggest a different dopaminergic function between LD and HD, and points to SIP as a behavioral model for distinguishing populations possibly vulnerable to dopaminergic function disorders.

Acquisition of schedule-induced polydipsia by rats in proximity to upcoming food delivery

Food-deprived rats that receive intermittent delivery of small amounts of food develop excessive drinking-specifically, schedule-induced polydipsia (SIP). A main characteristic of SIP is its occurrence at the beginning of interfood intervals. The purpose of this study was to demonstrate that SIP can be developed toward the end of interfood intervals, in closer proximity to upcoming than to preceding food delivery. In Experiment 1, two groups were exposed to a fixed-time (FT) 30-sec food schedule with water available during the first or the last 15 sec of each interfood interval. Two additional groups, which had access to water throughout, were exposed to FT 30-sec or FT 15-sec schedules of food presentation. The FT 30-sec group with free access to water developed the highest level of intake; similar and intermediate levels were induced in all the remaining groups. In Experiment 2, three groups of rats were exposed to an FT 90-sec food schedule with water available during the first, the second, or the last 30 sec of each interfood interval. One additional group with access to water throughout was exposed to the FT 90-sec schedule of food presentation. The group with free access to water developed a higher level of consumption than did the other groups, but by the end of training none of the four groups showed statistical differences in polydipsic drinking. Results show that adjunctive drinking can be developed in proximity to upcoming food delivery even with long interfood intervals.

Effects of d-Amphetamine on Temporal Distributions of Schedule-Induced Polydipsia

Food-deprived rats were divided into four groups according to the equal interval and time durations of a multiple fixed-interval, fixed-time schedule (15, 30, 60, and 120 s). Fixed-time components were signaled by a tone and lever withdrawal. d-Amphetamine (0.25-4.0 mg/kg) produced similar dose-dependent reductions in the drinking and licking induced by fixed-interval and fixed-time schedules. These dose-dependent decrements were a function of the interfood interval length. More licks occurred early in the interfood intervals with doses of d-amphetamine. Dose-dependent shifts to the left were observed in the distribution of licking, and there were dose-dependent decreases in the quarter-life, which were a function of fixed-interval and fixed-time lengths. The maximum lick rate within interfood intervals occurred at about the same absolute time in schedules up to 60 s; therefore, the effects of d-amphetamine were not mediated by its effects on temporal discrimination.

Effects of d-amphetamine and of diazepam on non-punished and punished schedule-induced drinking in rats.

&NA; Drinking induced in food‐deprived rats by a Fixed‐Time 1 min schedule of food presentation was measured by the amount of water consumed per session and the number of licks per inter‐food interval. Subsequently each lick initiated a 10‐sec signalled delay in the delivery of food, which led to a decrease in drinking (punishment). With three rats the effects of d ‐amphetamine (0.25, 0.5, 1.0, 2.0 mg/kg) were assessed on non‐punished and then on punished drinking. With another three rats, the effects of diazepam (0.5, 1.0, 2.0, 4.0 mg/kg) were assessed. The smaller doses of d ‐amphetamine had no consistent effect on overall measures of non‐punished schedule‐induced drinking, but the largest dose decreased them. With the signalled delay d ‐amphe‐tamine increased punished schedule‐induced drinking. Non‐punished drinking was increased by small doses of diazepam and decreased by the largest dose, but no dose of diazepam affected punished drinking.

The effects of food presentation at regular or irregular times on the development of activity-based anorexia in rats

Activity-based anorexia occurs when food availability is restricted to 1h of the day and a wheel is freely available to the rest of the time. Under such conditions rats run excessively and stop eating even during periods in which food is available. A defining characteristic of the excessive activity is that there is a peak of running in the anticipation of food availability. The present study was designed to test whether the occurrence of the food period at different times of the light phase of the light-dark cycle (from 08:00 to 20:00h) could impede or postpone the normal development of activity anorexia. We compared the effect of presenting the food at a fixed time of the light period with presenting it on a variable schedule. Far from impeding or postponing the development of activity-based anorexia, presenting food at irregular times resulted in a pronounced body-weight loss, a low food intake and an increase in locomotor activity. Animals ran excessively, with a peak at the start of the dark period, and again when lights were turned on in the experimental room (running in the anticipation of food). Both fixed and variable schedules of food availability resulted in the development of activity-based anorexia in rats.

Food-delay duration and the development of schedule-induced polydipsia in rats.

Twelve rats were exposed to a schedule that delivered a food pellet every 60 s (fixed time 60 s). The development of schedule-induced polydipsia was measured in terms of the water consumed and the licks per interpellet interval. Every lick by master rats initiated an unsignalled delay of 2 or 50 s in food delivery. Yoked-control rats received food at the same time as their masters, being unaffected by their own licking. Schedule-induced polydipsia developed in master rats exposed to 2-s delays, but more slowly and to a lesser extent than control animals. The development of polydipsia was prevented in master rats exposed to 50-s delays, however. When these delays were discontinued, polydipsia was obtained by master rats. The finding that the effect of the delays was modulated by their duration supports the view that the development of schedule-induced polydipsia is sensitive to control by its environmental consequences.