The discriminative stimulus properties and detection thresholds of intracranial self-stimulation: effects of d-amphetamine, morphine, and haloperidol

On the Similarity Between the Reinforcing and the Discriminative Properties of Intracranial Self-Stimulation

Rats work very hard for intracranial self-stimulation (ICSS) and tradeoff effort or time allocation for intensity and frequency parameters producing a sigmoidal function of the subjective reward magnitude of ICSS. Previous studies using electrical intracranial stimuli (ICS) as a discriminative cue focused on estimating detection thresholds or on the discrimination between intensities. To our knowledge, there is no direct comparison of the reinforcer tradeoff functions with the discriminative functions. Rats were trained to press and hold the lever for ICSS using the maximum reinforcing intensity below motor alterations or avoidance behavior. First, rats were trained to hold the lever for 1 s; after stability, they undergo trials where intensity or frequency was decreased on 0.1 log step. Thereafter, they undergo further training with a hold of 2 and later of 4 s to determine tradeoff with intensity or frequency. The same rats were trained on a discrimination task where the previously used ICSS signaled a lever where a 1 s hold response was followed by a reinforcing ICSS; on randomly alternating trials, a −0.6 log ICS signaled an alternate lever where a similar hold response led to a reinforcer. After mastering discrimination, generalization tests were carried out with varying intensity or frequency. Rats completed training with 2 and later 4 s hold response. After the completion of each task, the rats had different doses of a pimozide challenge while their intensity and hold-down requirement were varied. With regards to the rats’ tradeoff response time allocation as a function of intensity or frequency, sigmoid functions were displaced to the right when long responses were required. Rats that learned the discrimination task attained a discrimination index of 90–98%. Discrimination accuracy decreased slightly with the increase of hold requirement, but generalization gradients were not displaced to the right as a function of the response requirement. Pimozide induced a dose-dependent displacement of the time-allocation gradients, but it did not affect the generalization gradients. It is concluded that rats integrate response requirements as part of the reinforcement tradeoff function, but the response cost is not integrated into the discriminative function of ICSS.

Schedule-controlled brain self-stimulation: has it utility for behavioral pharmacology?

We review evidence that schedule-controlled intracranial self-stimulation (ICSS) has properties in common with conventional reinforcements, such as food and water, but unlike the latter, animals will respond for ICSS for long periods of time at a near-constant rate. Schedule-controlled ICSS has proven to be more sensitive to drug-induced changes than has ICSS on a continuous reinforcement schedule, and it permits a more fine-grained analysis of the pattern of responding that results in the reinforcement. Evidence is accumulating that the schedule of ICSS itself leads to neurochemical changes in areas of the brain, such as the nucleus accumbens, in which reward processes occur. Results obtained from schedule-controlled ICSS would complement those obtained by drug self-administration studies which generally use intermittent reinforcement. A systematic examination of ICSS schedules at different brain sites would greatly facilitate our interpretation of drug effects and this would have utility for behavioral pharmacology.

Interactions of diazepam and pentobarbital with RO 15-4513 on intracranial self-stimulation discrimination behavior in rats

Rats implanted with electrodes in the lateral hypothalamus were trained in a discrete trial procedure to make a differential response (right or left lever press) in the presence or absence of brain stimulation. When a high level of accuracy (95% correct) was attained in the discrimination, testing with vehicle, RO 15-4513, diazepam (1.0-10 mg/kg), diazepam plus RO 15-4513 (1.0 mg/kg), pentobarbital (1.0-17.5 mg/kg) and pentobarbital plus RO 15-4513 began. Diazepam, at 10 mg/kg, disrupted the discrimination behavior, and it also decreased the total number of lever-presses and increased the time to complete the session. These effects were blocked by the coadministration of 1.0 mg/kg RO 15-4513. Pentobarbital produced effects similar to those of diazepam, but these effects were only reversed to a limited extent by RO 15-4513. By itself, however, RO 15-4513 also decreased the total number of lever presses and increased the time to complete the session. Results were consistent with our previous findings with alcohol and RO 15-4513, and supported the notion that diazepam and alcohol have some similar effects at the GABA-benzodiazepine receptor complex.

Differential effects of cholinergic drugs on discriminative cues and self-stimulation produced by electrical stimulation of the ventral tegmental area

Cholinergic receptors have been shown to modulate a subset of discriminative cues produced by electrical stimulation of the ventral tegmental area (VTA) in rats. The present study identified the specific cholinergic receptor type modulating these electrical brain-stimulation (EBS) cues, and assessed whether these receptors also mediated the rewarding effects of VTA EBS. The EBS cues were enhanced by the acetylcholinesterase inhibitor physostigmine and the muscarinic receptor agonists pilocarpine and RS-86, whereas the nicotinic receptor agonist nicotine had no effect. The enhancing effects of pilocarpine or RS-86 were attenuated by the muscarinic antagonist scopolamine. The EBS cues were not affected when scopolamine was injected alone, although high doses disrupted discriminated responses. Intracranial self-stimulation (ICSS) rates were depressed by physostigmine and pilocarpine and increased by nicotine and scopolamine. These results indicated a facilitatory influence of muscarinic receptors on the EBS cues, and an inhibitory role in VTA ICSS. Nicotinic receptor activation did not affect the EBS cues, but facilitated ICSS. These differential effects of cholinergic receptor activation point to a dissociation of the specific EBS cues measured in this study from the rewarding effects of VTA stimulation.

An analysis of the effects of amphetamine on brain self-stimulation behavior

Rats were implanted with stimulating electrodes in the medial forebrain bundle-lateral hypothalamus, and trained in one of two brain self-stimulation (ICSS) procedures. One group of animals was trained in the auto-titration procedure which measured response rates and reinforcement thresholds for ICSS. When trained, the animals were tested with D-amphetamine (0.03-1.0 mg/kg) and saline. The second group of animals was trained in a procedure which established the discriminative properties of brain stimulation and which also measured the detection threshold for ICSS. When trained, the animals were tested with D-amphetamine and saline, and were subsequently tested with amphetamine and saline in an activity monitor. A graded decrease in mean reinforcement thresholds was produced by D-amphetamine. The two lower doses (0.03, 0.10 mg/kg) increased the rate of lever-pressing, but the highest dose (1.0 mg/kg) decreased it. No reliable changes in detection threshold occurred, although at the 0.3 mg/kg dose reductions were seen in two of five animals. Animals also showed a graded increase in motor activity with dose as well as increases in several performance measures. These results demonstrated that although amphetamine lowers reinforcement thresholds, it does not reliably alter detection thresholds, suggesting that the motivation to respond for ICSS is changed without any corresponding changes in the capacity to detect the presence of the stimulus.

Differential effects of physostigmine on cues produced by electrical stimulation of the ventral tegmental area using two discrimination procedures

Two procedures were employed to assess the effects of physostigmine on the discrimination of cues produced by either high or low intensity electrical brain stimulation (EBS) of the ventral tegmental area in rats. When the procedure involved frequent presentation of brief trials, physostigmine enhanced the perceived intensities of the cues, causing the rats to respond to low intensities as though they had higher values. In contrast, physostigmine had no effects on the discrimination when the trials were less frequent and extended in duration. These results confirm the existence of multiple substrates for cues produced by stimulation of the ventral tegmental area in rats and implicate cholinergic neurons as substrates for the non-dopaminergic cues identified in the companion paper.

Dissociation of dopaminergic and non-dopaminergic substrates for cues produced by electrical stimulation of the ventral tegmental area

The present study provides evidence for the existence of multiple substrates for cues produced by electrical stimulation of the ventral tegmental area in rats. Two different procedures were employed to assess the effects of amphetamine and haloperidol on the discrimination of high and low intensity cues produced by electrical brain stimulation (EBS). When the procedure involved frequent presentation of brief trials, amphetamine and haloperidol had no effect on the discrimination of EBS. In contrast, when the trials were less frequent and extended in duration, amphetamine enhanced the perceived intensity of the cues whereas haloperidol had the opposite effect. These results indicate that the use of different discrimination procedures may result in the measurement of separate dopaminergic and non-dopaminergic substrates for cue properties of EBS in the ventral tegmental area.

An automatic device for measuring speed of movement and time spent at rest: its application to testing dopaminergic drugs

We describe a device to measure speed of movement and time at rest for use with a commercially available infrared photobeam activity monitor. This system is a reliable substitute for a human observer and provides additional measures of activity that can help in interpreting how psychoactive drugs alter behavior. The effects of graded doses of d-amphetamine, haloperidol and (-)3-(3-hydroxyphenyl)-N-n-propylpiperidine, (-)-3-PPP, were studied with the device, and the results confirmed that these drugs differentially alter speed of movement and time at rest.