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

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.

The effects of reinforcer magnitude on timing in rats

The relation between reinforcer magnitude and timing behavior was studied using a peak procedure. Four rats received multiple consecutive sessions with both low and high levels of brain stimulation reward (BSR). Rats paused longer and had later start times during sessions when their responses were reinforced with low-magnitude BSR. When estimated by a symmetric Gaussian function, peak times also were earlier; when estimated by a better-fitting asymmetric Gaussian function or by analyzing individual trials, however, these peak-time changes were determined to reflect a mixture of large effects of BSR on start times and no effect on stop times. These results pose a significant dilemma for three major theories of timing (SET, MTS, and BeT), which all predict no effects for chronic manipulations of reinforcer magnitude. We conclude that increased reinforcer magnitude influences timing in two ways: through larger immediate after-effects that delay responding and through anticipatory effects that elicit earlier responding.

Effects of amphetamine and nomifensine on intracranial self-stimulation discrimination behavior in rats

Rats implanted with electrodes in the medial forebrain bundle-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 [intracranial self-stimulation (ICSS)]. When animals reached a high level of accuracy (95% correct) in the discrimination task, testing was begun. In the first experiment, we compared the effects of saline and 0.3 mg/kg d-amphetamine when the intertrial interval (ITI) was 1, 5, 10, and 15 s. In the second experiment, animals were tested either with saline, 0.3 mg/kg d-amphetamine, or 1, 3, or 10 mg/kg nomifensine and the ITI was held constant at 5 s. Increasing the ITI from 1-15 s did not produce a drug-induced change in the discriminative stimulus properties of ICSS, although it did produce changes in total numbers of lever presses and numbers of intertrial lever presses. In the second experiment, neither d-amphetamine nor nomifensine altered the discriminative stimulus properties of ICSS, but a dose-response increase occurred in the time to complete the test session and in total number of lever presses and in presses on the initiating lever. Under conditions known to increase extracellular dopamine (DA) levels in brain, both amphetamine and nomifensine produced large increases in locomotor activity, but neither drug produced changes in the detection threshold for ICSS. Results indicated that the internal cues produced by ICSS are different from those produced by these psychomotor stimulant drugs.

Discriminative properties of aversive electrical stimulations of the so-called "mesencephalic locomotor region": a parametric study

The classical 2-lever food-reward-reinforced discrimination paradigm, although already successfully applied to study discriminative properties of positively reinforcing electrical brain stimulations (EBS), has not yet been used for negatively reinforcing (aversive) EBS. This study was aimed at assessing if such a discrimination paradigm could be used to demonstrate discriminative properties of aversive EBS of the so-called "mesencephalic locomotor region" (MLR). Fourteen rats were trained for food reward to press one lever in the presence of EBS of the MLR and the other lever in the absence of EBS. Discriminative control by EBS of the MLR was obtained in 11 rats, which were subsequently subjected to a parametric study. It was found that the discriminative properties of EBS of the MLR covaried with changes in the intensity, frequency and pulse duration of the EBS. Our data also suggest that the aversive dimension of the stimulation might constitute a component of the discriminative cue induced by these EBS.

Amphetamine-like stimulus properties produced by electrical stimulation of reward sites in the ventral tegmental area

The present study examined whether the stimulus properties of D-amphetamine could be mimicked by electrical stimulation of the ventral tegmental area (VTA). Rats trained to discriminate 1.0 mg/kg D-amphetamine from saline were given generalization tests with a range of D-amphetamine doses administered either alone or in combination with VTA stimulation. The results suggested that the VTA stimulation could enhance the cueing effects of D-amphetamine, as levels of responding on the D-amphetamine-appropriate lever during stimulation trials were increased relative to tests without stimulation. Individual differences were observed in the amount of drug-lever responding elicited by the VTA stimulation during drug-free substitution tests, and the different levels of drug-lever responding correlated positively with the response rates obtained from these rats during subsequent intracranial self-stimulation tests. These finding suggest that VTA stimulation can have D-amphetamine-like stimulus properties and such stimulus properties may be related to the rewarding effects of the brain stimulation.

Rats' short-term memory for brief electrical stimulation of the ventral tegmental area of the brain

Short-term memory for brief electrical stimulation of rats' ventral tegmentum was assessed in a delayed conditional discrimination procedure in which trials began with a train of either low- or high-intensity stimulations. After a delay (i.e., retention interval) ranging from 0.01 to 4 sec, a lever was inserted, accompanied by a dim or a bright light. Lever-pressing was rewarded with food on low intensity-dim light and high intensity-bright light trials, but not on low-bright or high-dim trials. Discrimination ratios, based on relative responding on rewarded trials, indicated that subjects were able to remember the electrical stimulations, but that there was sometimes substantial forgetting after the 4-sec retention interval. The implications of these results for an understanding of the stimulus properties of electrical brain stimulation were discussed.

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.

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.