Parametric effects in the discrimination of intracranial stimulation: some methodological and analytical issues

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.

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

A two-choice discrimination task was used to evaluate the effects of psychoactive drugs on the discriminative stimulus properties of brain self-stimulation in rats. In these experiments, brain stimulation served both as a discriminative stimulus and as a reinforcing stimulus, but the two effects were manipulated separately. Animals were trained to a criterion of 95% correct in choosing between two levers, and when this level of accuracy was reached, the ability to choose correctly remained stable over an 8-month period. Increasing the current strength of the discriminative stimulus from zero to 100% of the training current produced a graded increase in the number of trials completed on the appropriate lever. The discriminative effects produced by brain stimulation were evaluated pharmacologically by using three prototypical psychoactive drugs in an attempt to change the detection threshold for the discriminative stimulus. Morphine, d-amphetamine, and haloperidol, drugs that reliably alter reinforcement thresholds for brain stimulation, failed to change detection thresholds. These results demonstrated that: brain stimulation produces potent and reliable discriminative effects and the effects of psychoactive drugs on detection thresholds can be dissociated from their effects on reinforcement thresholds for brain stimulation.

Discrimination between aversive brain stimulations: effect of stimulation parameters

Rats placed in a T maze were shown to be able to discriminate between two stimulations applied to one and the same brain site and inducing aversive effects of different magnitude. Discrimination occurred irrespective of whether the two stimulations differed in either intensity or frequency, and irrespective of whether they were applied to a medial hypothalamic or to a mesencephalic central gray site. Generalization experiments were performed in which either the stimulation parameter used to establish the discrimination or another parameter was varied. The results show that the animal's choice cannot be predicted on the sole basis of either the electrical charge/second applied or the escape latency induced.

Effects of drug-induced differences in reinforcement frequency on discriminative stimulus properties of fentanyl

Rats were trained to discriminate between fentanyl (0.04 mg/kg) and saline in a two-lever procedure. Using a FR 10 schedule of food reinforcement, drug-induced differences between the number of reinforcers obtained under fentanyl and saline conditions were observed. The effect of eliminating these differences on the outcome of generalization tests was investigated by different manipulations. In one group (N = 10), the FR 10 schedule used during saline sessions was changed to FR 6 during drug sessions. In a second group (N = 12), saline sessions ended after the number of reinforcers obtained was equal to the number obtained during the preceding drug session. A control group (N = 10) was trained using a FR 10 schedule under both conditions. Elimination of differences in reinforcement frequency accelerated the acquisition of the discrimination, diminished response bias, flattened the slope and reduced the ED50 value of generalization gradients of fentanyl, morphine and sufentanil and increased the ED50 value of naloxone in antagonizing 0.04 mg/kg fentanyl. It is concluded that the unconditioned effects of 0.04 mg/kg fentanyl on response rate in a FR 10 procedure lead to differences between saline and drug sessions which contribute to the apparent discriminative stimulus properties of fentanyl.