Self-training for brain stimulation in the medial forebrain bundle of rats: a comparison of saline with amphetamine

Bilateral responses of rat ventral striatum tonically active neurons to unilateral medial forebrain bundle stimulation

Unilateral medial forebrain bundle (MFB) stimulation is an extremely effective promoter of reinforcement learning irrespective of the conditioned cue's laterality. The effectiveness of unilateral MFB stimulation, which activates the mesolimbic pathway connecting the ventral tegmental area to the ventral striatum (vStr), is surprising considering that these fibers rarely cross to the contralateral hemisphere. Specifically, this type of biased fiber distribution entails the activation of brain structures that are primarily ipsilateral to the stimulated MFB, along with weak to negligible activation of the contralateral structures, thus impeding the formation of a cue-outcome association. To better understand the spread of activation of MFB stimulation across hemispheres, we studied whether unilateral MFB stimulation primarily activates the ipsilateral vStr or the vStr of both hemispheres. We simultaneously recorded neuronal activity in the vStr of both hemispheres in response to several sets of unilateral MFB stimulation in anesthetized and freely moving rats. Unilateral MFB stimulation evoked strong stimulus-dependent activation of vStr tonically active neurons (TANs), presumably the cholinergic interneurons, in both hemispheres. However, the TANs' activation patterns and responsiveness depended on whether the stimulus was delivered ipsilaterally or contralaterally to the recorded neuron. These findings indicate that unilateral MFB stimulation effectively activates the vStr in both hemispheres in a stimulus-dependent manner which may serve as neuronal substrate for the formation of cue-outcome associations during reinforcement learning.

Determinants of the slow acquisition of medical and sulcal prefrontal cortex self-stimulation: an individual differences approach

Stimulation-naive rats were tested for motor activity during noncontingent electrical stimulation of the medial prefrontal cortex (MPC) or sulcal prefrontal cortex (SPC). Defecation during stimulation was also measured. The rats were then tested using a conditioned taste aversion paradigm for aversion to a novel flavor (0.1% saccharin) paired with stimulation. Finally, the rats were trained to acquire self-stimulation over 26 days of training. Large individual differences were seen in motor activity, defecation, and conditioned taste aversion to initial stimulation and in the subsequent speed of self-stimulation acquisition. In the MPC-stimulated group, acquisition speed was positively correlated with motor activity to initial stimulation and negatively correlated with defecation to this stimulation. In the SPC-stimulated group, the same correlations were evident, but only when rats suffering seizures prior to self-stimulation acquisition were excluded from the analysis. Such preacquisition seizures, which were only found in the SPC-stimulated group, retarded self-stimulation acquisition. In most rats, MPC or SPC stimulation failed to condition a taste aversion to saccharin. These results suggest that the slow acquisition of MPC and SPC self-stimulation may be partly related to the motor suppressive, aversive, and convulsive properties of initial stimulation.

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.

Housing conditions alter the acquisition of brain self-stimulation and locomotor activity in adult rats

Rats were implanted with stimulating electrodes in the medial forebrain bundle-lateral hypothalamus (MFB-LH). After recovery from surgery, they were assigned to one of two groups equalized for body weight. One group was housed four animals to a cage and animals in the other group were housed individually. After four weeks in these housing conditions, animals were tested during the fifth week for locomotor activity in five daily 15-min sessions, and their reactivity to handling was also measured. During the sixth and seventh weeks, animals were allowed to self-train for brain stimulation (ICSS) during daily 15-min sessions. Animals housed in groups had higher levels of locomotor activity and were less reactive to handling than were animals housed individually. When tested for the acquisition of ICSS, animals housed as a group made more responses than did animals housed individually. These results indicate that, within the time frame of a few weeks, the housing conditions of adult animals caused differences in both unconditioned behavior, such as locomotor activity, and in the acquisition of a highly reinforcing brain self-stimulation task.

Effects of an alcohol diet on locomotor activity and the acquisition of brain self-stimulation in rats

Male rats (n = 45), implanted with stimulating electrodes in the lateral hypothalamus for intracranial self-stimulation (ICSS), were allocated to three groups equalized for body weights. One group was given an alcohol liquid diet (Lieber-DeCarli) diet as the sole source of food; a second group was given a control liquid diet in which carbohydrates were substituted for alcohol; and a third group was maintained on standard laboratory chow and water. After four weeks on these diets, all animals were tested during the fifth week of the diets for locomotor activity in five daily 15-min sessions, and their reactivity to handling was also measured. During the sixth and seventh weeks of the diets, animals were allowed to self-train for ICSS during daily 15-min sessions. In the eighth week blood alcohol levels were measured. There were no differences between groups in locomotor activity or reactivity to handling. However, when tested for the acquisition of ICSS, animals on the diet containing alcohol made fewer responses than did those receiving the two control diets (p less than 0.001). The mean blood level of alcohol in the alcohol-consuming group at the time of day of behavioral testing was 50.6 +/- 5.0 mg/dl. These results suggested that the chronic ingestion of an alcohol-containing diet which resulted in only moderate blood levels of alcohol may not affect simple behavior such as locomotor activity, but may produce decrements in the performance of a more complex task such as lever pressing for ICSS.