Amphetamine and increases in current intensity modulate reward in the hypothalamus and substantia nigra but not in the prefrontal cortex

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

Forebrain substrates of reward and motivation

Electrical stimulation of the medial forebrain bundle can reward arbitrary acts or motivate biologically primitive, species-typical behaviors like feeding or copulation. The subsystems involved in these behaviors are only partially characterized, but they appear to transsynaptically activate the mesocorticolimbic dopamine system. Basal function of the dopamine system is essential for arousal and motor function; phasic activation of this system is rewarding and can potentiate the effectiveness of reward-predictors that guide learned behaviors. This system is phasically activated by most drugs of abuse and such activation contributes to the habit-forming actions of these drugs.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Reciprocal information flow between prefrontal cortex and ventral tegmental area in an animal model of schizophrenia

The medial prefrontal cortex (PFC) is anatomically and functionally connected with the ventral tegmental area (VTA), the neuronal source of mesocorticolimbic system that is pathophysiologically related to schizophrenia-like symptoms. Methamphetamine (MAP) was applied to examine the functional relationship between PFC and VTA in an animal model of schizophrenia. Hyperactivity and stereotyped behavior were observed accompanied by a distinctive direction of information flow. In hyperactivity, information flow in the direction from PFC to VTA was dominant. Contrarily, dominant information flow from VTA to PFC was found in stereotyped behavior. These results indicate that dysfunctional interaction between PFC and VTA is the neuronal basis of MAP-induced schizophrenia-like psychosis. The information flow and its direction can be useful tool to explain the neurogenesis of these abnormal behaviors.

Synergistic effects of cocaine and dizocilpine (MK-801) on brain stimulation reward

The rewarding effects of lateral hypothalamic electrical stimulation were assessed in animals treated with the combination of cocaine and dizocilpine (MK-801), a noncompetitive N-methyl-D-aspartate antagonist. Eight male Long-Evans rats were trained to perform a lever-press operant to deliver trains of cathodal rectangular pulses directly into the lateral hypothalamus. Response rate was determined across the range of effective stimulation frequencies. For each rat the frequency threshold was defined as the lowest frequency that sustained minimal responding. After thresholds had stabilized each rat was tested under 4 treatment conditions; saline + saline, dizocilpine (0.05 mg/kg, i.p., 30 min before test) + saline, saline + cocaine (4 mg/kg, i.p., 5 min before test) and dizocilpine + cocaine. The saline + saline, dizocilpine + saline and saline + cocaine treatments each failed to cause significant changes in threshold or maximum response rates. The dizocilpine + cocaine treatment produced a large reduction in thresholds indicating a synergism between the two drugs and the rewarding stimulation. These synergistic effects of dizocilpine and cocaine stand in contrast to the putative antagonism by dizocilpine of cocaine's psychomotor-sensitizing action.

Classical conditioning of electrical self-stimulation of ventral tegmental area to brief visual stimuli in rats

Two groups of rats with bipolar electrodes implanted unilaterally into the left ventral tegmental area were trained to lever-press for the response contingent electrical stimulation. After preliminary lever-press training, two types of daily sessions were held on 10 consecutive days: type T+, current intensity set at the Threshold level and each response was accompanied by the visual signal (stimulus lights above the lever briefly went off): and type ST-, current set at the Sub Threshold level and no visual stimuli. On day 11, combination of the subthreshold current intensities and stimulus lights previously associated with the threshold stimulation (session type ST+) resulted in a significantly elevated response rates compared to the performance under the subthreshold current without visual stimuli (session type ST-). The extinction and recovery of this phenomenon are also demonstrated.

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.

Haloperidol attenuates conditioned place preferences produced by electrical stimulation of the medial prefrontal cortex

A Conditioned Place Preference test procedure [Ettenberg and Duvauchelle (13)] was used to investigate the effects of dopamine antagonist challenge on the rewarding properties of medial prefrontal cortex (MPFC) electrical stimulation. Rats exhibited strong preferences for the side of a two-compartment test apparatus in which they experienced sessions of experimenter-administered 0.5-s trains of MPFC sine-wave 60-Hz stimulation. Pretreatment with the neuroleptic dopamine antagonist drug, haloperidol (0.0, 0.15, or 0.3 mg/kg IP), resulted in a dose-dependent reduction in the magnitude of observed place preferences. Preference tests were conducted 24 hours after drug-conditioning trials and, hence, were not subject to motoric or other nonspecific actions of the neuroleptic treatments. In a control experiment, haloperidol did not block the place aversions produced by dorsomedial tegmental stimulation. Animals can, therefore, recall place-associations formed in the presence of haloperidol, a result which challenges "state-dependent learning" explanations of the drug's actions. Together, these results are consistent with the view that dopamine neurotransmission is involved in the rewarding consequences of electrical stimulation in the medial prefrontal cortex.

Amphetamine affects the extinction of self-stimulation differently in prefrontal cortex and posterior hypothalamus of rats

The effects of amphetamine on the extinction of intracranial self-stimulation (ICSS) and on postextinction ICSS performance were examined in rats implanted with electrodes either in medial prefrontal cortex (mPFC) or in the posterior hypothalamus-ventral tegmental area (PH-VTA). Lever-pressing for ICSS was allowed to stabilize in daily 15-minute sessions before each animal was exposed to 5 minutes of extinction (responding without reward). Animals were administered either 0.25 mg/kg d-amphetamine or saline before baseline, extinction and postextinction sessions. After amphetamine treatment, the number of lever presses during extinction was higher in mPFC animals and lower in PH-VTA animals compared with saline-treated controls. Rates did not change immediately after extinction but, one day later, rates had increased in all saline-treated animals (both PH-VTA and mPFC animals) and had decreased in all amphetamine-treated animals. These findings demonstrated that the effects of amphetamine on the extinction of ICSS were different in cortical and hypothalamic sites, possibly because of regional differences in stimulus-evoked reinforcement and inhibitory processes.

Behavioral characterization of intracranial self-stimulation from mesolimbic, mesocortical, nigrostriatal, hypothalamic and extra-hypothalamic sites in the non-inbred CD-1 mouse strain

A behavioral analysis of intracranial self-stimulation (ICSS) was provided for mesolimbic/mesocortical, nigrostriatal, hypothalamic and extrahypothalamic sites in the CD-1 mouse. Robust responding and rapid acquisition of mesocortical ICSS appeared dorsally along notably fluorescent sites in rostral and caudal planes. ICSS was diminished demonstrably in medial and ventral positions in posterior planes. Mesolimbic ICSS from the medial and ventral nucleus accumbens (Nas), was accompanied by significant elevations in locomotor activity, corresponding to regions of dopamine (DA) and cholecystokinin co-localization. Stimulation-induced seizures appeared from both the Nas as well as the mesocortex. ICSS from the ventral tegmental field (VTA) was evident along its medial, lateral and dorsal borders with longer pulse durations more likely to elicit responding. Seizure activity was absent from the VTA. Striatal ICSS was conspicuously poor in dorsal and medial locations; regions presumably devoid of tegmental innervation. ICSS emerged from both the ventrocaudal and anteromedial striatum; regions linked to innervation by the dorsolateral and ventromedial VTA. The red nucleus, a previously neglected self-stimulation site supported marked responding for ICSS. Regions supporting rubral ICSS were correlated with thalamic innervation sites; notably the ventrolateral thalamic nucleus and the parafascicular nucleus, regions found to support ICSS. The substantia nigra supported high rates of responding for ICSS when electrode placement was restricted to the dorsomedial portion of the pars compacta. Electrode deviations lateral and dorsal to the substantia nigra pars medialis induced a progressive decline in responding. Hypothalamic sites were found to support significant responding for ICSS, although such performance was frequently associated with seizure induction. Taken together these data (1) provide the first behavioral analysis of ICSS in mice responding from previously unexamined DA sites in the mesolimbic (e.g. VTA, Nas) and nigrostriatal systems (e.g. caudate, red nucleus) (2) suggest an anatomical reconsideration of the assumptions underlying the elicitation of ICSS from the frontal cortex (3) suggest that the neural circuitry underlying thalamic, caudate, rubral and frontal cortical ICSS are interrelated and (4) suggest that the Nas and the frontal cortex, like the hypothalamus, in the mouse appear to be particularly sensitive to stimulation-induced seizures.

Enhanced dopamine receptor activation in accumbens and frontal cortex has opposite effects on medial forebrain bundle self-stimulation

This study was undertaken to investigate the effects of activating dopamine receptors in accumbens and prefrontal cortex on self-stimulation behavior in the medial forebrain bundle. The experiments were carried out in rats chronically implanted with one stimulating electrode in medial forebrain bundle and two bilaterally-placed cannulas for giving injections into accumbens or prefrontal cortex. After completion of training, animals classified as responders and non-responders were given drug tests. The non-responders were tested to determine the effects of the treatment on motor activity. The self-stimulation task involved the depression of a lever to obtain a stimulus of 0.25 s duration, 60 Hz sine waves applied to the medial forebrain bundle. Dopamine receptor activation in accumbens or prefrontal cortex was induced with bilateral injections in these structures of a mixture containing 5 mg dopamine, 10 mg d-amphetamine sulfate and 5 mg pargyline mixed in 0.5 ml saline containing 0.1% ascorbic acid (dopamine + d-amphetamine sulfate + pargyline, the cocktail). Each injection was of 2 microliters/side, yielding a concentration of 20 micrograms of dopamine, 40 micrograms of d-amphetamine sulfate and 20 micrograms of pargyline/injection. The bilateral injections were given immediately before the self-stimulation session which lasted 12 h, starting in late afternoon. The effects of saline containing the ascorbate were determined in control sessions. Saline injected bilaterally in accumbens or prefrontal cortex of self-stimulators or non-self-stimulators had no effects on the response-rate of self-stimulators or on the gross motor activity of non-responders. In contrast, the cocktail of dopamine + d-amphetamine sulfate + pargyline injected in accumbens of self-stimulators induced a complex response which included first a facilitation, then a prolonged suppression and then again one or two episodes of facilitation interspersed with periods of suppression of self-stimulation and then a return to baseline rats. The same cocktail of dopamine + d-amphetamine sulfate + pargyline injected bilaterally in accumbens of non-self-stimulators resulted also in a complex response including as a first component a facilitation of responding, but the complex effect was of shorter duration and lower magnitude, never raising the rate of lever-pressing to levels meeting self-stimulation criteria. The same cocktail of dopamine + d-amphetamine sulfate + pargyline injected in prefrontal cortex of self-stimulators simply attenuated or suppressed responding, and the effect lasted for most of the session. The same effect was seen in non-self-stimulators indicating a decrease in gross motor activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple reward systems and the prefrontal cortex

Electrical stimulation of the major divisions of the prefrontal cortex, the mediodorsal and sulcal areas, can serve as a reinforcing stimulus. Studies of self-stimulation of the prefrontal cortex have produced behavioral, anatomical and pharmacological evidence that the substrate of these rewarding effects can be dissociated from that subserving self-stimulation of ventral diencephalic sites such as the lateral hypothalamus. Other studies indicate that within the prefrontal cortex itself, self-stimulation of the medial and sulcal divisions can be attributed to dissociable processes. These observations suggest the existence of multiple, largely autonomous prefrontal subsystems involved in reinforcement. This raises the question of the functional significance of such systems, and of their organization. An approach to this problem is to consider the relationship between the behavioral functions of the prefrontal divisions and the characteristics of stimulation-induced reward obtained at each site. Studies of the effects of restricted prefrontal lesions indicate that the medial and sulcal divisions can be dissociated according to their involvement in the control of distinct types of sensory and motor events. Further experiments indicate that damage to each division causes selective deficits in the learning of stimulus-reinforcer and response-reinforcer relations, depending in part on the nature of the reinforcing event. Conditioning experiments further show that the rewarding effects produced by stimulation of these areas are preferentially associated to sensory events which correspond to the functional specialization of each division. These data are interpreted to suggest that different rewarding events and/or different attributes of rewarding stimuli are processed by distinct systems which are reflected by the organization of dissociable self-stimulation pathways.

Hedonic interactions of medial prefrontal cortex and nucleus reticularis gigantocellularis

It has been shown that 'pure reward' and 'reward-escape' sites in the lateral hypothalamus (LH) of rats respectively ameliorate and exacerbate nucleus reticularis gigantocellularis (NGC) stimulation-induced aversion52. Conversely, the present studies found that 'rewarding' medial prefrontal cortex (MPFC) stimulation increased escape from NGC stimulation regardless of whether the MPFC site tested was 'pure reward' or 'reward-escape' in type. This suggested that a simple algebraic summation model of positive and negative affective processes may not adequately describe the NGC-MPFC interaction. In a subsequent study, rats were observed both to barpress less to obtain, and more to escape from, 'rewarding' MPFC stimulation during continuous NGC stimulation, supporting the hypothesis that the observed MPFC stimulation-mediated increase in NGC stimulation escape reflected an exacerbation of aversion. Finally, NGC stimulation was seen to increase barpressing to obtain 'subreward' MPFC current trains, indicating a potentiation of the reward value of such current. Results of this series of studies suggests a hedonic interaction model of NGC and MPFC characterized by reciprocal neuromodulation. The model is conceptualized as a 'neural opponent process' subserving affective 'balance' and 'feature enhancement', and its possible relevance to the putative role of the MPFC in cocaine use is discussed.

Effects of acute nicotine and ethanol on medial prefrontal cortex self-stimulation in rats

The acute effects of nicotine and ethanol were studied in low and high rates of intracranial self-stimulation (ICSS) of the medial prefrontal cortex (MPFC) in the rat. Nicotine tended to increase low ICSS rates but did not change or even reduced high ICSS rates. Independent of ICSS rate, ethanol tended to decrease ICSS, but only at high doses (1.0 g/kg). It is suggested that the effects of nicotine and ethanol on ICSS may be mediated by their effects on dopamine.

Measurement of intracranial self-stimulation thresholds using the post-reinforcement pause

A method for measuring the current threshold for intracranial self-stimulation (ICSS) using the post-reinforcement pause (PRP) is described. Rats trained to lever press on a fixed ratio schedule for ICSS in prefrontal cortex, medial forebrain bundle, ventral tegmental area or periaqueductal gray received stimuli of opposite polarity in an alternating fashion. Stimuli of one polarity were sufficient to maintain ICSS responding by themselves (maintaining stimuli). Stimuli of the other polarity were systematically varied in 5-10 microA steps (experimental stimuli). PRPs following both maintaining (Pm) and experimental (Pe) stimuli were measured, and the ratio Pm/Pe was calculated. The PRP threshold was defined as the lowest experimental stimulus current producing PRP ratios significantly less than the ratios produced by all lower current steps. After the PRP threshold for one polarity was determined in 5 daily sessions, the experimental and maintaining stimuli were reversed, and the PRP threshold was measured for the alternate polarity. Rates of ICSS on a continuous reinforcement schedule were subsequently measured at currents around the PRP threshold. Rates increased sharply at PRP threshold, suggesting a correlation between PRP threshold and reinforcement threshold. Similar results were obtained from all four ICSS sites indicating the broad applicability of the PRP threshold method.

The role of corticocortical projections in self-stimulation of the prelimbic and sulcal prefrontal cortex in rats

Four experiments were performed to assess the nature of the contribution of the corticocortical projections between the prelimbic and sulcal divisions of the rat prefrontal cortex to self-stimulation (SS) of these sites. The first experiment showed that transection of these projections by parasagittal knife cuts or bilateral electrolytic lesions of the prelimbic cortex had no effect on SS of the sulcal cortex. The second experiment demonstrated that SS of the prelimbic cortex could be obtained after transection of the corticocortical projection path. The third experiment demonstrated that the deficit in prelimbic SS, seen to follow such bilateral transections, is a function of the amount of exposure to the stimulation given to the animals after the lesion. The fourth experiment showed that the stimulation-dependent process underlying the acquisition of prelimbic and sulcal SS could be dissociated by the knife cuts. The discussion focused on the implications of these findings for an account of prefrontal self-stimulation behavior.

Acquisition of intracranial self-stimulation in medial prefrontal cortex of rats facilitated by amphetamine

Two groups of rats were trained to lever press for intracranial self-stimulation (ICSS) in the medial prefrontal cortex (mPFC) using a uniform amount of stimulation for all animals. One group acquired the lever pressing task very gradually during saline pretreatment but dramatically improved its rate of acquisition during the third week of training when pretreated with d-amphetamine (0.5 mg/kg). Administration of amphetamine to the other group of rats before each of the first five training sessions greatly facilitated acquisition of the ICSS task, and a significant improvement in performance over the saline control group appeared on the third day of training. After ICSS performance had stabilized, testing the animals revealed a significant amphetamine-induced increase in rate over the dose range of 0.25 to 1.0 mg/kg. These effects of amphetamine suggest that ICSS in mPFC is sensitive to changes in catecholamine neurotransmission during both the acquisition and maintenance of this behavior.

Distinct substrates influence the acquisition of self-stimulation of the hippocampus and the prefrontal cortex

Self-stimulation (SS) of both the medial prefrontal cortex (MPFC) and the dorsolateral hippocampus (HPC) is known to develop slowly, over a period of days. In both cases, the acquisition of bar-pressing can be markedly hastened by delivery of noncontingent electrical stimulation for several days prior to SS training. The similarity of these effects suggests that there might be a common substrate mediating the acquisition process. However, in the present experiment, pre-training noncontingent electrical stimulation of the MPFC had no effect on how rapidly rats acquired the bar-pressing response for HPC stimulation, or vice versa. A further dissociation of the elements governing the acquisition process for these two SS sites was suggested by the observation that pre-training noncontingent stimulation of the entorhinal cortex facilitated the speed of acquisition of SS of the HPC but not of the MPFC. It seems that the HPC and entorhinal cortex can be excluded from the subset of neural structures which are known to influence the acquisition process governing MPFC SS. These and other data suggest that the development of SS of the MPFC and HPC can be regarded, at least in part, as involving a process rooted in distinct substrates.

Post-stimulation excitability of mediodorsal thalamic self-stimulation

The post-stimulation excitability of the substrate for brain stimulation reward in the mediodorsal thalamus was assessed using equal- and unequal-pulse procedures. In 3 rats, refractory periods were found to begin no earlier than 1 ms and to end as late as 10 ms. Using test (T) pulses 1.5 times the amplitude of condition (C) pulses, the contribution of absolute and relative refractory periods was determined in one subject. No change in the slope of the recovery function was obtained in this condition, suggesting that several populations of neurons with different absolute refractory periods compose the behaviorally relevant substrate. A large supernormal contribution, evaluated by increasing the C amplitude to 1.5T, occurred between 3 and 10 ms with a peak at 7.5 ms. These results suggest that mediodorsal thalamic self-stimulation is mediated by a wide range of small, probably unmyelinated fibers.

Spatio-temporal integration in the substrate for self-stimulation of the prefrontal cortex

The number of stimulation pulses required to maintain a half maximal rate of self-stimulation of the prefrontal cortex (PFC) was determined for various currents. Over a restricted range, the effects of decreasing the stimulation frequency could be compensated for by increasing the current. This finding cannot easily be reconciled with the hypothesis that the rewarding impact of PFC stimulation is unaffected by increments in current. The minimum current that would support self-stimulation of the PFC at high frequencies was larger than has been reported at medial forebrain bundle sites.

The substrates for self-stimulation of the lateral hypothalamus and medial prefrontal cortex: a comparison of strength-duration characteristics

The directly activated substrates for self-stimulation of the lateral hypothalamus (LH) and medial prefrontal cortex (MPFC) were described by comparing their strength-duration characteristics. The current required to maintain a half-maximal rate of lever pressing was traded off against the pulse duration while all other stimulation parameters were kept constant. In this manner, cathodal strength-duration curves were obtained at four LH and eight MPFC sites; anodal curves were obtained at two of the LH and six of the MPFC sites. In general, the cathodal LH curves had lower rheobases than the cathodal MPFC curves and continued to descend after the MPFC curves had levelled off. At short pulse durations, the anodal curves lay above the cathodal curves, a finding more pronounced in the LH data. The two sets of curves converged at the longer pulse durations. The differences in the strength-duration curves are consistent with the notion that different directly stimulated neurons are responsible for the rewarding effects of LH and MPFC stimulation. Anatomical and physiological properties that could account for these differences are discussed.

Comparison of deficits in electrical self-stimulation after ibotenic acid lesion of the lateral hypothalamus and the medial prefrontal cortex

The aim of the present study was to compare the self-stimulation deficit produced by a unilateral injection of the neurotoxin, ibotenic acid, in the lateral hypothalamus (LH) to the deficit produced by the same unilateral injection in the medial prefrontal cortex (MPC). Four groups of adult male Sprague-Dawley rats were used: in two control groups, electrodes were bilaterally implanted in the LH (5 rats) or in the MPC (6 rats) and self-stimulation (ICSS) was obtained separately with the right and left electrodes. In the two experimental groups the intrinsic neurons of the LH (8 rats) or of the MPC (10 rats) were destroyed unilaterally by local injection of ibotenic acid (4 micrograms in 0.5 microliter); the other side served as the sham-lesioned control. Ten days later ICSS electrodes were implanted bilaterally, one in the lesioned area, the other in the contralateral region. As in the case of the control rats, ICSS was determined separately for each electrode, first by a rate dependent test (nose-poke) then by a 'rate-free' test (shuttle-box). In the LH and MPC control rats, ICSS responses were the same with stimulation on either side. In the LH-lesioned rats, the ICSS rates measured with the nose-poke test were significantly decreased with stimulation on the lesioned side, whereas rates with stimulation of the non-lesioned LH were normal. Likewise, while shuttle responses with stimulation of the non-lesioned LH were normal, the OFF-time was increased and the ON-time was decreased with stimulation of the lesioned LH. In the MPC-lesioned rats, ICSS (nose-poke) was totally suppressed and the shuttle responses were disorganized since neither the ON- nor the OFF-times changed in response to increasing current intensities. Nose-poke responses with stimulation of the non-lesioned MPC were just about normal. These results show that in the two brain regions studied local neurons are involved in ICSS. The difference in the magnitude of the deficit observed suggests, that the neuronal circuits involved in MPC self-stimulation are poorly represented whereas in the LH many neuronal circuits involved in these mechanisms overlap.

An investigation of the factors affecting development of frontal cortex self-stimulation

Intracranial self-stimulation (ICSS) of the medial prefrontal cortex (MFC) is acquired gradually, taking 4 or more days to establish. One explanation for this finding is that the stimulation becomes more rewarding with repetition. Four experiments were conducted to test this hypotheses. In Experiment 1, the MFC ICSS frequency thresholds remained constant over the first 3 weeks of testing while the rate of lever pressing response increased. In Experiment 2, it was found that acquisition of MFC ICSS was much more rapid when a motorically simpler response (nose-poking) was employed. Similarly, Experiments 3 and 4 further demonstrated that response factors such as task complexity may ultimately determine the rate of development of frontal cortex ICSS. Overall, these data suggest that independent of the rewarding effects of MFC stimulation there are other effects that initially interfere with learning of complex operant responses.

Selective neonatal depletion of dopamine has no effect on medial prefrontal cortex self-stimulation in the rat

The role of the dopaminergic input to the medial prefrontal cortex (MFC) on self-stimulation (SS) was investigated in adult rats injected neonatally with 6-hydroxydopamine (6-OHDA). Each subject on day 3 or 5 received bilateral intraventricular injection of 6-OHDA (total dose 200 micrograms, 50 micrograms/injection/2.5 microliters vehicle which contained 1 mg/ml ascorbic acid) or of the vehicle alone after pretreatment with desmethylimipramine (50 mg/kg i.p.) 30 min earlier. At 150 days of age, the animals were implanted with monopolar (100 microns) stainless steel electrodes in the MFC. One long (10 h) and 5 short (2 h) SS sessions resulted in similar percentages of responders for the brain reward in test and control subjects, and similar response rates in both groups. Biochemical assays of the levels of norepinephrine (NE) and dopamine (DA) in the frontal cortex showed depletion of DA 90% in the test animals, but no depletion of NE. Histochemical fluorescence visualization of the catecholamine input verified the biochemical results in the MFC. These results are viewed as negative evidence for the hypothesis that DA innervations in the MFC are critical neural substrates for SS, and suggest that activation of intrinsic neurons in the MFC are responsible for SS in the region.

Sensitization to the effects of repeated amphetamine administration on intracranial self-stimulation: evidence for changes in reward processes

The effects of daily administration of 1.0 mg/kg of D-amphetamine for 20 consecutive days on self-stimulation responding from the substantia nigra, and nucleus accumbens were evaluated at several current intensities. Raising current intensity increased rates of responding when electrodes were situated in these areas, and amphetamine significantly enhanced rates of responding from both brain regions. Moreover, the drug-induced response enhancements were facilitated further after repeated drug/test pairings. Although response sensitization was observed at several current intensities, it developed sooner at the lower current levels indicating that the sensitizing effect of repeated drug administration on self stimulation responding was not due to variations in locomotor activity or arousal levels induced by amphetamine treatment. Furthermore, sensitization was observed at current levels that engendered both high and low levels of responding, suggesting that the sensitization was unrelated to the rate dependent effects of the drug. Rather, it was argued that repeated amphetamine treatment sensitized animals to the rewarding properties of electrical brain stimulation. Possible neurochemical and behavioral mechanisms that may be involved in the development of reverse tolerance after repeated amphetamine treatment were discussed.

Dissociation of limbic structures by pharmacological effects of diazepam on electrical self-stimulation in the mouse

The effect of diazepam was tested on self-stimulation (SS) in 21 mice implanted with a bipolar electrode in the lateral hypothalamus (LH), the dorsolateral hippocampus (HPC) or the lateral entorhinal cortex (LEC). Diazepam, injected i.p. in doses of 0.5, 1 and 2 mg/kg, significantly increased SS rates with electrodes in LH while 4 and 8 mg/kg of diazepam had no significant effect. At low doses, similar increases were seen in mice with LEC electrodes but high doses produced a significant suppression. HPC animals showed an almost total suppression of SS beginning at 2 mg/kg of diazepam; lower doses had no significant effect. The results indicate that entorhinal and hippocampal SS are at least partly independent phenomena; in addition, the suppression of SS by moderate doses of diazepam remains specific to the HPC among the brain structures studied to date.

Brain stimulation reward and dopamine terminal fields. II. Septal and cortical projections

The boundaries and relative sensitivities of the substrates of septal and cortical brain stimulation reward were mapped in relation to the dopamine terminal fields in these regions using a dorsal-ventral moveable electrode. Brain stimulation was rewarding at all levels of the posterior lateral septum and not just in the region of dopamine terminal innervation. Reward thresholds, ease of training, maximum response rates and stability of responding were all unrelated to the proximity of the stimulating electrode to the band of dopamine terminals revealed by glyoxylic acid-induced dopamine fluorescence. Stimulation was also rewarding in the anterior lateral septum; the best sites were in the ventral portions of this region although dopamine terminal fluorescence was uniform throughout. Thus the anatomy of the brain stimulation reward substrate of the lateral septal nucleus does not bear a special relation to the anatomy of dopamine terminals within this region. Stimulation was also rewarding in each of the dopamine terminal fields of the cerebral cortex. The best self-stimulation was obtained with electrodes in the medial frontal cortex; sulcal frontal cortex was next best, entorhinal cortex was next, and pyriform cortex, though reliably positive, supported the weakest self-stimulation. Variations in self-stimulation threshold were seen as electrodes were moved through homogeneous regions of dopamine terminal density in some regions, while stable thresholds were associated with movements through areas of varying dopamine terminal density in others; thus, again, there was no special relation between goodness of self-stimulation and density of dopaminergic innervation. These data suggest that rewarding brain stimulation in these regions is not due to direct activation of either the dopaminergic terminals or the cells that they innervate.

The influence of amphetamine on preference for lateral hypothalamic versus prefrontal cortex or ventral tegmental area self-stimulation

Rats were trained to bar-press for intermittent reinforcement on a concurrent schedule offering self-stimulation (SS) at the animal's choice of one of two different brain loci. On the concurrent schedule, the relative reward value of the two reinforcers is evaluated by the way the subject divides its session time responding for these reinforcers, thus yielding a rate-free measure of reward in addition to response rate data. In animals with electrodes in the lateral hypothalamus (LH) and prefrontal cortex (PFC), amphetamine dose-dependently increased response rates as well as the proportion of time allotted to LH stimulation, demonstrating that the reward value of LH stimulation was increased relative to PFC stimulation. This finding supports the hypothesis that DA systems modulate the rewarding value of LH but not PFC SS, and it suggests that differing neural mechanisms underlie these two behaviors. In animals with LH/ventral tegmental area (VTA) implants, amphetamine had no effect on preference, although it produced an overall increase in rate. This suggests that the drug elevates the rewarding value of LH and VTA stimulation to a similar degree, and that the two regions may have a common DA-related reward substrate. Finally, it was found that when the two reinforcers were equally preferred (50% session time allotted towards each reinforcer), response rates for the two rewards were not necessarily equal. This confirms that SS response rate is not a simple function of reward magnitude.

Discriminating between reward and performance: a critical review of intracranial self-stimulation methodology

Despite numerous pharmacological investigations of intracranial self-stimulation (ICSS), the substrates of this behavior have yet to be completely understood. In view of the likelihood that inadequate methodology has hindered the quest for these substrates, the present review was undertaken. Criteria for ICSS methodology should include not only the ability to discriminate reward from gross performance deficit, but also adequate capacity (ability to generate experimental data at a reasonable rate). For numerous reasons, bar-pressing on a continuous reinforcement schedule fails the first criterion despite its ease and rapidity. The use of partial reinforcement schedules may alleviate some of these shortcomings. Analysis of drug-induced response decrement patterns can discriminate gross motoric incapacity from other variables, although the question of subtle response maintenance deficits remains to be answered. Measurements of response rates using alternative operants do not differentiate reward and performance adequately. More promising, "rate-free" measures using locomotion as an operant include the two-platform method of Valenstein and the "locus of rise" method. Comparison of drug effects on ICSS with those on alternate tasks are fraught with pitfalls including the problems of assuring equivalent rates and patterns of responding. The use of differential electrode placements is ideally suited for neurochemically well-characterized drugs, particularly if "double dissociations" can be established during studies of multiple placements. Presentation of different current intensities or frequencies permits the compilation of rate-intensity functions, and drug-induced shifts in these functions have considerable analytical power. Self-regulation of current intensity constitutes a powerful tool that has yet to realize its full potential in the pharmacological study of ICSS. Extensive studies involving self-regulation of stimulation duration ("shuttlebox" studies) suggest that this method may be highly versatile despite several practical difficulties. It is concluded that at least six of these methods appear to do a reasonable job of excluding gross performance deficit. However, the possible influences of other factors, such as subtle response maintenance deficit, incentive or arousal, remain to be resolved in view of the multifactorial nature of ICSS. Multiple tests for ICSS drug or lesion studies are advocated whenever feasible, as no single test appears capable of resolving all theoretical complexities.

Plasticity of the medial prefrontal cortex: facilitated acquisition of intracranial self-stimulation by pretraining stimulation

Prior electrical stimulation of the medial prefrontal cortex MFC facilitated the subsequent acquisition of intracranial self-stimulation (ICSS) from the same MFC electrode site. Stimulations that were spaced over a period of six days were more effective in producing this facilitation than the same number of stimulations delivered over a two day period. These data suggest that the rewarding effects of MFC stimulation may involve some process akin to the kindling phenomenon and as such may provide insights in the neuronal modifications thought to underlie learning and memory.