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

Prior extended daily access to cocaine elevates the reward threshold in a conditioned place preference test

We have previously shown that extended-access subjects exhibit heightened motivation for cocaine in the runway model, as reflected by reduced number of retreats. This heightened motivation could reflect either an increase in cocaine-induced reward or a decrease in cocaine-induced aversion. The current experiment was therefore devised to assess the cocaine-induced reward and aversion in extended-access rats using a place conditioning test. Rats trained to lever press for intravenous (IV) cocaine (0.25 mg/infusion) were provided 6-hour daily access to the drug over 10 days. Lever pressing in control subjects produced IV infusions of saline. Following drug self-administration, subjects underwent place conditioning for the immediate or delayed effects of cocaine (1.0 or 2.5 mg/kg, IV). In control subjects, the immediate effects of the low dose of cocaine produced conditioned places preferences (CPPs), while the delayed effects produced conditioned place aversions (CPAs). In contrast, the animals receiving low cocaine dose for 6 hours, exhibited place aversions but not preferences; an effect that was reversed when the dose of cocaine was increased. Additionally, in the 6-hour group, delayed conditioning was associated with a reduction in zif268 immunoreactivity in the medial prefrontal cortex and nucleus accumbens shell while immediate conditioning was associated with an increase in zif268-positive cells in the central nucleus of the amygdala. Collectively, these data suggest that extended daily access to cocaine produces a shift in the subject's perceived reward threshold that is paralleled by alterations in the activity of both the reward and stress pathways.

Tracing the Neuroanatomical Profiles of Reward Pathways with Markers of Neuronal Activation

Functional neuroanatomical tools have played an important role in proposing which structures underlie brain stimulation reward circuitry. This review focuses on studies employing metabolic markers of neuronal and glial activation, including 2-deoxyglucose, cytochrome oxidase, and glycogen phosphorylase, and a marker of cellular activation, the immediate early gene c-fos. The principles underlying each method, their application to the study of brain stimulation reward, and their strengths and limitations are described. The usefulness of this strategy in identifying candidate structures, and the degree of overlap in the patterns of activation arising from different markers is addressed in detail. How these data have contributed to an understanding of the organization of reward circuitry and directed our thinking towards an alternative framework of neuronal arrangement is discussed in the final section.

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.

Fos expression following self-stimulation of the medial prefrontal cortex

Self-stimulation of the medial prefrontal cortex and medial forebrain bundle appears to be mediated by different directly activated fibers. However, reward signals from the medial prefrontal cortex do summate with signals from the medial forebrain bundle, suggesting some overlap in the underlying neural circuitry. We have previously used Fos immunohistochemistry to visualize neurons activated by rewarding stimulation of the medial forebrain bundle. In this study, we assessed Fos immunolabeling after self-stimulation of the medial prefrontal cortex. Among the structures showing a greater density of labeled neurons in the stimulated hemisphere were the prelimbic and cingulate cortex, nucleus accumbens, lateral preoptic area, substantia innominata, lateral hypothalamus, anterior ventral tegmental area, and pontine nuclei. Surprisingly, little or no labeling was seen in the mediodorsal thalamic nucleus or the locus coeruleus. Double immunohistochemistry for tyrosine hydroxylase and Fos showed that within the ventral tegmental area, a substantial proportion of dopaminergic neurons did not express Fos. Despite previous suggestions to the contrary, comparison of the present findings with those of our previous Fos studies reveals a number of structures activated by rewarding stimulation of both the medial prefrontal cortex and the medial forebrain bundle. Some subset of activated cells in the common regions showing Fos-like immunoreactivity may contribute to the rewarding effect produced by stimulating either site.

Tests of executive functioning predict scores on the MacAndrew Alcoholism Scale

1. Previous work reported that tests of executive functioning (EF) predict the risk of alcoholism in subject populations selected for a "high density" of a family history of alcoholism and/or the presence of sociopathic traits. The current experiment examined the ability of EF tests to predict the risk of alcoholism, as measured by the MacAndrew Alcoholism Scale (MAC), in outpatient subjects referred to a general neuropsychological testing service. 2. Sixty-eight male and female subjects referred for neuropsychological testing were assessed for their past drinking histories and administered the Wisconsin Card Sorting Test, the Wechsler Adult Intelligence Scale-Revised, the Trails (Part B) Test, and the MAC. Principal Components analysis (PCA) reduced the number of EF tests to two measures, including one that loaded on the WCST, and one that loaded on the Similarities, Picture Arrangement, and Trails tests. Multiple hierarchical regression first removed the variance from demographic variables, alcohol consumption, and verbal (i.e., Vocabulary) and non-verbal (i.e., Block Design) IQ, and then entered the executive functioning factors into the prediction of the MAC. 3. Seventy-six percent of the subjects were classified as either light, infrequent, or non-drinkers on the Quantity-Frequency-Variability scale. The factor derived from the WCST on PCA significantly added to the prediction of risk on the MAC (p = .0063), as did scores on Block Design (p = .033). Relatively more impaired scores on the WCST factor and Block Design were predictive of higher scores on the MAC. The other factors were not associated with MAC scores. 4. These results support the hypothesis that decrements in EF are associated with risk factors for alcoholism, even in populations where the density of alcoholic behaviors are not unusually high. When taken in conjunction with other findings, these results implicate EF test scores, and prefrontal brain functioning, in the neurobiology of the risk for alcoholism.

Preferential localization of self-stimulation sites in striosomes/patches in the rat striatum

Histological sections of the mammalian striatum reveal a "matrix" that is histochemically distinguishable from patches, or "striosomes". The latter are cross sections of a compartment that consists primarily of tube-shaped structures radiating through the matrix. As a test of the hypothesis that the function of the striosome/patch compartment includes the mediation of behaviors related to reward, the present study examined electrical self-stimulation of the caudoputamen in rats with electrodes in either of the two compartments. Rats acquired and maintained bar-pressing responses that were contingent on stimulation through electrodes making contact with striosomes/patches more reliably than animals with electrodes terminating exclusively in the matrix. The results provide in vivo evidence that the striosome/patch compartment is functionally differentiated from the matrix compartment: Stimulation centered in or around the striosome/patch compartment but not in the matrix led to rapid acquisition of a new behavior.

Effect of adrenalectomy on cocaine facilitation of medial prefrontal cortex self-stimulation

Adrenalectomy (ADX) is known to block the acquisition of intravenous cocaine self-administration. A previous study therefore examined whether ADX decreases sensitivity of the 'brain reward system' in general, or its response to cocaine in particular, by measuring thresholds for intracranial self-stimulation with and without concurrent cocaine administration. ADX had no effect on thresholds for lateral hypothalamic self-stimulation (LHSS) and did not alter the cocaine dose-response curve for lowering the LHSS threshold. This result suggested that ADX does not affect sensitivity of the brain reward system. However, medial prefrontal cortex (MPFC) appears to be an important site in the mediation of cocaine reinforcing effects, and MPFC self-stimulation (MPFCSS) is mediated by a neural substrate that is largely independent of that which mediates LHSS. The present study therefore assessed whether ADX diminishes cocaine facilitation of MPFCSS. It was found that the threshold-lowering effect of cocaine (5.0, 10.0 and 20.0 mg/kg, i.p. ) did not differ between ADX rats maintained on 0.7% saline, ADX rats maintained on corticosterone (50 microg/ml) in 0.7% saline, and sham-operated controls. However, there was a trend toward desensitization of MPFCSS, itself, following ADX in the group that did not receive corticosterone supplementation. Based on this observation, and the similar responses of MPFCSS and cocaine self-administration to noncontingent priming stimulation, stress, and NMDA receptor antagonism, it is speculated that acquisition of MPFCSS and cocaine self-administration may be dependent upon a common sensitization process that is regulated by corticosterone.

Dorsal lesions of the prefrontal cortex: effects on alcohol consumption and subcortical monoaminergic systems

Male Wistar rats were subjected to either bilateral aspiration lesions of the dorsal regions of the prefrontal cortex (PFC) or sham lesions and placed on a 6-week, modified sucrose-fading procedure. At the time of sacrifice, the size of the lesion, both in anterior-posterior and medial-lateral dimensions, was measured. Following sacrifice, levels of dopamine (DA), serotonin (5-HT), norepinephrine (NE), and their metabolites were measured in the midbrain (raphe) and nucleus accumbens (NA). Lesioned animals had reductions in 5-HT in the NA, and DA and NE in the raphe. The lesioned group drank more of a solution of 5% alcohol than controls early in the sucrose fading, and less during the later stages. In the lesioned group, the size of the left- and right-hemisphere lesions predicted 5-HIAA levels in the NA, and 5-HT and 5-HIAA levels in the raphe. A laterality effect was noted, such that the size of left-hemisphere lesions were positively associated with raphe 5-HT and 5-HIAA levels, and negatively associated with 5-HT levels in the NA, while right-hemisphere lesions showed the opposite relationships. In addition, the width of the left-hemisphere lesion predicted some measures of alcohol intake. These results suggest that, in the rat, the dorsal PFC is involved in the regulation of monoamines in subcortical regions known to be important in the regulation of reinforced behaviors, and that this regulation differs between hemispheres and shows a laterality effect. In addition, the dorsal PFC appears to have a subtle involvement in the regulation of alcohol intake.

Human brain activity response to fentanyl imaged by positron emission tomography

Positron emission tomography (PET) is a noninvasive imaging technique that can be used to observe drug actions on human brain in vivo. We used 15O-water PET scanning in six volunteers to examine the effects on regional cerebral activity as reflected by regional cerebral blood flow (rCBF) of a small intravenous bolus of fentanyl. rCBF was compared between scans obtained after fentanyl or a placebo using three separate statistical criteria including a pixel-by-pixel t statistic; significance was stringently defined at P values < 0.01. Anatomic locations of regional cerebral activity changes were verified by aligning rCBF PET scans with cranial magnetic resonance images using mathematical coregistration. Fentanyl administration was associated with significant increases in rCBF consistent with regional neuronal activation in both cingulate and orbitofrontal and medial prefrontal cortices, as well as caudate nuclei. These areas are responsive to nociceptive stimuli and are involved in avoidance learning, reward and addiction, visceromotor control, maintenance of attention, and pain-related affective behavior. Significant decreases were noted in both frontal and temporal areas and the cerebellum, a distribution far less extensive than that of opiate receptors in general. These data indicate that fentanyl's effects are highly localized and specifically affect cerebral regions associated with a range of pain-related behaviors.

Characteristics of stimulation-induced feeding sites in the sulcal prefrontal cortex

Double pulse tests were used to infer the refractory periods of the substrate underlying stimulation-induced feeding in the sulcal prefrontal cortex of the rat. Eleven sites were examined, of which five supported the behaviour at currents of 250 to 400 microA; pulse duration was 100 microseconds. The average profile indicates a recovery function that begins at 0.5 ms and ends at 3.0 ms, with no apparent contribution from local potential summations. The mean effectiveness value corresponding to the asymptotic portion of the curves was 91%. These results suggest that there is substantial overlap in the excitability of neurons underlying stimulation-induced feeding in the sulcal prefrontal cortex and that reported for the medial forebrain bundle, but unlike the latter structure, there is no evidence of self-stimulation from the same sulcal prefrontal cortex placements. Response rates were collected for a maximum of 21 days from each of the cortical sites at which feeding was recorded and at the same currents used to evaluate the refractory periods underlying stimulation-induced feeding. The rates averaged between 0 and 3 responses per minute whether the current was available or not. These data represent the first demonstration of a site that supports stimulation-induced feeding in the absence of brain stimulation reward, at least at these specific placements and stimulation parameters.

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.

Behaviourally derived estimates of excitability in striatal and medial prefrontal cortical self-stimulation sites

The refractory periods of the substrate underlying brain-stimulation reward were investigated in three rats with moveable electrodes implanted in the rostral caudate-putamen and the medial prefrontal cortex. Acquisition of caudate-putamen self-stimulation occurred within the first session, while self-stimulation for medial prefrontal cortex was observed only after three sessions of caudate-putamen stimulation. The currents required for self-stimulation ranged from 300 to 800 microA (0.1 ms pulse duration) across animals; the maximum response rates averaged roughly 40 bar presses per minute for both structures. Refractory period estimates were obtained from ten caudate-putamen and four medial prefrontal cortex sites. The time course of recovery had the following profile: the curves began to rise at 0.65 ms and 0.95 ms for caudate-putamen and medial prefrontal cortex stimulation, respectively, thereafter increasing to approach an asymptote at 6.00 ms for the caudate-putamen and 6.25 ms for the medial prefrontal cortex. The mean effectiveness value corresponding to the asymptotic portion of the curves was 73% for the caudate-putamen and 69% for the medial prefrontal cortex. Like other forebrain structures, the behaviourally derived refractory periods underlying caudate-putamen and medial prefrontal cortex stimulation, at least at these particular sites, are significantly longer than those observed in most medial forebrain bundle areas, both beginning and ending later. One interpretation for the similarity in their refractory period profiles and the apparent facilitating effect of caudate-putamen stimulation on acquisition of medial prefrontal cortex self-stimulation is that these two regions form part of the same reward substrate.

The acquisition of self-stimulation of the medical prefrontal cortex following exposure to escapable or inescapable footshock

The effect of acute stress on the acquisition of an instrumental action reinforced by electrical stimulation of the medial prefrontal cortex (MPC) was investigated by exposing rats to either escapable, inescapable or no footshock prior to daily self-stimulation training sessions. Treatment with inescapable footshock did not affect the number of sessions required for acquisition of MPC self-stimulation but did increase the rate of responding over acquisition sessions compared with the no-shock group. When the treatment footshock was escapable, however, both a facilitation in acquisition, as indexed by a reduction in the number of sessions to criterion, and an increase in the rate of MPC self-stimulation was found. These data were interpreted as offering evidence for the operation of a dopaminergic mechanism in the acquisition of MPC self-stimulation. Further, they indicate, contrary to the reported effects of footshock on self-stimulation of other brain areas, that exposure to acute stress has a facilitatory effect on the rate of self stimulation of the MPC.

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.

Ketamine blocks the plasticity associated with prefrontal cortex self-stimulation

Intracranial self-stimulation (ICSS) at sites within the medial prefrontal cortex (MFC) is acquired slowly but can be hastened by prior exposure to a regimen of noncontingent stimulation delivered to the MFC ICSS electrode. The facilitatory effects of noncontingent MFC stimulation on subsequent ICSS acquisition were blocked by pretreatment with ketamine, a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor. These findings provide further support for the view that the NMDA receptor is importantly involved in mechanisms of neural plasticity.

Cocaine facilitates prefrontal cortex self-stimulation

It has been demonstrated that cocaine HCl lowers thresholds for and increases rates of medial forebrain bundle intracranial self-stimulation. The influence of cocaine on prefrontal cortex self-stimulation was assessed in the present experiment. The prefrontal cortex was chosen because evidence indicates that the neuroanatomical and pharmacological substrate for intracranial self-stimulation at this site may differ from the substrate for medial forebrain bundle self-stimulation. Cocaine significantly decreased train-duration thresholds and increased the rate of prefrontal cortex self-stimulation. It was concluded that cocaine facilitates both prefrontal cortex and medial forebrain bundle self-stimulation, perhaps by influencing neural activity in the mesocorticolimbic dopamine system. However, the role of dopamine in cocaine's effects at both sites remains speculative.

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)

Controllability of prestimulation of the medial prefrontal cortex determines the facilitation of self-stimulation and kindled seizures

Electrical stimulation of the medial prefrontal cortex (MPC) was administered according to the triadic design typically used to demonstrate learned helplessness. Three groups received either controllable, uncontrollable or no stimulation during the pretreatment phase. The effects of this pretreatment on the acquisition of self-stimulation at the same electrode site were investigated in the second phase of the experiment. Relative to unstimulated controls, both controllable and uncontrollable prestimulation facilitated the acquisition of self-stimulation and produced higher self-stimulation rates. In addition, compared with controllable stimulation, pretreatment with uncontrollable stimulation produced a greater facilitation in self-stimulation rate. The unambiguous demonstration of a behavioural facilitation produced by pretreatment with uncontrollable stimulation is, effectively, the inverse of the typical learned helplessness finding. It was also found, in the second phase of the experiment, that 6 of the 7 rats previously exposed to uncontrollable stimulation developed full class 5 seizures. No behavioural evidence of kindling was seen in any of the other rats or during the prestimulation procedure. These data are interpreted in terms of kindling and stress effects both proximal and distal to the site of stimulation.

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.

Electrical stimulation of the medial prefrontal cortex supports both 'pure reward' and 'reward-escape' behavior in rats

In female Sprague-Dawley rats, 8 of 12 medial prefrontal cortex (MPFC) sites that yielded criterion self-stimulation behavior supported only self-stimulation, i.e. were 'pure reward' in type. The remaining 4 sites supported behavior to escape from experimenter-administered stimulation of the same parameters as well, i.e. were 'reward-escape' in type. 'Pure reward' and 'reward-escape' sites in the MPFC were distinguished by both the magnitude and temporal form of the escape response functions generated, and by the prevalence of 'pounce-back', a vigorous and repetitive barpressing during the 3-s MPFC stimulation-escape interval produced by an effective barpress. The finding that both 'pure reward' and 'reward-escape' patterns of behavior can be elicited by stimulation of the MPFC provides a basis for further assessment of similarities and differences in medial prefrontal cortical and lateral hypothalamic (LH) 'reward' systems. It is suggested that 'reward-escape' in the MPFC may be mediated by the activity of 'reward' neurons which respond to stimulus offset, rather than by a secondary aversive process as is proposed to underlie 'reward-escape' in the LH.

Prefrontal cortex lesions attenuate substantia nigra self-stimulation: a reward summation analysis

A curve-shift paradigm was used to assess the effects of lesions of the prefrontal cortex on self-stimulation from electrode sites in the substantia nigra. Combined lesions of the medial and sulcal cortical regions severely attenuated substantia nigra self-stimulation. These results are discussed in the context of the frontal cortex and the substantia nigra as belonging to a reinforcement system that is largely independent of the medial forebrain bundle system.

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

Rats were implanted with stimulating electrodes in the medial forebrain bundle-lateral hypothalamus (MFB-LH). Following recovery from surgery, they were placed in 3 groups prior to brain self-stimulation training. This consisted of one 15-min session on each of 5 consecutive days. Animals in the first group (controls) were placed in a conventional, single lever operant chamber without any additional manipulation. There were no priming stimuli, there was no experimenter intervention of any kind, and no exteroceptive cues in the chamber to indicate the availability or otherwise of the reinforcement. Animals in the second group (saline-injected) were treated similarly to the first group except that they were weighed and injected subcutaneously with saline (1 ml/kg) immediately before being placed in the chamber. Animals in a third group (D-amphetamine-injected) were weighed and administered D-amphetamine (0.5 mg/kg in saline) immediately before being placed in the chamber. The number of lever-presses made per 15-min session was recorded. In addition, the time taken to achieve a lever-pressing rate of 10 presses per min was recorded. There were no significant differences between groups in the number of presses per 15-min session. Animals administered D-amphetamine reached the rate of 10 presses per min significantly more rapidly than animals administered saline, but the latter did so significantly more rapidly than controls. These results demonstrated that the simple manipulation, and perhaps the mild stress, associated with a saline injection strongly affected the acquisition of a brain self-stimulation task.

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.

Rat strain differences in the acquisition of hippocampal self-stimulation

The rate of acquisition of lever-pressing for electrical stimulation of the hippocampus (HPC) was compared in two strains of rat: barrier-sustained Wistar and Sprague-Dawley. Sprague-Dawley rats initially bar-pressed at very low rates and took a median of 11 days to self-stimulate, according to the criterion used. Wistar rats all reached the same criterion in the first test session. Differential sensitivity to the activating effects of stimulation as an explanation for this difference was ruled out by the observation that both strains decreased response rates at the same rate and to the same level if stimulation was made non-contingent on lever-pressing. Differential threshold for reward was ruled out by the observation that rate-intensity curves yielded the same threshold currents and peak rates in both strains. Finally, it was shown that the rate of development of kindled seizures in the two strains of rats is different: Wistars kindle to full seizures faster than do Sprague-Dawleys. The relationship between the quicker onset of self-stimulation and of kindled seizures in Wistars is discussed.

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.

Cortical and ventral tegmental systems exert opposing influences on self-stimulation from the prefrontal cortex

Intracranial self-stimulation (ICSS) was obtained from 3 areas of anteromedial cortex: the prelimbic area (Brodman's area 32), the anterior cingulate area and the posterior cingulate area. Electrical stimulation in the prelimbic and anterior cingulate areas also produces a behavioral inhibition which is most pronounced at anterior sites (i.e. prelimbic) and declines at increasingly more posterior sites. It was found that the acquisition of responding for ICSS and the magnitude of amphetamine's facilitation of ICSS were inversely related to the degree of behavioral inhibition. These data and the ability of amphetamine to reverse prefrontal stimulation-induced inhibition suggest an important interaction between the prefrontal cortex and the mesolimbic dopamine systems in the control of goal-directed behavior. A model involving cortical suppression of mesolimbic dopamine function is discussed.

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.

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.