The neurobiological basis of prefrontal cortex self-stimulation: a review and an integrative hypothesis

Dissecting the chain of information processing and its interplay with neurochemicals and fluid intelligence across development

Previous research has highlighted the role of glutamate and gamma-aminobutyric acid (GABA) in perceptual, cognitive, and motor tasks. However, the exact involvement of these neurochemical mechanisms in the chain of information processing, and across human development, is unclear. In a cross-sectional longitudinal design, we used a computational approach to dissociate cognitive, decision, and visuomotor processing in 293 individuals spanning early childhood to adulthood. We found that glutamate and GABA within the intraparietal sulcus (IPS) explained unique variance in visuomotor processing, with higher glutamate predicting poorer visuomotor processing in younger participants but better visuomotor processing in mature participants, while GABA showed the opposite pattern. These findings, which were neurochemically, neuroanatomically and functionally specific, were replicated ~21 mo later and were generalized in two further different behavioral tasks. Using resting functional MRI, we revealed that the relationship between IPS neurochemicals and visuomotor processing is mediated by functional connectivity in the visuomotor network. We then extended our findings to high-level cognitive behavior by predicting fluid intelligence performance. We present evidence that fluid intelligence performance is explained by IPS GABA and glutamate and is mediated by visuomotor processing. However, this evidence was obtained using an uncorrected alpha and needs to be replicated in future studies. These results provide an integrative biological and psychological mechanistic explanation that links cognitive processes and neurotransmitters across human development and establishes their potential involvement in intelligent behavior.

Differential release of dopamine in the nucleus accumbens evoked by low-versus high-frequency medial prefrontal cortex stimulation

The medial prefrontal cortex (mPFC) coordinates goal-directed behaviors, which may be mediated through mPFC regulation of dopamine release in the nucleus accumbens (NAc). Furthermore, frequency-specific oscillatory activity between the frontal cortex and downstream structures may facilitate inter-region communication. Although high-frequency (e.g., 60 Hz) mPFC stimulation is known to increase basal dopamine levels in the NAc, little is known about how phasic dopamine release is affected by mPFC stimulation. Understanding the frequency-specific control of phasic dopamine release by mPFC stimulation could elucidate mechanisms by which the mPFC modulates other regions. It could also inform optimization of deep brain stimulation for treatment of neurological disorders.

OBJECTIVE

The goal of this work was to characterize the frequency response of NAc dopamine release resultant from mPFC stimulation. We hypothesized that the magnitude of dopamine release in the NAc would increase with increasing stimulation frequency.

METHODS

Electrical stimulation of the mPFC of anesthetized rats was delivered at 4-60 Hz and at varying durations while measuring NAc dopamine release with fast-scan cyclic voltammetry.

RESULTS

mPFC stimulation resulted in phasic dopamine release in the NAc. Furthermore, 20 Hz stimulation evoked the largest peak response for stimulation intervals >5 s when compared to higher or lower frequencies.

CONCLUSIONS

Activation of the mPFC drives dopamine release in the NAc in a complex frequency- and duration-dependent manner. This has implications for the use of deep brain stimulation treatment of disorders marked by dopaminergic dysregulation, and suggest that mPFC may exert more specialized control over neuromodulator release than previously understood.

Differential roles of medial prefrontal subregions in the regulation of drug seeking

The prefrontal cortex plays an important role in shaping cognition and behavior. Many studies have shown that medial prefrontal cortex (mPFC) plays a key role in seeking, extinction, and reinstatement of cocaine seeking in rodent models of relapse. Subregions of mPFC appear to play distinct roles in these behaviors, such that the prelimbic cortex (PL) is proposed to drive cocaine seeking and the infralimbic cortex (IL) is proposed to suppress cocaine seeking after extinction. This dichotomy of mPFC function may be a general attribute, as similar dorsal-ventral distinctions exist for expression vs. extinction of fear conditioning. However, other results indicate that the role of mPFC neurons in reward processing is more complex than a simple PL-seek vs. IL-extinguish dichotomy. Both PL and IL have been shown to drive and inhibit drug seeking (and other types of behaviors) depending on a range of factors including the behavioral context, the drug-history of the animal, and the type of drug investigated. This heterogeneity of findings may reflect multiple subcircuits within each of these PFC areas supporting unique functions. It may also reflect the fact that the mPFC plays a multifaceted role in shaping cognition and behavior, including those overlapping with cocaine seeking and extinction. Here we discuss research leading to the hypothesis that dorsal and ventral mPFC differentially control drug seeking and extinction. We also present recent results calling the absolute nature of a PL vs. IL dichotomy into question. Finally, we consider alternate functions for mPFC that correspond less to response execution and inhibition and instead incorporate the complex cognitive behavior for which the mPFC is broadly appreciated.

Glutamatergic markers, age, intellectual functioning and psychosis in 22q11 deletion syndrome

RATIONALE

Patients with 22q11 deletion syndrome (22q11DS) have a high prevalence of intellectual disabilities and psychiatric disorders, including psychosis. Haplo-insufficiency of genes in the deleted region may offer a partial explanation for the increased vulnerability for psychosis and intellectual disability. One gene of particular interest is the gene coding for proline dehydrogenase (PRODH), an enzyme responsible for the conversion of proline into glutamate.

OBJECTIVES

Because abnormalities in glutamatergic signaling are thought to be responsible for cognition and psychosis in the general population, we hypothesized that PRODH haplo-insufficiency may underlie some of the cognitive and psychotic features seen in 22q11DS.

METHODS

In this explorative study, we investigated the relation between plasma proline, glutamate, and glutamine and age, intelligence, and psychosis in 64 adults with 22q11DS.

RESULTS

Hyperprolinemia was found in 31.3% of subjects with 22q11DS. A relation between glutamine, glutamate, proline, and presence of psychosis was not observed. Regression analysis revealed a positive relation between plasma glutamate and age, a positive relation of glutamate with antipsychotic drugs, a relation of glutamine and gender, and a positive relation of glutamine and mood stabilizing drugs, and a negative relation of the ratio glutamine/glutamate and age. The group with relatively lower IQ had higher glutamate levels compared to the group with relatively higher IQ.

CONCLUSIONS

Our results suggest that 22q11DS is accompanied by abnormalities in glutamatergic metabolism. Future longitudinal studies are needed to further investigate the glutamatergic system in 22q11DS and how this affects the development of cognitive problems and psychopathology.

Involvement of the GABAergic septo-hippocampal pathway in brain stimulation reward

The hippocampus is a structure related to several cognitive processes, but not very much is known about its putative involvement in positive reinforcement. In its turn, the septum has been related to instrumental brain stimulation reward (BSR) by its electrical stimulation with trains of pulses. Although the anatomical relationships of the septo-hippocampal pathway are well established, the functional relationship between these structures during rewarding behaviors remains poorly understood. To explore hippocampal mechanisms involved in BSR, CA3-evoked field excitatory and inhibitory postsynaptic potentials (fEPSPs, fIPSPs) were recorded in the CA1 area during BSR in alert behaving mice. The synaptic efficiency was determined from changes in fEPSP and fIPSP amplitudes across the learning of a BSR task. The successive BSR sessions evoked a progressive increase of the performance in inverse relationship with a decrease in the amplitude of fEPSPs, but not of fIPSPs. Additionally, we evaluated CA1 local field potentials (LFPs) during a preference task, comparing 8-, 20-, and 100-Hz trains of septal BSR. We corroborate a clear preference for BSR at 100 Hz (in comparison with BSR at 20 Hz or 8 Hz), in parallel with an increase in the spectral power of the low theta band, and a decrease in the gamma. These results were replicated by intrahippocampal injections of a GABAB antagonist. Thus, the GABAergic septo-hippocampal pathway seems to carry information involved in the encoding of reward properties, where GABAB receptors seem to play a key role. With regard to the dorsal hippocampus, fEPSPs evoked at the CA3-CA1 synapse seem to reflect the BSR learning process, while hippocampal rhythmic activities are more related to reward properties.

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.

Glutamate changes in healthy young adulthood

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system and has been associated with several cognitive functions that are known to change with age. In rodents and humans age-related glutamate changes have been found in several brain areas. In this cross-sectional study the presence and extent of age-associated glutamate changes in the medial frontal cortex of healthy young adults were measured. Proton magnetic resonance spectroscopy ((1)H-MRS) and brain imaging were performed at 7 T in a 2 × 2 × 2 cm(3) voxel in 33 participants between 18 and 31 years old. Glutamate concentrations and grey and white matter volume could be successfully determined at an ultra-high magnetic field strength. Glutamate concentrations were lower in older individuals (0.33 mM/year). This decline is in line with grey matter thinning in the medial frontal cortex, but could not be explained by cortical thinning alone. Therefore, the decrease in glutamate in young adulthood may be due to physiological changes rather than anatomical changes.

The GABAergic septohippocampal pathway is directly involved in internal processes related to operant reward learning

We studied the role of γ-aminobutyric acid (GABA)ergic septohippocampal projections in medial septum (MS) self-stimulation of behaving mice. Self-stimulation was evoked in wild-type (WT) mice using instrumental conditioning procedures and in J20 mutant mice, a type of mouse with a significant deficit in GABAergic septohippocampal projections. J20 mice showed a significant modification in hippocampal activities, including a different response for input/output curves and the paired-pulse test, a larger long-term potentiation (LTP), and a delayed acquisition and lower performance in the MS self-stimulation task. LTP evoked at the CA3-CA1 synapse further decreased self-stimulation performance in J20, but not in WT, mice. MS self-stimulation evoked a decrease in the amplitude of field excitatory postsynaptic potentials (fEPSPs) at the CA3-CA1 synapse in WT, but not in J20, mice. This self-stimulation-dependent decrease in the amplitude of fEPSPs was also observed in the presence of another positive reinforcer (food collected during an operant task) and was canceled by the local administration of an antibody-inhibiting glutamate decarboxylase 65 (GAD65). LTP evoked in the GAD65Ab-treated group was also larger than in controls. The hippocampus has a different susceptibility to septal GABAergic inputs depending on ongoing cognitive processes, and the GABAergic septohippocampal pathway is involved in consummatory processes related to operant rewards.

Comparison of metabolic profiling of cyanidin-3-O-galactoside and extracts from blueberry in aged mice

The metabonomics changes of plasma and brain tissue after dietary supplementation with blueberry extracts (BBE) and cyanidin-3-O-galactoside from blueberry (BBM) in aged mice were investigated by (1)H NMR technique. The mice received intragastric administration of BBE (200 mg/kg/day), BBM (50 mg/kg/day), and saline water (0.9%) for 6 weeks, respectively, in the BBE, BBM, and control groups. At the end of the experiment, plasma and brain samples were collected for NMR analysis. The results demonstrated that the level of choline in plasma from BBE and BBM groups were obviously elevated relative to the control group, whereas the levels of lactate and phosphocholine in plasma were remarkably reduced. Compared with those in the control group, the levels of choline and GABA in the brain from the BBE group were obviously increased, whereas glutamate and phosphocholine in the BBE group were significantly decreased. The level of taurine in the brain from the BBM group was particularly higher than that in the control group. These results indicated supplementation with BBE or BBM might induce similar changes of endogenous plasma and brain metabolic profiles in aged mice.

Understanding Obsessive–Compulsive Disorder: Focus on Decision Making

Current approaches to obsessive-compulsive disorder (OCD) have suggested that neurobiological abnormalities play a crucial role in the etiology and course of this psychiatric illness. In particular, a fronto-subcortical circuit, including the orbitofrontal cortex, basal ganglia and thalamus appears to be involved in the expression of OCD symptoms. Neuropsychological studies have also shown that patients with OCD show deficits in cognitive abilities that are strictly linked to the functioning of the frontal lobe and its related fronto-subcortical structures, such as executive functioning deficits and insufficient cognitive-behavioral flexibility. This article focuses on decision making, an executive ability that plays a crucial role in many real-life situations, whereby individuals choose between pursuing strategies of action that involve only immediate reward and others based on long-term reward. Although the role of decision-making deficits in the evolution of OCD requires further research, the collected findings have significant implications for understanding the clinical and behavioral heterogeneity that characterizes individuals with OCD.

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.

Functional heterogeneity of the rat medial prefrontal cortex: effects of discrete subarea-specific lesions on drug-induced conditioned place preference and behavioural sensitization

While the principal components of the brain reward system, the nucleus accumbens septi and the ventral tegmental area have received much attention, their efferent and afferent structures have not been investigated to the same degree. One major input to this system originates from the medial prefrontal cortex (mPFC) which is not a homogenous structure but can be divided into different subareas that can be distinguished on anatomical and possibly functional grounds. We examined the effects of discrete bilateral quinolinic acid lesions (45 nmol/0.5 micro(L)) of each of the mPFC subareas, the infralimbic (il), prelimbic (pl) and the anterior cingulate (cg) mPFC, on the conditioned place preference (CPP) and psychomotor activation induced by several drugs. Lesions of the il mPFC blocked CPP induced by morphine (10 mg/kg) and CGP37849 [DL-(E)-2-amino-4-methyl-5-phosphono-3-pentic acid, a competitive N-methyl-D-aspartate receptor antagonist; 10 mg/kg]. Lesions of the pl mPFC blocked CPP induced by cocaine (15 mg/kg) and CGP37849, and lesions of the cg mPFC only blocked CGP37849-induced CPP. Lesions of the whole mPFC blocked morphine-, cocaine- and CGP37849-induced CPP. None of the lesions affected DL-amphetamine (4 mg/kg)-induced CPP. During the conditioning period, none of the lesions affected amphetamine-induced psychomotor activation and sensitization, whereas both phenomena were attenuated by pl and whole mPFC lesions in the case of cocaine, and by il and whole mPFC lesions in the case of morphine. These results show that the different mPFC subregions have distinct functional roles in the generation of behavioural effects produced by different classes of drugs. This heterogeneity should be taken into account in future studies addressing the role of the mPFC in drug reward and sensitization.

Dopamine-glutamate interactions in the prefrontal cortex of the conscious rat: studies on ageing

The effects of apomorphine, a D1-D2 dopamine receptor agonist, on the extracellular concentration of glutamate were investigated in the medial prefrontal cortex of young, middle-aged and aged rats. In vivo intracerebral perfusions were undertaken in the conscious rat using a concentric push-pull cannula system. Glutamate concentration in the samples were determined by HPLC with fluorometric detection. Apomorphine produced an increase in extracellular concentration of glutamate in medial prefrontal cortex of young rats (178% of baseline) only at 10 microM, but not at 5 and 20 microM. This increase in glutamate concentration induced by apomorphine was significantly attenuated by blockade of D1-D2 dopamine receptors with haloperidol. Apomorphine, at 10 microM, failed to induce an increase in extracellular concentration of glutamate in the prefrontal cortex of middle-aged and aged rats. However, at 20 microM, apomorphine induced an increase in glutamate concentration in the prefrontal cortex of middle-aged rats, but not in aged rats. These data indicate that an interaction between dopamine and glutamate exists in the medial prefrontal cortex and that this interaction deteriorates with age.

Single neuronal responses in medial prefrontal cortex during cocaine self-administration in freely moving rats

Chronic single neuronal recording techniques were applied to investigate the involvement of the medial prefrontal cortex (mPFC) during cocaine self-administration in the rat. Rats were trained to press a lever for cocaine under continuous reinforcement and fixed ratio schedules. Different patterns of phasic neuronal activity changes were found to be associated with lever-pressing for cocaine. The neuronal responses could be classified into five categories: 1) increases in neuronal firing before the lever press (15 out of 121 neurons, 12.4%); 2) decreases in neuronal firing before the lever press (13 neurons, 10.7%); 3) increases in neuronal firing after cocaine infusion (4 neurons, 3.3%); 4) decreases in neuronal firing after cocaine infusion (32 neurons, 26.4%); and 5) no alteration of neuronal activity throughout the self-administration session (67 neurons, 55.4%). The anticipatory responses, i.e., neuronal activity appearing before the lever press, were observed for both the continuous reinforcement and fixed ratio schedules. In a few cases, alteration of firing rate was not observed for the first lever press but appeared before subsequent lever presses in fixed ratio schedules. Eliminating cocaine abolished the inhibitory neuronal responses observed after lever press, suggesting that these inhibitory responses after cocaine self-administration were attributable to the pharmacologic effect of cocaine. The data provide initial electrophysiological evidence that the mPFC may play a role in mediating the task sequencing which leads to cocaine self-administration.

Neuronal responses in prefrontal cortex and nucleus accumbens during heroin self-administration in freely moving rats

Chronic multi-channel single unit recordings of neuronal responses in prefrontal cortex (PFC) and nucleus accumbens (NAc) were made in 9 male Sprague Dawley rats to determine patterns of neuronal activity during heroin self-administration. Up to 32 neurons were recorded simultaneously in these two brain regions while rats lever pressed on a continuous reinforcement schedule for intravenous infusion of heroin (30 microg/kg/infusion). The variety of neuronal responses observed before and after each self-administered heroin infusion can be classified according to the following categories: (1) neurons that increased or (2) decreased their activity immediately before the lever press; (3) neurons that increased or (4) decreased their activity after the heroin infusion; and, (5) neurons that did not alter their activity either before or after the lever press for heroin infusion. The majority (69% in the PFC and 65% in the NAc) of neurons sampled fell into this last category of no change, indicating that a selected fraction becomes active during this specific task. In general, NAc neurons displayed more post-heroin responses than PFC neurons while the proportion of neurons showing responses before the lever press was similar in the mPFC and the NAc. This initial description of the responses of PFC and NAc neurons during heroin self-administration suggests that the neuronal circuit of the mesocorticolimbic system is involved in heroin self-administration. This circuit appears to contribute both to the initiation of drug-seeking behavior (pre-lever press phasic neuronal responses), as well as the action of heroin infusion itself (post-infusion phasic neuronal responses) by activation of different subsets of neurons.

Effects of ibotenic acid lesions of the ventral striatum and the medial prefrontal cortex on ethanol consumption in the rat

The purpose of this study was to to assess the effect on ethanol drinking of ibotenic acid lesions in the medial prefrontal cortex and the ventral striatum of female rats with continuous access to water and a 6% ethanol solution. Ibotenic acid infusions in the prefrontal cortex did not affect ethanol intake at any time, but a significant increase in water intake was observed on the third postoperative week. Ventral striatal lesions significantly increased ethanol intake during the first 2 postoperative weeks. On the third week consumption was not significantly different from vehicle-infused controls. Apparently, then, severe excitoxic injury to the ventral striatum is compatible with normal, or increased, intake of ethanol; in contrast, similar lesions reduce the intake of other drugs of abuse such as psychostimulants and opioids.