The behavioral neurochemistry of motivation: methodological and conceptual issues in studies of the dynamic activity of nucleus accumbens dopamine

Local dopamine production in the dorsal striatum restores goal-directed behavior in dopamine-deficient mice

To determine whether dopamine signaling in the dorsal striatum is sufficient for performance of goal-directed behaviors, local dopamine production was restored in the dorsal striatum of dopamine-deficient (DD) mice through viral-mediated gene therapy. Virally rescued DD (vrDD) mice were tested for learning of an appetitive T-maze task designed to measure goal-directed behavior. The results indicate that in contrast with the performance of DD mice that have dysregulated dopamine signaling, vrDD mice were able to perform the T-maze task and reverse their behavior as well as sham-operated control mice. The authors conclude that finely tuned dopaminergic signaling within the dorsal striatum is sufficient for performance of goal-directed behavior.

NMDA and muscarinic receptors of the nucleus accumbens have differential effects on taste memory formation

Animals recognize a taste cue as aversive when it has been associated with post-ingestive malaise; this associative learning is known as conditioned taste aversion (CTA). When an animal consumes a new taste and no negative consequences follow, it becomes recognized as a safe signal, leading to an increase in its consumption in subsequent presentations (attenuation of neophobia, AN). It has been shown that the nucleus accumbens (NAcc) has an important role in taste learning. To elucidate the involvement of N-methyl-D-aspartate (NMDA) and muscarinic receptors in the NAcc during safe and aversive taste memory formation, we administrated bilateral infusions of DL-2-amino-5-phosphonopentanoic acid (APV) or scopolamine in the NAcc shell or core respectively. Our results showed that pre-training injections of APV in the NAcc core and shell disrupted aversive but not safe taste memory formation, whereas pre-training injections of scopolamine in the NAcc shell, but not core, disrupted both CTA and AN. These results suggest that muscarinic receptors seem to be necessary for processing taste stimuli for either safe or aversive taste memory, whereas NMDA receptors are only involved in the aversive taste memory trace formation.

Association of multiple DRD2 polymorphisms with anorexia nervosa

To investigate whether the dopaminergic system plays a role in the etiology of anorexia nervosa (AN) via the dopamine D2 receptor, we investigated association and transmission disequilibrium at seven single-nucleotide polymorphisms (SNPs) spanning about 75 kbp of the gene DRD2. We studied 191 probands with a DSM-IV diagnosis of AN, 457 parents and affected relatives with a DSM-IV eating disorder diagnosis, and 98 unrelated, female, normal weight controls. The -141 C/- insertion/deletion (-141 Indel), previously shown to affect DRD2 transcription efficiency, and multiple exon seven polymorphisms, one of which has previously been shown to affect DRD2 transcript stability, exhibited statistically significant association with diagnosis in haplotype transmission disequilibrium and in haplotype case : control analyses. Significant linkage disequilibrium between the -141 Indel and two exon seven SNPs (939Y and 957Y) was observed over a distance of >50 kbp in the AN probands but not in the controls. Genetically transmitted variation in D2 dopamine receptor expression mediated by functional polymorphisms affecting transcription and translation efficiency may play a role in vulnerability to AN.

Heritability, correlations and in silico mapping of locomotor behavior and neurochemistry in inbred strains of mice

The midbrain dopamine system mediates normal and pathologic behaviors related to motor activity, attention, motivation/reward and cognition. These are complex, quantitative traits whose variation among individuals is modulated by genetic, epigenetic and environmental factors. Conventional genetic methods have identified several genes important to this system, but the majority of factors contributing to the variation remain unknown. To understand these genetic and environmental factors, we initiated a study measuring 21 behavioral and neurochemical traits in 15 common inbred mouse strains. We report trait data, heritabilities and genetic and non-genetic correlations between pheno-types. In general, the behavioral traits were more heritable than neurochemical traits, and both genetic and non-genetic correlations within these trait sets were high. Surprisingly, there were few significant correlations between the behavioral and the individual neurochemical traits. However, striatal serotonin and one measure of dopamine turnover (DOPAC/DA) were highly correlated with most behavioral measures. The variable accounting for the most variation in behavior was mouse strain and not a specific neurochemical measure, suggesting that additional genetic factors remain to be determined to account for these behavioral differences. We also report the prospective use of the in silico method of quantitative trait loci (QTL) analysis and demonstrate difficulties in the use of this method, which failed to detect significant QTLs for the majority of these traits. These data serve as a framework for further studies of correlations between different midbrain dopamine traits and as a guide for experimental cross designs to identify QTLs and genes that contribute to these traits.

Glycine receptors regulate dopamine release in the rat nucleus accumbens

BACKGROUND

The mesolimbic dopamine (DA) system seems to be centrally involved in regulating reward-related behavior and consequently has been implicated in addictive processes, such as alcoholism and drug addiction. This DA system has also been implicated in psychosis and in regulating hedonia/anhedonia, important components of mania and depression. Given the potentially great importance of the mesolimbic DA system for several psychiatric disorders, it is of major interest to delineate the mechanisms and dynamics underlying DA regulation and release. Recently strychnine-sensitive glycine receptors (GlyR) have attracted some interest in this matter.

METHODS

Western blot and in vivo microdialysis (couplied to high-pressure liquid chromatography with electrochemical detection), as well as reversed microdialysis, in awake, freely moving, adult male Wistar rats.

RESULTS

Here we demonstrate by means of Western blot that alpha GlyR subunit proteins are expressed in the rat nucleus accumbens (nAc), a major target of the mesolimbic DA system. We further show that reversed microdialysis of the competitive GlyR antagonist strychnine into the nAc concentration-dependently (2-200 microM) and in a reversible manner decreases accumbal extracellular DA levels. Conversely, reversed microdialysis of the agonist glycine increases accumbal DA levels in some rats but not others. The strychnine-induced depression of the accumbal DA levels is antagonized by simultaneous local perfusion of glycine.

CONCLUSIONS

The present results indicate that GlyRs in the nAc are tonically activated and of importance for regulating extracellular DA levels. The possibility of pharmacologically interfering with GlyRs to combat psychiatric disorders, in which the mesolimbic DA system is implicated, such as alcoholism, drug addiction, and psychosis, should be explored.

The GABA uptake inhibitor beta-alanine reduces pilocarpine-induced tremor and increases extracellular GABA in substantia nigra pars reticulata as measured by microdialysis

Substantia nigra pars reticulata (SNr) is a major output nucleus of the basal ganglia that receives GABAergic projections from neostriatum and globus pallidus. Previous research has shown that local pharmacological manipulations of GABA in SNr can influence tremulous jaw movements in rats. Tremulous jaw movements are defined as rapid vertical deflections of the lower jaw that resemble chewing but are not directed at a particular stimulus, and evidence indicates that these movements share many characteristics with parkinsonian tremor in humans. In order to investigate the role of GABA in motor functions related to tremor, the present study tested the GABA uptake blocker beta-alanine for its ability to reduce pilocarpine-induced tremulous jaw movements. In a parallel experiment, the effect of an active dose of beta-alanine on dialysate levels of GABA in SNr was assessed using microdialysis methods. GABA levels in dialysis samples were measured using high performance liquid chromatography with electrochemical detection. beta-Alanine (250-500 mg/kg) significantly reduced tremulous jaw movements induced by pilocarpine (4.0 mg/kg). Moreover, systemic administration of beta-alanine at a dose that reduced tremulous jaw movements (500 mg/kg) resulted in a substantial increase in extracellular levels of GABA in SNr compared to the pre-injection baseline. Thus, the present results are consistent with the hypothesis that GABAergic tone in SNr plays a role in the regulation of tremulous jaw movements. This research may lead to a better understanding of how parkinsonian symptoms are modulated by SNr GABA mechanisms.

Distinguishing whether dopamine regulates liking, wanting, and/or learning about rewards

To determine whether dopamine regulates liking, wanting, and/or learning about rewards during goal-directed behavior, the authors tested genetically engineered dopamine-deficient (DD) mice for acquisition of an appetitive T-maze task with and without endogenous dopamine signaling. Experiment 1 established that DD mice treated with L-dihydroxyphenylalanine (L-dopa [LD]) perform similarly to controls on a T-maze task designed to measure liking, wanting, and learning about rewards. Experiment 2, which tested saline-, caffeine-, and LD-treated DD mice on the T maze, separated performance factors from cognitive processes and revealed that dopamine is not necessary for mice to like or learn about rewards but is necessary for mice to seek (want) rewards during goal-directed behavior.

Locomotion induced by non-contingent intracranial electrical stimulation: Dopamine dependence and general characteristics

Intracranial self-stimulation (ICSS) is induced by delivery of electrical stimulation contingent upon a response such as bar pressing. This procedure has been widely used to investigate the brain reward system. Recent investigations, however, have noted that non-contingent electrical stimulation, also called experimenter applied stimulation (EAS), produces a unique set of locomotion behaviors that appear to be related to ICSS, and that these behaviors resemble locomotion similar to those elicited by dopamine enhancing drugs. However, little is known about the general characteristics of EAS-induced locomotion. While ICSS appears to be robust, long lasting, and highly rewarding in that the rat will invest vast amounts of time or energy to obtain the electrical stimulation, these parameters have not been explored for EAS. Moreover, the dopamine dependence of EAS-evoked locomotion is also not firmly established. Thus, the present study investigated dopamine dependence and general characteristics of the EAS-induced locomotion to determine its similarity to ICSS. Results suggested that motor and limbic systems were strongly activated by non-contingent EAS, and that the resulting locomotion was dopamine dependent, robust, continued across long time horizons, and was greater than that evoked by contingent electrical stimulation.

Brain activation during vaginocervical self-stimulation and orgasm in women with complete spinal cord injury: fMRI evidence of mediation by the vagus nerves

Women diagnosed with complete spinal cord injury (SCI) at T10 or above report vaginal-cervical perceptual awareness. To test whether the Vagus nerves, which bypass the spinal cord, provide the afferent pathway for this response, we hypothesized that the Nucleus Tractus Solitarii (NTS) region of the medulla oblongata, to which the Vagus nerves project, is activated by vaginal-cervical self-stimulation (CSS) in such women, as visualized by functional magnetic resonance imaging (fMRI). Regional blood oxygen level-dependent (BOLD) signal intensity was imaged during CSS and other motor and sensory procedures, using statistical parametric mapping (SPM) analysis with head motion artifact correction. Physiatric examination and MRI established the location and extent of spinal cord injury. In order to demarcate the NTS, a gustatory stimulus and hand movement were used to activate the superior region of the NTS and the Nucleus Cuneatus adjacent to the inferior region of the NTS, respectively. Each of four women with interruption, or "complete" injury, of the spinal cord (ASIA criteria), and one woman with significant, but "incomplete" SCI, all at or above T10, showed activation of the inferior region of the NTS during CSS. Each woman showed analgesia, measured at the fingers, during CSS, confirming previous findings. Three women experienced orgasm during the CSS. The brain regions that showed activation during the orgasms included hypothalamic paraventricular nucleus, medial amygdala, anterior cingulate, frontal, parietal, and insular cortices, and cerebellum. We conclude that the Vagus nerves provide a spinal cord-bypass pathway for vaginal-cervical sensibility in women with complete spinal cord injury above the level of entry into spinal cord of the known genitospinal nerves.

Cocaine- and amphetamine-regulated transcript in the nucleus accumbens participates in the regulation of feeding behavior in rats

The present studies aimed to determine whether cocaine- and amphetamine-regulated transcript (CART) peptide in the nucleus of accumbens shell (AcbSh) is implicated in the regulation of food intake. Bilateral intranuclear injections of CART peptide (55-102, 1 microg/microl/side) into the AcbSh decreased food intake with no change in locomotion activity and attenuated the orexigenic effect of the GABA(A) agonist muscimol (100 ng/microl/side) in male Sprague-Dawley rats. Decreased food intake after bilateral intranuclear injections of CART was more sustained in freely fed rats than in food-deprived rats, suggesting fuel availability is an important factor in modulating the function of CART in the regulation of feeding. Our anatomical findings indicate that in addition to the perifornical region and the arcuate nucleus, some neurons within the AcbSh also project within the AcbSh. Moreover, many of these efferent cells contain CART immunoreactivity, including those which reside within the AcbSh, suggesting that accumbal CART circuitry is involved in the central function of the nucleus accumbens. Furthermore, fasting suppressed CART mRNA levels in the AcbSh, paraventricular nucleus of the hypothalamus, arcuate nucleus, and the perifornical region, indicating that the Acb is sensitive to fuel availability to an extent similar to those regions in the hypothalamus. Our findings are the first to demonstrate that CART mRNA in the AcbSh is sensitive to metabolic challenges and that injection of CART peptide into the AcbSh has an inhibitory effect on food intake.

Increased extracellular dopamine in nucleus accumbens in response to unconditioned and conditioned aversive stimuli: studies using 1 min microdialysis in rats

Previous microdialysis studies measuring extracellular dopamine levels in response to unconditioned and conditioned aversive stimuli have used relatively long (e.g. 10 min) sample durations, such that more than one stimulus event occurred within a single dialysis sample. The present study used 1 min dialysate sampling to measure changes in dopamine levels in response to individual stimulus presentations. The changes evoked by mild footshock showed an initial enhancement from the first to the second presentation, after which there was a steady decline in the response over subsequent presentations. Compared to the responses to footshock alone, when the footshock was paired with an unfamiliar tone, there was no change in the response to the first stimulus presentation, but a significant augmentation of responses during subsequent presentations, giving weight to the view that dopamine is not involved in the learning per se, but rather in the processing of learned information. Whilst an unfamiliar tone had no measurable effect on extracellular dopamine levels, the same tone which had previously been paired with footshock evoked a significant increase in dopamine during the tone presentation, indicating that it is the aversive nature of the stimulus onset rather than the 'rewarding' nature of its offset which increases extracellular dopamine.

Dopamine operates as a subsecond modulator of food seeking

The dopamine projection to the nucleus accumbens has been implicated in behaviors directed toward the acquisition and consumption of natural rewards. The neurochemical studies that established this link made time-averaged measurements over minutes, and so the precise temporal relationship between dopamine changes and these behaviors is not known. To resolve this, we sampled dopamine every 100 msec using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in the nucleus accumbens of rats trained to press a lever for sucrose. Cues that signal the opportunity to respond for sucrose evoked dopamine release (67 +/- 20 nm) with short latency (0.2 +/- 0.1 sec onset). When the same cues were presented to rats naive to the cue-sucrose pairing, similar dopamine signals were not observed. Thus, cue-evoked increases in dopamine in trained rats reflected a learned association between the cues and sucrose availability. Lever presses for sucrose occurred at the peak of the dopamine surges. After lever presses, and while sucrose was delivered and consumed, no further increases in dopamine were detected. Rather, dopamine returned to baseline levels. Together, the results strongly implicate subsecond dopamine signaling in the nucleus accumbens as a real-time modulator of food-seeking behavior.

Assessing the effects of bupropion SR on mood dimensions of depression

BACKGROUND

We assessed the therapeutic effects of bupropion SR and placebo on mood and anxiety symptoms derived from the tripartite model of mood. Based on evidence indicating linkages between dopaminergic activity and the emotional dimension of positive affect/anhedonia, we hypothesized that the dopaminergic effects of bupropion SR would yield particularly pronounced effects on symptoms of anhedonia, relative to anxiety.

METHODS

Nineteen depressed outpatients were randomly assigned to treatment with either bupropion SR 300 mg/day or placebo during a 6-week initial treatment phase. This was followed by a second open-label phase in which patients previously treated with bupropion SR had their dose increased to 400 mg/day, and the placebo group was initiated on bupropion SR 300 mg/day.

RESULTS

Random regression analyses revealed that during the initial double-blind phase, bupropion SR elicited greater declines than placebo on all measures except those that assessed anxiety. By contrast, the weakest placebo effects were evident on anhedonia. Items assessing the low positive affect pole of the anhedonia dimension were more sensitive to earlier/lower dose bupropion SR treatment, whereas items assessing the high positive affect pole were more sensitive to later/higher dose bupropion SR treatment.

LIMITATIONS

Replication and extension using a larger sample size are mandated.

CONCLUSIONS

This study suggests that the catecholaminergic effects of bupropion SR tended to produce more robust effects on anhedonia/positive affect than placebo.

Brain substrates for increased drug seeking during protracted withdrawal

Studies are reviewed indicating that both increased anxiety and altered hedonic processing accompany protracted withdrawal from opiates. Increased anxiety may be most apparent in response to stress, whereas decreased motivation for natural rewards but increased interest in drugs reveals substantial alterations in hedonic values. Our recent work indicates that increased norepinephrine (NE) release in the bed nucleus of the stria terminalis (BNST) may underlie anxiety associated with protracted withdrawal. Altered plasticity in afferents to the ventral tegmental area (VTA; accumbens, amygdala and lateral hypothalamus), or in the VTA itself, may be involved in the altered hedonic processing that occurs during protracted withdrawal. We hypothesize that conditioned release of NE in the BNST in response to stressors (including drug-associated stimuli) may elevate anxiety which then augments the reward value of drugs by a negative reinforcement mechanism. We also propose that plasticity in VTA neurons and their afferents during chronic drug exposure and protracted withdrawal decreases the valence of natural rewards whereas sensitization occurs to the motivational effects of drugs that increases their motivational valence. The combination of anxiety, decreased valence of natural rewards, and sensitized incentive for drugs make a potent formula for relapse and drug seeking during protracted withdrawal.

Memantine presents different effects from MK-801 in motivational and physical signs of morphine withdrawal

Adaptive changes in neural systems due to chronic opiate exposure are related to the neural plasticity phenomenon, NMDA receptors being implicated in these processes, e.g. tolerance, dependence or withdrawal. In this work, we investigated the effect of two non-competitive NMDA antagonists, memantine and MK-801, in motivational (Conditioned Place Aversion paradigm, CPA) and physical aspects of morphine withdrawal. After the induction of morphine dependence, animals in which the CPA was studied, received memantine (5 and 10 mg/kg) or MK-801 (0.3-0.006 mg/kg) either during the acquisition (conditioning) or expression (test) phase of this procedure. Both drugs were capable of inhibiting conditioned aversion when administered in any phase. In a second experiment, the effects of these drugs were evaluated in the intensity of the physical signs of withdrawal, only memantine administration being efficient. In addition to these studies, the intensity of morphine dependence was investigated under the blockade of NMDA receptors, i.e. morphine was co-administered with memantine or MK-801. These animals did not develop CPA and present less intensity in the physical signs of morphine withdrawal. Our results support the idea that NMDA receptors are involved in the behavioural changes and therefore in the neural adaptations produced by repeated morphine administration.

Substantia nigra pars reticulata GABA is involved in the regulation of operant lever pressing: pharmacological and microdialysis studies

Substantia nigra pars reticulata (SNr) is an important mesencephalic nucleus that functions as a relay area for basal ganglia output. SNr receives GABAergic inputs from the neostriatum and globus pallidus, and in turn sends projections to a variety of motor areas. Although a large number of studies have focused upon the behavioral functions of basal ganglia dopamine, much less is known about the behavioral functions of SNr GABA. The present studies were undertaken to investigate the role of SNr GABA in lever pressing behavior. In the first experiment, the GABA(A) antagonist bicuculline was infused locally into SNr to determine if blockade of GABA receptors interfered with the performance of lever pressing on a fixed ratio 5 schedule. SNr injections of bicuculline produced a dose-related suppression of operant responding. Analysis of interresponse time bins showed that SNr bicuculline produced a response slowing characterized by a relative reduction in the number of fast interresponse times, and an increase in the relative number of pauses. In an additional experiment, microdialysis methods were used to determine if extracellular GABA is elevated during the performance of fixed ratio five lever pressing. During the 30 min lever pressing session, extracellular GABA showed a significant and substantial increase relative to baseline levels. These data support the hypothesis that SNr GABA is involved in the regulation of motor output, and indicate that GABA release in this structure is increased during behavioral stimulation.

Differential sensitivity to acute administration of Ritalin, apomorphine, SCH 23390, but not raclopride in mice selectively bred for hyperactive wheel-running behavior

RATIONALE

Previous studies of mice ( Mus domesticus) selectively bred for high voluntary wheel running have suggested that the hyperactivity is associated with dysfunction in the dopaminergic neuromodulatory system and that high-running mice may represent a useful genetic model for attention deficit hyperactivity disorder (ADHD).

OBJECTIVES

We tested the hypothesis that mice from the four replicate hyperactive lines would respond differently to methylphenidate (Ritalin), apomorphine (non-selective dopamine agonist), SCH 23390 (selective D1-like dopamine antagonist), and raclopride (selective D2-like dopamine antagonist) than individuals from the four replicate, randomly bred, control lines.

METHODS

After animals were habituated (3 weeks) to their cages with attached wheels, drugs were administered via intraperitoneal injections, at night, during peak wheel-running activity. Revolutions on wheels 10-70 min post-injection were used to quantify drug responses.

RESULTS

Ritalin (15 mg/kg and 30 mg/kg) increased wheel running in control lines but decreased running in selected lines. A low-dose (0.125 mg/kg) of apomorphine reduced wheel running by a similar amount in control and selected lines; however, higher doses of apomorphine (0.25 mg/kg and 0.5 mg/kg) produced greater reductions in wheel running in the control lines. SCH 23390 (0.025, 0.05, and 0.1 mg/kg) caused greater reductions in wheel running in control than in selected lines. Raclopride (0.5, 1, and 2 mg/kg) reduced wheel running by a similar amount in control and selected lines.

CONCLUSIONS

These results support the interpretation that genetically determined hyperactive wheel-running behavior is associated with altered dopaminergic function in this mouse model. More specifically, results suggest that D1-like (D1 or D5), but not D2-like (D2, D3, or D4), dopamine receptors have reduced function in the high-running mice. The fact that Ritalin decreased wheel running in selected lines further supports their use as an animal model of ADHD.

Nucleus accumbens dopamine and the regulation of effort in food-seeking behavior: implications for studies of natural motivation, psychiatry, and drug abuse

For several decades, it has been suggested that dopamine (DA), especially in nucleus accumbens, mediates the primary reinforcing characteristics of natural stimuli such as food, as well as drugs of abuse. Yet, several fundamental aspects of primary food reinforcement, motivation, and appetite are left intact after interference with accumbens DA transmission. Recent studies have shown that accumbens DA is involved in responsiveness to conditioned stimuli and activational aspects of motivation. In concurrent choice tasks, accumbens DA depletions cause animals to reallocate their choice behavior in the direction of instrumental behaviors that involve less effort. Also, an emerging body of evidence has demonstrated that the effects of accumbens DA depletions on instrumental food-seeking behavior can vary greatly depending upon the task. For example, some schedules of reinforcement are insensitive to the effects of DA depletions, whereas others are highly sensitive (e.g., large fixed ratios). Accumbens DA depletions slow the rate of operant responding, blunt the rate-facilitating effects of moderate-sized ratios, and enhance the rate-suppressing effects of very large ratios (i.e., produce ratio strain). Accumbens DA may be important for enabling rats to overcome behavioral constraints, such as work-related response costs, and may be critical for the behavioral organization and conditioning processes that enable animals to engage in vigorous responses, such as barrier climbing, or to emit large numbers of responses in ratio schedules in the absence of primary reinforcement. The involvement of accumbens DA in activational aspects of motivation has implications for energy-related disorders in psychiatry, as well as aspects of drug-seeking behavior.

Attenuation of cue-controlled cocaine-seeking by a selective D3 dopamine receptor antagonist SB-277011-A

Conditioned stimuli (CS) previously paired with drugs of abuse can elicit cravings in humans, relapse to drug use, and can also reinforce drug-seeking behavior in both humans and animals, events that are believed to be subserved in part by activation of the mesolimbic dopamine system. Converging anatomical, pharmacological, and behavioral evidence implicates dopamine D(3) receptors in the mechanisms underlying cue-controlled behaviors. The purpose of the present study was therefore to investigate the effects on cocaine-seeking behavior of a novel D(3) receptor antagonist, SB-277011-A, which is 100-fold more selective for D(3) over D(2) dopamine receptors. We have established previously that second-order schedules of reinforcement provide an animal model of cue-controlled drug-seeking both prior to and after cocaine has been self-administered. SB-277011-A dose-dependently decreased cocaine-seeking maintained by a cocaine-associated conditioned reinforcer in both the first, drug-free interval and also following self-administration of cocaine. At higher doses, SB-277011-A also increased the latency to receive the first CS presentation and cocaine infusion, thereby decreasing the number of cocaine infusions self-administered under the second-order schedule of reinforcement. SB-277011-A had no effect on cocaine intake under an FR-1 schedule of reinforcement, or on responding for sucrose under a second-order schedule of reinforcement, at any dose tested. These results therefore suggest that D(3) dopamine receptors may be critically involved in cue-controlled drug-seeking behavior independently of any interaction with the reinforcing effects of cocaine itself, and may therefore provide a therapeutic target in the treatment of relapse to cocaine use induced by CSs.

Modeling fast dopamine neurotransmission in the nucleus accumbens during behavior

Recent advances in electrophysiology and voltammetry permit monitoring of dopamine (DA) neuronal activity in real time in the brain of awake animals. Studies using these approaches demonstrate that behaviorally relevant events elicit characteristic patterns of electrical activity in midbrain DA neurons as well as large, transient changes in extracellular DA in the nucleus accumbens (NAc). In addition to providing insight into the role of the DA system in the processing of motor, motivational, and sensory information, the new findings also shed light on fast DA neurotransmission in a behavioral context. This report, (1). summarizes the information obtained by electrophysiological and real-time voltammetric approaches and (2). describes a general model of phasic DA signaling in the NAc that links the observed changes in DA electrical activity and extracellular dynamics. The analysis demonstrates that the behaviorally evoked DA transients are governed by similar mechanisms as those produced by short trains of electrical stimulation. Thus, action potential-dependent release and presynaptic uptake are primary determinants of functional DA levels in the brain during behavior. Interestingly, the model predicts that the same burst of electrical activity generated at DA cell bodies produces markedly different DA dynamics in forebrain projection fields. The distinct changes result from heterogeneous release and uptake rates and may underlie region-specific effects of DA. Auto- and heteroreceptors, as well as other sites of presynaptic control, could further modulate the DA transients.

Studies of the interaction between behavioral stereotypes and the effects of activation of presynaptic dopamine receptors during extinction and amnesia in mice

The involvement of presynaptic dopamine receptors in the retention of a conditioned passive avoidance reflex through extinction and amnesia was studied in C57BL/6J mice selected in a 20-day aggressive conflict test for aggressive and submissive behavioral stereotypes. These experiments showed that in aggressive mice, activation of presynaptic receptors with the agonist (+)3-(3-hydroxyphenyl)-N-n-propylpiperidine [(+)3PPP] at a dose of 2 mg/kg degraded learning, significantly decreased the retention time of the acquired conditioned habit in extinction, and increased the effect of an amnesia-inducing treatment. Mice showing submissive behavior in daily confrontations with aggressors responded to administration of (+)3PPP with long-lasting reproduction of the conditioned passive avoidance reaction during extinction and showed no changes in the development of amnesia. These data on the relationship between the effects of activating presynaptic dopamine receptors in reproduction of the memory trace in conditions of trace disruption on the one hand and behavioral status on the other are assessed from the point of view of different basal levels of dopaminergic system operation in aggressive and submissive mice.

Exposure to female rats produces differences in c-fos induction between sexually-naïve and experienced male rats

Sexual incentive-induced Fos-like immunoreactivity (Fos-Li) within six neural regions implicated in male sexual behavior was investigated in both sexually-naïve and experienced male rats. Sexual experience was limited to one copulation culminating in ejaculation 24 h prior to testing. On test-day, subjects were placed within a cylindrical arena for 15 min on the opposite side of a perforated, Plexiglas partition from one of three targets: an uninhabited area, a non-estrous female, or an estrous female. Then 1 h later, each subject was sacrificed and its brain prepared for subsequent immunocytochemical staining. Analyses revealed a main effect of target stimulus on c-fos expression within the nucleus accumbens shell and core of male subjects. In addition, sexually-experienced subjects demonstrated significantly more Fos-Li within the nucleus accumbens shell in response to an estrous female versus a non-estrous female. There was also greater estrous cue-induced Fos-Li in the nucleus accumbens shell of experienced subjects when compared to naïve subjects. These data support previous suggestions implicating the nucleus accumbens in the generation of male sexual motivation. In addition, copulatory experience, even when limited to one ejaculation, seems to mediate long-term changes in the response properties of nucleus accumbens neurons that may reflect the value enhancement of primary female incentives.

Transient changes in mesolimbic dopamine and their association with 'reward'

Mesolimbic dopaminergic neurons modulate complex circuitry in the ventral forebrain involved in reward processing, although the precise function of the dopaminergic input is debated. Electrophysiological measurements have revealed that mesolimbic dopaminergic neurons can fire in either tonic or phasic modes, and that phasic firing accompanies the alerting or anticipatory phases of reward. However, the neurochemical relevance of this rapid neuronal discharge within the reward processing circuitry is not yet clear, in part because of difficulty in interpretation of extracellular dopamine measurements. Herein, the nature of the information provided by different neurochemical techniques is critically discussed. Classical methods of monitoring dopamine reveal changes in extracellular dopamine resulting from tonic neuronal activity, but do not have the temporal resolution to distinguish concentration transients. However, recent advances in dopamine sensors now enable transient dopamine concentrations resulting from phasic firing to be positively identified and followed on a physiologically relevant timescale. This has enabled demonstrations of discrete, phasic dopamine signals accompanying rewarding or alerting stimuli. Thus, enhanced dopamine release at terminals appears to be coincident with phasic electrical activity at cell bodies. These accumulating data promise to help unravel the precise role of phasic dopamine transmission in reward processing.

Differential Expression of Motivational Stimulus Properties by Dopamine in Nucleus Accumbens Shell versus Core and Prefrontal Cortex

The response of extracellular dopamine (DA) and its relationship to motivational valence (positive or negative) and novelty of motivational stimuli was investigated by brain microdialysis in the nucleus accumbens (NAc) shell and core and prefrontal cortex (PFCX) of rats. Stimuli were elicited by intraoral infusion of 20% sucrose, sucrose plus chocolate, quinine, and NaCl solutions, feeding of a palatable food (Fonzies), or smelling of a predator (red fox) urine. Sucrose elicited appetitive reactions and increased DA in the PFCX but not in the NAc shell. An unfamiliar appetitive taste such as that of sweet chocolate and Fonzies, increased DA in all three areas. Habituation of the stimulatory DA response to intraoral chocolate or to Fonzies feeding was observed in the NAc shell after a single pre-exposure to the same taste or food; no habituation was observed in the NAc core nor in the PFCX. Aversive taste stimuli (quinine, saturated NaCl solutions) rapidly increased DA in the PFCX and in the NAc core, and this response did not undergo one-trial habituation. In the NAc shell, instead, no effect (10 min exposure) or a delayed, transitory increase of DA (5 min exposure) sensitive to one-trial habituation was obtained in response to the aversive taste (quinine and saturated NaCl) or olfactory (red fox urine) stimuli. These observations indicate that DA responsiveness is an integrated function of the motivational valence and novelty of stimuli in the NAc shell and an expression of generic motivational value in the NAc core and PFCX.

The neurosteroid allopregnanolone increases dopamine release and dopaminergic response to morphine in the rat nucleus accumbens

Neurosteroids are a subclass of steroids that can be synthesized in the central nervous system independently from peripheral sources. Clinical studies in humans have associated these hormones with depression and postpartum mood disorders. In rodents, allopregnanolone (AlloP) has been shown to have anxiolytic and rewarding properties. These observations suggest that neurosteroids could interact with mood and motivation. However, the possible neural substrates of these effects remain unknown. In this report, we have studied the action of AlloP on the activity of the mesencephalic dopaminergic (DA) projection to the nucleus accumbens, which is considered one of the biological substrates of motivation and reward. This study was conducted by measuring extracellular concentrations of dopamine (DA) in the nucleus accumbens by means of microdialysis in freely moving rats. We studied both the direct effect of AlloP and the influence of this hormone on the DA response to an injection of morphine. AlloP dose-dependently increased the release of DA in the nucleus accumbens. Furthermore, this hormone doubled the DA response to morphine. These effects were observed for AlloP doses of 50 and 100 pmol injected intracerebroventricularly. These results suggest that the stimulatory effect of AlloP on DA could mediate some of the behavioural effects of neurosteroids and, in particular, the interaction of these hormones with mood and motivation.

Differential modulation of nucleus accumbens synapses

The nucleus accumbens (NAcc) is a brain region involved in functions ranging from motivation and reward to feeding and drug addiction. The NAcc is typically divided into two major subdivisions, the shell and the core. The primary output neurons of both of these areas are medium spiny neurons (MSNs), which are quiescent at rest and depend on the relative input of excitatory and inhibitory synapses to determine when they fire action potentials. These synaptic inputs are, in turn, regulated by a number of neurochemical signaling agents that can ultimately influence information processing in the NAcc. The present study characterized the ability of three major signaling pathways to modulate synaptic transmission in NAcc MSNs and compared this modulation across different synapses within the NAcc. The opioid [Met](5)enkephalin (ME) inhibited excitatory postsynaptic currents (EPSCs) in shell MSNs, an effect mediated primarily by micro-opioid receptors. Forskolin, an activator of adenylyl cyclase, potentiated shell EPSCs. An analysis of miniature EPSCs indicated a primarily presynaptic site of action, although a smaller postsynaptic effect may have also contributed to the potentiation. Adenosine and an adenosine A(1)-receptor agonist inhibited shell EPSCs, although no significant tonic inhibition by endogenous adenosine was detected. The effects of these signaling agents were then compared across four different synapses in the NAcc: glutamatergic EPSCs and GABAergic inhibitory postsynaptic currents (IPSCs) in both the core and shell subregions. ME inhibited all four of these synapses but produced a significantly greater inhibition of shell IPSCs than the other synapses. Forskolin produced an increase in transmission at each of the synapses tested. However, analysis of miniature IPSCs in the shell showed no sign of a postsynaptic contribution to this potentiation, in contrast to the shell miniature EPSCs. Tonic inhibition of synaptic currents by endogenous adenosine, which was not observed in shell EPSCs, was clearly present at the other three synapses tested. These results indicate that neuromodulation can vary between the different subregions of the NAcc and between the different synapses within each subregion. This may reflect differences in neuronal interconnections and functional roles between subregions and may contribute to the effects of drugs acting on these systems.

Sexually conditioned incentives: attenuation of motivational impact during dopamine receptor antagonism

The motivational impact of sexually conditioned incentives was examined in two experiments. In Experiment 1, male Long-Evans rats copulated to ejaculation in the presence of one of two scents (orange or almond extract) on five separate occasions. On alternating days, subjects spent an equal amount of time in social isolation with the opposing scent. Following the 10-day conditioning regimen, subjects ran more rapidly down an operant runway toward a goalbox containing the sex-paired scent (CS+) compared to trials on which the isolation-paired scent (CS-) or no scent was provided. In Experiment 2, comparably conditioned male rats were first given a baseline runway trial with an unscented goalbox. The following day, subjects were pretreated with one of four doses of haloperidol (0.0, 0.075, 0.15, or 0.30 mg/kg i.p.) 45 min prior to being tested in the runway for their motivation to approach either the CS+ or CS- scents. Control subjects given vehicle injections performed comparably to subjects from Experiment 1, taking significantly less time to approach the CS+ than an unscented goalbox. This decrease in run latency was not observed in subjects within the 0.075 and 0.15 mg/kg haloperidol groups. Subjects in the 0.30 mg/kg haloperidol groups took significantly more time to approach both the CS+ and CS- compared to their baseline run times. These data reveal that an olfactory cue associated with sexual reward becomes a conditioned incentive capable of eliciting approach behavior, and that dopamine receptor antagonism (at moderate but not high doses) selectively attenuates this cue-induced motivation.

The dopamine mesocorticolimbic pathway is affected by deficiency in n-3 polyunsaturated fatty acids

BACKGROUND

Several findings in humans support the hypothesis of links between n-3 polyunsaturated fatty acid (PUFA) status and psychiatric diseases.

OBJECTIVE

The involvement of PUFAs in central nervous system function can be assessed with the use of dietary manipulation in animal models. We studied the effects of chronic dietary n-3 PUFA deficiency on mesocorticolimbic dopamine neurotransmission in rats.

DESIGN

Using dual-probe microdialysis, we analyzed dopamine release under amphetamine stimulation simultaneously in the frontal cortex and the nucleus accumbens. The messenger RNA (mRNA) expression of vesicular monoamine transporter(2) and dopamine D(2) receptor was studied with the use of in situ hybridization. The protein expression of the synthesis-limiting enzyme tyrosine 3-monooxygenase (tyrosine 3-hydroxylase) was studied with the use of immunocytochemistry.

RESULTS

Dopamine release was significantly lower in both cerebral areas in n-3 PUFA-deficient rats than in control rats, but this effect was abolished in the frontal cortex and reversed in the nucleus accumbens by reserpine pretreatment, which depletes the dopamine vesicular storage pool. The mRNA expression of vesicular monoamine transporter(2) was lower in both cerebral areas in n-3 PUFA-deficient rats than in control rats, whereas the mRNA expression of D(2) receptor was lower in the frontal cortex and higher in the nucleus accumbens in n-3 PUFA-deficient rats than in control rats. Finally, tyrosine 3-monooxygenase immunoreactivity was higher in the ventral tegmental area in n-3 PUFA-deficient rats than in control rats.

CONCLUSIONS

Our results suggest that the mesolimbic dopamine pathway is more active whereas the mesocortical pathway is less active in n-3 PUFA-deficient rats than in control rats. This provides new neurochemical evidence supporting the effects of n-3 PUFA deficiency on behavior.

A functional neuroanatomical investigation of the role of the medial preoptic area in neural circuits regulating maternal behavior

The medial preoptic area (MPOA) is essential for normal maternal behavior in the rat. Hormone stimulation of the MPOA facilitates the behavior and lesions of the MPOA and the adjoining ventral part of the bed nucleus of the stria terminalis (vBST) disrupt the behavior. The MPOA/vBST also show increases in Fos protein expression during maternal behavior. The present study examines the larger neural circuitry within which the MPOA/vBST might operate to influence maternal behavior. Combining Fos immunocytochemistry with unilateral excitotoxic amino acid lesions or lateral knife cuts of the MPOA/vBST, we sought to identify brain regions which might be under the influence of Fos expressing neurons in the MPOA/vBST. Two brain regions, the shell of the nucleus accumbens (NAs), and the intermediate part of the lateral septum (LSi) were identified. Both the NAs and LSi exhibited elevated Fos expression during maternal behavior, while unilateral MPOA/vBST damage resulted in an ipsilateral reduction of maternal behavior-induced Fos expression in each area, suggesting that MPOA/vBST neurons modulate Fos expression and associated neural activity in both of these structures during maternal behavior. Importantly, these unilateral preoptic lesions also depressed maternal behavior-induced Fos expression in the ipsilateral MPOA and vBST. The effects of these lesions on Fos expression in the periaqueductal gray (PAG) and other brain regions are also presented.

D1 dopamine receptor: a putative neurochemical and behavioral link to cocaine action

Cocaine is one of the most abused psychostimulants known to man and as such, researchers have been steadfast in their attempts to understand the neurobiological mechanisms responsible for its abuse. Cocaine undoubtedly wreaks havoc on a number of mammalian neuronal neurotransmitter systems, and it is maintained that this dysregulatory effect supports cocaine abuse. Cocaine's mechanism of action has been well described. Cocaine binds differentially to the dopamine, serotonin, and norepinephrine transport proteins and directly prevents the re-uptake of dopamine, serotonin, and norepinephrine into pre-synaptic neurons (Heikkila et al., 1975, Biochem Pharmacol 24(8):847-852; Reith et al., 1986, Biochem Pharmacol 35(7):1123-1129; Ritz et al., 1987, Science 237:1219-1223). Inhibition of re-uptake subsequently elevates the synaptic concentrations of each of these neurotransmitters. In addition to this direct effect, cocaine also produces a number of indirect actions, which alter other neuromodulatory systems (i.e., opioidergic, glutamatergic, and GABAergic systems). Many of these effects are just beginning to be elucidated, but nonetheless contribute to this agent's diverse pharmacological profile. Interestingly, it is the indirect actions of this mellifluous molecule, which mediate most of its sought and unsought effects. The intricacy with which cocaine produces neuronal alterations beyond its direct effects on neurotransmitter re-uptake appear to be most relevant to cocaine abuse, and hence the phenomenon of addiction. In light of cocaine's multifarious effects on numerous neuronal systems, its effect on dopaminergic neurotransmission has attracted the most attention, particularly because of the implicated role of dopamine in brain reward. Pharmacologically, the psychostimulant effects of cocaine appear to be mediated by its ability to enhance dopaminergic activity within the mesocorticolimbic circuit (Roberts et al., 1977, Pharmacol Biochem Behav 6(6):615-620). Additionally, it is the intensity with which cocaine produces alterations in dopaminergic circuitry that have enabled this drug to prevail as one of the most addictive substances known to man. This review will summarize findings relevant to cocaine-induced alterations in dopamine-mediated signal transduction. Specifically, it will concentrate on the D1 dopamine receptor and intracellular signaling mediated by this receptor subtype. It will describe cocaine-induced cellular and behavioral alterations relevant to this pathway and how these changes potentially effect gene transcription and protein expression. This article too will review a common behavioral manifestation associated with repeated cocaine exposure, sensitization, and why the D1 dopamine receptor and its associated signaling pathway have been implicated in this phenomenon. Lastly, this article will discuss how targeting the D1 dopamine receptor and its signaling pathway may offer some insight into understanding cocaine addiction, a somewhat elusive brain disease.

Polyunsaturated fatty acids and cerebral function: focus on monoaminergic neurotransmission

More and more reports in recent years have shown that the intake of polyunsaturated fatty acids (PUFA) constitutes an environmental factor able to act on the central nervous system (CNS) function. We recently demonstrated that the effects of PUFA on behavior can be mediated through effects on the monoaminergic neurotransmission processes. Supporting this proposal, we showed that chronic dietary deficiency in alpha-linolenic acid in rats induces abnormalities in several parameters of the mesocortical and mesolimbic dopaminergic systems. In both systems, the pool of dopamine stored in presynaptic vesicles is strongly decreased. This may be due to a decrease in the number of vesicles. In addition, several other factors of dopaminergic neurotransmission are modified according to the system affected. The mesocortical system seems to be hypofunctional overall [e.g., decreased basal release of dopamine (DA) and reduced levels of dopamine D2 (DAD2) receptors]. In contrast, the mesolimbic system seems to be hyperfunctional overall (e.g., increased basal release of DA and increased levels of DAD2 receptors). These neurochemical changes are in agreement with modifications of behavior already described with this deficiency. The precise mechanisms explaining the effects of PUFA on neurotransmission remain to be clarified. For example, modifications of physical properties of the neuronal membrane, effects on proteins (receptors, transporters) enclosed in the membrane, and effects on gene expression and/or transcription might occur. Whatever the mechanism, it is therefore assumed that interactions exist among PUFA, neurotransmission, and behavior. This might be related to clinical findings. Indeed, deficits in the peripheral amounts of PUFA have been described in subjects suffering from neurological and psychiatric disorders. Involvement of the monoaminergic neurotransmission function has been demonstrated or hypothesized in several of these diseases. It can therefore be proposed that functional links exist among PUFA status, neurotransmission processes, and behavioral disorders in humans. Animal models are tools of choice for the understanding of such links. Improved prevention and complementary treatment of neurological and psychiatric diseases can be expected from these studies.

Sexual behavior induction of c-Fos in the nucleus accumbens and amphetamine-stimulated locomotor activity are sensitized by previous sexual experience in female Syrian hamsters

Dopamine transmission in the nucleus accumbens can be activated by drugs, stress, or motivated behaviors, and repeated exposure to these stimuli can sensitize this dopamine response. The objectives of this study were to determine whether female sexual behavior activates nucleus accumbens neurons and whether past sexual experience cross-sensitizes neuronal responses in the nucleus accumbens to amphetamine. Using immunocytochemical labeling, c-Fos expression in different subregions (shell vs core at the rostral, middle, and caudal levels) of the nucleus accumbens was examined in female hamsters that had varying amounts of sexual experience. Female hamsters, given either 6 weeks of sexual experience or remaining sexually naive, were tested for sexual behavior by exposure to adult male hamsters. Previous sexual experience increased c-Fos labeling in the rostral and caudal levels but not in the middle levels of the nucleus accumbens. Testing for sexual behavior increased labeling in the core, but not the shell, of the nucleus accumbens. To validate that female sexual behavior can sensitize neurons in the mesolimbic dopamine pathway, the locomotor responses of sexually experienced and sexually naive females to an amphetamine injection were then compared. Amphetamine increased general locomotor activity in all females. However, sexually experienced animals responded sooner to amphetamine than did sexually naive animals. These data indicate that female sexual behavior can activate neurons in the nucleus accumbens and that sexual experience can cross-sensitize neuronal responses to amphetamine. In addition, these results provide additional evidence for functional differences between the shell and core of the nucleus accumbens and across its anteroposterior axis.

The role of the brain reward system in depression

The goal of this review is to familiarize the reader about the potential involvement of the brain reward system (BRS) in symptoms of Major Depressive Disorder (MDD). The authors introduce a novel approach to study the pathophysiology of MDD that includes pharmacological probing of BRS pathways (e.g. d-amphetamine, hydromorphone) together with an elicited and measurable behavioral component (e.g. pleasant effects, increased energy, altered cognition). To this date, the major focus of MDD pathophysiology studies has been to characterize biological differences between healthy subjects and depressed patients such as alteration in the monoaminergic and endocrine systems. The relative importance of the various biological changes has not been elucidated, that is, linking these with specific behavioral manifestations in MDD have rarely been attempted. One core symptom of MDD is a decreased experience of pleasure or interest in previously enjoyed activities (i.e. anhedonia) such as work or hobbies, and is accompanied by decreased motivation or drive. The BRS consists of the neural pathways involved in eliciting rewarding experiences in animals and humans. The hypothesis is that altered BRS function may be an underlying brain mechanism of the loss of pleasure/interest experienced in MDD, and will be manifested through an altered response to a BRS probe. The authors have examined BRS function in MDD by introducing a pharmacological probe (i.e. d-amphetamine/d-amph). Amphetamine is defined as a probe due to its ability to release dopamine within major components of the BRS (i.e. the mesocorticolimbic dopamine system.) In addition to the objective pharmacological effects (e.g. altered heart rate), BRS probes like d-amph elicit reliable and measurable behavior, that is, the hedonic effects. A review of the neurobiology of MDD, the BRS, the rationale for implicating the BRS in depressive symptoms, and preliminary data, are presented in this article.

Dopamine antagonism attenuates the unconditioned incentive value of estrous female cues

The role of dopaminergic transmission in the incentive-motivational processes involved in the generation of male sexual behavior was examined. Three groups of sexually naïve Long-Evans male rats traversed a straight alley for one of three goalbox targets: an empty goalbox, a nonestrous female, or an estrous female. A Plexiglas partition within the goalbox allowed for the perception of visual, auditory, and olfactory cues, but prevented physical contact. Baseline run times revealed that subjects returned to the goalbox significantly faster for an estrous female than for a nonestrous female, replicating our earlier work on the inherent incentive value of primary female cues. When subjects were then pretreated with the dopamine receptor antagonist, haloperidol (0.0, 0.075, or 0.15 mg/kg), they expressed decreased sexual motivation as reflected by increased run times for estrous female targets. Subjects' run times for the empty goalbox condition were unaffected by haloperidol, suggesting that the drug did not reliably impair motoric capacity. Results support the contention that central dopaminergic systems are involved in the regulation of the positive, unconditioned incentive value of estrous female cues.

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.

Parallel analyses of nociceptive neurones in rat superior colliculus by using c-fos immunohistochemistry and electrophysiology under different conditions of anaesthesia

Multiple sensory inputs to the superior colliculus (SC) play an important role in guiding head and eye movements toward or away from biologically significant stimuli. Much is now known about the visual, auditory, and somatosensory response properties of SC neurones that mediate these behavioural reactions. Rather less is known about the responses of SC neurones to noxious stimuli, and thus far, most of this information has been obtained in anaesthetised animals. Therefore, the purpose of the present study was to use the c-fos immunohistochemical technique and standard extracellular electrophysiology as parallel measures of nociceptive activity in the SC under different conditions of anaesthesia. In unanaesthetised animals, experimental and control treatments induced a qualitatively similar pattern of Fos-like immunoreactivity (FLI) in the SC, which was quantitatively related to the severity or biologic salience of the treatment; thus, baseline control < control injections of saline < a nonpainful stressor (immobilisation) < noxious injections of formalin. Compared with baseline levels, urethane and avertin anaesthesia induced FLI expression in the SC intermediate layers, although the FLI response to both noxious stimulation and control conditions was differentially suppressed in different layers of the SC by anaesthesia. Parallel electrophysiologic recordings found that anaesthesia was associated with high levels of spontaneous activity in the SC intermediate layers, often in neurones which were also nociceptive. High rates of background spike activity were also induced in the SC intermediate layers by noxious stimulation in chronically recorded awake animals. Although these results point to some differences between the nociceptive responses of SC neurones in anaesthetised and unanaesthetised animals, both data sets support the view that there are different populations of nociceptive neurones in the rodent SC that may be related to different adaptive functions of pain.

Neurosteroids and reward: allopregnanolone produces a conditioned place aversion in rats

The neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) has been reported to have rewarding properties in mice tested for place conditioning. Another study found that allopregnanolone reduced dopamine (DA) output in the nucleus accumbens (NAc) of rats. As many rewarding stimuli increase accumbens DA, these results may appear contradictory. Thus, the present study examined the rewarding properties of allopregnanolone in rats tested for place conditioning using an unbiased conditioning procedure. In control studies, a place preference was observed following conditioning with intraperitoneal (2.0 mg/kg) or intracerebroventricular (i.c.v.) (100 microg/0.5 microl) amphetamine. Conditioning with i.c.v. allopregnanolone produced a significant aversion at a dose of 5.0 microg (in 5.0 microl) and a near aversion at 25.0 microg (in 8.3 microl); doses of 0 microg (i.e., vehicle alone, in 10 microl) or 30.0 microg (in 10 microl) produced little effect on place preference. During conditioning, locomotor activity was stimulated by amphetamine using either route of administration, but allopregnanolone had no significant main effect on locomotor activity. Thus, there was a dissociation between the effects of drugs on locomotor activity vs. place conditioning. Results show that i.c.v. amphetamine produces a place preference, whereas allopregnanolone produces either no effect or an aversion, depending on the dose.

Tactile stimulation activates dopamine release in the lateral septum

Little is known about the functional properties of the dopamine innervation of the lateral septum. In this study, the feasibility of using microdialysis to assess action-potential mediated release of dopamine in the lateral septum was established. A mild stressor, in the form of handling, significantly increased septal dopamine levels, implicating a role for dopamine in sensory-related processing associated with the septal complex.

Insulin and leptin: dual adiposity signals to the brain for the regulation of food intake and body weight

Insulin and leptin are hypothesized to be 'adiposity signals' for the long-term regulation of body weight by the brain. Accordingly, a change in the plasma levels of leptin or insulin indicates a state of altered energy homeostasis and adiposity, and the brain responds by adjusting food intake to restore adipose tissue mass to a regulated level. The candidate site for the brain's detection of leptin adiposity signaling is the hypothalamic arcuate nucleus, where leptin inhibits expression neuropeptide Y and increases expression of the pro-opiomelanocortin (POMC) precursor of alphaMSH. Insulin also inhibits arcuate nucleus expression of neuropeptide Y but its effects on other hypothalamic signaling systems are not known. Leptin-responsive neurons in the arcuate nucleus are hypothesized to project to the paraventricular nucleus and lateral hypothalamic area where they are proposed to influence the expression of peptides that regulate food intake. Future development of this model will incorporate brain pathways for integration of leptin and insulin adiposity signaling to the hypothalamus with meal-related signals that act in the caudal brainstem. Recent research showing that leptin and insulin enhance the satiety action of peripheral CCK, thereby causing meals to be terminated earlier and reducing cumulative food intake, suggests that hypothalamic pathways that are sensitive to leptin and insulin adiposity signals have anatomical connections with caudal brainstem neurons that respond to meal-related signals and regulate meal size. The recent findings that insulin alters the expression and function of neural transporters for dopamine and norepinephrine indicate that adiposity signals may influence food intake by acting on non-peptide neurotransmitter systems.

Different behavioral functions of dopamine in the nucleus accumbens and ventrolateral striatum: a microdialysis and behavioral investigation

Three experiments were conducted to investigate the behavioral functions of dopamine in the nucleus accumbens and ventrolateral striatum. In the first experiment, dialysis probes were implanted in the nucleus accumbens or ventrolateral striatum of rats previously trained to respond on fixed interval lever pressing schedules for food reinforcement. During the dialysis test session, both schedule- and site-dependent effects on dopamine release were observed. Overall, lever pressing on a fixed interval 30-s schedule produced a greater increase in extracellular dopamine than did responding on a fixed interval 120-s schedule. The fixed interval 30-s schedule was also accompanied by a higher rate of lever pressing. Rats with nucleus accumbens probe placements showed significantly higher increases in dopamine release than rats with ventrolateral striatal placements. An additional dialysis experiment showed that baseline levels of dopamine were suppressed by 1.0 microM tetrodotoxin to a similar extent in the nucleus accumbens and ventrolateral striatum. In the third experiment, 6-hydroxydopamine was injected locally into either the nucleus accumbens or the ventrolateral striatum in order to deplete dopamine. Nucleus accumbens dopamine depletions produced only a minor decrease in operant responding, whereas rats with ventrolateral striatal dopamine depletions showed low levels of responding that differed from both the control group and from the group that had accumbens dopamine depletions. Thus, these results are somewhat paradoxical, in that the structure that showed the greatest increase in dopamine release (i.e. the nucleus accumbens) was also the terminal region at which dopamine depletions had very little effect on operant responding. Ventrolateral striatal dopamine appears to be largely permissive over lever pressing, in that normal levels of dopamine in the ventrolateral striatum are critical for responding, although dopamine levels do not fluctuate much during behavioral sessions.

Yohimbine interacts with the dopaminergic system to reverse sexual satiation: further evidence for a role of sexual motivation in sexual exhaustion

The possible interaction of yohimbine with the dopaminergic system in the mediation of sexual behaviour expression in sexually exhausted male rats was investigated. The behavioural effects of the simultaneous injection of yohimbine (500 microg/kg) plus apomorphine (50 microg/kg) and those of the combined treatment of haloperidol (125 microg), a nonspecific dopamine receptor antagonist, with an effective dose of yohimbine (2000 microg/kg) on sexually satiated rats were evaluated. Data show that yohimbine and apomorphine, per se, dose-dependently reverse sexual exhaustion by increasing the percentage of sexually satiated rats copulating and resuming copulation after ejaculation. Injection of haloperidol simultaneous to an effective dose of yohimbine, blocked the ability of the latter to reverse sexual satiation. The combined treatment with subthreshold doses of apomorphine and yohimbine synergised to reverse the sexual inhibition characteristic of sexual exhaustion. Data suggest that the dopaminergic system might be the final pathway for the yohimbine-induced sexual behaviour expression in satiated rats. The possible role of sexual motivation in the sexual exhaustion phenomenon is discussed.

The locomotor effects of MK801 in the nucleus accumbens of developing and adult rats

This developmental study was an investigation of locomotion induced by the NMDA receptor antagonist, (+)MK-801 hydrogen maleate [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine], at doses of 0, 3 or 10 microg injected bilaterally into the nucleus accumbens of rats at 11, 21, 31, or 61-66 days of age. During a 2-h test session, only a few 11-day-old pups responded to either dose of MK801; they displayed short bouts of obstinate progression. In contrast, 21- and 31-day-olds were not affected by 3 microg MK801 but exhibited robust activation after 10 microg MK801. The activation was greatest in 21-day-olds and also occurred after mid-striatal injections in 21- but not 31-day-old rats. Adult rats injected with MK801 were not robustly activated, but they maintained their initial level of activity throughout the test session, instead of habituating to the test monitor, as controls did. Ontological changes in MK801-induced activity are likely to reflect maturation of glutamate transmission in the nucleus accumbens.

Sexual experience sensitizes mating-related nucleus accumbens dopamine responses of female Syrian hamsters

We examined the effects of prior sexual experience on extracellular concentrations of dopamine in the nucleus accumbens of female hamsters. Nucleus accumbens dopamine was measured by in vivo microdialysis during mating in female Syrian hamsters that had previously been given six prior sexual encounters with a male, three prior encounters, or were sexually naive. High levels of sexual behavior were observed in all three groups, which were accompanied by increases in dialysate dopamine during periods when the male was present. However, females that received six prior sexual encounters had significantly elevated and prolonged increases in dialysate dopamine compared with those of the sexually naive females or females with only three prior sexual encounters with a male. The data indicate that the mesolimbic system can be sensitized by repeated experiences associated with a motivated behavior.

Facilitation of sexual behavior and enhanced dopamine efflux in the nucleus accumbens of male rats after D-amphetamine-induced behavioral sensitization

Behavioral sensitization caused by repeated and intermittent administration of psychostimulants, such as cocaine and D-amphetamine, is accompanied by enhanced function in limbic-motor circuitry that is involved in the generation of motivated behavior. The present microdialysis study investigated the effect of D-amphetamine-induced sensitization on dopamine (DA) efflux in the nucleus accumbens (NAC) of male rats during sexual behavior. Male rats were given one injection of D-amphetamine (1.5 mg/kg, i.p.) or saline every other day for a total of 10 injections. Three weeks after discontinuation of drug treatment, rats were tested for sexual behavior during a test in which microdialysis was performed. There was an augmented efflux of DA in the NAC of D-amphetamine-sensitized rats compared with nonsensitized control rats when a receptive female was present behind a screen (35 vs 17%). Sensitized rats exhibited facilitated sexual behavior when the screen was removed, as indicated by a significantly shorter latency to mount and an overall increase in the amount of copulatory behavior. Although there was a significant increase in NAC DA concentrations from baseline in both sensitized and nonsensitized rats during copulation, there was a greater increase in DA efflux in the NAC of sensitized rats during the first 10 min copulatory sample (60 vs 37%). These results demonstrate that behavioral sensitization caused by repeated psychostimulant administration can "cross-sensitize" to a natural behavior, such as sex, and that increased NAC DA release may contribute to the facilitation of appetitive and consummatory aspects of this behavior.

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.

A microdialysis study of nucleus accumbens core and shell dopamine during operant responding in the rat

This investigation examined dopamine release and metabolism in nucleus accumbens core and shell during three operant tasks in the rat. Rats were trained to lever press on a fixed-ratio 5, variable-interval 30 s, or a tandem variable interval 30/fixed-ratio 5 schedules; these three schedules were chosen because they generate a wide range of response and reinforcement rates. After several weeks of training, dialysis probes were implanted into nucleus accumbens core or shell subregions. A single 30 min behavioural session was conducted during the dialysis test session. Rats lever pressing on each of the three operant schedules showed a significant increase in extracellular dopamine relative to the food-deprived control group during the behavioural session. In addition, increases in dopamine in nucleus accumbens shell were found to be significantly greater than in the core during the lever pressing period. Across all three schedules, extracellular dopamine in the nucleus accumbens was significantly correlated with the number of lever presses performed, but was not correlated with the number of food pellets delivered. Analysis of covariance, which used amount of food consumed as the covariate, showed an overall group difference, indicating that dopamine levels increased in lever pressing animals even if one corrected for the amount of food consumed. These results indicate that dopamine release was more responsive in the nucleus accumbens shell than in the core during operant responding, and that increases in extracellular dopamine in nucleus accumbens are related to response rate rather than reinforcement magnitude.

The role of basal ganglia in the control of generalized absence seizures

During the last two decades, evidence has accumulated to demonstrate the existence, in the central nervous system, of an endogenous mechanism that exerts an inhibitory control over different forms of epileptic seizures. The substantia nigra and the superior colliculus have been described as key structures in this control circuit; inhibition of GABAergic neurons of the substantia nigra pars reticulata results in suppression of seizures in various animal models of epilepsy. The role in this control mechanism of the direct GABAergic projection from the striatum to the substantia nigra and of the indirect pathway, from the striatum through the globus pallidus and the subthalamic nucleus, was examined in a genetic model of absence seizures in the rat. In this model, pharmacological manipulations of both the direct and indirect pathways resulted in modulation of absence seizures. Activation of the direct pathway or inhibition of the indirect pathway suppressed absence seizures through disinhibition of neurons in the deep and intermediate layers of the superior colliculus. Dopamine D1 and D2 receptors in the nucleus accumbens, appear to be critical in these suppressive effects. Along with data from the literature, our results suggest that basal ganglia circuits play a major role in the modulation of absence seizures and provide a framework to understand the role of these circuits in the modulation of generalized seizures.

Electrophysiological responses of nucleus accumbens neurons to novelty stimuli and exploratory behavior in the awake, unrestrained rat

To establish a physiological correlate of goal-directed and reward-seeking non-operant behaviors, we studied the electrophysiological activity of NAcc neurons in unrestrained, naive Sprague-Dawley rats. Our results showed an inhibitory response in 21% (7/34) of the recorded NAcc neurons during spontaneous nosepoking behavior and in 16% (4/25) of the NAcc neurons when rats were fed with a favorite novel food morsel (popcorn). These data suggest that a subgroup of NAcc neurons is actively modulated during motivated behavior and during consummatory events resulting in a suppression of neuronal activity.