Tail-pinch stress increases extracellular DOPAC levels (as measured by in vivo voltammetry) in the rat nucleus accumbens but not frontal cortex: antagonism by diazepam and zolpidem

Role of central oxytocin and dopamine systems in nociception and their possible interactions: suggested hypotheses

Central oxytocin and dopamine have an important role in the process of nociception at the spinal level as well as supraspinal structures, e.g. anterior cingulate cortex, insular cortex, amygdala, nucleus accumbens, and hypothalamus. Many studies have pointed out the importance of both systems in the pain descending modulatory system and in pain-related symptoms in some chronic disorders, e.g. Parkinson disease and fibromyalgia. The interaction between oxytocin and dopamine systems has been addressed in some motivational behaviors, e.g. maternal and sexual behaviors, pair bonding, and salience. In this aspect, we propose that an oxytocin-dopamine interaction could be present in nociception, and we also explain the possible hypotheses of such an interaction between these systems.

Emotional, physical and sexual abuse are associated with a heightened limbic response to cocaine cues

Drug-reward cues trigger motivational circuitry, a response linked to drug-seeking in animals and in humans. Adverse life events have been reported to increase sensitivity to drug rewards and to bolster drug reward signaling. Therefore, we hypothesized that cocaine-dependent individuals with prior emotional, physical and sexual abuse might have a heightened mesolimbic brain response to cues for drug reward in a new brief-cue probe. Cocaine-dependent human individuals (N = 68) were stabilized in an inpatient setting and then completed an event-related blood-oxygen-level dependent functional magnetic resonance imaging task featuring 500-ms evocative (cocaine, sexual, aversive) and comparator (neutral) cues. Responses to three questions about emotional, physical and sexual abuse from the Addiction Severity Index were used to divide the patients into subgroups (history of Abuse [n = 40] versus No Abuse [n = 28]). When subjects were grouped by the historical presence or absence of emotional, physical or sexual abuse, the Abuse group showed a heightened midbrain, thalamic, caudate, and caudal orbitofrontal cortex response to cocaine cues; a similar result was found in other evocative cues, as well. These findings are the first reported for a 500-ms cocaine-cue probe, and they highlight the ability of very brief evocative cues to activate the brain's motivational circuitry. Although all participants had severe cocaine use disorders, individuals reporting prior abuse had a heightened mesolimbic response to evocative cues. To our knowledge, this is the first study in humans linking a history of abuse to a brain vulnerability (heightened mesolimbic response to drug cues) previously shown to contribute to drug-seeking.

Unseen scars: Cocaine patients with prior trauma evidence heightened resting state functional connectivity (RSFC) between the amygdala and limbic-striatal regions

BACKGROUND

Substance use disorder (SUD) patients with a history of trauma exhibit poorer treatment outcome, greater functional impairment and higher risk for relapse. Endorsement of prior trauma has, in several SUD populations, been linked to abnormal functional connectivity (FC) during task-based studies. We examined amygdala FC in the resting state (RS), testing for differences between cocaine patients with and without prior trauma.

METHODS

Patients with cocaine use disorder (CUD; n=34) were stabilized in an inpatient setting prior to a BOLD fMRI scan. Responses to Addiction Severity Index and the Mini-International Neuropsychiatric Interview were used to characterize the No-Trauma (n=16) and Trauma (n=18) groups. Seed-based RSFC was conducted using the right and left amygdala as regions of interest. Examination of amygdala RSFC was restricted to an a priori anatomical mask that incorporated nodes of the limbic-striatal motivational network.

RESULTS

RSFC was compared for the Trauma versus No-Trauma groups. The Trauma group evidenced greater connectivity between the amygdala and the a priori limbic-striatal mask. Peaks within the statistically significant limbic-striatal mask included the amygdala, putamen, pallidum, caudate, thalamus, insula, hippocampus/parahippocampus, and brain stem.

CONCLUSIONS

Results suggest that cocaine patients with prior trauma (versus without) have heightened communication within nodes of the motivational network, even at rest. To our knowledge, this is the first fMRI study to examine amygdala RSFC among those with CUD and trauma history. Heightened RSFC intralimbic connectivity for the Trauma group may reflect a relapse-relevant brain vulnerability and a novel treatment target for this clinically-challenging population.

Early adversity disrupts the adult use of aversive prediction errors to reduce fear in uncertainty

Early life adversity increases anxiety in adult rodents and primates, and increases the risk for developing post-traumatic disorder (PTSD) in humans. We hypothesized that early adversity impairs the use of learning signals -negative, aversive prediction errors–to reduce fear in uncertainty. To test this hypothesis, we gave adolescent rats a battery of adverse experiences then assessed adult performance in probabilistic Pavlovian fear conditioning and fear extinction. Rats were confronted with three cues associated with different probabilities of foot shock: one cue never predicted shock, another cue predicted shock with uncertainty, and a final cue always predicted shock. Control rats initially acquired fear to all cues, but rapidly reduced fear to the non-predictive and uncertain cues. Early adversity rats were slower to reduce fear to the non-predictive cue and never fully reduced fear to the uncertain cue. In extinction, all cues were presented in the absence of shock. Fear to the uncertain cue in discrimination, but not early adversity itself, predicted the reduction of fear in extinction. These results demonstrate early adversity impairs the use of negative aversive prediction errors to reduce fear, especially in situations of uncertainty.

Brainstem system of hippocampal theta induction: The role of the ventral tegmental area

This article summarizes the results of studies concerning the influence of the ventral tegmental area (VTA) on the hippocampal theta rhythm. Temporary VTA inactivation resulted in transient loss of the hippocampal theta. Permanent destruction of the VTA caused a long-lasting depression of the power of the theta and it also had some influence on the frequency of the rhythm. Activation of glutamate (GLU) receptors or decrease of GABAergic tonus in the VTA led to enhancement of dopamine release and increased hippocampal theta power. High time and frequency cross-correlation was detected for the theta band between the VTA and hippocampus during paradoxical sleep and active waking. Thus, the VTA may belong to the broad network involved in theta rhythm regulation. This article also presents a model of brainstem-VTA-hippocampal interactions in the induction of the hippocampal theta rhythm. The projections from the VTA which enhance theta rhythm are incorporated into the main theta generation pathway, in which the septum acts as the central node. The neuronal activity that may be responsible for the ability of the VTA to regulate theta probably derives from the structures associated with rapid eye movement (sleep) (REM) sleep or with sensorimotor activity (i.e., mainly from the pedunculopontine and laterodorsal tegmental nuclei and also from the raphe).

Reward responsiveness in patients with chronic pain

BACKGROUND

It is proposed that changes in reward processing in the brain are involved in the pathophysiology of pain based on experimental studies. The first aim of the present study was to investigate if reward drive and/or reward responsiveness was altered in patients with chronic pain (PCP) compared to controls matched for education, age and sex. The second aim was to investigate the relationship between reward processing and nucleus accumbens volume in PCP and controls. Nucleus accumbens is central in reward processing and its structure has been shown to be affected by chronic pain conditions in previous studies.

METHODS

Reward drive and responsiveness were assessed with the Behavioral Inhibition Scale/Behavioral Activation Scale, and nucleus accumbens volumes obtained from T1-weighted brain MRIs obtained at 3T in 19 PCP of heterogeneous aetiologies and 20 age-, sex- and education-matched healthy controls. Anhedonia was assessed with Beck's Depression Inventory II.

RESULTS

The PCP group had significantly reduced scores on the reward responsiveness, but not reward drive. There was a trend towards smaller nucleus accumbens volume in the PCP compared to control group. There was a significant positive partial correlation between reward responsiveness and nucleus accumbens volume in the PCP group adjusted for anhedonia, which was significantly different from the same relationship in the control group.

CONCLUSIONS

Reward responsiveness is reduced in chronic pain patients of heterogeneous aetiology, and this reduction was associated with nucleus accumbens volume. Reduced reward responsiveness could be a marker of chronic pain vulnerability, and may indicate reduced opioid function.

Litter size reduction alters insulin signaling in the ventral tegmental area and influences dopamine-related behaviors in adult rats

Postnatal overfeeding is a well-known model of early-life induced obesity and glucose intolerance in rats. However, little is known about its impact on insulin signaling in specific brain regions such as the mesocorticolimbic system, and its putative effects on dopamine-related hedonic food intake in adulthood. For this study, rat litters were standardized to 4 (small litter - SL) or 8 pups (control - NL) at postnatal day 1. Weaning was at day 21, and all tests were conducted after day 60 of life in male rats. In Experiment 1, we demonstrated that the SL animals were heavier than the NL at all time points and had decreased AKT/pAKT ratio in the Ventral Tegmental Area (VTA), without differences in the skeletal muscle insulin signaling in response to insulin injection. In Experiment 2, the standard rat chow intake was addressed using an automated system (BioDAQ, Research Diets(®)), and showed no differences between the groups. On the other hand, the SL animals ingested more sweet food in response to the 1 min tail-pinch challenge and did not develop conditioned place preference to sweet food. In Experiment 3 we showed that the SL rats had increased VTA TH content but had no difference in this protein in response to a sweet food challenge, as the NL had. The SL rats also showed decreased levels of dopamine D2 receptors in the nucleus accumbens. Here we showed that early postnatal overfeeding was linked to an altered functioning of the mesolimbic dopamine pathway, which was associated with altered insulin signaling in the VTA, suggesting increased sensitivity, and expression of important proteins of the dopaminergic system.

NMDA-glutamatergic activation of the ventral tegmental area induces hippocampal theta rhythm in anesthetized rats

Glutamate afferents reaching the ventral tegmental area (VTA) affect dopamine (DA) cells in this structure probably mainly via NMDA receptors. VTA appears to be one of the structures involved in regulation of hippocampal theta rhythm, and this work aimed at assessing the role of glutamatergic activation of the VTA in the theta regulation. Male Wistar rats (n=17) were divided into groups, each receiving intra-VTA microinjection (0.5 μl) of either solvent (water), glutamatergic NMDA agonist (0.2 μg) or antagonist (MK-801, 3.0 μg). Changes in local field potential were assessed on the basis of peak power (Pmax) and corresponding peak frequency (Fmax) for the delta (0.5-3 Hz) and theta (3-6 Hz) bands. NMDA microinjection evoked long-lasting hippocampal theta. The rhythm appeared with a latency of ca. 12 min post-injection and lasted for over 30 min; Pmax in this band was significantly increased for 50 min, while simultaneously Pmax in the delta band remained lower than in control conditions. Theta Fmax and delta Fmax were increased in almost entire post-injection period (by 0.3-0.5 Hz and 0.3-0.7 Hz, respectively). MK-801 depressed the sensory-evoked theta: tail pinch could not induce theta for 30 min after the injection; Pmax significantly decreased in the theta band and at the same time it increased in the delta band. Theta Fmax decreased 10 and 20 min post injection (by 0.4-0.5 Hz) and delta Fmax decreased in almost entire post injection period (by 0.3-0.7 Hz). NMDA injection generates theta rhythm probably through stimulation of dopaminergic activity within the VTA.

Oxytocin, motivation and the role of dopamine

The hypothalamic neuropeptide oxytocin has drawn the attention of scientists for more than a century. The understanding of the function of oxytocin has expanded dramatically over the years from a simple peptide adept at inducing uterine contractions and milk ejection to a complex neuromodulator with a capacity to shape human social behavior. Decades of research have outlined oxytocin's ability to enhance intricate social activities ranging from pair bonding, sexual activity, affiliative preferences, and parental behaviors. The precise neural mechanisms underlying oxytocin's influence on such behaviors have just begun to be understood. Research suggests that oxytocin interacts closely with the neural pathways responsible for processing motivationally relevant stimuli. In particular, oxytocin appears to impact dopaminergic activity within the mesocorticolimbic dopamine system, which is crucial not only for reward and motivated behavior but also for the expression of affiliative behaviors. Though most of the work performed in this area has been done using animal models, several neuroimaging studies suggest similar relationships may be observed in humans. In order to introduce this topic further, this paper will review the recent evidence that oxytocin may exert some of its social-behavioral effects through its impact on motivational networks.

Adolescent rat circadian activity is modulated by psychostimulants

Circadian pattern of activity regulates many aspects of mammalian physiology and behavior to particular times of the day by entraining the circadian clocks to external environmental signals. Since circadian rhythms are sensitive to many pharmacological agents, it is important to understand if the repetitive use of psychostimulants such as amphetamine will alter the circadian rhythm behavioral activity pattern. The present study uses male Sprague-Dawley rats to study the long-term effects of amphetamine on the locomotor circadian rhythm activity pattern. Rats were randomly assigned to a testing cage that recorded their locomotor activity nonstop for eleven days using the open field assay, as follows: one day of baseline activity was recorded and then the experimental group was injected with amphetamine (0.6mg/kg) for 6days, no treatment for 3days (i.e., washout days) and then re-challenged with amphetamine for one more day while the control group was treated similarly with saline. The Cosine Curve Statistical Analysis (CCSA) test was used to fit a 24-hour curve to activity pattern. Results indicate that repetitive daily amphetamine injections cause behavioral sensitization and a significant change of circadian rhythm of locomotor activity pattern, and elicit behavioral expectation to receive the drug or expression of withdrawal during the washout days. The results suggest that either changes in circadian rhythm caused sensitization and withdrawal or sensitization and withdrawal caused the change in circadian rhythm activity.

Place conditioning and neurochemical responses elicited by the aftereffect of acute stressor exposure involving an elevated stand

Acute exposure to an elevated stand has been used as an inescapable mild stressor for rats. The present study examined the effects of this stressor using a place conditioning behavioral test and neurochemical assays of dopamine and its metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), in the medial prefrontal cortex and nucleus accumbens. The behavioral data showed that a conditioned place preference was formed as an aftereffect of the elevated stand stressor. In a separate experiment, neurochemical assay showed an immediate increase of dopamine in the nucleus accumbens after 30min exposure to the elevated stand stressor. In addition, the DOPAC content in the nucleus accumbens was significantly increased at 30min after this stressor. No significant change in dopamine or DOPAC levels in the medial prefrontal cortex was detected for up to 60min after stressor manipulation. These results suggest that an increase in dopamine activity in the nucleus accumbens is involved in the development of conditioned place preference elicited by the aftereffects of the elevated stand stressor.

Marked for life? Effects of early cage-cleaning frequency, delivery batch, and identification tail-marking on rat anxiety profiles

Daily handling of preweanling rats reduces their adult anxiety. Even routine cage-cleaning, involving handling, reduces adult anxiety compared with controls. Cage-cleaning regimes differ between animal breeders, potentially affecting rodent anxiety and experimental results. Here, 92 adult male rats given different cage-cleaning rates as pups, were compared on plus-maze, hyponeophagia, corticosterone, and handling tests. They were pair-housed and half were tail-marked for identification. Anxiety/stress profiles were unaffected by cage-cleaning frequency, suggesting that commercial-typical differences in husbandry contribute little variance to adult rat behavior. However, delivery batch affected some elevated plus-maze measures. Also, tail-marked rats spent three times longer on the plus-maze open arms than their unmarked cagemates, suggesting reduced anxiety, yet paradoxically they showed greater chromodacryorrhoea responses to handling, implying increased aversion to human contact. A follow-up study showed that rats avoided the odor released from the marker pen used. Thus, apparently trivial aspects of procedure can greatly affect experimental results.

The mGlu2/3 receptor agonist, LY354740, blocks immobilization-induced increases in noradrenaline and dopamine release in the rat medial prefrontal cortex

The metabotropic glutamate (mGlu2/3) receptor agonist, LY354740, exhibits anxiolytic-like properties in a number of rodent models. The present study utilized in vivo microdialysis to examine the effects of LY354740 on extracellular monoamine levels in the medial prefrontal cortex (mPFC) of animals subjected to 30 min immobilization stress. Immobilization stress significantly elevated extracellular levels of noradrenaline (NA) and dopamine (DA) in the mPFC, while systemic administration of LY354740 (30 mg/kg, s.c.) significantly attenuated immobilization-induced increases in both NA and DA. Reverse-dialysis of LY354740 (30 microm) into the mPFC significantly attenuated immobilization-induced increases in NA, but not DA without affecting basal levels of either amine. In separate studies in the presence of citalopram (1 microm; reverse dialysis into the mPFC), systemic administration of LY354740 attenuated immobilization-induced increases in NA and DA, but had no effect on serotonin (5-HT) levels. Co-administration of the selective mGlu2/3 receptor antagonist, LY341495, partially or fully reversed the attenuation in NA and DA levels produced by LY354740, respectively. Taken together, these data suggest that LY354740 may produce anti-stress actions, in part, by blocking stress-related increases in catecholamines in the mPFC via mGlu2/3 receptor stimulation.

Fear and feeding in the nucleus accumbens shell: rostrocaudal segregation of GABA-elicited defensive behavior versus eating behavior

This study examined localization of positive versus negative motivational functions mediated by GABA circuits within the accumbens shell. Microinjections of a GABA(A) agonist (0, 25, 75, and 225 ng/0.5 microl muscimol) in rostral shell sites elicited appetitive increases in eating behavior. In contrast, microinjections in caudal shell sites elicited defensive burying or paw-treading behavior. Rats whose microinjections landed bilaterally outside of the accumbens shell did not display either behavior. Defensive treading elicited by caudal shell muscimol microinjection appeared to be a negative motivated response to threat (similar in parameters and orientation to normal defensive burying of a threatening electrified shock prod). The nucleus accumbens shell thus appears functionally heterogeneous in coding motivational valence. The demonstration that muscimol elicits positive eating behavior from rostral shell versus negative defensive behavior from caudal shell suggests in particular that GABAergic substrates of positive and negative types of motivated behavior in the nucleus accumbens shell are segregated along a rostrocaudal gradient.

Engagement in a non-escape (displacement) behavior elicits a selective and lateralized suppression of frontal cortical dopaminergic utilization in stress

Although the preferential activation of the prefrontal cortical (PFC) dopaminergic system is generally observed in stress, limited exceptions to this have been observed. Certain non-escape behaviors have been demonstrated to attenuate physiological indices of stress (e.g., coping or displacement responses). One well-characterized non-escape behavior observed in stress is chewing, or gnawing, of inedible objects. Engagement in this behavior attenuates stress-related activation of the hypothalamopituitary-adrenal axis, in a variety of species. We examined the degree to which engagement in this non-escape behavior modulates stressor-induced activation of the PFC dopamine (DA) system. Rats and mice were exposed to a brightly lit novel environment (novelty stress) in the presence or absence of inedible objects. Following novelty exposure, various dopaminergic terminal fields were collected and dopamine and its major catabolite, DOPAC, were measured using HPLC with electrochemical detection. DOPAC/DA ratios were calculated as an index of DA utilization. In some cases serotonin (5-HT) and its major catabolite, 5-HIAA, were also measured. In animals that did not chew, novelty exposure elicited significant increases in DOPAC/DA levels within PFC, nucleus accumbens (shell and core subdivisions), and striatum (relative to quiet-controls). DOPAC/DA responses were greater in the right PFC than in the left PFC. Animals that chewed displayed significantly lower DOPAC/DA responses in PFC, but not other dopaminergic terminal fields. This effect of chewing was always observed in the right PFC and less consistently in the left PFC. Chewing did not alter novelty-induced increases in PFC 5-HIAA/5-HT responses. Thus, engagement in this non-escape behavior elicits a neuroanatomically and neurochemically specific attenuation of the PFC DA response in stress. Given the pivotal role of the PFC in certain cognitive and affective processes, behavioral regulation of PFC DA utilization may modulate cognitive and/or affective function in stress.

Individual differences in stress-induced dopamine release in the nucleus accumbens are influenced by corticosterone

Stressful experiences, glucocorticoids hormones and dopaminergic neurons seems to interact in determining a higher propensity to develop drug abuse. In this report, we studied the acute interaction between these three factors. For this purpose, we compared stress-induced dopamine release in intact rats and in rats in which stress-induced corticosterone secretion was experimentally blocked. Ten-minute tail-pinch was used as a stressor and dopamine release estimated in the nucleus accumbens by using the microdialysis technique. Individual differences were also taken into account by comparing rats identified as either predisposed (HRs) or resistant (LRs) to develop self-administration of drugs of abuse, on the basis of their locomotor response to novelty. It was found that suppression of stress-induced corticosterone secretion significantly decreased stress-induced dopamine release. However, such an effect greatly differed between HR and LR rats. When corticosterone secretion was intact HR animals had a higher and longer dopamine release in response to stress than LRs. The blockade of stress-induced corticosterone secretion selectively reduced the dopaminergic response of HRs that did not differ from LRs anymore. These findings strength the idea that glucocorticoids could be involved in determining propensity to develop drug self-administration. In particular, these hormones could play a role in determining the higher dopaminergic activity that characterizes drug proned individuals.

N(G)-nitro-L-arginine methyl ester reduces stress-related feeding in the rat tail-pinch model

A possible role of nitric oxide (NO) in stress-related feeding was investigated in male rats using the tail-pinch (TP) model, in within-subjects experimental designs. An initial experiment demonstrated a dose-related reduction in TP-induced solid food intake over a 10-min test period with increasing dose (10.25, and 50 mg/kg SC) of the NO-synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), reaching statistical significance at 25 mg/kg L-NAME when compared to vehicle control (p < 0.05). Pattern analysis further revealed a decrease both in total duration of food-directed oral behavior and in percentage of longer duration (> 60 s) oral behavior bouts with increasing dose of L-NAME; both measures reached statistical significance at 50 mg/kg (p < 0.01). Pretreatment with 500 mg/kg of the NO precursor, L-arginine (L-arg), resulted in partial but not significant reversal of the attenuating effect of 25 mg/kg L-NAME on food intake. Latency to begin eating or gnawing was not significantly affected by L-NAME. In a subsequent experiment, L-arg alone (500 and 750 mg/kg) did not significantly alter TP-induced food intake. It is cautiously suggested that these results implicate involvement of NO in TP-induced feeding.

Differential effects of dopaminergic drugs on anxiety and arousal in healthy volunteers with high and low anxiety

1. The appearance of frontal midline theta activity (Fm theta), the distinct EEG theta rhythm in the frontal midline area during performance of a mental task, indicates relief from anxiety in humans. 2. The authors examined the effects of bromocriptine and sulpiride on anxiety and arousal in 24 male university students with (Fm theta group, n = 12) and without (non-Fm theta group, n = 12) Fm theta. Subjects were given placebo, 2.5 mg bromocriptine and 100 mg sulpiride in a double-blind crossover design. 3. Blood samples were obtained, STAI scores were determined, and EEGs were recorded before and during the performance of an arithmetic addition task. The test was repeated twice: before and 1 hr after drug administration. 4. Bromocriptine reduced the HVA concentration in both groups; sulpiride caused an increase in both groups. In the Fm theta group, bromocriptine did not alter the appearance time of Fm theta, the state anxiety score or the task performance; sulpiride increased the Fm theta amount and reduced the state anxiety but did not affect the task performance. In the non-Fm theta group, bromocriptine increased the Fm theta duration and reduced the state anxiety score but did not influence the task performance, while sulpiride reduced Fm theta and increased the state anxiety but had no effect on the task performance. 5. These results suggest that the sensitivity of presynaptic D2 receptors is higher in high-anxiety subjects compared with low-anxiety subjects, and that anxiolytic effects in high-anxiety humans and those in low-anxiety humans may be caused by decreased and increased DA activity, respectively. In addition, the stimulation of DA function may cause anxiogenic effects in high-anxiety individuals.

Behavioral functions of nucleus accumbens dopamine: empirical and conceptual problems with the anhedonia hypothesis

Nucleus accumbens (DA) has been implicated in a number of different behavioral functions, but most commonly it is said to be involved in "reward" or "reinforcement". In the present article, the putative reinforcement functions of accumbens DA are summarized in a manner described as the "General Anhedonia Model". According to this model, the DA innervation of the nucleus accumbens is conceived of as a crucial link in the "reward system", which evolved to mediate the reinforcing effects of natural stimuli such as food. The reward system is said to be activated by natural reinforcing stimuli, and this activation mediates the reinforcing effects of these natural stimuli. According to this view, other stimuli such as brain stimulation and drugs can activate this system, which leads to these stimuli being reinforcing as well. Interference with DA systems is said to blunt the reinforcing effects of these rewarding stimuli, leading to "extinction". This general model of the behavioral functions of accumbens DA is utilized widely as a theoretical framework for integrating research findings. Nevertheless, there are several difficulties with the General Anhedonia Model. Several studies have observed substantial differences between the effects of extinction and the effects of DA antagonism or accumbens DA depletions. Studies involving aversive conditions indicate that DA antagonists and accumbens DA depletions can interfere with avoidance behavior, and also have demonstrated that accumbens DA release is increased by stressful or aversive stimuli. Although accumbens DA is important for drug abuse phenomena, particularly stimulant self-administration, studies that involve other reinforcers are more problematic. A large body of evidence indicates that low doses of dopamine antagonists, or depletions of accumbens DA, do not impair fundamental aspects of food motivation such as chow consumption and simple instrumental responses for food. This is particularly important, in view of the fact that many behavioral researchers consider the regulation of food motivation to be a fundamental aspect of food reinforcement. Finally, studies employing cost/benefit analyses are reviewed, and in these studies considerable evidence indicates that accumbens DA is involved in the allocation of responses in relation to various reinforcers. Nucleus accumbens DA participates in the function of enabling organisms to overcome response costs, or obstacles, in order to obtain access to stimuli such as food. In summary, nucleus accumbens DA is not seen as directly mediating food reinforcement, but instead is seen as a higher order sensorimotor integrator that is involved in modulating response output in relation to motivational factors and response constraints. Interfering with accumbens DA appears to partially dissociate the process of primary reinforcement from processes regulating instrumental response initiation, maintenance and selection.

Tail pinch induces fos immunoreactivity within several regions of the male rat brain: effects of age

Brief, intermittent stressors, such as low-level foot shock or tail pinch, induce a general excitement and autonomic arousal in rats that increases their sensitivity to external incentives. Such stimulation can facilitate a variety of behaviors, including feeding, aggression, sexual activity, parental behavior, and drug taking if the appropriate stimuli exist in the environment. However, the ability of tail pinch to induce general arousal and incentive motivation appears to diminish with age. Here we report on the ability of tail pinch to induce Fos immunoreactivity within several brain regions as a function of age. Young (2-3 months) and middle-aged (12-13 months) male rats were administered either five tail pinches (one every 2 min), one tail pinch, or zero (sham) tail pinches (n = 4 per stimulation condition). Rats were sacrificed 75 min following the onset of stimulation, and their brains were prepared for immunocytochemical detection of Fos protein. Fos immunoreactivity was induced by one and five tail pinches in several brain regions, including the anterior medial preoptic area (mPOA), paraventricular nucleus of the hypothalamus (PVN), paraventricular nucleus of the thalamus (PV-Thal), medial amygdala (MEA), basolateral amygdala (BLA), lateral habenula (LHab), and ventral tegmental area (VTA), of young rats compared with those that received zero tail pinches. In contrast to young rats, middle-aged rats had significantly less Fos induced by one and five tail pinches in the mPOA, PVN, MEA, BLA, and VTA, but an equivalent amount induced in the LHab. Fos immunoreactivity was not found within the medial prefrontal cortex, nucleus accumbens, striatum, lateral septum, or locus coeruleus in either young or old rats. Tail pinch appears to activate regions of the brain known to be involved in behavioral responses to both incentive cues and stressors. The lower level of cellular reactivity to tail pinch in middle-aged rats suggests a diminished neural responsiveness to incentives and stressors.

Acute and repeated activation of male sexual behavior by tail pinch: opioid and dopaminergic mechanisms

We studied the effect of tail pinch on male sexual behavior and its neurochemical basis. Male rats were gonadectomized and maintained on low doses of testosterone propionate (20.0 micrograms/day). Tail pinch significantly increased the percentage of males that mounted, intromitted, and ejaculated within a 30-min test, and these increases were attenuated by both pimozide (1.0 mg/kg, i.p.) and by naloxone (0.5, 1.0, and 2.0 mg/kg, s.c.). Moreover, tail pinch in the presence of an estrous female led to significantly increased female-directed behavior 48 h later during a test without tail pinch. Repeated tail pinch tests led to progressively more sexual activity, and the development of this behavioral sensitization was prevented by naloxone. These findings suggest that tail pinch increases the salience of the incentive characteristics of the female. Furthermore, during subsequent tests, with or without tail pinch, the increased salience of the female remains, as measured by the continued increases in sexual activity. These acute and sensitized behavioral increases might result from tail pinch-induced activation of the midbrain dopamine system via an opioid mechanism; either preventing tail pinch-induced dopamine activation (by an opioid antagonist) or blocking the effects of dopamine activation (by a dopamine antagonist) attenuated the long-term facilitation of sexual behavior seen after pairing the female with tail pinch.

Medial prefrontal cortical D1 receptor modulation of the meso-accumbens dopamine response to stress: an electrochemical study in freely-behaving rats

Voltammetry was used to study the role of prefrontal cortex (PFC) dopamine (DA) in modulating the nucleus accumbens (NAcc) DA response to stress. Signal increases elicited in NAcc by 15 min of restraint were monitored in freely-behaving rats following intra-PFC microinjections of D1 and D2 receptor-selective drugs. The exact site of injection was first determined by assessing the electrochemical response to stress at two dorsal-ventral levels of PFC. Consistent with previous reports, a pronounced stress response was observed ventrally at sites within the infralimbic PFC but not dorsally within the superficial layers of PFC. When microinjected into the infralimbic PFC, the D1 receptor antagonist SCH 23390 significantly enhanced the NAcc stress response. While the D1 receptor agonist SKF 38393 tended to decrease the NAcc stress response, it failed to do so reliably. Neither sulpiride (D2 receptor antagonist) nor quinpirole (D2 receptor agonist) had a significant effect. Finally, systemic administration of the selective DA uptake inhibitor GBR 12909 dose-dependently potentiated stress-induced signal increases in NAcc and in PFC, indicating that the electrochemical responses to stress in both regions were due primarily to increases in extracellular DA levels. Together, these data add to other evidence indicating that the PFC exerts an inhibitory influence on subcortical DA transmission. Specifically, the present results suggest that the NAcc DA response to stress is dampened by the concurrent activation of meso-PFC DA neurons and that this action is mediated, at least in part, by D1 receptors in PFC.

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

Considerable experimental and clinical evidence links forebrain dopamine (DA) systems to the performance of motor activities and to motivational processes. Much of the support for this conclusion was obtained from studies utilizing lesions or drugs to manipulate aspects of central dopaminergic function. Although such experiments yield important information concerning the behavioral consequences of interference with DA systems in brain, they do not demonstrate any relation between the dynamic activity of DA neurons and the level or type of motor function exhibited by the organism. This review discusses the emerging field of behavioral neurochemistry, and provides an overview of recent studies investigating the relation between nucleus accumbens DA release and behavior. Particular emphasis is placed upon current research involving microdialysis, voltammetry and electrophysiology. These different methods are viewed as complementary techniques for investigating the activity of DA systems in behaving animals. Evidence indicates that DA activity is most reliably activated by stimuli that trigger instrumental behavior and during the preparatory or instrumental phase of motivated behavior. The effects of consummatory reactions to positive reinforcers are somewhat equivocal; with food consumption, dialysis studies have yielded inconsistent results, while some voltammetric and electrophysiological studies have shown that DA activity in accumbens or ventral tegmental area actually decreases during consumption of food reinforcement. Moreover, the responsiveness of accumbens DA activity during behavioral stimulation is not unique to appetitive conditions, as several studies have shown that aversive or stressful conditions also stimulate accumbens DA release or metabolism. It is reasonable to suggest at this time that accumbens DA neurons are activated by a variety of different motivational conditions, but that the consequence of that activation is to modulate the behavioral reactivity of the organism. This type of function is seen as representing an area of overlap between motor and motivational processes.

Selective sparing of human nucleus accumbens in aging and anoxia

OBJECTIVE

To investigate the effects of aging and anoxia on the nucleus accumbens.

METHODS

The number of neurons in nucleus accumbens and caudate nucleus in 35 patients over 65 and 35 under 65 years, all without neurological or psychiatric disease were counted.

RESULTS

There was no statistically significant difference between the number of neurons in the accumbens in the two groups, but there was a decrease in the number of neurons in the elderly group. There was no reduction in volume of the neuronal nucleoli of the accumbens measured in 12 elderly patients compared to controls. These data suggest a sparing of the accumbens from changes associated with aging. There was relative preservation of the nucleus accumbens in 3 patients with anoxic encephalopathy.

CONCLUSIONS

These results show that accumbens was resistant to both aging and anoxia, the mechanism of which is discussed.

Nucleus accumbens dopamine release increases during instrumental lever pressing for food but not free food consumption

This experiment was undertaken to investigate the role of nucleus accumbens dopamine (DA) in instrumental and consummatory responses for food. In vivo microdialysis methods were used to study DA release and metabolism in the nucleus accumbens of behaving rats. Four behavioral conditions were used: performance on a fixed ratio 5 (FR 5) schedule of food reinforcement, consumption of Bioserve food pellets, consumption of laboratory chow, and food deprivation control. Groups of rats that were previously exposed to these conditions were implanted with dialysis probes in the nucleus accumbens and tested the day after implantation. The rats that pressed a lever on a FR 5 schedule showed significant increases in extracellular DA and DA metabolites compared to food-deprived control rats. In further analyses, rats that responded on the FR5 schedule were divided into three groups based upon their response rates. The rats with low response rates did not significantly differ from control rats, whereas rats with medium and high rates of responding showed significant increases in DA release relative to the control group. Rats that received massed presentation of food pellets or laboratory chow consumed large quantities of food, but showed no significant increases in DA release. This experiment demonstrated that performance of lever pressing behavior is accompanied by an increase in accumbens DA release and metabolism, and that DA release in nucleus accumbens is more closely related to the performance of highly active instrumental responses than it is to consumption of large quantities of food.

Opposite effects of midazolam and beta-carboline-3-carboxylate ethyl ester on the release of dopamine from rat nucleus accumbens measured by in vivo microdialysis

This report describes the effects of midazolam and beta-carboline-3-carboxylate ethyl ester (beta-CCE) on extracellular concentrations of dopamine in the nucleus accumbens of freely moving rats measured by in vivo microdialysis. The two compounds had opposite effects, midazolam (0.075 and 0.15 mg/kg i.v.) dose dependently decreasing, and beta-CCE (3 and 10 mg/kg i.p.) dose dependently increasing, dialysate concentrations of dopamine. Flumazenil (6 micrograms/kg i.v.) did not affect the efflux of dopamine but it prevented the effects of both midazolam and beta-CCE on dopamine efflux. N6-Cyclohexyladenosine (0.1, and 1 mg/kg i.p.), a selective adenosine A1 agonist, dose dependently increased the efflux of dopamine. This effect was blocked by 8-cyclopentyl-1,3-dipropylxanthine (25 mg/kg i.p.), a selective adenosine A1 receptor antagonist, a dose which given alone did not affect dopamine efflux; responses to midazolam were not affected. 3,7-Dimethyl-1-propargylxanthine (1 and 3 mg/kg i.p.), a selective adenosine A2 receptor antagonist, did not mimic the effects of beta-CCE. The results suggest that midazolam and beta-CCE modulate dopamine release in the nucleus accumbens by an action at the benzodiazepine binding site associated with the GABAA receptor complex.

The involvement of nucleus accumbens dopamine in appetitive and aversive motivation

In recent years, considerable emphasis has been placed upon the putative role of nucleus accumbens dopamine systems in appetitive motivation and positive reinforcement. However, considerable evidence indicates that brain dopamine in general, and nucleus accumbens dopamine in particular, is involved in aspects of aversive motivation. Administration of dopamine antagonists or localized interference with nucleus accumbens dopamine systems has been shown to disrupt active avoidance behavior. In addition, accumbens dopamine release and metabolism is activated by a wide variety of stressful conditions. A review of the literature indicates that there are substantial similarities between the characteristics of dopaminergic involvement in appetitive and aversive motivation. There is conflicting evidence about the role of dopamine in emotion, and little evidence to suggest that the profound and consistent changes in instrumental behavior produced by interference with DA systems are due to direct dopaminergic mediation of positive affective responses such as hedonia. It is suggested that nucleus accumbens dopamine is involved in aspects of sensorimotor functions that are involved in both appetitive and aversive motivation.

The role of nucleus accumbens dopamine in responding on a continuous reinforcement operant schedule: a neurochemical and behavioral study

Two experiments were undertaken to investigate the role of nucleus accumbens dopamine (DA) in instrumental lever pressing on a continuous reinforcement (CRF) schedule. Rats trained to press a lever for food reinforcement on a CRF schedule, and food-deprived control rats, were implanted with dialysis probes in the nucleus accumbens. The day after implantation, rats were tested and dialysis samples were assayed for DA and the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC). Performance of the lever-pressing task resulted in significant increases in extracellular levels of DA and DOPAC relative to control rats. The increases in extracellular DA were significantly correlated (r = 0.92) with the number of lever press responses committed. In the second experiment, the neurotoxic agent 6-hydroxydopamine was infused directly into the nucleus accumbens to investigate the effects of DA depletion on lever-pressing performance. DA depletion had only a modest effect on the total number of lever presses, and there was a significant effect on total lever presses only on the first test day (third day postsurgery). Analyses also were performed on responding across the 45-min session by breaking down the session into three 15-min periods. There was a significant group x time interaction, with DA-depleted rats showing a significant reduction in the numbers of responses in the first 15-min period, but no significant effects over the second or third 15 min in the session. This initial slowing of response rate was present across all 5 test days. These results indicate that DA release and metabolism increases in rats performing on a CRF schedule, and that DA depletion produces a slowing of initial response rate.

Serotonergic innervation of the hippocampus and nucleus accumbens septi and the anxiolytic-like action of midazolam and 5-HT1A receptor agonists

An involvement of serotonergic innervation of the hippocampus (HP) and the nucleus accumbens septi (NAS) in anxiolytic activity of benzodiazepine midazolam and 5-HT1A receptor agonists was studied in two different animal models of anxiety. Injection of midazolam (10.0 and 20.0 micrograms) or 8-OH-DPAT (0.5 and 1.0 micrograms) into the hippocampus increased punished consumption of water in the Vogel conflict test. Buspirone given at 0.1, 0.5 and 1.0 microgram was ineffective in the Vogel test, while at 5.0 micrograms it enhanced shock-induced suppression of drinking. In the open-field test midazolam (0.01 and 0.1 microgram), 8-OH-DPAT (0.1, 0.5 and 1.0 microgram) and buspirone (2.5 and 5.0 micrograms) increased the number of entries into the central part of the open-field and the time spent in the central sector. Depletion of 5-HT had no influence on the anxiolytic-like effect in the open-field test of intrahippocampally-administered 8-OH-DPAT (0.5 microgram), but the drug tended to increase motor activity in lesioned animals. Midazolam and buspirone injected into the NAS did not have an anxiolytic effect in the Vogel test. A small increase in punished drinking was observed after 8-OH-DPAT (1.0 and 2.5 micrograms). Following intra-NAS injection, midazolam, 8-OH-DPAT and buspirone all failed to produce any marked anxiolytic-like effect in the open-field test. It appears that the hippocampus, rather than the NAS, is involved in mediating anxiolytic-like effects of 5-HT1A receptor agonists. Hippocampal postsynaptic 5-HT1A receptors may account for the anti-emotional influence of this group of drugs. The results indicate some similarities in the psychotropic profile of 5-HT1A receptor agonists and midazolam.

Serotonergic innervation of the hippocampus and nucleus accumbens septi and the anxiolytic-like action of the 5-HT3 receptor antagonists

The roles of hippocampus (HP) and the nucleus accumbens septi (NAS) in the anxiolytic activity of two 5-HT3 receptor antagonists were studied in two animal models of anxiety, in rats. Injection of tropisetron (0.005 and 0.01 microgram) or ondansetron (1.0 and 2.5 micrograms) into the hippocampus increased punished consumption of water in the Vogel conflict test. In the open field test neither 5-HT3 receptor antagonists had anxiolytic-like effects. Tropisetron (0.01 and 0.025 microgram) injected into the NAS caused a marked increase in punished drinking, while ondansetron (0.01-15.0 micrograms) had no effect. In the open field test, tropisetron (0.001, 0.005 and 0.01 microgram) and ondansetron (1.0 and 2.5 micrograms) given to the NAS increased the number of entries into the central part of the open-field, and the time spent in the central sector of the arena. Depletion of 5-HT significantly enhanced the anxiolytic-like effect of intra-NAS-injected tropisetron in the open field, at the dose of 0.005 microgram. Moreover, 5,7-DHT lesions produced a tendency to increase motor activity in tropisetron-treated rats. Both hippocampal and accumbens 5-HT3 receptors seem to contribute to the anxiolytic-like effects of 5-HT3 antagonists in the Vogel test. It also appears that this effect of 5-HT3 receptor antagonists is related to their action on postsynaptic 5-HT3 receptors within the NAS, and depends on the functional state of the 5-HT innervation ascending from the raphe nuclei. Thus, the present data add more arguments for the more specific involvement of this limbic nucleus in emotional control.

Functional studies on monoaminergic transmitter release in parkinsonism

1. In vivo pulse voltammetry and apomorphine induced circling behaviour were used to study the effect of antiparkinsonian drugs and neurotoxins on striatal, extraneuronal dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations which are a measure of dopamine (DA) release/DA metabolism and serotonin (5-HT) release, respectively. 2. The DA precursor dihydroxyphenylalanine (DOPA, i.p.) increased extraneuronal DOPAC and reduced 5-HIAA levels whereas the opposite effect was induced by the 5-HT precursor 5-hydroxytryptophan (5-HTP, i.p.). Tryptophan, i.p., decreased the extraneuronal DOPAC levels without significant effect on 5-HT release. 3. The monoamine oxidase (MAO) inhibitors pargyline, i.p., and deprenyl, i.p., as well as the DA agonist apomorphine, i.p., decreased the catechol signal. The DA antagonist haloperidol, i.p., increased extraneuronal DOPAC. 4. In longterm studies unilateral application of the neurotoxins 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetra-hydroxypyridine (MPTP), and 1-methyl-4-phenylpyridinium cation (MPP+) into the substantia nigra pars compacta abolished the DOPAC signal in the striatum at the lesioned side. This effect can be partially or fully restored by DOPA depending on the time elapsed after neurotoxin administration. 5. In accordance with the voltammetric recorded unilateral lesion of the dopaminergic system the apomorphine stimulated circling behaviour was significantly enhanced in MPTP and MPP+ treated rats as compared with controls. 6. The results obtained indicate that antiparkisonian drugs and neurotoxins besides their effect on total catecholamine and 5-HT concentrations change specifically the extraneuronal levels of the transmitter (metabolites). Moreover the results suggest that neurotoxin-treated rats can be used as a model to study Parkinson-like effects with regard to the pathogenesis and treatment of this disease.

A neurochemical and behavioral investigation of the involvement of nucleus accumbens dopamine in instrumental avoidance

Two experiments were conducted to investigate the role of nucleus accumbens dopamine in the performance of a lever press avoidance task in the rat. In this task, 0.5 mA shock was presented for 5 s every 30 s, but the rat could escape shock presentation, or avoid the shock for 30 s, by pressing a lever. In the first experiment, dialysis probes were implanted into the nucleus accumbens of rats previously trained on the avoidance procedure, and also two groups of untrained rats. On the day after dialysis probe implantation, rats trained to press a lever to avoid shock (n = 10) were exposed to a 45 min avoidance test session. Untrained rats were either exposed to periodic shock (n = 9) or the control procedure in which lights were dimmed but no shock was presented (n = 8). Performance of the avoidance task led to significant increases in extracellular dopamine and dopamine metabolites. There was a significant positive correlation between increases in extracellular dopamine and the number of avoidance responses emitted. In the second experiment, groups of rats were trained on the lever press avoidance procedure. After training, rats received intra-accumbens injections of the neurotoxic agent 6-hydroxydopamine or ascorbic acid vehicle. Dopamine depletion produced by 6-hydroxydopamine injection led to a substantial decrease in lever pressing to avoid or escape shock. These results indicate that dopamine in nucleus accumbens is important for operant avoidance responding, and that the involvement of accumbens dopamine in instrumental behavior is not unique to positively reinforced responses.

Higher and longer stress-induced increase in dopamine concentrations in the nucleus accumbens of animals predisposed to amphetamine self-administration. A microdialysis study

Individual vulnerability to the reinforcing effects of drugs appears to be a crucial factor in the development of addiction in humans. In the rat, individuals at risk for psychostimulant self-administration (SA) may be identified from their locomotor reactivity to a stress situation such as exposure to a novel environment. Animals with high locomotor responses to novelty (high responders, HR) acquire amphetamine SA, while animals with low responses (low responders, LR) do not. In this study we examined by microdialysis whether stress-induced extracellular dopamine (DA) concentrations in the nucleus accumbens differed between these two groups of animals. This neurotransmitter was studied because it is thought to be involved in the reinforcing effects of psychostimulants. Furthermore, previous studies have shown that HR animals have a higher basal DOPAC/DA ratio in the nucleus accumbens and higher extracellular concentrations of dopamine in this structure in response to cocaine. The stress procedure used in this experiment consisted of a 10 min tail-pinch. HR animals displayed a higher and longer stress-induced changes in DA concentrations than the LR group. Regression analysis showed that stress-induced changes in DA levels accounted for 75% of the variance observed in the locomotor response to a novel environment. Since higher DA activity in the nucleus accumbens has been reported in animals in which the propensity to psychostimulant SA is induced by brain lesions or life events, this biochemical modification may be one neurobiological substrate of the predisposition to acquire psychostimulant self-administration.

Effects of intracisternal vs. intrahypothalamic 5,7-DHT on feeding elicited by hypothalamic infusion of NE

A variety of evidence has led to suggestions that brain serotonin (5-HT) and norepinephrine (NE) interact within the medial hypothalamus to control food intake. To test the possibility that chronic decrements in 5-HT might enhance NE-induced feeding, adult male rats were prepared with permanently indwelling cannulae aimed at the paraventricular nucleus (PVN), then received either intracisternal (IC) or PVN injections of the 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) vs. its vehicle, 1% ascorbic acid. Over a 4-week period, IC-5,7-DHT rats showed no signs of enhanced daily feeding or drinking. However, in 40-min intake tests, feeding but not drinking was enhanced by injecting 20 nmol NE into the PVN commencing 2 weeks after neurotoxin treatment. Terminal monoamine assays confirmed that IC-5,7-DHT produced large (80-90%) depletions of brain regional 5-HT. A functional index of 5-HT terminal damage was also implied by the impaired short-term feeding responses IC-5,7-DHT rats showed to the systemic administration of the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) when tested between 3 and 4 weeks after IC treatment. Over a comparable 4-week period, PVN-5,7-DHT rats also showed no tendencies to overeat or overdrink on a daily basis. However, in contrast to IC-5,7-DHT rats, they also showed no differences in their feeding or drinking responses to NE injections into the PVN. This was so despite reliable depletions of 5-HT in the hypothalamus (-28%) and hippocampus (-71%). These results support earlier work showing that neither widespread nor localized hypothalamic damage to brain 5-HT neurons produce chronic overeating. However, the data suggest that phasic enhancements of PVN NE activity may trigger enhanced feeding when there is widespread damage to brain 5-HT neurons, although the PVN does not appear to be the brain site mediating this effect.

Involvement of nucleus accumbens dopamine in the motor activity induced by periodic food presentation: a microdialysis and behavioral study

Two experiments were undertaken to investigate the role of accumbens dopamine (DA) in food-related motor activities. Although presentation of large amounts of food elicits feeding behavior, periodic food presentation (PFP; e.g. a 45-mg pellet every 45 s) induces considerable locomotion, rearing and other motor activities in food-deprived rats. In the first experiment, in vivo microdialysis methods were used to study DA release and metabolism in the nucleus accumbens of behaving rats exposed to periodic food presentation. Four behavioral conditions were used: high rate of PFP (one pellet per 45 s), low rate of PFP (one pellet per 4 min), massed food presentation and food deprivation control. The rats that received a high rate of PFP showed substantial increases in locomotor activity, and also showed significant increases in extracellular DA and DA metabolites. Rats that received massed presentation of food pellets consumed large quantities of food, but showed no significant increases in locomotor activity or DA release. Although the group that received the high rate of PFP showed the highest motor activity and the largest increase in DA release, there was only a modest correlation (r = 0.34) between motor activity and increased DA release. In the second experiment, the neurotoxic agent 6-hydroxydopamine (6-OHDA) was injected into the nucleus accumbens in order to assess the effects of DA depletion of PFP-induced motor activity. DA depletion significantly reduced PFP-induced motor activity in the first week after surgery, but by the second week DA-depleted rats had recovered normal levels of motor activity.(ABSTRACT TRUNCATED AT 250 WORDS)

High-speed chronoamperometric measurements of mesolimbic and nigrostriatal dopamine release associated with repeated daily stress

The effects of repeated, once daily exposure to either restraint or tail pinch stress on extracellular levels of dopamine in nucleus accumbens and striatum were electrochemically monitored in conscious rats using high-speed chronoamperometry. Acutely, both tail pinch and restraint increased extracellular dopamine levels in both regions. However, the effect of restraint on mesolimbic and, to some extent, also on nigrostriatal dopamine neurotransmission increased progressively with each daily exposure. While increases in extracellular dopamine elicited by tail pinch varied across test days, no reliable daily enhancement of electrochemical responses to this stress were observed in either of the regions studied. Pretreatment with dopamine autoreceptor-specific doses of apomorphine (50 and 100 micrograms/kg s.c.) potently inhibited stress-elicited responses in nucleus accumbens, indicating that dopamine was the primary electroactive species contributing to the electrochemical signal. The results of this study indicate that the magnitude of stress-elicited increases in levels of extracellular dopamine is determined by the number of previous exposures to stress and are consistent with reports of sensitization to the behavioral effects of stress with repeated testing. The study also provides pharmacological data that are consistent with electrophysiological evidence of increased mesolimbic dopamine cell firing during exposure to stress.

Regional differences in the regulation of dopamine and noradrenaline release in medial frontal cortex, nucleus accumbens and caudate-putamen: a microdialysis study in the rat

Dopamine (DA) and noradrenaline (NA) extracellular levels have been measured by microdialysis in the medial frontal cortex (MFC), nucleus accumbens (NAc) and caudate-putamen (CP) under baseline conditions in awake and halothane-anaesthetized rats, and after application of three types of stimuli which are likely to activate the brainstem catecholaminergic systems: mild stressors (handling and tail pinch), rewarded behavior (eating palatable food without prior food deprivation) and electrical stimulation of the lateral habenular nucleus. Changes were studied with and without uptake blockade (10 microM nomifensine in the perfusion fluid). The influence of calcium concentration (1.2 or 2.3 mM in the perfusion fluid) on DA and NA overflow was tested in some cases. Handling and tail pinch stimulated both DA and NA overflow in MFC, and enhanced NA overflow in NAc. By contrast, these mildly stressful stimuli had only marginal effects on DA overflow in NAc and no effects on either DA or NA overflow in CP. Eating behavior was accompanied by increased DA and NA overflow in MFC but had no effect in NAc. These regional differences were similar also when the manipulations were applied under uptake blockade, which indicates that the more pronounced changes seen in MFC did not simply reflect a more sparse innervation (i.e. lower density of uptake sites) in the MFC compared to the more densely innervated NAc and CP areas. Stimulation of the lateral habenula induced a 2-3-fold increase in NA overflow in both MFC, NAc and CP but had no consistent effect on DA overflow in any region. The effect on NA release was abolished by a transection of the ipsilateral fasciculus retroflexus (which carries the efferent output of the lateral habenula). The results show that the forebrain DA and NA projections to cortical and striatal targets are differentially regulated during ongoing behavior, that the mesocortical and mesostriatal DA systems respond quite differently to stressful and rewarding stimuli; and that the NA projection to MFC (like the dopaminergic one) is more responsive to stressful and rewarding stimuli than the ones innervating the striatum (NAc and CP). The results support the view that environmental stimuli evoking emotional arousal (whether aversive or non-aversive) are accompanied by increased DA and NA release above all in the MFC and only to a minor extent in limbic and striatal areas.

Evidence for two different afferent pathways carrying stress-related information (noxious and amygdala stimulation) to the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) is an important nucleus involved in mediating amygdala-regulated endocrine effects. Since the amygdala is important in mediating the endocrine response to noxious somatosensory stimuli and olfactory stimulation, this experiment studies whether noxious input (tail pinch, TP) and stress-related input (amygdala stimulation, AmygS) will modulate BNST neuronal activity. One hundred and fifty-eight BNST neurons were studied following AmygS, TP and cutaneous stroke. AmygS was effective in 66% of BNST neurons and produced one of the following five responses: oligosynaptic excitation (43%), polysynaptic excitation (5%), time-locked inhibition (4%), generalized increase in firing rate (8%), or generalized decrease in firing rate (6%). TP produced an increase in firing rate in 27% of BNST neurons tested. Analysis of a contingency table constructed to determine the degree of correspondence between neurons responsive to AmygS and neurons responsive to TP showed that the distributions of reactivity to these stimuli in BNST neurons are independent of each other. This suggests that although AmygS and TP are both capable of altering the firing rate of BNST neurons, the pathways by which they reach BNST differ.

Complex motor and sensorimotor functions of striatal and accumbens dopamine: involvement in instrumental behavior processes

The suggestions that dopamine (DA) systems are involved in "motor control" and "reward" represent the classic working hypotheses on the behavioral functions of these systems. The research generated by these hypotheses has yielded results that are far more complicated than the simplest form of either hypothesis would indicate. Pharmacological or lesion-induced interference with DA function does not suppress all aspects of movement control, nor all aspects of reward, nor all aspects of motivation. The deficits produced by interference with DA systems are selective and dissociative in nature, affecting some aspects of motor or motivational function, but leaving others basically intact. In some sense the hypotheses that DA is involved in "motor" or "reward" or "motivational" processes are partly correct, but the processes to which these terms refer are too broad to offer an accurate and detailed description of the behavioral functions of brain DA. A review of the literature on the behavioral pharmacology of DA suggests that the behaviors most easily disrupted by DA antagonists are highly activated and complex learned instrumental responses that are elicited or supported by mild conditioned stimuli, and maintained for considerable periods of time. It is proposed that DA in accumbens and striatum modulates the ability of neocortical and limbic areas involved in sensory, associative, and affective processes to influence complex aspects of motor function, and also modulates the execution of complex motor acts organized by the neocortex. Thus, interference with DA systems produces a "subcortical apraxia", which dissociates complex stimulus processes from complex motor processes, but leaves aspects of those processes intact.

Anxiogenic drugs beta-CCE and FG 7142 increase extracellular dopamine levels in nucleus accumbens

Two experiments were conducted to study the effects of anxiogenic drugs on dopamine release and metabolism in nucleus accumbens. Microdialysis probes were implanted into the nucleus accumbens, and rats were tested the day after implantation. In the first experiment, groups of rats received injections of saline, 1.25 or 2.5 mg/kg beta-CCE. In the second experiment, groups of rats received injections of saline, 10.0, 20.0 or 30.0 mg/kg FG-7142. Both drugs produced significant increases in dopamine release and metabolism in nucleus accumbens. Neither drug had significant effects on locomotor activity. These experiments indicate that exposure to anxiogenic drugs increases accumbens dopamine activity, an effect that is consistent with other studies showing that the mesolimbic dopamine system is responsive to stressful stimuli. In addition, these results demonstrate that drug-induced increases in accumbens dopamine release are not unique to drugs of abuse.

Desmethylimipramine promotes recovery of self-stimulation from the prefrontal cortex following footshock

Intracranial self-stimulation (ICSS) was assessed from the prefrontal cortex in CD-1 mice immediately (0 h), 24 h and 168 h following exposure to uncontrollable footshock. Marked reductions in ICSS rates were observed in all mice immediately following the stressor. Although the ICSS alterations were transient in some animals, ICSS rates were reduced in the majority of animals 24-h and 168-h poststressor. Mice of either the shock or no shock treatment groups were administered either saline or desmethylimipramine (DMI, 5 mg/kg x 2) for 20 consecutive days. Chronic DMI ameliorated the stressor-induced ICSS deficits from the prefrontal cortex. Potential explanations for the stressor-provoked variations in ICSS and the effects of DMI are discussed.

Individual differences in behavioral measures: correlations with nucleus accumbens dopamine measured by microdialysis

Rats were placed in one of two novel test environments for behavioral observation. In one, exploratory behavior (assessed by hole pokes) and locomotion were assessed during a 10-min test session. In the other, the chewing of varied objects on the cage floor was rated over a 20-min session. Within 2-18 days, animals were anesthetized and microdialysis probes were implanted into the nucleus accumbens for measurement of basal and d-amphetamine-stimulated levels of dopamine (DA). These measures were then correlated with the individual behavioral rating collected earlier from the drug-free animals. We found a significant correlation between duration of exploratory behavior and amphetamine-induced DA release. Locomotor activity did not correlated with either basal or amphetamine-stimulated DA release. Duration of chewing episodes correlated with basal levels of DA, as well as with amphetamine-induced DA release. Our studies indicate that differences in the dopaminergic responsivity of the nucleus accumbens (or other circuitry influencing nucleus accumbens DA function) may contribute to individual differences in certain behaviors displayed by the animals when placed in a novel environment.

Sex-related olfactory stimuli induce a selective increase in dopamine release in the nucleus accumbens of male rats. A voltammetric study

Changes in the dopaminergic (DA) transmission in the nucleus accumbens were investigated in male rats exposed to sociosexual olfactory stimuli from different conspecifics: receptive female, non-receptive female and intact male. DAergic transmission was assessed by measurement of extracellular levels of DA and dihydroxyphenylacetic acid (DOPAC). Both compounds were recorded by using differential normal pulse voltammetry (DNPV) with electrochemically pretreated carbon fiber electrodes and numerical analysis of the catechol peak. Exposition to receptive female odors induced a marked and selective increase in DA release compared to control values. Exposition to non-receptive female odors and male odors induced an increase in DA release not significantly different from that following the change of environment. In conclusion, mesencephalic DAergic neurons reaching the nucleus accumbens appear to be involved in the perception of behaviorally significant olfactory cues.