In vivo modulation of acetylcholine in the nucleus accumbens of freely moving rats: I. Inhibition by serotonin

Microdialysis was used to characterize the effect of serotonergic input on cholinergic interneurons in the nucleus accumbens (NAC) of freely moving rats. Local infusion of 5-hydroxytryptamine (5-HT) or the serotonin reuptake blocker fluoxetine significantly decreased extracellular acetylcholine (ACh) in the NAC. This decrease in ACh was blocked by the 5-HT₁ (and β-adrenergic) antagonist propranolol. To test β-adrenergic effects, it was found that a β-adrenergic agonist isoproterenol had no measurable effect on extracellular ACh in the NAC. This suggests that 5-HT inhibits ACh interneurons via one of the 5-HT₁ receptor types. The 5-HT_1A agonist 8-OH-DPAT given systematically again decreased extracellular levels of ACh, and the effect was dose-dependent. The 5-HT_1A effect was probably exerted in the NAC, because local infusion of 8-OH-DPAT mimicked systemic injections. These microdialysis results are similar to in vitro studies which suggest an inhibitory impact of 5-HT on ACh release in basal ganglia slices and homogenates. The decrease in extracellular ACh as measured in vivo is apparently mediated, at least in part, through a 5-HT_1A receptor in the accumbens. Given the role of the NAC in behavior reinforcement, this 5HT-ACh interaction may be involved in serotonergic treatment of depression.     

Caudate-putamen and nucleus accumbens extracellular acetylcholine responses to methamphetamine binges

Exposure of experimental animals to an escalating dose, multiple binge pattern of methamphetamine administration results in the progressive emergence of a unique behavioral profile, which includes a significant decrease in the duration of the stereotypy phase as well as a profound increase and qualitative change in the locomotor activation. This behavioral profile is associated with a selective decrease in the caudate-putamen but not nucleus accumbens extracellular dopamine response. Since the acetylcholine interneurons in these regions are partly under the control of the mesostriatal and mesoaccumbens dopamine inputs, changes in the activation of these interneurons should parallel the regionally differential dopamine responses during multiple binge treatment. Therefore, we characterized the caudate-putamen and nucleus accumbens extracellular acetylcholine responses to escalating-dose, multiple binge methamphetamine administration. An acute methamphetamine binge decreased acetylcholine levels in caudate-putamen, but had no effect on levels in nucleus accumbens. Furthermore, corresponding to the selective decrease in the dopamine response, the caudate-putamen but not nucleus accumbens extracellular acetylcholine response exhibited tolerance with repeated binge exposures; i.e. the decrease in acetylcholine associated with the acute methamphetamine binge was attenuated with multiple binge exposure. These results are consistent with our hypothesis and suggest that the regionally differential acetylcholine responses reflect one functional consequence of the escalating-dose, multiple binge stimulant treatment.     

Colocalization of CART peptide with prodynorphin and dopamine D1 receptors in the rat nucleus accumbens

CART peptide is a peptidergic neurotransmitter that is expressed in brain regions involved in critical biological processes such as feeding and stress, and in areas associated with drug reward and abuse including the dopamine-rich nucleus accumbens (NAcc), which can be considered part of the basal ganglia. Because CART has been shown to colocalize with substance P, a marker of the basal ganglia direct pathway, we now test for colocalization with other markers of the direct pathway to determine if CART colocalizes with dynorphin and dopamine D1 receptors. In the NAcc, CART peptide immunoreactivity (IR) was colocalized with prodynorphin-IR in neurons. Approximately 80.1% of CART-IR cells colocalized with prodynorphin-IR, while only 27.6% of prodynorphin-IR neurons contained CART-IR, suggesting that CART cells are a subset of dynorphin cells. In contrast, only about 25% of CART-IR cell bodies demonstrated dopamine D1 receptor-IR. Because dynorphin and D1 receptors are markers for the basal ganglia direct pathway, from the NAcc to the basal ganglia output nuclei, and because CART significantly colocalizes with these markers, some CART neurons are part of the direct pathway or some comparable pathway in the accumbens. The presence of CART in NAcc neurons and the fact that NAcc projection neurons have extensive local collaterals suggest that CART may have effects in both terminal and cell body regions of the accumbens and may therefore affect information processing in the NAcc by modulating accumbal neurons.     

Melatonin acts on the nucleus accumbens to increase acetylcholine release and modify the motor activity pattern of rats

Brain microdialysis coupled to high performance liquid chromatography with electrochemical detection (HPLC–ED) was used to evaluate the influence of melatonin on extracellular concentration of acetylcholine (ACh) in the nucleus accumbens (NAc) of rats. Motor activity was simultaneously monitored during the dialysis sessions with an activity meter. Melatonin and prazosin were administered locally through the dialysis probe. It was found that melatonin dose-dependently increased accumbens ACh. Melatonin (3 μM) decreased horizontal activity and increased vertical activity, while another dose (100 μM) enhanced both horizontal and vertical activity. Prazosin, a putative melatonin antagonist, blocked the effects of melatonin on both motor activity and ACh release when given 20 min before melatonin. Overall, these results suggest that melatonin modulates the release of ACh in the NAc and the pattern of motor activity in the rat.     

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)     

Dissociations between the effects of intra-accumbens administration of amphetamine and exposure to a novel environment on accumbens dopamine and cortical acetylcholine release

Previous research has demonstrated an interaction between the effects of amphetamine and exposure to a novel environment on the activity of neurons in the nucleus accumbens. Given a model in which these accumbens efferents gate the excitability of basal forebrain cholinergic corticopetal neurons, the administration of intra-accumbens amphetamine was hypothesized to potentiate the increase in cortical acetylcholine produced by introduction to a novel environment. Dual probe microdialysis revealed no synergistic interactions between exposure to a novel environment and amphetamine on nucleus accumbens dopamine or cortical acetylcholine efflux. This finding indicates that exposure to a novel environment failed to recruit the telencephalic activation of the nucleus accumbens presumably necessary to reveal modulatory effects of accumbens dopaminergic transmission on cortical acetylcholine release.     

Cocaine administration increases the fraction of CART cells in the rat nucleus accumbens that co-immunostain for c-Fos

In order to further test whether or not psychostimulant drugs activate CART peptide-containing cells in the nucleus accumbens, we examined the fraction of CART positive cells that co-immunostained for c-Fos after administration of saline or cocaine (10 and 25 mg/kg i.p.). There was about a 45% increase in the fraction of cells that stained for both CART and c-Fos after administration of cocaine, but there was no change in the fraction after administration of saline. Moreover, the increase was not found 24h after injection and is therefore reversible. These results support the notion that psychostimulant drugs activate CART cells in the nucleus accumbens, even under conditions where it is difficult to show a change in CART levels.     

Atropine decreases drinking but not feeding and induces less hypothalamic acetylcholine release in diabetic rats

Drinking, feeding and hypothalamic extracellular acetylcholine (ACh) release was measured before and after the administration of several doses of atropine sulfate in streptozotocin (STZ)-diabetic and normal rats. Drinking but not feeding was dose-relatedly decreased by i.p. or intrahypothalamic injections of atropine in STZ-diabetic rats. Hypothalamic ACh release, as measured by microdialysis, increased less (dose-related) in diabetic than normal rats following an i.p. administration of atropine. Ach basal levels were the same in both groups. These results are discussed in terms of a hyperactive hypothalamic cholinergic (muscarinic) system involved in the diabetic polydipsia.     

Microdialysis evidence that acetylcholine in the nucleus accumhens is involved in morphine withdrawal and its treatment with clonidine

This study used microdialysis to measure changes in extracellular acetylcholine (ACh) content in the nucleus accumbens (NAC) of freely moving rats during acute and chronic morphine treatment, and following naloxone-precipitated withdrawal. Morphine injection (20 mg/kg, i.p.) caused a significant decrease in extracellular ACh which was not apparent after repeated exposure to the opiate for 7 days. Basal recovery of ACh was not altered by chronic morphine treatment. On day 8, after morphine dependence had been established, naloxone caused a large increase in ACh levels accompanied by withdrawal symptoms such as wet dog shakes, diarrhea and teeth-chattering. Pretreatment with clonidine (200 μg/kg, i.p.) reduced these withdrawal symptoms and eliminated the ACh response. These results suggest that accumbens ACh is involved in some of the aversive aspects of opiate withdrawal.     

Response to novelty predicts the locomotor and nucleus accumbens dopamine response to cocaine.

The relationship between a rat's locomotor response to a novel environment and its behavioral and dopaminergic responses to cocaine was examined. Subjects were divided into two groups based on their locomotor response to a novel environment. Subjects who had a novelty response above the median were classified as high responders (HR), while those with a novelty response below the median were classified as low responders (LR). Following administration of cocaine-HCl (0, 2.5, 5.0, 10.0, or 15.0 mg/kg), HR rats showed a greater locomotor response than LR rats. Moreover, there was a significant correlation between a subject's locomotor response to the novel environment and the locomotor response to either 10.0 (r = 0.65) or 15.0 (r = 0.92) mg/kg cocaine. In a separate experiment, the extracellular concentration of dopamine in the nucleus accumbens (NACC) was monitored using microdialysis procedures. Following cocaine administration (15.0 mg/kg) HR rats showed a larger NACC dopamine response and greater locomotor activity than LR rats. In addition, there was a threefold greater locomotor activity to dopamine ratio in HR rats than in LR rats. A correlation between a subject's locomotor response to a novel environment and the dopaminergic response to cocaine was also evident. These results suggest that differences in the locomotor response to cocaine can, to some degree, be predicted by a rat's locomotor response to a novel environment, and that variations in dopamine-dependent mechanisms of the NACC may underlie these individual differences.     

Dopamine regulation of extracellular glutamate in the nucleus accumbens

The capacity of dopamine to alter extracellular glutamate in the nucleus accumbens was examined by passing 1, 10 and 100 μM of amphetamine, the D_2/3 agonist, quinpirole, or the D₁ agonist, SKF-82958 through a microdialysis probe. It was found that amphetamine and quinpirole produced a dose-dependent reduction in the basal levels of extracellular glutamate, while SKF-82958 was without significant effect. The capacity of the D₁ antagonist, SCH-23390 (1.0 mg/kg, i.p.) or the D₂ antagonist, sulpiride (10 mg/kg, i.p.) to block the reduction in extracellular glutamate by amphetamine (100 μM) was examined. Both SCH-23390 and sulpiride prevented the reduction in extracellular glutamate by amphetamine. The data indicate that, similar to the striatum, glutamate release in the nucleus accumbens is modulated by presynaptic dopamine receptors.     

Tetrodotoxin-dependent glutamate release in the rat nucleus accumbens during concurrent presentation of appetitive and conditioned aversive stimuli

In vivo microdialysis combined with a high-performance liquid chromatography was used to monitor extracellular glutamate (GLU) levels in the nucleus accumbens (N.Acc) of Sprague-Dawley rats during their behavioral responses to the concurrent presentation of appetitive and conditioned aversive stimuli. The presentation of a highly palatable diet followed by a tone previously paired with footshock to rats trained to take a pellet of the diet under these experimental conditions resulted in a marked and short lasting increase in extracellular glutamate, whereas the tone alone had no effect. A similar increase of the glutamate release was observed during the presentation of a piece of rubber instead of the diet. In both cases, the increase in extracellular glutamate was completely prevented by intra-accumbal infusions through the dialysis probe of 1 μM tetrodotoxin (a voltage-dependent Na⁺ channel blocker), whereas (S)-4-carboxyphenylglycine (a cystine/glutamate exchange blocker, 5 μM) had no effect.

The data obtained suggest that behavioral responses to unpredicted change in motivational value of expected reward appear to be associated with an increase of the extracellular glutamate level in the nucleus accumbens, and impulse-dependent synaptic release, rather than non-vesicular glutamate release via cystine/glutamate exchange, is responsible for this phenomenon.     

Spontaneous and graft-induced behavioral recovery after 6-hydroxydopamine lesion of the nucleus accumbens in the rat

In the present study the long-term evolution of behavioral deficits following a local lesion of the dopaminergic innervation of the nucleus accumbens with 6-hydroxydopamine (6-OHDA) was compared in two groups of rats: lesioned animals and animals bearing a dopaminergic implant in the nucleus accumbens. Lesioned animals gradually recovered on various behavioral tests (amphetamine-induced locomotion, exploration, hoarding) and were indistinguishable from the control group on most parameters by 10 months postlesion. The deficits were, however, reinstated by a second intra-accumbens 6-OHDA lesion, a finding which suggests a role for dopaminergic reinnervation in the observed recovery. Conversely, grafted animals still displayed marked deficits even 10 months after grafting, although the lesioned areas were well reinnervated by the graft. These results indicate that the graft, while being unable on its own to compensate for part of the deficits, can nevertheless impair and compete with endogenous processes leading to behavioral recovery following a local lesion.     

Blockade of GABAA receptors in the medial ventral pallidum elicits feeding in satiated rats

γ-aminobutyric acid (GABA)-containing fibers from the nucleus accumbens shell (AcbSh) terminate in the medial ventral pallidum (VPm) and neurons in the VPm project to the lateral hypothalamus (LH). Therefore, the VPm is anatomically interposed between the AcbSh and LH, two functionally related brain regions that mediate food intake. The present study demonstrates that blockade of GABA_A receptors in the VPm by local administration of bicuculline greatly increases food intake in satiated rats. The data suggest that an AcbSh-VPm-LH circuit may be involved in the control of feeding behavior.     

Cholecystokinin-induced release of dopamine in the nucleus accumbens of the spontaneously hypertensive rat

Changes in dopamine neurotransmission in the nucleus accumbens of the spontaneously hypertensive rat (SHR) may be involved in the pathogenesis of hypertension. This investigation tested the hypothesis that the sulfated octapeptide cholecystokinin (CCK8S) induced release of dopamine is greater in the SHR than in its normotensive control, the Wistar-Kyoto rat (WKY). Dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were sampled using microdialysis in the caudal half of the nucleus accumbens of 10-week-old anesthetized SHRs and WKYs. Samples were collected in the following order: 3 baseline, 3 CCK8S (10 μmol/l), and 3 postdrug samples. The samples were then analyzed using high pressure liquid chromatography with electrochemical detection. CCK8S increased dopamine and DOPAC levels in both the SHR and WKY with a larger increase in basal dopamine in the SHR (greater than 200%). Perfusion of the nucleus accumbens with 1 μmol/l of CCK8S or the nonsulfated form of CCK8 (CCK8US, 10 μmol/l) produced no significant increase in the release of dopamine in the SHR. These results indicate that CCK8S-induced release of dopamine in the nucleus accumbens is greater in the SHR. Changes in CCK8S neurotransmission/receptor function may be responsible for the alterations in dopaminergic function of the SHR and the pathogenesis of hypertension.     

Reward, memory and substance abuse: functional neuronal circuits in the nucleus accumbens

The firing patterns of neurons in the nucleus accumbens (NA) are examined and discussed with respect to different types of rewards and reward conditions. Comparisons and contrasts between individually identified NA neuron responses to cocaine self-administration and water reinforcement are presented with an emphasis on the fact that the same neurons do not respond in a phasic manner to both types of rewards. However, the phasic firing patterns, even though segregated for each reinforcer, are quite similar, suggesting that the method of differentiation between rewarding stimuli in the NA is by sorting cell populations into distinct ensembles or networks for each type of reinforcer. These neural networks appear to be ‘tuned’ to respond to particular associative behavioral contexts that couple response execution to reward delivery, and in the process acquire a reciprocity to firing within reward contexts. This maintains the specificity of each reinforcer for the response and associated stimuli that produce it and, makes it possible to attach different NA networks to different reinforcing circumstances. Comparisons of cocaine and water reinforced NA cell firing patterns during rapid switching between these two reinforcers suggests that the networks are negatively coupled and mutually inhibit each other to maintain accurate encoding of immediately experienced, as well as expected (i.e. future) reward contingencies.     

N-Methyl-d-aspartic acid-induced lesions of the nucleus accumbens and/or ventral pallidum fail to attenuate lateral hypothalamic self-stimulation reward

The role of ventral striatum in the maintenance and transmission of a hypothalamic intracranial self-stimulation (ICSS) reward signal was investigated using the rate-frequency multiple-curve paradigm. The excitotoxinN-methyl-d-aspartic acid (NMDA) was bilaterally administered into the nucleus accumbens (15 μg per side), the ventral pallidum (15 μg per side) or the juncture between the two structures (20 μg per side) creating three lesion groups. Both the nucleus accumbens (NAC) lesion group and the ventral pallidum (VP) lesion group displayed substantial NMDA-induced damage which was generally restricted to the intended limbic structure. The NMDA lesions in the third group displayed extensive damage to both the NAC and VP, as intended, but also typically diffused into adjacent medial structures. NMDA-induced lesions in all groups caused a suppression in motor/performance activity at all currents tested. Contrary to motor effects, reward efficacy was relatively unaffected for the NAC and VP groups. The lack of reward effects may be due to plasticity of neuronal systems and redundancy of circuit connections. However, this explanation is questionable given the fact that NMDA lesions which encompassed both the NAC and VP had little effect on reward efficacy. The above data suggests that the nucleus accumbens and the ventral pallidum are not critical for ICSS rewards stimulation and that hypothalamic ICSS reward signals are processed downstream from these limbic structures.     

Purinergic modulation of extracellular glutamate levels in the nucleus accumbens in vivo

In the present study, the P2 receptor-mediated modulation of the extracellular glutamate concentration was investigated by microdialysis in the nucleus accumbens (NAc) of freely moving rats. Because of the known interference of dopaminergic and glutamatergic mechanisms in this area the experiments were performed with animals intra-accumbally treated with 6-hydroxydopamine (6-OHDA) to deplete dopamine pools. Perfusion of the NAc with the prototypic P2 receptor agonist 2-methylthioadenosine 5'-triphosphate (2-MeSATP, 0.1, 1 and 10mM) concentration-dependently increased the extracellular level of glutamate in this area. Pretreatment with the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 0.1mM) decreased the basal extracellular glutamate concentration and inhibited the 2-MeSATP-induced outflow of glutamate. In rats treated with 6-OHDA, 2-MeSATP increased the total extracellular glutamate to an extent about fivefold larger than in sham-lesioned rats. The perfusion of the dopamine-depleted NAc with the D(2)/D(3) dopamine receptor agonist quinpirole (0.1mM) diminished the basal concentration of glutamate and reduced the effect of 2-MeSATP on the extracellular glutamate. These results provide evidence that the stimulation of P2 receptors is involved in the increase of accumbal extracellular glutamate in vivo. This behaviourally relevant mechanism depends on a dopamine D(2) receptor-mediated tone in the nucleus accumbens. Furthermore, the inhibition of P2 receptors may prevent, at least partly, glutamate-mediated neurodegeneration.     

海洛因成瘾大鼠伏隔核毁损术前后觅药行为及多巴胺神经递质的变化

目的研究海洛因成瘾大鼠毁损伏隔核前后觅药行为及多巴胺神经递质的变化。方法建立海洛因成瘾大鼠模型，毁损大鼠双侧伏隔核，利用条件性位置偏好实验测定成瘾前、后，术前、术后成瘾大鼠觅药行为的变化，利用高效液相方法测定边缘系统多巴胺神经递质的变化。结果毁损大鼠双侧伏隔核能够完全消除条件性地点偏好。成瘾大鼠边缘系统多巴胺含量较对照组明显增高。毁损前后大鼠边缘系统多巴胺含量无明显变化。结论伏隔核是调节强化作用的重要位置，长期使用海洛因可使大鼠边缘系统多巴胺含量明显增高，但毁损前后无明显变化，与条件性位置偏好表现不同步。     

Dexmedetomidine decreases extracellular dopamine concentrations in the rat nucleus accumbens

The effects of dexmedetomidine, a highly selective alpha(2)-adrenoceptor agonist, on extracellular dopamine (DA) concentrations in the nucleus accumbens of awake rats were collected via in vivo cerebral microdialysis and measured using HPLC with electrochemical detection. The administration of dexmedetomidine (DEX) at a low dose (2 microg/kg bolus i.v. over 2 min followed by a continuous infusion of 0.1 microg/kg per min) and a high dose (20 microg/kg bolus i.v. over 2 min followed by a continuous infusion of 1 microg/kg per min), significantly decreased extracellular dopamine concentrations in the nucleus accumbens. The observed decrease was dose-dependent, occurring sooner and to a greater magnitude in the rats receiving a high dose of DEX. This inhibitory modulation of accumbal dopamine was receptor-specific, as the decrease in extracellular DA produced by DEX was no longer evident following pre-treatment and co-infusion with the highly selective alpha(2)-adrenoceptor antagonist, atipamezole (ATZ). Thus, these data suggest that adrenoceptor agonists and antagonists may modulate dopaminergic neurotransmission via mechanisms that are specific to the alpha(2)-adrenoceptor.