Electrophysiological evidence of modulatory interaction between dopamine and cholecystokinin in the nucleus accumbens

Dopamine has been shown to modulate responses of accumbens neurones to excitatory inputs from the amygdala. The demonstration that cholecystokinin (CCK) co-exists and appears to be co-released with dopamine in the accumbens suggests that the modulatory action of dopamine in the accumbens may in turn be modified by CCK. This possibility was investigated in the present study. Single unit recordings were obtained in the medial and caudal accumbens of urethane-anaesthetized rats. These neurones were strongly excited by amygdala stimulation, and concurrent stimulation of the ventral tegmental area (VTA) at 10 Hz attenuated the responses, presumably due to dopamine release. Iontophoretic application of proglumide (PRG) at 30 nA enhanced the attenuating effect of VTA stimulation on the excitatory response to amygdala stimulation. Exogenous dopamine produced a similar attenuation in response and the attenuation was in turn suppressed by concurrent iontophoresis of sulphated CCK fragments applied at a current titrated not to produce significant effect on the spontaneous activity of the neurone nor its response to amygdala stimulation. These results demonstrate that exogenous and endogenous CCK can modify the postsynaptic action of dopamine in the nucleus accumbens in addition to modulating its release shown in other studies, and further suggests that CCK is likely an endogenous functional antagonist of dopamine, serving a comodulatory role in regulating synaptic transmission in the ventral striatum.     

CCK-8 modulates D2 receptor agonist-induced hypermotility in the nucleus accumbens

The influence of CCK-8 on locomotor effects associated with independent D2 receptor stimulation was studied. To selectively stimulate mesolimbic D2 receptors LY 171555 was injected into the nucleus accumbens of awake rats. Locomotor activity was measured in the open-field test. LY 171555 induced a biphasic effect: low doses stimulated, whereas higher doses inhibited locomotor activity. CCK-8 injected into the posteromedial part of the nucleus accumbens suppressed hyperlocomotion induced by LY 171555. The CCK-8 effect was prevented by the CCK-antagonist L 364,718. Our results indicate that CCK-8 modulates D2 receptor-mediated effects in the mesolimbic system.     

Differential effects of dopamine agonists on evoked dopamine release from slices of striatum and nucleus accumbens in rats

The effects of dopamine receptor agonists on electrically evoked dopamine release from slices of nucleus accumbens were compared with the effects on release from striatal slices in rats. Apomorphine, which has equal potency at the dopamine D₂ and D₃ receptors, reduced the evoked dopamine release from both regions to the same extent (ED₅₀, 0.42 μM for nucleus accumbens; ED₅₀, 0.46 μM for striatum). Quinpirole of 7-[³H]hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OHDPAT), which are much more potent at the D₃ receptor than at the D₂ receptor, reduced the evoked dopamine release from the nucleus accumbens (ED₅₀, 0.12 μM for quinpirole; 0.02 μM for 7-OHDPAT) much more than the release from the striatum (ED₅₀, 1.6 μM for quinpirole; 0.55 μM for 7-OHDPAT). These results suggest that the contribution of D₃ receptors in nucleus accumbens to regulate dopamine release from dopamine nerve terminals is much greater than that in striatum.     

Jaw muscle activity, the nucleus accumbens and dopaminergic agonists: A new approach

A method was developed to analyze electromyographic (EMG) signals in terms of power, viz., a measure for overall muscle activity, and number of seconds marked by distinct frequency ranges. With the help of this method, the effects of intraaccumbens administration of distilled water, the D1 receptor agonist SK&F 38393 (SKF; 5 micrograms), the D2 receptor agonist LY 171555 (LY; 10 micrograms), and their combination upon the EMG signals of the masseter and the digastric muscle were analysed in freely moving rats. Only the combined treatment affected the power: The noted increase was limited to the digastric muscle. The time/frequency analysis was limited to frequency ranges 3-4 Hz (class A), 4-5 and 5-6 Hz (class B), and 6-7, 7-8, ..., 12-13, and 13-14 Hz (class C). Apart from a small effect of SKF alone and of SKF in combination with LY on class B of the masseter muscle, neither SKF nor LY affected class A or B. SKF and LY increased and decreased, respectively, class C in both muscles. The data suggest that SKF and LY elicited both opposite and synergistic effects. The method is a new tool to analyze EMG signals in freely moving rats.     

Interactions of cholecystokinin octapeptide and dopamine on nucleus accumbens neurons

Local microiontophoretic administration of cholecystokinin octapeptide (CCK) increased the firing rates of neurons in the dorsomedial nucleus accumbens (NAc), but exerted little to no effect on lateral NAc neurons. This regionally defined CCK-effect corresponds to the topographical distribution of CCK-like immunoreactive nerve terminal fiber networks and CCK receptors within the NAc. The excitatory effects of CCK were selectively antagonized by the CCK antagonist proglumide. Dopamine (DA) decreased the firing of NAc cells and reversed CCK-induced excitation. These results suggest that CCK and DA may interact to influence the activity of neurons within the dorsomedial NAc.     

Serotonin 5-HT2C agonists selectively inhibit morphine-induced dopamine efflux in the nucleus accumbens

In vivo microdialysis was used to compare the effects of serotonergic drugs on morphine- and cocaine-induced increases in extracellular dopamine (DA) concentrations in the rat nucleus accumbens (NAc). Systemic administration of the 5-HT_2A/2C receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (2.5 mg/kg, s.c.) prevented the increase in extracellular DA in the NAc produced by morphine (5 mg/kg, i.p.). In contrast, this dose of DOI had no effect on the ability of cocaine (10 mg/kg, i.p.) to increase extracellular DA concentrations in the NAc. A preferential 5-HT_2C receptor agonist, 6-chloro-2-[1-piperazinyl]-pyrazine (MK-212, 5 mg/kg, s.c.) also inhibited morphine-induced increases in extracellular DA concentrations in the NAc. Pretreatment of rats with the selective 5-HT_2A antagonist, amperozide, had no effect on morphine-induced elevation of NAc DA concentrations. In order to determine if inhibition of the firing of 5-HT neurons contributes to the serotonin agonist-mediated inhibition of morphine-induced accumbens DA release, rats were pretreated with the 5-HT_1A agonist, 8-OHDPAT. At a dose of 100 μg/kg (s.c.), 8-OHDPAT did not interfere with morphine's ability to increase DA concentrations in the NAc. These results suggest that the activation of 5-HT_2C receptors selectively inhibits morphine-induced DA release in the NAc in a manner which is independent of the inhibition of 5-HT neurons.     

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。     

Distribution of amygdala input to the nucleus accumbens septi: An electrophysiological investigation

Summary The nucleus accumbens septi (NAS) receives afferent input from the amygdala via the stria terminalis and from the hippocampus via the fimbria. Extracellular recordings from 196 NAS neurons in halothane-anesthetized rats revealed heterogeneous response patterns following stimulation of the amygdala. The observation that 30% of anterior NAS units but only 16% of posterior NAS units were responsive to amygdala stimulation suggested a topographical arrangement of amygdala efferents. Comparing the effects of amygdala and fimbria stimulation revealed that the two afferent pathways converge onto individual NAS neurons, but that the two sites of stimulation can differentially influence other neurons. The present results clarify the topographical distribution of amygdala input to the NAS, confirm that inputs from two limbic structures are integrated within the NAS, and further illustrate the electrophysiological heterogeneity of NAS neurons.     

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.     

Prefrontal cortex and basolateral amygdala modulation of dopamine-mediated locomotion in the nucleus accumbens core

The prefrontal cortex (PFC) and the basolateral amygdala (BLA) play a critical role in the production of normal and abnormal goal-oriented behaviors. Though this may be of critical importance to better understand the neural mechanisms of motivated behaviors and certain psychiatric diseases, the specific role of the glutamatergic afferents arising from the PFC and the BLA in the modulation of locomotion produced by activation in the nucleus accumbens (NAcc) of D1-like receptors or D2-like postsynaptic receptors yet has not been examined. Here, we investigated how focal administration of lidocaine in the PFC or the BLA modulated hyperlocomotion induced by injection in the NAcc core of (i) the selective D1-like receptor agonist, SKF 38393, (ii) co-injection of SKF 38393 and of the selective D2-like receptor agonist LY 171555, a pharmacological condition required for the full expression of the postsynaptic effects of D2-like receptor agonists and believed to produce a locomotor response mainly mediated by D2-like postsynaptic receptors (iii) amphetamine, a psychoactive drug that possesses catecholamine and other neurotransmitters releasing effects. We show that reversible inhibition by lidocaine of the PFC potentiated hyperlocomotion induced by d-amphetamine or activation of D2-like postsynaptic receptors. Contrasting with these effects, inhibition by lidocaine of the BLA inhibited hyperlocomotion induced by D1-like receptor activation and amphetamine, but not by D2-like receptor activation. These data demonstrate that the glutamatergic inputs arising from the PFC and the BLA specifically control D2-like- and D1-like-mediated locomotor responses, respectively.     

Response of nucleus accumbens neurons to amygdala stimulation and its modification by dopamine

Extracellular single unit recordings were obtained from the nucleus accumbens of urethane anesthetized rats. It was found that electrical stimulation of the basal lateral and basal medial nuclei of the amygdala produced strong excitatory responses in neurons of the nucleus accumbens, in particular the medial region. Latencies of activation were relatively short with a mean of 10.7 ms.Dopamine applied iontophoretically had a marked attenuating effect on the excitatory response of nucleus accumbens neurons to amygdala stimulation. The spontaneous activity of all neurons recorded from the nucleus accumbens was also suppressed by dopamine, but the excitatory response was more sensitive to dopamine inhibition than the spontaneous activity.Neurons in the nucleus accumbens showed a variety of responses to single-pulse electrical stimulation of the ventral tegmental area (VTA). Some units in the nucleus accumbens received convergent inputs from both the amygdala and the VTA. Stimulation of the VTA also attenuated the response of nucleus accumbens neurons to excitatory inputs from the amygdala. A train of 10 pulses (0.15 ms, 200–600 αA) at 10 Hz delivered to the VTA at 100 ms before stimulation of the amygdala caused attenuation of the original excitatory response. The attenuating effect could be observed irrespective of whether individual single-pulse stimulation of the VTA elicited a response in that particular accumbens neuron or not. 6-Hydroxydopamine injected into the VTA 2 days prior to the recording experiment, or haloperidol injected intraperitoneally 1 h before the recording session, abolished this attenuating effect. However, responses to single-pulse stimulations of the VTA were not abolished. The results suggest that the attenuation of the excitatory response to amygdala stimulation was due to the release of dopamine from mesolimbic dopaminergic neurons. Responses to single-pulse stimulations of the VTA were probably due to activation of non-dopaminergic neurons projecting from the same area.It is suggested as a working hypothesis that this inhibitory effect of dopamine may be an important function of the mesolimbic dopamine pathway in modulating the extent to which limbic structures can exert an influence on the motor system through the accumbens.     

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.     

Neurotensin and cholecystokinin microinjected into the ventral tegmental area modulate microdialysate concentrations of dopamine and metabolites in the posterior nucleus accumbens

Neurotensin and cholecystokinin, neuropeptides which coexist with dopamine in many ventral tegmental neurons, were microinjected into the ventral tegmental area during in vivo microdialysis in the posterior nucleus accumbens. Neurotensin significantly elevated concentrations of dopamine and its metabolites at doses of 10 pmol, 1 nmol, and 10 nmol, while cholecystokinin significantly elevated dopamine metabolite concentrations only at a dose of 10 nmol. These data suggest that neurotensin potently mediates the release of dopamine from the mesolimbic pathway via direct actions on the cell body.     

Effects of d-amphetamine injected into the nucleus accumbens on ethanol reinforced behavior

Rats, initiated to self-administer 10% (v/v) ethanol in an operant situation using the sucrose-fading procedure, received bilateral n. accumbens microinjections of d-amphetamine prior to operant sessions. Doses of 4 micrograms, 10 micrograms and 20 micrograms/brain were administered and some animals also received a 4 microgram/brain dose of LY171555. Three different effects were observed: increased, decreased and no change in total session responding. There was no clear relation between injection area in the n. accumbens and type of effect observed. For either an increase or decrease in total session responding, momentary response rates were decreased. Both d-amphetamine and LY171555 produced similar results. The data support the hypothesis that dopamine in the n. accumbens is involved with ethanol reinforced operant responding but in a complex manner.     

Basolateral amygdala modulation of the nucleus accumbens dopamine response to stress: role of the medial prefrontal cortex

The basolateral amygdala (BLA) is involved in modulating affective responses to stress and, along with the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC), receives a stress-responsive dopamine (DA) projection from the ventral tegmental area. The present study was undertaken to characterize the role of BLA DA D1 and D2/D3 receptor subtypes in modulating the NAc and mPFC DA responses to stress. Voltammetry was used to monitor, in freely behaving rats, stress-induced DA release in NAc or mPFC after injection of D1 (SCH 23390) or D2/D3 (raclopride) receptor antagonist into BLA. Intra-BLA SCH 23390 injection potentiated stress-induced NAc DA release but attenuated the mPFC DA stress response; raclopride had no effect on either the NAc or mPFC DA responses to stress. Based on these results, we also examined the possibility that BLA can indirectly modulate the NAc DA stress response via its projection to mPFC. To do so we studied the effects of intra-mPFC co-administration of D1 (SKF 38393) and D2/D3 (quinpirole) receptor agonists on the potentiated NAc DA stress response resulting from intra-BLA SCH 23390 injection. Alone, mPFC D1 and D2/D3 receptor co-activation had no effect on stress-induced NAc DA release, but did prevent the potentiated NAc DA stress response produced by BLA D1 receptor blockade. These findings indicate that BLA DA modulates the NAc and mPFC DA stress responses via activation of the D1 receptor subtype. They also suggest that BLA DA modulates stress-induced NAc DA release indirectly by modulating the mPFC DA response to stress.     

D1 dopamine receptor stimulation enables the postsynaptic, but not autoreceptor, effects of D2 dopamine agonists in nigrostriatal and mesoaccumbens dopamine systems

Possible functional interactions between D1 and D2 dopamine (DA) receptors were examined using extracellular single-cell recording with microiontophoretic application of selective D1 and D2 receptor agonists both postsynaptically, in the rat nucleus accumbens (NAc) and caudate-putamen (CPu), and presynaptically, at impulse-regulating somatodendritic DA autoreceptors in the ventral tegmental area (A10) and substantia nigra pars compacta (A9). In addition, synthesis-modulating nerve terminal DA autoreceptors were studied in both the CPu and NAc using the gamma-butyrolactone (GBL) neurochemical model of isolated nerve terminal autoreceptor function in vivo. In both the NAc and CPu, the inhibition of neurons produced by iontophoresis of the D2 receptor agonists quinpirole or RU-24213 was attenuated by acute DA depletion via the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (AMPT). However, during iontophoresis of the selective D1 DA receptor agonist SKF 38393, the inhibitory effects of the D2 agonists were again evident, suggesting that the attenuation of D2 agonist-induced inhibition was due to decreased D1 receptor activation. In contrast, the inhibitory effects produced by the non-selective D1/D2 agonist apomorphine or by SKF 38393 were unaffected by AMPT pretreatment. Thus, D1 receptor activation appears necessary for D2 receptor-mediated inhibition of NAc and CPu neurons, whereas D2 receptor activation is not required for the inhibition produced by D1 receptor stimulation. In contrast to postsynaptic D2 receptors, the ability of DA agonists to stimulate D2 DA autoreceptors was not altered by manipulations of D1 receptor occupation. Enhancing D1 receptor stimulation with SKF 38393 or reducing D1 receptor occupation with either the selective D1 receptor antagonist SCH 23390 or AMPT failed to alter the rate-inhibitory effect of i.v. quinpirole on A9 or A10 DA neurons. Similarly, iontophoresis of SKF 38393 failed to alter the inhibitory effects of iontophoretic quinpirole. SKF 38393 also failed to affect the inhibition of GBL-induced increases in DOPA accumulation (tyrosine hydroxylase activity) produced by quinpirole in either the NAc or CPu. Furthermore, reversal of GBL-induced increases in DOPA accumulation by apomorphine or quinpirole was unaffected by pretreatment with SCH 23390. Therefore, D1 receptor occupation appears to be necessary for the expression of the functional effects of postsynaptic D2 receptor stimulation but not presynaptic D2 DA autoreceptor stimulation.     

The in vivo microdialysis recovery of dopamine is altered independently of basal level by 6-hydroxydopamine lesions to the nucleus accumbens

The present study was designed to test the hypothesis that the active neurotransmitter processes of release and uptake affect the in vivo microdialysis recovery of dopamine (DA) in the nucleus accumbens (N ACC) of the rat. The in vivo recovery for DA was established for rats which had received either unilateral infusions of the neurotoxin 6-hydroxydopamine (6-OHDA, 8 μg) or vehicle (0.2 μg ascorbate). In the quantitative dialysis method used (point of no net flux method), DA is added to the perfusate at concentrations above and below the expected extracellular concentration (0, 5, 10 and 20 nM) and DA is measured in the dialysate from the brain to generate a series of points. A linear fit is performed, the slope of which is the in vivo recovery of the dialysis probe. The in vivo recovery of the 6-OHDA group was 30 ± 3% which was significantly lower (P < 0.002) than the in vivo recovery of the control group which was 60 ± 3% (mean ± SEM; n = 6/group). The zero intercept of this regression is the point of no net flux, which is the extracellular concentration of DA independent of the probe sampling characteristics. The extracellular DA concentration for the 6-OHDA group was 7.8 ± 1.1nM, which was not significantly different than the control group which was 6.9 ± 0.7nM. The tissue DOPAC/DA ratios of the 6-OHDA lesioned hemispheres were significantly higher than the contralateral hemispheres of the same animals (0.62 ± 0.1vs.0.27 ± 0.1; P < 0.02) while the DOPAC/DA ratios in the control group were not significantly different (0.24 ± 0.1vs.0.27 ± 0.1). The fractional DA efflux from the terminals in the 6-OHDA group was significantly higher than the fractional DA efflux of the control group (0.52 ± 0.08vs.0.03 ± 0.003; P < 0.0001), indicating that the remaining terminals have increased turnover of DA. Despite the increased turnover, however, the number of remaining release and uptake sites are not sufficient to maintain the high in vivo recovery observed in the control group.     

吗啡戒断后焦虑症状大鼠伏核和杏仁核突触结构可塑性

目的 探讨吗啡依赖戒断焦虑行为与大鼠伏核、杏仁核突触形态结构可塑性变化之间的相关性.方法 采用剂量递增法建立大鼠吗啡依赖模型,应用高架十字迷宫检测焦虑行为,应用透射电镜技术结合图像分析系统比较对照组、模型组和治疗组(每组均6只)大鼠伏核、杏仁核突触体视学、界面结构参数的数据.结果 (1)行为学:模型组开放臂的次数和时间均少于对照组和治疗组(P＜0.01或P＜0.05).(2)突触体视学:伏核模型组数密度(Nv)[(1.012±0.036)个/μm3]较对照组[(0.701±0.138)个/μm3]和治疗组[(0.751±0.245)个/μm3]增加(P＜0.01),面密度(Sv)和突触连接带平均面积(S)3组间比较差异无统计学意义;杏仁核模型组Nv[(0.427±0.178)个/μm3]较对照组[(0.247±0.117)个/μm3]和治疗组[(0.246±0.116)个/μm3]增加(P＜0.01或P＜0.05),模型组Sv[(0.047±0.018)μm2/μm3]较对照组[(0.030±0.012)μm2/μm3]和治疗组[(0.030±0.015)μm2/μm3]增加(P＜0.01),模型组S[(0.124±0.066)μm2]较对照组[(0.157±0.119)μm2]和治疗组[(0.159±0.114)μm2]减小(P＜0.05).(3)突触界面结构:伏核和杏仁核各自突触的突触后致密物厚度、突触活性区长度、突触间隙宽度和突触界面曲率在模型组、对照组和治疗组间比较差异均无统计学意义.结论 吗啡依赖戒断所产生的焦虑与伏核和杏仁核突触形态结构可塑性的改变有一定相关性.

Abstract：

Objective The possible correlations between morphological contracture and plastic variability of synaptic structure in nucleus accumbens and amygdala neurons were surveyed in anxious symptom rats suffered from morphine withdrawal. Methods The escalating doses of morphine and the elevated plus-maze were applied to validate anxiety-like behavior in rats. The electron microscope was applied to detect the parameters involving the synaptic stereology and structural plasticity of synaptic interface structure of the nucleus accumbens and amygdala neurons in the control group, the morphine-withdrawal group and the cured group ( n = 6 ), associated with the stereological ways. Results ( 1 ) Compared with the control group and the cured group, reductions of frequency and time of open-arm were observed in the morphine-withdrawal group ( P ＜ 0. 01 or P ＜ 0. 05 ). ( 2 ) Higher numerical density ( Nv ) [( 1. 012 ±0. 036 )/μm3] of synapses of nucleus accumbens was detected in the anxious rats ( P ＜ 0. 01 ) than in the controls [( 0. 701 ±0. 138 )/μm3] and the cured rats [( 0. 751 ±0. 254 )/μm3] . No significant difference between the surface density ( Sv ) and the mean profile area ( S ) of synapse of the nucleus accumbens was discovered. Compared with the control group [( 0. 247 ± 0. 117 )/μm3] and the cured one [( 0. 246 ±0. 116 )/μm3] , higher values of Nv [( 0. 427 ±0. 178 )/μm3] in amygdala were detected in anxious rats ( P＜0.01 or P＜0.05 ). Similarly, higher score of Sv [( 0.047 ±0.018 )μm2/μm3] in amygdala was observed in the anxious rats ( P ＜ 0. 01 ) than those of the control group [( 0. 030 ±0. 012 )μm2/μm3] and cured group [( 0. 030 ±0. 015 )μm2/μm3] . However, anxious rats [( 0. 124 ±0. 066 )μm2] appear to be lower S of synapse in amygdale ( P ＜ 0. 05 ) than those of the control group [( 0. 157 ±0. 119 )μm2] and the cured group [( 0. 159 ±0. 114 )μm2] . ( 3 )No significant difference among postsynaptic density, length of synaptic thickening, widths in synaptic interface structure on junctions and curvature of synaptic cleft region was detected in the nucleus accumbens and amygdala neurons. Conclusion In the present study, the results suggest that anxious rats suffered from morphine withdrawal could possibly be related to the plastic variability of synaptic morphological structure in nucleus accumbens and amygdale.     

Chronic administration of morphine is associated with a decrease in surface AMPA GluR1 receptor subunit in dopamine D1 receptor expressing neurons in the shell and non-D1 receptor expressing neurons in the core of the rat nucleus accumbens

The nucleus accumbens (Acb) is an extensively studied neuroanatomical substrate of opiate reward and the neural plasticity associated with chronic opioid use. The cellular mechanisms mediating opioid-dependent plasticity are uncertain, however AMPA-type glutamate receptor trafficking in dopamine D1 dopamine receptor (D1R) expressing neurons may be a potential cellular pathway for these adaptations, although there is no evidence for this possibility. Immunogold electron microscopy was used to quantify the surface expression of the AMPA GluR1 subunit in dendritic profiles of neurons in the Acb in response to intermittent 14-day non-contingent injections of escalating doses of morphine, a model that parallels opioid self-administration. To determine if changes in GluR1 trafficking occurred in neurons potentially sensitive to dopamine-induced D1R activation, immunoperoxidase labeling of D1R was combined with immunogold labeling of GluR1. Immunogold quantification was performed in two distinct Acb subregions, the shell, an area involved in processing incentive salience related to rewarding stimuli, and the core, an area involved in reward-seeking behaviors. We provide the first report that chronic morphine administration is associated with a receptor-phenotypic decrease in surface trafficking of GluR1 in Acb subregions. When compared to saline injected animals, morphine produced a decrease in plasma membrane GluR1 labeling in medium- and large-sized D1R expressing dendritic profiles in the Acb shell. In contrast, in the Acb core, surface GluR1 was decreased in small-sized dendrites that did not express the dopamine receptor. These results indicate that chronic intermittent injection of escalating doses of morphine is accompanied by ultrastructural plasticity of GluR1 in neurons that are responsive to glutamate and dopamine-induced D1R activation in the Acb shell, and neurons capable of responding to glutamate but not D1R receptor stimulation in the Acb core. Thus, AMPA receptor trafficking associated with chronic opiate exposure in functionally distinct areas of the Acb may be distinguished by D1R receptor activation, suggesting the potential for differing neural substrates of reward and motor aspects of addictive processes involving glutamate and dopamine signaling.