Ultrastructural features of the nitric oxide synthase-containing interneurons in the nucleus accumbens and their relationship with tyrosine hydroxylase-containing terminals

Response of nitrergic system in the brain of rat conditioned to intracranial self-stimulation

The role of nitrergic system in modulating the action of psychostimulants on reward processing is well established. However, the relevant anatomical underpinnings and scope of the involved interactions with mesolimbic dopaminergic system have not been clarified. Using immunohistochemistry, we track the changes in neuronal nitric oxide synthase (nNOS) containing cell groups in the animals conditioned to intracranial self-stimulation (ICSS) via an electrode implanted in the lateral hypothalamus-medial forebrain bundle (LH-MFB) area. An increase in the nNOS immunoreactivity was noticed in the cells and fibers in the ventral tegmental area (VTA) and nucleus accumbens shell (AcbSh), the primary loci of the reward system. In addition, nNOS was up-regulated in the nucleus accumbens core (AcbC), vertical limb of diagonal band (VDB), locus coeruleus (LC), lateral hypothalamus (LH), superficial gray layer (SuG) of the superior colliculus, and periaqueductal gray (PAG). The brain tissue fragments drawn from these areas showed a change in nNOS mRNA expression, but in opposite direction. Intracerebroventricular (icv) administration of nNOS inhibitor, 7-nitroindazole (7-NI) showed decreased lever press activity in a dose-dependent manner in ICSS task. While an increase in the dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) efflux was noted in the microdialysates collected from the AcbSh of ICSS rats, pre-administration of 7-NI (icv route) attenuated the response. The study identifies nitrergic centers that probably mediate sensory, cognitive, and motor components of the goal-directed behavior.

Sodium nitroprusside enhances stepping test performance and increases medium spiny neurons responsiveness to cortical inputs in a rat model of Levodopa-induced dyskinesias

Levodopa (L-DOPA) is the classical gold standard treatment for Parkinson's disease. However, its chronic administration can lead to the development of L-DOPA-induced dyskinesias (LIDs). Dysregulation of the nitric oxide-cyclic guanosine monophosphate pathway in striatal networks has been linked to deficits in corticostriatal transmission in LIDs. This study investigated the effects of the nitric oxide (NO) donor sodium nitroprusside (SNP) on behavioural and electrophysiological outcomes in sham-operated and 6-hydroxydopamine-lesioned rats chronically treated with vehicle or L-DOPA, respectively. In sham-operated animals, systemic administration of SNP increased the spike probability of putative striatal medium spiny neurons (MSNs) in response to electrical stimulation of the primary motor cortex. In 6-hydroxydopamine-lesioned animals, SNP improved the stepping test performance without exacerbating abnormal involuntary movements. Additionally, SNP significantly increased the responsiveness of putative striatal MSNs in the dyskinetic striatum. These findings highlight the critical role of the NO signalling pathway in facilitating the responsiveness of striatal MSNs in both the intact and dyskinetic striata. The study suggests that SNP has the potential to enhance L-DOPA's effects in the stepping test without exacerbating abnormal involuntary movements, thereby offering new possibilities for optimizing Parkinson's disease therapy. In conclusion, this study highlights the involvement of the NO signalling pathway in the pathophysiology of LIDs.

Pathologic role of nitrergic neurotransmission in mood disorders

Mood disorders are chronic, recurrent mental diseases that affect millions of individuals worldwide. Although over the past 40 years the biogenic amine models have provided meaningful links with the clinical phenomena of, and the pharmacological treatments currently employed in, mood disorders, there is still a need to examine the contribution of other systems to the neurobiology and treatment of mood disorders. This article reviews the current literature describing the potential role of nitric oxide (NO) signaling in the pathophysiology and thereby the treatment of mood disorders. The hypothesis has arisen from several observations including (i) altered NO levels in patients with mood disorders; (ii) antidepressant effects of NO signaling blockers in both clinical and pre-clinical studies; (iii) interaction between conventional antidepressants/mood stabilizers and NO signaling modulators in several biochemical and behavioral studies; (iv) biochemical and physiological evidence of interaction between monoaminergic (serotonin, noradrenaline, and dopamine) system and NO signaling; (v) interaction between neurotrophic factors and NO signaling in mood regulation and neuroprotection; and finally (vi) a crucial role for NO signaling in the inflammatory processes involved in pathophysiology of mood disorders. These accumulating lines of evidence have provided a new insight into novel approaches for the treatment of mood disorders.

Electrical stimulation of the hippocampal fimbria facilitates neuronal nitric oxide synthase activity in the medial shell of the rat nucleus accumbens: Modulation by dopamine D1 and D2 receptor activation

The medial shell region of the nucleus accumbens (msNAc) is a key center for the regulation of goal-directed behavior and is likely to be dysfunctional in neuropsychiatric disorders such as addiction, depression and schizophrenia. Nitric oxide (NO)-producing interneurons in the msNAc are potently modulated by dopamine (DA) and may play an important role in synaptic integration in msNAc networks. In this study, neuronal NO synthase (nNOS) activity was measured in anesthetized rats using amperometric microsensors implanted into the msNAc or via histochemical techniques. In amperometric studies, NO oxidation current was recorded prior to and during electrical stimulation of the ipsilateral fimbria. Fimbria stimulation elicited a frequency and intensity-dependent increase in msNAc NO efflux which was attenuated by systemic administration of the nNOS inhibitor N^G-propyl-l-arginine. Parallel studies using NADPH-diaphorase histochemistry to assay nNOS activity produced highly complementary outcomes. Moreover, systemic administration of either a DA D1 receptor agonist or a DA D2 receptor antagonist potentiated nNOS activity in the msNAc elicited by fimbria stimulation. These observations demonstrate for the first time that NO synthesis in nNOS expressing interneurons in the msNAc is facilitated by robust activation of hippocampal afferents in a manner that is differentially modulated by DA D1 and D2 receptor activation.

Involvement of nitric oxide in anticompulsive-like effect of agmatine on marble-burying behaviour in mice

In view of the reports that nitric oxide modulates the neurotransmitters implicated in obsessive-compulsive disorder (OCD), patients with OCD exhibit higher plasma nitrate levels, and drugs useful in OCD influence nitric oxide. Agmatine is a polyamine and widely distributed in mammalian brain which interacts with nitrergic systems. Hence, the present study was carried out to understand the involvement of nitrergic systems in the anticompulsive-like effect of agmatine. We used marble-burying behaviour (MBB) of mice as the animal model of OCD, and nitric oxide levels in hippocampus (HC) and cortex homogenate were measured. Results revealed that, agmatine (20 and 40mg/kg, i.p) significantly inhibited the MBB. Intraperitoneal administration of nitric oxide enhancers viz. nitric oxide precursor - l-arginine (l-ARG) (400mg/kg and 800mg/kg) increased MBB as well as brain nitrites levels, whereas treatment with N(G)-nitro-l-arginine methyl ester (l-NAME) neuronal nitric oxide synthase inhibitor (30mg/kg and 50mg/kg, i.p.) and 7-nitroindazole (7-NI) (20mg/kg and 40mg/kg) attenuated MBB and nitrites levels in brain. Further, in combination studies, the anticompulsive-like effect of agmatine (20mg/kg, ip) was exacerbated by prior administration of l-ARG (400mg/kg) and conversely l-NAME (15mg/kg) or 7-NI (10.0mg/kg) attenuated OCD-like behaviour with HC and cortex changes in the levels of NO. None of the above treatment had any significant influence on locomotor activity. In conclusion, Agmatine is effective in ameliorating the compulsive-like behaviour in mice which appears to be related to nitric oxide in brain.

Role of nitric oxide in the regulation of motor function. An overview of behavioral, biochemical and histological studies in animal models

A compelling body of evidence suggests that nitric oxide (NO), a unique gaseous neurotransmitter and neuromodulator plays a key role in the regulation of motor function. Recently, the interest of researchers concentrates on the NO - soluble guanylyl cyclase (sGC) - cyclic GMP (cGMP) signaling pathway in the striatum as a new target for the treatment of Parkinson's disease (PD). The aim of the study is to review the available literature referring to the role of NO in the integration of basal ganglia functions. First, attention has been focused on behavioral effects of NO donors and neuronal nitric oxide synthase (nNOS) inhibitors in the modulation of motor behavior. Then, disturbances in the nitrergic neurotransmission in PD and its 6-OHDA animal model have been presented. Moreover, the most current data demonstrating the contribution of both dopamine and glutamate to the regulation of NO biosynthesis in the striatum have been analyzed. Finally, the role of NO in the tonic and phasic dopamine release as well as in the regulation of striatal output pathways also has been discussed.

AMPA receptor upregulation in the nucleus accumbens shell of cocaine-sensitized rats depends upon S-nitrosylation of stargazin

Behavioral sensitization to cocaine is associated with increased AMPA receptor (AMPAR) surface expression in the nucleus accumbens (NAc). This upregulation is withdrawal-dependent, as it is not detected on withdrawal day (WD) 1, but is observed on WD7-21. Its underlying mechanisms have not been clearly established. Nitric oxide (NO) regulates AMPAR trafficking in the brain by S-nitrosylation of the AMPAR auxiliary subunit, stargazin, leading to increased AMPAR surface expression. Our goal was to determine if stargazin S-nitrosylation contributes to AMPAR upregulation during sensitization. First, we measured stargazin S-nitrosylation in NAc core and shell subregions on WD14 after 8 daily injections of saline or 15 mg/kg cocaine. Stargazin S-nitrosylation was markedly increased in NAc shell but not core. To determine if this is associated with AMPAR upregulation, rats received 8 cocaine or saline injections followed by twice-daily treatments with vehicle or the nitric oxide synthase inhibitor l-NAME (50 mg/kg) on WD1-6, the time when AMPAR upregulation is developing in cocaine-exposed rats. Cocaine/vehicle rats showed elevated stargazin and GluA1 surface expression on WD7 compared to saline/vehicle rats; the GluA1 increase was more robust in core, while stargazin increased more robustly in shell. These effects of cocaine were attenuated in shell but not core when cocaine injections were followed by l-NAME treatment on WD1-6. Together, these results indicate that elevated S-nitrosylation of stargazin contributes to AMPAR upregulation during sensitization selectively in the NAc shell. It is possible that AMPAR upregulation in core involves a different TARP, γ4, which also upregulates in the NAc of sensitized rats.

Investigations into Potential Extrasynaptic Communication between the Dopaminergic and Nitrergic Systems

Nitric oxide is unconstrained by cell membranes and can therefore act along a broad distance as a volume transmitter. Spillover of nitric oxide between neurons may have a major impact on central nervous system diseases and particularly on neurodegeneration. There is evidence whereby communication between nitrergic and dopaminergic systems plays an essential role in the control of the nigrostriatal pathway. However, there is sparse information for either the coexistence or overlap of nitric oxide and dopaminergic structures. The dual localization of immunoreactivity for nitric oxide synthase (NOS) and tyrosine hydroxylase, enzymes responsible for the synthesis of nitric oxide and dopamine, respectively, was examined in neurons of the nigrostriatal pathway in the rat brain by means of a double-immunohistochemical method and confocal laser scanning microscopy, acquired at the resolution limit. After perfusional fixation, the brains were cut and double-immunostained. A proximity analysis of tyrosine hydroxylase and NOS structures was done using binary masks generated from the respective maximum projections, using confocal laser microscopy. Unrevealed regions were determined somatodendritic positive for both NOS and tyrosine hydroxylase, within an image limit resolution at 2 μm-wide margin. The described interconnected localization of nNOS(+) and TH(+) containing neuronal fibers and cells bodies in the nigrostriatal pathway propose a close anatomical link between the two neurotransmitters.

Dopamine release in the basal ganglia

Dopamine (DA) is a key transmitter in the basal ganglia, yet DA transmission does not conform to several aspects of the classic synaptic doctrine. Axonal DA release occurs through vesicular exocytosis and is action potential- and Ca²⁺-dependent. However, in addition to axonal release, DA neurons in midbrain exhibit somatodendritic release by an incompletely understood, but apparently exocytotic, mechanism. Even in striatum, axonal release sites are controversial, with evidence for DA varicosities that lack postsynaptic specialization, and largely extrasynaptic DA receptors and transporters. Moreover, DA release is often assumed to reflect a global response to a population of activities in midbrain DA neurons, whether tonic or phasic, with precise timing and specificity of action governed by other basal ganglia circuits. This view has been reinforced by anatomical evidence showing dense axonal DA arbors throughout striatum, and a lattice network formed by DA axons and glutamatergic input from cortex and thalamus. Nonetheless, localized DA transients are seen in vivo using voltammetric methods with high spatial and temporal resolution. Mechanistic studies using similar methods in vitro have revealed local regulation of DA release by other transmitters and modulators, as well as by proteins known to be disrupted in Parkinson's disease and other movement disorders. Notably, the actions of most other striatal transmitters on DA release also do not conform to the synaptic doctrine, with the absence of direct synaptic contacts for glutamate, GABA, and acetylcholine (ACh) on striatal DA axons. Overall, the findings reviewed here indicate that DA signaling in the basal ganglia is sculpted by cooperation between the timing and pattern of DA input and those of local regulatory factors.

Nitric oxide donors enhance the frequency dependence of dopamine release in nucleus accumbens

Dopamine (DA) neurotransmission in the nucleus accumbens (NAc) is critically involved in normal as well as maladaptive motivated behaviors including drug addiction. Whether the striatal neuromodulator nitric oxide (NO) influences DA release in NAc is unknown. We investigated whether exogenous NO modulates DA transmission in NAc core and how this interaction varies depending on the frequency of presynaptic activation. We detected DA with cyclic voltammetry at carbon-fiber microelectrodes in mouse NAc in slices following stimuli spanning a full range of DA neuron firing frequencies (1-100 Hz). NO donors 3-morpholinosydnonimine hydrochloride (SIN-1) or z-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]diazen-1-ium-1,2-diolate (PAPA/NONOate) enhanced DA release with increasing stimulus frequency. This NO-mediated enhancement of frequency sensitivity of DA release was not prevented by inhibition of soluble guanylyl cyclase (sGC), DA transporters, or large conductance Ca(2+)-activated K(+) channels, and did not require glutamatergic or GABAergic input. However, experiments to identify whether frequency-dependent NO effects were mediated via changes in powerful acetylcholine-DA interactions revealed multiple components to NO modulation of DA release. In the presence of a nicotinic receptor antagonist (dihydro-β-erythroidine), NO donors increased DA release in a frequency-independent manner. These data suggest that NO in the NAc can modulate DA release through multiple GC-independent neuronal mechanisms whose net outcome varies depending on the activity in DA neurons and accumbal cholinergic interneurons. In the presence of accumbal acetylcholine, NO promotes the sensitivity of DA release to presynaptic activation, but with reduced acetylcholine input, NO will promote DA release in an activity-independent manner through a direct action on dopaminergic terminals.

Nitric Oxide-Soluble Guanylyl Cyclase-Cyclic GMP Signaling in the Striatum: New Targets for the Treatment of Parkinson's Disease?

Striatal nitric oxide (NO)-producing interneurons play an important role in the regulation of corticostriatal synaptic transmission and motor behavior. Striatal NO synthesis is driven by concurrent activation of NMDA and dopamine (DA) D1 receptors. NO diffuses into the dendrites of medium-sized spiny neurons which contain high levels of NO receptors called soluble guanylyl cyclases (sGC). NO-mediated activation of sGC leads to the synthesis of the second messenger cGMP. In the intact striatum, transient elevations in intracellular cGMP primarily act to increase neuronal excitability and to facilitate glutamatergic corticostriatal transmission. NO–cGMP signaling also functionally opposes the inhibitory effects of DA D2 receptor activation on corticostriatal transmission. Not surprisingly, abnormal striatal NO–sGC–cGMP signaling becomes apparent following striatal DA depletion, an alteration thought to contribute to pathophysiological changes observed in basal ganglia circuits in Parkinson's disease (PD). Here, we discuss recent developments in the field which have shed light on the role of NO–sGC–cGMP signaling pathways in basal ganglia dysfunction and motor symptoms associated with PD and l-DOPA-induced dyskinesias.

Nitric oxide signaling via cGMP-stimulated phosphodiesterase in striatal neurons

Nitric oxide (NO) acts in the nervous system to activate guanylyl cyclase and increase cGMP. One target for cGMP appears to be the cGMP-stimulated phosphodiesterase (PDE2A), which is widely expressed in the brain and provides a molecular mechanism for NO to regulate cAMP levels. We have found that PDE2A is highly expressed in the medium spiny neurons of the striatum, which project to the pallidum and substantia nigra. These cells express dopamine-stimulated adenylyl cyclase, and we have found that increases in cAMP in these neurons, produced by activation of the D1-type dopamine receptor, are dramatically enhanced by the general phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine and the PDE2A-selective inhibitor erythro-p-(2-hydroxyl-3-nonyl)adenine (EHNA). These results indicate that PDE2A plays a major role in regulating dopamine-stimulated cAMP production in striatal neurons. EHNA also enhances NO-induced increases in striatal cGMP. In addition, dopamine appears to act via another receptor, activated by the agonist SKF83959, to increase striatal cGMP in a NO-dependent manner. Together, these observations indicate that striatal NO producing interneurons can act via the PDE2A in the medium spiny neurons to regulate the cAMP response to dopamine stimulation.

Tolerance to the cataleptic effect that follows repeated nitric oxide synthase inhibition may be related to functional enzymatic recovery

Systemic or intra-striatal acute administration of nitric oxide synthase (NOS) inhibitors causes catalepsy in rodents. This effect disappears after sub-chronic treatment. The aim of the present study was to investigate if this tolerance is related to changes in the expression of NOS or dopamine-2 (D2) receptor or to a recovery of NOS activity. Male albino Swiss mice (25-30 g) received single or sub-chronic (once a day for 4 days) i.p. injections of saline or L-nitro-arginine (L-NOARG, 40 mg/kg), a non-selective inhibitor of neuronal nitric oxide synthase (nNOS). Twenty-four hours after the last injection, the animals were killed and their brains were removed for immunohistochemistry assay to detect the presence of nNOS or for 'in-situ' hybridisation study using (35)S-labeled oligonucleotide probe complementary to D2 receptor mRNA. The results were analysed by computerised densitometry. Independent groups of animals received the same treatment, but were submitted to the catalepsy test and had their brain removed to measure nitrite and nitrate (NOx) concentrations in the striatum. Acute administration of L-NOARG caused catalepsy that disappeared after sub-chronic treatment. The levels of NOx were significantly reduced after acute L-NOARG treatment. The decrease in NOx after drug injection suffered a partial tolerance after sub-chronic treatment. The catalepsy time after acute or sub-chronic treatment with L-NOARG was negatively (r = -0.717) correlated with NOx levels. Animals that received repeated L-NOARG injections also showed an increase in the number of nNOS-positive neurons in the striatum. No change in D2 receptor mRNA expression was found in the dorsal striatum, nucleus accumbens and substantia nigra. Together, these results suggest that tolerance to L-NOARG cataleptic effects do not depend on changes in D2 receptors. They may depend, however, on plastic changes in nNOS neurons resulting in partial recovery of NO formation in the striatum.

Nitric oxide-soluble guanylyl cyclase signaling regulates corticostriatal transmission and short-term synaptic plasticity of striatal projection neurons recorded in vivo

Striatal medium-sized spiny neurons (MSNs) contain the highest levels of soluble guanylyl cyclase (sGC) in the brain. Striatal sGC signaling is activated by nitric oxide (NO) and other neuromodulators. MSNs also express cGMP-dependent protein kinase and other components of the cGMP signaling system which are critically involved in integrating corticostriatal transmission and regulating synaptic plasticity in striatal networks. However, the influence of tonic and phasic activation of this signaling pathway on striatal MSN activity is poorly understood. The present study examined the impact of systemic administration of the selective sGC inhibitor [1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one] (ODQ) on spike activity evoked using low and high frequency electrical stimulation of the frontal cortex. MSN activity was monitored using single-unit extracellular recordings in urethane-anesthetized rats. ODQ administration significantly decreased spike activity evoked by low frequency cortical stimulation in a stimulus intensity- and time-dependent manner. Additionally, ODQ administered along with the neuronal NO synthase inhibitor 7-nitroindazole (7-NI) potently decreased the incidence of excitatory responses observed during high-frequency train stimulation of the contralateral frontal cortex. The short-term depression of cortically-evoked spike activity induced by train stimulation was enhanced following pretreatment with ODQ in MSNs exhibiting an excitatory response during cortical train stimulation. Unexpectedly, this effect of ODQ was reversed in animals receiving co-administration of ODQ and 7-NI. 7-NI/ODQ co-administration also reversed measures of short-term depression observed in MSNs exhibiting an inhibitory response during cortical train stimulation. These observations extend previous studies showing that tonic and phasic NO-sGC signaling modulates the responsiveness of MSNs to corticostriatal input. Moreover, phasic activation of NO signaling is likely to regulate short-term changes in corticostriatal synaptic plasticity via complex mechanisms involving both sGC-cGMP-dependent and independent pathways.

Impact of dopamine-glutamate interactions on striatal neuronal nitric oxide synthase activity

RATIONALE

It is known that dopamine (DA) D1 receptor activation stimulates striatal nitric oxide (NO) synthesis, whereas D2 receptor activation produces the opposite effect. However, the mechanisms involved in the dopaminergic modulation of nitric oxide synthase (NOS) are unknown.

OBJECTIVES

We hypothesized that the effects of DA on striatal NO signaling are dependent on ongoing glutamatergic activation of NOS. Therefore, the current study examined whether intact N-methyl-D-aspartic acid (NMDA) receptor activation is required for the dopaminergic modulation of NOS activity.

METHODS

We assessed the impact of pharmacological manipulations of D1, D2, and NMDA receptors on NOS activity in the dorsal striatum and motor cortex using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry. Drugs were administered systemically to conscious animals and NADPH-d staining was quantified in these regions using ex vivo measurements of tissue optical density.

RESULTS

Administration of the neuronal NOS inhibitor N (G)-propyl-L-arginine (NPA), the D1 receptor antagonist SCH 23390, and the NMDA receptor antagonist 3-phosphonopropyl-piperazine-2-carboxylic acid (CPP) all attenuated staining selectively in the striatum. Administration of the D2 receptor agonist quinpirole decreased NADPH-d staining in both the striatum and cortex. Striatal NADPH-d staining elicited by administration of the D1 receptor agonist SKF 81297 or the D2 receptor antagonist eticlopride was attenuated by NPA, SCH 23390, and CPP pretreatment. Quinpirole pretreatment also abolished the facilitatory effect of SKF 81297.

CONCLUSIONS

These studies show for the first time that ongoing NMDA receptor activation is necessary for the modulation of striatal NOS activity by both facilitatory (D1 receptor activation) and inhibitory (D2 receptor activation) dopaminergic signaling mechanisms.

Cortico-Basal Ganglia reward network: microcircuitry

Many of the brain's reward systems converge on the nucleus accumbens, a region richly innervated by excitatory, inhibitory, and modulatory afferents representing the circuitry necessary for selecting adaptive motivated behaviors. The ventral subiculum of the hippocampus provides contextual and spatial information, the basolateral amygdala conveys affective influence, and the prefrontal cortex provides an integrative impact on goal-directed behavior. The balance of these afferents is under the modulatory influence of dopamine neurons in the ventral tegmental area. This midbrain region receives its own complex mix of excitatory and inhibitory inputs, some of which have only recently been identified. Such afferent regulation positions the dopamine system to bias goal-directed behavior based on internal drives and environmental contingencies. Conditions that result in reward promote phasic dopamine release, which serves to maintain ongoing behavior by selectively potentiating ventral subicular drive to the accumbens. Behaviors that fail to produce an expected reward decrease dopamine transmission, which favors prefrontal cortical-driven switching to new behavioral strategies. As such, the limbic reward system is designed to optimize action plans for maximizing reward outcomes. This system can be commandeered by drugs of abuse or psychiatric disorders, resulting in inappropriate behaviors that sustain failed reward strategies. A fuller appreciation of the circuitry interconnecting the nucleus accumbens and ventral tegmental area should serve to advance discovery of new treatment options for these conditions.

Effects of blockade of dopamine D2 receptors on extracellular citrulline levels in the nucleus accumbens during performance of a conditioned reflex fear response

Vital microdialysis studies on Sprague-Dawley rats using HPLC showed that performance of a conditioned reflex fear response was accompanied by an increase in the extracellular level of citrulline (a coproduct of nitric oxide synthesis) in the nucleus accumbens. Administration of the dopamine D(2) receptor antagonist raclopride (10 microM) into the nucleus accumbens decreased the magnitude of the increase in the extracellular citrulline level in this structure during performance of the conditioned reflex fear response but had no effect on its behavioral measures (the level of freezing). Doses increased investigative activity in a novel context which had been inhibited by acquisition of the conditioned reflex fear response, without affecting the investigative behavior of control animals. These data suggest that the dopaminergic input and dopamine D(2) receptors control the activity of the NO-ergic system of the nucleus accumbens during performance of the conditioned reflex fear response and may control "transfer" of fear to another behavioral situation.

The ventral basal ganglia, a selection mechanism at the crossroads of space, strategy, and reward

The basal ganglia are often conceptualised as three parallel domains that include all the constituent nuclei. The 'ventral domain' appears to be critical for learning flexible behaviours for exploration and foraging, as it is the recipient of converging inputs from amygdala, hippocampal formation and prefrontal cortex, putatively centres for stimulus evaluation, spatial navigation, and planning/contingency, respectively. However, compared to work on the dorsal domains, the rich potential for quantitative theories and models of the ventral domain remains largely untapped, and the purpose of this review is to provide the stimulus for this work. We systematically review the ventral domain's structures and internal organisation, and propose a functional architecture as the basis for computational models. Using a full schematic of the structure of inputs to the ventral striatum (nucleus accumbens core and shell), we argue for the existence of many identifiable processing channels on the basis of unique combinations of afferent inputs. We then identify the potential information represented in these channels by reconciling a broad range of studies from the hippocampal, amygdala and prefrontal cortex literatures with known properties of the ventral striatum from lesion, pharmacological, and electrophysiological studies. Dopamine's key role in learning is reviewed within the three current major computational frameworks; we also show that the shell-based basal ganglia sub-circuits are well placed to generate the phasic burst and dip responses of dopaminergic neurons. We detail dopamine's modulation of ventral basal ganglia's inputs by its actions on pre-synaptic terminals and post-synaptic membranes in the striatum, arguing that the complexity of these effects hint at computational roles for dopamine beyond current ideas. The ventral basal ganglia are revealed as a constellation of multiple functional systems for the learning and selection of flexible behaviours and of behavioural strategies, sharing the common operations of selection-by-disinhibition and of dopaminergic modulation.

Regulation of striatal nitric oxide synthesis by local dopamine and glutamate interactions

Nitric oxide (NO) is a key neuromodulator of corticostriatal synaptic transmission. We have shown previously that dopamine (DA) D1/5 receptor stimulation facilitates neuronal NO synthase (nNOS) activity in the intact striatum. To study the impact of local manipulations of D1/5 and glutamatergic NMDA receptors on striatal nNOS activity, we combined the techniques of in vivo amperometry and reverse microdialysis. Striatal NO efflux was monitored proximal to the microdialysis probe in urethane-anesthetized rats during local infusion of vehicle or drug. NO efflux elicited by systemic administration of SKF-81297 was blocked following intrastriatal infusion of: (i) the D1/5 receptor antagonist SCH-23390, (ii) the nNOS inhibitor 7-nitroindazole, (iii) the non-specific ionotropic glutamate receptor antagonist kynurenic acid, and (iv) the selective NMDA receptor antagonist 3-phosphonopropyl-piperazine-2-carboxylic acid. Glycine co-perfusion did not affect SKF-81297-induced NO efflux. Furthermore, intrastriatal infusion of SKF-81297 potentiated NO efflux evoked during electrical stimulation of the motor cortex. The facilitatory effects of cortical stimulation and SKF-81297 were both blocked by intrastriatal infusion of SCH-23390, indicating that striatal D1/5 receptor activation is necessary for the activation of nNOS by corticostriatal afferents. These studies demonstrate for the first time that reciprocal DA-glutamate interactions play a critical role in stimulating striatal nNOS activity.

Physiology and pharmacology of striatal neurons

The basal ganglia occupy the core of the forebrain and consist of evolutionarily conserved motor nuclei that form recurrent circuits critical for motivation and motor planning. The striatum is the main input nucleus of the basal ganglia and a key neural substrate for procedural learning and memory. The vast majority of striatal neurons are spiny GABAergic projection neurons, which exhibit slow but temporally precise spiking in vivo. Contributing to this precision are several different types of interneurons that constitute only a small fraction of total neuron number but play a critical role in regulating striatal output. This review examines the cellular physiology and modulation of striatal neurons that give rise to their unique properties and function.

The neuronal nitric oxide synthase (nNOS) gene contributes to the regulation of tyrosine hydroxylase (TH) by cocaine

Recently, we demonstrated that intact nitric oxide (NO) signaling is essential for the development of cocaine behavioral sensitization in adulthood [M.A. Balda, K.L. Anderson, Y. Itzhak, Differential role of the nNOS gene in the development of behavioral sensitization to cocaine in adolescent and adult B6;129S mice, Psychopharmacology (Berl) 200 (2008) 509-519]. Given the requirement of dopamine (DA) transmission in cocaine-induced behavioral sensitization and the interactions between NO and DA systems, the present study investigated the role of the neuronal nitric oxide synthase (nNOS) gene and the effect of cocaine on the expression of tyrosine hydroxylase (TH)-immunoreactive (-ir) neurons. Adult (postnatal day 80) wild type (WT) and nNOS knockout (KO) mice received saline or a sensitizing regimen of cocaine (20mg/kg) for 5 days. After 24h, TH immunoreactivity was assessed in the ventral tegmental area (VTA) and the dorsal striatum (dST) using stereology and Western blotting, respectively. We report that (a) nNOS KO mice express lower levels of TH-ir neurons in the VTA compared to WT counterparts, (b) cocaine administration to WT mice significantly increased striatal TH expression, and (c) the same cocaine administration to nNOS KO mice significantly decreased striatal TH expression. Thus, the nitrergic system may contribute to cocaine-induced behavioral sensitization by regulating dopaminergic neurotransmission.

Dopamine D1 receptor-dependent regulation of extracellular citrulline level in the rat nucleus accumbens during conditioned fear response

Nucleus accumbens (N.Acc) contains a subclass of nitric oxide (NO)-generating interneurons that are presumably regulated by the dopamine input. Receptor mechanisms underlying dopamine-NO interaction in the N.Acc are poorly understood. In the current study, we used in vivo microdialysis combined with high-performance liquid chromatography to examine participation of dopamine D1 receptors in regulation of extracellular levels of citrulline (an NO co-product) in the medial N.Acc of Sprague-Dawley rats during both pharmacological challenge and a conditioned fear response. The intraaccumbal infusion of the D1 receptor agonist SKF-38393 (100-500 microM) increased dose-dependently the local dialysate citrulline levels. The SKF-38393-induced increase in extracellular citrulline was prevented by intraaccumbal infusions of 500 microM 7-nitroindazole, a neuronal NO synthase inhibitor. In behavioral microdialysis experiment, the accumbal levels of extracellular citrulline markedly increased in rats given a mild footshock paired with tone. The presentation of the tone previously paired with footshock (the conditioned fear response) produced a "conditioned" rise of extracellular citrulline levels in the N.Acc which was attenuated by intraaccumbal infusion of 100 microM SCH-23390, a dopamine D1 receptor antagonist, and prevented by intraaccumbal infusion of 500 microM 7-nitroindazole. The results suggest that in the N.Acc, the dopamine D1 receptors might regulate the neuronal NO synthase activity; this dopamine-dependent mechanism seems to participate in activation of the neuronal NO synthase and probably NO formation in this brain area during the conditioned fear response.

NO synthase-dependent increases in extracellular citrulline levels in the nucleus accumbens in an emotional conditioned reflex

Intracerebral microdialysis/HPLC studies in Sprague-Dawley rats showed that the acquisition and execution of an emotional conditioned reflex was accompanied by an increase in the extracellular citrulline level in the nucleus accumbens; citrulline is a co-product of nitric oxide synthesis. The increase in the citrulline level evoked by execution of this reflex decreased after injection of 7-nitroindazole (0.5 mM), a selective inhibitor of neuronal NO synthase, into the nucleus accumbens, and was completely blocked by injection of N-nitroarginine (0.5 mM), a non-selective inhibitor NO synthase. The increase in the nucleus accumbens citrulline level seen during execution of the emotional conditioned reflex was prevented by administration of both of these NO synthase inhibitors. These data suggest that during the acquisition and execution of the emotional conditioned reflex, there is an increase in nitric oxide production in the nucleus accumbens, which arises predominantly as a result of activation of neuronal NO synthase.

Dopamine D2 receptor-dependent modulation of striatal NO synthase activity

RATIONALE

Striatal nitric oxide (NO)-producing interneurons receive synaptic contacts from midbrain dopamine (DA) neurons and are regulated by phasic DA transmission. Classic antipsychotic drugs elevate neuronal NO synthase (NOS) expression in the rat striatum. Given that NO signaling potently modulates the membrane excitability of striatal projection neurons, it is plausible that up-regulation of NOS activity after DA D2 receptor blockade contributes to the therapeutic efficacy and/or motor side effects associated with antipsychotic drugs.

OBJECTIVES

This study assessed the impact of DA D(2) receptor activation on striatal NOS activity in vivo. Characterization of the dopaminergic regulation of striatal NO signaling will be relevant for understanding the mechanism(s) of action of antipsychotic drugs.

MATERIALS AND METHODS

Striatal NO efflux, evoked via electrical stimulation of the substantia nigra (SN) or systemic administration of the DA D(1) receptor agonist SKF 81297, was assessed in anesthetized rats using an NO-selective amperometric microsensor.

RESULTS

The facilitatory effect of SN stimulation on striatal NO efflux was attenuated by systemic administration of the DA D(2) receptor agonist quinpirole. Conversely, administration of the DA D(2) receptor antagonist eticlopride augmented evoked NO efflux. NO efflux induced by systemic administration of SKF 81297 was attenuated by quinpirole and restored by co-administration of quinpirole and eticlopride. The facilitatory effect of SKF 81297 on NO efflux was also significantly attenuated after pretreatment with the non-specific NOS inhibitor methylene blue.

CONCLUSIONS

Activation of NO synthesis by phasic DA transmission is down-regulated via a DA D2 receptor-dependent mechanism. DA D(2) receptor activation opposes DA D(1) receptor activation of NO synthesis at a site postsynaptic to the DA terminal. Further studies examining NO-DA dynamics may have potential to reveal novel therapeutic strategies to treat various brain disorders.

Effects of N-methyl-d-aspartate on extracellular citrulline level in the rat nucleus accumbens

In vivo microdialysis combined with high-performance liquid chromatography and electrochemical detection was used to study effects of intraaccumbal infusion of N-methyl-D-aspartate (NMDA) on the content of extracellular citrulline (a nitric oxide co-product) in the medial nucleus accumbens of Sprague-Dawley rats. The intraaccumbal NMDA infusion (10-1000 microM) dose-dependently increased the local dialysate citrulline levels (193+/-7% and 258+/-7% versus basal for the 100 and 1000 microM, respectively). The NMDA-induced increase of extracellular citrulline was completely prevented by intraaccumbal infusions through the dialysis probe both of 50 microM dizocilpine maleate (an NMDA antagonist) and of 0.5 mM N-nitro-L-arginine (a nitric oxide synthase inhibitor). Local infusion of N-nitro-L-arginine (0.5 mM) slightly decreased basal citrulline levels in the nucleus accumbens throughout the entire period of the infusion, whereas dizocilpine maleate (50 microM) had no long-lasting effect. These results suggest that NMDA receptor stimulation of the medial nucleus accumbens might cause a local nitric oxide synthase activation resulting in nitric oxide production in this brain area.

Influence of nitric oxide on morphine-induced amnesia and interactions with dopaminergic receptor agents

The interactions of dopaminergic receptors and nitric oxide (NO) with morphine-induced memory of passive avoidance have been investigated in mice. Pre-training administration of morphine (1, 3 and 5 mg/kg, s.c.) dose-dependently decreased the learning of a one-trial passive avoidance task. Pre-training administration of L-arginine, a nitric oxide precursor (50, 100 and 200 mg/kg, i.p.), alone did not affect memory formation. The drug (100 and 200 mg/kg) decreased significantly amnesia induced by pre-training morphine (5 mg/kg). Pre-training administration of L-NAME (N(G)-nitro-L-arginine methyl ester), a nitric oxide synthase (NOS) inhibitor (20 and 30 mg/kg, i.p.), dose-dependently impaired memory formation. In addition, co-pretreatment of different doses of L-NAME (10, 20 and 30 mg/kg) with lower dose of morphine (1 mg/kg), which did not induce amnesia by itself, caused inhibition of memory formation. Pre-training administration of apomorphine, a dopaminergic receptor agonist (0.25, 0.5 and 1 mg/kg, i.p.), alone also did not affect memory formation, but morphine-induced amnesia was significantly inhibited by pretreatment with apomorphine (0.5 and 1 mg/kg, 5 min, i.p.). On the other hand, the inhibition of morphine-induced amnesia by L-arginine (200 mg/kg, i.p.) was significantly decreased by pretreatment with different doses of dopamine D1 receptor antagonist, SCH 23390 (0.001, 0.01 and 0.1 mg/kg, i.p.) or D2 receptor antagonist, sulpiride (12.5, 25, 50 and 100 mg/kg, i.p.). However, the dopamine receptor antagonists could not affect memory formation by themselves. It may be concluded that the morphine-induced impairment of memory formation can be prevented by nitric oxide donor and, in this effect, dopaminergic mechanism is involved.

Phasic dopaminergic transmission increases NO efflux in the rat dorsal striatum via a neuronal NOS and a dopamine D(1/5) receptor-dependent mechanism

Dysfunctional neurotransmission within striatal networks is believed to underlie the pathophysiology of several neurological and psychiatric disorders. Nitric oxide (NO)-producing interneurons have been shown to play a critical role in modulating striatal synaptic transmission. These interneurons receive synaptic contacts from midbrain dopamine (DA) neurons and may be regulated by DA receptor activation. In the current study, striatal NO efflux was measured in anesthetized male rats using an NO-selective electrochemical microsensor and the role of DA in modulating NO synthase (NOS) activity was assessed during electrical or chemical (bicuculline) stimulation of the substantia nigra (SN). Electrical stimuli were patterned to approximate the natural single spike or burst firing activity of midbrain DA neurons. Electrical stimulation of the SN at low frequencies induced modest increases in striatal NO efflux. In contrast, train stimulation of the SN robustly increased NO efflux in a stimulus intensity-dependent manner. NO efflux evoked by SN stimulation was similar in chloral hydrate- and urethane-anesthetized rats. The facilitatory effect of train stimulation on striatal NO efflux was transient and attenuated by systemic administration of the neuronal NOS inhibitor 7-nitroindazole and the nonselective NOS inhibitor methylene blue. Moreover, the increase in NO efflux observed during chemical and train stimulation of the SN was attenuated following systemic administration of the DA D(1/5) receptor antagonist SCH 23390. SCH 23390 also blocked NO efflux induced by systemic administration of the D(1/5) agonist SKF 81297. These results indicate that neuronal NOS is activated in vivo by nigrostriatal DA cell burst firing via a DA D(1/5)-like receptor-dependent mechanism.

NOS-positive local circuit neurons are exclusively axo-dendritic cells both in the neo- and archi-cortex of the rat brain

Neuronal nitric oxide synthase (nNOS)-containing neurons and axon terminals were examined in the rat somatosensory and temporal neocortex, in the CA3/a-c areas of Ammon's horn and in the hippocampal dentate gyrus. In these areas, only nonpyramidal neurons were labeled with the antibody against nNOS. Previous observations suggested that all nNOS-positive nonpyramidal cells are GABAergic local circuit neurons, which form exclusively symmetric synapses. In agreement with this, nNOS-positive axon terminals in the hippocampal formation formed symmetric synapses exclusively with dendritic shafts. In the neocortex, in contrast, in addition to the nNOS-positive axon terminals that formed synapses with unlabeled spiny and aspiny dendrites and with nNOS-positive aspiny dendrites, a small proportion of the nNOS-positive axon terminals formed symmetric synapses with dendritic spines. These results suggest that nNOS-positive local circuit neurons form a distinct group of axo-dendritic cells displaying slightly different domain specificity in the archi- and neocortex. However, nNOS-positive cells show no target selectivity, because they innervate principal cells and local circuit neurons. Afferents to the NOS-positive cells display neither domain nor target selectivity, because small unlabeled terminals formed synapses with both the soma or dendrites of nNOS-positive neurons and an adjacent unlabeled dendrite or spine in both the hippocampal formation and in neocortex.

Nucleus accumbens nitric oxide immunoreactive interneurons receive nitric oxide and ventral subicular afferents in rats

The nitric oxide generating neurons of the nucleus accumbens exert a powerful influence over striatal function, in addition, these nitrergic inputs are in a position to regulate the dopaminergic and glutamatergic inputs on striatal projection neurons. It was the aim of this study to establish the source of the glutamatergic drive to nitric oxide synthase interneurons of the nucleus accumbens. The nucleus accumbens nitric oxide-generating neurons receive asymmetrical, excitatory, presumably glutamatergic inputs. Possible sources of these inputs could be the limbic and cortical regions known to project to this area. To identify sources of the excitatory inputs to the nitric oxide synthase-containing interneurons of the nucleus accumbens in the rat we first examined the ultrastructural morphology of asymmetrical synaptic specializations contacting nitric oxide synthase-immunohistochemically labeled interneurons in the nucleus accumbens. Neurons were selected from different regions of the nucleus accumbens, drawn using camera lucida, processed for electron microscopic analysis, and the boutons contacting nitric oxide synthase-labeled dendrites were photographed and correlated to the drawings. Using vesicle size as the criterion the source was predicted to be either the prefrontal cortex or the ventral subiculum of the hippocampus. To examine this prediction, a further study used anterograde tracing from both the prefrontal cortex and the ventral subiculum, and nitric oxide synthase immunohistochemistry with correlated light and electron microscopy. Based on appositions by anterogradely labeled fibers, selected nitric oxide synthase-labeled neurons within the nucleus accumbens, were examined with electron microscopic analysis. With this technique we confirmed the prediction that subicular afferent boutons make synaptic contact with nitric oxide synthase interneurons, and demonstrated anatomically that nitric oxide synthase boutons make synaptic contact with the dendritic arbors of nitric oxide synthase interneurons. We suggest that the subicular input may excite the nitric oxide synthase neurons synaptically, while the nitric oxide synthase-nitric oxide synthase interactions underlie a nitric oxide signaling network which propagates hippocampal information, and expands the hippocampus's influence on 'gating' information flow across the nucleus accumbens.

Y+ and y+ L arginine transporters in neuronal cells expressing tyrosine hydroxylase

Arginine is a semi-essential amino acid that serves as sole substrate for enzymes involved in diverse cell processes including redox balance via nitric oxide synthase (NOS) and cell proliferation via arginase. Neurons that express nNOS require intracellular arginine to generate nitric oxide (NO). Using a TH+ neuronal cell line (CAD cells), we show that neuronal NO production is largely dependent on extracellular arginine. Although a small intracellular pool exists in CAD cells, the lack of mRNA for argininosuccinate synthase (AS), a rate limiting enzyme for arginine recycling, suggests that intracellular pools are not re-supplied by this mechanism in this sub-class of neurons. Rather, arginine is taken up from the extracellular media by two primary transport systems, the y+ and the y+ L systems. The expression of CAT1, CAT3, y+ LAT1 and y+ LAT2 mRNAs supports the presence of each system. CAD cell arginine transport is depressed by increased extracellular K+ levels and demonstrates that variations in membrane potential control neuronal arginine uptake. Short term exposure to the oxidizing agents, rotenone and Angeli's salt, but not FeSO4, increases arginine transport. The regulation of arginine uptake by physiological factors suggests that arginine supply adapts in a moment-to-moment fashion to the changing needs of the neuron.

Nitric oxide production and regulation of neuronal NOS in tyrosine hydroxylase containing neurons

CAD cells are a murine CNS catecholaminergic (tyrosine hydroxylase-positive; TH+) neuronal cell line that undergoes morphological differentiation to resemble CNS catecholaminergic neurons upon serum deprivation. We show here that CAD cells also express neuronal nitric oxide synthase (nNOS) mRNA and protein and produce readily measurable levels of NO. Since both NO and catecholamines (L-DOPA; dopamine; norepinephrine) are redox active molecules, their production within the same cell may affect the cell's vulnerability to insult. Thus, we examined the regulation of NO production by CAD cells and the effect of NO on cell survival. NO is generated in a dose-dependent fashion by treatment with agents (ionomycin; A23817; KCl) known to increase calcium entry across the cell membrane. The NO level can be increased further by pretreatment with sepiapterin, a membrane permeable precursor for BH4 synthesis, suggesting that the BH4 levels or access required for nNOS activation is limited in CAD cells. Reducing mitochondrial Ca2+ uptake using ruthenium red (RuR) increased ionomycin-mediated NO production over ionomycin alone and indicates a critical role for mitochondria in nNOS regulation. Cell death was significantly increased by ionomycin treatment alone or in conjunction with reduced mitochondrial Ca2+ uptake. However, NO was not the primary mediator of cell death since NOS inhibitors rescued only less than 10% of the cells. These data suggest that endogenous NO production by nNOS is not a major factor in CAD cell death under these conditions.

The nitric oxide-guanylyl cyclase signaling pathway modulates membrane activity States and electrophysiological properties of striatal medium spiny neurons recorded in vivo

Nitric oxide (NO)-releasing interneurons are believed to regulate the activity of striatal medium spiny neurons (MSNs) that contain the NO effector enzyme guanylyl cyclase (GC). The involvement of NO-GC signaling in modulating steady-state membrane activity of striatal MSNs was examined using in vivo intracellular recordings in rats. Intrastriatal infusion of a neuronal NO synthase inhibitor or a NO scavenger via reverse microdialysis consistently decreased the amplitude of spontaneously occurring depolarized plateau potentials (up events). Intrastriatal infusion of a NO scavenger also decreased the amplitude of EPSPs evoked during electrical stimulation of the orbital prefrontal cortex. The effect of the NO scavenger on spontaneous up events was partially reversed by coperfusion with a cell-permeable cGMP analog. Intracellular injection of MSNs with a soluble GC inhibitor resulted in large decreases in the following: (1) spontaneous up-event amplitude, (2) responsiveness to depolarizing current, (3) action potential amplitude, and (4) input resistance. These effects were partially reversed by coinjection of cGMP. Conversely, intracellular injection of a phosphodiesterase inhibitor increased MSN neuron membrane excitability. These results indicate that, in the intact animal, the NO signaling pathway exerts a powerful tonic modulatory influence over the membrane activity of striatal MSNs via the activation of GC and stimulation of cGMP production.

7-Nitroindazole blocks conditioned place preference but not hyperactivity induced by morphine

The effects of 7-nitroindazole (7-NI), a neural nitric oxide synthase (nNOS) inhibitor, on spontaneous locomotor activity, morphine-induced hyperactivity, acquisition of place conditioning and morphine-induced conditioned place preference (CPP) were evaluated in male mice. In experiment 1, animals treated with 7-NI (25, 50 and 100mg/kg), morphine (40 mg/kg) or morphine (40 mg/kg) plus 7-NI (25, 50 or 100mg/kg) were placed in an actimeter for 3h. In experiment 2, animals treated with the same drugs and doses were conditioned following an unbiased procedure. 7-NI did not affect the spontaneous locomotor activity or hyperactivity induced by morphine. However, the moderate and high doses of 7-NI produced conditioned place aversion (CPA) and the lowest dose blocked morphine-induced CPP. Our results suggest that nitric oxide is involved in the rewarding properties of morphine but not in its motor effects.

Direct Physiological Evidence for Synaptic Connectivity Between Medium-Sized Spiny Neurons in Rat Nucleus Accumbens In Situ

Dual whole cell patch-clamp recordings in rat nucleus accumbens, the main component of the ventral striatum, were made to assess the presence of synaptic interconnections between medium-sized spiny neurons, a group of GABAergic and peptidergic neurons that constitute the principal cells of the striatum. Neurons were stained with biocytin for subsequent morphological analysis. Electrical activity of cells was recorded in current- and voltage-clamp mode; the characteristics of medium-sized spiny neurons were confirmed by electrophysiological and morphological properties. Thirteen of 38 medium-sized spiny neuron pairs (34%) showed a synaptic connection. In these pairs, suprathreshold stimulation with current injection evoked a train of action potentials in the presynaptic cell, which in turn elicited depolarizing postsynaptic potentials (dPSPs) in the postsynaptic cell. Twelve of these 13 pairs were connected unilaterally. The onset latency of the postsynaptic response was 1.7 ± 0.7 ms. dPSPs were blocked by 12.5 μM bicuculline, suggesting they were mediated by GABAA receptors. A linear fit of the current-voltage relationship of GABAergic currents crossed the voltage axis near the value of -20 mV, in agreement with the Cl- equilibrium potential predicted from the composition of the artificial cerebrospinal fluid and pipette medium. No evidence for electrotonic coupling was found. Paired-pulse facilitation and depression were induced when the amplitude of the first IPSC of a pair was relatively small and large, respectively. No clear dependence of paired-pulse facilitation or depression was found on the width of the spike interval, which ranged between 100 and 380 ms. Conversely, 1- to 2-s trains of dPSPs showed marked frequency facilitation at low presynaptic frequencies, but frequency depression at high firing rates. These data show that intra-accumbens synaptic communication between medium-sized spiny neurons exists, is mediated by GABAA receptors, and exhibits spike train-dependent short-term dynamics.

Effects of electrolytic and 6-hydroxydopamine lesions of rat nigrostriatal pathway on nitric oxide synthase and nicotinamide adenine dinucleotide phosphate diaphorase

The aim of the present study was to assess degenerative changes in the nitric oxide (NO) system of basal ganglia in animals with experimentally induced Parkinson's disease. In one procedure, rats were stereotaxically injected with 6-hydroxydopamine (6-OHDA) in the right medial forebrain bundle; in a second procedure, electrodes were implanted in the right substantia nigra pars compacta (SNc). After 15 and 30 days animals were tested for rotational asymmetry induced by apomorphine. Apomorphine induced rotation in lesioned animals, towards the ipsilateral side after electrolytic lesion and towards contralateral side in 6-OHDA animals. Structural deficits in basal ganglia were quantified by nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry and by nitric oxide synthase (NOS) immunoreactivity. 6-OHDA and electrolytic lesions induced a significant decrease in the number of NADPH-d/NOS positive cells in the lesion ipsilateral to SNc, in contrast with cell number increase in the ipsilateral dorsal striatum. By contrast, 6-OHDA-treated animals showed a decrease in the number of NOS immunoreactive cells in the contralateral nucleus accumbens. We conclude that populations of NO-synthesizing neurons are differentially regulated in Parkinson's disease induced by different experimental procedures.

Electrophysiological Interactions between Striatal Glutamatergic and Dopaminergic Systems

Glutamatergic and dopaminergic systems play a primary role in frontal-subcortical circuits involved in motor and cognitive functions. Considerable evidence has emerged indicating that the complex interaction between these neurotransmitter systems within the dorsal striatum and nucleus accumbens is critically involved in the gating of information flow in these highly integrative brain regions. As a result, disruptions of the interaction between glutamate and dopamine has been proposed as a pathological basis for a number of disorders, including the pathophysiology of schizophrenia. In this chapter, we discuss recent studies that have significantly advanced our understanding of the reciprocal interactions between glutamatergic and dopaminergic systems within the striatal complex in the normal brain and in pathological states.

D1 dopamine receptor stimulation increases GluR1 surface expression in nucleus accumbens neurons

The goal of this study was to understand how dopamine receptors, which are activated during psychostimulant administration, might influence glutamate-dependent forms of synaptic plasticity that are increasingly recognized as important to drug addiction. Regulation of the surface expression of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit GluR1 plays a critical role in long-term potentiation, a well-characterized form of synaptic plasticity. Primary cultures of rat nucleus accumbens neurons were used to examine whether dopamine receptor stimulation influences cell surface expression of GluR1, detected using antibody to the extracellular portion of GluR1 and fluorescence microscopy. Surface GluR1 labeling on processes of medium spiny neurons and interneurons was increased by brief (5-15 min) incubation with a D1 agonist (1 microm SKF 81297). This effect was attenuated by the D1 receptor antagonist SCH 23390 (10 microm) and reproduced by the adenylyl cyclase activator forskolin (10 microm). Labeling was decreased by glutamate (10-50 microm, 15 min). These results are the first to demonstrate modulation of AMPA receptor surface expression by a non-glutamatergic G protein-coupled receptor. Normally, this may enable ongoing regulation of AMPA receptor transmission in response to changes in the activity of dopamine projections to the nucleus accumbens. When dopamine receptors are over-stimulated during chronic drug administration, this regulation may be disrupted, leading to inappropriate plasticity in neuronal circuits governing motivation and reward.

Nitric oxide signalling system in rat brain stem: immunocytochemical studies

Nitric oxide is a free radical, which is produced in several tissues of the body and is thought to be the first of a new class of neural messenger molecules and a retrograde modulator of synaptic transmission in the brain. Nitric oxide synthase (NOS) is the enzyme that produces nitric oxide from the substrate l-arginine. The pattern of the distribution of neuronal isoform of NOS was investigated in neurones and fibres in the brain stem using standard immunocytochemistry. In our results, NOS positive neurones and processes were seen in the spinal trigeminal nucleus, gracile nucleus, nucleus of the solitary tract, nucleus ambiguus, reticular nuclei and lateral to the pyramidal tract of the medulla. In the pons, heavily labelled NOS containing neurones were seen in the pedunuclopontine tegmental nucleus, ventral tegmental nucleus and in the laterodorsal tegmental nucleus. The localization of neuronal NOS expressing neurones suggests a widespread neuromodulatory role for the nitric oxide in the central nervous system of rat.

Regulation of striatal dopamine neurotransmission by nitric oxide: effector pathways and signaling mechanisms

An important role for the reactive gas nitric oxide (NO) in regulating striatal dopaminergic neurotransmission was identified shortly after initial observations indicated that this unorthodox neurotransmitter mediates many of the influences of glutamatergic neurotransmission in the cerebellum, cortex, and hippocampus. While the precise actions of NO on striatal presynaptic and postsynaptic elements remain to be fully characterized, the recent application of sophisticated anatomical, neurochemical, and electrophysiological approaches to the study of nitrergic signaling has revealed that NO exerts a powerful influence both on tonic extracellular dopamine (DA) levels and phasic DA neuron spike activity via the modulation of intrinsic striatal mechanisms and striatonigral feedback loops. Although the nature of the NO-mediated modulatory influence on DA neurotransmission was initially clouded by seemingly conflicting neurochemical observations, a growing body of literature and understanding of the diverse signaling mechanisms and effector pathways utilized by NO indicates that NO exerts a primary facilitatory influence over tonic and phasic dopaminergic neurotransmission under physiological conditions. A review of neurochemical and electrophysiological studies examining the influence of endogenous and exogenous NO on DA neurotransmission indicates that NO signaling exerts multiple effects on local striatal circuits and projection neurons involved in regulating basal ganglia output and nigrostriatal DA neuron activity. In addition to summarizing these influences, the current review focuses on the mechanisms utilized by striatal NO signaling pathways involved in modulating DA transmission at the level of the terminal and cell body and attempts to integrate these observations into a functional model of NO-dependent regulation of basal ganglia systems.

Colocalization of neuronal nitric oxide synthase and neurokinin-1 receptor in striatal interneurons in the rat

It has been established that nitric oxide synthase (NOS)-containing aspiny neurons constitute one class of interneurons in the striatum, and that substance P (SP)-containing projection neurons give off many axon collaterals within the striatum. In the present study, we investigated a morphological substrate of possible influences of SP-containing projection neurons upon NOS-containing interneuron in the rat striatum; colocalization of immunoreactivities for NOS and NK-1 type tachykinin receptor (NK1R: SP receptor) was examined by a double-immunofluorescence histochemistry. The vast majority (94.6%) of NOS-positive neurons showed NK1R immunoreactivity, whereas only smaller cells of NK1R-positive neurons (61.2% of NK1R-positive neurons) displayed NOS immunoreactivity. The results indicated that the NOS-containing interneurons were under direct control of SP-containing projection neurons in the striatum.