NMDA-induced acetylcholine release in mouse striatum: role of NO synthase isoforms

d-Serine Increases Release of Acetylcholine in Rat Submandibular Glands

d-serine has been observed in submandibular gland tissue in rats, but its functions remain to be clarified. Oral administration of d-serine, but not l-serine, increased its concentrations in the submandibular gland and pilocarpine-induced salivary secretion. In vivo microdialysis was used to collect the d- and l-enantiomers of amino acids from local interstitial fluid in the rat submandibular gland. The proportion of the d-form of serine in interstitial fluid was higher than that in plasma or saliva. Perfusion of the rat submandibular gland with d-serine and l-glutamic acid via the submandibular gland artery resulted in a significant increase in salivary secretion after stimulation of muscarinic receptors with carbachol. In vivo microdialysis applied to the submandibular glands of rats showed that infusion of d-serine along with l-glutamate through the microdialysis probe significantly elevated acetylcholine levels in local interstitial fluids in the submandibular glands of anesthetized rats as compared to that with l-glutamate alone in an N-methyl-d-aspartate receptor glycine site antagonist-sensitive manner. These results indicate that d-serine augments salivary secretion by increasing acetylcholine release in the salivary glands.

Effect of an inhibitor of neuronal nitric oxide synthase 7-nitroindazole on cerebral hemodynamic response and brain excitability in urethane-anesthetized rats

The role of neuronal nitric oxide synthase (nNOS) in the pathophysiology of epilepsy and seizures remains disputable. One of the reasons why results from the acute in vivo studies display controversies might be the effect on the regulation of cerebral blood flow (CBF) during pharmacologically induced alterations of NO system. We examined neurovascular coupling in the rat sensorimotor cortex in response to transcallosal stimulation under nNOS inhibition by 7-nitroindazole (7-NI). Adult Wistar rats were anesthetized with urethane and epidural silver EEG electrodes were implanted over sensorimotor cortices. Regional CBF was measured by Laser Doppler Flowmetry (LDF). We catheterized a common carotid artery to measure arterial blood pressure (BP). 7-NI did not significantly affect blood pressure and heart rate. Electrophysiological recordings of evoked potentials (EPs) revealed no effect on their amplitude, rhythmic potentiation or depression of EPs. Transcallosal stimulation of the contralateral cortex induced a frequency dependent rise in CBF. Although 7-NI did not significantly affect basal CBF and cortical excitability, hemodynamic responses to the transcallosal stimulation were diminished implicating a role of nNOS in neurovascular coupling. Urethane anesthesia is suitable for future epileptological experiments. Our findings demonstrate that NO contributes to the hemodynamic response during brain activation.

Possible synergistic effect of direct angiotensin II type 2 receptor stimulation by compound 21 with memantine on prevention of cognitive decline in type 2 diabetic mice

Type 2 diabetes mellitus (T2DM) is known to be associated with increased risk of cognitive impairment including Alzheimer disease. Recent studies have suggested an interaction between angiotensin II and N-methyl-d-aspartic acid (NMDA) glutamate receptors. We previously reported that stimulation of the angiotensin II type 2 (AT2) receptor exerts brain protective effects. A newly developed AT2 receptor agonist, compound 21 (C21), has enabled examination of the direct effect of AT2 receptor stimulation in vivo. Accordingly, we examined the possible synergistic effect of C21 and memantine on cognitive impairment in T2DM mice, KKAy. KKAy were divided into four groups; (1) control, (2) treatment with C21 (10 μg/kg/day), (3) treatment with memantine (20mg/kg/day), and (4) treatment with both for 4 weeks, and subjected to Morris water maze tasks. Treatment with C21 or memantine alone at these doses tended to shorten escape latency compared to that in the control group. C21 treatment increased cerebral blood flow (CBF), but memantine did not influence CBF. Treatment with C21 or C21 plus memantine increased hippocampal field-excitatory postsynaptic potential (f-EPSP). Moreover, treatment with memantine or C21 increased acetylcholine level, which was lower in KKAy than in wild-type mice, and C21 plus memantine treatment enhanced memantine or C21-induced acetylcholine secretion. This study provides an insight into new approaches to understand the interaction of angiotensin II and neurotransmitters. We can anticipate a new therapeutic approach against cognitive decline using C21 and memantine.

Nitric oxide modulates the cardiovascular effects elicited by acetylcholine in the NTS of awake rats

Microinjection of acetylcholine chloride (ACh) in the nucleus of the solitary tract (NTS) of awake rats caused a transient and dose-dependent hypotension and bradycardia. Because it is known that cardiovascular reflexes are affected by nitric oxide (NO) produced in the NTS, we investigated whether these ACh-induced responses depend on NO in the NTS. Responses to ACh (500 pmol in 100 nl) were strongly reduced by ipsilateral microinjection of the NOS inhibitor NG-nitro-l-arginine methyl ester (l-NAME; 10 nmol in 100 nl) in the NTS: mean arterial pressure (MAP) fell by 50 ± 5 mmHg before l-NAME to 9 ± 4 mmHg, 10 min after l-NAME, and HR fell by 100 ± 26 bpm before l-NAME to 20 ± 10 bpm, 10 min after l-NAME (both P < 0.05). Microinjection of the selective inhibitor of neuronal nitric oxide synthase (nNOS), 1-(2-trifluoromethylphenyl) imidazole (TRIM; 13.3 nmol in 100 nl), in the NTS also reduced responses to ACh: MAP fell from 42 ± 3 mmHg before TRIM to 27 ± 6 mmHg, 10 min after TRIM ( P < 0.05). TRIM also tended to reduce ACh-induced bradycardia, but this effect was not statistically significant. ACh-induced hypotension and bradycardia returned to control levels 30–45 min after NOS inhibition. Control injections with d-NAME and saline did not affect resting values or the response to ACh. In conclusion, injection of ACh into the NTS of conscious rats induces hypotension and bradycardia, and these effects may be mediated at least partly by NO produced in NTS neurons.

Nitric oxide modulates the discharge rate of basal forebrain neurons

RATIONALE

During prolonged wakefulness, the concentrations of nitric oxide (NO) and adenosine (AD) increase in the basal forebrain (BF). AD inhibits neuronal activity via adenosine (A1) receptors, thus providing a potential mechanism for sleep facilitation. Although NO in the BF increases adenosine and promotes sleep, it is not clear whether the sleep promotion by NO is mediated through adenosine increase, or NO independently of adenosine could modulate sleep.

OBJECTIVE

The objective of the study was to clarify whether NO modulates the discharge rate of BF neurons and whether this effect is mediated via AD.

MATERIALS AND METHODS

We measured the discharge rates of BF neurons in anesthetized rats during microdialysis infusion of NO donor alone or in combination with A1 receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine.

RESULTS

NO dose dependently modulated the discharge rate of BF neurons. NO donor (0.5 mM) increased the discharge rates in 48% of neurons and decreased it in 22%. A 1-mM dose decreased it in 55% and increased in 18%. Tactile stimulus affected the discharge rates of most neurons: 60% increased (stimulus-on) it and 14% decreased it (stimulus-off). A 1-mM NO donor predominantly inhibited neurons of both stimulus related types. A small proportion of stimulus-on (23%) neurons but none of the stimulus-off neurons were activated by NO donor. The blockade of A1 receptors partly prevented the inhibitory effect of NO on most of the neurons. This response was more prominent in stimulus-on than in stimulus-off neurons.

CONCLUSION

NO modulates the BF neuronal discharge rates in a dose-dependent manner. The inhibitory effect is partly mediated via adenosine A1 receptors.

In vivo electrophysiology of dopamine-denervated striatum: focus on the nitric oxide/cGMP signaling pathway

Within the striatum, the gaseous neurotransmitter nitric oxide (NO) is produced by a subclass of interneurons containing the neuronal NO synthase (nNOS). NO promotes the second messenger cGMP through the activation of the soluble guanyl cyclase (sGC) and plays a crucial role in the integration of glutamate (GLU) and DA transmission. The aim of this study was to characterize the impact of 6-hydroxyDA (6-OHDA) lesion of the rat nigrostriatal pathway on NO/cGMP system. In vivo extracellular single units recordings were performed under urethane anesthesia to avoid any potentially misleading contributions of cortically-driven changes on endogenous NO. Hence, no electrical extrastriatal stimulation was performed and great attention was paid to the effects of 3-morpholinosydnonimine (SIN-1, a NO donor), N(G)-nitro-L-arginine methyl ester (L-NAME, a nonselective NOS inhibitor) and Zaprinast (a PDE inhibitor) delivered by iontophoresis upon the main striatal phenotypes. The latter were operationally distinguished in silent medium spiny-like neurons (MSN), with negligible spontaneous activity but displaying glutamate-induced firing discharge at rest and spontaneously active neurons (SAN), representing to a large extent nonprojecting interneurons. SANs were excited by SIN-1 and Zaprinast while MSNs showed a clear inhibition during local iontophoretic application of SIN-1 and Zaprinast. In 6-OHDA animals, SIN-1-induced excitation in SANs was significantly increased (on the contrary, the inhibitory effect of L-NAME was less effective). Interestingly, in DA-denervated animals, a subclass of MSNs (40%) displayed a peculiar excitatory response to SIN-1. These findings support the notion of an inhibitory modulatory role exerted by endogenous NO on control striatal projection cells. In addition, these findings suggest a functional cross-talk between NO, spontaneously active interneurons, and projection neurons that becomes critical in the parkinsonian state.

NO synthase-positive striatal interneurons are decreased in schizophrenia

The gaseous messenger NO has repeatedly been suggested to play a role in the pathogenesis of psychoses. Following a pilot study, we investigated whether the number of nitrinergic neurons in the putamen of patients suffering from schizophrenia, bipolar disorder or major depression is altered. Post-mortem striatum sections of 15 brains from patients with either disease were examined by NADPH-diaphorase staining, which selectively labels NO synthase-positive interneurons. Quantification of these cells revealed significantly lower numbers of NO synthase-containing neurons in the putamen of schizophrenic patients. Our results suggest that striatal nitrinergic interneurons are involved in the pathophysiology of at least some forms of schizophrenia, such as e.g. catatonic schizophrenia.

Expression of the cGMP-specific phosphodiesterases 2 and 9 in normal and Alzheimer's disease human brains

We studied the mRNA expression of cGMP-hydrolysing phosphodiesterases (PDEs) in selected brain areas of normal elderly people and patients with Alzheimer's disease. Using radioactive in-situ hybridization histochemistry we found a widespread distribution of the mRNA for PDE2 and PDE9, whereas no specific hybridization signal was observed for PDE5. We observed PDE2 and PDE9 mRNA in all cortical areas studied (insular cortex, entorhinal cortex and visual cortex), although to a different extent. PDE2 mRNA was high in the claustrum, whereas PDE9 mRNA was moderate. PDE2 and PDE9 mRNAs was present in the putamen. No cGMP-hydrolysing PDE expression was observed in the globus pallidus. PDE2 and PDE9 mRNA was observed in all subareas of the hippocampus; however, there were significant differences in the amount of expression. In the Purkinje and cerebellar granule cells only PDE9 expression was observed. PDE2 and PDE9 mRNA expression was not significantly different in Alzheimer's disease brains.

Soluble amyloid beta1-42 reduces dopamine levels in rat prefrontal cortex: relationship to nitric oxide

Several studies suggest a pivotal role of amyloid beta (Abeta)(1-42) and nitric oxide (NO) in the pathogenesis of Alzheimer's disease. NO also possess central neuromodulatory properties. To study the soluble Abeta(1-42) effects on dopamine concentrations in rat prefrontal cortex, microdialysis technique was used. We showed that i.c.v. injection or retrodialysis Abeta(1-42) administration reduced basal and K(+)-stimulated dopamine levels, measured 2 and 48 h after peptide administration. Immunofluorescent experiments revealed that after 48 h from i.c.v. injection Abeta(1-42) was no longer detectable in the ventricular space. We then evaluated the role of NO on Abeta(1-42)-induced reduction in dopamine concentrations. Subchronic L-arginine administration decreased basal dopamine levels, measured either 2 h after i.c.v. Abeta(1-42) or on day 2 post-injection, whereas subchronic 7-nitroindazole administration increased basal dopamine concentrations, measured 2 h after i.c.v. Abeta(1-42) injection, and decreased them when measured on day 2 post-Abeta(1-42)-injection. No dopaminergic response activity was observed after K(+) stimulation in all groups. These results suggest that the dopaminergic system seems to be acutely vulnerable to soluble Abeta(1-42) effects. Finally, the opposite role of NO occurring at different phases might be regarded as a possible link between Abeta(1-42)-induced effects and dopaminergic dysfunction.

Haplotype analysis of endothelial nitric oxide synthase (NOS3) genetic variants and tardive dyskinesia in patients with schizophrenia

OBJECTIVES

Several studies have indicated the involvement of nitric oxide (NO) in the pathogenesis of tardive dyskinesia (TD), an incapacitating adverse movement disorder associated with long-term antipsychotic treatment. In human brain, the NO could be generated by endothelial nitric oxide synthase (NOS3). In this study, we studied whether the genetic variants in human NOS3 gene is associated with TD in patients with schizophrenia.

METHODS

Two hundred and eighty-two chronic inpatients with schizophrenia treated with typical antipsychotics were recruited in this study. The patients were further grouped by the presence of TD or not according to the Research and Diagnostic Criteria for TD. The genetic variants in the NOS3 gene investigated in this study were -786T > C in the promotor region, 27-bp variable number of tandem repeats (27-bp VNTR) in intron 4, and Glu298Asp in exon 7. The frequencies of genotypes, alleles and haplotypes of the three markers were compared between the TD (n = 153) and non-TD (n = 129) groups.

RESULTS

There were no significant associations between the genotypes and alleles of the three markers and TD. However, in the haplotype-based case-control analysis, the frequency of haplotype T-4b-Glu was significantly higher in non-TD than in TD group (TD vs. non-TD = 72.7% vs. 81.0%, permutation P value = 0.021, OR = 0.648, 95% CI = 0.432-0.973).

CONCLUSIONS

We found that the haplotype T-4b-Glu represents a protective haplotype against TD after long-term antipsychotic treatment. This finding suggests that human NOS3 gene may be involved in the pathogenesis of TD.

Microdialysis as a tool in local pharmacodynamics

In many cases the clinical outcome of therapy needs to be determined by the drug concentration in the tissue compartment in which the pharmacological effect occurs rather than in the plasma. Microdialysis is an in vivo technique that allows direct measurement of unbound tissue concentrations and permits monitoring of the biochemical and physiological effects of drugs throughout the body. Microdialysis was first used in pharmacodynamic research to study neurotransmission, and this remains its most common application in the field. In this review, we give an overview of the principles, techniques, and applications of microdialysis in pharmacodynamic studies of local physiological events, including measurement of endogenous substances such as acetylcholine, catecholamines, serotonin, amino acids, peptides, glucose, lactate, glycerol, and hormones. Microdialysis coupled with systemic drug administration also permits the more intensive examination of the pharmacotherapeutic effect of drugs on extracellular levels of endogenous substances in peripheral compartments and blood. Selected examples of the physiological effects and mechanisms of action of drugs are also discussed, as are the advantages and limitations of this method. It is concluded that microdialysis is a reliable technique for the measurement of local events, which makes it an attractive tool for local pharmacodynamic research.

The physiology and pathophysiology of nitric oxide in the brain

Nitric oxide (NO) is a molecule with pleiotropic effects in different tissues. NO is synthesized by NO synthases (NOS), a family with four major types: endothelial, neuronal, inducible and mitochondrial. They can be found in almost all the tissues and they can even co-exist in the same tissue. NO is a well-known vasorelaxant agent, but it works as a neurotransmitter when produced by neurons and is also involved in defense functions when it is produced by immune and glial cells. NO is thermodynamically unstable and tends to react with other molecules, resulting in the oxidation, nitrosylation or nitration of proteins, with the concomitant effects on many cellular mechanisms. NO intracellular signaling involves the activation of guanylate cyclase but it also interacts with MAPKs, apoptosis-related proteins, and mitochondrial respiratory chain or anti-proliferative molecules. It also plays a role in post-translational modification of proteins and protein degradation by the proteasome. However, under pathophysiological conditions NO has damaging effects. In disorders involving oxidative stress, such as Alzheimer's disease, stroke and Parkinson's disease, NO increases cell damage through the formation of highly reactive peroxynitrite. The paradox of beneficial and damaging effects of NO will be discussed in this review.

Rationale for and use of NMDA receptor antagonists in Parkinson's disease

N-Methyl-d-aspartate (NMDA) glutamate receptors are a class of excitatory amino acid receptors, which have several important functions in the motor circuits of the basal ganglia, and are viewed as important targets for the development of new drugs to prevent or treat Parkinson's disease (PD). NMDA receptors are ligand-gated ion channels composed of multiple subunits, each of which has distinct cellular and regional patterns of expression. They have complex regulatory properties, with both agonist and co-agonist binding sites and regulation by phosphorylation and protein-protein interactions. They are found in all of the structures of the basal ganglia, although the subunit composition in the various structures is different. NMDA receptors present in the striatum are crucial for dopamine-glutamate interactions. The abundance, structure, and function of striatal receptors are altered by the dopamine depletion and further modified by the pharmacological treatments used in PD. In animal models, NMDA receptor antagonists are effective antiparkinsonian agents and can reduce the complications of chronic dopaminergic therapy (wearing off and dyskinesias). Use of these agents in humans has been limited because of the adverse effects associated with nonselective blockade of NMDA receptor function, but the development of more potent and selective pharmaceuticals holds the promise of an important new therapeutic approach for PD.

cGMP: a second messenger for acetylcholine in the brain?

Already 30 years ago, it became apparent that there exists a relationship between acetylcholine and cGMP in the brain. Acetylcholine plays a role in a great number of processes in the brain, however, the role of cGMP in these processes is not known. A review of the data shows that, although the connection between NO-mediated cGMP synthesis and acetylcholine is firmly established, the complexities of the heterosynaptic pathways and the oligosynaptic structures involved preclude a clear definition of the role of cGMP in the functioning of acetylcholine presently.

Identification of neurons with acetilcholinesterase and NADPH-diaphorase activities in the centrifugal visual system of the chick

The isthmo-optic nuclei (ION) and ectopic neurons, which constitute the centrifugal visual system (CVS), are thought to be cholinoceptive and nitrergic. However, it is not clear which neurons express these markers, namely the ones that project to the retina rather than in neurons that only participate in a local circuit. Therefore, to characterize the neurochemical patterns of the centrifugal visual system in the post-hatched chick, retinopetal cells of the isthmo-optic nuclei and the ectopic region were identified via immunolabeling for cholera toxin, a neuronal tracer, which has been injected in the ocular globe. Then, double labeled with acetylcholinesterase histochemistry to reveal cholinergic synapses, or NADPH-diaphorase histochemistry as a nitrergic marker. Briefly, acetylcholinesterase activity was present mainly in cholera toxin labeled cell bodies of the isthmo-optic nucleus and the ectopic region indicating that retinal projecting neurons of centrifugal visual system comprise a cholinoceptive pathway. On the other hand, NADPH-diaphorase histochemistry was present in the neuropile and sparse cell bodies inside of the isthmo-optic nucleus and in ectopic neurons which were not cholera toxin positive suggesting their role in an intrinsic circuit of the centrifugal visual system. These data support the idea that these two neurochemical systems are present in distinct neuronal populations in the centrifugal visual system.

Effects of the nitric oxide donor, DEA/NO on cortical spreading depression

Cortical spreading depression (CSD) is a transient disruption of local ionic homeostasis that may promote migraine attacks and the progression of stroke lesions. We reported previously that the local inhibition of nitric oxide (NO) synthesis with Nomega-nitro-L-arginine methyl ester (L-NAME) delayed markedly the initiation of the recovery of ionic homeostasis from CSD. Here we describe a novel method for selective, controlled generation of exogenous NO in a functioning brain region. It is based on microdialysis perfusion of the NO donor, 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA/NO). As DEA/NO does not generate NO at alkaline pH, and as the brain has a strong acid-base buffering capacity, DEA/NO was perfused in a medium adjusted at alkaline (but unbuffered) pH. Without DEA/NO, such a microdialysis perfusion medium did not alter CSD. DEA/NO (1, 10 and 100 microM) had little effect on CSD by itself, but it reversed in a concentration-dependent manner the effects of NOS inhibition by 1 mM L-NAME. These data demonstrate that increased formation of endogenous NO associated with CSD is critical for subsequent, rapid recovery of cellular ionic homeostasis. In this case, the molecular targets for NO may be located either on brain cells to suppress mechanisms directly involved in CSD genesis, or on local blood vessels to couple flow to the increased energy demand associated with CSD.