3,4-Diaminopyridine-evoked noradrenaline release in rat hippocampal slices: facilitation by endogenous or exogenous nitric oxide

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.

Beta-adrenergic receptors link NO/sGC/PKG signaling to BDNF expression during the consolidation of object recognition long-term memory

The nitric oxide (NO)/soluble guanylyl cyclase (sGC)/protein kinase G (PKG) pathway is important for memory processing, but the identity of its downstream effectors as well as its actual participation in the consolidation of nonaversive declarative long-term memory (LTM) remain unknown. Here, we show that training rats in an object recognition (OR) learning task rapidly increased nitrites/nitrates (NOx) content in the CA1 region of the dorsal hippocampus while posttraining intra-CA1 microinfusion of the neuronal NO synthase (nNOS) inhibitor L-NN hindered OR LTM retention without affecting memory retrieval or other behavioral variables. The amnesic effect of L-NN was not state dependent, was mimicked by the sGC inhibitor LY83583 and the PKG inhibitor KT-5823, and reversed by coinfusion of the NO donor S-nitroso-N-acetylpenicillamine (SNAP) and the PKG activator 8-bromoguanosine 3',5'-cyclic monophosphate (8Br-cGMP). SNAP did not affect the amnesic effect of LY83583 and KT-5823. Conversely, 8Br-cGMP overturned the amnesia induced by LY83583 but not that caused by KT-5823. Intra-CA1 infusion of the beta-adrenergic receptor blocker timolol right after training hindered OR LTM and, although coadministration of noradrenaline reversed the amnesia caused by L-NN, LY83583, and KT5823, the amnesic effect of timolol was unaffected by coinfusion of 8Br-cGMP or SNAP, indicating that hippocampal beta-adrenergic receptors act downstream NO/sGC/PKG signaling. We also found that posttraining intra-CA1 infusion of function-blocking anti-brain-derived neurotrophic factor (BDNF) antibodies hampered OR LTM retention, whereas OR training increased CA1 BDNF levels in a nNOS- and beta-adrenergic receptor-dependent manner. Taken together, our results demonstrate that NO/sGC/PKG signaling in the hippocampus is essential for OR memory consolidation and suggest that beta-adrenergic receptors link the activation of this pathway to BDNF expression during the consolidation of declarative memories.

Cognitive deficits in aged rats correlate with levels of L-arginine, not with nNOS expression or 3,4-DAP-evoked transmitter release in the frontoparietal cortex

Aging is associated with altered neurotransmitter function in the brain. In this study, we measured release parameters for acetylcholine (ACh), norepinephrine and serotonin in the frontoparietal cortex of young and aged rats. We also determined cortical amino acid concentrations and nitric oxide (NO) synthase function. Prior to sacrifice, the rats had been tested for Morris water-maze performance. In aged, compared with young rats, we observed a reduction in both uptake of choline and acetylcholine release. Serotonin release and L-arginine concentrations (a precursor of NO) showed an aging-related increase; however, L-citrulline/L-arginine ratios were decreased in aged rats. Moreover, while most age-related changes in transmitter release or neurochemical markers were not related to the learning performance, L-arginine concentrations were positively correlated to cognitive deficits. NO synthase concentrations were not affected by aging. It is suggested that events related to L-arginine-to-L-citrulline/NO metabolism in the frontoparietal cortex may take part in age-related cognitive deficits.

3,4-DAP-evoked transmitter release in hippocampal slices of aged rats with impaired memory

Based on a slice superfusion technique, this study investigated the release of acetylcholine, noradrenaline and serotonin in the hippocampus of aged rats (25-27 months) showing no or severe deficits in a spatial reference-memory task (water maze). Young adults (3-5 months) were used as controls. 3,4-diaminopyridine (3,4-DAP), a potassium channel antagonist which increases neuronal excitability, was used to evoke the overflow of the three neurotransmitters. The release of [3H]noradrenaline induced by stimulation of presynaptic nicotinic receptors was also assessed. The experiment compared the accumulation and 3,4-DAP-evoked (or nicotine-evoked) overflow of [3H] in hippocampal slices preincubated with [3H]choline, [3H]noradrenaline, or [3H]serotonin. In aged rats, only the accumulation of [3H]serotonin was reduced significantly (-17%). In percent of tissue-[3H], the 3,4-DAP-evoked overflow of [3H]serotonin was increased (+28%), and that of [3H]acetylcholine was reduced (-23%) in the aged rats. The nicotine-evoked overflow of [3H]noradrenaline was not altered in aged rats. There was a significant correlation of water-maze performance (distance to platform) and evoked overflow of [3H]serotonin. It is concluded that hippocampal cholinergic functions are more altered by aging than noradrenergic or serotonergic ones. Excessive excitability of serotonergic terminals, perhaps in addition to cholinergic dysfunction, might be a crucial factor accounting for age-related cognitive deficits in the present population of rats.

Raphé grafts and 3,4-diaminopyridine-evoked overflow of serotonin in the rat hippocampus after 5,7-dihydroxytryptamine lesions: evidence for 5-HT1A autoreceptors

A first experiment verified that the overflow of 5-HT evoked by 75 microM 3,4-diaminopyridine in superfused hippocampal slices was calcium-dependent, tetrodotoxin-sensitive and modulable by drugs acting on 5-HT autoreceptors. Subsequently, the technique was used in rats to investigate the effects of 5,7-dihydroxytryptamine lesions and intrahippocampal serotonergic grafts. The lesions reduced the accumulation (-81%) and relative evoked overflow (-23%; absolute evoked overflow -86%) of [ H]5-HT, but increased the relative baseline overflow (+23%; absolute baseline overflow -78%). Grafts partially compensated for these effects. In slices from grafted rats, the evoked overflow was reduced by application of a 5-HT receptor agonist (8-OH-DPAT), a response not found in sham-operated and lesion-only rats. Although the graft-induced effects were less marked than in previous studies, they were beneficial and modulated by a mechanism that normally does not operate in the intact hippocampus.

Differential contributions of nitric oxide synthase isoforms at hippocampal formation to negative feedback regulation of penile erection in the rat

We established previously that a novel negative feedback mechanism for the regulation of penile erection, which is triggered by ascending sensory inputs initiated by tumescence of the penis, exists in the hippocampal formation (HF). This study further evaluated the participation of nitric oxide (NO) and the contribution of nitric oxide synthase (NOS) isoforms at the HF in this process. Adult, male Sprague-Dawley rats that were anaesthetized and maintained with chloral hydrate were used, and intracavernous pressure (ICP) recorded from the corpus cavernosum of the penis was employed as our experimental index for penile erection. Microinjection bilaterally of a NO donor, S-nitroso-N-acetylpenicillamine (0.25 or 1 nmoles), or the NO precursor, L-arginine (1 or 5 nmoles), into the hippocampal CA1 or CA3 subfield or dentate gyrus elicited a significant reduction in baseline ICP. Bilateral hippocampal application of a NO trapping agent, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (10 nmoles), significantly potentiated the elevation in ICP induced by intracavernous administration of papaverine (400 microg). Microinjection bilaterally into the HF of equimolar doses (0.5 or 2.5 pmoles) of two selective neuronal NOS inhibitors, 7-nitroindazole or N(omega)-propyl-L-arginine; or equimolar doses (50 or 250 pmoles) of two selective inducible NOS inhibitors, aminoguanidine or S-methylisothiourea, significantly enhanced the magnitude and/or duration of the papaverine-induced elevation in ICP. In contrast, hippocampal application of a potent endothelial NOS inhibitor, N5-(1-iminoethyl)-L-ornithine (18 or 92 nmoles), was ineffective. Neither of these inhibitors, furthermore, affected baseline ICP. These results suggest that NO generated via both neuronal and inducible NOS at the HF may participate in negative feedback regulation of penile erection.

Nitric oxide and peroxynitrite as factors to stimulate neurotransmitter release in the CNS

This review summarizes the stimulatory potentials of NO and peroxynitrite (OONO-) on neurotransmitter release in the central nervous system. Exogenous and endogenous NO stimulates to release neurotransmitter. NO synthesized intracellularly diffuses out through neuronal membrane and acts on the outer side of membrane to depolarize neuronal membrane, which triggers neurotransmitter release. NO-induced release of neurotransmitters is mediated by Ca2+-dependent and -independent processes. The latter process is operated by reverse process of the Na+-dependent carrier-mediated neurotransmitter uptake system or by unknown mechanisms. Ca2+-dependent release of neurotransmitter occurs in part subsequent to increase in Ca2+ influx via VDCCs, although N-type VDCCs may not involve in this action of NO because of suppression of Ca2+ influx through N-type VDCCs by NO. Participation of cGMP formation by NO on neurotransmitter release is controversial. A superoxide scavenger, Ca2+, Zn(2+)-superoxide dismutase, abolishes NO-induced neurotransmitter release and synthesized OONO- induces neurotransmitter release, indicating that OONO- participates in NO-evoked neurotransmitter release.

Role of nitric oxide in the regulation of monoaminergic neurotransmission

Data accumulated in the last decade indicate that nitric oxide (NO) participates in the regulation of neurotransmission in the central nervous system. Due to its physicochemical properties, NO is an ideal mediator of nonsynaptic interactions. The importance of monoaminergic systems in the function of the brain is clearly shown by the number of severe neuropsychiatric diseases (e.g. depression, Parkinson's disease) caused by the impairment of monoaminergic neurotransmission. Because of their neuroanatomical characteristic, monoaminergic systems participate mainly in nonsynaptic interactions. Since NO is a potential nonsynaptic modulator, it may have an important role in the regulation of monoaminergic systems. The aim of the present review is to survey the literature on the effect of NO on dopaminergic, noradrenergic and serotonergic neurotransmission. The potential mechanisms of action are summarized. Since there is no agreement in the literature on the nature of the effect of NO exerted on monoaminergic neurotransmission, and there are contradictory data concerning the mechanisms involved, the possible reasons for this unusual inconsistency are also discussed.

Capsaicin-evoked release of immunoreactive calcitonin gene-related peptide from the spinal cord is mediated by nitric oxide but not by cyclic GMP

Recent data support a role for nitric oxide (NO) in pain processing at the level of the spinal cord, possibly via regulation of neuropeptide release. The goal of this study was to determine whether capsaicin, which selectively activates primary afferent neurons and evokes neuropeptide release, acts in an NO-dependent manner. Our results indicate that capsaicin (1 microM)-evoked release of immunoreactive calcitonin gene-related peptide (iCGRP) is significantly reduced in the presence of the NO synthase inhibitor, L-NAME (10-400 nM; F(3,45)=68.38; P<0.001) and, the selective nNOS inhibitor, 3-bromo-7-nitroindazole (170-680 nM; F(5,48)=56.2; P<0. 01). D-NAME (200 nM) had no effect on capsaicin-evoked iCGRP release. Hemoglobin (an extracellular scavenger of NO; 3 mg/ml) significantly reduced the effect of capsaicin on the release of iCGRP (F(1,8)=9.12; P<0.05). The NOS substrate, L-arginine, effectively reversed the inhibitory effect of 3-bromo-7-nitroindazole on capsaicin-evoked iCGRP release. To determine whether the NO-mediated release was NMDA-driven, we superfused spinal cord slices with competitive and non-competitive NMDA antagonists in the presence and absence of capsaicin. MK-801 (0. 1-10 microM; F(4,33)=8.49; P<0.0001) and AP-5 (0.01-10 microM; F(4, 38)=3.34; P<0.05) reduced capsaicin-evoked iCGRP release. CNQX, an AMPA/kainate antagonist (10 nM-10 microM), significantly decreased capsaicin-evoked release of iCGRP (F(6,42)=8.76; P<0.01) in a dose-dependent fashion. Additionally, our results demonstrate that while capsaicin-evoked release is significantly reduced in the presence of LY-83583 (10 microM; F(2,18)=3.46; P<0.01; a cyclic GMP lowering agent), there is no effect of ODQ (a potent and selective inhibitor of guanylate cyclase). Moreover, the application of a cell permeable analog of cyclic GMP (8-bromo-cGMP; 0.01-1000 microM) is without effect on both basal and evoked iCGRP release. Finally, we observed no colocalization of immunoreactive neuronal NOS (nNOS) with CGRP in the dorsal horn. In summary, these data indicate that capsaicin evokes the release of iCGRP, in part, via the production of NO which enters the extracellular space prior to having an effect. Moreover, iCGRP and nNOS are produced in distinct populations of neurons within the dorsal horn. We conclude that capsaicin-evoked release involves the activation of the NMDA receptor but is also modified by the activation of AMPA or kainate receptors. Finally, these data suggest that while capsaicin-evoked iCGRP release is modified by NO, this release does not require the activation of guanylate cyclase and subsequent production of cyclic GMP.

Involvement of Hg2+-sensitive sulfhydryl groups in regulating noradrenaline release induced by S-nitrosocysteine in rat brain slices

Nitric oxide has been shown to regulate neurotransmitter release. Previously, we reported that S-nitrosothiols such as S-nitrosocysteine (SNC) stimulate noradrenaline (NA) release in rat hippocampus in vivo and in vitro. To examine the role of sulfhydryl groups in SNC-induced NA release, the effects of metal ions such as Hg2+ and N-ethylmaleimide (NEM, a sulfhydryl alkylating agent) on [3H]NA release from labeled rat brain slices (hippocampus and cerebral cortex) were studied and compared with the effects of SNC. The addition of 200 microM HgCl2, but not Pb2+, Zn2+, or Cd2+, stimulated [3H]NA release from both types of slices in the presence of extracellular CaCl2. p-Chloromercuribenzoic acid (p-CMBA) also stimulated [3H]NA release. NEM stimulated [3H]NA release from both types of slices in the presence and absence of extracellular CaCl2. The effect of 200 microM NEM was enhanced, but the effect of 200 microM SNC was inhibited by co-addition of 200 microM p-CMBA in the absence of extracellular CaCl2. The concentration-response curve of SNC shifted to the right after co-addition of 200 microM p-CMBA or 100 microM HgCl2, although the effect of 200 microM NEM was additive to the effect of SNC. These findings demonstrate that SNC acts as a sulfhydryl agent on proteins that regulate NA release, and that SNC may share the same sulfhydryl groups with Hg compounds. The effect of T-588 ¿(R)-(-)-(benzo[b]thiophen-5-yl)-2-[2-(N,N-diethylamino)ethoxy]eth anol hydrochloride¿, a novel cognitive enhancer and a stimulator of NA release, was compared with the effects of sulfhydryl reagents.

Nitric oxide modulates release of noradrenaline in guinea-pig gastric fundus

The interaction between nitric oxide (NO) and the release of [(3)H]noradrenaline ([(3)H]NA) in conditions of non-activated and activated nicotinic receptors in guinea-pig gastric fundus preincubated with [(3)H]NA was studied. Nicotinic receptor agonist, 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) (100 microM) significantly increased the resting release of [(3)H]NA. NO-synthase inhibitor, N(omega)-nitro-L-arginine (L-NNA) (100 microM) significantly decreased DMPP-induced release of [(3)H]NA. Field electrical stimulation (FES) (2Hz; 1 ms; 360 st) significantly increased the release of [(3)H]NA above the basal levels. L-NNA significantly decreased the stimulation-evoked release of [(3)H]NA. DMPP increased the stimulation-evoked release of [(3)H]NA, effect which was significantly decreased by L-NNA. The data suggests that endogenous NO increases the release of [(3)H]NA, evoked either by activation of the nicotinic receptors or by electrical stimulation in guinea-pig gastric fundus.

Nitric oxide inhibits the positive chronotropic and inotropic responses to sympathetic nerve stimulation in the isolated guinea-pig atria

This study was designed to determine whether nitric oxide (NO) modulates the positive chronotropic and inotropic (in paced atria) responses to cardiac sympathetic nerve stimulation (SNS) in the isolated guinea-pig double atrial/right stellate ganglion preparation. The ganglion was stimulated at 1, 2, 3 and 5 Hz at constant voltage and the changes in heart rate or force of contraction were measured. The selective neuronal NO synthase (nNOS) inhibitors TRIM (1-(2-trifluoromethylphenyl) imidazole; 100 microM) and 7-NiNa (Na+ salt of 7-nitroindazole; 100 microM) significantly enhanced the positive chronotropic and inotropic responses to SNS. Similar results for heart rate were seen with the non-isoform-selective NOS inhibitor N(omega)nitro-L-arginine (L-NA; 100 microM). All effects were reversed with L-arginine (1 mM). The NO donor sodium nitroprusside (SNP; 100 microM) increased baseline heart rate and force of contraction, and attenuated the positive chronotropic and inotropic responses to SNS. SNP also decreased the positive chronotropic response to bath-applied noradrenaline (NA; 1 microM). In contrast, 7-NiNa did not alter the increase in heart rate with bath-applied NA (0.1 or 1 microM). The guanylyl cyclase inhibitor ODQ (10 microM) enhanced (mimicking nNOS inhibition) and the cyclic GMP (guanosine 3':5'-cyclic monophosphate) analogue 8-Br-cGMP (8-bromoguanosine 3':5'-cyclic monophosphate; 1 mM) attenuated (mimicking exogenous NO) the positive inotropic response to SNS. Taken together, these results are consistent with endogenous NO, synthesized from nNOS, inhibiting the positive chronotropic and inotropic responses evoked by cardiac SNS via a cyclic GMP-dependent pathway.

Effects of nitric oxide donors on basal and K+-evoked release of [3H]noradrenaline from rat cerebral cortex synaptosomes

We investigated the effects of nitric oxide (NO) donors, S-nitroso-N-acetylpenicillamine and sodium nitroprusside on basal and K+-evoked release of [3H]noradrenaline from superfused synaptosomes from the rat cerebral cortex. Both substances produced concentration-dependent increases in the release of the labeled transmitter under basal and depolarized conditions. The effects of the donors on basal release were Ca2+-independent but were not inhibited by the carrier-uptake blocker, desipramine; the effects were abolished by hemoglobin (an NO scavenger). Thirty-five minutes after stimulation with sodium nitroprusside, the synaptosomes were still responsive to KCl stimulation, indicating that the donor's effects were not caused by damage to the synaptosome membrane. The cGMP analogue, 8-bromo-cGMP, had no effect on basal release, and the enhanced release produced by sodium nitroprusside was not inhibited by the specific inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one, indicating that NO's effects on basal release of the neurotransmitter are guanylate cyclase-independent. Both of the NO donors had more marked effects on release of [3H]noradrenaline during K+-stimulated depolarization. The NO-mediated increase in this case was partially antagonized by 10 microM LH-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one, and 8-Br-cGMP was also capable of producing concentration-dependent increases in the K+-stimulated release of the transmitter. These findings indicate that the effects of the NO donors on [3H]noradrenaline release during depolarization are partially mediated by the activation of guanylate cyclase.

Intravenous morphine increases release of nitric oxide from spinal cord by an alpha-adrenergic and cholinergic mechanism

Systemic opioids produce analgesia in part by activating bulbospinal noradrenergic pathways. Spinally released norepinephrine (NE) has been suggested to produce analgesia in part by stimulating alpha2-adrenoceptors on cholinergic spinal interneurons to release acetylcholine (ACh). We hypothesized that this spinally released ACh would stimulate synthesis of nitric oxide (NO), and that spinally released NO after intravenous (IV) opioid injection thus would depend on a cascade of noradrenergic and cholinergic receptor stimulation. To test these hypotheses, IV morphine was administered to anesthetized sheep, and neurotransmitters in dorsal horn interstitial fluid were measured by microdialysis. IV morphine increased NE and ACh in dorsal horn microdialysates, and these increases were inhibited by IV naloxone or cervical spinal cord transection. IV morphine also increased dorsal horn microdialysate concentrations of nitrite, a stable metabolite of NO. Increases in NE, ACh, and nitrite were antagonized by prior intrathecal injection of the alpha2-adrenergic antagonist idazoxan, the muscarinic antagonist atropine, or the NO synthase inhibitor N-methyl--arginine (NMLA). To examine the concentration-dependent effects of spinal adrenergic stimulation, isolated rat spinal cord tissue was perfused with the alpha2-adrenergic agonist clonidine. Clonidine increased nitrite in the spinal cord tissue perfusate, an effect blocked by coadministration of idazoxan, atropine, and NMLA. These data support a previously hypothesized cascade of spinally released NE and ACh after systemic opioid administration. These data also suggest that spinally released NO plays a role in the analgesic effects of systemic opioids. In addition, these data imply a positive feedback whereby spinally released nitric oxide increases NE release and that has not previously been described.

Properties of 3,4-diaminopyridine-evoked dopamine and acetylcholine release in rabbit caudate nucleus slices: involvement of facilitatory adenosine A2 receptors or nitric oxide?

The 3H-overflow from slices of the rabbit caudate nucleus preincubated with tritiated dopamine (DA), or choline, and then superfused and stimulated twice with 3,4-diaminopyridine (3,4-DAP; 25 microM, 1 min), was explored as an in vitro model for evoked release of DA, or acetylcholine (ACh), respectively. In both cases the 3,4-DAP-evoked 3H-overflow was tetrodotoxin-sensitive and Ca(2+)-dependent and hence most probably represents action potential-induced exocytotic release of DA or ACh, respectively. Using pairs of preferential agonists/antagonists it was shown, that evoked DA release was inhibited via presynaptic D2 autoreceptors (quinpirole/domperidone) and kappa-opioid receptors (U-50488H/norbinaltorphimine). No evidence was found for the presence of presynaptic adenosine A1 or A2 receptors on dopaminergic terminals. Moreover, 3,4-DAP-evoked DA release was unaffected by increased intracellular cyclic AMP levels or by drugs affecting the NO/guanylate cyclase pathway. In a similar manner it was shown that 3,4-DAP-evoked ACh release was inhibited via presynaptic muscarine autoreceptors (oxotremorine/atropine) and dopamine D2 heteroreceptors (quinpirole/domperidone). Again, no evidence for the involvement of the NO/guanylate cyclase system in the modulation of ACh release was found, whereas the presence of inhibitory adenosine A1 receptors, but not of facilitatory A2 receptors, could be clearly established. It is concluded, that 3,4-DAP-evoked 3H-overflow from rabbit caudate nucleus slices preincubated with [3H]DA or [3H]choline, represents a simple and useful in vitro model for action potential-induced DA or ACh release, respectively. Moreover, at least in this model or rabbit brain region, facilitatory adenosine A2 receptors and the NO/guanylate cyclase system seem not to be involved in the release of these transmitters.