Involvement of nitric oxide in nitrous oxide anxiolysis in the elevated plus-maze

Nitrous Oxide, a Rapid Antidepressant, Has Ketamine-like Effects on Excitatory Transmission in the Adult Hippocampus

BACKGROUND

Nitrous oxide (N2O) is a noncompetitive inhibitor of NMDA receptors that appears to have ketamine-like rapid antidepressant effects in patients with treatment-resistant major depression. In preclinical studies, ketamine enhances glutamate-mediated synaptic transmission in the hippocampus and prefrontal cortex. In this study, we examined the effects of N2O on glutamate transmission in the hippocampus and compared its effects to those of ketamine.

METHODS

Glutamate-mediated synaptic transmission was studied in the CA1 region of hippocampal slices from adult albino rats using standard extracellular recording methods. Effects of N2O and ketamine at subanesthetic concentrations were evaluated by acute administration.

RESULTS

Akin to 1 μM ketamine, 30% N2O administered for 15-20 minutes resulted in persistent enhancement of synaptic responses mediated by both AMPA receptors and NMDA receptors. Synaptic enhancement by both N2O and ketamine was blocked by co-administration of a competitive NMDA receptor antagonist at saturating concentration, but only ketamine was blocked by an AMPA receptor antagonist. Synaptic enhancement by both agents involved TrkB (tropomyosin receptor kinase B), mTOR (mechanistic target of rapamycin), and NOS (nitric oxide synthase) with some differences between N2O and ketamine. N2O potentiation occluded enhancement by ketamine, and in vivo N2O exposure occluded further potentiation by both N2O and ketamine.

CONCLUSIONS

These results indicate that N2O has ketamine-like effects on hippocampal synaptic function at a subanesthetic, but therapeutically relevant concentration. These 2 rapid antidepressants have similar, but not identical mechanisms that result in persisting synaptic enhancement, possibly contributing to psychotropic actions.

The role of brain gaseous neurotransmitters in anxiety

Although anxiety is perhaps one of the most significant current medical and social problems, the neurochemical mechanistic background of this common condition remains to be fully understood. Multifunctional regulatory gasotransmitters are novel, atypical inorganic factors of the brain that are involved in the mechanisms of anxiety responses. Nitric oxide (NO) signaling shows ambiguous action in animal models of anxiety, while NO donors exert anxiogenic or anxiolytic effect depending on their chemical structure, dose, treatment schedule and gas release rapidity. The majority of NO synthase inhibitors act as a relatively potent axiolytic agents, while hydrogen sulfide (H₂S) and carbon monoxide (CO) delivered experimentally in the form of "slow" or "fast" releasing donors have recently been considered as anxiolytic neurotransmitters. In this comprehensive review we critically summarize the literature regarding the intriguing roles of NO, H₂S and CO in the neuromolecular mechanisms of anxiety in the context of their putative, yet promising therapeutic application. A possible mechanism of gasotransmitter action at the level of anxiety-related synaptic transmission is also presented. Brain gasesous neuromediators urgently require further wide ranging studies to clarify their potential value for the current neuropharmacology of anxiety disorders.

Nitrous oxide as a putative novel dual-mechanism treatment for bipolar depression: Proof-of-concept study design and methodology

Introduction

Depressive symptoms predominate in the course of bipolar disorder (BD) and there is an urgent need to evaluate novel application of repurposed compounds that act on pre-specified treatment targets. Several lines of reasoning suggest that nitrous oxide (N₂O) is an ideal medication to study as a potential treatment and as a strategy to identify the underlying pathophysiology of bipolar depression. N₂O is a potent cerebral vasodilator and there is compelling evidence of reduced frontal cerebral blood flow (CBF; i.e. hypoperfusion) in depression. Therefore, N₂O may increase CBF and thereby improve symptoms of depression. The goal of this randomized, double-blind trial is to study the effect of a single administration of N₂O versus the active comparator midazolam on mood and CBF in adults with treatment-resistant bipolar depression.

Methods

Participants with BD-I/-II currently experiencing a major depressive episode will be randomized to one of two conditions (n = 20/group): 1) inhaled N₂O plus intravenous saline, or 2) inhaled room air plus intravenous midazolam. Montgomery-Asberg Depression Rating Scale scores will serve as the primary endpoint. CBF will be measured via arterial spin labelling magnetic resonance imaging.

Conclusions

N₂O is a potential novel treatment for bipolar depression, as it causes cerebral vasodilation. This proof-of-concept study will provide valuable information regarding the acute impact of N₂O on mood and on CBF. If N₂O proves to be efficacious in future larger-scale trials, its ubiquity, safety, low cost, and ease of use suggest that it has great potential to become a game-changing acute treatment for bipolar depression.

Nitrergic signaling modulation by ascorbic acid treatment is responsible for anxiolysis in mouse model of anxiety

The present study was designed to investigate the effect of ascorbic acid (AA) treatment on the anxiety related behavioral and neurochemical alterations. AA (50, 100 and 200 mg/kg, i.p.) was administered to the mice and anxiety related behavior and levels of glutamate and nitrite in the brain of mice were determined. The results obtained revealed that the administration of AA (100 mg/kg, i.p.) significantly reduced the anxiety related behavior and the levels of nitrite in the brain of mice. Nitrergic interactions were further determined by the pretreatment of mice with nitric oxide (NO) modulator and AA treatment followed by behavioral and neurochemical measurements. The results obtained suggested that NO inhibition potentiated the anxiolytic like activity of AA in mice. It was also observed that the glutamate and nitrite level in the brain of mice were significantly reduced by the NO inhibitor pretreatment. Thus, the present study demonstrated the possible nitrergic pathways modulation in the anxiolytic like activity of AA in mice.

Role of the endogenous nitric oxide inhibitor asymmetric dimethylarginine (ADMA) and brain-derived neurotrophic factor (BDNF) in depression and behavioural changes: clinical and preclinical data in chronic kidney disease

BACKGROUND

Patients with chronic kidney disease (CKD) exhibit a high prevalence of neuropsychiatric alterations, including depression and behavioural changes. CKD is also associated with decreased physical activity not fully explained by co-morbidities. In patients without CKD, the brain-derived neurotropic factor (BDNF) as well as the endogenous NOS inhibitor asymmetric dimethylarginine (ADMA) had been suspected to be involved in major depression. The aim of our study was to examine the role of ADMA and BDNF in the behaviour of haemodialysis patients (CKD5D) as well as in a rat model of 5/6 nephrectomy and chronic ADMA infusion alone.

METHODS

Eleven (5F/6M) CKD5D patients underwent Beck Depression Inventory (BDI) testing along with analysis of ADMA and BDNF. Male Sprague-Dawley rats were randomly assigned to four groups: (i) saline infusion; (ii) ADMA (250 µg/kg/day) infusion via osmotic mini pumps; (iii) 5/6 nephrectomy; (iv) untreated controls. After 28 days, the animals underwent behavioural tests measuring anxiety, locomotion and investigative behaviour. Animals were sacrificed, blood samples were drawn and analysed and hippocampal immunohistology for BDNF was performed.

RESULTS

In CKD5D patients, decreased BDNF levels correlated with higher scores of depression (Pearson r = -0.8156, P = 0.002). ADMA infusion led to a significant decrease of BDNF while the decrease of BDNF in 5/6 nephrectomy was not significant. However, an attenuated hippocampal BDNF expression could be detected in 5/6 nephrecomized animals. Decreased spontaneous locomotor activity was shown in ADMA-infused rats [15.9 (13.5-26.1) lines crossed/min] and 5/6 nephrectomy [14.6 (6.1-20.2) lines crossed/min] when compared with controls [32.5 (15.3-42.4) lines crossed/min]. Anxiety-like behaviour tested by hole investigation time was significantly more pronounced in 5/6 nephrectomy [24 (6-44) s] when compared with ADMA infusion [64 (28-93) s] and controls [33 (26-65) s].

CONCLUSIONS

Progressive renal failure in rats is accompanied by a marked increase of ADMA and a decrease in BDNF. 5/6 nephrectomy leads to significantly decreased exploratory behaviour and locomotion. Both behaviours could be reproduced by ADMA infusion alone. Indicators of anxiety were more pronounced in ADMA-infused animals when compared with 5/6 nephrectomized rats. Furthermore, an inverse relationship of BDNF and BDI in 11 CKD5D patients was shown.

Treatment-Resistant Major Depression: Rationale for NMDA Receptors as Targets and Nitrous Oxide as Therapy

Major depressive disorder (MDD) remains a huge personal and societal encumbrance. Particularly burdensome is a virulent subtype of MDD, treatment resistant major depression (TMRD), which afflicts 15-30% of MDD patients. There has been recent interest in N-methyl-d-aspartate receptors (NMDARs) as targets for treatment of MDD and perhaps TMRD. To date, most pre-clinical and clinical studies have focused on ketamine, although psychotomimetic and other side effects may limit ketamine's utility. These considerations prompted a recent promising pilot clinical trial of nitrous oxide, an NMDAR antagonist that acts through a mechanism distinct from that of ketamine, in patients with severe TRMD. In this paper, we review the clinical picture of TRMD as a subtype of MDD, the evolution of ketamine as a fast-acting antidepressant, and clinical and basic science studies supporting the possible use of nitrous oxide as a rapid antidepressant.

Zaprinast and rolipram enhances spatial and emotional memory in the elevated plus maze and passive avoidance tests and diminishes exploratory activity in naive mice

Background

Phosphodiesterase (PDE) inhibitors in the central nervous system have been shown to stimulate neuronal functions and increase neurogenesis in Alzheimer disease (AD) patients.

Material/Methods

The aim of this study was to investigate the effects of zaprinast, a PDE₅ inhibitor, and rolipram, a PDE₄ inhibitor, on learning and memory in elevated plus maze (EPM) and passive avoidance (PA) tests in naive mice. Male Balb-c mice received short-term treatment with zaprinast (3 and 10 mg/kg) and rolipram (0.05 and 0.1 mg/kg) before the acquisition trial of the EPM and PA tests. The exploratory activity of the animals was also investigated in the Hughes box test.

Results

Both zaprinast (10 mg/kg) and rolipram (0.1 mg/kg) significantly decreased second-day latency compared to the control group in the EPM test, while only rolipram (0.1 mg/kg) significantly increased second-day latency in the PA test. Both zaprinast (10 mg/kg) and rolipram (0.1 mg/kg) significantly decreased the number of entries to new areas and time spent in new areas in the Hughes box test.

Conclusions

Our study revealed that both zaprinast and rolipram enhanced spatial memory in EPM, while rolipram seemed to have more emotional memory-enhancing effects in the PA test compared to zaprinast. Both zaprinast and rolipram diminished exploratory activity in the Hughes box test, which can be attributed to the drugs’ anxiogenic effects.

Effects of zaprinast and rolipram on olfactory and visual memory in the social transmission of food preference and novel object recognition tests in mice

The role of phosphodiesterase (PDE) inhibitors in central nervous system has been investigated and shown to stimulate neuronal functions and increase neurogenesis in Alzheimer patients. The aim of this study is to investigate effect of PDE5 inhibitor zaprinast and PDE4 inhibitor rolipram on visual memory in novel object recognition (NOR) test, on olfactory memory in social transmission of food preference (STFP) test, and also on locomotion and anxiety in open field test in naive mice. Male Balb-c mice were treated intraperitoneally (i.p.) with zaprinast (3 and 10 mg/kg), rolipram (0.05 and 0.1 mg/kg), or physiological saline. Zaprinast (10 mg/kg) significantly increased cued/non-cued food eaten compared to control group, while rolipram had a partial effect on retention trial of STFP test. Zaprinast (10 mg/kg) and rolipram (0.05 and 0.1 mg/kg) significantly increased ratio index (RI) compared to control group in retention trial of NOR test. There was no significant effect of zaprinast and rolipram on total distance moved, speed, and center zone duration in open field test. Results of this study revealed that both zaprinast and rolipram enhanced visual memory in NOR test, however zaprinast exerted a significant memory-enhancing effect compared to rolipram in STFP test in mice.

Effects of 7-NI and ODQ on memory in the passive avoidance, novel object recognition, and social transmission of food preference tests in mice

Background

Nitric oxide (NO) is an intercellular messenger that plays a critical role in learning and memory processes. Effects of nitric oxide synthase (NOS) inhibitors and guanylate cyclase (GC) inhibitors on cognitive function remain controversial.

Material/Methods

The aim of this study was to investigate effects of an NOS inhibitor, 7-nitroindazole (7-NI), and a GC inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), on different aspects of memory in passive avoidance (PA), novel object recognition (NOR), and social transmission of food preference (STFP) tests. Male Balb-c mice were treated intraperitoneally with 7-NI (15 mg/kg), ODQ (3,10 mg/kg), L-arginine (100 mg/kg) + 7-NI (15 mg/kg), or physiological saline.

Results

ODQ (10 mg/kg) and 7-NI (15 mg/kg) significantly decreased second-day latency in PA test. 7-NI (15 mg/kg) and ODQ (10 mg/kg) significantly decreased the ratio index in the NOR test. 7-NI and ODQ (10 mg/kg) decreased cued/non-cued food eaten in STFP test. Amount of time spent in center zone significantly increased in ODQ (10 mg/kg) and 7-NI (15 mg/kg) groups in open field test, but there was no effect on total distance moved and speed of animals. ODQ (10 mg/kg) significantly increased number of entries into new compartments in exploratory activity apparatus, while 7-NI had no effect. Administration of L-arginine (100 mg/kg) before 7-NI reversed 7-NI-induced effects, supporting the role of NO in cognition.

Conclusions

Our results confirm that inhibition of NO/cGMP/GS pathway might disturb emotional, visual, and olfactory memory in mice. Also, 7-NI and ODQ had anxiolytic effects in open field test, and ODQ also enhanced exploratory activity.

An anxiolytic-like effect of hyperbaric oxygen in the mouse light/dark exploration test

AIMS

We studied whether hyperbaric oxygen (HBO(2)) treatment, which is known to increase production of nitric oxide (NO) in the brain, might also produce an NO-dependent anxiolytic-like behavioral response.

MAIN METHODS

Male NIH Swiss mice (20-25g) were subjected to a 60-min HBO(2) treatment at different absolute atmospheres, and anxiety was assessed using the light/dark exploration test at different time intervals following the cessation of HBO(2) treatment. To ascertain the underlying mechanism of action, other groups of mice were pretreated with the NO synthase inhibitor N(G)-monomethyl-l-arginine acetate, the NO scavenger 2-(4-carboxyphenyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3-oxide (carboxy-PTIO), the soluble guanylyl cyclase-inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or the benzodiazepine antagonist flumazenil to determine their influence on the HBO(2)-induced anxiolytic-like effect.

KEY FINDINGS

A 60-min HBO(2) treatment at 3.0 absolute atmospheres increased the time spent by mice in the light compartment that lasted up to 90 min following the end of HBO(2) treatment. This anxiolytic effect of HBO(2) was significantly reduced by pretreatment with L-NMMA, carboxy-PTIO, ODQ and flumazenil.

SIGNIFICANCE

Based on these findings, we conclude that a 60-min HBO(2) treatment is capable of inducing an anxiolytic effect that possibly involves NO, cyclic GMP and the benzodiazepine binding site.

Inhibition of neuronal nitric oxide reduces anxiety-like responses to pair housing

Many psychological disorders are characterized by anxiety and alterations in social interactions. Recent studies demonstrate that the chemical messenger nitric oxide (NO) can regulate both anxiety and social behaviours. We tested whether an enzyme that produces NO in the brain, neuronal nitric oxide synthase (nNOS), serves as an interface between social interactions and anxiety-like behaviour. Several investigators have observed that mice increase anxiety-like responses in the elevated plus-maze after pair housing. nNOS gene deletion and 3-Bromo-7-Nitroindazole were used to inhibit the production of neuronal NO. Similar to previous studies, pair housing reduced open arm exploration in the elevated plus-maze. Pair housing also increased corticotropin-releasing hormone (CRH) immunoreactive cells in the paraventricular nucleus (PVN) of the hypothalamus. Inhibition of NO production increased open arm exploration in pair-housed mice but decreased open arm exploration in individually housed mice. These results suggest that the effect of nNOS inhibition on anxiety-like responses is context dependent and that behavioural responses to social housing are altered after nNOS inhibition. This research suggests that NO may play an important role in mediating the effect social interactions have on anxiety.

Advances in understanding the actions of nitrous oxide

Nitrous oxide (N(2)O) has been used for well over 150 years in clinical dentistry for its analgesic and anxiolytic properties. This small and simple inorganic chemical molecule has indisputable effects of analgesia, anxiolysis, and anesthesia that are of great clinical interest. Recent studies have helped to clarify the analgesic mechanisms of N(2)O, but the mechanisms involved in its anxiolytic and anesthetic actions remain less clear. Findings to date indicate that the analgesic effect of N(2)O is opioid in nature, and, like morphine, may involve a myriad of neuromodulators in the spinal cord. The anxiolytic effect of N(2)O, on the other hand, resembles that of benzodiazepines and may be initiated at selected subunits of the gamma-aminobutyric acid type A (GABA(A)) receptor. Similarly, the anesthetic effect of N(2)O may involve actions at GABA(A) receptors and possibly at N-methyl-D-aspartate receptors as well. This article reviews the latest information on the proposed modes of action for these clinical effects of N(2)O.

Neurobiology and treatment of anxiety: signal transduction and neural plasticity

The stress-dependence and chronic nature of anxiety disorders along with the anxiolytic effectiveness of antidepressant drugs suggests that neuronal plasticity may play a role in the pathophysiology of anxiety. Intracellular signaling pathways are known in many systems to be critical links in the cascades from surface signals to the molecular alterations that result in functional plasticity. Chronic antidepressant treatments can regulate intracellular signaling pathways and can induce molecular, cellular, and structural changes over time. These changes may be important to the anxiolytic effectiveness of these drugs. In addition, the signaling proteins implicated in the actions of chronic antidepressant action, such as cAMP response element binding protein (CREB), have also been implicated in conditioned fear and in anxiety. The cellular mechanisms underlying conditioned fear indicate roles for additional signaling pathways; however, less is known about such mechanisms in anxiety. The challenge to identify intracellular signaling pathways and related molecular and structural changes that are critical to the etiology and treatment of anxiety will further establish the importance of mechanisms of neuronal plasticity in functional outcome and improve treatment strategies.

Involvement of cyclic GMP-dependent protein kinase in nitrous oxide-induced anxiolytic-like behavior in the mouse light/dark exploration test

The second messenger cyclic GMP (cGMP) plays a role in the anxiolytic-like behavioral response of mice to nitrous oxide (N2O). This study was conducted to determine whether this behavioral effect of N2O is affected by inhibition of cGMP-dependent protein kinase (PKG). N2O-induced behavior in the light/dark exploration test was significantly attenuated by the PKG inhibitors H-8 and Rp-8-pCPT-cGMPS but not Rp-8-pCPT-cAMPS, an inhibitor of cAMP-dependent protein kinase. These findings implicate PKG in the mediation or modulation of the anxiolytic-like behavioral response to N2O.

Role of nitric oxide on motor behavior

The present review paper describes results indicating the influence of nitric oxide (NO) on motor control. Our last studies showed that systemic injections of low doses of inhibitors of NO synthase (NOS), the enzyme responsible for NO formation, induce anxiolytic effects in the elevated plus maze whereas higher doses decrease maze exploration. Also, NOS inhibitors decrease locomotion and rearing in an open field arena. These results may involve motor effects of this compounds, since inhibitors of NOS, NG-nitro-L-arginine (L-NOARG), N(G)-nitro-L-arginine methylester (L-NAME), N(G)-monomethyl-L-arginine (L-NMMA), and 7-Nitroindazole (7-NIO), induced catalepsy in mice. This effect was also found in rats after systemic, intracebroventricular or intrastriatal administration. Acute administration of L-NOARG has an additive cataleptic effect with haloperidol, a dopamine D2 antagonist. The catalepsy is also potentiated by WAY 100135 (5-HT1a receptor antagonist), ketanserin (5HT2a and alfal adrenergic receptor antagonist), and ritanserin (5-HT2a and 5HT2c receptor antagonist). Atropine sulfate and biperiden, antimuscarinic drugs, block L-NOARG-induced catalepsy in mice. L-NOARG subchronic administration in mice induces rapid tolerance (3 days) to its cataleptic effects. It also produces cross-tolerance to haloperidol-induced catalepsy. After subchronic L-NOARG treatment there is an increase in the density NADPH-d positive neurons in the dorsal part of nucleus caudate-putamen, nucleus accumbens, and tegmental pedunculupontinus nucleus. In contrast, this treatment decreases NADPH-d neuronal number in the substantia nigra compacta. Considering these results we suggest that (i) NO may modulate motor behavior, probably by interfering with dopaminergic, serotonergic, and cholinergic neurotransmission in the striatum; (ii) Subchronic NO synthesis inhibition induces plastic changes in NO-producing neurons in brain areas related to motor control and causes cross-tolerance to the cataleptic effect of haloperidol, raising the possibility that such treatments could decrease motor side effects associated with antipsychotic medications. Finally, recent studies using experimental Parkinson's disease models suggest an interaction between NO system and neurodegenerative processes in the nigrostriatal pathway. It provides evidence of a protective role of NO. Together, our results indicate that NO may be a key participant on physiological and pathophysiological processes in the nigrostriatal system.

Altered diurnal variation of nitric oxide production in patients with panic disorder

The aim of this prospective study was to investigate the diurnal change in serum nitric oxide (NO) levels in active and remission phases of patients with panic disorder. This study included 15 patients fulfilling the criteria for panic disorder of Diagnostic and Statistical Manual of Mental Disorders--Fourth Edition and 15 healthy controls matched for age and sex. All patients were receiving a selective serotonin reuptake inhibitor at therapeutic doses. The serum nitrite and nitrate levels of subjects were determined at 10:00 a.m. after overnight fasting and at 3:00 p.m. 2 hours after lunch. NO levels of all patients measured in the morning were significantly higher than those of controls. The patients were also divided into active and remission groups according to clinical status and Panic Agoraphobia Scale's cut-off point. There were no statistically significant differences in serum nitrite and nitrate levels of the active group between the 10:00 a.m. and 3:00 p.m. measurements. In contrast, statistically significant differences were found in the serum levels of nitrite (p<0.05) and nitrate (p<0.05) in the remission group. Notably, the afternoon nitrite and nitrate levels of the remission group were higher than those of the morning levels as seen in control subjects. Thus, diurnal variation of NO production is altered in patients with panic disorder but is resumed in the remission phase. The present study suggests that serum NO levels are a good marker for evaluation of panic disorder.

New pharmacological treatment approaches for anxiety disorders

New developments in the pharmacological treatment of anxiety disorders will have distinct backgrounds: characterization of pathophysiological processes including evolving techniques of genomics and proteomics will generate new drug targets. Drug development design will generate new pharmacological substances with specific action at specific neurotransmitter and neuropeptide receptors or affecting their reuptake and metabolism. New anxiolytic drugs may target receptor systems that only recently have been linked to anxiety-related behavior. This includes the N-methyl-D-aspartate (NMDA), S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and the cannabinoid receptors. In addition, signal transduction pathways, neurotrophic factors, and gases such as nitric oxide or carbon monoxide may be new drug targets. Combining psychopharmacological and psychotherapeutical interventions is a further field where benefits for the treatment of anxiety disorders could be achieved. Although the road of drug development is arduous, improvements in the pharmacological treatment of anxiety disorders are expected for the near future.

Assessment of heat production, heat loss, and core temperature during nitrous oxide exposure: a new paradigm for studying drug effects and opponent responses

Studies using core temperature (T(c)) have contributed greatly to theoretical explanations of drug tolerance and its relationship to key features of addiction, including dependence, withdrawal, and relapse. Many theoretical accounts of tolerance propose that a given drug-induced psychobiological disturbance elicits opponent responses that contribute to tolerance development. This proposal and its theoretical extensions (e.g., conditioning as a mechanism of chronic tolerance) have been inferred from dependent variables, such as T(c), which represent the summation of multiple underlying determinants. Direct measurements of determinants could increase the understanding of opponent processes in tolerance, dependence, and withdrawal. The proximal determinants of T(c) are metabolic heat production (HP) and heat loss (HL). We developed a novel system for simultaneously quantifying HP (indirect calorimetry), HL (direct gradient layer calorimetry), and T(c) (telemetry) during steady-state administrations of nitrous oxide (N(2)O), an inhalant with abuse potential that has been previously used to study acute and chronic tolerance development to its hypothermia-inducing property. Rats were administered 60% N(2)O (n = 18) or placebo gas (n = 16) for 5 h after a 2-h placebo baseline exposure. On average, N(2)O rapidly but transiently lowered HP and increased HL, each by approximately 16% (P < 0.001). On average, rats reestablished and maintained thermal equilibrium (HP = HL) at a hypothermic T(c) (-1.6 degrees C). However, some rats entered positive heat balance (HP > HL) after becoming hypothermic such that acute tolerance developed, i.e., T(c) rose despite continued drug administration. This work is the first to directly quantify the thermal determinants of T(c) during administration of a drug of abuse and establishes a new paradigm for studying opponent processes involved in acute and chronic hypothermic tolerance development.

Antagonism by NOS inhibition of the behavioral effects of benzodiazepine and GABAA receptor agonists in the mouse elevated plus-maze

Earlier we implicated nitric oxide (NO) in mediation of the behavioral effects of benzodiazepines. Since benzodiazepines work through facilitation of GABAergic inhibitory neurotransmission, this study was designed to determine whether the direct-acting gamma-aminobutyric acidA (GABAA) receptor agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) evokes behavioral effects similar to those of benzodiazepines and whether behavioral effects of THIP are also NO dependent. When challenged with either chlordiazepoxide or THIP in an elevated plus-maze paradigm, male NIH Swiss mice exhibited a dose-related increase in open-arm activity. The chlordiazepoxide-induced effects were sensitive to antagonism by a benzodiazepine antagonist, and the effects of THIP were blocked by a GABAA receptor antagonist. Pretreatment with the NO synthase (NOS) inhibitor L-NG-nitro arginine antagonized the effects of both chlordiazepoxide and THIP; similar pretreatment with the D-isomer, D-NG-nitro arginine, which is inactive as an NOS inhibitor, was without effect on chlordiazepoxide and THIP. These findings indicate that chlordiazepoxide and THIP evoke similar behavioral effects in mice in the elevated plus-maze through actions on different parts of the GABAA receptor, and that NO appears to play a key role in mediation of the behavioral effects of both chlordiazepoxide and THIP.

Role of cyclic GMP in nitrous-oxide-induced anxiolytic-like behavior in the mouse light-dark exploration test

Nitric oxide (NO) has been implicated in the anxiolytic-like behavioral effects of nitrous oxide (N2O). This study was conducted to determine whether NO activates a soluble guanylyl cyclase (sGC)-cyclic guanosine monophosphate (cGMP) pathway in the behavioral response to N2O in the light-dark exploration test. In mice pretreated with an sGC inhibitor, the increased light-compartment activity normally induced by N2O was significantly attenuated. Pretreatment with a cGMP phosphodiesterase inhibitor antagonized the anxiogenic effect of 15% N2O and enhanced the anxiolytic effect of 25% N2O, implying that cGMP reduces anxiety. These preliminary findings suggest that a signaling pathway involving NO and cGMP may mediate the behavioral effects of N2O.

Putative anxiety-linked effects of the nitric oxide synthase inhibitor L-NAME in three murine exploratory behavior models

The aim of the current study was to extend investigation into possible linkage between nitric oxide (NO) and anxiety-linked behavior using a battery of tests. Effects of the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) were investigated in three murine models of anxiety-the light-dark, hole-board and elevated plus-maze-in between-groups designs. Treatment groups included L-NAME (0 [vehicle, or Veh], 10, 25, and 50 mg/kg) and 50 mg/kg of the inactive isomer N(G)-nitro-D-arginine methyl ester (D-NAME) injected subcutaneously. Mice exhibited a robust anxiogenic-like response profile reflected by dose-related decreases in both light-dark (transitions and time in lighted area) and hole-board (head dips and time spent head dipping) test measures, reaching statistical significance at 25 and 50 mg/kg L-NAME when compared to Veh controls (P<.05 or.01; Dunnett's t test), while distance traveled and rearing showed no significant differential pattern in either model. In both models, there was a strong dissociation between nonspecific locomotion and putative exploratory behaviors. D-NAME was not significantly different from Veh condition in either model, indicating a stereospecific action and supporting NO involvement. A dose-related decrease was also observed for several traditional and ethological measures in the plus-maze; however, the effect was limited and relatively weak or absent; with the exception of open-arm and percent open-arm entries, putative anxiety-sensitive measures reached statistical significance only at the highest dose. Reductions in motor activity compromised ability to dissociate an anxiety linkage from a nonspecific motor effect in most measures. It is concluded that the hole-board and light-dark tests provide indication of anxiogenic-like action of NOS inhibition, suggesting that NO has an anxiolytic action. Data from the plus-maze are unclear, owing to a confounding motor influence in most measures.

Antisense knockdown of neuronal nitric oxide synthase antagonizes nitrous oxide-induced behavior

The behavioral effects of nitrous oxide (N(2)O) were antagonized by non-specific inhibitors of nitric oxide synthase (NOS). To identify the isoform of NOS involved in this response, mice were pretreated with an antisense oligodeoxynucleotide (AS-ODN) against neuronal NOS, and then tested in a light/dark exploration paradigm. The AS-ODN but not the mismatch ODN significantly antagonized N(2)O induced behavior and also reduced NOS activity in the cerebellum and hippocampus. These results implicate neuronal NOS in the N(2)O response.

Antagonism of nitrous oxide-induced anxiolytic-like behavior in the mouse light/dark exploration procedure by pharmacologic disruption of endogenous nitric oxide function

RATIONALE

Previous studies have shown the anxiolytic-like effects of nitrous oxide (N(2)O) to be sensitive to antagonism by non-specific inhibitors of nitric oxide synthase (NOS).

OBJECTIVES

The present study was conducted to demonstrate further the involvement of nitric oxide (NO) and ascertain whether a specific isoform of NOS is involved in N(2)O-induced behavior in mice.

METHODS

Male NIH Swiss mice were tested in the light/dark exploration test to determine how N(2)O-induced behavior was affected by the following pretreatments: the NO scavenger hemoglobin (Hb); the selective nNOS-inhibitor S-methyl- l-thiocitrulline (SMTC); the selective eNOS-inhibitor N(5)-(1-iminoethyl)- l-ornithine ( l-NIO); and the selective iNOS-inhibitor 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT). Furthermore, NOS activity was assessed in the whole brain as well as five brain areas of N(2)O- versus room air-exposed mice to determine the effects of N(2)O on NOS activity.

RESULTS

The behavioral effects of N(2)O in the light/dark exploration test were significantly attenuated following pretreatment with Hb (2.0 nmol, i.c.v.), SMTC (0.3 micro g and 1.0 micro g per mouse, i.c.v.) and the higher dose of l-NIO (30 mg/kg, s.c.). However, the N(2)O-induced behavioral effect was unaltered by pretreatment with either the lower dose of l-NIO (10 mg/kg, s.c.) or AMT (1.0 mg/kg and 3.0 mg/kg, s.c.). Finally exposure to 50% N(2)O for 15 min significantly increased NOS activity in the cerebellum and corpus striatum but not in other brain regions or whole brain.

CONCLUSION

These findings provide further support for the hypothesis that NO is involved in N(2)O-induced anxiolytic-like behavior and that this NO is the product of nNOS enzyme activity.

Stimulation of P2Y1 receptors causes anxiolytic-like effects in the rat elevated plus-maze: implications for the involvement of P2Y1 receptor-mediated nitric oxide production

The widespread and abundant distribution of P2Y receptors in the mammalian brain suggests important functions for these receptors in the CNS. To study a possible involvement of the P2Y receptors in the regulation of fear and anxiety, the influences of the P2Y(1,11,12) receptor-specific agonist adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS), the P2X(1,3) receptor agonist alpha,beta-methylene ATP (alpha,betameATP), the unspecific P2 receptor antagonist pyridoxalphosphate-6-azopheny l-2',4'-disulfonic acid (PPADS), and the specific P2Y(1) receptor antagonist N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS 2179) on the elevated plus-maze behavior of the rat were investigated. All tested compounds were given intracerebroventricularly (0.5 microl). ADPbetaS (50 and 500 fmol) produced an anxiolytic-like behavioral profile reflected by an increase of the open arm exploration. The anxiolytic-like effects were antagonized by pretreatment with PPADS (5 pmol) or MRS 2179 (5 pmol). Both compounds caused anxiogenic-like effects when given alone. Furthermore, the anxiolytic-like effects of ADPbetaS could be antagonized by pretreatment with the nitric oxide synthase (NOS) inhibitor N(w)-nitro-L-arginine methyl ester (L-NAME). In addition, the anxiogenic-like effects of PPADS were reversed by the pretreatment with L-arginine (500 pmol), which is the natural substrate for NOS, but not by D-arginine (500 pmol), which is not. Immunofluorescence staining revealed the presence of P2Y(1) receptors on neurons in different brain regions such as hypothalamus, amygdala, hippocampus and the periaqueductal gray. Furthermore, the colocalization of P2Y(1) receptors and neuronal NOS (nNOS) on some neurons in these regions could be demonstrated. The highest density of P2Y(1)- and nNOS-immunoreactivity was detected in the dorsomedial hypothalamic nucleus. Taken together, the present results suggest that P2Y(1) receptors are involved in the modulation of anxiety in the rat. The anxiolytic-like effects after stimulation of P2Y(1) receptors seem to be in close connection with the related nitric oxide production.

Effects of a nitric oxide donor on behavior and interaction with nitrous oxide in the mouse light/dark exploration test

There is controversy whether endogenous nitric oxide (NO) is involved in anxiogenesis or anxiolysis. This study was conducted to determine the influence of the NO donor, 3-morpholinosyndnonimine (SIN-1), on resting and nitrous oxide (N(2)O)-induced behaviors in the mouse light/dark exploration test. I.c.v. doses of 0.3 and 1.0 microg SIN-1 both increased the time spent in the light compartment. When pretreated with 0.1 microg SIN-1, mice responded to N(2)O with an apparent additive increase in the time spent in the light compartment. These findings further support a functional role of NO in regulation of anxiety and mediation of N(2)O-induced behavior.

Effects of nitric oxide synthase inhibitors 7-NI, L-NAME, and L-NOARG in staircase test

BACKGROUND

The objective of the study was to investigate the effects of the nitric oxide synthase (NOS) inhibitors 7-nitroindazole (7-NI), N(G)-nitro-L-arginine (L-NOARG), and N(G)-nitro-L-arginine methyl ester (L-NAME) on the behavior of mice in the staircase test.

METHODS

NOS inhibitors 7-NI (20-120 mg/kg), L-NOARG (20 and 40 mg/kg), and L-NAME (20 and 40 mg/kg) were administered intraperitoneally (i.p.) 30 min prior to the staircase test. Staircase test consisted of placing a mouse in an enclosed staircase with five steps and recording the number of rearings made and the number of steps climbed during a 3-min period.

RESULTS

7-NI and L-NOARG did not have a significant effect on the behavior of mice in the staircase test. L-NAME caused a decrease in the number of rearings without changes in the number of steps taken.

CONCLUSIONS

NOS inhibitor L-NAME but not 7-NI or L-NAME induced an anxiolytic effect in the staircase test.

Neurobiology of nitrous oxide-induced antinociceptive effects

Nitrous oxide (N2O), or laughing gas, has been used for clinical anesthesia for more than a century and is still commonly used. While the anesthetic/hypnotic mechanisms of N2O remain largely unknown, the underlying mechanisms of its analgesic/antinociceptive effects have been elucidated during the last several decades. Evidence to date indicate that N2O induces opioid peptide release in the periaqueductal gray area of the midbrain leading to the activation of the descending inhibitory pathways, which results in modulation of the pain/nociceptive processing in the spinal cord. The types of opioid peptide induced by N2O and the subtypes of opioid receptors that mediate the antinociceptive effects of N2O appear to depend on various factors including the species and/or strain, the regions of the brain, and the paradigms of behavior testing used for the experiments. Among three types of descending inhibitory pathways, the descending noradrenergic inhibitory pathway seems to play the most prominent role. The specific elements involved are now being resolved.

Effects of NG-nitro-L-arginine methyl ester, 7-nitro indazole, and agmatine on pentylenetetrazol-induced discriminative stimulus in Long-Evans rats

This study was undertaken to determine any role that nitric oxide (NO) may play in the discriminative stimuli produced by pentylenetetrazol (PTZ). The PTZ-induced discriminative stimulus is pharmacologically similar to anxiety in humans and is used in a behavioral assay of anxiety (the PTZ model of anxiety). In the present study, effects of L-N(G)-nitro arginine methyl ester (L-NAME), 7-nitroindazole (7-NI) and agmatine, NO synthase (NOS) inhibitors, on PTZ-induced discriminative stimulus were investigated in male Long-Evans rats (330-350 g). Rats were trained to discriminate PTZ (16 mg/kg) from saline using a two-lever, food-reinforced choice procedure (FR 10). The rats that met the training criteria were injected with L-NAME (15, 30, and 60 mg/kg), 7-NI (15 and 30 mg/kg), agmatine (20, 40, and 60 mg/kg), and saline or vehicle intraperitoneally before each test. They were tested for the PTZ-discrimination to determine if the NOS inhibitors produce discriminative stimulus similar to PTZ or if they block PTZ-induced discrimination. Treatment with the NOS inhibitory drugs neither substituted for PTZ nor altered the PTZ lever selection in any other way. These findings suggest that PTZ-induced discriminative stimulus may not be related to NO-mediated central mechanisms.

Unaltered radial maze performance and brain acetylcholine of the endothelial nitric oxide synthase knockout mouse

Proceeding from previous findings of a beneficial effect of endothelial nitric oxide synthase (eNOS) gene inactivation on negatively reinforced water maze performance, we asked whether this improvement in place learning capacities also holds for a positively reinforced radial maze task. Unlike its beneficial effects on the water maze task, eNOS gene inactivation did not facilitate radial maze performance. The acquisition performance over the days of place learning did not differ between eNOS knockout (eNOS-/-) and wild-type mice (eNOS+/+). eNOS-/- mice displayed a slight and eNOS+/+ mice a more severe working memory deficit in the place learning version of the radial maze compared to the genetic background C57BL/6 strain. Possible differential effects of eNOS inactivation, related to differences in reinforcement contingencies between the Morris water maze and radial maze tasks, behavioral strategy requirements, or to different emotional and physiological concomitants inherent in the two tasks are discussed. These task-unique characteristics might be differentially affected by the reported anxiogenic and hypertensional effects of eNOS gene inactivation. Post-mortem determination of acetylcholine concentrations in diverse brain structures revealed that acetylcholine and choline contents were not different between eNOS-/- and eNOS+/+ mice, but were increased in eNOS+/+ mice compared to C57BL/6 mice in the frontal cortex. Our findings demonstrate that phenotyping of learning and memory capacities should not rely on one learning task only, but should include tasks employing both negative and positive reinforcement contingencies in order to allow valid statements regarding differences in learning capacities between rodent strains.

The effects of the nitric oxide synthase inhibitors on the behaviour of small-platform-stressed mice in the plus-maze test

Effects of the nitric oxide synthase (NOS) inhibitors 7-nitroindazole (7-NI), N(G)-nitro-L-arginine (L-NOARG) and N(G)-nitro-L-arginine methyl ester (L-NAME) on the behaviour of control and small-platform (SP)-stressed mice in the plus-maze test were studied. SP stress was induced by placing mice on SPs (3.5 cm diameter) surrounded by water for 24 h. This model contains several factors of stress like rapid eye movement (REM) sleep deprivation, isolation, immobilization and falling into the water. The plus-maze test was carried out with control and SP-stressed mice. SP stress induced an anxiolytic-like effect that was evidenced by increased percentage of time spent on the open arms of the plus-maze. The administration of NOS inhibitors 7-NI (20.0-120.0 mg/kg) and L-NOARG (20.0 and 40.0 mg/kg) induced an anxiolytic effect and the administration of L-NAME (20.0 and 40.0 mg/kg)--an anxiogenic effect in control mice. In SP-stressed mice, the effects of NOS inhibitors were changed. Contrary to control mice, 7-NI at a dose of 20.0 mg/kg induced an anxiogenic effect in SP-stressed mice and other doses of 7-NI, with exception of 80.0 mg/kg, as well as L-NOARG and L-NAME were without any effect. On the basis of these data, we can propose that SP stress induced changes in the function of L-arginine-NOS-NO pathways. It is also proposed that the behavioural effects of NOS inhibitors can be changed in stressed animals.

Association of serum nitric oxide levels with depressive symptoms: a study with end-stage renal failure patients

BACKGROUND

Nitric oxide (NO) is a soluble gas produced by the activity of an enzyme found in neurons. It has been implicated in a great number of normal physiological functions (such as noradrenaline and dopamine release, memory and learning, regulation of the cerebrovascular system, modulation of wakefulness, modulation of nociception, olfaction, food intake and drinking) as well as pathologies (Alzheimer's, Huntington's disease, cerebral ischemia, stroke). Two reports have addressed the involvement of NO in depression.

METHODS

The objective of the study was to examine the association between NO and specific depressive symptoms. For this purpose, in a sample of 28 end-stage renal failure patients (who have increased NO levels), we tested the hypothesis that the subgroup of patients with these specific depressive symptoms was differentiated from the patients without these symptoms with regard to serum levels of NO metabolites. The depressive symptoms were assessed using the Zung self-rating scale.

RESULTS

Our study revealed an association of NO with the following depressive symptoms: sexual dysfunction, weight loss, psychomotor retardation, indecisiveness and irritability.

CONCLUSION

The association between NO system and symptoms of depression does not necessarily imply a pathogenetic association between NO and depressive disorder. Further research is needed to verify these findings and study their possible pathogenetic implications.

Comparison of N2O- and chlordiazepoxide-induced behaviors in the light/dark exploration test

Earlier research has demonstrated similarities in the behavioral effects of nitrous oxide (N2O) and benzodiazepine (BZ) drugs such as chlordiazepoxide (CP). The present research was conducted to compare the behavioral effects of N2O and CP in mice in the light/dark exploration test. When challenged with either N2O or CP, mice exhibited significant dose-dependent increases in the time spent in the light compartment and also in the number of transitions between the light and dark compartments. Pretreatment with BZ receptor antagonist flumazenil (FLU), the GABA(A) receptor antagonist SR-95531 or the selective neuronal nitric oxide (NO) synthase (nNOS) inhibitor 7-nitroindazole (7-NI) all antagonized anxiolytic effects of N20 and CP. Based on these findings, it was concluded that N20 and CP evoke similar behavioral effects in the light/dark exploration test that are similar in their interaction with BZ and GABA(A) receptor antagonists. There also appears to be a specific role for nNOS in generating the NO involved in mediation of these effects.

Superior water maze performance and increase in fear-related behavior in the endothelial nitric oxide synthase-deficient mouse together with monoamine changes in cerebellum and ventral striatum

Nitric oxide (NO) has been implicated in the control of emotion, learning, and memory. We have examined endothelial NO synthase-deficient mice (eNOS-/-) in terms of habituation to an open field, elevated plus-maze behavior, Morris water maze performance, and changes in cerebral monoamines. In the open field, eNOS-/- animals were less active than wild-type controls but showed unimpaired habituation. In the plus-maze, an anxiogenic effect was observed. Proceeding from previous findings of deficits in hippocampal and neocortical long-term potentiation (LTP) in our eNOS-/- mice, we investigated whether these animals also express deficits in learning tasks that have been linked to hippocampal function and LTP. Unexpectedly, eNOS gene disruption led to accelerated place learning in the water maze. Furthermore, during long-term retention and reversal learning, eNOS-/- mice showed improved performance. In a cued version of the water maze task, eNOS-/- and control mice did not differ, implying that the superior performance of eNOS-/- animals on the former tasks cannot be attributed solely to differences in sensorimotor capacities. The neurochemical evaluation of the eNOS-/- mice revealed increases in the concentrations of the serotonin metabolite 5-HIAA in the cerebellum, together with an accelerated serotonin turnover in the frontal cortex. Furthermore, eNOS-/- mice had a higher dopamine turnover in the ventral striatum. These findings are discussed in terms of possible concomitant effects on physiological parameters, such as a decreased reactivity of GABAergic neurotransmission or changes in vascular functions, and effects on behavioral processes related to reinforcement, learning, and emotion.

The nitric oxide synthesis inhibitor L-NAME produces anxiogenic-like effects in the rat elevated plus-maze test, but not in the social interaction test

The effects of the nitric oxide synthase inhibitor, Nw-nitro-L-arginine methyl ester (L-NAME) were investigated in two animal models of anxiety: the elevated plus-maze and the social interaction test. In the elevated plus-maze, L-NAME (12.5-50 mg/kg) had an anxiogenic-like profile as indicated by dose-dependent reductions in the time spent on the open arms, open arm entries, the percentage of open arm entries and head dips, but there was no significant effect on the number of stretch attend postures. In contrast, L-NAME (12.5-50 mg/kg) failed to modify time spent in social interaction but did reduce a measure of vertical activity. The differential effects of L-NAME in the two anxiety paradigms are discussed.

Anxiolytic-like effects of 7-nitroindazole in the rat plus-maze test

It is considered that nitric oxide (NO) is one of the most interesting research subjects. Because the actual role of NO in the mechanism of anxiety is still unclear, in this study, the involvement of NO in the mechanism of anxiety was investigated, using the plus-maze test. 7-Nitroindazole (7-NI) (15, 30, 60, 90, and 120 mg/kg), a new nitric oxide synthase (NOS) inhibitor was studied. The time spent on open arms and open-arm visits was evaluated. 7-NI, at 15-120 mg/kg doses potently increased the time spent on open arms and open-arm visits. However, at 120 mg/kg it attenuated the time spent on the open arms, compared to at 90 mg/kg. This effect was attributed to decreased locomotor activity in the higher dose group. Neither L-arginine, nor D-arginine (100 mg/kg) significantly affected any of the behavioral parameters measured in the rat elevated plus-maze test. Neither drugs revealed any effect on locomotion. L-Arginine but not D-arginine given 10 min before 7-NI, reversed the 7-NI induced anxiolytic-like effects. These data support an involvement of NO in the process of anxiety, and further suggest that the anxiolytic-like effect of 7-NI may be attributable to the inhibition of NO synthesis.

Narcotic effects produced by nitrous oxide and hyperbaric nitrogen narcosis in rats performing a fixed-ratio test

Narcosis is a neurological syndrome that reduces capacities of divers. Although this phenomenon appeared at the end of 19th century, the mechanisms are not yet elucidated. The greatest technical problem is that these studies are carried out under hyperbaric conditions. Nitrous oxide is known to be an inducer of narcosis, at atmospheric pressure. The aim of this study is to compare two narcotic environments; a normobaric narcosis under several percentages of nitrous oxide, and an hyperbaric narcosis under 0.9 MPa of Nitrox (N2O2 mixture). This comparison is realized on rats submitted to a fixed-ratio 15 test, in which they have to press a lever to get rewarded. The results show significant performances decreases: the number of pressed lever are reduced by 50% under Nitrox and by 70% under N2O. Nitrous oxide could be considered as a normobaric model of hyperbaric narcosis.

Inhibition of nitric oxide synthesis with L-NAME suppresses isolation-induced ultrasounds in rat pups

The present experiments examined the impact of manipulating the NO system on production of isolation-induced ultrasonic vocalizations (USVs) in 10- and 11-day-old rat pups. Pups were tested under both high- and low-baseline USV emission; the latter was accomplished by pretest administration of cocaine, a drug known to suppress USVs. Treatment with 10, 50, or 100 mg/kg (but not 1 mg/kg) of the nitric oxide synthase (NOS) inhibitor L-nitro-arginine methyl ester (L-NAME) significantly attenuated USV production, as did injection of 10 mg/kg cocaine; combined treatment with both drugs did not result in greater suppression, perhaps due to a floor effect. Although cocaine increased locomotor activity, treatment with L- or D-NAME alone did not alter activity levels. Exposure to L-NAME induced some hypothermia, although these alterations in body temperature were not systematically related to the drug-induced suppression of USVs. Alterations in USV production by L-NAME were not evident after pretreatment with the less active isomer D-NAME, evidence supporting the importance of NO synthesis inhibition per se in the marked L-NAME-induced suppression of USVs in isolated infant rats.

Neurochemical studies of narcosis: a comparison between the effects of nitrous oxide and hyperbaric nitrogen on the dopaminergic nigro-striatal pathway

Inert gas narcosis is a neurological syndrome inducing several psychomotor disorders. Nitrogen narcosis represents the major cause of performances decrease concerning divers, in the depth range of 30 to 90 meters (0.3 to 0.9 MegaPascal). As narcosis affects motor functions, we chose to study the nigro-striatal dopaminergic pathway owing to its involvement in psychomotor disorders. The aim of this study is to compare, in the Sprague-Dawley rats striatium, changes in extracellular concentrations of Dopamine and its metabolites: Dihydroxyphenylacetic Acid (DOPAC) and Homovanillic Acid (HVA) under a normobaric narcosis (20; 40, and 60% of Nitrous Oxide (N2O)) on one hand, and under 0.9 MegaPascal of Nitrox (Nitrogen Oxygen normoxic mixture) on the other hand. In fact, if these two conditions are similar, normobaric narcosis would allow us to explain nitrogen narcosis mechanisms without any pressure effect. The first emergence of Dopamine and metabolites variations occurs around 40% of N2O. Dopamine decreases by 45% and is accompanied by a DOPAC diminution of 7% while HVA concentrations remain constant. Under 60% N2O, these decrease have a greater amplitude. The Dopamine variations obtained under 0.9 Mpa of Nitrox are closed to alterations induced by 60% of N2O (DA decreases by 70%).

Neural basis of inhalant abuse

It should be apparent from this review that far less is known about the neural basis for inhalant abuse than for other forms of drug abuse. This reflects a lack of research interest in this area (Balster, 1997). Indeed, conclusions are difficult to draw. In the case of the volatile alkyl nitrites, the most reasonable hypothesis at this time is that the cellular basis for their abuse resides in their actions on smooth muscles to produce vasodilation and relaxation, however, direct effects on the brain cannot be ruled out. Although there is some evidence that analgesic effects of nitrous oxide may involve opiate systems, even this conclusion is controversial. There is no evidence that opiate systems play a role in nitrous oxide intoxication or reinforcement. The mechanisms for these effects are unknown. They may reflect the same actions on lipid membranes or on hydrophobic sites on unspecified proteins that have been proposed as mechanisms for nitrous oxide anesthesia. In the case of the volatile solvents, fuels and anesthetics we are faced with a wide variety of specific chemicals which may produce different profiles of pharmacological effects. There is evidence that the prototypic abused solvents toluene and trichloroethane produce acute effects similar to subanesthetic concentrations of general anesthetics, as well as to the effects of classical CNS depressant drugs, such as alcohol and the barbiturates. For the anesthetics, evidence suggests that enhancement of GABAergic inhibition may be an important cellular target for their acute effects, just as it is for alcohol and other depressant drugs. For toluene, as with alcohol, recent evidence suggests a possible role for inhibition of glutamatergic neurotransmission involving NMDA receptors. Toluene has also been shown to have some dopaminergic effects which may be important to its abuse. As for the large number of other abused vapors, practically no information can be found on their cellular actions, and certainly not on actions that may be relevant to their abuse. This entire area would seem an important direction for future research.

Effects of L-NOARG on plus-maze performance in rats

Nitric oxide (NO) synthase, the enzyme responsible for NO formation, is located in brain regions such as amygdala and dorsolateral central grey, regions which are known to be involved in anxiety. To investigate the possible role of NO in anxiety, rats received acute i.p. injections of NG-nitro-l-arginine (L-NOARG, 7.5-120 mg kg-1), an inhibitor of NO synthase, and were tested in the elevated plus maze, an animal model of anxiety. The drug, at doses of 30-120 mg kg-1, decreased the percentage of entries and time spent on the open arms of the maze, but these doses, with exception of 30 mg, also decreased the number of entries into enclosed arms. These effects disappeared when the animals were tested after chronic L-NOARG treatment (3.75 to 60 mg kg-1 i.p., twice a day for four days). The effects of acute i.p. injection of 30 mg kg-1 of L-NOARG were blocked by i.c.v. pretreatment with 1000 nmol of l-arginine (but not 500 nmol). Thus, inhibition of NO formation in the central nervous system seems to decrease exploration of the elevated plus maze, an effect that disappears after four days of chronic (twice a day) L-NOARG administration.