Nitric oxide is involved in nicotine-induced burst firing of rat ventral tegmental area dopamine neurons

Ghrelin activates the mesolimbic dopamine system via nitric oxide associated mechanisms in the ventral tegmental area

Besides enhanced feeding, the orexigenic peptide ghrelin activates the mesolimbic dopamine system to cause reward as measured by locomotor stimulation, dopamine release in nucleus accumbens shell (NAcS), and conditioned place preference. Although the ventral tegmental area (VTA) appears to be a central brain region for this ghrelin-reward, the underlying mechanisms within this area are unknown. The findings that the gaseous neurotransmitter nitric oxide (NO) modulate the ghrelin enhanced feeding, led us to hypothesize that ghrelin increases NO levels in the VTA, and thereby stimulates reward-related behaviors. We initially demonstrated that inhibition of NO synthesis blocked the ghrelin-induced activation of the mesolimbic dopamine system. We then established that antagonism of downstream signaling of NO in the VTA, namely sGC, prevents the ability of ghrelin to stimulate the mesolimbic dopamine system. The association of ghrelin to NO was further strengthened by in vivo electrochemical recordings showing that ghrelin enhances the NO release in the VTA. Besides a GABA_B -receptor agonist, known to reduce NO and cGMP, blocks the stimulatory properties of ghrelin. The present series of experiments reveal that ablated NO signaling, through pharmacologically inhibiting the production of NO and/or cGMP, prevents the ability of ghrelin to induced reward-related behaviors.

Managing idiopathic intracranial hypertension in pregnancy: practical advice

Idiopathic intracranial hypertension (IIH) is more common in women of reproductive age who have obesity, yet there is little information on its management specifically in pregnancy. Women with IIH should plan their pregnancy including discussing contraception before pregnancy, recognising that hormonal contraceptives are not contraindicated. Potentially teratogenic medications including acetazolamide and topiramate are not recommended during pregnancy or in those with immediate plans to conceive; prescribing acetazolamide in pregnancy must only follow discussion with the patient and their obstetrician. Ideally, patients should aim to achieve disease remission or control before pregnancy, through optimising their weight. Although weight gain is expected in pregnancy, excessive weight gain may exacerbate IIH and increase maternal and fetal complications; evidence-based recommendations for non-IIH pregnancies may help in guiding optimal gestational weight gain. The vast majority of women with IIH can have a normal vaginal delivery, with spinal or epidural anaesthesia if needed, provided the papilloedema is stable or the IIH is in remission.

Human Pluripotent Stem Cell-Derived Neural Cells and Brain Organoids Reveal SARS-CoV-2 Neurotropism

Neurological complications are common in patients with COVID-19. While SARS-CoV-2, the causal pathogen of COVID-19, has been detected in some patient brains, its ability to infect brain cells and impact their function are not well understood, and experimental models using human brain cells are urgently needed. Here we investigated the susceptibility of human induced pluripotent stem cell (hiPSC)-derived monolayer brain cells and region-specific brain organoids to SARS-CoV-2 infection. We found modest numbers of infected neurons and astrocytes, but greater infection of choroid plexus epithelial cells. We optimized a protocol to generate choroid plexus organoids from hiPSCs, which revealed productive SARS-CoV-2 infection that leads to increased cell death and transcriptional dysregulation indicative of an inflammatory response and cellular function deficits. Together, our results provide evidence for SARS-CoV-2 neurotropism and support use of hiPSC-derived brain organoids as a platform to investigate the cellular susceptibility, disease mechanisms, and treatment strategies for SARS-CoV-2 infection.

A glucagon-like peptide-1 receptor agonist reduces intracranial pressure in a rat model of hydrocephalus

Current therapies for reducing raised intracranial pressure (ICP) under conditions such as idiopathic intracranial hypertension or hydrocephalus have limited efficacy and tolerability. Thus, there is a pressing need to identify alternative drugs. Glucagon-like peptide-1 receptor (GLP-1R) agonists are used to treat diabetes and promote weight loss but have also been shown to affect fluid homeostasis in the kidney. We investigated whether exendin-4, a GLP-1R agonist, is able to modulate cerebrospinal fluid (CSF) secretion at the choroid plexus and subsequently reduce ICP in rats. We used tissue sections and cell cultures to demonstrate expression of GLP-1R in the choroid plexus and its activation by exendin-4, an effect blocked by the GLP-1R antagonist exendin 9-39. Acute treatment with exendin-4 reduced Na+- and K+-dependent adenosine triphosphatase activity, a key regulator of CSF secretion, in cell cultures. Finally, we demonstrated that administration of exendin-4 to female rats with raised ICP (hydrocephalic) resulted in a GLP-1R-mediated reduction in ICP. These findings suggest that GLP-1R agonists can reduce ICP in rodents. Repurposing existing GLP-1R agonist drugs may be a useful therapeutic strategy for treating raised ICP.

Role of Nitric Oxide on Dopamine Release and Morphine-Dependency

The catastrophic effects of opioids use on public health and the economy are documented clearly in numerous studies. Repeated morphine administration can lead to either a decrease (tolerance) or an increase (sensitization) in its behavioral and rewarding effects. Morphine-induced sensitization is a major problem and plays an important role in abuse of the opioid drugs. Studies reported that morphine may exert its effects by the release of nitric oxide (NO). NO is a potent neuromodulator, which is produced by nitric oxide synthase (NOS). However, the exact role of NO in the opioid-induced sensitization is unknown. In this study, we reviewed the role of NO on opioid-induced sensitization in 2 important, rewarding regions of the brain: nucleus accumbens and ventral tegmentum. In addition, we focused on the contribution of NO on opioid-induced sensitization in the limbic system.

Brain nitric oxide metabolites in rats preselected for nicotine preference and intake

Nicotine addiction is a serious health problem resulting in millions of preventable deaths worldwide. The gas messenger molecule nitric oxide (NO) plays a critical role in addiction, and nicotine increases nitric oxide metabolites (NOx) in the brain. Understanding the factors which underlie individual differences in nicotine preference and intake is important for developing effective therapeutic strategies for smoking cessation. The present study aimed to assess NO activity, by measuring its stable metabolites, in three brain regions that express high levels of nicotinic acetylcholine receptors in rats preselected for nicotine preference. Rats (n=88) were exposed to two-bottle, free choice of oral nicotine/water starting either as adolescents or adults; control animals received only water under identical conditions. Following 12 or six weeks of exposure, levels of NOx (nitrite+nitrate), were determined in the hippocampus, frontal cortex, and amygdala. Since the rats were singly housed during oral nicotine treatment, naïve rats were also included in the study to evaluate the effect of isolation stress. Isolation stress increased NOx in the hippocampus. Nicotine preference did not have a significant effect on NO activity, but rats with adolescent exposure had higher NOx levels in the frontal cortex compared to adult-onset rats. Our findings suggest that nicotine exposure during adolescence, regardless of the amount of nicotine consumed, results in higher NO activity in the frontal cortex of rats, which persists through adulthood.

Nicotine withdrawal upregulates nitrergic and galaninergic activity in the rat dorsal raphe nucleus and locus coeruleus

The dorsal raphe nucleus (DRN), a major source of forebrain serotonin, mediates various neural functions including anxiety. The nucleus locus coeruleus (LC) is likewise involved in mediating central components of the stress response and anxiety. An anxiety-reducing effect is widely believed to underlie many cases of nicotine dependence. While much is known about nicotine-serotonin interactions, little is known about how nicotine engages the DRN non-serotonergic domain in specific physiological functions that influence organismal behavior. The aim of this study was to determine how chronic nicotine withdrawal influences neuronal nitric oxide (NO) synthase (nNOS) and galanin immunoreactivity in the DRN and LC of adult rats. Compared with saline, nicotine increased nicotinamide adenine dinucleotide phosphate diaphorase profiles within distinct DRN subregions and also enhanced intensity in nNOS and galanin cell bodies in the rostral DRN as well as galanin in the LC. Nicotine-induced nNOS/galanin staining of somata was abundant in the rostral ventromedial DRN. Galanin-positive terminals surrounded nNOS-containing cell bodies in the DRN lateral wing subregions. These observations suggest that the DRN NOS-galanin domain and galanin in the LC are engaged in the organism's neural adaptation to chronic nicotine exposure. Hence NO and galanin synthesized or released within the DRN and LC or at the respective target sites might regulate the whole animal behavioral response to nicotine exposure.

Phasic D1 and tonic D2 dopamine receptor signaling double dissociate the motivational effects of acute nicotine and chronic nicotine withdrawal

Nicotine, the main psychoactive ingredient of tobacco smoke, induces negative motivational symptoms during withdrawal that contribute to relapse in dependent individuals. The neurobiological mechanisms underlying how the brain signals nicotine withdrawal remain poorly understood. Using electrophysiological, genetic, pharmacological, and behavioral methods, we demonstrate that tonic but not phasic activity is reduced during nicotine withdrawal in ventral tegmental area dopamine (DA) neurons, and that this pattern of signaling acts through DA D2 and adenosine A2A, but not DA D1, receptors. Selective blockade of phasic DA activity prevents the expression of conditioned place aversions to a single injection of nicotine in nondependent mice, but not to withdrawal from chronic nicotine in dependent mice, suggesting a shift from phasic to tonic dopaminergic mediation of the conditioned motivational response in nicotine dependent and withdrawn animals. Either increasing or decreasing activity at D2 or A2A receptors prevents the aversive motivational response to withdrawal from chronic nicotine, but not to acute nicotine. Modification of D1 receptor activity prevents the aversive response to acute nicotine, but not to nicotine withdrawal. This double dissociation demonstrates that the specific pattern of tonic DA activity at D2 receptors is a key mechanism in signaling the motivational effects experienced during nicotine withdrawal, and may represent a unique target for therapeutic treatments for nicotine addiction.

Electrophysiological and neurochemical characterization of 7-nitroindazole and molsidomine acute and sub-chronic administration effects in the dopaminergic nigrostrial system in rats

Nitric oxide (NO) plays an important role in the integration of information processed by the basal ganglia nuclei. Accordingly, considerable evidence has emerged indicating a role for NO in pathophysiological conditions such as Parkinson's disease (PD) and other neurodegenerative disorders. Despite these recent advances, the nitrergic modulation of the dopamine (DA) nigrostriatal system is still unclear. In order to fill this gap, in this study we used in vivo electrophysiology and ex vivo neurochemical analysis to further investigate the effect of NO signaling in rat substantia nigra pars compacta (SNc) and the striatum. Acute and subchronic (4 days) pharmacological manipulation of the NO system using 7-nitroindazole (7-NI, 50 mg kg(-1) i.p.) and molsidomine (MOL, 40 mg kg(-1) i.p.) treatment caused significant changes in both DA SNc neurons electrophysiological properties and striatal DA and 3,4-dihydroxyphenylacetic acid (DOPAC) levels. It is worth noting that acute inhibition of NO production decreased DA nigrostriatal neurotransmission while its subchronic inhibition was instead excitatory. Thus, a crucial role for NO in the modulation of nigrostriatal DA function is suggested together with a potential role for inhibitors of NO sythase in the treatment of PD.

Critical role of nitric oxide on nicotine-induced hyperactivation of dopaminergic nigrostriatal system: Electrophysiological and neurochemical evidence in rats

Nicotine, the main psychoactive ingredient in tobacco, stimulates dopamine (DA) function, increasing DA neuronal activity and DA release. DA is involved in both motor control and in the rewarding and reinforcing effects of nicotine; however, the complete understanding of its molecular mechanisms is yet to be attained. Substantial evidence indicates that the reinforcing properties of drugs of abuse, including nicotine, can be affected by the nitric oxide (NO) system, which may act by modulating central dopaminergic function. In this study, using single cell recordings in vivo coupled with microiontophoresis and microdialysis in freely moving animals, the role of NO signaling on the hyperactivation elicited by nicotine of the nigrostriatal system was investigated in rats. Nicotine induced a dose-dependent increase of the firing activity of the substantia nigra pars compacta (SNc) DA neurons and DA and 3,4-dihydroxyphenylacetic acid (DOPAC) release in the striatum. Pharmacological manipulation of the NO system did not produce any change under basal condition in terms of neuronal discharge and DA release. In contrast, pretreatments with two NO synthase (NOS) inhibitors, N-omega-nitro-l-arginine methyl ester (l-NAME) and 7-nitroindazole (7-NI) were both capable of blocking the nicotine-induced increase of SNc DA neuron activity and DA striatal levels. The effects of nicotine in l-NAME and 7-NI-pretreated rats were partially restored when rats were pretreated with the NO donor molsidomine. These results further support the evidence of an important role played by NO on modulation of dopaminergic function and drug addiction, thus revealing new pharmacological possibilities in the treatment of nicotine dependence and other DA dysfunctions.

Chronic nicotine exposure switches the functional role of mesolimbic dopamine transmission in the processing of nicotine's rewarding and aversive effects

The mammalian ventral tegmental area (VTA) and associated mesolimbic dopamine (DA) system are critical neural substrates for processing nicotine's motivational effects. Considerable evidence suggests that the role of DA transmission may be altered as a function of nicotine exposure. Using a combination of in vivo neuronal recording and behavioral conditioning, we report that chronic nicotine exposure induces a functional switch in the role of mesolimbic DA transmission. Thus, in nicotine-naive subjects, blockade of DA transmission potentiates the rewarding effects of sub-reward-threshold doses of nicotine and reverses the motivational valence of nicotine from aversive to rewarding. However, in animals treated chronically with nicotine, DA blockade switches previously sub-reward-threshold or rewarding doses of nicotine into aversion signals. Neuronal VTA recordings similarly revealed a functional switch in this DAergic neuronal circuit resulting in strongly increased sensitivity of the VTA DAergic system to nicotine administration and a tonic reduction in the baseline activity of VTA DAergic neurons. These results demonstrate a functional switch in the role of DAergic transmission during the acute versus chronic phases of nicotine exposure and suggest that mesolimbic DA transmission plays qualitatively distinct roles in the processing of nicotine's motivational effects as a function of drug exposure.

Minocycline reduced craving for cigarettes but did not affect smoking or intravenous nicotine responses in humans

In recent preclinical studies, the role of nitric oxide (NO) in nicotine dependence has become increasingly evident. Inhibition of NO synthesis blocks acquisition of conditioned place preference, and attenuates the nicotine abstinence syndrome in rodents. These findings have not been followed up in human studies. In order to obtain preliminary data on NO inhibition in human smokers, we conducted a randomized, double-blind, crossover study (N=12) of minocycline, a tetracycline derivative antibiotic, that inhibits the neuronal nitric oxide (NO) synthase enzyme with resultant inhibition of NO production. Medication effects were assessed through a smoking choice procedure as well as subjective and physiological responses to nicotine administered via the intravenous route (IV). Minocycline treatment did not affect smoking self-administration in our choice procedure and did not affect most of the subjective responses to IV nicotine or sample smoking. Following IV nicotine administration, there was a greater reduction in craving for cigarettes under minocycline, compared to placebo. Similarly, smokers had greater reduction in their craving for cigarettes following sample smoking under minocycline treatment. These findings provide limited support for the potential use of minocycline as a treatment of nicotine dependence.

Behavioral and neuropharmacological characterization of nicotine as a conditional stimulus

In rats, the pharmacological (interoceptive) effects of 0.4 mg/kg nicotine can serve as a conditional stimulus in a Pavlovian conditioning task. Nicotine administration is paired with intermittent access to a liquid sucrose unconditional stimulus; sucrose is withheld on saline sessions. An increase in sucrose receptacle entries (goal tracking) on nicotine sessions indicates conditioning. Rats were trained on a nicotine dose ((-)-1-Methyl-2-(3-pyridyl)pyrrolidine; 0.1, 0.2, or 0.4 mg base/kg, s.c.). Generalization was examined using 0.025, 0.05, 0.1, 0.2, and 0.4 mg/kg nicotine and saline. Some behavioral effects of nicotine have been attributed to dopamine and glutamate. Accordingly, potential blockade of the nicotine cue via the dopamine system was examined by administering (R)-(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390; 0.005, 0.01, and 0.03 mg/kg), 3-Chloro-5-ethyl-N-[[(2S)-1-ethyl-2-pyrrolidinyl)methyl]-6-hydroxy-2-methoxy-benzamide hydrochloride (eticlopride; 0.01, 0.03, 0.1, and 0.3 mg/kg), or N-[(1-Butyl-2-pyrrolidinyl)methyl]-4-cyano-1-methoxy-2-naphthalenecarboxamide (nafadotride; 0.03, 0.1, 0.3, 1, and 3 mg/kg) before nicotine. 2-Methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP; 0.3, 1, and 3 mg/kg) and (5S,10R)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801; 0.01, 0.03, 0.1, and 0.2 mg/kg; dizocilpine) were used to examine possible glutamatergic components. Substitution tests were conducted with MPEP and nafadotride. Differential conditioned responding was acquired in the 3 groups. Conditioned responding generally decreased as the nicotine test dose moved away from the training dose; responding increased when 0.4 mg/kg trained rats were tested with 0.2 mg/kg. SCH-23390, eticlopride, nafadotride, and MPEP decreased conditioned responding on nicotine at doses that also decreased chamber activity. In contrast, MK-801 decreased goal tracking on nicotine without decreasing chamber activity, indicating a role for N-methyl-D-aspartate receptors in expression of nicotine-evoked conditioned responding.

Effect of nitric oxide synthase inhibitor and NMDA receptor antagonist on the development of nicotine sensitization of nucleus accumbens dopamine release: an in vivo microdialysis study

We have previously found that the neuronal nitric oxide synthase inhibitor N-nitro-L-arginine (L-NNA) and the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 prevent behavioral sensitization to nicotine. This study aimed to investigate the effect of L-NNA and MK-801 on a neurochemical component of nicotine sensitization by evaluating the effect of the drugs on nicotine sensitization of nucleus accumbens dopamine (DA) release. Sprague-Dawley rats were pretreated with L-NNA (15 mg/kg, i.p.), MK-801 (0.3mg/kg, i.p.), or saline 30 min before injection of nicotine (0.4 mg/kg, s.c., once daily) for seven consecutive days. Twenty-four hours after the last drug injection, animals were challenged with local perfusion of 5 mM nicotine into the shell of nucleus accumbens for 60 min and DA release was monitored using in vivo microdialysis. In rats treated with repeated nicotine, acute nicotine challenge induced a greater increase of accumbal DA release than in saline-treated animals (maximal DA response=969+/-235% (mean+/-S.E.M.) of basal level versus 520+/-93%, p=0.042). Co-administration of L-NNA or MK-801 with nicotine attenuated an increase of DA release elicited by acute nicotine challenge, compared with nicotine alone (maximal DA response=293+/-58% and 445+/-90% of basal level, respectively versus 969+/-235%, p=0.004 and p=0.013, respectively). These data demonstrate that L-NNA and MK-801 block the development of nicotine sensitization of nucleus accumbens DA release, further supporting the involvement of nitric oxide and NMDA receptors in the development of behavioral sensitization to nicotine.

Mediation of glutamatergic receptors and nitric oxide on striatal dopamine release evoked by anatoxin-a. An in vivo microdialysis study

In this work, the involvement of ionotropic glutamatergic receptors and nitric oxide on striatal dopamine release induced by anatoxin-a was investigated in conscious and freely-moving rats. To study the participation of glutamatergic receptors, the effects of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine (MK-801) and d(-)-2-amino-5-phosphonopentanoic acid (APV), were examined. The perfusion of 3.5 mM anatoxin-a increased the extracellular dopamine levels to 701% relative to the basal. When CNQX was administered with 3.5 mM anatoxin-a, the increase of dopamine levels was 29% smaller than that observed with anatoxin-a alone. When MK-801 and APV were administered, the effect of anatoxin-a was attenuated 26% and 25% respectively in terms of that observed with anatoxin-a alone. And with CNQX plus MK-801, the effect of anatoxin-a was 53% inhibited in terms of the effect of anatoxin-a alone. These results suggest that the striatal dopamine release induced by anatoxin-a is partly mediated by activation of both ionotropic glutamatergic receptors. Since the neuronal form of nitric oxide synthase (nNOS) produces nitric oxide (NO) primarily in response to activation of NMDA receptors, it was tested if NO could play any role in the effect of anatoxin-a. Treatment with NOS inhibitors, L-nitro-arginine methyl ester (L-NAME) and d(-)-2-amino-5-phosphonopentanoic acid (7-NI), induced decreased anatoxin-a effects of 22% and 26% respectively. In conclusion, the present in vivo results demonstrate that anatoxin-a induced an indirect activation of ionotropic glutamatergic receptors (NMDA and AMPA/kainite receptors), which stimulate striatal dopamine release. On the other hand, activation of NMDA receptors may elicit NO increased levels enhancing dopamine release.

Nicotine-induced changes of glutamate and arginine in naive and chronically alcoholized rats: An in vivo microdialysis study

The effects of nicotine, when administered either acutely or chronically, at doses of 0.15, 0.3 or 0.6 mg/kg, on the release of glutamate and arginine in the rat nucleus accumbens have been studied in microdialysis experiments. Glutamate release significantly increased after acute nicotine injection, 0.3 mg/kg, which was accentuated if there was a priming regime of saline for the previous 27 days. This is possibly related to the rewarding effects of nicotine. Five hours after cessation of chronic oral nicotine administration, there were significant increases in glutamate content, which was possibly reflective of a withdrawal process. Significant decreases in nucleus accumbens arginine release were evident, between 1 and 2 h, after chronic nicotine administration. When nicotine was co-administered to rats during chronic ethanol intoxication, at either 0.15 mg/kg or 0.3 mg/kg doses, glutamate release did not increase during the first 12 h of withdrawal. However, a decrease in arginine microdialysate content was still observed with all nicotine doses. The nicotine-induced changes in glutamate and arginine release in nucleus accumbens highlights the complex neuropharmacological interactions evoked by this compound and also identified its possible modulating effect on glutamate release during the initial stages of chronic ethanol withdrawal.

High doses of simvastatin upregulate dopamine D1 and D2 receptor expression in the rat prefrontal cortex: possible involvement of endothelial nitric oxide synthase

This study aims to investigate whether or not long-term statin treatment causes upregulation of D1 and D2 receptor gene expression with concomitant increase in endothelial nitric oxide synthase (eNOS) expression in Sprague-Dawley rats. Serum triglyceride levels were dose dependently reduced in the simvastatin-treated rats reaching statistical significance at the highest dose (49% reduction), while pravastatin caused similar effects (52%) at the same dose. Cholesterol levels remained unchanged in both groups at all doses. Simvastatin, 10 or 30 mg kg(-1) day(-1), increased D1 and D2 receptor expressions in the prefrontal cortex. Similar upregulation was observed neither with simvastatin in the striatum nor with pravastatin in both brain regions. Simvastatin (10 mg kg(-1) day(-1)) also increased eNOS expression in the prefrontal cortex but not neuronal NOS or inducible NOS. D1 receptor activation by chloro-APB (5 microM) increased cAMP levels in synaptosomes prepared from the prefrontal cortex of control and simvastatin-treated rats by 88 and 285%, respectively. This effect was markedly attenuated by the selective D1 antagonist SCH-23390 (25 microM). D2 receptor activation by quinpirole (5 microM) had no effect on the basal cAMP levels in synaptosomes prepared from the prefrontal cortex of control and simvastatin-treated rats, while the same concentration of quinpirole completely abolished the D1 receptor-mediated increase. These results suggest that lipophilic statins can alter dopaminergic functions in the prefrontal cortex possibly via a central mechanism. The possibility of a nitric oxide mechanism involving eNOS requires further investigation.

Subchronic treatment with kynurenine and probenecid: effects on prepulse inhibition and firing of midbrain dopamine neurons

Acute elevation of the endogenous NMDA-receptor antagonist kynurenic acid (KYNA) is associated with an increased neuronal activity of rat ventral tegmental area (VTA) dopamine (DA) neurons and disruption in prepulse inhibition (PPI). In the present study, the effects of subchronic exposure to kynurenine and probenecid (20 mg/kg/day and 10 mg/kg/day, respectively for 14 days), aiming at increasing brain KYNA turnover, on rat VTA dopaminergic firing and on PPI were investigated. This treatment increased neuronal firing of VTA DA neurons, changed the response of these neurons to systemically administered nicotine (3-400 microg/kg, i.v.) and tended to disrupt PPI. Present results show that the effect on firing of VTA DA neurons by acutely elevated levels of brain KYNA also persists following subchronic exposure. In addition, no adaptive changes seem to occur with regard to the electrophysiological effects of KYNA on VTA DA neurons following subchronic treatment with kynurenine and probenecid.

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

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