Regional distribution of binding sites for the nitric oxide synthase inhibitor L-[3H]nitroarginine in rat brain

Distribution of neuronal nitric oxide synthase immunoreactivity in adult male Sprague-Dawley rat brain

Neuronal NOS (nNOS) accounts for most of the NO production in the nervous system that modulates synaptic transmission and neuroplasticity. Although previous studies have selectively described the localisation of nNOS in specific brain regions, a comprehensive distribution profile of nNOS in the brain is lacking. Here we provided a detailed morphological characterization on the rostro-caudal distribution of neurons and fibres exhibiting positive nNOS-immunoreactivity in adult Sprague-Dawley rat brain. Our results demonstrated that neurons and fibres in the brain regions that exhibited high nNOS immunoreactivity include the olfactory-related areas, intermediate endopiriform nucleus, Islands of Calleja, subfornical organ, ventral lateral geniculate nucleus, parafascicular thalamic nucleus, superior colliculus, lateral terminal nucleus, pedunculopontine tegmental nucleus, periaqueductal gray, dorsal raphe nucleus, supragenual nucleus, nucleus of the trapezoid body, and the cerebellum. Moderate nNOS immunoreactivity was detected in the cerebral cortex, caudate putamen, hippocampus, thalamus, hypothalamus, amygdala, and the spinal cord. Finally, low NOS immunoreactivity were found in the corpus callosum, fornix, globus pallidus, anterior commissure, and the dorsal hippocampal commissure. In conclusion, this study provides a comprehensive view of the morphology and localisation of nNOS immunoreactivity in the brain that would contribute to a better understanding of the role played by nNOS in the brain.

Alterations in postnatal neurogenesis and dopamine dysregulation in schizophrenia: a hypothesis

An increasing number of studies demonstrate the important role of several susceptibility genes for schizophrenia, such as neuregulin-1 and DISC1, in early postnatal and adult neurogenesis. Its significance for the pathophysiology of the disease, including its relation to neurotransmitter systems implicated in schizophrenia (like the dopamine system), remains, however, unknown. Here, we review molecular and cellular components of the dopamine system associated with postnatal neurogenesis and plasticity, both in rodents and in primates, and discuss their possible implication in schizophrenia. We focus mainly on the islands of Calleja, complex aggregations of granule cells in the ventral striatum, generated early postnatally in the subventricular zone. In contrast to the involution of the primate olfactory bulb, the islands of Calleja attain their maximal development in humans, an evolution paralleled by a larger ventral subventricular zone and more connections with other structures, including temporal cortical areas. The islands of Calleja express high levels of neuronal nitric oxide (NO) synthase and D3 dopamine receptors and are densely interconnected by dopaminergic projections with the ventral tegmental area. D3 receptors modulate subventricular zone neurogenesis and dopamine release. Their genetic deletion induces striatal hyperdopaminergia. We review data indicating a high plasticity of postnatal islands of Calleja, potentially facilitating susceptibility to schizophrenia-related risk factors. In this context, we propose a new pathophysiological model, where altered neurogenesis of the islands of Calleja may contribute to dysfunction of the dopamine and NO systems and psychosis through convergence of genetic and environmental disease-associated factors.

Different response to exogenous l-arginine in nitric oxide production between hippocampus and striatum of conscious rats: a microdialysis study

We previously showed that systemic administration of a nitric oxide (NO) precursor, L-arginine (L-Arg), failed to reverse suppression by NO synthase (NOS) inhibitors of chemically induced shaking behavior in rats, leading to the hypothesis that exogenous L-Arg might be non-uniformly supplied to brain regions susceptible to NOS inhibitors. In the present study, therefore, we examined the effect of exogenous L-Arg on the extracellular levels of the oxidative nitric oxide (NO) products, nitrite (NO2-) and nitrate (NO3-), in two different brain regions, the hippocampus and the striatum, of conscious rats by means of in vivo brain microdialysis. The basal NO2- levels in the two brain regions were comparable, while the NO3- level was significantly lower in the hippocampus than the striatum. The addition of 10 mM L-Arg, but not D-Arg, to the perfusing solution significantly increased NO2- and NO3- in the hippocampus and NO2- alone in the striatum. These increases were abolished by 1 mM N(omega)-nitro-L-arginine, an NOS inhibitor. L-Arg at 1mM was able to significantly increase NO2-, but not NO3-, in the hippocampus to a level comparable with that at 10 mM L-Arg, while it had no effect in the striatum. L-Arg (500 mg/kg, i.p.) induced a significant increase in NO2- and NO3- in the hippocampus, but not in the striatum. These results suggest that the striatum may have a lower ability to enhance NO production by utilising exogenous L-Arg than the hippocampus, despite higher basal NO production.

Rat basal forebrain cholinergic lesion affects neuronal nitric oxide synthase activity in hippocampal and neocortical target regions

Nitric oxide (NO)-mediated mechanisms have been assigned a role in cortical perfusion, learning and memory as well as in neuronal plasticity. Dysfunction of cortical cholinergic transmission has also been associated with reduced cortical cerebral blood flow and impaired performance in learning and memory tasks suggesting a link between the basal forebrain cholinergic system and cortical NO-mediated mechanisms. The aim of this study was therefore to study the influence of cholinergic input on neuronal NO-synthase (nNOS) activity in cortical cholinoceptive target neurons. A nearly complete loss of rat basal forebrain cholinergic cells was induced by a single intracerebroventricular application of the cholinergic immunotoxin 192IgG-saporin. Basal forebrain cholinergic hypofunction resulted in reduced catalytic and substrate binding activity of nNOS in a number of hippocampal and neocortical subregions 7 days after lesion as revealed by NADPH-diaphorase enzyme histochemistry and quantitative autoradiography of [3H]L-N(G)-nitro-arginine binding, respectively. The total amount of nNOS protein assayed by Western analysis, was not affected in the cortical and hippocampal regions examined. The data indicate that cortical cholinergic deafferentation results in reduced nNOS activity in select cholinoceptive neocortical and hippocampal neurons. As the total amount of cortical nNOS protein was not affected by basal forebrain cholinergic lesion, the results suggest that the ratio of catalytically active and inactive cortical nNOS is driven by basal forebrain cholinergic input presumably via M1-muscarinic cholinergic receptors.

Axonal transport of NADPH-diaphorase and [(3)H]nitro-L-arginine binding, but not [(3)H]cGMP binding, by the rat vagus nerve

Previous studies have shown that the NO(ccirf)-cGMP pathway may be functionally relevant in the nodose ganglion and at afferent terminations of the vagus nerve. The technique of unilateral vagal ligations, using double ligatures, was combined with the techniques of NADPH-diaphorase histochemistry, as an index of nitric oxide synthase (NOS) activity, and autoradiography using the radioligands [(3)H]nitro-L-arginine and [(3)H]cGMP, to examine axonal transport of NOS and cGMP-dependent effectors by the rat vagus nerve. A population of perikarya in the nodose ganglia was NADPH-diaphorase positive, and binding of both [(3)H]nitro-L-arginine and [(3)H]cGMP was found on the nodose ganglia. Following vagal ligation, NADPH-diaphorase reactivity accumulated proximal to the proximal ligature and distal to the distal ligature. Vagus nerve transection beyond the distal ligature eliminated NADPH-diaphorase reactivity at the distal ligature. Similarly, [(3)H]nitro-L-arginine binding was found over the nodose ganglion; and after vagal ligation, an accumulation of [(3)H]nitro-L-arginine binding was seen adjacent to the proximal ligature, though little binding was found adjacent to the distal ligature. No accumulation of [3H]cGMP binding was found adjacent to either the proximal or the distal ligatures. These findings suggest that the rat vagus nerve bidirectionally transports NOS, the enzyme involved in biosynthesis of NO(ccirf) by nitroxidergic nerves. As anticipated, [(3)H]nitro-L-arginine, a competitive inhibitor of the amino acid precursor for NO(ccirf), binds only to a centrifugally transported moiety that we conjecture is NOS, while cGMP apparently is not subject to transport. These data further support the use of NO(&z.ccirf;) in transmission at vagal afferent terminals.

The ascending reticular activating system--from aminergic neurons to nitric oxide

The cholinergic neurons of the laterodorsal and pedunculopontine tegmental neurons are thought to comprise an important portion of the ascending reticular activating system. More recent work has demonstrated that the neurons of this cell group also released a number of neruoactive peptides and can produce nitric oxide in response to increases in intracellular calcium. The release of NO from the nerve terminals of these cells within the thalamus varies with behavioural state, being much lower during slow wave sleep than during wake and paradoxical sleep states. The NO release in the thalamus appears to act via the type II cGMP-dependent protein kinase present at high levels in the thalamic neurons. Thus the NO-cGMP signal transduction system can play an important role in regulating thalamic activity across behavioural states.

Traumatic injury to rat brain upregulates neuronal nitric oxide synthase expression and L-[3H]nitroarginine binding

Overstimulation of N-methyl-D-aspartate (NMDA) receptors is felt to precipitate the neuronal damage following traumatic brain injury (TBI). NMDA receptor-mediated, glutamate-induced excitotoxicity is thought to be mediated via nitric oxide (NO) formed by neuronal nitric oxide synthase (nNOS). The present study examined the mRNA and protein levels of nNOS in the ipsilateral and contralateral cortex of rats as a function of time (5 minutes to 1 week) after controlled cortical impact (CCI) brain injury. Sham-operated rats served as controls. TBI resulted in a significant increase in the levels of nNOS mRNA (1.5- to 2.8-fold, p < .05) between 2 and 4 hours after the injury. There was also a significant increase in the levels of nNOS protein (by 55% to 90%, p < .05) and binding densities of the nNOS-specific ligand L-[3H]nitroarginine (L-[3H]NOARG) (by 35% to 59%, p < .05) between 2 and 12 hours after the injury. Increased nNOS expression and function may contribute to the concomitant excitotoxic neuronal death after TBI.

Increased neuronal nitric oxide synthase expression in brain following portacaval anastomosis

It has previously been suggested that increases of L-arginine uptake into brain following portacaval shunting may result in increased activities of constitutive neuronal nitric oxide synthase (nNOS). In order to further address this issue, nNOS protein and gene expression were studied by Western blot analysis using a monoclonal nNOS antibody and RT-PCR respectively in the brains of rats following portacaval shunting or sham operation. Portacaval shunting resulted in a 2-fold increase (P < 0.01) in nNOS protein and a concomitant 2.4-fold increase (P < 0.01) in nNOS mRNA. Increased nNOS activity in brain and the resulting increase in nitric oxide production could contribute to the increased cerebral blood flow and to the pathogenesis of hepatic encephalopathy in chronic liver disease.