L-citrulline conversion to L-arginine in sphenopalatine ganglia and cerebral perivascular nerves in the pig

Depletion of arginine by recombinant arginine deiminase induces nNOS-activated neurotoxicity in neuroblastoma cells

The abnormal regulation of inducible nitric oxide synthase (iNOS) and neuronal nitric oxide synthase (nNOS) is associated with neurodegenerative disorders. Recombinant arginine deiminase (rADI) is a selective NO modulator of iNOS and eNOS in endothelial cells, and it also exhibits neuroprotective activity in an iNOS-induced neuron-microglia coculture system. However, the effect of rADI on nNOS remains unknown. Addressing this issue is important for evaluating the potential application of rADI in neurodegenerative diseases. SH-SY5Y cells were treated with N-methyl-D-aspartic acid (NMDA) to activate nNOS. NMDA increased NO production by 39.7 ± 3.9% via nNOS under arginine-containing conditions, but there was no significant increase in both arginine-free and rADI pretreated arginine-containing (citrulline) buffer. Subsequently, neither NMDA nor rADI alone caused cytotoxicity, whereas cotreatment with NMDA and rADI resulted in dissipation of the cell mitochondrial membrane potential and decreased cell viability. The mechanism of rADI cytotoxicity in the presence of NMDA is caused by the inhibition of NO production via nNOS mediated by the NMDA receptor, which was abolished when extracellular arginine was absent, even in the presence of citrulline. rADI not only reduced NO production but also caused cellular toxicity in nNOS-activated SH-SY5Y cells, suggesting a dual role for rADI in NOS-mediated neurotoxicity.

A novel central pathway links arterial baroreceptors and pontine parasympathetic neurons in cerebrovascular control

1. We tested the hypothesis that arterial baroreceptor reflexes modulate cerebrovascular tone through a pathway that connects the cardiovascular nucleus tractus solitarii with parasympathetic preganglionic neurons in the pons. 2. Anesthetized rats were used in all studies. Laser flowmetry was used to measure cerebral blood flow. We assessed cerebrovascular responses to increases in arterial blood pressure in animals with lesions of baroreceptor nerves, the nucleus tractus solitarii itself, the pontine preganglionic parasympathetic neurons, or the parasympathetic ganglionic nerves to the cerebral vessels. Similar assessments were made in animals after blockade of synthesis of nitric oxide, which is released by the parasympathetic nerves from the pterygopalatine ganglia. Finally the effects on cerebral blood flow of glutamate stimulation of pontine preganglionic parasympathetic neurons were evaluated. 3. We found that lesions at any one of the sites in the putative pathway or interruption of nitric oxide synthesis led to prolongation of autoregulation as mean arterial pressure was increased to levels as high as 200 mmHg. Conversely, stimulation of pontine parasympathetic preganglionic neurons led to cerebral vasodilatation. The second series of studies utilized classic anatomical tracing methods to determine at the light and electron microscopic level whether neurons in the cardiovascular nucleus tractus solitarii, the site of termination of baroreceptor afferents, projected to the pontine preganglionic neurons. Fibers were traced with anterograde tracer from the nucleus tractus solitarii to the pons and with retrograde tracer from the pons to the nucleus tractus solitarii. Using double labeling techniques we further studied synapses made between labeled projections from the nucleus tractus solitarii and preganglionic neurons that were themselves labeled with retrograde tracer placed into the pterygopalatine ganglion. 4. These anatomical studies showed that the nucleus tractus solitarii directly projects to pontine preganglionic neurons and makes asymmetric, seemingly excitatory, synapses with those neurons. These studies provide strong evidence that arterial baroreceptors may modulate cerebral blood flow through direct connections with pontine parasympathetic neurons. Further study is needed to clarify the role this pathway plays in integrative physiology.

Segregation of VIPergic-nitric oxidergic and cholinergic-nitric oxidergic innervation in porcine middle cerebral arteries

The distribution of nitric oxide synthase (NOS)-, choline acetyltransferase (ChAT)-, and vasoactive intestinal polypeptide (VIP)-immunoreactivities, and nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd)-reactivities in the sphenopalatine ganglia (SPG), and perivascular nerves in middle cerebral arteries of the pig was investigated by double-staining techniques using combined immunofluorescence and histochemistry methods. In the SPG, almost all ganglionic cells were NOS-immunoreactive (I) and NADPHd-positive, and both NOS immunoreactivities and NADPHd reactivities were completely co-localized. ChAT-I ganglionic cells accounted for 75%, while VIP-I ganglionic cells represented 42% of all ganglionic cells. Almost all VIP immunoreactivities were co-localized with ChAT immunoreactivities, and all ganglionic cells that were VIP-I and/or ChAT-I were NOS-I and NADPHd-reactive. None of the ganglionic cells in the SPG were immunoreactive to calcitonin gene-related peptide (CGRP). CGRP immunoreactivities, however, were found to surround some ganglionic cells. In middle cerebral arteries, all adventitial NOS-I bundles and fine fibers were coincident with NADPHd fibers. Almost all adventitial ChAT-I bundles and thin fibers, and VIP-I mesh-like fibers stained positively for NADPHd, while the mesh-like NADPHd fine fibers were not ChAT-I. Simultaneous labeling using antibodies against VIP and ChAT further indicated that VIP-I fibers were closer than ChAT-I fibers to the smooth muscle. In rare occasions, perivascular fibers were found to be stained for both ChAT and VIP, showing that most ChAT-I and VIP-I fibers were not coincident. These results suggest that ChAT and VIP are rarely co-localized in perivascular nerves in middle cerebral arteries, and point out that the neurotransmitter and the modulator that are co-localized within the same nerve cell body may distribute totally independently and differently at the terminal level. The present results also indicate that in cerebral perivascular nerves, the combination of nitric oxide (NO) and acetylcholine (ACh), as well as the combination of NO and VIP, are localized in the same nerve with different axons containing either NO plus ACh, or NO plus VIP. These findings support the hypothesis that ACh and VIP may act as modulators in regulating presynaptic release of NO, and therefore, cerebral neurogenic vasodilation, from their respective perivascular cholinergic-nitric oxidergic and VIPergic-nitric oxidergic nerves.