NO-generating neurons in the medullary cardiovascular centers of rodents and carnivores

Distribution of nitric oxide in the rock cavy (Kerodon rupestris) brain II: The brainstem

In a recent paper, we described the distribution of Nitric oxide (NO) in the diencephalon of the rock cavy (Kerodon rupestris). This present paper follows this work, showing the distribution of NO synthesizing neurons in the rock cavy's brainstem. For this, we used immunohistochemistry against the neuronal form of nitric oxide synthase (NOS) and NADPH diaphorase histochemistry. In contrast to the diencephalon in the rock cavy, where the NOS neurons were seen to be limited to some nuclei in the thalamus and hypothalamus, the distribution of NOS in the brainstem is widespread. Neurons immunoreactive to NOS (NOS-ir) were seen as rostral as the precommissural nuclei and as caudal as the caudal and gelatinous parts of the spinal trigeminal nucleus. Places such as the raphe nuclei, trigeminal complex, superior and inferior colliculus, oculomotor complex, periaqueductal grey matter, solitary tract nucleus, laterodorsal tegmental nucleus, pedunculopontine tegmental, and other nuclei of the reticular formation are among the locations with the most NOS-ir neurons. This distribution is similar, but with some differences, to those described for other rodents, indicating that NO also has an important role in rock cavy's physiology.

P2 purinergic receptor activation of neuronal nitric oxide synthase and guanylyl cyclase in the dorsal facial area of the medulla increases blood flow in the common carotid arteries of cats

In the dorsal facial area (DFA) of the medulla, an activation of either P2 purinergic receptor or nitric oxide synthase (NOS) results in the release of glutamate, leading to an increase in blood flow of the common carotid artery (CCA). It is not known whether activation of the P2 receptor by ATP may mediate activation of NOS/guanylyl cyclase to cause glutamate release and/or whether L-Arg (nitric oxide (NO) precursor) may also cause ATP release from any other neuron, to cause an increase in CCA flow. We demonstrated that microinjections of P2 receptor agonists (ATP, α,β-methylene ATP) or NO precursor (L-arginine) into the DFA increased CCA blood flow. The P2-induced CCA blood flow increase was dose-dependently reduced by pretreatment with NG-nitro-arginine methyl ester (L-NAME, a non-specific NOS inhibitor), 7-nitroindazole (7-NI, a relatively selective neuronal NOS inhibitor) or methylene blue (MB, a guanylyl cyclase inhibitor) but not by that with D-NAME (an isomer of L-NAME) or N5-(1-iminoethyl)-L-ornithine (L-NIO, a potent endothelial NOS inhibitor). Involvement of glutamate release in these responses were substantiated by microdialysis studies, in which perfusions of ATP into the DFA increased the glutamate concentration in dialysates, but co-perfusion of ATP with L-NAME or 7-NI did not. Nevertheless, the arginine-induced CCA blood flow increase was abolished by combined pretreatment of L-NAME and MB, but not affected by pretreatment with a selective P2 receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS). In conclusion, ATP activation of the P2 receptor in the DFA induced activation of neuronal NOS/guanylyl cyclase, which causes glutamate release leading to an increase in CCA blood flow. However, arginine activation of neuronal NOS/guanylyl cyclase, which also caused glutamate release and CCA blood flow increase, did not induce activation of P2 receptors. These findings provide important information for drug design and/or developing therapeutic strategies for the diseases associated with CCA blood flow that supplies intra- and extra-cranial tissues.

Neurochemicals involved in medullary control of common carotid blood flow

The common carotid artery (CCA) supplies intra- and extra-cranial vascular beds. An area in the medulla controlling CCA blood flow is defined as the dorsal facial area (DFA) by Kuo et al. in 1987. In the DFA, presynaptic nitrergic and/or glutamatergic fibers innervate preganglionic nitrergic and/or cholinergic neurons which give rise to the preganglionic fibers of the parasympathetic 7th and 9th cranial nerves. Released glutamate from presynaptic nitrergic and/or glutamatergic fibers can activate N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors on preganglionic nitrergic and/or cholinergic neurons. By modulating this glutamate release, several neurochemicals including serotonin, arginine, nitric oxide, nicotine, choline and ATP in the DFA regulate CCA blood flow. Understanding the neurochemical regulatory mechanisms can provide important insights of the physiological roles of the DFA, and may help develop therapeutic strategies for diseases involving CCA blood flow, such as migraine, hypertensive disease, Alzheimer’s disease and cerebral ischemic stroke.

Regulation of common carotid arterial blood flow by nitrergic neurons in the medulla of cats

Glutamate stimulation of the dorsal facial area, an area located dorsal to the facial nucleus, increases common carotid arterial blood flow. Nitrergic neurons are important in cardiovascular regulatory areas. We investigated whether the nitrergic neurons might be present and play a role in the dorsal facial area to regulate the arterial blood flow. Injections of L-arginine (an NO precursor) and sodium nitroprusside (an NO donor) into the area caused dose-dependent increases in the arterial blood flow. Injection of N(G)-nitro-arginine methyl ester (L-NAME, an NO synthase inhibitor) or methylene blue (a guanylate cyclase inhibitor) decreased the arterial blood flow. Nitrergic neurons and fibers were found in the dorsal facial area by histochemical staining of nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase, a maker of NO synthase. In conclusion, nitrergic neurons are present in the dorsal facial area and appear to release NO tonically in stimulating the area to cause increase in common carotid arterial blood flow.