Role of neuronal nitric oxide synthase on cardiovascular functions in physiological and pathophysiological states

Abstract

This review describes and summarizes the role of neuronal nitric oxide synthase (nNOS) on the central nervous system, particularly on brain regions such as the ventrolateral medulla (VLM) and the periaqueductal gray matter (PAG), and on blood vessels and the heart that are involved in the regulation and control of the cardiovascular system (CVS). Furthermore, we shall also review the functional aspects of nNOS during several physiological, pathophysiological, and clinical conditions such as exercise, pain, cerebral vascular accidents or stroke and hypertension. For example, during stroke, a cascade of molecular, neurochemical, and cellular changes occur that affect the nervous system as elicited by generation of free radicals and nitric oxide (NO) from vulnerable neurons, peroxide formation, superoxides, apoptosis, and the differential activation of three isoforms of nitric oxide synthases (NOSs), and can exert profound effects on the CVS. Neuronal NOS is one of the three isoforms of NOSs, the others being endothelial (eNOS) and inducible (iNOS) enzymes. Neuronal NOS is a critical homeostatic component of the CVS and plays an important role in regulation of different systems and disease process including nociception. The functional and physiological roles of NO and nNOS are described at the beginning of this review. We also elaborate the structure, gene, domain, and regulation of the nNOS protein. Both inhibitory and excitatory role of nNOS on the sympathetic autonomic nervous system (SANS) and parasympathetic autonomic nervous system (PANS) as mediated via different neurotransmitters/signal transduction processes will be explored, particularly its effects on the CVS. Because the VLM plays a crucial function in cardiovascular homeostatic mechanisms, the neuroanatomy and cardiovascular regulation of the VLM will be discussed in conjunction with the actions of nNOS. Thereafter, we shall discuss the up-to-date developments that are related to the interaction between nNOS and cardiovascular diseases such as hypertension and stroke. Finally, we shall focus on the role of nNOS, particularly within the PAG in cardiovascular regulation and neurotransmission during different types of pain stimulus. Overall, this review focuses on our current understanding of the nNOS protein, and provides further insights on how nNOS modulates, regulates, and controls cardiovascular function during both physiological activity such as exercise, and pathophysiological conditions such as stroke and hypertension.

Keywords: Antioxidants; Arteriosclerosis; Autonomic nervous system; Baroreceptor; Blood pressure; Cerebral vascular accident; Endothelial cells; Exercise; Exercise pressor reflex; GABA; Glutamate; Heart rate; Hypertension; Hypothalamus; Nociception; Periaqueductal gray matter; Reactive oxygen species; Stroke; Thrombus; Ventrolateral medulla.

Figure 1:

Bidomain structure of nNOS isoform and the splicing variants (nNOSβ and nNOSγ). The electrons donated by NADPH (nicotinamide adenine dinucleotide phosphate) to the reductase domain reach the oxygenase domain via two redox carriers, FAD (flavin-adenine dinucleotide) and FMN (flavin mononucleotide). In the oxygenase domain, the electrons interact with heme and BH ₄ (tetrahydrobiopterin) to cause arginine oxidation to citrulline and NO (nitric oxide).

Figure 2.. Difference between nNOS α and nNOS β splice variants.

Serine, Threonine, Arginine), Tetrahydrobiopterin, Calmodulin, Flavin mononucleotide, Flavin adenine dinucleotide, and Nicotinamide adenine dinucleotide phosphate are common structural components present in all the splice variants of nNOS (neuronal nitric oxide synthase). The arrows indicate the direction of phosphorylation at the respective sites on NOS (nitric oxide synthase) activity.

Figure 3.. Negative feedback loop for nNOS control of Ca²⁺ entry through NMDA receptor.

The entry of Ca²⁺ into the cell is coupled to nNOS (neuronal nitric oxide synthase) through the PDZ domain, PSD-95. As the level of Ca²⁺ in the cell increases, nNOS-derived NO (nitric oxide) acts on glutamate receptors (Both NMDA (N-methyl-D-aspartate) and AMPA) in order to inhibit further entry of Ca²⁺ by S-nitrosylation reaction.

Figure 4:. Effect of nNOS on calcium handling.

The process of excitation-contraction coupling begins with the entry of calcium (Ca²⁺) into the cell via the calcium current that triggers the further release of Ca²⁺ from sarcoplasmic reticulum. This stimulation of intracellular Ca²⁺ initiates and propagates contraction. Neuronal nitric oxide synthase (nNOS) have significant effects on myocyte contraction under basal conditions and via Beta-1 adrenergic receptor stimulation.

Figure 5.. The role of nNOS in autonomic control of cardiac function.

(Top) In the central nervous system, NO (nitric oxide) derived from nNOS (neuronal nitric oxide synthase) and eNOS (endothelial nitric oxide synthase) causes inhibition of sympathetic outflow and stimulation of vagal outflow via augmentation of GABA (γ-amino butyric acid)-mediated neuronal activity and inhibition of glumatergic neurons, respectively. (Bottom) In the periphery, NO derived from nNOS decreases the release of NE (norepinephrine) and possibly glutamate and stimulates the release of ACh (acetylcholine) and GABA that results in the inhibition of cardiac responses to sympathetic nerve stimulation and the facilitation of negative chronotropic action of vagal stimulation, respectively.

Figure 5.. The role of nNOS in autonomic control of cardiac function.

(Top) In the central nervous system, NO (nitric oxide) derived from nNOS (neuronal nitric oxide synthase) and eNOS (endothelial nitric oxide synthase) causes inhibition of sympathetic outflow and stimulation of vagal outflow via augmentation of GABA (γ-amino butyric acid)-mediated neuronal activity and inhibition of glumatergic neurons, respectively. (Bottom) In the periphery, NO derived from nNOS decreases the release of NE (norepinephrine) and possibly glutamate and stimulates the release of ACh (acetylcholine) and GABA that results in the inhibition of cardiac responses to sympathetic nerve stimulation and the facilitation of negative chronotropic action of vagal stimulation, respectively.

Figure 6:

A picture of a gel with Western blots of the 155 KDa recombinant nNOS band and nNOS proteins as derived from each quadrant of the right and left rostral (RVLM) and caudal ventrolateral medulla (CVLM) homogenates of intact or Sham and middle cerebral artery occluded (MCAO) rats. Equal amounts of proteins (20 μg) were loaded in each lane in the same gel. Recombinant nNOS DNA (5μg: Sigma Chemicals, USA) was loaded as control.