As mentioned earlier, once activated, microglia release large amounts of NO and superoxide
as a cytotoxic attack mechanism (Colton and Gilbert, 1987
). ROS and RNS derived from NO and superoxide may also cause local cellular
damage by reacting with proteins, lipids and nucleic acids (Valko et al.
). These chemicals can directly damage cells
and lead to neuronal cell death. As a result, elevated NO levels can cause a wide array of
medical problems, many of which are found in ASD. Although an ASD diagnosis is defined by
three core features (impairment in communication and socialization, and behavioral issues),
other features, more physical or systemic in nature, are associated with an ASD diagnosis.
For example, a recent analysis of the National Health Interview Survey, 2006–2010, that
included 375 children with autism by Schieve et al.
), found that children with autism were more likely to have
headaches/migraines, respiratory and food allergies, physician visits and to be taking
prescription medication than children without autism. Children with ASD are also, according
to a study by Atladóttir et al.
), more likely to be hospitalized for an infectious disease. These medical
diseases could possibly be a result of, or associated with, or exacerbated by elevated NO
levels. Some examples are as follows:
As mentioned, children with ASD have a higher rate of infection (Atladóttir et
). There is evidence that abundant
NO at an inflammatory site may reduce and impair natural killer (NK) cell function
(Takabayashi et al.
may provide an explanation for the frequent infections that a large subset of children with
autism suffer from (Nicolson et al.
). Studies have found low NK function in ASD (Enstrom et al.
). Vojdani et al.
), for example, found that at least 45% of children
with autism suffer from low NK cell activity.
Seizures are common in autism, occurring in 20–30% of patients based on the majority of
studies. Epileptiform EEG abnormalities are present in 10.3–72.4% of patients (Danielsson
). Several studies
have found that microglial activation can result in seizures (Radewicz et
; Somera-Molina et
). In a study by Kovács et al.
) the researchers propose that NO-dependent
enhancement of synaptic transmission is a key promoting factor for the initiation of
seizures. In addition, NO might exert long-term effects in epilepsy. NO-dependent inhibition
of mitochondrial electron transport chain activity (Brown, 2001
), disruption of the mitochondrial networks (Yuan et al.
), and blockade of mitochondrial trafficking
(Rintoul et al.
contribute to metabolic impairment as described for the epileptic hippocampus (Kunz
; Kann et
In the National Health Interview Survey, 2006–2010, Schieve et al.
) also reported a higher rate of asthma and
bronchitis in children with intellectual disabilities (ID), including ASD. In a review by
), it was reported that an increase in
the exhaled NO has been shown to accompany eosinophilic inflammation and to correlate with
other indices of inflammation in asthma. Exhaled NO increases during exacerbation and
decreases with recovery in patients with asthma. Yates (2001
) also reported that asthma is characterized by chronic airway inflammation
and increased synthesis of NO and other highly reactive and toxic substances (ROS).
Pro-inflammatory cytokines such as TNF-α and IL-1β are secreted in asthma and result in
inflammatory cell recruitment, but also induce calcium- and calmodulin-iNOS and perpetuate
the inflammatory response within the airways. NO is released by several pulmonary cells
including epithelial cells, eosinophils and macrophages, and NO has been shown to be
increased in conditions associated with airway inflammation, such as asthma and viral
Research suggests relatively high rates of eczema and food allergies in ASD as compared to
TD children (Schieve et al.
). It has been suggested that NO is an important player in eczema, food
allergies, and intestinal inflammation. Eczema is characterized by inflammation of the skin
and is commonly associated with food allergy. The results of a study, by Devenney et
), were able to support
previous studies indicating that the homeostasis of nitrogen radicals is disturbed in
As mentioned, Adams et al.
and Wang et al.
) found that
there is a correlation of gastrointestinal symptoms with autism severity indicating that
children with more severe autism are likely to have more severe gastrointestinal symptoms
and vice versa. Schieve et al.
) found that children with autism were 70% more likely than children in the ID
group, two times more likely than children in the attention-deficit hyperactivity disorder
and learning disabled/other developmental delay groups, and seven times more likely than
children without developmental delays (DDs) to have had frequent diarrhea/colitis in the
last 12 months. Research shows that exaggerated or uncontrolled expression of iNOS itself
becomes detrimental to the gastrointestinal tract (Calatayud et al.
), and that large amounts of NO can increase gut
permeability and induce apoptosis (Dijkstra et al.
). Inflammatory bowel disease (IBD) and irritable bowel syndrome
(IBS) are chronic diseases that cause inflammation of the intestines. A study by Reinders
) found that NO was
low in healthy control subjects, and variations over time were small. In IBS patients NO was
slightly elevated, whereas patients with active IBD or collagenous colitis had greatly
increased NO levels. Rectal NO correlated with disease activity in IBD and collagenous
colitis and decreased markedly in IBD patients responding to anti-inflammatory treatment.
A statistically significant global reduction of cerebral blood flow (CBF) is found in
autistic children (Burroni et al.
). Recent studies on brain circulation have provided evidence that CBF is
impaired by decreased formation of NO from endothelial cells, autonomic nitrergic nerves or
brain neurons and also by increased production of ROS. The NO–ROS interaction is an
important topic in discussing blood flow and cell viability in the brain (Toda et
In the recent study, Giulivi et al.
) found that children with autism were more likely to have mitochondrial
dysfunction than TD children. Evidence has also been provided that mitochondrial dysfunction
can be induced by elevated levels of NO (Stewart and Heales, 2003
). NO and its toxic metabolite peroxynitrite (ONOO(-)) can inhibit
the mitochondrial respiratory chain, leading to energy failure and ultimately cell death.
ROS and RNS derived from NO and superoxide may also inhibit mitochondrial brain energy
metabolism (Valko et al.
preventing the production of adenosine triphosphate (Bolaños et al.
Abnormal eating patterns and eating disorders are associated with ASD (Maenner et
; Tang et al.
). A team of researchers in Italy provided
evidence on the possible actions of NO on the etiology of eating disorders (Vannacci
). In this study,
plasma nitrite and cyclic guanosine monophosphate levels were significantly higher in eating
disorder patients than in healthy controls.