Showing posts with label nicotine. Show all posts
Showing posts with label nicotine. Show all posts

Sunday 26 March 2017

Sensory Gating in Autism, Particularly Asperger's

Sensory gating is an issue in autism, schizophrenia and ADHD.   It is the neurological process of filtering out redundant or unnecessary stimuli in the brain; like the child who sits in his classroom and gets bothered by the noise of the clock on the wall.  He is unable to filter out and ignore this sound. He becomes preoccupied by the sound and cannot concentrate on his work.
There are also sometimes advantages to not filtering out environmental stimuli, because you would have more situational awareness and notice things that others miss.
An example of sensory gating is the fact that young children are not waken by smoke detectors that have high pitched siren, but are waken by a recorded human voice telling them there is a fire and to wake up.
There may be times when sensory overload in autism is not a case of too much volume from each of the senses, but rather too many inputs being processed by the brain, instead of some just being ignored.  It is more a case of information overload.
Note that this blog has already covered hypokalemic sensory overload in some depth, which is treatable.
Much is known about sensory gating because it has long been known to be a problem in schizophrenia.
An EEG (Electroencephalography) test measures your brain waves / neural oscillations. Many people with autism have EEGs, but mainly those in which epilepsy is a consideration.
In the world of the EEG, the P50 is an event occurring approximately 50 millisecond after the presentation of an auditory click.  The P50 response is used to measure sensory gating, or the reduced neurophysiological response to redundant stimuli.
Abnormal P50 suppression is a biomarker of schizophrenia, but is present in other disorders, including Asperger’s, post-traumatic stress disorder (PTSD) and traumatic brain injury (TBI).
In more severe autism abnormal P50 suppression was found not to be present in one study.  This might be because cognition and the senses are dimmed by the excitatory-inhibitory imbalance.
More broadly, sensory gating is seen as an issue in wider autism and ADHD.

Correcting P50 gating
It is known that α7 nicotinic acetylcholine receptor (α7 nAChR) agonists can correct the impaired P50 gating. It is also known that people with schizophrenia have less expression of this receptor in their brains than typical people.

One short term such agonist is the nicotine released from smoking.  This likely contributes to why people with schizophrenia can be heavy smokers.  The effect is thought to last for about 30 minutes.
Clinical trials using Tropisetron, a drug that is a α7 nAChR agonist and used off-label to treat fibromyalgia, have shown that it can correct defective P50 gating and improve cognitive function in schizophrenia.

An alternative α7 nAChR agonist that is widely available is varenicline, a drug approved to help people stop smoking.
So you might expect varenicline to improve P50 gating and improve cognition. You might also expect it to help people with fibromyalgia and indeed some other people with chronic inflammation, as shown by elevated inflammatory cytokines.

You may recall that the α7 nAChR is the key to stimulating the vagus nerve and this should be beneficial to many people with inflammatory conditions (from arthritis to fibromyalgia).

Abnormalities in CHRNA7, the alpha7-nicotinic receptor gene, have been reported in autism spectrum disorder. These genetic abnormalities potentially decrease the receptor’s expression and diminish its functional role. This double-blind, placebo-controlled crossover study in two adult patients investigated whether an investigational receptor-specific partial agonist drug would increase the inhibitory functions of the gene and thereby increase patients’ attention. An electrophysiological biomarker, P50 inhibition, verified the intended neurobiological effect of the agonist, and neuropsychological testing verified a primary cognitive effect. Both patients perceived increased attention in their self-ratings. Alpha7-nicotinic receptor agonists, currently the target of drug development in schizophrenia and Alzheimer Disease, may also have positive clinical effects in autism spectrum disorder.

A role for H3 and HI histamine receptors
It has also been suggested that histamine plays a role in sensory gating via the H1 and H3 receptors.

It had also been thought H3 receptors could be targeted to improve cognition in schizophrenia, but that research really did not go anywhere.

Histamine H1 receptor systems have been shown in animal studies to have important roles in the reversal of sensorimotor gating deficits, as measured by prepulse inhibition (PPI). H1-antagonist treatment attenuates the PPI impairments caused by either blockade of NMDA glutamate receptors or facilitation of dopamine transmission. The current experiment brought the investigation of H1 effects on sensorimotor gating to human studies. The effects of the histamine H1 antagonist meclizine on the startle response and PPI were investigated in healthy male subjects with high baseline startle responses and low PPI levels. Meclizine was administered to participants (n=24) using a within-subjects design with each participant receiving 0, 12.5, and 25 mg of meclizine in a counterbalanced order. Startle response, PPI, heart rate response, galvanic skin response, and changes in self-report ratings of alertness levels and affective states (arousal and valence) were assessed. When compared with the control (placebo) condition, the two doses of meclizine analyzed (12.5 and 25 mg) produced significant increases in PPI without affecting the magnitude of the startle response or other physiological variables. Meclizine also caused a significant increase in overall self-reported arousal levels, which was not correlated with the observed increase in PPI. These results are in agreement with previous reports in the animal literature and suggest that H1 antagonists may have beneficial effects in the treatment of subjects with compromised sensorimotor gating and enhanced motor responses to sensory stimuli.

The aim of this study was to investigate an established rat model of decreased PPI induced by administration of the NMDA antagonist, dizocilpine and the reversal of this PPI impairment by the histaminergic H1-antagonist, pyrilamine. H1-antagonism is a potential mechanism of the therapeutic effects of the atypical antipsychotic, clozapine, which improves PPI following dizocilpine administration in rats as well as in patients with schizophrenia. In the present study we show that chronic pyrilamine administration prevents the PPI impairment induced by chronic dizocilpine administration, an effect that is correlated with a reduction in ligand-binding potential of H1 receptors in the anterior cingulate and an increase in nicotinic receptor α7 subunit binding in the insular cortex. In light of the functional anatomical connectivity of the anterior cingulate and insular cortex, both of which interact extensively with the core PPI network, our findings support the inclusion of both cortical areas in an expanded network capable of regulating sensorimotor gating.

The brain histamine system has been implicated in regulation of sensorimotor gating deficits and in Gilles de la Tourette syndrome. Histamine also regulates alcohol reward and consumption via H3 receptor (H3R), possibly through an interaction with the brain dopaminergic system. Here, we identified the histaminergic mechanism of sensorimotor gating and the role of histamine H3R in the regulation of dopaminergic signaling. We found that H3R knockout mice displayed impaired prepulse inhibition (PPI), indicating deficiency in sensorimotor gating. Histamine H1 receptor knockout and histidine decarboxylase knockout mice had similar PPI as their controls. Dopaminergic drugs increased PPI of H3R knockout mice to the same level as in control mice, suggesting that changes in dopamine receptors might underlie deficient PPI response when H3R is lacking. Striatal dopamine D1 receptor mRNA level was lower, and D1 and D2 receptor-mediated activation of extracellular signal-regulated kinase 1/2 was absent in the striatum of H3R knockout mice, suggesting that H3R is essential for the dopamine receptor-mediated signaling. In conclusion, these findings demonstrate that H3R is an important regulator of sensorimotor gating, and the lack of H3R significantly modifies striatal dopaminergic signaling. These data support the usefulness of H3R ligands in neuropsychiatric disorders with preattentional deficits and disturbances in dopaminergic signaling.


Other than nicotine, varenicline would seem a good potential therapy for sensory gating.  There are α7-nicotinic acetylcholine receptor agonists in development.
There are many H1 histamine antagonists.  Histamine release in the brain triggers secondary release of excitatory neurotransmitters such as glutamate and acetylcholine via stimulation of H1 receptors. Centrally acting H1 antihistamines are sedating.

H3 antagonists have stimulant and nootropic effects. Betahistine is an approved drug in this class, there are many research drugs.

The aim of this study is to investigate the role of the neurotransmitter histamine in sensory and cognitive deficits as they often occur in schizophrenia patients (e.g. hearing voices, planning and memory problems). The ideal location to conduct the study and to obtain a unique learning experience is at the Institute of Psychiatry, London, United Kingdom, where staff comprises of leading experts in the field of schizophrenia and Magnetic Resonance Imaging of pharmacological effects. Current pharmacological treatment of psychotic symptoms including sensory and cognitive deficits remains partially unsuccessful due to side effects and treatment resistance. The neurotransmitter histamine seems to be a very promising target for new treatments. It has been found that histamine neurotransmission is altered in brains of schizophrenics, which may contribute to both the hallucinatory and cognitive symptoms. However, this specific role of histamine has not been investigated before. I will assess the effects of increased histaminergic activity, by administration of betahistine to healthy volunteers, on performance (sensory gating, executive functioning or planning and memory) and associated brain activity using fMRI. Altered performance and brain activity would support the importance of histamine in schizophrenia and would provide a research model and target for new treatments.

Thursday 13 October 2016

Multigenerational Epigenetic Change Stimulating Inflammatory Disease

Multigenerational transmission of nicotine-induced effects. The diagram illustrates the experimental design and findings of Rehan et al. [4]. Pregnant dams (F0 generation) are injected with nicotine or nicotine + rosiglitazone. The lungs and gonads of both male and female offspring (F1 generation) of nicotine-treated dams exhibit epigenetic changes, and the lungs show an asthma-like functional phenotype (blue nicotine-induced changes). These nicotine effects are not seen in the offspring of animals treated with nicotine + rosiglitazone. Offspring of F1 mated pairs (F2 generation) exhibit the same nicotine-induced changes to lung function as their parents, even though they were not exposed to drug.

Today’s post is again filling in some gaps in this blog to date.

A big question in autism is whether the incidence is increasing or not.  According to the now best-selling autism author Silberman, incidence is not increasing at all; it is just that diagnosis is much better than it was half a century ago.  So it is not an “autism epidemic”, rather a “diagnosis epidemic”.

I did not buy Siberman’s book and while I would like to believe he has accurately assessed the facts, in this case he really has not.

Psychiatrists have done none of us any favours by constantly changing the definition of autism and clinicians have never adequately collated data on those who match those criteria.

It does actually matter whether or not incidence of autism is increasing, because this would then stimulate research as to why.  In time this better understanding would lead to therapeutic avenues.

Being neither a professional researcher, nor a best-selling author, my level of evidence can be a little lower.  In earlier posts we saw incidence of ASD (autism, Asperger’s and PDD-NOS) is around one percent of both the child and adult population.  Many adults with Asperger’s and milder dysfunctions were never diagnosed as children, because they did not have speech delay or great cognitive difficulties.

The autism figures are always of low quality, but there is an opinion that underlying them is a real increase in severe autism, as well as the increased diagnosis of milder autism due to lowering of the diagnostic threshold.

The data I would like to see is the incidence of severe autism over the last few decades, but it does not exist.  All we have is anecdotes.

I remember asking my retired doctor mother how many patients had autism in her medical practice of about 10,000, where she saw all the children.  They did not have any and apparently until the Wakefield autism-MMR business nobody even talked about autism.

Hidden away in a group of 10,000 there “should be” about 100 with some degree of autism.  About 30 might have quite severe autism, many with MR/ID and epilepsy. 30 sounds a lot, but it is only one or two births a year.  People with severe autism live half as long as typical people, so you would not see many past middle age. I suppose it was easy to just diagnose mental retardation and then put the child into “care” when the parents could not cope.  

When a friend of mine from graduate school asked our alumni group of 200 how many had a child with autism there were six responses.  None were Asperger’s, all were strictly defined autism (SDA).

Some disease surprisingly does correlate with educational level.  I recently read that IBS/IBD is much more common among more educated people.

So my take is that hidden in all those poor quality statistics is a rise in the incidence of strictly defined autism (SDA).  Just as it is known that there has been a rise in inflammatory disease like asthma.

Asthma and COPD are really well researched and we know at least some of the reason why they have become more common.  I think the same general mechanism is behind the increase in SDA.

By understanding this mechanism you can then try and reverse it.  This is already being done in COPD research and some of the single gene autisms like Pitt Hopkins.

The mechanism is epigenetics, where you can modify when genes turn on, or turn off.  COPD is a severe disease because an environmental factor (normally smoking) has caused the body's oxidative stress response genes to be turned off.  Pitt Hopkins is caused by an insufficient expression of the TCF4 gene.  This was unlikely to have been caused by epigenetic changes, but could potentially be treated by using epigenetics to turn on the TCF4 gene.

Today’s post highlights pretty convincing research that shows how an environmental factor, smoking in this case, can cause heritable epigenetic changes.  It shows how a Grandparent smoking increases asthma incidence in the grandchildren.

Other than sending the message that smoking can affect the health of your future grandchildren, it becomes clear that many other environmental insults could also be heritable.  The accumulation of these insults over generations affects the incidence of certain diseases, particularly those complex ones often caused by multiple hits (cancer, autism etc.).
This makes me recall how it is theorized that epilepsy can develop as an acquired channelopathy.  We saw how the threshold for a person’s first seizure is quite high, but after the first seizure the threshold falls.  The proposed mechanism is called an acquired channelopathy.  This means that one of the many ion channels whose dysfunction is known to lead to epilepsy has been permanently disturbed.  The ion channel can now behave aberrantly with little provocation,

Ion channel diseases are classified as ‘acquired’ or ‘genetic’. Genetic ion channel disorders of the brain generally manifest as epilepsy, migraine, paroxysmal dyskinesia or episodic ataxia.

Acquired channelopathies can be caused by antibodies which target specific ion channels or by toxins which block voltage-gated ion channels. Altered transcription of ion channels may contribute to many acquired neurological ion channel disorders.

Mutations in genes which encode subunits of CNS sodium, potassium, calcium channels, GABAA and nicotinic receptors have been reported in association with various epilepsy syndromes.

While genetic (inherited) ion channel disorders may be the cause of most people’s epilepsy, it is suggested that acquired channelopathies are also involved.  Perhaps both are present?

 the “acquired channelopathy” hypothesis suggests that proepileptic channel characteristics develop during epilepsy.

In summary, cell type-specific information on epilepsy-related ion channel modifications can explain and support AED strategies. Precisely those inhibitory ion channels which appear to be effective AED targets in preclinical tests are the ones upregulated in DG GCs during TLE. These data indicate that cell-endogenous ion channel homeostasis mechanisms could be used as “channelacoid” archetypes in the search of antiepileptic strategies. In particular, the enhancement of static shunt via combined K/Cl/cation leak channel support appears to be a promising strategy.

The science, though complex, is still in its infancy.  You do wonder if acquired channelopathy cannot be caused by epigenetic changes to the genes encoding the ion channel.

Nicotine, your genes and those of your heirs

Finally, the subject of today’s post, the research showing the epigenetic effects of nicotine. In place of nicotine you could likely substitute other environment damage such as intense air pollution in cities like Beijing.  Another example below is lead pollution. 

 First the easier to read article:-

"Our results therefore indicate that the increased disease risk associated with smoking is partly caused by epigenetic changes. A better understanding of the molecular mechanism behind diseases and reduced body function might lead to improved drugs and therapies in the future," 

Now the more interesting study that shows how the effect of nicotine is passed down the generations to non-smokers.

Multigenerational transmission of nicotine-induced effects. The diagram illustrates the experimental design and findings of Rehan et al. [4]. Pregnant dams (F0 generation) are injected with nicotine or nicotine + rosiglitazone. The lungs and gonads of both male and female offspring (F1 generation) of nicotine-treated dams exhibit epigenetic changes, and the lungs show an asthma-like functional phenotype (blue nicotine-induced changes). These nicotine effects are not seen in the offspring of animals treated with nicotine + rosiglitazone. Offspring of F1 mated pairs (F2 generation) exhibit the same nicotine-induced changes to lung function as their parents, even though they were not exposed to drug.

A recent preclinical study has shown that not only maternal smoking but also grandmaternal smoking is associated with elevated pediatric asthma risk. Using a well-established rat model of in utero nicotine exposure, Rehan et al. have now demonstrated multigenerational effects of nicotine that could explain this 'grandmother effect'. F1 offspring of nicotine-treated pregnant rats exhibited asthma-like changes to lung function and associated epigenetic changes to DNA and histones in both lungs and gonads. These alterations were blocked by co-administration of the peroxisome proliferator-activated receptor-γ agonist, rosiglitazone, implicating downregulation of this receptor in the nicotine effects. F2 offspring of F1 mated animals exhibited similar changes in lung function to that of their parents, even though they had never been exposed to nicotine. Thus epigenetic mechanisms appear to underlie the multigenerational transmission of a nicotine-induced asthma-like phenotype. These findings emphasize the need for more effective smoking cessation strategies during pregnancy, and cast further doubt on the safety of using nicotine replacement therapy to reduce tobacco use in pregnant women.

More on epigenetic changes related to heart disease.

Finally the effect down the generations of lead, a known neurotoxin.

We report that the DNA methylation profile of a child’s neonatal whole blood can be significantly influenced by his or her mother’s neonatal blood lead levels (BLL). We recruited 35 mother-infant pairs in Detroit and measured the whole blood lead (Pb) levels and DNA methylation levels at over 450,000 loci from current blood and neonatal blood from both the mother and the child. We found that mothers with high neonatal BLL correlate with altered DNA methylation at 564 loci in their children’s neonatal blood. Our results suggest that Pb exposure during pregnancy affects the DNA methylation status of the fetal germ cells, which leads to altered DNA methylation in grandchildren’s neonatal dried blood spots. This is the first demonstration that an environmental exposure in pregnant mothers can have an epigenetic effect on the DNA methylation pattern in the grandchildren.


As regards autism, heritable epigenetic changes could well explain the increase in strictly defined autism (SDA) that cannot be explained away in terms of widening diagnostic criteria and awareness.

With respect to many diseases it is hardly surprising that they are becoming more prevalent if we accumulate the environmental insults experienced by our ancestors, via heritable epigenetic changes.  Where this will lead in future generations?

There are further studies looking at the role of PPAR gamma agonists (the rosiglitazone given to protect the mouse from epigenetic change) and HDAC inhibitors, which together can do very clever things regarding epigenetics.

You may recall the broccoli sprout extract being given by John Hopkins researchers to protect Beijing residents from the effects of severe air pollution.  The sulforaphane produced is an HDAC inhibitor.  

The mouse studies showed how to protect a mouse from epigenetic change occurring, what would be more interesting would be studies looking at reversing that change, once it has already occurred.

The only bad thing in the Mediterranean diet/lifestyle is smoking; just imagine how healthy the Greeks would be without smoking 2,000 cigarettes per adult per year, compared to 1,000 in the US.

Monday 15 September 2014

Antabine (Anatabloc) and Autism - a Supplement or a Drug?

This is another post prompted by a comment received on this blog.

My 15 year-old daughter has classic regressive type ASD. I started her on an anti-inflammatory, Anatabloc, over a year ago and it allowed me to take her off atypical anti-psychotics ( she was on them for aggression) Do you know anyone else using this dietary supplement?  

I found this very interesting and so I did some quick research.

Anatabloc was until very recently sold in the US as a supplement, it was withdrawn from sale by the producer following a corruption trial and a dispute with the FDA over approvals.  Nobody is saying the supplement does not work, rather it is a drug.


Anatabloc was sold as an anti-inflammatory supplement based on a substance called Anatabine, found in tobacco and in lower concentrations in green tomatoes, green potatoes, ripe red peppers, tomatillos, and sundried tomatoes.

Anatabine has been studied in animal models and in cells to see if it might be useful for treating nicotine addiction and inflammation, and has been studied in models of diseases characterized by inflammation, such as Alzheimer's Disease, thyroiditis, and multiple sclerosis.

On a biochemical level, it appears to be active against certain nicotinic acetylcholine receptors.

Regular readers will recall extensive earlier posts on the cholinergic system and nicotinic acetylcholine receptors.

The conclusion of all those posts was that, most definitely, in some people’s autism, an effective strategy is to adjust the cholinergic system.  Possible methods include:-

·        Vagus nerve stimulation – still in development
·        Nicotine patches – cheap and effective in some people
·        Two Alzheimer's drugs Donepezil and Galantamine, that are acetylcholinesterase inhibitors

So at first it seemed that Anatabloc may be “just another” cholinergic drug.  However on analyzing the patent submitted by the producer, it seems there may be an alternative mode of action.

Patent for Antabine use in Autism

32| Anatabine is an alkaloid present in tobacco and, in lower concentrations, in a variety of foods, including green tomatoes, green potatoes, ripe red peppers, tomatillos, and sundried tomatoes. Without being bound by this explanation, data presented in Examples I and 2 below indicate that anatabine reduces transcription mediated by nuclear factor B (NFKB). NFKB is a transcription factor which operates in cells involved in inflammatory and immune reactions.

The nuclear factor NF-κB pathway

NF-κB is seen as being clinically significant in cancer and inflammation.

The NF-κB pathway has long been considered a prototypical proinflammatory signaling pathway, largely based on the role of NF-κB in the expression of proinflammatory genes including cytokines, chemokines, and adhesion molecules.  

NF-κB has long been considered the “holy grail” as a target for new anti-inflammatory drugs.

Because NF-κB controls many genes involved in inflammation, it is not surprising that NF-κB is found to be chronically active in many inflammatory diseases, such as inflammatory bowel disease, arthritis, sepsis, gastritis, asthma, atherosclerosis and others. It is important to note though, that elevation of some NF-κB inhibitors, such as osteoprotegerin (OPG), are associated with elevated mortality, especially from cardiovascular diseases.  Elevated NF-κB has also been associated with schizophrenia.

I take the, perhaps unconventional, view that schizophrenia is adult-onset autism.  It has already shown that in terms of genetics, there is a great overlap between these two conditions.


Schizophrenic patients showed activation of the cytokine system and immune disturbance. NF-kappaB activation may play a pivotal role in schizophrenia through interaction with cytokines

The nuclear factor NF-κB pathway has long been considered a prototypical proinflammatory signaling pathway, largely based on the role of NF-κB in the expression of proinflammatory genes including cytokines, chemokines, and adhesion molecules. In this article, we describe how genetic evidence in mice has revealed complex roles for the NF-κB in inflammation that suggest both pro- and anti-inflammatory roles for this pathway. NF-κB has long been considered the “holy grail” as a target for new anti-inflammatory drugs; however, these recent studies suggest this pathway may prove a difficult target in the treatment of chronic disease. In this article, we discuss the role of NF-κB in inflammation in light of these recent studies.

Clinical trials using Anatabloc

The producer behind Anatabloc is well advanced with clinical trials, as you can see below.

I suspect that Anatabloc will disappear as a supplement and reappear a few years later on as an FDA approved drug for various conditions.


It is a pity that Anatabloc has been taken off the market.

It looks plausible that it could be effective in other people’s autism, not just the reader of this blog.

For the time-being, other than taking up smoking, a good source would be those tasty sun-dried tomatoes.


Having re-read this post and taken a closer look at the patent and the company, I wonder if the original comment is genuine.  The patent is not very convincing and in Table 1 on page 44 it is quoting Wakefield et al, as one of only two sources that link inflammation to autism.  I could have written a much better patent application myself.

Thursday 14 November 2013

Clonidine, ADHD and Autism

Clonidine has been used for more than half a century as an antihypertensive drug, to lower blood pressure.

It later found favour as a treatment for ADHD, drug withdrawal treatment, tobacco withdrawal treatment and a wide range of psychiatric disorders.  Off label usage of Clonidine includes autism.

Until recently it appeared to researchers to be a centrally acting α2 adrenergic agonist, but recent research indicates than instead it is a centrally as an imidazoline receptor agonist.  This would account for its actions other than lowering blood pressure. Maybe it is both.  The good thing is that it is centrally acting (i.e. acting on the brain and the CNS) and it does appear to work. 

Adrenergic Agonist
As a centrally-acting α-adrenergic receptor agonist, Clonidine has more affinity for α2 than α1. It selectively stimulates receptors in the brain that monitor catecholamine (epinephrine, norepinephrine and dopamine) levels in the blood. These receptors close a negative feedback loop that begins with descending sympathetic nerves from the brain that controls the production of catecholamines.  By fooling the brain into believing that catecholamine levels are higher than they really are, clonidine causes the brain to reduce its signals to the adrenal medulla, which in turn lowers catecholamine production and blood levels. The result is a lowered heart rate and blood pressure.

Imidazoline Receptors
There are three classes of imidazoline receptors:
  • I1 receptor – mediates the sympatho-inhibitory actions of imidazolines to lower blood pressure
  • I2 receptor – an allosteric binding site of monoamine oxidase and is involved in pain modulation and neuroprotection.
  • I3 receptor – regulates insulin secretion from pancreatic beta cells

L-Monoamine oxidases (MAO)
MAOs are enzymes that act as catalysts.  There are two types of MAO: MAO-A and MAO-B
MAO- A is an enzyme that degrades amine neurotransmitters such as dopamine (DA), norepinephrine (NE), and serotonin (5-HT).

MAO-B is an enzyme that catalyzes the oxidation of arylalkylamine neurotransmitters, including dopamine (DA).
The differences between the selectivity of the two enzymes are utilized clinically.  MAO- A inhibitors have been used in the treatment of depression, and MAO-B inhibitors are used in the treatment of Parkinson's disease

Selective MAO-B inhibitors preferentially inhibit MAO-B, which mostly metabolizes DA. If MAO-B is inhibited, then more DA is available for proper neuronal function, especially in Parkinson's Disease. 

Clinical significance
Because of the vital role that MAOs play in the inactivation of neurotransmitters, MAO dysfunction (too much or too little MAO activity) is thought to be responsible for a number of psychiatric and neurological disorders. For example, unusually high or low levels of MAOs in the body have been associated with schizophrenia, depression, attention deficit disorder, substance abuse, migraines, and irregular sexual maturation.
MAO inhibitors are one of the major classes of drug prescribed for the treatment of depression, although they are often last-line treatment due to risk of the drug's interaction with diet or other drugs. Excessive levels epinephrine, norepinephrine or dopamine may lead to a hypertensive crisis, and excessive levels of serotonin may lead to serotonin syndrome.
MAO-A inhibitors act as antidepressant and antianxiety agents, whereas MAO-B inhibitors are used to treat Alzheimer’s and Parkinson’s diseases.

Clonidine in ADHD
In the US, the FDA has licensed clonidine for use in children with ADHD.
Pediatric doses of clonidine are calculated based on the child's body weight. Clonidine dosage for ADHD in children is 5 micrograms per kilogram of body weight per day orally in four divided doses. Children who require a daily dosage of 0.2 mg usually can use the 0.3 mg trans-dermal patch. If ADHD is associated with sleep disturbances, low to moderate doses of clonidine can be taken at bedtime.

Clonidine in Autism
Not surprisingly, since clonidine is effective in ADHD, it also shows promise in autism. 

Other ADHD drugs, like Ritalin, have problematic side effects.  The US Center for Disease Control reported in 2012 that an estimated 6.4 million children ages 4 to 17 had been diagnosed with ADHD at some point, a 53 percent increase over the past decade. Approximately two-thirds of those currently diagnosed have been prescribed drugs such as Ritalin or Adderall. Those drugs can help patients with both mild and severe symptoms, but they can also cause addiction, anxiety and psychosis.  In the UK, it is suggested that about 3% of children may have ADHD.  Drug use is far lower than in the US, but 657,000 prescriptions were written by doctors for drugs like Ritalin in 2012.
There have been studies of clonidine in autism; here a fairly recent one:-
Perhaps even more interesting is a lively debate among parents who have tried it:-
It does seem to work, but nobody seems to be following it up.

Clonidine Stimulation Test
Regular readers will know my interest in TRH and GH.  At least there is no doubt about Clonidine’s effect on GH (growth hormone).  If you want to test pituitary function to see how well GH is being produced, the standard test is the:-
For those interested in GH, if you were to take Clonidine, smoke a cigarette and then have your GH measured, the Endocrinologist would have a surprise.

“These findings suggest that in man nicotinic cholinergic and adrenergic mechanisms might interact in the stimulation of GH secretion.”

Interestingly, one of the milder side effects of the ADHD drug Ritalin is growth retardation. According to Professor Tim Kendal, who created the national guidelines in the UK for treating ADHD: - “In children, without doubt, if you take Ritalin for a year, it's likely to reduce your growth by about three-quarters of an inch.

Clonidine looks like another old drug that has been stumbled upon by somebody doing some off label experimentation.  It does seem to have good results in ADHD and Autism.  The good thing is that it is FDA approved and is available in both oral and time release transdermal forms.
I do not think anybody really understands how it works in ADHD or other psychiatric disorders; undoubtedly, there is another, as yet unidentified, mode of action.
For those who want more info:- 

Note ulcerative colitis, ADD and even growth delay.