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.