8 Histaminergic system
<p>From: <a href="http://fblt.cz/en/skripta/regulacni-mechanismy-2-nervova-regulace/5-neurotransmisni-systemy/">4. Neurotransmitter Systems • Functions of Cells and Human Body</a>
Histaminergic system</p>
<p><strong>Histamine</strong> plays many important roles in the human body, as a neurotransmitter it regulates sleep, arousal and hormone secretion in the hypothalamo-pituitary system.</p>
<p>Structurally it is a biogenic amine formed by a decarboxylation of histidine (catalysed by histidine decarboxylase). Once released, histamine is degraded by diamine oxidase.</p>
<p>The effect of histamine is mediated by three subtypes of histamine receptors. They are found in the CNS and in peripheral tissues.</p>
<p><strong>1) H1 receptors</strong></p>
<p>are expressed mainly in the hypothalamus and mamillary bodies and their effect is excitatory. The activation of H1 receptors decreases membrane permeability for potassium and thus causes depolarisation. Most antihistamines produce sleepiness by their antagonistic effect on H1 receptors.</p>
<p><strong>2) H2 receptors</strong></p>
<p>These are metabotropic receptors couple to a Gs protein, they are expressed in neurons in many cortical areas, also in glial cells and the endothelium of cerebral capillaries.</p>
<p><strong>3) H3 receptors</strong></p>
<p>Found on presynaptic terminals they act as autoreceptors and limit further release of histamine. They are also present as heteroreceptors on noradrenergic, dopaminergic and cholinergic neurons, where they inhibit the release of the respective neurotransmitter.</p>