Cognitive function histamine effects

Adverse reactions with antihistamines are the result of multiple mechanisms (see Figure 13-3). Muscarinic, a-adrenergic, and serotonin receptor blockade may result in mydriasis, dry mouth and eyes, urinary retention, constipation, and dizziness in first-generation antihistamines. When the neurotransmitter effect of histamine is interrupted, various CNS adverse reactions may occur. These side effects include increased sedation, decreased cognitive function, decreased psychomotor function, and headache. 

Blockade of histamine autoreceptors increases histamine synthesis and release and may support higher CNS functions such as arousal, cognition and learning. Peripheral histamine heteroreceptors on C liber and on postganglionic sympathetic fiber terminals diminish neuropeptide and noradrenaline release, respectively. Both inhibititory effects are beneficial in myocardial ischemia. The inhibition of neuropeptide release also explains the antimigraine effects of some agonists of presynaptic histamine receptors. 

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