M7.A5: Content and Video Playlist: Histamine and First-Generation H1 Antihistamines

Diphenhydramine

- significant anticholinergic activity - constipation - tachycardia - dilated pupils - flushing

Diphenhydramine

- significant anticholinergic activity - dry mouth - dry mucous membranes - blurred vision - urinary retention

BBB does not like..

...carboxylic acids but it likes amines.

1st generation antihistamines have..

..anticholinergic activity

H4

Eosinophils, neutrophils, mast cells, CD4 T cells

H2

Gastric mucosa (gastric parietal cells), cardiac muscle, mast cells, brain

H1 Antihistamines

H1 antihistamines are inverse agonists Reversible Selective for H1 receptor

H3

Presynaptic autoreceptors and heteroreceptors: brain, myenteric plexus, other neurons

Histamine receptors

all GPCRs 7 transmembrane spanning regions differ in their secondary messengers & tissue distribution

H1 antihistamine

bind to inactive state.

adverse effects of diphenhydramine

crosses BBB, gets into the CNS, causes marked sedation via H1 inverse agonist activity in the CNS

scopolamine

for motion sickness & vertigo central antimuscarinic drug

Diephenhydramine

like all 1st generation histamines - cause sedation they get into the CNS they have inverse agonist activity on H1 receptors

H1

smooth muscle, endothelium, brain (Tissue Distribution)

Dimenhydrinate (Dramamine)

use to Tx motion sickness