many of the antihistamines exert serotonin action, actually. In fact, SSRIs were developed specifically to try to mimic the serotonin action of the first-generation antihistamines without mimicking the anticholergenic effects. That's where "selective" came from....
All I could find was this "In the
cortex, activation of H1 receptors leads to inhibition of cell membrane
potassium channels. This depolarizes the neurons and increases the resistance of the neuronal cell membrane, bringing the cell closer to its firing threshold and increasing the excitatory voltage produced by a given excitatory current. H1 receptor antagonists, or
antihistamines, produce drowsiness because they oppose this action, reducing neuronal excitation.
[4]"
"Second-generation H1-antihistamines are newer drugs that are much more selective for peripheral H1 receptors as opposed to the
central nervous system H1 receptors and cholinergic receptors. This selectivity significantly reduces the occurrence of adverse drug reactions, such as sedation, while still providing effective relief of allergic conditions. The reason for their peripheral selectivity is that most of these compounds are
zwitterionic at physiological pH (around pH 7.4). As such, they are very polar, meaning that they do not cross the
blood–brain barrierand act mainly outside the central nervous system."