In rabbit's aqueous humor,
norepinephrine,
epinephrine,
dopamine and
serotonin were detected simultaneously by a high performance liquid chromatography with electrochemical detection. Furthermore, the changes in
catecholamine levels in aqueous humor were evaluated after topical application of
moxonidine, an imidazoline1/alpha 2 receptor agonist, in the presence and absence of
efaroxan. The level of
norepinephrine in aqueous humor was reduced by
moxonidine treatment. However, under the same set of conditions, there were no significant changes in the levels of
dopamine,
epinephrine or
serotonin. Pretreatment with
efaroxan antagonized
moxonidine-induced suppression of
norepinephrine levels. In other in vivo experiments,
moxonidine caused a decrease in intraocular pressure which was antagonized by pretreatment with
efaroxan. In the superior cervical ganglion preparation,
norepinephrine release was increased 5-fold by the presence of a high K+ medium. The K(+)-evoked
norepinephrine secretion was reduced by 55% by
moxonidine. Pretreatment with
efaroxan blocked the
moxonidine-induced inhibition of
norepinephrine release. It is concluded that inhibition of
norepinephrine release from the superior cervical ganglion and suppression of aqueous
norepinephrine levels contribute to the
moxonidine-induced lowering of intraocular pressure. Moreover, the antagonism of
moxonidine's in vivo and in vitro effects by
efaroxan suggests the involvement of imidazoline1 receptors, but does not preclude activity on alpha 2
adrenoceptors.