Pharmacological Characterization of Prejunctional Serotonin Receptors in Mammalian Anterior Uvea
Serotonin (5-hydroxytiyptamine, 5-HT) and its receptors have been demonstrated in ocular tissues including the cornea, iris-ciliary body and retina of various species. The overall objective of the present study was two-fold: (a) to examine the effect of exogenous 5-HT on the release of radiolabeled stores of this amine ([3H]-5-HT) via an effect on prejunctional autoreceptors and (b) to investigate the role of prejunctional 5-HT heteroreceptors in regulation of radiolabeled norepinephrine ([3H]NE) release. Isolated bovine and human hemi-irides were prepared for studies of electrically-evoked [3H]-5-HT and [3H]NE release using the superfusion method.|Electrical field stimulation of iris-ciliary bodies preloaded with [3H]-5-HT caused an increase in tritium efflux over baseline. Since chromatographic separation of metabolized and unmetabolized [3H]-5-HT released from isolated bovine-iris-ciliary bodies showed that more than 60% was unmetabolized [3H]-5-HT, the observed stimulated tritium efflux was designated as [3H]-5-HT release. Both tetrodotoxin (TTX) and zero calcium buffer attenuated electrically-evoked [3H]-5-HT release without affecting basal tritium efflux, indicating a neuronal origin of the released [3H]-5-HT. Exogenously applied 5-HT (10 nM - 1 pM) produced a concentration-related enhancement of evoked [3H]-5-HT overflow without affecting basal tritium efflux in bovine and human iris-ciliary bodies.|Agonists selective for 5-HT6/7, 5-HT3, 5-HTia, 5-HTib/id, 5-HT2 and non- selective agonists all caused concentration-related enhancement in evoked [3H]-5-HT release from bovine tissues with the following rank order of potency (as measured by equipotent concentration ratio): 5-CT > m-CPBG > 2-methyl-5-HT > 5-MeO-DMT > 5-HT > 5-MeOT » L-694, 247 > CGS 12066A > 8-OH-DPAT. In human iris-ciliary bodies, both 5-HT and m-CPBG had excitatory effect on evoked [3H]-5-HT release, however this tissue was less sensitive to the effects of these agonists than bovine tissue. The agonists selective for 5-HT6/7 and 5-HT3 were more potent in eliciting an excitatory effect in bovine iris-ciliary bodies. Therefore, to further confirm the nature of the 5-HT receptor involved in mediating the release of [3H]-5-HT, the effect of antagonists active at 5-HT7, 5-HT6/7 and 5-HT3 receptors was examined, on the standard 5-HT response. The rank order of potency of the antagonists was (as measured by IC50): mesulergine > SB 258719 > clozapine > MDL-72222. Taken together, these findings indicate that the subtype of 5-HT prejunctional autoreceptors mediating the excitatory effect of this amine in the bovine iris-ciliary body belong to the 5-HT7 subclass.|To study the pharmacology of prejunctional 5-HT heteroreceptors in bovine and human iris-ciliary bodies, the effect of exogenous 5-HT was examined on the release of [3H]NE. The effect of various 5-HT receptor agonists were also tested for their excitatory or inhibitory action on the evoked release of [3H]NE. Both 5-HT and m-CPBG caused enhancementin the field-stimulated release of [3H]NE from bovine tissues. However, both 5-CT and 2-methyl-5-HT elicited a slight inhibitory effect, whereas the 5-HTia receptor agonist, 8-hydroxy-dipropylaminotetralin (8-OH- DPAT), caused significant dose-related inhibition of evoked [3H]NE release. In human iris-ciliary bodies 5-HT caused a biphasic response on electrically-evoked [3H]NE release. Since 5-HT3 agonists, m-CPBG and 2- methyl-5-HT, produced a differential effect on 5-HT heteroreceptors release of [3H]NE, it cannot be stated unequivocally that 5-HT3 receptors are involved in regulating the excitatory effect of 5-HT in bovine iris- ciliary body. To further confirm the nature of the prejunctional heteroreceptors regulating [3H]NE release, the effect of 5-HT3 and 5-HTz receptor antagonists was examined on a standard response to 5-HT. Both mesulergine and MDL-72222 caused a concentration-dependent inhibition of the response elicited by the standard 5-HT-induced response. The rank order of potency of the antagonists (as measured by IC30): mesulergine > MDL-72222. The high potency exhibited by mesulergine would support the classification of the prejunctional 5-HT heteroreceptors as belonging to the 5-HT7 subclass.|In conclusion, excitatory prejunctional 5-HT auto- and heteroreceptors are present in bovine and human iris-ciliary bodies. Both prejunctional 5-HT receptors present at serotonergic and adrenergic neurons to belong to the 5-HT7 subclass.
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