Brooks H. Rohde

Effect of Permeation Enhancers on Beta-Endorphin Systemic Uptake after Topical Application to the Eye

Solutions of human beta-endorphin were applied topically to the eyes of rabbits. Atrial blood was collected and plasma beta-endorphin was assayed using a Nichols Diagnostics radioimmunoassay kit. Levels peaked rapidly (within 10-20 min) at greater than 5 ng/ml with 1% beta-endorphin and no enhancer, and then returned to background within 2 h. When the permeation enhancers Brij-78 or BL-9 were added to the formulation applied topically, plasma levels were elevated above 30 ng/ml and remained elevated above 1 ng/ml over 1 h. Plasma levels were actually higher after topical application using either enhancer than after intravenous injection at the collection points of 45 min and later. The data suggest that topical application to the eyes can be an efficacious method of systemic delivery of peptide drugs.

Effects of Melatonin and Haloperidol Given via Vortex Vein on the Intraocular Pressure

Melatonin injected into the vortex vein of a rabbit eye produced an increase in intraocular pressure (IOP) which lasted for up to 5 h. Injection of haloperidol caused a decrease in IOP; this effect was totally reversed by melatonin. It is probable that these effects are caused by physiological antagonisms. Injection of the mu opiate agonist (D-ala2-n-methyl-ph2-gly5-ol) enkephalin caused a decrease in the IOP of artificially ventilated rabbits and a decrease in melatonin levels in iris, iris root-ciliary body, and retina of the rabbit eye. Melatonin levels did not decrease after anesthesia with rompunketamine. It is probable that the decrease in melatonin levels is specific to certain classes of opioids, and endogenous opioids may play a role in the regulation of ocular melatonin levels and hence IOP.

Effects of Some Opiates and Opioid Peptide Eyedrops on Ocular Melatonin Regulation in Rabbits

Levels of melatonin in rabbit eye tissues were detected by radioimmunoassay. Solutions of met-enkephalin, leu-enkephalin, alpha-endorphin and beta-endorphin were given topically. Met-Enkephalin and alpha-endorphin lowered levels of melatonin in the iris, iris root-ciliary body, retina and choroid; leu-enkephalin raised levels in the retina and lowered them in other tissues. beta-Endorphin only lowered levels in the iris root-ciliary body. DAGO (a mu agonist) given i.v. lowered levels of melatonin in the iris, iris root-ciliary body and retina. The delta and sigma agonists given i.v. only lowered levels in the iris root-ciliary body, and a kappa agonist given i.v. raised levels in the ciliary body. No opiate binding sites could be detected in the rabbit iris or iris root-ciliary body for any class of receptor. Our data suggest opioids may be useful for treating glaucoma.

Is Ocular Melatonin Regulated by the Adrenergic System

There is evidence supporting a role for melatonin as an endogenous agent regulating intraocular pressure, but little is known about whether ocular levels of melatonin are regulated by the sympathetic nervous system, as they are in the rat pineal gland. The effects of topically applied L-timolol, norepinephrine, forskolin, domperidone and pilocarpine on levels of melatonin in regions of rabbit and chicken eyes were determined. None of these drugs altered levels of melatonin in the rabbit iris-ciliary body or in the chicken iris or ciliary body, suggesting that the actions of these drugs were not mediated through an action on melatonin. Topical forskolin did not alter N-acetyltransferase activities in rabbit iris-ciliary body. Chemical sympathectomy with 6-hydroxydopamine had no effect on nighttime levels of melatonin in rabbit iris-ciliary body or on daytime levels in chicken eyes. These results suggest that melatonin in the eyes does not appear to be under the solitary control of the adrenergic sympathetic nervous system. The data suggested a transmitter other than norepinephrine regulates N-acetyltransferase activities and melatonin levels.

Characterization of unique ADTN-catecholamine binding sites in the iris root-ciliary body of rabbits.

A binding site for tritiated 2-amino-6, 7-dihydroxy-1, 2,3,4-tetrahydronaphthalene (ADTN) has been partially characterized in the rabbit iris root-ciliary body. Binding of ADTN is proportional to protein content and requires at least 60 minutes to reach equilibrium. Binding is saturable, with a Kd of 27 +/- 1 nM and a Bmax of 2.1 +/- .3 pmol/mg protein (mean +/- SEM). Dopamine competes for this site with a Ki of 100 nM and apomorphine with a Ki of 180 nM. This site is not blocked by L-timolol, phenoxybenzamine, or by several DA1 and DA2 antagonists. It appears to be a new type of catecholamine binding site, of a type not observed outside the anterior eye. It is possible that some of the effects of dopamine on intraocular pressure are mediated through this binding site.