Lacrimal Secretion Stimulants: Sigma Receptors and Drug Implications

Abstract

3H-DTG (1.3-di(2-[5-3H]tolyl)guanidine) or 3H-haloperidol was added to areceptors (25 nM) in the presence of 25 nM splperone and incubated with increasing concentrations of bromhexine derivatives (phenylalkylamines; 10~9 to 10~2 M) in membrane homogenate suspensions. IC50 values for two derivatives ranged from 3.2 to 8.8 nM for both radioligands. A preferred derivative, 7A (N, N'-dimethy1-2-phenylethylamine) , yielded an IC50 of 7.8 nM for 3H-haloperidol but showed much less affinity in displacing 3H-DTG (IC50=900 nM). Applying the technic of Bromberg [Exp. Eye Res. , 40:313-320, 1985] , in vitro protein secretion rates were measured following stimulation of either lacrimal gland slices or isolated, intact lacrimocytes with the compounds, in vitro protein secretion rates exhibit a dose-response relationship with increases in protein release up to a concentration of 10~8 to 10"4 M for various derivatives of bromhexine and ÎO-4 M for carbachol. By means of Schirmer strips, tear fluid was collected over a five minute period at 10 and 60 minutes post-dosing following the topical application (50/il) to the right eye of New Zealand white rabbits (n=20-24) of 7A at various concentrations. Incubation of lacrimocytes with 7A alone (10"'' M) , with haloperidol (lO"4 M) alone or in combination show that 7A is acting as an agonist to stimulate protein release, whereas haloperidol is acting as an antagonist to inhibit release. In vivo protein secretion rates also show a doseresponse curve (at both 10 and 60 minutes post-dosing) for 7A that reach a statistically significant maximum in the dosed eye at a concentration of 0.15% w/v. Analysis of protein extracts using size exclusion HPLC shows an increase in secretory proteins, particularly tear-specific prealbumin.

Expression pattern of sigma receptor 1 mRNA and protein in mammalian retina

“…These data are extremely important as sigma receptor 1 ligands may have potential therapeutic use. For example, lacrimal secretion has been stimulated with sigma receptor 1 ligands [33], making these compounds of possible therapeutic use in dry eye syndrome [27]. Topical administration of sigma receptor 1-site agonists lowered intraocular pressure, hence they may offer a novel class of agents effective in control of ocular hypertension [6].…”

Determination of Tolerance to Tear Protein Release Following a Twice a Day Topical Application of N,N-dimethyl-2-phenylethylamine HCl

“…This leads to overexposure and if the drug stimulates more than one receptor, a disproportionality in response may develop. Although the release of protein occurs following the stimulation of acini receptors, a number of receptors may be responsible, such as cAMP-dependent (15), muscarinic (15), alpha-adrenergic (15), or, as we believe in the case of AF2975, sigma receptors (2,3). Nevertheless, regardless of the receptor that is responsible, it is important to determine if tolerance can develop for the protein secretion rate, In addition, AF2975 has shown an uncharacteristic dose response curve on topical administration to the albino rabbit eye.…”

“…The protein content of each Schirmer test strip was measured by using a modified Coomassie solution (2). Bovine serum albumin (BSA, Sigma Chemical Co. # A-4503) was used as the standard.…”

“…Among the various analogs, N,N-Dimethy1-2-phenylethy lamine hydrochloride (AF2975) stimulated protein secretion from intact isolated lacrymocytes (acini) in vitro and was also found to promote protein release in tears following topical administration to the albino rabbit eye in vivo (2,3). Dry eye is a chronic disease characterized by an unstable tear film that generally requires long-term treatment (4).…”

“…Nevertheless, regardless of the receptor that is responsible, it is important to determine if tolerance can develop for the protein secretion rate, In addition, AF2975 has shown an uncharacteristic dose response curve on topical administration to the albino rabbit eye. We have observed that the response (measured as average percentage change in protein secretion rate) first increases as single doses of 0.075% and 0.15 % are administered, then decreases as concentration is increased (2,3). For these reasons, a regimen was devised in normal albino rabbits for AF2975 to determine if tolerance to repeated instillation could cause a diminution of the protein secretion rate.…”