The effects of sigma ligands on protein release from lacrimal acinar cells: a potential agonist/antagonist assay

Abstract

Sigma (σ) receptor antagonists have been proposed as leading clinical candidates for use in various psychotic disorders. Prior to clinical testing, it is imperative that a new agent be correctly identified as an antagonist and not an agonist since the latter may worsen the psychosis. For σ-ligands many behavioral and pharmacological assays have been developed in an attempt to classify agonist/antagonist activity. These assays evaluate a response or a behavior in an animal model that can be related to clinical efficacy. However, is the action by the presumed antagonist a consequence of σ-receptor activity? Previously we have identified σ-receptors in acinar cells of the main lacrimal gland of the New Zealand white rabbit and have measured protein release after the addition of various N,N-disubstituted phenylalkylamine derivatives known to be σ-ligands by receptor binding studies. Although protein release from acinar cells has been attributed to either muscarinic or a-adrenergic stimulation, protein release from σ-receptor stimulation was also confirmed. In the reported studies here, we isolated and incubated acinar cells with varying concentrations of known σ-ligands and measured protein concentration. A knowledge of the receptor profile for the disubstituted phenylalkylamines permitted experiments to be designed in which various α, muscarinic, serotonergic and dopaminergic antagonists could be added in equimolar concentrations. Under the conditions of these experiments, statistically significant increases in protein release for σ-ligands could be attributed to stimulation of a-receptors. Haloperidol, an apparent σ-antagonist, caused a statistically significant decrease in protein release and also inhibited protein release when tested with a known σ-ligand, AF2975 [N,N-dimethyl-2-phenylethylamine]. In this system, stimulation and inhibition of protein release were defined as agonist and antagonist behavior, respectively. Of particular interest were the results for BMY 14802 and +/- pentazocine, both of which were found to be agonists. Various antipsychotic and antidepressant drugs were measured for their agonist/antagonist behavior. Because of multireceptors present in acini, their agonist or antagonist behavior could not be attributed solely to interaction with the σ-receptor unless specific antagonists were added.