Introduction

Melanophores from fish scales can aggregate or disperse their pigment granules (melanosomes) within seconds as a means of background adaptation or social signalling. The scales from the teleost fish cuckoo wrasse (Labrus ossifagus L.) can be isolated and the melanophores can be used as a simple model to answer various pharmacological and physiological questions. The melanophores are sympathetically innervated, and pigment aggregation occurs when postsynaptic 2-adrenoceptors are activated by electrical stimulation of the nerve endings surrounding the melanophores, which entails the concomitant release of noradren-aline, or by adding different pharmacological substances to the surrounding medium (1).

For each experiment, scales are removed from the dark area of the dermis and transferred to a saline buffer solution and then subjected to different treatments. The effects are investigated under a light microscope and evaluated by using a slight modification of the melanophore index of Hogben and Slome (2); 1 stands for maximal pigment aggregation and 5 stands for a state of complete pigment dispersion (Figure 1).

Denervation of the melanophores can be accomplished either by chemical treatment or by allowing degeneration, occurring naturally when the scales are isolated in cell culture medium (3,4). However, the melanophores remain able to aggregate and disperse their pigment granules for weeks. After denervation, the

Figure 1. A schematic illustration of the Hogben and Slome melanophore index. An index of 5 stands for a fully dispersed melanophore, an index of 3 for partly aggregated melanophores and an index of 1 for fully aggregated melanophores.

Figure 1. A schematic illustration of the Hogben and Slome melanophore index. An index of 5 stands for a fully dispersed melanophore, an index of 3 for partly aggregated melanophores and an index of 1 for fully aggregated melanophores.

melanophores become supersensitive and a contemporaneous change occurs in the pharmacology.

Experimental data from earlier pharmacological studies on cuckoo wrasse melanophores suggest a two-site receptor model: the two agonists noradrenaline and B-HT 920 have different pharmacological properties and seem to bind to two different sites. For example, yohimbine does not interact with noradrenaline and B-HT 920 in the same way, and the equivalent is true for amiloride (5) and UK 14,304 (6). Furthermore, signal transduction mechanisms are not the same for B-HT 920 and nora-drenaline. Neither of these studies implicated melatonin as a substance of interest. As a result of these studies, a new receptor model was suggested (7).

A receptor model consists in most cases of a receptor with an active site, which binds the endogenous ligand, and, in doing so, activates a transducer, usually a G-protein. The G-protein mediates the signal to a second messenger, which in turn activates the intracellular response. The sensitivity of the system is dependent on occupancy, efficacy and the number of spare receptors. Occupancy refers to the number of receptors engaged by an agonist; efficacy reflects how effective the agonist is in inducing a cellular response; and there are spare receptors if it is not necessary that all receptors be occupied to produce a maximal response. In an in vitro system, such as the prepared cuckoo wrasse melanophores, different agonists react differently to the indicated parameters. This may be explained by the suggested model comprising local and non-local receptor signaling (Figure 2). An agonist with less efficacy may induce only a local physiological response, and an agonist with greater efficacy could cause a global response. However, the model does not reveal whether the local or non-local response originates from the same receptor with different binding sites or if two receptors are present.

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