Sympathetic nervous control of the iris sphincter of the atlantic cod,Gadus morhua

Abstract

The autonomic nervous control of the cod iris has been studied. The pharmacological properties of the smooth muscles of the iris have been elucidated by agonist/antagonist studies on isolated strip preparations. Electrical stimulation of parasympathetic and sympathetic pathways to the eye have been carried out, with recordings of the movements of the iris margin. Additions of cholinergic and adrenergic antagonists in selective concentrations were made to investigate the nature of the autonomic nerve fibres controlling the iris. Isolated strip preparations of the iris sphincter contracted in response to cholinergic orα-adrenoceptor agonists. There appear to be no radial muscular elements in the cod iris. The effect of carbachol on the iris sphincter could be competitively antagonized by atropine, suggesting the presence of muscarinic receptors of the smooth muscles. The effect of adrenaline was similarly antagonized by phentolamine. The effect of phentolamine, and the order of potency for the adrenergic agonists, shows the presence ofα-adrenoceptors in the iris sphincter.β-adrenoceptors of minor importance are also suggested by the inhibitory effects of isoprenaline on preparations pre-contracted by carbachol. The indirectly acting adrenergic agonist tyramine also contracts the isolated sphincter preparations. This effect is probably due to release of nervously stored catecholamines, since tyramine lacks effect on preparations from animals pre-treated with 6-hydroxydopamine. Preparations from 6-hydroxydopamine pre-treated animals also show a 10-fold increase in the affinity for adrenaline, demonstrating the development of a pre-synaptic supersensitivity due to the destruction of adrenergic nerve terminals of the iris. Stimulation of the sympathetic chain or ciliary nerves produces a constriction of the pupil of the same side. Application of selective concentrations of the antagonists atropine and phentolamine shows that the sympathetic constrictory innervation is solely adrenergic. In some preparations a small pupillo-dilatory effect of nerve stimulation is evident after the constrictory effect has been abolished by phentolamine. This inhibitory effect can be abolished by propranolol, indicating the presence of aβ-adrenoceptor mediated inhibitory control of minor importance. Stimulation of the oculomotor nerve produces no consistent responses of the cod iris. Illumination of one eye produces a pupilloconstriction comparable to that seen after sympathetic nerve stimulation. The light induced response is insensitive to atropine, phentolamine and tetrodotoxin, showing a direct effect on the smooth muscles of the sphincter. There is no consensual reflex in the cod.