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Specificity of serotonin uptake by bovine retina: Comparison with tryptamine

1980; Elsevier BV; Volume: 31; Issue: 1 Linguagem: Inglês

10.1016/0014-4835(80)90088-3

1096-0007

Neville N. Osborne, George S. Richardson,

Neuroscience and Neuropharmacology Research

Isolated bovine retinas possess the ability to concentrate tryptamine from an external medium. The entry mechanism is independent of sodium and other cations, but is dependent on temperature. Kinetic analysis of the tryptamine uptake shows a single mechanism with Km and Vmax values of 2 × 10−5m and 50 pmol/mg/min respectively. A lack of specificity in the process is indicated by its insensitivity to a number of metabolic inhibitors and various analogues. Autoradiography has shown that specific neuronal processes, associated with the inner plexiform layer, take up serotonin (5-hydroxytryptamine) and that this accumulation can be inhibited by p-chlorophenylalanine. Tryptamine is not taken up by any specific components. It is also demonstrated that synaptosomes derived from the inner plexiform layer take up more serotonin than tryptamine. Furthermore, radioactivity is rapidly released from tissue loaded with [3H]tryptamine, and this process cannot be stimulated by potassium chloride. This contrasts with tissue loaded with [3H]serotonin, where the spontaneous release of the substance is not so rapid but can nevertheless be stimulated by potassium chloride treatment. It is concluded that the total uptake of tryptamine resembles the low affinity unspecific uptake mechanism for serotonin. The specificity of the high affinity uptake mechanism for serotonin, together with the autoradiographical and release experiments, can be taken to support the idea that serotonin is a transmitter in the retina. On the other hand, there is no evidence of tryptamine being a likely transmitter in the retina.