INVESTIGATIONS INTO WHETHER 5-HYDROXYTRYPTAMINE IS A NEUROTRANSMITTER IN THE RETINA OF RABBIT AND CHICKEN

Abstract

A system of indoleamine-accumulating neurons exists in the retina of several species, but the exact transmitter of these neurons is unknown. 5-Hydroxytryptamine (5-HT) is a likely candidate, but reports on its amount in the retina conflict. The amounts of 5-HT were determined with 2 sensitive, specific and independent methods. Because the content was lower than expected for a neurotransmitter, the rate-limiting enzyme in the 5-HT synthesis was estimated, as was the effect of 5-HT on cyclic[c]AMP content of the retina. In the rabbit the 5-HT content was 25-35 ng/gm wet wt, with no difference between light- and dark-adapted animals. No detectable activity exists of the rate-limiting enzyme tryptophan hydroxylase. The 5-HT concentration was not raised by treating the rabbits with a monoamine oxidase inhibitor and L-tryptophan. 5-HT induced no change in the cAMP content of the rabbit retina. In newly hatched and older chicken retina, the 5-HT concentration was higher than in rabbit. The 5-HT concentrations in all retinas were lower than expected for a monoamine neurotransmitter. 5-HT apparently is not the neurotransmitter of the indoleamine-accumulating.