Histamine metabolism in the visual system of the horseshoe crab Limulus polyphemus

There is now strong evidence that arthropod photoreceptors use histamine as a neurotransmitter. The synthesis, storage and release of histamine from arthropod photoreceptors have been demonstrated, and the postsynaptic effects of histamine and the endogenous neurotransmitter are similar. However, a full understanding of these photoreceptor synapses also requires knowledge of histamine inactivation and metabolism. Relatively little is known about histamine metabolism in the nervous system of arthropods, and mechanisms appear to differ with the species. This study focuses on histamine metabolism in visual tissues of the horseshoe crab Limulus polyphemus, a chelicerate. We present two major findings: (1) histamine is metabolized to imidazole acetic acid and to gamma-glutamyl histamine. (2) relatively low levels of histamine metabolites accumulate in Limulus visual tissues.