Octopamine enhances dark-adaptation in Limulus ventral photoreceptors.

Limulus ventral photoreceptors receive synaptic input from fibers that emerge from the brain and that appear to use octopamine as a transmitter. Exogenous application of octopamine has been shown to increase levels of intracellular cAMP in ventral photoreceptors, but the resulting physiological effects have been unclear. In this report, we show that octopamine increases the rate of dark-adaptation following a bright light. Since a similar increase in the rate of dark-adaptation is produced by IBMX (1 mM) and forskolin (100 microM), drugs shown previously to raise the concentration of cAMP, the octopamine effect may be mediated by cAMP. Our results suggest that dark-adaptation, a fundamental process of photoreceptors, is under efferent neural control.