Circadian rhythms in light-evoked responses of the fly's compound eye, andthe effects of neuremodulators 5-HT and the peptide PDF

Two sets of wide-field neurons extend neurites into the fly's optic lamina, where monopolar cells receive photoreceptor input. They exhibit immunoreac tivity to antibodies raised against either 5-hydroxytryptamine or the crust acean peptide PDH, respectively. Both are proposed whole-field neuromodulat ors of vision, apparently regulating a circadian rhythm of monopolar cell s ize. Seeking functional correlates, we have re-examined the electroretinogr am for circadian rhythmicity, and for responses to locally injected 5-hydro xytryptamine and peptide. Long-term electroretinogram recordings from Calli phora entrained to a light/dark cycle and then transferred to constant dark ness, uncovered a gradual, modest increase during the subjective night in t he electroretinogram's ON- and OFF-transients, from the lamina's monopolar cells. Five to twenty nl of 5-hydroxytryptamine (10(-3) mol.l(-1)) injected into the head haemolymph strongly enhanced the electroretinogram transient s, an action reversed by 5-hydroxytryptamine antagonists. Injected into the eye, 5-hydroxytryptamine (10(-4) mol.l(-1)) had the opposite effect, the r apid onset there suggests direct action, whilst the opposing effect from ha emolymph injection suggests a different receptor site. Pigment-dispersing h ormone (2.2 x 10(-5) mol.l(-1)) injected into the haemolymph increased the electroretinogram transients along a biphasic course, with a slow partial r ecovery; injected into the eye, it lacked effect.