Prolonged exposure of chicks to light or darkness differentially affects the quinpirole-evoked suppression of serotonin N-acetyltransferase activity in chick retina: an impact on dopamine D4-like receptor.

Dopamine plays an important role in regulation of melatonin biosynthesis in retinas of several vertebrate species. In the avian retina, the dopamine receptor that controls melatonin production represents a D4-like subtype. Stimulation of this receptor by quinpirole (QNP) results in a dose-dependent decline of the nighttime activity of serotonin N-acetyltransferase (NAT, a key regulatory enzyme in melatonin biosynthesis) and melatonin level of the retina. The present study was undertaken to determine whether the ability of QNP to suppress nocturnal NAT activity of chick retina was affected by prolonged adaptation of animals to light and darkness. In the retina of chicks kept under a light:dark (LD) illumination cycle, dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) levels measured at the end of the light phase were significantly higher than those found in the middle of the dark phase. In animals maintained under continuous light (LL) or darkness (DD) dopamine and DOPAC contents of the retina measured at these two time points were similar and resembled levels found during, respectively, the light and dark phase in the retina of chicks kept under LD illumination cycle. Adaptation of chicks to LL and DD resulted in an attenuated and enhanced, respectively, response of the retinal NAT activity to the suppressive action of QNP. When compared to the LD group, a parallel shift to the right (LL group) or left (DD group) of the dose-response curve for QNP was observed, and the ED50 values for this dopamine receptor agonist were 3.4-times higher (LL) or 2.8-lower (DD) than those calculated for the control LD animals. It is suggested that prolonged exposure to light or darkness, by altering the level of the retinal dopaminergic neurotransmission, may modify the reactivity of the D4-like dopamine receptors regulating NAT activity of the chick retina.