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

Abstract

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.

Pineal control of the dopamine D2‐receptor gene and dopamine release in the retina of the chicken and their possible relation to growth rhythms of the eye

“…The only addition to this model from the present study is that the D 2 -receptor, which was assumed to be involved and to be present in the RPE [Rohrer et al, 1993;Ohngemach et al, 1997], could not be located, since no D 2 mRNA was found, neither in the RPE nor in the choroid. It is possible that the D 4 -receptor is involved instead, with similar properties [Zawilska et al, 1997]. Since DA acts on many different retinal cells via different DA receptors, it is difficult to track its selective function in the visual control of eye growth.…”

Influence of melatonin on the development of functional nicotinic acetylcholine receptors in cultured chick retinal cells

“…Melatonin is continuously synthesized by cultured retinal cells (15), and this synthesis can be blocked by stimulating D4 dopamine receptors with quinpirole (14). On the other hand, the effect of melatonin on chick retina is blocked by luzindole, a classical inhibitor of melatonin G-protein-coupled receptors.…”

“…Cultured cells from embryonic chick retina have been used to study neurodevelopment (14) and melatonin production by photoreceptors (15). Melatonin has a paracrine function in the retina and is produced by photoreceptors since embryonic day 7.…”

“…In the early stages melatonin is synthesized tonically; the rhythmic synthesis of melatonin only starts at day 20 (16). Light-induced inhibition of melatonin biosynthesis is mediated by lightinduced release of dopamine, which acts on D4 dopamine receptors expressed on the membrane of cells that synthesize melatonin (14). Accordingly, melatonin synthesis by photoreceptors is inhibited by quinpirole, a D4 dopamine receptor agonist.…”

Melatonin inhibitory effect on cAMP accumulation in the chick retina development

“…The most important function is the signaling of circadian and seasonal rhythms (Barrenetxe et al, 2004; Iuvone et al, 2005). In the retina, melatonin synthesis occurs in the photoreceptors (Haque et al, 2003) and is blocked by dopamine (Zawilska et al, 1997). In this tissue, melatonin is related to modulation of dopamine release, activation of rod shedding, enhancing of horizontal cells sensitivity and modulation of the circadian electroretinogram rhythms (Dubocovich, 1985; Wiechmann et al, 1988; Nguyen‐Legros and Hicks, 2000; Peters and Cassone, 2005).…”