Photomechanical movements of cultured embryonic photoreceptors: regulation by exogenous neuromodulators and by a regulable source of endogenous dopamine

Abstract

In the retina of nonmammalian vertebrates, light regulates photoreceptor morphology by causing rod photoreceptor elongation and cone photoreceptor contraction. The opposite photomechanical movements occur in the dark, and proceed with a circadian rhythm in many species in vivo. Using dissociated cultures of embryonic chick retina cells, we have recently demonstrated that photoreceptor cells that differentiate in vitro acquire the capacity of responding to light/dark cycles with photomechanical movements (Stenkamp and Adler, 1993). Here we report that the putative neuromodulators melatonin and dopamine can mimic the effects of darkness and light, respectively, on in vitro photomechanical movement. Pharmacological studies showed that dopamine appears to function by means of a D2-type receptor negatively coupled to adenylate cyclase. The effects of light on the cultured photoreceptors were inhibited by dopamine D2 receptor antagonists, and were attenuated by the dopaminergic neurotoxin 6-hydroxydopamine and by the dopamine synthesis inhibitor alpha-methyl-p-tyrosine. The possible existence of an endogenous source of dopamine in the cultures was also suggested by the presence of tyrosine hydroxylase-like immunoreactivity, and of an Na(+)-dependent mechanism for the accumulation of 3H-dopamine, which was predominantly associated with nonphotoreceptor cells. Additionally, 3H-dopamine release occurred in vitro through a Ca(2+)-dependent mechanism, as well as through reverse function of a nomifensine-sensitive dopamine transporter. Both of these putative release mechanisms appeared to be regulated by light and by melatonin, suggesting a mechanism whereby the putative dopaminergic cells may interact with other cells present in the cultures. These studies suggest that complex paracrine neuromodulatory mechanisms can differentiate in low-density embryonic cell culture, that dopaminergic activities exist in vitro, and that they are important for mediating photomechanical movements.

Photoreceptor Coupling Is Controlled by Connexin 35 Phosphorylation in Zebrafish Retina

“…A large body of evidence indicates that adenylyl cyclase/cAMP signaling pathways, driven in photoreceptors by adenosine at night and dopamine during the day, play an important role in this transition. The adaptive mechanisms in photoreceptors controlled by these pathways include opposing photomechanical movements of rods and cones (Dearry and Burnside, 1985; Pierce and Besharse, 1985; Dearry et al , 1990; Stenkamp et al , 1994; Rey and Burnside, 1999), modulation of calcium currents (Stella and Thoreson, 2000; Stella et al , 2002; Stella et al , 2003) and chloride currents (Thoreson et al , 2002), and changes in rod-cone (Yang and Wu, 1989; Krizaj et al , 1998; Ribelayga et al , 2008) and cone-cone (Copenhagen and Green, 1987) gap junctional coupling. The net effect of most of these adaptive changes is to reduce rod input and increase cone input to post-receptoral neurons during the day or when adapted to bright light (Witkovsky et al , 1988; Witkovsky et al , 1989; Wang and Mangel, 1996; Ribelayga et al , 2002).…”

Circadian regulation of ion channels and their functions

“…Exposure to dopamine or D 2 agonists causes a significant decrease in the apparent affinity of CNGCs at night but has no effect on these channels during the day. It is well established that D 2 receptors are pertussis toxin sensitive G protein‐coupled receptors that mediate the inhibition of adenylate cyclase and cause a decrease in cAMP formation in the retina (Iuvone 1986, 1990; Stenkamp et al. 1994).…”

Visual photoreceptor subtypes in the chicken retina: melatonin-synthesizing activity and in vitro differentiation

“…Melatonin is an indolic hormone derived from tryptophan that acts locally in the eye, where it regulates many physiological processes in different species (Besharse and Dunis 1983;Dubocovich 1983;Pierce and Besharse 1985). Specifically in the chicken retina, melatonin inhibits cyclic AMP accumulation and dopamine synthesis (Nowak et al 1992;Iuvone and Gan 1994) and regulates the photomechanical movements of cones (Stenkamp et al 1994). Melatonin synthesis from its precursor serotonin, requires two enzymes: arylalkylamine-N-acetyltransferase (AANAT: EC 2.3.1.87) and hydroxyindole-O-methyltransferase (HIOMT: EC 2.1.1.4, also named acetylserotonin-methyltransferase: ASMT).…”