Journal of Neuroscience, Vol 14, 3083-3096, Copyright © 1994 by Society for Neuroscience
Photomechanical movements of cultured embryonic photoreceptors: regulation by exogenous neuromodulators and by a regulable source of endogenous dopamine
DL Stenkamp, PM Iuvone and R Adler
Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287.
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.
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