Calcium channel heterogeneity among cone photoreceptors in the tree shrew retina

Retinal photoreceptors are depolarized in darkness and release neurotransmitter tonically. They respond to light by hyperpolarization and a concomitant reduction in transmitter release. The calcium-dependent release of transmitter is coupled to graded changes in membrane potential by L-type calcium channels in the photoreceptor terminals. This paper reports the immuno-localization of an L-type channel alpha1D subunit to most, but not all, synaptic terminals of cones in the tree shrew retina. Double labelling for the alpha1D subunit and the plasma membrane Ca2+-ATPase, which has been shown to be present in all tree shrew cones, revealed a subpopulation of cone terminals that did not react with the alpha1D antibody. The nonimmunoreactive synaptic terminals represented approximately 5.8% of the total and formed a highly regular array across the retina reminiscent of the blue cones. Double-staining for the alpha1D subunit and blue cone opsin confirmed that these are the blue cones. The observed differences in calcium channel immunoreactivity between long and short wavelength cones points to previously unsuspected heterogeneity in the molecular machinery governing transmitter release from spectrally different cone types.