Signal transmission from pineal photoreceptors to luminosity-type ganglion cells in the lamprey, Lampetra japonica

Functional heterogeneity in the pineal projection neurons of zebrafish

The zebrafish pineal organ is a photoreceptive structure containing two main neuronal populations (photoreceptors and projections neurons). Here we describe a subpopulation of projection neurons that expresses the melanopsin gene, opn4xa. This new pineal cell type, that displays characteristics of both projection neurons and photoreceptors, share a similar dependency for BMP and Notch signalling pathways with classical non-photosensitive projection neurons (PN). Functionally, however, whereas classical, opn4xa-negative PNs display an achromatic LIGHT OFF response, the novel cell type we describe exhibit a LIGHT ON character that is elicited by green and blue light. Taken together, our data suggest a previously unanticipated heterogeneity in the projection neuron population in the zebrafish pineal organ raising the question of the importance of these differences in pineal function.

Stabilization of spontaneous neurotransmitter release at ribbon synapses by ribbon-specific subtypes of complexin

Ribbon synapses of tonically releasing sensory neurons must provide a large pool of releasable vesicles for sustained release, while minimizing spontaneous release in the absence of stimulation. Complexins are presynaptic proteins that may accomplish this dual task at conventional synapses by interacting with the molecular machinery of synaptic vesicle fusion at the active zone to retard spontaneous vesicle exocytosis yet facilitate release evoked by depolarization. However, ribbon synapses of photoreceptor cells and bipolar neurons in the retina express distinct complexin subtypes, perhaps reflecting the special requirements of these synapses for tonic release. To investigate the role of ribbon-specific complexins in transmitter release, we combined presynaptic voltage clamp, fluorescence imaging, electron microscopy, and behavioral assays of photoreceptive function in zebrafish. Acute interference with complexin function using a peptide derived from the SNARE-binding domain increased spontaneous synaptic vesicle fusion at ribbon synapses of retinal bipolar neurons without affecting release triggered by depolarization. Knockdown of complexin by injection of an antisense morpholino into zebrafish embryos prevented photoreceptor-driven migration of pigment in skin melanophores and caused the pigment distribution to remain in the dark-adapted state even when embryos were exposed to light. This suggests that loss of complexin function elevated spontaneous release in illuminated photoreceptors sufficiently to mimic the higher release rate normally associated with darkness, thus interfering with visual signaling. We conclude that visual system-specific complexins are required for proper illumination-dependent modulation of the rate of neurotransmitter release at visual system ribbon synapses.

Immunohistochemical characterization of a parapinopsin-containing photoreceptor cell involved in the ultraviolet/green discrimination in the pineal organ of the river lamprey Lethenteron japonicum

In the pineal organ, two types of ganglion cell exhibit antagonistic chromatic responses to UV and green light, and achromatic responses to visible light. In this study, we histologically characterized UV-sensitive photoreceptor cells that contain a unique non-visual UV pigment, lamprey parapinopsin, in order to elucidate the neural network that is associated with antagonistic chromatic responses. These characteristics were compared with those of lamprey rhodopsin-containing cells, most of which are involved in achromatic responses. RT-PCR analysis revealed that lamprey parapinopsin was expressed in the pineal organ but not in the retina, unlike lamprey rhodopsin, which was expressed in both. Lamprey parapinopsin and lamprey rhodopsin were immunohistochemically localized in the dorsal and ventral regions of the pineal organ, respectively. The two pigments were localized in distinct photoreceptor cells throughout the pineal organ, namely the dorsal and ventral regions as well as the peripheral region, which corresponds to the dorso-ventral border region. The ratio of the number of lamprey parapinopsin-containing cells to lamprey rhodopsin-containing cells around the peripheral region was higher than in the central region. Electron-microscopic analysis revealed that lamprey parapinopsin-containing dorsal cells have outer segments and synaptic ribbons similar to those of ventral photoreceptor cells. However, unlike lamprey rhodopsin-containing cells, lamprey parapinopsin-containing cells connected with each other in a wide area of dorsal and peripheral portions and made direct contact with ganglion cells, mainly in the peripheral portion. These results suggest that UV light information captured by lamprey parapinopsin-containing photoreceptor cells is converged and directly transmitted to chromatic-type ganglion cells in the peripheral region to generate antagonistic chromatic responses.

Molecular identification and developmental expression of UV and green opsin mRNAs in the pineal organ of the Atlantic halibut

The pineal organ is the only differentiated photoreceptor organ present in embryos and early larvae of the Atlantic halibut (Hippoglossus hippoglossus). We investigated the molecular identity of opsins in the pineal organ, and their expression during different life stages. Using RT-PCR we identified two 681-bp gene sequences, named HPO1 and HPO4, in cDNA from adult pineal and whole embryos. The predicted amino acid sequences showed highest identity to the transmembrane regions of teleostean RH2 green cone opsins (HPO1, 72-91%) and SWS-1 UV cone opsins (HPO4, 71-83%). In situ hybridization revealed expression of HPO1 and HPO4 mRNA transcripts in photoreceptors in the pineal organ of embryos, larvae and adults. HPO1 and HPO4 mRNA transcripts were also expressed in the larval retina. Our study provides molecular evidence for short and middle wavelength light sensitive photoreceptors in the pineal organ of Atlantic halibut throughout life, and suggests that pineal photoreception may play an important role during embryonic and larval life stages, especially at the time when the retina does not possesses corresponding photoreceptor capacity.

Cloning and characterization of the pineal gland-specific opsin gene of marine lamprey (Petromyzon marinus)

We have cloned and sequenced the pineal gland-specific opsin (P opsin) gene, p(Pm), of marine lamprey, P. marinus. The deduced P opsin, P(Pm), consisting of 444 aa, is about 90 aa longer than the orthologous opsins of chicken, pigeon, and American chameleon. The evolution of P(Pm) is characterized not only by a significantly higher rate of aa replacement than the other P opsins but also by a larger number of unique aa replacements at highly conserved sites in the nonTM region. These changes in P(Pm) might have been important in achieving marine lamprey-specific interaction between P(Pm) and other proteins in the pineal gland.

Spectral sensitivity and mechanism of interaction between inhibitory and excitatory responses of photosensory pineal neurons

The characteristics and distribution of chromatic-type neurons in the photosensory pineal organ of the river lamprey, Lampetra japonica, were investigated electrophysiologically. Neuronal activity was inhibited by light of short wavelengths and excited by middle to long wavelengths. The maximum sensitivities of the inhibitory and excitatory responses were at about 380 nm and 540 nm respectively. The spike activity of the neurons during steady illumination for a 10-min period was measured. Although a flash of short-wavelength light caused a strong inhibition in the neuron, this effect was not sustained during 10 min of photic stimuli. It was found that the inhibitory effect continued when excitatory (middle-wavelength) light was delivered together with inhibitory (short-wavelength) light. The result supports the hypothesis of photoregeneration in the pineal photoreceptor, which occurs when photoreceptors having high sensitivity to short wavelengths receive middle-wavelength light. Contrary to the inhibitory response, the excitatory one caused by middle wavelengths continued during stimulation. Spike frequency of the neuron was determined by the spectral composition of the light. Since environmental light contains both inhibitory and excitatory components, the neuron would keep both sensitivities during the daytime and could measure the variation in the spectral composition. Judging from the recording sites, the chromatic-type neurons are distributed in the peripheral part of the pineal organ.