|Retinal amino acid neurochemistry of the southern hemisphere lamprey, Geotria australis.|
Nivison-Smith, L; Collin, SP; Zhu, Y; Ready, S; Acosta, ML; Hunt, DM; Potter, IC; Kalloniatis, M
Lampreys are one of the two surviving groups of the agnathan (jawless) stages in vertebrate evolution and are thus ideal candidates for elucidating the evolution of visual systems. This study investigated the retinal amino acid neurochemistry of the southern hemisphere lamprey Geotria australis during the downstream migration of the young, recently-metamorphosed juveniles to the sea and during the upstream migration of the fully-grown and sexually-maturing adults to their spawning areas. Glutamate and taurine were distributed throughout the retina, whilst GABA and glycine were confined to neurons of the inner retina matching patterns seen in most other vertebrates. Glutamine and aspartate immunoreactivity was closely matched to Müller cell morphology. Between the migratory phases, few differences were observed in the distribution of major neurotransmitters i.e. glutamate, GABA and glycine, but changes in amino acids associated with retinal metabolism i.e. glutamine and aspartate, were evident. Taurine immunoreactivity was mostly conserved between migrant stages, consistent with its role in primary cell functions such as osmoregulation. Further investigation of glutamate signalling using the probe agmatine (AGB) to map cation channel permeability revealed entry of AGB into photoreceptors and horizontal cells followed by accumulation in inner retinal neurons. Similarities in AGB profiles between upstream and downstream migrant of G. australis confirmed the conservation of glutamate neurotransmission. Finally, calcium binding proteins, calbindin and calretinin were localized to the inner retina whilst recoverin was localized to photoreceptors. Overall, conservation of major amino acid neurotransmitters and calcium-associated proteins in the lamprey retina confirms these elements as essential features of the vertebrate visual system. On the other hand, metabolic elements of the retina such as neurotransmitter precursor amino acids and Müller cells are more sensitive to environmental changes associated with migration.
|Differential expression of synapsin in visual neurons of the locust Schistocerca gregaria.|
Gerd Leitinger, Maria Anna Pabst, F Claire Rind, Peter J Simmons
The Journal of comparative neurology
In many taxa, photoreceptors and their second-order neurons operate with graded changes in membrane potential and can release neurotransmitter tonically. A common feature of such neurons in vertebrates is that they have not been found to contain synapsins, a family of proteins that indicate the presence of a reserve pool of synaptic vesicles at synaptic sites. Here, we provide a detailed analysis of synapsin-like immunoreactivity in the compound eye and ocellar photoreceptor cells of the locust Schistocerca gregaria and in some of the second-order neurons. By combining confocal laser scanning microscopy with electron microscopy, we found that photoreceptor cells of both the compound eye and the ocellus lacked synapsin-like immunostaining. In contrast, lamina monopolar cells and large ocellar L interneurons of the lateral ocellus were immunopositive to synapsin. We also identified the output synapses of the photoreceptors and of the L interneurons, and, whereas the photoreceptor synapses lacked immunolabeling, the outputs of the L interneurons were clearly labeled for synapsin. These findings suggest that the photoreceptors and the large second-order neurons of the locust differ in the chemical architecture of their synapses, and we propose that differences in the time course of neurotransmission are the reason for this.