Abstract
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. J. Comp. Neurol. 480:89–100, 2004. © 2004 Wiley‐Liss, Inc.