Differential expression of synapsin in visual neurons of the locust Schistocerca gregaria

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.