Fine structure of squid (Loligo pealei) optic lobe synapses

Cephalopod optic lobes are a well-known source of cholinergic nerve endings [Dowdall and Whittaker (1973) J. Neurochem, 20, 921-935]. In order to utilize this property for subsequent analyses of cholinergic mechanisms of transmission in the CNS, we describe the ultrastructure of the entire optic lobe of the squid (Loligo pealei) and relate the morphology of synaptosomes to the intact tissue. In the cortex, chemical junctions were found showing two basic forms. The first was an invaginated synapse, appearing only between presynaptic bags and spines which may originate from the trunks of amacrine cells of the outer granule layer. The second was that of a typical synapse, found in almost all layers except the upper portion of the first radial layer. Synapses in the medulla were predominantly of the second type, although a few photoreceptor endings extended to this region as well. The different types of terminals observed in the intact squid optic lobe corresponded to the different types of endings recognized in a synaptosome fraction derived from these lobes. Because of its high content of cholinergic endings and distinct synaptic types, the squid optic lobe may contribute to the elucidation of the mechanisms of cholinergic transmission in the central nervous system. In addition, electrotonic synapses were found between photoreceptor processes in the cortex, as well as other elements of the neuropil.