THE JOURNAL OF COMPARATIVE NEUROLOGY 361~617-632 (1995) Ultrastructure of the Synaptic Terminals of the Dorsal Giant Serotonin-IR Neuron and Deutocerebral Commissure Interneurons in the Accessory and Olfactory Lobes of the Crayfish

The olfactory and accessory lobes in the crayfish are large spherical neuropils found on each side of its brain. The olfactory lobes receive the afferent axons of chemoreceptors that are located along the outer branches of the biramous first antennae. The accessory lobes receive a large input from interneurons whose axons lie in the deutocerebral commissure. A pair of large serotonergic neurons (the dorsal giant neurons) branch unilaterally in the accessory and olfactory lobes of each side. From physiological recordings, it has been proposed that the deutocerebral commissure interneurons synapse with elements in the accessory lobes that in turn connect to the dorsal giant neuron. It has also been proposed that the dorsal giant neuron is activated by inputs in the accessory lobe and that its output is in the olfactory lobe. This ultrastructural study tests this hypotheses by examining the polarity of synaptic terminals on dorsal giant neurons and deutocerebral interneurons that have been filled with neurobiotin. In double-labelled preparations, we found the deutocerebral interneurons to be presynaptic to elements in the accessory lobes, but none of these postsynaptic elements was identifiable as the dorsal giant neuron. The dorsal giant neurons receive many more synaptic inputs in the accessory lobes than in the olfactory lobe. Very few giant serotonin neuron output synapses were found in either lobe. o 1995 wiley-Liss, Inc. Indexing terms: crustacea, central nervous system, olfaction, serotonin, synapses Freshwater crayfish possess an olfactory system consisting of rows of specialized chemoreceptive sensilla on the antennules that project exclusively to discrete areas of glomerular neuropil in the olfactory lobes of the brain. Adjacent to the olfactory lobes, and evidently also part of the olfactory system, are an additional pair of large glomerular neuropils, the accessory lobes. Four groups of interneurons are known to be associated with the olfactory and accessory lobes: (1) local interneurons with small somata (in cluster 9, see Sandeman et al., 1992) and with terminals in both the olfactory and accessory lobes (Mellon and Nones, 1993); (2) projection neurons with small somata (in cluster 10) whose axons extend in a single large tract (olfactory globular tract) from the olfactory and accessory lobes to the hemiellipsoid body in the lateral protocerebrum; (3) midbrain cells with larger somata (in cluster ll), whose axons terminate predominantly in the olfactory lobe; and (4) deutocerebral commissure neurons with small somata (also in cluster l l ) , with axons in the deutocerebral commissure and their terminals in the accessory lobe glomeruli (Sandeman and Luff, 1973; Arbas et al., 1988; Sandeman et al., 1992, 1993; Mellon and Nones, 1993). Like other arthropods, crayfish have serotonin-immunoreactive (serotonin-IR) neurons that branch extensively among the olfactory lobe glomeruli, where they may exert a modulatory effect on the olfactory processing (Nassel and Elekes, 1984; Sandeman and Sandeman, 1987; Schmidt and Ache, 1992; Sun et al., 1993). Among the serotonin-IR neurons projecting to the crayfish olfactory lobes are a pair of very large neurons, one on each side of the brain, the dorsal giant neurons (DGNs; Sandeman and Sandeman, Accepted April 4, 1995. A.H.D. Watson’s present address: Anatomy Unit, School of Molecular Medicine and Biosciences, University of Wales College of Cardiff, Cardiff, Wales. Address reprint requests to R.E. Sandeman, School of Biological Science, University of New South Wales, P.O. Box 1, Kensington, NSW 2033,