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

Abstract

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.

Immunocytochemical analysis of glomerular regionalization and neuronal diversity in the olfactory deutocerebrum of the spiny lobster

“…The most prominent local interneuron we identified, the "dorsal giant", has not been characterized by intracellular recordings previously. In most morphological features -giant soma, arborizations in OL, AL and OGTN -as well as in serotonin-like immunoreactivity it corresponds very closely to the "dorsal giant neuron" of crayfish Sandeman 1987, 1994;Sandeman et al 1988, D. Sandeman et al 1995R.E. Sandeman et al 1995).…”

“…A giant "serotonergic" neuron has also been demonstrated in the clawed lobster (Beltz et al 1990(Beltz et al , 1992 and thus in all decapod taxa equipped with an AL. This, together with the likely direction of information flow from the AL to the OL in this neuron (Sandeman and Sandeman 1994;R.E. Sandeman et al 1995), argues that the "dorsal giant" serves a specific function connected to the presence of the AL.…”

“…Sparse arborizations violating glomerular boundaries have been observed in AL projection neurons of crayfish and spiny lobster (Mellon et al 1992;Schmidt and Ache 1996;Wachowiak et al 1996), while tuft-like arborizations have been found in other neurons of the crayfish and spiny lobster AL (e.g., Sandeman and Luft 1973;Sandeman and Sandeman 1994;D. Sandeman et al 1995;R.E. Sandeman et al 1995;Wachowiak et al 1996).…”

“…Some minor features, however, differ: the "dorsal giant" of crayfish does not show additional FMRF amide-like immunoreactivity (M. Schmidt, unpublished work) and it arborizes in all AL glomeruli (Sandeman and Sandeman 1994;D. Sandeman et al 1995;R.E. Sandeman et al 1995).…”

Distribution of synapses on two local auditory interneurones, ON1 and ON2, in the prothoracic ganglion of the cricket: relationships with GABA-immunoreactive neurones

“…The ultrastructure of serotonergic neurones has been studied in several arthropods (Nässel et al 1985;Salecker andDistler 1990, Sun et al 1993;Sandeman et al 1995), including among the Orthoptera, locusts but not crickets. Though vesicle compliment in serotonin-immunoreactive processes varies in different 241 Fig.…”