ELECTRICAL RESPONSES AND SYNAPTIC CONNECTIONS OF GIANT SEROTONIN-IMMUNOREACTIVE NEURONS IN CRAYFISH OLFACTORY AND ACCESSORY LOBES

Five pairs of identified 5HT-IR cells in the deutocerebrum of the cray fish Cherax are known to have their synaptic endings in the accessory and olfactory lobes. Two of these cells, one on each side of the brain , are significantly larger than the others. Dye fills of these ''giant '' cells reveal each to be an interneuron with its branches confined t o, but distributed throughout, the olfactory and accessory lobes on th e side of the brain ipsilateral to its cell body and with no branches to the contralateral side. Intracellular recordings from the giant cel ls were made while stimulating the olfactory afferents and tracts with in the brain in an attempt to discover the inputs and outputs to the c ells. Electrical stimulation of chemoreceptor sensilla on the outer br anch of the antennule does not excite the giant 5HT neurons. Focal ext racellular electrical stimulation of the olfactory globular tract cont aining the axons of projection neurons from the olfactory and accessor y lobes produces excitatory synaptic potentials and action potentials in the giant cells. Focal extracellular electrical stimulation of the deutocerebral commissure, the axons of which terminate in the glomerul i of the accessory lobes, also results in excitation of the giant cell s. We conclude that the input to the giant cells is via axons in the d eutocerebral commissure and collaterals from the projection neurons, e nding in the glomeruli of the accessory lobes. The output of the giant cells is to the olfactory lobes, where it may serve to modulate olfac tory signals. (C) 1994 Wiley-Liss, Inc.