Serotonin depletion in vivo inhibits the branching of olfactory projectionneurons in the lobster deutocerebrum

Serotonin depletion during embryogenesis has been shown previously to retar d the growth of the olfactory and accessory lobes of the lobster deutocereb rum (Benton et al., 1997). The present study was undertaken to determine wh ether morphological changes in the interneurons innervating these lobes con tribute to this growth retardation. We examined the effects of in vivo sero tonin depletion using 5,7-dihydroxytryptamine (5,7-DHT) on the morphology o f the olfactory projection neurons, one of two major classes of interneuron s that innervate both lobes. Intracellular dye fills of olfactory projectio n neurons in normal embryos showed that each neuron extensively innervates either the olfactory or accessory lobe before projecting to neuropil region s in the protocerebrum. In embryos injected with 5,7-DHT, however, the deut ocerebral arbors of 13.5% of the olfactory projection neurons examined were either markedly reduced compared with normal neurons or absent. Affected n eurons also exhibited a number of additional aberrant morphological feature s suggesting that these neurons represent cells that were affected during t heir initial morphogenesis. Olfactory projection neurons with aberrant morp hologies were also encountered, although less frequently (7.5% of the neuro ns examined), in control (sham-injected) embryos indicating that the sham i njections can affect the development of the brain. This observation provide s insights into the nature of effects seen in control embryos in previous e xperiments (Benton et al., 1997). The results of the present study indicate that in vivo serotonin depletion inhibits the branching of olfactory proje ction neurons and suggest, therefore, that one of the functions of serotoni n during normal development is to promote the ingrowth of these neurons int o the deutocerebral neuropils.