Serotonin DepletionIn VivoInhibits the Branching of Olfactory Projection Neurons in the Lobster Deutocerebrum

Abstract

Serotonin depletion during embryogenesis has been shown previously to retard the growth of the olfactory and accessory lobes of the lobster deutocerebrum (Benton et al., 1997). The present study was undertaken to determine whether morphological changes in the interneurons innervating these lobes contribute to this growth retardation. We examined the effects of in vivo serotonin depletion using 5,7-dihydroxytryptamine (5,7-DHT) on the morphology of the olfactory projection neurons, one of two major classes of interneurons that innervate both lobes. Intracellular dye fills of olfactory projection neurons in normal embryos showed that each neuron extensively innervates either the olfactory or accessory lobe before projecting to neuropil regions in the protocerebrum. In embryos injected with 5,7-DHT, however, the deutocerebral arbors of 13.5% of the olfactory projection neurons examined were either markedly reduced compared with normal neurons or absent. Affected neurons also exhibited a number of additional aberrant morphological features suggesting that these neurons represent cells that were affected during their initial morphogenesis. Olfactory projection neurons with aberrant morphologies were also encountered, although less frequently (7.5% of the neurons examined), in control (sham-injected) embryos indicating that the sham injections can affect the development of the brain. This observation provides insights into the nature of effects seen in control embryos in previous experiments (Benton et al., 1997). The results of the present study indicate that in vivo serotonin depletion inhibits the branching of olfactory projection neurons and suggest, therefore, that one of the functions of serotonin during normal development is to promote the ingrowth of these neurons into the deutocerebral neuropils.

Transient uptake of serotonin by newborn olfactory projection neurons

“…Further experiments showed that, although mature projection neurons are not serotonin-immunoreactive in lobsters (24,25), newborn olfactory projection neurons are transiently able to take up serotonin. This phenomenon may be related to the role serotonin plays in promoting the ingrowth of new projection neurons into the olfactory neuropils of lobsters (23) and͞or to the effects of serotonin on the regulation of neurogenesis that are reported here.…”

“…These results suggest that the proliferation, growth, or morphogenesis of the local and projection neurons may be affected by serotonin depletion. Dye-injection studies have shown that the differentiation of a subset of the olfactory projection neurons is indeed affected by serotonin depletion: ϳ14% of the projection neurons sampled in serotonin-depleted embryos send axons toward the lateral protocerebrum via the olfactory globular tract (OGT), as expected, but do not innervate the OLs or ALs, as in normal embryos (23). Hence, it would seem that serotonin depletion in vivo inhibits the branching of olfactory projection neurons in lobsters, suggesting that one of the functions of serotonin during normal development is to promote, directly or indirectly, the growth of these neurons into the olfactory lobe glomeruli and associated brain neuropils.…”

Adult neurogenesis and cell cycle regulation in the crustacean olfactory pathway: from glial precursors to differentiated neurons

“…Towards this end, we have examined serotonin and nitric oxide as possible endogenous modulators of neurogenesis. Various in vitro and in vivo studies have shown that increases in serotonin levels result in an upregulation of neurogenesis in both the MPZ and LPZ of juvenile clawed lobsters (Benton and Beltz 2001a;Beltz et al 2001) and that decreases inhibit the branching of newborn projection neurons (Sullivan et al 2000). Furthermore, brain serotonin levels are under circadian control (Wildt et al 2004).…”

“…The OL glomeruli are sites of synaptic contact between the ORNs, olfactory projection neurons and several classes of local interneurons. In crayfish and lobsters, the latter two classes of neurons also innervate an additional glomerular neuropil, the accessory lobe (AL), which lies adjacent to the OL (Arbas et al 1988;Mellon and Alones 1994;Wachowiak et al 1996;Sullivan et al 2000;Beltz 2001a,b, 2005b). In contrast to the OL, which receives only primary olfactory inputs, the AL receives higher-order olfactory, visual and mechanosensory inputs Sullivan and Beltz 2005b).…”

“…Anatomical studies in eureptantian decapods have shown that individual cluster 10 projection neurons innervate either the OL or the AL (Wachowiak and Ache 1994;Schmidt and Ache 1996;Wachowiak et al 1996;Sullivan et al 2000). Cluster 10 in these animals, therefore, is comprised of at least two groups of interneurons (OL projection neurons, AL projection neurons) with distinct functional identities .…”