Ontogenetic alteration in peptidergic expression within a stable neuronal population in lobster stomatogastric nervous system

Abstract

In the adult lobster, Homarus gammarus, the stomatogastric ganglion (STG) contains two well-defined motor pattern generating networks that receive numerous modulatory peptidergic inputs from anterior ganglia. We are studying the appearance of extrinsic peptidergic inputs to these networks during ontogenesis. Neuron counts indicate that as early as 20% of development (E20) the STG neuronal population is quantitatively established. By using immunocytochemical detection of 5-bromo-2'-deoxyuridine incorporation, we found no immunopositive cells in the STG by E70. We concluded that the STG neuronal population remains quantitatively stable from mid-embryonic life until adulthood. We then investigated the ontogeny of FLRFamide- and proctolin-like peptides in the stomatogastric nervous system, from their first appearance until adulthood by using whole mount immunocytochemistry. Numerous FLRFamide-like-immunoreactive STG neuropilar ramifications were observable as early as E45 and remain thereafter. From E50 to the first larval stage, one to three STG somata stained, while somatic staining was not observed in larval stage II and subsequent stages. From E50 and thereafter, the STG neuropilar area was immunopositive for proctolin. One to two proctolinergic somata were detected in the STG of the three larval stages but were not seen in embryos, the post-larval stage or in adults. Thus, peptidergic inputs to the STG are present from mid-embryonic life. Moreover, whereas in the adult, STG neurons only contain glutamate or acetylcholine, some neurons transiently express peptidergic phenotypes during development. Although this system expresses an ontogenetic peptidergic plasticity, the STG neurons produce a single stable embryonic-larval motor output (Casasnovas and Meyrand [1995] J. Neurosci. 15:5703-5718).

Ontogeny of Modulatory Inputs to Motor Networks: Early Established Projection and Progressive Neurotransmitter Acquisition

“…Moreover, central projection neurons play a key role in the ontogeny of STNS networks because their presence masks preexisting adultlike phenotypes (Le Feuvre et al, 1999). In addition, immunocytochemical characterization of these inputs has shown that modulatory substances appear gradually in the STG during embryonic development (Cournil et al, 1995;Fenelon et al, 1998aFenelon et al, , 1999Kilman et al, 1999). However, such results do not exclude that some projection neurons may reach their target networks without expressing their final neurotransmitter phenotype.…”

“…Neuromodulatory systems involved in the control of central pattern generators (CPGs) have been extensively studied in terms of neuronal population, neurotransmitter phenotype, and alteration of network activity. A large body of work has been devoted to the progressive developmental acquisition of projection neurons using retrograde dye migration (Cabana and Martin, 1984;van Mier and ten Donkelaar, 1984;Okado and Oppenheim, 1985;Kudo et al, 1993) and of neuromodulatory substances using immunocytochemistry (Senba et al, 1982;Commissiong, 1983;Sako et al, 1986;van Mier et al, 1986;Rajaofetra et al, 1989;Pindzola et al, 1990;Fenelon et al, 1998aFenelon et al, , 1999Kilman et al, 1999). Using these approaches, it has been concluded that progressive acquisition of neuromodulatory substances by a target network is associated with axonal ingrowth of descending neurons (Sako et al, 1986;van Mier et al, 1986;Pindzola et al, 1990).…”

“…Many of the projection neurons are well characterized in terms of electrical activity, neurotransmitter phenotype, soma location (CoG, OG, ivn, and lvn), morphology, and projection pattern (Table 1). In the embryo, recent immunocytochemical detection of neuromodulators suggests that projection inputs to the STG appear gradually throughout development, although some are present very early in the development (Cournil et al, 1995;Fenelon et al, 1998aFenelon et al, , 1999Kilman et al, 1999). Furthermore, electrophysiological studies indicate that some of these projection neurons are present and functional early in development (Le Feuvre et al, 1999).…”

“…This contrasts with previous demonstrations that neuromodulatory phenotypes are acquired gradually throughout development (Fenelon et al, 1998aKilman et al, 1999). Therefore, ontogenetic plasticity seems to depend on changes in transmitter phenotype expression within the same early established population of projection neurons rather than on a progressive acquisition of new projection fibers or neurons.…”

“…We have shown previously that among the earliest detectable modulatory substances in the embryo [FLRFamide, proctolin, and Red Pigment concentrating hormone (RPCH)], only FLRFamide is commonly expressed in the adult by identified neurons such as GPR cells , GN1/2 neurons (Meyrand et al, 2000), and PS neurons (Fenelon et al, 1998a). In contrast, although RPCH and proctolin immunoreactivity have been detected in the STG, so far the detection of these substances has not been coupled to electrophysiological identification of known projection neurons.…”