Serotonin induces or inhibits neuritic regeneration of leech CNS neurons depending on neuronal identity

Abstract

Recovery of motor function after central nervous system (CNS) injury is dependent on the regeneration capacity of the nervous system, which is a multifactorial process influenced, among other things, by the role of neuromodulators such as serotonin. The neurotransmitter serotonin can promote neuronal regeneration but there are also reports of it causing restriction, so it is important to clarify these divergent findings in order to understand the direct scope and side effects of potential pharmacological treatments. We evaluated the effect of serotonin on the extent of neuritic outgrowth and morphology of three different neuronal types in the leech Haementeria officinalis during their regeneration in vitro: Retzius interneurons (Rz), annulus erector (AE) motoneurons, and anterolateral number 1 (AL1) CNS neurons. Neurons were isolated and cultured in L15 medium, with or without serotonin. Growth parameters were registered and quantified, and observed differences were analyzed. The addition of serotonin was found to induce AL1 neurons to increase their average growth dramatically by 8.3-fold (P=0.02; n=5), and to have no clear effect on AE motoneurons (P=0.44; n=5). For Rz interneurons, which normally do not regenerate their neurites, the addition of concanavaline-A causes substantial growth, which serotonin was found to inhibit on average by 98% (P=0.02; n=5). The number of primary neurites and their branches were also affected. These results reveal that depending on the neuronal type, serotonin can promote, inhibit, or have no effect on neuronal regeneration. This suggests that after CNS injury, non-specific pharmacological treatments affecting serotonin may have different effects on different neuronal populations.

Comparative Aspects of Annelid Regeneration: Towards Understanding the Mechanisms of Regeneration

“…The repertoire of these molecules in annelids is poorly characterized, but some pioneering data suggest their relevance for regeneration [46,118,119], and so do the observations on the suppression of proliferation in a denervated wound [10,40,82]. Neurotransmitters are an important NS-derived factor, but their role in annelid regeneration remains almost unexplored [120,121]. The identification of specific neurotrophic factors is clearly one of the most promising directions for understanding the causality of annelid regeneration.…”

"5HT1A Receptor Transcripts are Found in Retzius Cells; Interneurons Inhibit Neuritic Regeneration by Serotonin"

“…In every ganglion, Rz interneurons harbor serotonin and control many features of the animal´s behavior; AE motor neurons control the circular muscle annuli of the body, and the AL1 neurons possess great neuritic regeneration capacity. These 3 neuronal types respond differently in culture to serotonin by extensively growing or inhibiting their growth according to their identity [19]. In this study, we cloned and designed probes for the 1A and 2 serotonin receptors, to hybridize their genes in the CNS ganglia; we demonstrated that RZ neurons possess transcripts for the serotonin 1A receptor.…”

“…The Phylum Annelida was used for this study: Haementeria officinalis is a Mexican leech from our Neuronal Regeneration Laboratory animal colony (Figure 1A). The procurement and maintenance of Haementeria officinalis samples have been previously described [19]. These organisms were originally collected in the lakes and dams of the central Mexican plateau The animals were anesthetized with 9% ethanol, for 20 minutes.…”

"Monoaminergic Neurotransmitters Promote the Percentage of Recovery of Swimming and Ambulation Behaviors following Central Nervous System Lesion in the Leech"

“…Haementeria officinalis is a Mexican leech from our Neuronal Regeneration Laboratory (NRL) animal colony. The procurement and maintenance of Haementeria officinalis samples has been previously described [26]. These organisms were originally collected in the lakes and dams of the central Mexican plateau…”

“…The addition of serotonin to their environmental water, triggers swimming in animals depleted of this neurotransmitter after 5, 7-dihydroxytryptamine administration [25]. Likewise, in culture, adding serotonin to specific leech neuron cultures induces neuritic regeneration, as in AL1 cells that increase their number of primary neurites, their length, and their ramification; however, in other cells such as Retzius cells, serotonin has an inhibitory effect [26].…”