Serotonin and Synaptic Transmission at Invertebrate Neuromuscular Junctions

Abstract

The serotonergic system in vertebrates and invertebrates has been a focus for over 50 years and will likely continue in the future. Recently, genomic analysis and discovery of alternative splicing and differential expression in tissues have increased the knowledge of serotonin (5-HT) receptor types. Comparative studies can provide useful insights to the wide variety of mechanistic actions of 5-HT responsible for behaviors regulated or modified by 5-HT. To determine cellular responses and influences on neural systems as well as the efferent control of behaviors by the motor units, preparations amenable to detailed studies of synapses are beneficial as working models. The invertebrate neuromuscular junctions (NMJs) offer some unique advantages for such investigations; action of 5-HT at crustacean NMJs has been widely studied, and leech and Aplysia continue to be key organisms. However, there are few studies in insects likely due to the focus in modulation within the CNS and lack of evidence of substantial action of 5-HT at the Drosophila NMJs. There are only a few reports in gastropods and annelids as well as other invertebrates. In this review we highlight some of the key findings of 5-HT actions and receptor types associated at NMJs in a variety of invertebrate preparations in hopes that future studies will build on this knowledge base.

Serotonin in Animal Cognition and Behavior

“…In the larval stages, 5-HT is involved in turning behavior [79], whereas in adult flies, acute activation of 5-HT neurons disrupts normal locomotor activity [64]. In light of the absence of 5-HT at the neuromuscular junction [80], such an effect implies an important regulatory role of the central 5-HT systems over the motor programs. This has been further evaluated by using mutations in the 5-HT biosynthetic enzymes and by knocking down 5-HTRs expressed in brain mushroom bodies (MBs) (d5-HT 1A , d5-HT 1B , d5-HT 2 , and d5-HT 7 ) using RNAi strategy.…”

Extensive projections of myenteric serotonergic neurons suggest they comprise the central processing unit in the colon

“…5‐Hydroxytryptamine (5‐HT, serotonin) is a widespread biogenic amine found in vertebrates, invertebrates, and plants that has been the focus of research for over 50 years . Most of the body's 5‐HT (~95%) is in the GI tract, where it is synthesized in enterochromaffin cells (ECC) and in ~1% of myenteric interneurons .…”

Serotonergic regulation of the buccal (feeding) rhythm of the pond snail,Lymnaea stagnalis. An immunocytochemical, biochemical and pharmacological approach

“…Among the regulatory substances serotonin (5-HT) has been considered as one of the major neuromodulators in invertebrates. Its role in a number of physiological and behavioral processes has well been established [1,10,19,23,35,61], including a pivotal role in the regulation of the feeding behavior of gastropods [8,18]. In connection with a diverse set of feeding functions the role of 5-HT was addressed including the sensory initiation of feeding [41], central motor pattern generation [11,12,49], as well as the cellular mechanisms underlying different forms of feeding related behavior, such as arousal [29], satiety, [30] and learning [3,26].…”