Invertebrate Neuropeptides

Abstract

Invertebrate neuropeptides are found in neurosecretory cells, interneurons and motor neurons, as well as in endocrine cells of the intestine and in glandular cells of nonnervous tissue. Neuropeptides can act as neurohormones released into the circulation and/or as local neurohormones released in a paracrine fashion within the nervous system or at muscles or glands.

Keywords: neurohormone; neuromodulator; neurotransmitter; peptide receptor; peptide evolution; prohormone processing; neuropeptide genes

Figure 1.

A generalized outline of known regulatory steps underlying sequential initiation of preecdysis and ecdysis behaviours in insects. Twenty hydroxyecdysone (20HE) steroid levels fall in the haemolymph, which triggers activation of corazonin neurons in the CNS (possibly via other unknown upstream activators (???); 20HE may also act directly at other levels in this circuit. Cor initiates limited ETH release from peripheral Inka cells. ETH release prompts initial display of preecdysis (PE) behaviour and release of EH from central Vm cells. ETH and EH releases are under strong positive feedback regulation leading to massive release of both. Both peptides prompt CCAP release from central neurons; CCAP inhibits PE behaviour and together with ETH helps trigger ecdysis (E) behaviour. EH also acts on some aspect of ETH action (dashed arrow) as it is required for Drosophila to respond to ETH injections with premature ecdysis behaviour. Adapted from Kim YJ, Spalovska‐Valachova II, Cho KH, et al. (2004) Corazonin receptor signaling in ecdysis initiation. Proceedings of the National Academy of Sciences of the USA101: 6704–6709 and Ewer and Reynolds .

Figure 2.

Concerted actions of the neuropeptide proctolin, tachykinin‐related peptide (CabTRP) and the inhibitory transmitter GABA in different neurons of the crab stomatogastric system. The distinct stomatogastric ganglion motor patterns elicited by the three proctolin containing projection neurons (MCN1, MCN7 and MPN) are shown. This includes a summary of their transmitter content and additional synaptic actions by which they elicit the indicated stomatogastric ganglion (STG) motor patterns. These additional actions include: (1) MCN1‐presynaptic inhibition of MCN1 by the LG neuron in the STG and electrical coupling to the LG neuron; (2) MCN7‐strong excitation of the IC neuron; (3) MPN‐synaptic inhibition of projection neurons in the commissural ganglia. Lower panels: rhythmic impulse bursts in STG neurons are represented by labelled boxes. Abbreviations: Nerves: pdn, pyloric dilator nerve; mvn, medial ventricular nerve; lgn, lateral gastric nerve; dgn, dorsal gastric nerve; Neurons: PD, pyloric dilator neuron; IC, inferior cardiac neuron; VD, ventricular dilator neuron; LG, lateral gastric neuron; DG, dorsal gastric neuron. Legend: t‐bar, transmitter‐mediated excitation; filled circle, transmitter mediated inhibition; resistor, electrical coupling. Modified from Nusbaum et al..

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Nässel, Dick R, and Taghert, Paul H(Jan 2006) Invertebrate Neuropeptides. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0004080]