Enkephalinergic control of the secretory activity of neurons producing stereoisomers of crustacean hyperglycemic hormone in the eyestalk of the crayfish Orconectes limosus

Abstract

A subgroup of neurons in the classical X organ sinus gland neuroendocrine system of the crayfish (Orconectes limosus) eyestalk produces two chiral forms of the crustacean hyperglycemic hormone (CHH) in two different types of neurons: CHH in 22 cells and D Phe(3) CHH in eight cells. Previous reports have demonstrated that release of CHH from the sinus gland is inhibited by enkephalins. Here, we have addressed the questions of 1) whether this inhibition affects one or both types of CHH neurons, 2) where the site of enkephalinergic control of CHH and/or D Phe(3) CHH is, and 3) whether the inhibitory effect is due to direct or indirect interactions of enkephalinergic neurons with CHH cells. In vitro incubations of neurosecretory complexes followed by immunoassays of CHH isoforms indicated that both methionine and leucine enkephalins inhibit release of the two CHH isoforms from crayfish eyestalks, by a receptor mediated process. Whole mount double or triple immunofluorescence labelings combined with confocal microscopy revealed enkephalin immunostaining in all neuropils of the eyestalk, except in the sinus gland. Virtual thin confocal sections showed many close appositions between terminals of enkephalinergic neurons and dendritic arborizations of specific CHH immunoreactive cells in the medulla terminalis neuropil. This provides the first evidence for direct inputs from enkephalinergic neurons into dendrites of both CHH cell types, which suggests that enkephalins inhibit release of both CHH isoforms via synaptic contacts.

Neuroanatomical distribution of angiotensin-II-like neuropeptide within the central nervous system of the crab Chasmagnathus; physiological changes triggered by water deprivation

“…The pattern of labelling for ANGII-ir in the nervous system of Chasmagnathus resembles that of some neuronal processes previously described for numerous neurotransmitters and neuropeptides in crustaceans, including various families, such as the enkephalins (Fingerman et al 1985), substance P (Sandeman et al 1990;Schmidt and Ache 1994;Schmidt 1997), crustacean-SIF amide (Polanska et al 2007), serotonin (Sandeman et al 1988) and CCH (Ollivaux et al 2002). In the lamina, medulla and lobula, we have found strong ANGII-ir in the tangential layers of the lamina and in some medullary centrifugal cells in the medulla and lobula.…”

Crustacean neuropeptides

“…While Carcinus maenas is the only crustacean from which enkephalins have been fully characterized, biochemical/ immunohistochemical data suggest they are broadly conserved in the taxon [129][130][131][132][133][134][135][136]; these data also suggest that the enkephalins function both as locally released autocrines/ paracrines and circulating hormones in crustaceans.…”

“…Several lines of evidence suggest that the modulatory activity of enkephalin on carbohydrate metabolism results from their involvement in the regulation of CHH release from the SG. For example, both d-and b-opioid receptors have been identified in the eyestalk ganglia of crustaceans [144], opioid-binding sites have been localized to CHH-containing terminals of the SG [132], the hypo-/hyperglycemic actions of enkephalins are absent in eyestalk-ablated animals [139][140][141][142][143], and the peptides have been shown/implicated in the inhibition of CHH release in animals exhibiting hypoglycemic responses [135,138]. Additionally, the enkephalins appear to play roles in the control of pigment granule migration in chromatophores, likely mediated via their regulation of release of other peptide hormones, e.g., PDH or RPCH [145][146][147], and they have been implicated in the modulatory control of both locomotion [146] and ovarian development [146,[148][149][150][151][152][153].…”

Circadian clocks in crustaceans: identified neuronal and cellular systems

“…Nonetheless, it would be interesting to examine whether or not NDH is similar or in fact also part of a large family of proven myoinhibitory and neuroinhibitory allatostatin-A-type neuropeptides in crayfish O. limosus (Orcostatins; 281) that occur identically or in many isoforms in several other crustaceans (208,(282)(283)(284). Another type of NDH may belong to a family of small inhibitory opioid neuropeptides, the enkephalins, which are known to occur in several neurons and fibre networks of the eyestalk ganglia including the medulla terminalis of lobsters (285), crabs (286)(287)(288), and crayfish O. limosus (289), and are co-localised with MIH in the crab XOSG-neurons (288). Identified authentic Leu-and Met-enkephalins are known to inhibit the secretory activity of crab and crayfish CHH-neurons and thereby hyperglycaemia, effects that can be antagonised by the opioidreceptor antagonist naloxone (289, 290).…”