The presence of enkephalin-like substances in the eyestalk and brain of the land crab Gecarcinus lateralis

Nociceptive Biology of Molluscs and Arthropods: Evolutionary Clues About Functions and Mechanisms Potentially Related to Pain

Important insights into the selection pressures and core molecular modules contributing to the evolution of pain-related processes have come from studies of nociceptive systems in several molluscan and arthropod species. These phyla, and the chordates that include humans, last shared a common ancestor approximately 550 million years ago. Since then, animals in these phyla have continued to be subject to traumatic injury, often from predators, which has led to similar adaptive behaviors (e.g., withdrawal, escape, recuperative behavior) and physiological responses to injury in each group. Comparisons across these taxa provide clues about the contributions of convergent evolution and of conservation of ancient adaptive mechanisms to general nociceptive and pain-related functions. Primary nociceptors have been investigated extensively in a few molluscan and arthropod species, with studies of long-lasting nociceptive sensitization in the gastropod, Aplysia, and the insect, Drosophila, being especially fruitful. In Aplysia, nociceptive sensitization has been investigated as a model for aversive memory and for hyperalgesia. Neuromodulator-induced, activity-dependent, and axotomy-induced plasticity mechanisms have been defined in synapses, cell bodies, and axons of Aplysia primary nociceptors. Studies of nociceptive sensitization in Drosophila larvae have revealed numerous molecular contributors in primary nociceptors and interacting cells. Interestingly, molecular contributors examined thus far in Aplysia and Drosophila are largely different, but both sets overlap extensively with those in mammalian pain-related pathways. In contrast to results from Aplysia and Drosophila, nociceptive sensitization examined in moth larvae (Manduca) disclosed central hyperactivity but no obvious peripheral sensitization of nociceptive responses. Squid (Doryteuthis) show injury-induced sensitization manifested as behavioral hypersensitivity to tactile and especially visual stimuli, and as hypersensitivity and spontaneous activity in nociceptor terminals. Temporary blockade of nociceptor activity during injury subsequently increased mortality when injured squid were exposed to fish predators, providing the first demonstration in any animal of the adaptiveness of nociceptive sensitization. Immediate responses to noxious stimulation and nociceptive sensitization have also been examined behaviorally and physiologically in a snail (Helix), octopus (Adopus), crayfish (Astacus), hermit crab (Pagurus), and shore crab (Hemigrapsus). Molluscs and arthropods have systems that suppress nociceptive responses, but whether opioid systems play antinociceptive roles in these phyla is uncertain.

Crustacean neuropeptides

Crustaceans have long been used for peptide research. For example, the process of neurosecretion was first formally demonstrated in the crustacean X-organ-sinus gland system, and the first fully characterized invertebrate neuropeptide was from a shrimp. Moreover, the crustacean stomatogastric and cardiac nervous systems have long served as models for understanding the general principles governing neural circuit functioning, including modulation by peptides. Here, we review the basic biology of crustacean neuropeptides, discuss methodologies currently driving their discovery, provide an overview of the known families, and summarize recent data on their control of physiology and behavior.

Delta and kappa opioid receptors in eyestalk ganglia of a crustacean

Crustacean eyestalk ganglia are part of the protocerebrum and have been demonstrated to produce numerous neurohormones. 3H(2-D-Pen, 5-D-Pen)-enkephalin, 3H-(-)-ethylketocyclazocine and 3H(D-Ala2-NMePhe-Glyol5)-enkephalin were used as ligands for opioid receptors on neuronal membrane preparations of eyestalk ganglia under consideration of their stereospecific binding properties. In context with saturation binding isotherms, association and dissociation plots, we demonstrate here two opioid receptors; a delta-type receptor with high affinity (Bmax 68.5 fmol/mg protein, Kd = 4.0 nM) and low affinity (Bmax 493 fmol/mg, Kd = 83.6 nM) and a second receptor of kappa-type with Bmax of 3.1 pmol/mg protein and Kd = 68.6 nM.

Isolation and structural characterization of enkephalins in the brain of the rhynchobdellid leech Theromyzon tessulatum

This paper reports the purification of four peptides related to enkephalins from the brain of the leech Theromyzon tessulatum. After reverse-phase HPLC purification, the sequence of the enkephalins (YGGFM, YGGFL, FM, FL) was established by a combination of automated Edman degradation, electrospray mass spectrometry measurement, and co-elution experiments in reverse-phase HPLC with synthetic peptides. ELISA titrations performed on each purified peptide indicated that the major amount was borne by the leucine-enkephalin. The ratio of leucine-enkephalin and methionine-enkephalin of 2:1 is in line with previous immunocytochemical data obtained on T. tessulatum brains. The presence of enkephalins in T. tessulatum, an animal belonging to the oldest group of coelomate metazoans (the Annelida) establishes the very ancient phylogenetic origin of opioids and their conservation in the course of evolution.

Autoradiographic localization of opioid binding sites combined with immunogold detection of Leu-enkephalin, crustacean hyperglycaemic hormone and moult inhibiting hormone at the electron microscopic level in the sinus gland of the shore crab, Carcinus maenas

Double labelling experiments were performed on the same tissue section at the electron microscopic level, in order to show the involvement of the opioid leucine-enkephalin (Leu-enk) in the modulation of crustacean hyperglycaemic hormone (CHH) mobilization. Both neuropeptides were stored in distinct axon terminals of the sinus gland of Carcinus maenas. A post-embedding immunogold cytochemical technique for Leu-enk, CHH and the CHH neurohormone related moult inhibiting hormone (MIH) was combined with a scintillator intensified autoradiographic method to demonstrate binding of the opioid antagonist [3H] naloxone. Ultrathin sections were successively incubated with antisera against Leu-enk, CHH or MIH, and the corresponding colloidal gold labelled antisera, followed by autoradiographic processing. At the ultrastructural level [3H] naloxone binding sites were easily recognized by their silver tracks after development. Opioid binding sites for [3H] naloxone were visualized only at membranes of CHH-containing axon terminals. These results provide morphological evidence for direct enkephalinergic control of CHH containing neurons in the sinus gland of C. maenas and are furthermore the first autoradiographic demonstration of opioid binding sites in the nervous system of invertebrates.

Isolation, sequence analysis, and physiological properties of enkephalins in the nervous tissue of the shore crab Carcinus maenas L

[Leu]- and [Met]enkephalin from thoracic ganglia of the shore crab Carcinus maenas have been purified to homogeneity by a reversed-phase HPLC procedure. Automated gas-phase sequencing revealed a primary structure identical with that of enkephalins from vertebrates. The immunoreactive material in different parts of the nervous system of C. maenas was measured by RIA. Isolated crustacean [Leu]enkephalin shows physiological properties identical to synthetic [Leu]enkephalin in our bioassay. The enkephalinergic effect has been monitored by two bioassays: (i) inhibition of release of crustacean hyperglycemic hormone from isolated eyestalks of C. maenas, and (ii) decreased effect on blood glucose level in the fiddler crab Uca pugilator. The decrease in blood sugar level induced by [Leu]enkephalin was antagonized by naloxone, indicating a receptor-mediated process. In sequence analysis, the thoracic ganglion yielded 3 pmol of [Leu]enkephalin and approximately 1 pmol of [Met]enkephalin. The presence of enkephalins in crustaceans shows an earlier phylogenetic origin of opioids than presumed.

Behavioral and electrophysiological effects of crustacean neurohormone on freely moving cats

The behavior of freely moving cats was assessed in an observation chamber during prolonged periods of time. Four patterns of behavior were consistently scored during the mid-day period: a) exploration, b) attention, c) grooming and d) drowsiness. Intracerebroventricular injections of crustacean neurodepressing hormone (NDH) greatly extended the time spent in drowsiness. The threshold dose of NDH for this effect was 300 units. The effect was established a few minutes after the injections and lasted for several hours. During this time the animals sat quietly and showed complete or semicomplete closure of the eyelids. Conspicuous changes in brain electrical activity were also observed under NDH. At low doses, the predominant electrophysiological pattern matches the activity recorded under spontaneous lapses of drowsiness, i.e., spindle bursts in trains of 8-16 Hz in cortical areas and mesencephalic reticular formation. At higher doses, the brain electrical activity changes into a nonconvulsive spiking activity in limbic areas. The time course of the effects differs in the various structures recorded. These results suggest a multiple substrate of NDH activity.

Evidence for dopaminergic and opioid involvement in the regulation of locomotor activity in the land crab Gecarcinus lateralis

1. Computerized analysis of the crabs locomotor behavior revealed an initial increase in activity followed by a gradual decrease over a 12 min observation period. 2. Dopamine, in a dose-dependent manner, inhibits locomotor activity. The effect can be antagonized with the dopamine antagonist, haloperidol. This suggests that dopaminergic influences are involved with locomotor mechanisms. 3. FK 33,824, a stable opioid analog, significantly enhances the initial excitatory locomotor activity. Naloxone, a potent opiate antagonist, can block the excitatory action induced by FK 33 824. This suggests the presence of an opioid modulation mechanism in the regulation of locomotor activity. 4. Concomitant administration of the various agents results in the behavioral characteristics of the agonist appearing when the appropriate antagonist is not present. Thus, administration of dopamine + FK 33,824 + haloperidol results in enhanced locomotor activity. 5. Concomitant dopamine and FK 33,824 administration results in enhanced locomotor activity. This suggests that the opioid mechanism is closer to the last step in affecting the organism's locomotion or in initiating activity.