In vitro evidence and age-related changes for nicotinic but not muscarinic acetylcholine receptors in the central nervous system of Sepia officinalis

Structural aspects of the aging invertebrate brain

Aging is characterized by a decline in neuronal function in all animal species investigated so far. Functional changes are accompanied by and may be in part caused by, structurally visible degenerative changes in neurons. In the mammalian brain, normal aging shows abnormalities in dendrites and axons, as well as ultrastructural changes in synapses, rather than global neuron loss. The analysis of the structural features of aging neurons, as well as their causal link to molecular mechanisms on the one hand, and the functional decline on the other hand is crucial in order to understand the aging process in the brain. Invertebrate model organisms like Drosophila and C. elegans offer the opportunity to apply a forward genetic approach to the analysis of aging. In the present review, we aim to summarize findings concerning abnormalities in morphology and ultrastructure in invertebrate brains during normal aging and compare them to what is known for the mammalian brain. It becomes clear that despite of their considerably shorter life span, invertebrates display several age-related changes very similar to the mammalian condition, including the retraction of dendritic and axonal branches at specific locations, changes in synaptic density and increased accumulation of presynaptic protein complexes. We anticipate that continued research efforts in invertebrate systems will significantly contribute to reveal (and possibly manipulate) the molecular/cellular pathways leading to neuronal aging in the mammalian brain.

The Gastric Ganglion of Octopus vulgaris: Preliminary Characterization of Gene- and Putative Neurochemical-Complexity, and the Effect of Aggregata octopiana Digestive Tract Infection on Gene Expression

The gastric ganglion is the largest visceral ganglion in cephalopods. It is connected to the brain and is implicated in regulation of digestive tract functions. Here we have investigated the neurochemical complexity (through in silico gene expression analysis and immunohistochemistry) of the gastric ganglion in Octopus vulgaris and tested whether the expression of a selected number of genes was influenced by the magnitude of digestive tract parasitic infection by Aggregata octopiana. Novel evidence was obtained for putative peptide and non-peptide neurotransmitters in the gastric ganglion: cephalotocin, corticotrophin releasing factor, FMRFamide, gamma amino butyric acid, 5-hydroxytryptamine, molluscan insulin-related peptide 3, peptide PRQFV-amide, and tachykinin-related peptide. Receptors for cholecystokininA and cholecystokininB, and orexin2 were also identified in this context for the first time. We report evidence for acetylcholine, dopamine, noradrenaline, octopamine, small cardioactive peptide related peptide, and receptors for cephalotocin and octopressin, confirming previous publications. The effects of Aggregata observed here extend those previously described by showing effects on the gastric ganglion; in animals with a higher level of infection, genes implicated in inflammation (NFκB, fascin, serpinB10 and the toll-like 3 receptor) increased their relative expression, but TNF-α gene expression was lower as was expression of other genes implicated in oxidative stress (i.e., superoxide dismutase, peroxiredoxin 6, and glutathione peroxidase). Elevated Aggregata levels in the octopuses corresponded to an increase in the expression of the cholecystokininA receptor and the small cardioactive peptide-related peptide. In contrast, we observed decreased relative expression of cephalotocin, dopamine β-hydroxylase, peptide PRQFV-amide, and tachykinin-related peptide genes. A discussion is provided on (i) potential roles of the various molecules in food intake regulation and digestive tract motility control and (ii) the difference in relative gene expression in the gastric ganglion in octopus with relatively high and low parasitic loads and the similarities to changes in the enteric innervation of mammals with digestive tract parasites. Our results provide additional data to the described neurochemical complexity of O. vulgaris gastric ganglion.

Nicotinic acetylcholine receptors containing the α7-like subunit mediate contractions of muscles responsible for space positioning of the snail, Helix pomatia L. tentacle

Three recently discovered tentacle muscles are crucial to perform patterned movements of upper tentacles of the terrestrial snail, Helix pomatia. The muscles receive central and peripheral excitatory cholinergic innervation lacking inhibitory innervation. Here, we investigate the pharmacology of acetylcholine (ACh) responses in muscles to determine the properties of the ACh receptor (AChR), the functional availability of which was assessed using isotonic contraction measurement. Using broad spectrum of nicotinic and muscarinic ligands, we provide the evidence that contractions in the muscles are attributable to the activation of nAChRs that contain the α7-like subunit. Contractions could be evoked by nicotine, carbachol, succinylchloride, TMA, the selective α7-nAChR agonist choline chloride, 3-Bromocytisine and PNU-282987, and blocked by nAChR selective antagonists such as mytolon, hexamethonium, succinylchloride, d-tubocurarine, hemicholinium, DMDA (decamethonium), methyllycaconitine, α-Bungarotoxin (αBgTx) and α-Conotoxin IMI. The specific muscarinic agonist oxotremorine and arecoline failed to elicit contractions. Based on these pharmacological properties we conclude that the Na⁺ and Ca²⁺ permeable AChRs of the flexor muscle are nicotinic receptors that contain the α7-like subunit. Immunodetection experiments confirmed the presence of α7- or α7-like AChRs in muscle cells, and α4-AChRs in nerves innervating the muscle. These results support the conclusion that the slowly desensitizing αBgTx-sensitive responses obtained from flexor muscles are produced by activation of α7- like AChRs. This is the first demonstration of postsynaptic expression and an obligatory role for a functional α7-like nAChR in the molluscan periphery.

Immunohistochemical localization of two types of choline acetyltransferase in neurons and sensory cells of the octopus arm

Cholinergic structures in the arm of the cephalopod Octopus vulgaris were studied by immunohistochemistry using specific antisera for two types (common and peripheral) of acetylcholine synthetic enzyme choline acetyltransferase (ChAT): antiserum raised against the rat common type ChAT (cChAT), which is cross-reactive with molluscan cChAT, and antiserum raised against the rat peripheral type ChAT (pChAT), which has been used to delineate peripheral cholinergic structures in vertebrates, but not previously in invertebrates. Western blot analysis of octopus extracts revealed a single pChAT-positive band, suggesting that pChAT antiserum is cross-reactive with an octopus counterpart of rat pChAT. In immunohistochemistry, only neuronal structures of the octopus arm were stained by cChAT and pChAT antisera, although the pattern of distribution clearly differed between the two antisera. cChAT-positive varicose nerve fibers were observed in both the cerebrobrachial tract and neuropil of the axial nerve cord, while pChAT-positive varicose fibers were detected only in the neuropil of the axial nerve cord. After epitope retrieval, pChAT-positive neuronal cells and their processes became visible in all ganglia of the arm, including the axial and intramuscular nerve cords, and in ganglia of suckers. Moreover, pChAT-positive structures also became detectable in nerve fibers connecting the different ganglia, in smooth nerve fibers among muscle layers and dermal connective tissues, and in sensory cells of the suckers. These results suggest that the octopus arm has two types of cholinergic nerves: cChAT-positive nerves from brain ganglia and pChAT-positive nerves that are intrinsic to the arm.

Pre- and postsynaptic excitation and inhibition at octopus optic lobe photoreceptor terminals; implications for the function of the 'presynaptic bags'

Synaptic transmission was examined in the plexiform zone of Octopus vulgaris optic lobes using field-potential recording from optic lobe slices. Stimulation of the optic nerve produced pre- and postsynaptic field potentials. Transmission was abolished in calcium-free seawater, L- glutamate or the AMPA/Kainate receptor blocker CNQX (EC(50), 40 microm), leaving an intact presynaptic field potential. ACh markedly reduced or blocked and d-tubocurarine augmented both pre- and postsynaptic field potentials, while alpha-bungarotoxin and atropine were without effect. Paired-pulse stimulation showed short-term depression of pre- and postsynaptic components with a half-time of recovery of approximately 500 ms. The depression was partially relieved in the presence of d-tubocurarine (half-time of recovery, 350 ms). No long-term changes in synaptic strength were induced by repetitive stimulation. A polyclonal antibody raised against a squid glutamate receptor produced positive staining in the third radial layer of the plexiform zone. No positive staining was observed in the other layers. Taking into account previous morphological data and our results, we propose that the excitatory terminations of the photoreceptors are in the innermost layer of the plexiform zone where the transmitter is likely to be glutamate and postsynaptic receptors are AMPA/kainate-like. Thus, the function of the terminal bags is to provide a location for a presynaptic cholinergic inhibitory shunt. The results imply that this arrangement provides a temporal filter for visual processing and enhances the perception of moving vs. stationary objects.

Comparative effects of aluminum and ouabain on synaptosomal choline uptake, acetylcholine release and (Na+/K+)ATPase

Closing the gap between adverse health effects of aluminum and its mechanisms of action still represents a huge challenge. Cholinergic dysfunction has been implicated in neuronal injury induced by aluminum. Previously reported data also indicate that in vivo and in vitro exposure to aluminum inhibits the mammalian (Na(+)/K(+))ATPase, an ubiquitous plasma membrane pump. This study was undertaken with the specific aim of determining whether in vitro exposure to AlCl(3) and ouabain, the foremost utilized selective inhibitor of (Na(+)/K(+))ATPase, induce similar functional modifications of cholinergic presynaptic nerve terminals, by comparing their effects on choline uptake, acetylcholine release and (Na(+)/K(+))ATPase activity, on subcellular fractions enriched in synaptic nerve endings isolated from rat brain, cuttlefish optic lobe and torpedo electric organ. Results obtained show that choline uptake by rat synaptosomes was inhibited by submillimolar AlCl(3), whereas the amount of choline taken up by synaptosomes isolated from cuttlefish and torpedo remained unchanged. Conversely, choline uptake was reduced by ouabain to a large extent in all synaptosomal preparations analyzed. In contrast to ouabain, which modified the K(+) depolarization evoked release of acetylcholine by rat, cuttlefish and torpedo synaptosomal fractions, AlCl(3) induced reduction of stimulated acetylcholine release was only observed when rat synaptosomes were challenged. Finally, it was observed that the aluminum effect on cuttlefish and torpedo synaptosomal (Na(+)/K(+))ATPase activity was slight when compared to its inhibitory action on mammalian (Na(+)/K(+))ATPase. In conclusion, inhibition of (Na(+)/K(+))ATPase by AlCl(3) and ouabain jeopardized the high-affinity (Na(+)-dependent, hemicholinium-3 sensitive) uptake of choline and the Ca(2+)-dependent, K(+) depolarization evoked release of acetylcholine by rat, cuttlefish and torpedo synaptosomal fractions. The effects of submillimolar AlCl(3) on choline uptake and acetylcholine release only resembled those of ouabain when rat synaptosomes were assayed. Therefore, important differences were found between the species regarding the cholinotoxic action of aluminum. The variability of (Na(+)/K(+))ATPase sensitivity to aluminum of cholinergic neurons might contribute to their differential susceptibility to this neurotoxic agent.