Effect of age on acetylcholinesterase and other hemolymph proteins in Aplysia

An age-related decline in the cholinergic synaptic response may cause the firing pattern in the jaw-closing motor neurons, which resembles the aversive taste response in the feeding behavior of old Aplysia kurodai

Anorexia due to aging is recognized as a syndrome of animal feeding behavior. Age-related functional disorders of the brain often cause behavioral changes. We used Aplysia kurodai to study this neural mechanism, following our previous study on food preference behaviors. The age of each wild animal was defined by a previously described method, and a significant age-related decline in food intake was observed. In this study, we explored the effects of aging on a specific inhibitory synaptic response in jaw-closing (JC) motor neurons produced by cholinergic multiaction (MA) neurons, the size of which determines the delay between MA and JC firings and this delay is reduced during aversive taste responses; in our analyses, we found a significant age-related decline in the synaptic response. Thereafter, we further explored whether such functional decline affects the JC firing pattern during the normal feeding response. During the feeding-like rhythmic responses induced by electrical nerve stimulation, the firing of the JC motor neurons advanced toward that of the MA burst, which typically happens during aversive taste responses. These results suggest that the age-related decline in the cholinergic synaptic response may partly cause the JC firing patterns that resemble the aversive taste response in old animals.

Trace elements in wild edible Aplysia species: Relationship with the desaturation-elongation indexes of fatty acids

The profile of essential and non-essential elements was traced in the edible sea hares Aplysia depilans Gmelin, Aplysia fasciata Poiret and Aplysia punctata Cuvier. Manganese (Mn), iron (Fe), zinc (Zn), copper (Cu) and selenium (Se) were identified as the major essential elements. Risk assessment evidenced that the levels of cadmium (Cd) and lead (Pb) did not exceed the maximum limit value established by the European Regulation, the contents of chromium (Cr), nickel (Ni) and arsenic (As) being also below the levels established by the FDA guide. A correlation between trace elements levels and desaturation-elongation indexes of fatty acids was found. While Cd, Se and molybdenum (Mo) seem to promote the desaturation-elongation process involved on the production of C20:4n-6c, Ni, Cr and Fe may potentiate the conversion of C18:3n-3c to C20:5n-3c. Furthermore, cobalt (Co), Ni and Cu appear to decrease Δ9 desaturation index. Besides the suggested biosynthetic switch modulated by trace elements, the nutritional value of the species is further strengthened.

Effects of aging on the food intake in the feeding behavior of Aplysia kurodai

In wild Aplysia, the birthdate of animals can typically not be determined. Therefore, we sought a reliable index of old age by taking into consideration the distinguished Japanese seasons. Large amounts of eggs and dead bodies were present on the coast during and after the second half of May (MayS). Body mass decreased after May. We roughly classified animals collected before and after the MayS as mature and old animals. Plots of internalized shell length (S) against body mass (B) gave distinct best-fit curves for mature and old animals. The B/S significantly decreased in the second half of June, suggesting that body mass decreases with age but shell length is maintained in each animal. Therefore, the collected animals were classified into mature and old animals using the best-fit curves for animals classified by the collection period. We examined the amount of food intake every 2 h up to 8 h after providing food. The amounts increased linearly, and the rate was significantly lower in old animals than in mature animals. The amount of 1-day food intake was also significantly lower in old animals. These results suggest that food intake may decline with age and this may cause mass loss in old animals.

Regeneration of Aplysia bag cell neurons is synergistically enhanced by substrate-bound hemolymph proteins and laminin

We have investigated Aplysia hemolymph as a source of endogenous factors to promote regeneration of bag cell neurons. We describe a novel synergistic effect between substrate-bound hemolymph proteins and laminin. This combination increased outgrowth and branching relative to either laminin or hemolymph alone. Notably, the addition of hemolymph to laminin substrates accelerated growth cone migration rate over ten-fold. Our results indicate that the active factor is either a high molecular weight protein or protein complex and is not the respiratory protein hemocyanin. Substrate-bound factor(s) from central nervous system-conditioned media also had a synergistic effect with laminin, suggesting a possible cooperation between humoral proteins and nervous system extracellular matrix. Further molecular characterization of active factors and their cellular targets is warranted on account of the magnitude of the effects reported here and their potential relevance for nervous system repair.

Isolation and characterization of haemoporin, an abundant haemolymph protein from Aplysia californica

In the present study, we show the isolation and characterization of the protein haemoporin, which constitutes the second most abundant protein fraction in the haemolymph of the marine gastropod Aplysia californica. Although Aplysia is commonly used to investigate the molecular basis of learning, not much is known about the proteins in its haemolymph, which is in contact with the neurons owing to the open circulatory system of molluscs. In the native state, haemoporin is a macromolecular complex forming a cylinder with a central solvent-filled pore. The native complex most probably is a homopentamer made up from 70 kDa subunits with a molecular mass of 360 kDa and a sedimentation coefficient of 11.7 S. Prediction of the secondary structure by CD spectroscopy revealed that haemoporin contains 36% alpha-helices and 19% beta-strands. An absorption band in the 300-400 nm region indicates that haemoporin probably contains a bound substance. Haemoporin also contains a below average amount of tryptophan as evident from absorption and fluorescence spectra. The specific absorption coefficient at 280 nm (a (280 nm, 1 mg/ml)) varies between 0.42 and 0.59 l x g(-1) x cm(-1) depending on the method. The function of the protein is not yet known, but there are structural parallels between haemoporin and a pore protein reported previously in the haemolymph of another marine gastropod Megathura crenulata. The alanine-rich N-terminal sequence (AAVPEAAAEATAEAAPVSEF) is unique among protein sequences and indicates an alpha-helical structure. Whereas one side of the helix is hydrophobic and faces the interior of the protein, the other side contains a glutamic cluster, which may form the channel of the pore in the quaternary structure. Thus both proteins might belong to a new class of haemolymph proteins present in the haemolymph of marine gastropods.

Effects of organophosphates on cholinesterase activity and neurite regeneration in Aplysia

In Aplysia, a marine mollusc, acetylcholinesterase (AChE) is present in cholinergic and non-cholinergic neurons and in hemolymph. Aplysia hemolymph has a very high level of AChE which promotes neurite growth in primary cultures of dopaminergic neurons via a non-catalytic mechanism. In contrast, AChE is known to facilitate neurite growth in cholinoceptive neurons by hydrolyzing ACh which inhibits neurite growth. In order to test whether AChE's site-specific neurotrophic action varies with the neuronal phenotype, we investigated the effects of active-site inhibited hemolymph AChE on neurite growth of cholinergic neurons of Aplysia in primary culture. Organophosphates being long-acting active site inhibitors of AChE were chosen for this study. The effects of active site inhibited hemolymph AChE was tested on large cholinergic neurons, R2 (abdominal ganglion) and LPL1 (left pleural ganglion) as well as small cholinergic neurons (buccal ganglion) of Aplysia, maintained in culture. Partially purified hemolymph AChE was inhibited by either 10 microM of echothiophate or 5 microM of paraoxon. Neurons were maintained in (1) L15 (defined medium) alone; (2) L15 + echothiophate; (3) L-15 + paraoxon; (4) L-15 + hemolymph AChE; (5) L15 + hemolymph AChE + echothiophate; and (6) L-15 + hemolymph AChE + paraoxon. Addition of uninhibited hemolymph AChE significantly increased neurite growth of cultured neurons compared to L15 alone. In the presence of echothiophate-inhibited or praoxon-inhibited AChE, neurite growth was significantly reduced when compared to L15 + uninhibited AChE. While the presence of echothiophate by itself did not reduce survival or neurite growth when compared to L-15 alone, the presence of paraoxon by itself markedly reduced survival and neurite growth of cultured neurons. The results show that AChE's catalytic action contributes to enhance neurite growth in cholinergic neurons and the effects of paraoxon appears to differ from that of echothiophate on cholinergic neurons of Aplysia.

Acetylcholinesterase promotes regeneration of neurites in cultured adult neurons of Aplysia

Aplysia, a marine mollusc, has significant amounts of acetylcholinesterase in its hemolymph, reaching maximum levels in the adults with reproductive maturity [Srivatsan M., et al. (1992) J. comp. Physiol. 162, 29-37]. Since hemolymph of mature Aplysia is neurotrophic to Aplysia neurons in culture [Schacher S. and Proshanski E. (1983) J. Neurosci. 3, 2403-2413], we examined whether acetylcholinesterase is a hemolymph neurotrophic factor. Dopaminergic neurons from the pedal ganglia of young adult Aplysia were maintained in culture in defined medium or defined medium supplemented with hemolymph. After 24 h, neurons in defined medium supplemented with hemolymph were well attached to the substratum and exhibited multiple, long neurites. In contrast, neurons in defined medium alone attached poorly and exhibited one or two short neurites. When acetylcholinesterase was inhibited with a specific, membrane-impermeable inhibitor (1,5-bis(4-allyldimethylammoniumphenyl)-pentan-3-one dibromide) which binds to its catalytic and peripheral anionic sites, the neurotrophic effect of hemolymph was significantly reduced. However, inhibition of the catalytic site alone with membrane impermeable echothiophate still resulted in enhanced neurite growth. An analogue of acetylcholine, carbachol, which is not hydrolysed by acetylcholinesterase, did not interfere with neurite growth when added to the supplemented medium. Acetylcholinesterase isolated from the hemolymph and highly purified human acetylcholinesterase also promoted neurite growth in Aplysia neurons. These results show that i) acetylcholinesterase circulating in the hemolymph promotes neurite growth of adult neurons in culture; ii) the growth promoting action of acetylcholinesterase is independent of its function of hydrolysing acetylcholine and iii) the peripheral anionic site of acetylcholinesterase appears to be involved in neurite regeneration.

Serotonin levels as a function of age in Aplysia californica

The neurotransmitter serotonin (5-HT) plays an important role in a number of behaviors in Aplysia californica some of which have been shown to vary with age. We were thus interested in examining the age-dependence of 5-HT in A. californica. Because animals of the same age can have very different weights, and weight alone is reliably known for wild-caught animals, we also examined the variation of 5-HT with weight. Serotonin was measured in the ring and abdominal ganglia combined, in lab-reared animals from 3 to 12 months post-hatch across a wide weight range. Serotonin increased rapidly from 4 to 6 months, and more slowly from 6 to 13 months. Serotonin scaled by soluble ganglion protein increased from 3 to 6-7 months, reached a maximum, and then decreased again. Serotonin, but not scaled 5-HT, increased significantly with weight across the whole weight range. Animals of the same weight, but different ages, had different 5-HT levels, as did young animals of the same age but different weight. Serotonin varied significantly with both age and weight, with the age-dependence being the more significant.

Chronic stimulation increases acetylcholinesterase activity in old Aplysia

In the marine mollusc Aplysia, a reduced level of activity of circulating AChE (acetylcholinesterase) signals the onset of aging [28], as it does in mammals [23,25]. In old Aplysia, coincident with the reduced AChE activity is impaired neuron function [17], which chronically applied sensory stimulation (CSS) improves [35]. As a first step to establish the link between the CSS and improved neuronal function, we investigated if CSS alters the level of AChE activity in old Aplysia. Before and after 4 weeks of CSS of the siphon-gill withdrawal reflex (S/GWR), we measured circulating and neural levels of AChE and behaviors involving the gill in freely moving mature and old Aplysia. Only in old animals did the CSS produce increased AChE activity levels in both the CNS and serum, and the increased levels were correlates of a change in the S/GWR, the behavior elicited by the CSS. This result shows that aging animals are able to up regulate enzymatic activity in response to specific sensory input. It also suggests that age influences how the level of AChE activity responds to persistent changes in sensory input. Parallels exist between the results here and those in higher vertebrates and are discussed.

Effect of acetylcholinesterase inhibition on behavior is age-dependent in freely moving Aplysia

The significance of age-dependent changes in acetylcholinesterase (AChE) activity level is poorly understood. Reported here is one approach to understand AChE's function as it relates to age: to investigate how inhibition of AChE affects behavior in freely moving Aplysia of two age groups, mature and old. The siphon/gill withdrawal reflex (S/GWR) and gill pumping movement (GPM) were examined to assay the effects of AChE inhibition by BW284c51, a specific and reversible AChE inhibitor. In mature Aplysia AChE inhibition by 2 microM and 5 microM of BW284c51 resulted in a significantly shortened S/GWR duration, and in suppression of habituation and dishabituation. In old animals, AChE inhibition by 2 microM of BW284c51 did not affect S/GWR and only dishabituation was suppressed by inhibition by 5 microM of BW284c51. AChE inhibition reduced the GPM rate significantly only in mature animals. AChE inhibition did not alter the decrement in GPM rate which is regularly observed in both age groups during repetitive exposure to acidified sea water. Thus both S/GWR and GPM were affected by AChE inhibition, and a significant age effect on the two behaviors was observed. Comparisons of the results of AChE inhibition which would elevate acetylcholine (ACh) levels with those of carbachol administration revealed that AChE inhibition affects both cholinergic and non-cholinergic mechanisms underlying the two behaviors.