Metabolism of acetylcholine in the nervous system of Aplysia californica. III. Studies of an indentified cholinergic neuron

Do different neurons age differently? Direct genome-wide analysis of aging in single identified cholinergic neurons

Aplysia californica is a powerful experimental system to study the entire scope of genomic and epigenomic regulation at the resolution of single functionally characterized neurons and is an emerging model in the neurobiology of aging. First, we have identified and cloned a number of evolutionarily conserved genes that are age-related, including components of apoptosis and chromatin remodeling. Second, we performed gene expression profiling of different identified cholinergic neurons between young and aged animals. Our initial analysis indicates that two cholinergic neurons (R2 and LPl1) revealed highly differential genome-wide changes following aging suggesting that on the molecular scale different neurons indeed age differently. Each of the neurons tested has a unique subset of genes differentially expressed in older animals, and the majority of differently expressed genes (including those related to apoptosis and Alzheimer's disease) are found in aging neurons of one but not another type. The performed analysis allows us to implicate (i) cell specific changes in histones, (ii) DNA methylation and (iii) regional relocation of RNAs as key processes underlying age-related changes in neuronal functions and synaptic plasticity. These mechanisms can fine-tune the dynamics of long-term chromatin remodeling, or control weakening and the loss of synaptic connections in aging. At the same time our genomic tests revealed evolutionarily conserved gene clusters associated with aging (e.g., apoptosis-, telomere- and redox-dependent processes, insulin and estrogen signaling and water channels).

Ion and transmitter movements during spreading cortical depression

A reaction--diffusion system of equations whose components are the extracellular concentrations of K+, Ca++, Na+, Cl-, an excitatory neurotransmitter and an inhibitory neurotransmitter, is developed in order to model the movements of these substances through various kinds of membrane in brain structures. Expressions are derived from probabilistic arguments for the conductances induced in subsynaptic membrane by transmitter substances at various concentrations, for one-way active transport rates and for an exchange pump rate. These expressions are employed in the reaction terms of the system. The meaning of the many constants is explained and, with appropriate choices for their values, the model predicts subthreshold responses to small enough local elevations of KCl or glutamate and stable propagating SD waves if the local elevations of these chemicals is sufficient. The SD waves consist of elevated K+ and transmitter substances and diminished Ca++, Na+ and Cl-, the velocity of propagation in cortex being about 0.6 mm/min. This is in the experimental range, the K+-amplitude being 17 mM relative to a baseline of 3 mM, as the model developed ignores the effects of action potentials. There is no SD response to either NaCl or GABA. The effects of no K+ and no glutamate diffusion are investigated, both being manifest as a failure in propagation of the stable SD waves. The wave solutions are analysed in terms of phase portraits. The roles of various amino acid uptake and release processes by neurons and glia are discussed, as are the complications with regard to their incorporation in a model for SD. The roles of neurons and glia are analysed and the six basic fluxes of K+ are outlined. It is postulated that under some circumstances, in cortex treated with TTX, there may be practically no transmitter release, but SD may propagate if TTX does not completely abolish gNa for nonsynaptic membrane, a corresponding system of model equations being developed. The data and ideas of K+-based and glutamate-based SD of Van Harreveld are discussed and interpreted in terms of which reaction terms are operative in the K+ equation. An appendix contains the values of the parameters used in the numerical calculations.

Fast axonal transport of foreign transmitters in an identified serotonergic neurone of Aplysia californica

1. Radioactively labelled compounds, several of which are neurotransmitters, were injected with pressure into the soma of the serotonergic giant cerebral neurone (g.c.n.) of Aplysia californica. The compounds injected were [3H]dopamine, [3H]DL-octopamine, [3H]histamine, [3H]gamma-aminobutyric acid and [3H]choline. 2. Substantial amounts of radioactivity appeared in the axons of the g.c.n. with all of the injected compounds. Except for [3H]choline, the amounts were similar to the amount appearing when [3H]serotonin is injected. 3. The biogenic amines, [3H]dopamine, DL-[3H]octopamine and [3H]histamine, all moved in the axon at velocities similar to that of [3H]serotonin. In contrast, the radioactivity in axons of cells injected with [3H]gamma-aminobutyric acid and [3H]choline moved much more slowly. In addition, the shapes of the spatial distributions of radioactivity in the axons of cells injected with the biogenic amines resembled that obtained when [3H]serotonin is injected. This distribution is characteristic of fast axonal transport. The spatial distributions of radioactivity in the axons of cells injected with [3H]gamma-aminobutyric acid and [3H]choline were markedly different. We thus conclude that [3H]dopamine, DL-[3H]octopamine and [3H]histamine move by fast axonal transport, whereas the radioactivity in the axons of cells injected with [3H]gamma-aminobutyric acid and [3H]choline does not. 4. Injection of large amounts of dopamine and octopamine reduced the export into the axon of [3H]serotonin injected into the same cell. Large amounts of choline did not reduce export of [3H]serotonin. We conclude that the biogenic amines compete with serotonin for the vesicular storage site and that they move by fast transport because they are sequestered by the serotonergic storage vesicle. 5. The specificity of uptake into the storage vesicle as assayed with this in vivo system is similar to the specificity of uptake into aminergic vesicles as previously studied in vitro.

Intra-axonal diffusion of [3H]acetylcholine and [3H]gamma-aminobutyric acid in a neurone of Aplysia

1. [3H]acetylcholine (ACh) or [3H]gamma-aminobutyric acid (GABA) was injected into the cell body of a cholinergic neurone of Aplysia kurodai. 2. [3H]ACh moved down the axon at a maximum speed of 2.5 mm/sec at 20 degrees C. 3. 20 mM-colchicine suppressed this movement, but some passive movement of radioactivity was noted along the axon. 4. Profiles of the passive movement coincided with theoretically obtained diffusion profiles. 5. The diffusion coefficient of ACh in the axoplasm was estimated. It was 3 x 10(-6) cm2/sec at 5 degrees C, 4 X 10(-6) cm2/sec at 15 degrees C and 6.5 x 10(-6) cm2/sec at 30 degrees C. The Q10 was 1.35, the activation energy was about 5 kcal/degrees C. These diffusion coefficients were lower than that of free diffusion of ACh (8 x 10(-6) cm2/sec at 18 degrees C, Fatt, 1954), and assumed to be reasonable, if one considers frictional resistivity of axoplasm in the diffusion of ACh molecules. 6. [3H]GABA diffused similarly to ACh, and the diffusion coefficients agreed with the estimated value when the molecular size differences were taken into account. 7. Both ACh and GABA seemed to diffuse in soluble form or as single molecules in the axoplasm. 8. Intra-axonal diffusion is very effective for short distances.

Transmitter release: ruthenium red used to demonstrate a possible role of sialic acid containing substrates

1. The possible function of sialic acid-containing substrates (SACS) in synaptic terminals of Aplysia was studied by intracellular injection of ruthenium red and of neuraminidase. 2. Ruthenium red, a dye known to have sialic acid as a molecular target, blocked transmission irreversibly in both cholinergic (buccal ganglion) and non-cholinergic (cerebral ganglion) synapses. 3. An intracellular site of action is likely because much less ruthenium red was necessary to block transmission when it was injected intracellularly than when it was presented by bath perfusion. 4. Ca2+ spikes recorded in the presence of tetrodotoxin or in Na+-free solution were not modified by ruthenium red or neuraminidase injections or perfusions. It is therefore improbable that these substances blocked transmission by blocking voltage-dependent Ca2+ influx. 5. Strong electrotonic depolarization of a pre-synaptic interneurone in the presence of 10(-4) M-tetrodotoxin caused a sustained post-synaptic response, which was abolished by ruthenium red. This result eliminates axonal conduction block as the principal mechanism of ruthenium red action. 6. Post-synaptic responses to ionophoretically applied acetylcholine (ACh) were not modified by bath perfusion of 2 x 10(-2) M-ruthenium red. 7. Biochemical analysis of pools of [3H]ACh was performed after injection of a precursor, [3H]acetate, into an identified interneurone. Ruthenium red appeared to increase significantly the 'free' (cytoplasmic) ACh pool without any change of 'bound' (vesicular) [3H]ACh-pool. 8. A model is proposed in which SACS act as intracellular Ca2+ receptors involved in transmitter release.

In vivo turnover rate of acetylcholine in rat brain parts at elevated steady-state concentration of plasma choline

Cortical and striatal turnover rates of acetylcholine TR(ACh) were estimated by applying steady-state tracer kinetics in rats killed by microwave irradiation following a constant i.v. infusion of 3H-Ch. In control rats TR(ACh) was 3.6 nmol . g-1 . min-1 in the cortex and 23.8 nmol . g-1 . min-1 in the striatum. When steady-state plasma concentrations of Ch were increased from 17 to 140 mumol . 1-1 by a 15-min infusion of unlabelled Ch the corresponding TR(ACh) were 3.6 nmol . g-1 . min-1 and 21.4 nmol . g-1 . min-1, respectively. These results indicate that increased plasma levels of Ch are not accompanied by increased synthesis of brain ACh.

Metabolism of acetylcholine in the nervous system of Aplysia californica. IV. Studies of an identified cholinergic axon

[3H]Choline, injected directly into the major axon of the identified cholinergic neuron R2, was readily incorporated into [3H]acetylcholine. Its metabolic fate was similar to that of [3H]choline injected into the cell body of R2. Over the range injected, we found that the amounts of acetylcholine formed were proportional to the amounts injected; the synthetic capability was not exceeded even when 88 pmol of [3H]choline were injected into the axon. Newly synthesized acetylcholine moved within the axon with the kinetics expected of diffusion. We could not detect any selective orthograde or retrograde transport from the site of the injection. In contrast, as indicated by experiments with colchicine, 30% of the [3H]acetylcholine formed after intrasomatic injection was selectively exported from the cell body and transported along the axon. Most of the [3H]acetylcholine was recovered in the soluble fraction after both intra-axonal and intrasomatic injection of [3H]choline; only a small fraction was particulate. The significance of large amounts of soluble acetylcholine in R2 is uncertain, and some may occur physiologically. The concentrations of choline introduced by intraneuronal injection into both cell body and axon were, however, greater than those normally available to choline acetyltransferase in the cholinergic neuron; nevertheless, these large concentrations were efficiently converted into the transmitter. The synthetic capacity of the neuron supplied with injected choline may exceed the capacity of storage vesicles and of the axonal transport process.

Axonal transport of [3H]serotonin in an identified neuron of Aplysia californica

A population of characteristic ellipsoidal dense-core vesicles was identified in axons of the giant cerebral neuron of the mollusc Aplysia. We injected [3H]serotonin into the cell body of this identified serotonergic neuron in the isolated central nervous system in order to study the subcellular components associated with the neurotransmitter. Subcellular fractionation by differential centrifugation indicated that injected serotonin was rapidly taken up into particulate form. [3H]Serotonin appeared in the axons within 2 h after injection, and export continued at a constant rate of 6% of the total in the neuron/h thereafter. The dependence of the total amounts of [3H]serotonin which appeared in the axons in 6 h (export from the cell body) on the amounts injected was consistent with the idea that export is a saturable process, possibly depending on the capacity of somatic vesicles or of some unidentified carrier for serotonin. [3H]Serotonin moved into both major branches of the axon, where it was translocated rapidly. The transmitter, which was shown by autoradiography to be restricted to the axons of the injected cell, was distributed along axons in accumulations of radioactivity; in contrast, its precursor, [5-3H]hydroxytryptophan, moved slowly along axons in a smooth, declining curve, its kinetics consistent with diffusion. Quantitative electron microscope autoradiography revealed that the dense-core vesicles and the cytosol of axons fixed with glutaraldehyde were labeled with [3H]serotonin.

Metabolism of acetylcholine in the nervous system of Aplysia californica. I. Source of choline and its uptake by intact nervous tissue

Although acetylcholine is a major neurotransmitter in Aplysia, labeling studies with methionine and serine showed that little choline was synthesized by nervous tissue and indicated that the choline required for the synthesis of acetylcholine must be derived exogenously. Aanglia in the central nervous system (abdominal, cerebral, and pleuropedals) all took up about 0.5 nmol of choline per hour at 9 muM, the concentration of choline we found in hemolymph. This rate was more than two orders of magnitude greater than that of synthesis from the labeled precursors. Ganglia accumulated choline by a process which has two kinetic components, one with a Michaelis constant between 2-8 muM. The other component was not saturated at 420 muM. Presumably the process with the high affinity functions to supply choline for synthesis of transmitter, since the efficiency of conversion to acetylcholine was maximal in the range of external concentrations found in hemolymph.

Metabolism of acetylcholine in the nervous system of Aplysia californica. II. Reginal localization and characterization of choline uptake

The choline required for synthesis of acetylcholine is derived exogenously by Aplysia ganglia. Under physiological conditions choline was taken up primarlily by neuropile and nerves and not by cholinergic cell bodies. In addition, compared with their contents of choline acetyltransferase, those components of nervous tissue which contain nerve terminals and axons synthesized acetylcholine far more efficiently. Choline was accumulated by high and low affinity uptake processes; the high affinity process appeared to be characteristic of cholinergic nuerons (Swartz, J. H., M. L. Eisenstadt, and H. Cedar.1975. J. Gen. Physiol. 65:255). The two uptake processes were similarly affected by temperature with a Q10 of 2.8. Both were dependent on a variety of ions in a complicated manner. High affinity uptake seemed to be more dependent on Na+, showed greater inhibition by ouabain, and was selectively inhibited by oxotremorine. We found that the functional state of neurons did not alter uptake of radioactive choline by either process, nor did it change the conversion to radioactive acetylcholine.