Isolation of synaptic vesicles from Narcine brasiliensis electric organ: some influences on release of vesicular acetylcholine and ATP

Acetylcholine and ATP are coreleased from the electromotor nerve terminals of Narcine brasiliensis by an exocytotic mechanism

Although the exocytotic mechanism for quantal acetylcholine (ACh) release has been widely accepted for many years, it has repeatedly been challenged by reports that ACh released upon stimulation originates from the cytosol rather than synaptic vesicles. In this report, two independent experimental approaches were taken to establish the source of ACh released from the electromotor system of Narcine brasiliensis. Since ATP is colocalized with ACh in the cholinergic vesicle, the exocytotic theory predicts the corelease of these two components with a stoichiometry identical to that of the vesicle contents. The stimulated release of ATP from isolated synaptosomes could be accurately quantitated in the presence of the ATPase inhibitor adenosine 5'-[alpha, beta-methylene]triphosphate (500 microM), which prevented degradation of the released ATP. Various concentrations of elevated extracellular potassium (25-75 mM), veratridine (100 microM), and the calcium ionophore ionomycin (5 microM) all induced the corelease of ACh and ATP in a constant molar ratio of 5-6:1 (ACh/ATP), a stoichiometry consistent with that established for the vesicle content. In parallel to these stoichiometry studies, the compound 2-(4-phenylpiperidino)cyclohexanol (AH5183) was used to inhibit specifically the vesicular accumulation of newly synthesized (radiolabeled) ACh without affecting cytosolic levels of newly synthesized ACh in cholinergic nerve terminals. Treatment with AH5183 (10 microM) was shown to inhibit the release of newly synthesized ACh without markedly affecting total ACh release; thus, the entry of newly synthesized ACh into the synaptic vesicle is essential for its release. We conclude that ACh released upon stimulation originates exclusively from the vesicular pool and is coreleased stoichiometrically with other soluble vesicle contents.

Ultrastructural changes in adrenaline- and SGC-cells after morphine coincide with alterations of adrenaline and dopamine levels

The effects of morphine on chromaffin cell ultrastructure and catecholamine contents were studied using the adrenal glands from male mice (ICR strain). After 2 h adrenaline was increased 25% from 8.1 to 11.6 mumol/g tissue, followed by a 50% decrease to 5.2 mumol/g between 8-24 h and low values persisting at 72 h. Dopamine increased initially, reaching peak values of 0.5 mumol/g between 8-24 h, but had returned towards control values of 0.29 mumol/g by 72 h. Noradrenaline remained unchanged at 2.5 mumol/gram. Naloxone prevented alterations in adrenaline and dopamine levels. Ultrastructural examination revealed several types of catecholamine-storing cells. Of these the adrenaline and small-granule chromaffin (SGC) cells were more affected by morphine than noradrenaline cells. While the initial elevation of adrenaline 2 h after morphine was not accompanied by significant ultrastructural changes, the decrease after 8 and 24 h was paralleled by a significant (p less than 0.001) loss of adrenaline granules. Signs of active membrane turnover included an increase in the number of vacuoles, and the appearance of numerous coated omega profiles and coated (77.7 +/- 0.6 nm) vesicles. Clusters of synaptic-like vesicles (59.8 +/- 8.2 nm), slightly larger than neuronal vesicles (45.4 +/- 6.4), increased in the SGC-cells. After 72 h, the chromaffin granules in adrenaline cells remained low in numbers and were heterogeneous in electron density. Many synaptic-like vesicles were aligned along the SGC-cell membranes where only few chromaffin granules (109.3 +/- 20 nm) remained. Thus, continuous morphine exposure for 8-72 h increases the turnover of storage granules in adrenaline and SGC-cells with less effect on the noradrenaline cells which maintain catecholamine levels as indicated by biochemical analysis.

Regulation of endogenous calcium-dependent synaptic membrane phospholipase A2

Synaptic plasma membrane preparations from brain tissue have endogenous Ca2+-dependent phospholipase A2 activity. Characterization of this activity revealed that it was maximally active at 10(-7)-10(-5) M Ca2+ and pH 7.0. The enzyme had a Km of 62.0 microM and a Vmax of 98.0 nmol/mg/h. Calmodulin and prostaglandin F2 alpha stimulated phospholipase A2 activity, whereas prostaglandin E2, cyclic AMP and ATP were inhibitory. Addition of exogenous phospholipase A2 to synaptic plasma membrane and synaptic vesicle preparations led to their disruption and/or lysis. We suggest that Ca2+-dependent regulation of phospholipase A2 activity may be required for synaptic vesicle and synaptic plasma membrane interaction.

Exocytosis from large and small dense cored vesicles in noradrenergic nerve terminals

The noradrenergic nerve terminals and their vesicle populations have been subject to much ultrastructural research which has served to support and extend data from biochemical analysis and pharmacological-physiological experiments. Although a great deal of information has been collected, there are still many problems which need further investigation before the various aspects of large and small dense-cored vesicle function and their possible relationship to the various types of clear vesicles and vacuoles in the terminals can be fully explained. In particular, the ontogeny of microvesicles (30-40 nm), intermediate clear vesicles (45-55 nm) and large vacuoles or cisternae (150-200 nm) needs clarification before they can be fruitfully compared to similar structures assigned various roles in other types of terminals. From the data presently available the following conclusions can be drawn (Fig. 32): (1) both large and small dense-cored vesicles participate in exocytosis. (2) The various steps of the exocytotic process from vesicle fusion to vesicle membrane retrieval can be captured by ultrastructural methods but the mode(s) of membrane recapture remains to be resolved. (3) A morphologically heterogeneous population of clear vesicles probably reflects organelles of different ontogeny some being formed via terminal membrane endocytosis and some representing smooth endoplasmic reticulum or small "dense-cored" vesicles devoid of noradrenaline. (4) The small dense-cored vesicles may be formed in the cell body and transported to the terminals as this type of vesicle can be seen in the axons (where they probably are not undergoing retrograde transport as suggested for some of the small clear vesicles). Some small dense-cored vesicles may also be formed from the permanganate and dichromate-positive tubular structures common in rodent terminals. (5) There is no morphological evidence for the presence of large protein molecules such as dopamine beta-hydroxylase in the dense core of small vesicles. Their staining properties appear to reflect mainly noradrenaline and small molecules that can leak out from the vesicles in parallel with the transmitter. (6) Dopamine beta-hydroxylase and opioid peptides secreted from the noradrenergic terminals most probably originate from the large vesicles. (7) The hypothesis that only the large dense-cored vesicles contain and secrete dopamine beta-hydroxylase and opioids can help to explain the species differences in the concentrations of these substances observed in certain tissues.