The subsynaptosomal distribution and release of [3H]acetylcholine synthesized by rat cerebral cortical synaptosomes

Inhibition of mitochondrial Ca2+ release diminishes the effectiveness of methyl mercury to release acetylcholine from synaptosomes

The interaction of methyl mercury (MeHg) with nerve-terminal mitochondria as a potential mechanism for its effects on the release of acetylcholine (ACh) was studied using rat brain synaptosomes. The primary goal was to assess the relative contribution of extracellular Ca2+ and Ca2+ released from nerve-terminal mitochondria to the previously described stimulatory effects of MeHg on spontaneous release of ACh. A secondary goal was to address possible mechanisms by which MeHg might interact with nerve-terminal mitochondria to elicit Ca2+ discharge and subsequent release of ACh. MeHg depressed the high-affinity uptake of [3H]choline into synaptosomes by approximately 25 and 45% when synaptosomes were incubated with [3H]choline in the presence of 10 and 100 microM MeHg, respectively. In Ca(2+)-containing solutions, 10 and 100 microM MeHg increased the release of [3H]ACh from [3H]choline-loaded synaptosomes by 10 and 30%, respectively; this effect was maximal at 10 sec. Excluding Ca2+ from the reaction medium diminished the effectiveness of both 10 and 100 microM MeHg for inducing [3H]ACh release by about 30 and 25%, respectively, from that of Ca(2+)-containing solutions; however, significant increases still occurred in nominally Ca(2+)-free solutions. Ruthenium red (RR), an inhibitor of mitochondrial Ca2+ transport, was tested for its ability to disrupt MeHg-induced release. RR alone increased [3H]ACh release by 8-10 and 10-13% at 20 and 60 microM, respectively. RR-induced release was attenuated only slightly in Ca(2+)-free solutions. Preincubation of [3H]choline-loaded synaptosomes with RR reduced the stimulatory effect of MeHg on release of [3H]ACh both in the presence and in the absence of Ca2+. The fluorescent potentiometric carbocyanine dye diS-C2(5) was used to assess the ability of RR to prevent MeHg-induced depolarization of intrasynaptosomal mitochondria. RR (20 microM) itself did not depolarize the mitochondrial membrane potential, nor did it prevent MeHg from depolarizing the mitochondria. The results indicate that extracellular Ca2+ contributes only partially to MeHg-induced spontaneous release of ACh. The results with RR suggest that MeHg interacts with mitochondria to induce release of bound intraterminal Ca2+ stores, resulting ultimately in stimulated release of ACh. The ability of RR to prevent release of mitochondrial Ca2+ and, subsequently, ACh is not due to prevention of access of MeHg to the mitochondria, nor to stabilization of the mitochondrial membrane. Finally, MeHg reduces choline uptake into nerve terminals. Thus, MeHg could interfere with cholinergic neurotransmission by affecting the regulatory step in ACh synthesis and by increasing the spontaneous release of transmitter.(ABSTRACT TRUNCATED AT 400 WORDS)

Different approaches to study acetylcholine release: endogenous ACh versus tritium efflux

Superfused slices of guinea-pig cerebral cortex (CC), caudate nucleus (CN) and thalamus (Th) were used to compare i) the resting and electrically-evoked release of endogenous acetylcholine (ACh) in the presence of physostigmine (Phys) and ii) the resting and electrically-evoked tritium efflux (after preloading with 3H choline) in the absence or in the presence of Phys and hemicholinium (HC-3). In addition, the effect of GABA, morphine and their antagonists on both effluxes was investigated. By matching the ACh and tritium outflow on a molar basis, an average ratio of 100: 2-4 was found. When expressed as a percentage of tissue content, the ACh release at 2 Hz (2 min) was 4.1 in CN, 0.92 in CC and 0.44 in Th. Lower percent values in the same rank order, were found for tritium outflow with Phys. Thus, CN has the highest secretory activity. Tritium evoked outflow in the presence of Phys was nearly halved in comparison with the normal values (without Phys). Therefore, the autoreceptor-mediated negative feed-back seems to be similar in the three areas. Tritium evoked outflow in the presence of HC-3 was more than doubled in Th (less so in CC and CN) in comparison with the normal values. A second stimulation at 2 Hz (2 min) gave rise to the same outflow in Th while an evident fall in radiolabel efflux was found in CN. Therefore the blockade of high affinity choline uptake plays a different role in Th and CN.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigation of the rate-limiting step in the synthesis of acetylcholine by the human placenta

The uptake of [3H]choline and its conversion to [3H]acetylcholine were investigated in term human placental tissue in vitro. Although the net uptake of [3H]choline increased throughout a 45 min incubation period, intracellular [3H]choline levels reach a plateau after 2 min. There was a constant increase in [3H]acetylcholine levels throughout the incubation period. After 45 min, 36.5 per cent of the total intracellular tritium was recovered as acetylcholine by high-voltage electrophoresis. The effects of the choline acetyltransferase inhibitors, 2-benzoylethyltrimethyl-ammonium chloride (BETA) and 4-naphthylvinyl pyridine (NVP), and an inhibitor of choline uptake, hemicholinium-3 (HC-3), were also investigated for their influence on the uptake and metabolism of [3H]choline. A significant depression in both [3H]choline uptake and [3H]acetylcholine synthesis could be demonstrated with all three compounds, although with somewhat different time courses and activities. An analysis of the accumulation of [3H]acetylcholine in relation to the uptake and intracellular levels of [3H]choline as well as the patterns of inhibition produced by the inhibitors indicates that, unlike nervous tissue, the rate-limiting step in the synthesis of acetylcholine in human placental tissue is the transacetylation reaction catalysed by choline acetyltransferase.