Ultrastructural distribution of calcium in cutaneous electroreceptor organs of teleost fish

Plasticity of synaptic ribbons of the rat pineal gland in vitro--minor effects of electrical stimulation

Synaptic ribbons (SRs) of mammalian pinealocytes exhibit day/night changes in number and size, changes that are apparently regulated by the suprachiasmatic nucleus via postganglionic sympathetic nerve fibres. Since the neural control of SR changes is far from clear and as pinealocytes produce action potentials, we undertook to investigate whether electrical stimulation affects SR changes. Isolated rat pineal glands removed during the daytime were kept in vitro for 0, 30, 60, 90 or 120 min, with or without continuous electrical stimulation (1 mA, 1 Hz), followed by the quantification of SR profiles (SRPs) by transmission electron microscopy. SRs were categorised as to whether they lay less than 100 nm away from the pinealocyte plasmalemma (SRPs(near)) or more distant from it (SRPs(dist)) and the lengths of the profiles were measured. Cultured pineal organs showed a significant numerical depression of SRPs(near), irrespective of whether the organs had been electrically stimulated or not. SRPs(near) length revealed a significant increase at 60 min in unstimulated control tissue and at 30 min in electrically stimulated glands. SRPs(dist) length decreased significantly at 30 min in control glands and after 60 min in electrically stimulated glands. Thus, action potentials inside the pineal gland appear to be minor factors regulating SR numbers. In future pineal studies, SRPs(near) and SRPs(dist) should be considered separately as they differ in plasticity.

Control of the calcium concentration involved in acetylcholine release and its facilitation: an additional role for synaptic vesicles?

2,5-Diterbutyl-1,4-benzohydroquinone, a specific blocker of Ca2+-ATPase pumps, increased acetylcholine release from an identified synapse of Aplysia, as well as from Torpedo and mouse caudate nucleus synaptosomes. Because 2,5-diterbutyl-1,4-benzohydroquinone does not change the presynaptic Ca2+ influx, the enhancement of acetylcholine release could be due to an accumulation of Ca2+ in the terminal. This possibility was further checked by studying the effects of 2,5-diterbutyl-1,4-benzohydroquinone on twin pulse facilitation, classically attributed to residual Ca2+. While preventing the fast sequestration of Ca2+ by presynaptic organelles, 2,5-diterbutyl-1,4-benzohydroquinone magnified both twin pulse facilitation observed under low extracellular Ca2+ concentration and twin pulse dysfacilitation observed under high extracellular Ca2+ concentration. Thus, it is concluded that 2,5-diterbutyl-1,4-benzohydroquinone, by preventing Ca2+ buffering near transmitter release sites, modulates acetylcholine release. As 2,5-diterbutyl-1,4-benzohydroquinone was also shown to decrease by 50% the uptake of 45Ca2+ by isolated synaptic vesicles, we propose that synaptic vesicles can control the presynaptic Ca2+ concentration triggering the release of neurotransmitter.