Calcium influx in resting conditions in a preparation of peptidergic nerve terminals isolated from the rat neurohypophysis

Voltage-induced Ca2+ release by ryanodine receptors causes neuropeptide secretion from nerve terminals

Depolarisation-secretion coupling is assumed to be dependent only on extracellular calcium ([Ca²⁺ ]_o ). Ryanodine receptor (RyR)-sensitive stores in hypothalamic neurohypophysial system (HNS) terminals produce sparks of intracellular calcium ([Ca²⁺ ]_i ) that are voltage-dependent. We hypothesised that voltage-elicited increases in intraterminal calcium are crucial for neuropeptide secretion from presynaptic terminals, whether from influx through voltage-gated calcium channels and/or from such voltage-sensitive ryanodine-mediated calcium stores. Increases in [Ca²⁺ ]_i upon depolarisation in the presence of voltage-gated calcium channel blockers, or in the absence of [Ca²⁺ ]_o , still give rise to neuropeptide secretion from HNS terminals. Even in 0 [Ca²⁺ ]_o , there was nonetheless an increase in capacitance suggesting exocytosis upon depolarisation. This was blocked by antagonist concentrations of ryanodine, as was peptide secretion elicited by high K⁺ in 0 [Ca²⁺ ]_o . Furthermore, such depolarisations lead to increases in [Ca²⁺ ]_i . Pre-incubation with BAPTA-AM resulted in > 50% inhibition of peptide secretion elicited by high K⁺ in 0 [Ca²⁺ ]_o . Nifedipine but not nicardipine inhibited both the high K⁺ response for neuropeptide secretion and intraterminal calcium, suggesting the involvement of CaV1.1 type channels as sensors in voltage-induced calcium release. Importantly, RyR antagonists also modulate neuropeptide release under normal physiological conditions. In conclusion, our results indicate that depolarisation-induced neuropeptide secretion is present in the absence of external calcium, and calcium release from ryanodine-sensitive internal stores is a significant physiological contributor to neuropeptide secretion from HNS terminals.

Modulation/physiology of calcium channel sub-types in neurosecretory terminals

The hypothalamic-neurohypophysial system (HNS) controls diuresis and parturition through the release of arginine-vasopressin (AVP) and oxytocin (OT). These neuropeptides are chiefly synthesized in hypothalamic magnocellular somata in the supraoptic and paraventricular nuclei and are released into the blood stream from terminals in the neurohypophysis. These HNS neurons develop specific electrical activity (bursts) in response to various physiological stimuli. The release of AVP and OT at the level of neurohypophysis is directly linked not only to their different burst patterns, but is also regulated by the activity of a number of voltage-dependent channels present in the HNS nerve terminals and by feedback modulators. We found that there is a different complement of voltage-gated Ca(2+) channels (VGCC) in the two types of HNS terminals: L, N, and Q in vasopressinergic terminals vs. L, N, and R in oxytocinergic terminals. These channels, however, do not have sufficiently distinct properties to explain the differences in release efficacy of the specific burst patterns. However, feedback by both opioids and ATP specifically modulate different types of VGCC and hence the amount of AVP and/or OT being released. Opioid receptors have been identified in both AVP and OT terminals. In OT terminals, μ-receptor agonists inhibit all VGCC (particularly R-type), whereas, they induce a limited block of L-, and P/Q-type channels, coupled to an unusual potentiation of the N-type Ca(2+) current in the AVP terminals. In contrast, the N-type Ca(2+) current can be inhibited by adenosine via A(1) receptors leading to the decreased release of both AVP and OT. Furthermore, ATP evokes an inactivating Ca(2+)/Na(+)-current in HNS terminals able to potentiate AVP release through the activation of P2X2, P2X3, P2X4 and P2X7 receptors. In OT terminals, however, only the latter receptor type is probably present. We conclude by proposing a model that can explain how purinergic and/or opioid feedback modulation during bursts can mediate differences in the control of neurohypophysial AVP vs. OT release.

Ca(2+) homeostasis, Ca(2+) signalling and somatodendritic vasopressin release in adult rat supraoptic nucleus neurones

Multiple mechanisms that maintain Ca(2+) homeostasis and provide for Ca(2+) signalling operate in the somatas and neurohypophysial nerve terminals of supraoptic nucleus (SON) neurones. Here, we examined the Ca(2+) clearance mechanisms of SON neurones from adult rats by monitoring the effects of the selective inhibition of different Ca(2+) homeostatic molecules on cytosolic Ca(2+) ([Ca(2+)](i)) transients in isolated SON neurones. In addition, we measured somatodendritic vasopressin (AVP) release from intact SON tissue in an attempt to correlate it with [Ca(2+)](i) dynamics. When bathing the cells in a Na(+)-free extracellular solution, thapsigargin, cyclopiazonic acid (CPA), carbonyl cyanide 3-chlorophenylhydrazone (CCCP), and the inhibitor of plasma membrane Ca(2+)-ATPase (PMCA), La(3+), all significantly slowed down the recovery of depolarisation (50 mM KCl)-induced [Ca(2+)](i) transients. The release of AVP was stimulated by 50 mM KCl, and the decline in the peptide release was slowed by Ca(2+) transport inhibitors. In contrast to previous reports, our results show that in the fully mature adult rats: (i) all four Ca(2+) homeostatic pathways, the Na(+)/Ca(2+) exchanger, the endoplasmic reticulum Ca(2+) pump, the plasmalemmal Ca(2+) pump and mitochondria, are complementary in actively clearing Ca(2+) from SON neurones; (ii) somatodendritic AVP release closely correlates with intracellular [Ca(2+)](i) dynamics; (iii) there is (are) Ca(2+) clearance mechanism(s) distinct from the four outlined above; and (iv) Ca(2+) homeostatic systems in the somatas of SON neurones differ from those expressed in their terminals.

Flunarizine blocks elevation of free cytosolic calcium in synaptosomes following sustained depolarization

Gerbil cerebral cortical synaptosomes loaded with the fluorescent calcium probe FURA-2 were used to study depolarization-induced presynaptic cytosolic free calcium concentration, as an in vitro model of cerebral ischemia. The depolarization-induced increase in intrasynaptosomal cytosolic free calcium concentration is not sodium-dependent or sodium channel-dependent and may be due to an influx of extrasynaptosomal calcium resulting from a cadmium- and omega-conotoxin-sensitive, nickel-, nifedipine-, and nimodipine-insensitive voltage-regulated channel. The depolarization-induced increase in intrasynaptosomal free cytosolic calcium concentration is also inhibited by flunarizine, a calcium antagonist that has protective effects in animal models of cerebral anoxia and ischemia. Our results suggest that presynaptic calcium uptake following depolarization may be mediated in part by an N-type channel. Flunarizine may block presynaptic calcium accumulation, in part, by blocking this N-type channel; this blockade may be just one of several mechanisms by which flunarizine exerts protective effects following cerebral ischemia.

Regulation of 109Cd2+ uptake into isolated neurohypophysial peptidergic nerve terminals

Cadmium (109Cd2+) uptake was studied in a preparation of isolated neurohypophysial nerve terminals. By use of a filter separation method, together with a permeabilizing agent (Triton X-100), two cellular Cd2+ pools have been distinguished. The uptake into the intraterminal pool was governed mainly by a process that displayed saturable kinetics, with a Vmax of 0.15 nmol of Cd2+/mg of protein/min and a Km of 0.18 mM, consistent with a transport system. The superficially bound Cd2+ pool (Triton insensitive), which represented 30-50% of the total Cd2+ bound to the cellular system, was very sensitive to the ionic composition of the incubation medium. Reducing the extracellular Ca2+ or Na+ concentration caused a significant increase in the size of the Triton-insensitive Cd2+ pool. Whereas Na+ did not affect Cd2+ uptake, Ca2+ induced a small, but significant, increase of Cd2+ uptake into the terminals. It is concluded that there is a significant intraterminal uptake of Cd2+, which could explain several physiological effects of this ion.

Effect of sodium and calcium on basal secretory activity of rat neurohypophysial peptidergic nerve terminals

1. The release of arginine vasopressin (AVP) from a perifused preparation of peptidergic nerve terminals isolated from rat neurohypophyses was studied during manipulations of the external sodium and calcium concentrations. Intracellular concentrations of these two ions were manipulated by use of ouabain and a calcium ionophore, respectively. 2. Removal of extracellular Na+ caused, in a concentration-dependent manner, a significant decrease of secretory activity. Conversely, graded addition of Na+ to a Na(+)-free perifusion medium increased secretion. Half-maximal activation of secretory activity was attained at ca 75 mM [Na+]o. 3. Manipulations of extracellular Ca2+ did not affect the level of hormonal secretion in the absence of extracellular Na+. However, when Na+ was present in the perifusion medium, removal of extracellular Ca2+ induced an increase of secretory activity. 4. The effects of manipulations of [Na+]o were not dependent on the presence of Ca2+ in the perifusion medium nor on the nature of the Na+ replacement used (i.e. choline or mannitol). 5. Ouabain (0.1 mM) increased the basal secretory activity and potentiated the secretory response to removal of Ca2+ from the perifusion medium. 6. The Ca2+ ionophore A23187 stimulated, in a concentration-dependent fashion, the secretory activity of the peptidergic nerve terminals and this stimulation was strictly dependent on the presence of Ca2+ in the perifusion medium. 7. These results show that basal secretion is directly dependent on [Na]o and indicate that intracellular Na+ is an important factor in the control of secretory mechanisms. Evidence is presented in regard to a possible antagonistic effect of extracellular Ca2+ and Na+ on secretion.