Article

Differential modulation of N-type calcium channels by micro-opioid receptors in oxytocinergic versus vasopressinergic neurohypophysial terminals.

Department of Physiology & Program in Neuroscience, University of Massachusetts Medical School, Worcester, Massachusetts 01655, USA.
Journal of Cellular Physiology (Impact Factor: 3.87). 10/2010; 225(1):276-88. DOI: 10.1002/jcp.22263
Source: PubMed

ABSTRACT Opioids modulate the electrical activity of magnocellular neurons (MCN) and inhibit neuropeptide release at their terminals in the neurohypophysis. We have previously shown that micro-opioid receptor (MOR) activation induces a stronger inhibition of oxytocin (OT) than vasopressin (AVP) release from isolated MCN terminals. This higher sensitivity of OT release is due, at least in part, to the selective targeting of R-type calcium channels. We now describe the underlying basis for AVP's weaker inhibition by MOR activation and provide a more complete explanation of the complicated effects on neuropeptide release. We found that N-type calcium channels in AVP terminals are differentially modulated by MOR; enhanced at lower concentrations but increasingly inhibited at higher concentrations of agonists. On the other hand, N-type calcium channels in OT terminals were always inhibited. The response pattern in co-labeled terminals was analogous to that observed in AVP-containing terminals. Changes in intracellular calcium concentration and neuropeptide release corroborated these results as they showed a similar pattern of enhancement and inhibition in AVP terminals contrasting with solely inhibitory responses in OT terminals to MOR agonists. We established that fast translocation of Ca(2+) channels to the plasma membrane was not mediating current increments and thus, changes in channel kinetic properties are most likely involved. Finally, we reveal a distinct Ca-channel beta-subunit expression between each type of nerve endings that could explain some of the differences in responses to MOR activation. These results help advance our understanding of the complex modulatory mechanisms utilized by MORs in regulating presynaptic neuropeptide release.

2 Bookmarks
 · 
100 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A successful pregnancy requires multiple adaptations in the mother's brain that serve to optimise fetal growth and development, protect the fetus from adverse prenatal programming and prevent premature delivery of the young. Pregnancy hormones induce, organise and maintain many of these adaptations. Steroid hormones play a critical role and of particular importance is the progesterone metabolite and neurosteroid, allopregnanolone. Allopregnanolone is produced in increasing amounts during pregnancy both in the periphery and in the maternal and fetal brain. This review critically examines a role for allopregnanolone in both the maternal and fetal brain during pregnancy and development in protecting pregnancy and birth outcomes, with particular emphasis on its role in relation to stress exposure at this time. Late pregnancy is associated with suppressed stress responses. Thus, we begin by considering what is known about the central mechanisms in the maternal brain, induced by allopregnanolone, that protect the fetus(es) from exposure to harmful levels of maternal glucocorticoids as a result of stress during pregnancy. Next we discuss the central mechanisms that prevent premature secretion of oxytocin and consider a role for allopregnanolone in minimising the risk of preterm birth. Allopregnanolone also plays a key role in the fetal brain, where it promotes development and is neuroprotective. Hence we review the evidence about disruption to neurosteroid production in pregnancy, through prenatal stress or other insults, and the immediate and long-term adverse consequences for the offspring. Finally we address whether progesterone or allopregnanolone treatment can rescue some of these deficits in the offspring.
    Progress in Neurobiology 09/2013; · 10.30 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: μ-Opioid agonists have no effect on calcium currents (ICa) in neurohypophysial terminals when recorded using the classic whole-cell patch-clamp configuration. However, μ-opioid receptor (MOR)-mediated inhibition of ICa is reliably demonstrated using the perforated-patch configuration. This suggests that the MOR-signaling pathway is sensitive to intraterminal dialysis and is therefore mediated by a readily diffusible second messenger. Using the perforated patch-clamp technique and ratio-calcium-imaging methods, we describe a diffusible second messenger pathway stimulated by the MOR that inhibits voltage-gated calcium channels in isolated terminals from the rat neurohypophysis (NH). Our results show a rise in basal intracellular calcium ([Ca(2+)]i) in response to application of [d-Ala(2)-N-Me-Phe(4),Gly5-ol]-Enkephalin (DAMGO), a MOR agonist, that is blocked by d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a MOR antagonist. Buffering DAMGO-induced changes in [Ca(2+)]i with BAPTA-AM completely blocked the inhibition of both ICa and high-K(+)-induced rises in [Ca(2+)]i due to MOR activation, but had no effect on κ-opioid receptor (KOR)-mediated inhibition. Given the presence of ryanodine-sensitive stores in isolated terminals, we tested 8-bromo-cyclic adenosine diphosphate ribose (8Br-cADPr), a competitive inhibitor of cyclic ADP-ribose (cADPr) signaling that partially relieves DAMGO inhibition of ICa and completely relieves MOR-mediated inhibition of high-K(+)-induced and DAMGO-induced rises in [Ca(2+)]i. Furthermore, antagonist concentrations of ryanodine completely blocked MOR-induced increases in [Ca(2+)]i and inhibition of ICa and high-K(+)-induced rises in [Ca(2+)]i while not affecting KOR-mediated inhibition. Antagonist concentrations of ryanodine also blocked MOR-mediated inhibition of electrically-evoked increases in capacitance. These results strongly suggest that a key diffusible second messenger mediating the MOR-signaling pathway in NH terminals is [Ca(2+)]i released by cADPr from ryanodine-sensitive stores.
    Journal of Neuroscience 03/2014; 34(10):3733-42. · 6.75 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Highly localized Ca(2+) release events have been characterized in several neuronal preparations. In mouse neurohypophysial terminals (NHTs), such events, called Ca(2+) syntillas, appear to emanate from a ryanodine-sensitive intracellular Ca(2+) pool. Traditional sources of intracellular Ca(2+) appear to be lacking in NHTs. Thus, we have tested the hypothesis that large dense core vesicles (LDCVs), which contain a substantial amount of calcium, represent the source of these syntillas. Here, using fluorescence immunolabeling and immunogold-labeled electron micrographs of NHTs, we show that type 2 ryanodine receptors (RyRs) are localized specifically to LDCVs. Furthermore, a large conductance nonspecific cation channel, which was identified previously in the vesicle membrane and has biophysical properties similar to that of an RyR, is pharmacologically affected in a manner characteristic of an RyR: it is activated in the presence of the RyR agonist ryanodine (at low concentrations) and blocked by the RyR antagonist ruthenium red. Additionally, neuropeptide release experiments show that these same RyR agonists and antagonists modulate Ca(2+)-elicited neuropeptide release from permeabilized NHTs. Furthermore, amperometric recording of spontaneous release events from artificial transmitter-loaded terminals corroborated these ryanodine effects. Collectively, our findings suggest that RyR-dependent syntillas could represent mobilization of Ca(2+) from vesicular stores. Such localized vesicular Ca(2+) release events at the precise location of exocytosis could provide a Ca(2+) amplification mechanism capable of modulating neuropeptide release physiologically.
    The Journal of General Physiology 06/2014; 143(6):693-702. · 4.57 Impact Factor

Full-text (2 Sources)

Download
40 Downloads
Available from
May 23, 2014