Voltage-dependent, pertussis toxin insensitive inhibition of calcium currents by histamine in bovine adrenal chromaffin cells

Serotonergic Modulation of Spontaneous and Evoked Transmitter Release in Layer II Pyramidal Cells of Rat Somatosensory Cortex

As axons from the raphe nuclei densely innervate the somatosensory cortex, we investigated how serotonin (5-HT) modulates transmitter release in layer II pyramidal cells of rat barrel cortex. In the presence of tetrodotoxin and gabazine, 10 μM 5-HT caused a waxing and waning in the frequency of miniature excitatory postsynaptic currents (mEPSC) with no effect on amplitude. Specifically, within 15 min of recording the mEPSC frequency initially increased by 28 ± 7%, then dropped to below control (-15 ± 3%), before resurging back to 27 ± 7% larger than control. These changes were seen in 47% of pyramidal cells (responders) and were mediated by 5-HT2C receptors (5-HT2CR). Waxing resulted from phospholipase C activation, IP3 production, and Ca2+ release from presynaptic stores. Waning was prevented if PKC was blocked. In contrast, in paired recordings, the unitary EPSC amplitude was reduced by 50 ± 3% after 5-HT exposure in almost all cases with no significant effect on paired-pulse ratio and synaptic dynamics. This sustained EPSC reduction was also caused by 5-HT2R, but was mediated by presynaptic Gβγ subunits likely limiting influx through CaV2 channels. EPSC reduction, together with enhanced spontaneous noise in a restricted subset of inputs, could temporarily diminish the signal-to-noise ratio and affect the computation in the neocortical microcircuit.

Roles of Na+, Ca2+, and K+ channels in the generation of repetitive firing and rhythmic bursting in adrenal chromaffin cells

Adrenal chromaffin cells (CCs) are the main source of circulating catecholamines (CAs) that regulate the body response to stress. Release of CAs is controlled neurogenically by the activity of preganglionic sympathetic neurons through trains of action potentials (APs). APs in CCs are generated by robust depolarization following the activation of nicotinic and muscarinic receptors that are highly expressed in CCs. Bovine, rat, mouse, and human CCs also express a composite array of Na⁺, K⁺, and Ca²⁺ channels that regulate the resting potential, shape the APs, and set the frequency of AP trains. AP trains of increasing frequency induce enhanced release of CAs. If the primary role of CCs is simply to relay preganglionic nerve commands to CA secretion, why should they express such a diverse set of ion channels? An answer to this comes from recent observations that, like in neurons, CCs undergo complex firing patterns of APs suggesting the existence of an intrinsic CC excitability (non-neurogenically controlled). Recent work has shown that CCs undergo occasional or persistent burst firing elicited by altered physiological conditions or deletion of pore-regulating auxiliary subunits. In this review, we aim to give a rationale to the role of the many ion channel types regulating CC excitability. We will first describe their functional properties and then analyze how they contribute to pacemaking, AP shape, and burst waveforms. We will also furnish clear indications on missing ion conductances that may be involved in pacemaking and highlight the contribution of the crucial channels involved in burst firing.

Ca(2+) signaling mechanisms in bovine adrenal chromaffin cells

Calcium (Ca(2+)) is a crucial intracellular messenger in physiological aspects of cell signaling. Adrenal chromaffin cells are the secretory cells from the adrenal gland medulla that secrete catecholamines, which include epinephrine and norepinephrine important in the 'fight or flight' response. Bovine adrenal chromaffin cells have long been used as an important model for secretion -(exocytosis) not only due to their importance in the short-term stress response, but also as a neuroendocrine model of neurotransmtter release, as they have all the same exocytotic proteins as neurons but are easier to prepare, culture and use in functional assays. The components of the Ca(2+) signal transduction cascade and it role in secretion has been extensively characterized in bovine adrenal chromaffin cells. The Ca(2+) sources, signaling molecules and how this relates to the short-term stress response are reviewed in this book chapter in an endeavor to generally -overview these mechanisms in a concise and uncomplicated manner.

Inhibition of Ca2+ channels and adrenal catecholamine release by G protein coupled receptors

Catecholamines and other transmitters released from adrenal chromaffin cells play central roles in the "fight-or-flight" response and exert profound effects on cardiovascular, endocrine, immune, and nervous system function. As such, precise regulation of chromaffin cell exocytosis is key to maintaining normal physiological function and appropriate responsiveness to acute stress. Chromaffin cells express a number of different G protein coupled receptors (GPCRs) that sense the local environment and orchestrate this precise control of transmitter release. The primary trigger for catecholamine release is Ca2+ entry through voltage-gated Ca2+ channels, so it makes sense that these channels are subject to complex regulation by GPCRs. In particular G protein βγ heterodimers (Gbc) bind to and inhibit Ca2+ channels. Here I review the mechanisms by which GPCRs inhibit Ca2+ channels in chromaffin cells and how this might be altered by cellular context. This is related to the potent autocrine inhibition of Ca2+ entry and transmitter release seen in chromaffin cells. Recent data that implicate an additional inhibitory target of Gβγ on the exocytotic machinery and how this might fine tune neuroendocrine secretion are also discussed.

G protein betagamma subunits modulate the number and nature of exocytotic fusion events in adrenal chromaffin cells independent of calcium entry

G-protein-coupled receptors (GPCR) play important roles in controlling neurotransmitter and hormone release. Inhibition of voltage-gated Ca(2+) channels (Ca(2+) channels) by G protein betagamma subunits (Gbetagamma) is one prominent mechanism, but there is evidence for additional effects distinct from those on calcium entry. However, relatively few studies have investigated the Ca(2+)-channel-independent effects of Gbetagamma on transmitter release, so the impact of this mechanism remains unclear. We used carbon fiber amperometry to analyze catecholamine release from individual vesicles in bovine adrenal chromaffin cells, a widely used neurosecretory model. To bypass the effects of Gbetagamma on Ca(2+) entry, we stimulated secretion using ionomycin (a Ca(2+) ionophore) or direct intracellular application of Ca(2+) through a patch pipette. Activation of endogenous GPCR or transient transfection with exogenous Gbetagamma significantly reduced the number of amperometric spikes (the number of vesicular fusion events). The charge ("quantal size") and amplitude of the amperometric spikes were also significantly reduced by GPCR/Gbetagamma. We conclude that independent from effects on calcium entry, Gbetagamma can regulate both the number of vesicles that undergo exocytosis and the amount of catecholamine released per fusion event. We discuss possible mechanisms by which Gbetagamma might exert these novel effects including interaction with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex.

Lysophospholipids regulate excitability and exocytosis in cultured bovine chromaffin cells

Bioactive lysophospholipids (LPLs) are released by blood cells and can modulate many cellular activities such as angiogenesis and cell survival. In this study, the effects of sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) on excitability and exocytosis in bovine chromaffin cells were investigated using the whole-cell configuration of the patch-clamp. Voltage-gated Ca(2+) current was inhibited by S1P and LPA pre-treatment in a concentration-dependent manner with IC(50)s of 0.46 and 0.79 mumol/L, respectively. Inhibition was mostly reversible upon washout and prevented by suramin, an inhibitor of G-protein signaling. Na(+) current was inhibited by S1P, but not by LPA. However, recovery of Na(+) channels from inactivation was slowed by both LPLs. The outward K(+) current was also significantly reduced by both LPLs. Chromaffin cells fired repetitive action potentials in response to minimal injections of depolarizing current. Repetitive activity was dramatically reduced by LPLs. Consistent with the reduction in Ca(2+) current, exocytosis elicited by a train of depolarizations and the ensuing endocytosis were both inhibited by LPL pre-treatments. These data demonstrate the interaction between immune and endocrine systems mediated by the inhibitory effects of LPLs on the excitability of adrenal chromaffin cells.

Potentiation of exocytosis by phospholipase C-coupled G-protein-coupled receptors requires the priming protein Munc13-1

The vesicle priming protein Munc13-1 is regulated by diacylglycerol (DAG) and is therefore hypothesized to play a role in the control of neurotransmitter release by phospholipase C (PLC)-coupled receptors. We combined voltage-clamp recordings of voltage-gated Ca2+ channels (VGCCs) and high-resolution capacitance measurements to investigate the mechanism of receptor-mediated modulation of exocytosis in bovine chromaffin cells. Activation of endogenous H1 G(q)-protein-coupled receptors (G(q)PCRs) by histamine potentiated stimulus-coupled secretion despite concurrently inhibiting Ca2+ influx through VGCCs. Histamine increased the size of the readily releasable pool of vesicles and in particular a subpool of fusion-competent vesicles localized in close proximity to VGCCs. Pharmacological characterization showed that potentiation of exocytosis depended on the activation of PLC but not protein kinase C. Overexpression of wild-type Munc13-1 by adenoviral infection had no effect on histamine-induced potentiation per se, whereas DAG-insensitive Munc13-1(H567K) completely abolished it. This is the first endogenous mammalian G(q)PCR signaling pathway identified that engages Munc13-1 to increase stimulus-coupled secretion by recruiting vesicles to the immediately releasable pool. G(q)PCRs are therefore able to control exocytosis at the level of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex formation to produce rapid, short-term potentiation of the secretory output of neurons and endocrine cells.

Low frequency stimulation of mouse adrenal slices reveals a clathrin-independent, protein kinase C-mediated endocytic mechanism

Evidence suggests that chromaffin cells employ separate mechanisms for evoked endocytosis and granule recycling when stimulated at basal (approximately 0.5 Hz) and stress-activated (approximately 15 Hz) rates. Previous studies have focused mainly on elucidating the cellular mechanisms responsible for membrane recycling under conditions similar to the stress-activated state and indicate a clathrin/dephosphin-mediated retrieval via coated pits. However, the mechanism for membrane internalisation at basal stimulus intensity remains largely unexplored. We electrically stimulated chromaffin cells in adrenal tissue slices at the sympathetic basal firing rate and measured cell capacitance in the perforated voltage clamp configuration. A new method for the separation of non-secretory from secretory cell capacitance signals is presented. Simultaneous catecholamine release was measured electrochemically to isolate the exocytic from endocytic components of the capacitance responses. Using this approach we demonstrate that firing patterns that mimic basal sympathetic input results in rapid and graded membrane retrieval. We show that block of the calcium-mediated protein phosphatase 2B, a common step in clathrin-mediated processes, did not alter endocytosis elicited at basal firing levels. We further blocked clathrin-mediated retrieval with a clathrin/dephosphin-disrupting peptide (PP-19) and found endocytosis to be blocked at 15 Hz stimulation but complete and indistinguishable from control cells at 0.5 Hz stimulation. Lastly, pharmacological treatments show that conventional isoforms of protein kinase C (cPKC) are required for the 0.5 Hz-evoked retrieval mechanism. From these data we conclude that unlike endocytosis evoked under stress conditions, basal firing activity results in a clathrin-independent rapid membrane retrieval mediated through conventional isoforms of PKC.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

TRPC4 can be activated by G-protein-coupled receptors and provides sufficient Ca(2+) to trigger exocytosis in neuroendocrine cells

Transient receptor potential (TRP) channels form a large family of plasma membrane cation channels. Mammalian members of the "short" TRP family (TRP channel (TRPC) 1-7 are Ca(2+)-permeant, non-selective cation channels that are widely expressed in various cell types, including neurons. TRPC activity is linked through unknown mechanisms to G-protein-coupled receptors or receptor tyrosine kinases that activate phospholipase C. To investigate the properties and function of TRPC4 in neuronally derived cells, we transiently expressed mouse TRPC4 and histamine H(1) receptor in mouse adrenal chromaffin cells and PC12 cells. Histamine, but not thapsigargin, stimulated Mn(2+) influx in transfected cells. In the whole-cell patch clamp mode, histamine triggered a transient current in TRPC4-expressing cells. No current was evoked by perfusion with inositol 1,4,5-trisphosphate. When exocytosis was monitored with the capacitance detection technique, the magnitude of the membrane capacitance increase (Delta C(m)) on application of histamine in H(1) receptor/TRPC4-expressing chromaffin cells was comparable with that triggered by a train of depolarizing pulses. Our results indicate that TRPC4 channels behave as receptor, but not store-operated, channels in neuronally derived cells. TRPC4 channels can provide sufficient Ca(2+) influx to trigger a robust secretory response in voltage-clamped neurosecretory cells. Similar mechanisms may modulate exocytosis in other neuronal systems.

Histamine promotes excitability in bovine adrenal chromaffin cells by inhibiting an M-current

The current study has investigated the electrophysiological responses evoked by histamine in bovine adrenal chromaffin cells using perforated-patch techniques. Histamine caused a transient hyperpolarization followed by a sustained depolarization of 7.2 +/- 1.4 mV associated with an increase in spontaneous action potential frequency. The hyperpolarization was abolished after depleting intracellular Ca(2+) stores with thapsigargin (100 nM), and was reduced by 40 % with apamin (100 nM). Membrane resistance increased by about 60 % during the histamine-induced depolarization suggesting inhibition of a K(+) channel. An inward current relaxation, typical of an M-current, was observed in response to negative voltage steps from a holding potential of -30 mV. This current reversed at -81.6 +/- 1.8 mV and was abolished by the M-channel inhibitor linopirdine (100 microM). During application of histamine, the amplitude of M-currents recorded at a time corresponding with the sustained depolarization was reduced by 40 %. No inward current rectification was observed in the range -150 to -70 mV, and glibenclamide (10 microM) had no effect on either resting membrane potential or the response to histamine. The results show that an M-current is present in bovine chromaffin cells and that this current is inhibited during sustained application of histamine, resulting in membrane depolarization and increased discharge of action potentials. These results demonstrate for the first time a possible mechanism coupling histamine receptors to activation of voltage-operated Ca(2+) channels in these cells.

Prostaglandin E(2) inhibits calcium current in two sub-populations of acutely isolated mouse trigeminal sensory neurons

Prostaglandins are important mediators of pain and inflammation. We have examined the effects of prostanoids on voltage-activated calcium currents (I(Ca)) in acutely isolated mouse trigeminal sensory neurons, using standard whole cell voltage clamp techniques. Trigeminal neurons were divided into two populations based on the presence (Type 2) or absence (Type 1) of low voltage-activated T-type I(Ca). The absence of T-type I(Ca) is highly correlated with sensitivity to mu-opioid agonists and the VR1 agonist capsaicin. In both populations of cells, high voltage-activated I(Ca) was inhibited by PGE(2) with an EC(50) of about 35 nM, to a maximum of 30 %. T-type I(Ca) was not inhibited by PGE(2). Pertussis toxin pre-treatment abolished the effects of PGE(2) in Type 2 cells, but not in Type 1 cells, whereas treatment with cholera toxin prevented the effects of PGE(2) in Type 1 cells, but not in Type 2 cells. Inhibition of I(Ca) by PGE(2) was associated with slowing of current activation and could be relieved with a large positive pre-pulse, consistent with inhibition of I(Ca) by G protein betagamma subunits. Reverse transcription-polymerase chain reaction of mRNA from trigeminal ganglia indicated that all four EP prostanoid receptors were present. However, in both Type 1 and Type 2 cells the effects of PGE(2) were only mimicked by the selective EP(3) receptor agonist ONO-AE-248, and not by selective agonists for EP(1) (ONO-DI-004), EP(2) (ONO-AE1-259) and EP(4) (ONO-AE1-329) receptors. These data indicate that two populations of neurons in trigeminal ganglia differing in their calcium channel expression, sensitivity to mu-opioids and capsaicin also have divergent mechanisms of PGE(2)-mediated inhibition of calcium channels, with Gi/Go type G proteins involved in one population, and Gs type G proteins in the other.

Voltage-independent inhibition of P/Q-type Ca2+ channels in adrenal chromaffin cells via a neuronal Ca2+ sensor-1-dependent pathway involves Src family tyrosine kinase

In common with many neurons, adrenal chromaffin cells possess distinct voltage-dependent and voltage-independent pathways for Ca(2+) channel regulation. In this study, the voltage-independent pathway was revealed by addition of naloxone and suramin to remove tonic blockade of Ca(2+) currents via opioid and purinergic receptors due to autocrine feedback inhibition. This pathway requires the Ca(2+)-binding protein neuronal calcium sensor-1 (NCS-1). The voltage-dependent pathway was pertussis toxin-sensitive, whereas the voltage-independent pathway was largely pertussis toxin-insensitive. Characterization of the voltage-independent inhibition of Ca(2+) currents revealed that it did not involve protein kinase C-dependent signaling pathways but did require the activity of a Src family tyrosine kinase. Two structurally distinct Src kinase inhibitors, 4-amino-5-(4-methylphenyl)7-(t-butyl)pyrazolo[3,4-d] pyrimidine (PP1) and a Src inhibitory peptide, increased the Ca(2+) currents, and no further increase in Ca(2+) currents was elicited by addition of naloxone and suramin. In addition, the Src-like kinase appeared to act in the same pathway as NCS-1. In contrast, addition of PP1 did not prevent a voltage-dependent facilitation elicited by a strong pre-pulse depolarization indicating that this pathway was independent of Src kinase activity. PPI no longer increased Ca(2+) currents after addition of the P/Q-type channel blocker omega-agatoxin TK. The alpha(1A) subunit of P/Q-type Ca(2+) channels was immunoprecipitated from chromaffin cell extracts and found to be phosphorylated in a PP1-sensitive manner by endogenous kinases in the immunoprecipitate. A high molecular mass (around 220 kDa) form of the alpha(1A) subunit was detected by anti-phosphotyrosine, suggesting a possible target for Src family kinase action. These data demonstrate a voltage-independent mechanism for autocrine inhibition of P/Q-type Ca(2+) channel currents in chromaffin cells that requires Src family kinase activity and suggests that this may be a widely distributed pathway for Ca(2+) channel regulation.