Two types of calcium channels coexist in peptide-releasing vertebrate nerve terminals

Stac protein regulates release of neuropeptides

Neuropeptides are important for regulating numerous neural functions and behaviors. Release of neuropeptides requires long-lasting, high levels of cytosolic Ca²⁺ However, the molecular regulation of neuropeptide release remains to be clarified. Recently, Stac3 was identified as a key regulator of L-type Ca²⁺ channels (CaChs) and excitation-contraction coupling in vertebrate skeletal muscles. There is a small family of stac genes in vertebrates with other members expressed by subsets of neurons in the central nervous system. The function of neural Stac proteins, however, is poorly understood. Drosophila melanogaster contain a single stac gene, Dstac, which is expressed by muscles and a subset of neurons, including neuropeptide-expressing motor neurons. Here, genetic manipulations, coupled with immunolabeling, Ca²⁺ imaging, electrophysiology, and behavioral analysis, revealed that Dstac regulates L-type CaChs (Dmca1D) in Drosophila motor neurons and this, in turn, controls the release of neuropeptides.

Advances in the neurophysiology of magnocellular neuroendocrine cells

Hypothalamic magnocellular neuroendocrine cells have unique electrical properties and a remarkable capacity for morphological and synaptic plasticity. Their large somatic size, their relatively uniform and dense clustering in the supraoptic and paraventricular nuclei, and their large axon terminals in the neurohypophysis make them an attractive target for direct electrophysiological interrogation. Here, we provide a brief review of significant recent findings in the neuroplasticity and neurophysiological properties of these neurones that were presented at the symposium "Electrophysiology of Magnocellular Neurons" during the 13th World Congress on Neurohypophysial Hormones in Ein Gedi, Israel in April 2019. Magnocellular vasopressin (VP) neurones respond directly to hypertonic stimulation with membrane depolarisation, which is triggered by cell shrinkage-induced opening of an N-terminal-truncated variant of transient receptor potential vanilloid type-1 (TRPV1) channels. New findings indicate that this mechanotransduction depends on actin and microtubule cytoskeletal networks, and that direct coupling of the TRPV1 channels to microtubules is responsible for mechanical gating of the channels. Vasopressin neurones also respond to osmostimulation by activation of epithelial Na⁺ channels (ENaC). It was shown recently that changes in ENaC activity modulate magnocellular neurone basal firing by generating tonic changes in membrane potential. Both oxytocin and VP neurones also undergo robust excitatory synapse plasticity during chronic osmotic stimulation. Recent findings indicate that new glutamate synapses induced during chronic salt loading express highly labile Ca²⁺ -permeable GluA1 receptors requiring continuous dendritic protein synthesis for synapse maintenance. Finally, recordings from the uniquely tractable neurohypophysial terminals recently revealed an unexpected property of activity-dependent neuropeptide release. A significant fraction of the voltage-dependent neurohypophysial neurosecretion was found to be independent of Ca²⁺ influx through voltage-gated Ca²⁺ channels. Together, these findings provide a snapshot of significant new advances in the electrophysiological signalling mechanisms and neuroplasticity of the hypothalamic-neurohypophysial system, a system that continues to make important contributions to the field of neurophysiology.

Electrophysiological properties of identified oxytocin and vasopressin neurones

To understand the contribution of intrinsic membrane properties to the different in vivo firing patterns of oxytocin (OT) and vasopressin (VP) neurones, in vitro studies are needed, where stable intracellular recordings can be made. Combining immunochemistry for OT and VP and intracellular dye injections allows characterisation of identified OT and VP neurones, and several differences between the two cell types have emerged. These include a greater transient K⁺ current that delays spiking to stimulus onset, and a higher Na⁺ current density leading to greater spike amplitude and a more stable spike threshold, in VP neurones. VP neurones also show a greater incidence of both fast and slow Ca²⁺ -dependent depolarising afterpotentials, the latter of which summate to plateau potentials and contribute to phasic bursting. By contrast, OT neurones exhibit a sustained outwardly rectifying potential (SOR), as well as a consequent depolarising rebound potential, not found in VP neurones. The SOR makes OT neurones more susceptible to spontaneous inhibitory synaptic inputs and correlates with a longer period of spike frequency adaptation in these neurones. Although both types exhibit prominent Ca²⁺ -dependent afterhyperpolarising potentials (AHPs) that limit firing rate and contribute to bursting patterns, Ca²⁺ -dependent AHPs in OT neurones selectively show significant increases during pregnancy and lactation. In OT neurones, but not VP neurones, AHPs are highly dependent on the constitutive presence of the second messenger, phosphatidylinositol 4,5-bisphosphate, which permissively gates N-type channels that contribute the Ca²⁺ during spike trains that activates the AHP. By contrast to the intrinsic properties supporting phasic bursting in VP neurones, the synchronous bursting of OT neurones has only been demonstrated in vitro in cultured hypothalamic explants and is completely dependent on synaptic transmission. Additional differences in Ca²⁺ channel expression between the two neurosecretory terminal types suggests these channels are also critical players in the differential release of OT and VP during repetitive spiking, in addition to their importance to the potentials controlling firing patterns.

Voltage-dependent calcium channels in the neurosecretory cells of cerebral ganglia of the mud crab, Scylla paramamosain

Voltage-dependent calcium channels (VDCCs) play a critical role in stimulus-secretion coupling in neurosecretory cells (NSCs). The crustacean cerebral ganglion plays a crucial role in neuromodulation and controls neuropeptide release. The present study used patch-clamp and Illumina sequencing techniques to investigate the potential features of VDCC in the cerebral ganglia of the mud crab (Scylla paramamosain). The electrophysiological characteristics of VDCC were analyzed in three types of NSCs with a patch clamp. The thresholds for activation of Ca channel current recorded from all the three types of NSCs were all above -40 mV, with peak amplitudes occurring around 0 mV. Therefore, it was concluded that the currents recorded in NSCs were mediated by high-voltage-activated Ca channels. Ca channel current densities in I type NSCs were significantly lower than those in II and III type NSCs. Four VDCC subunits derived from three transcripts were predicted from a transcriptome database of the cerebral ganglia. Among these transcripts, Cavα1, Cavβ, and Cavα2/δ were predicted to encode 1674, 554, and 776 amino acids, respectively, and they shared conservative domains with VDCC subunits in other species. Overall, these findings provide an important basis for further studies on the neuroendocrine mechanisms in crustaceans.

μ-Opioid inhibition of Ca2+ currents and secretion in isolated terminals of the neurohypophysis occurs via ryanodine-sensitive Ca2+ stores

μ-Opioid agonists have no effect on calcium currents (I(Ca)) in neurohypophysial terminals when recorded using the classic whole-cell patch-clamp configuration. However, μ-opioid receptor (MOR)-mediated inhibition of I(Ca) 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 I(Ca) 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 I(Ca) 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 I(Ca) 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.

The Ca2+ channel β2 subunit is selectively targeted to the axon terminals of supraoptic neurons

The assembly of high voltage-activated Ca(2+) channels with different β subunits influences channel properties and possibly subcellular targeting. We studied β subunit expression in the somata and axon terminals of the magnocellular neurosecretory cells, which are located in the supraoptic nucleus (SON) and neurohypophysis, respectively. Antibodies directed against the 4 Ca(V)β subunits (Ca(V)β(1)-Ca(V)β(4)) were used for immunoblots and for immunostaining of slices of these two tissues. We found that all 4 β subunits are expressed in both locations, but that Ca(V)β(2) had the highest relative expression in the neurohypophysis. These data suggest that the Ca(V)β(2) subunit is selectively targeted to axon terminals and may play a role in targeting and/or regulating the properties of Ca(2+) channels.

Expression of CaV 2.2 and splice variants of CaV 2.1 in oxytocin- and vasopressin-releasing supraoptic neurones

The magnocellular neurosecretory cells (MNCs) release vasopressin (VP) and oxytocin (OT) from their axon terminals into the circulation and from their somata and dendrites to exert paracrine effects on other MNCs. MNCs express several types of voltage-gated Ca(2+) channels, including Ca(V)2.1 and Ca(V)2.2. These two channels types are similar in structure and function in other cells, but although influx of Ca(2+) through Ca(V)2.2 triggers the release of both OT and VP into the circulation, Ca(V)2.1 is involved in stimulating the release of VP but not OT. Release of OT from MNC somata is also triggered by Ca(V)2.2 but not Ca(V)2.1. These observations could be explained by differences in the level of expression of Ca(V)2.1 in VP and OT MNCs or by differences in the way that the two channels interact with the exocytotic apparatus. We used immunohistochemistry to confirm earlier work suggesting that MNCs express variants of Ca(V)2.1 lacking portions of an internal loop that enables the channels to interact with synaptic proteins. We used an antibody that would recognise both the full-length Ca(V)2.1 and the deletion variants to show that OT MNCs express fewer Ca(V)2.1 channels than do VP MNCs in both somata and axon terminals. We used the reverse transcriptase-polymerase chain reaction and immunocytochemistry to test whether MNCs express similar deletion variants of Ca(V)2.2 and were unable to find any evidence to support this. Our data suggest that the different roles that Ca(V)2.1 and Ca(V)2.2 play in MNC secretion may be a result of the different levels of expression of Ca(V)2.1 in VP and OT MNCs, as well as the expression in MNCs of deletion variants of Ca(V)2.1 that do not interact with exocytotic proteins and therefore may be less likely to mediate exocytotic release.

Modulation of presynaptic Ca(2+) currents in frog motor nerve terminals by high pressure

Presynaptic Ca(2+) -dependent mechanisms have already been implicated in depression of evoked synaptic transmission by high pressure (HP). Therefore, pressure effects on terminal Ca(2+) currents were studied in Rana pipiens peripheral motor nerves. The terminal currents, evoked by nerve or direct stimulation, were recorded under the nerve perineurial sheath with a loose macropatch clamp technique. The combined use of Na(+) and K(+) channel blockers, [Ca(2+) ]o changes, voltage-dependent Ca(2+) channel (VDCC) blocker treatments and HP perturbations revealed two components of presynaptic Ca(2+) currents: an early fast Ca(2+) current (ICaF ), possibly carried by N-type (CaV 2.2) Ca(2+) channels, and a late slow Ca(2+) current (ICaS ), possibly mediated by L-type (CaV 1) Ca(2+) channels. HP reduced the amplitude and decreased the maximum (saturation level) of the Ca(2+) currents, ICaF being more sensitive to pressure, and may have slightly shifted the voltage dependence. HP also moderately diminished the Na(+) action current, which contributed to the depression of VDCC currents. Computer-based modeling was used to verify the interpretation of the currents and investigate the influence of HP on the presynaptic currents. The direct HP reduction of the VDCC currents and the indirect effect of the action potential decrease are probably the major cause of pressure depression of synaptic release.

The involvement of voltage-operated calcium channels in somato-dendritic oxytocin release

Magnocellular neurons of the supraoptic nucleus (SON) secrete oxytocin and vasopressin from axon terminals in the neurohypophysis, but they also release large amounts of peptide from their somata and dendrites, and this can be regulated both by activity-dependent Ca²⁺ influx and by mobilization of intracellular Ca²⁺. This somato-dendritic release can also be primed by agents that mobilise intracellular Ca²⁺, meaning that the extent to which it is activity-dependent, is physiologically labile. We investigated the role of different Ca²⁺ channels in somato-dendritic release; blocking N-type channels reduced depolarisation-induced oxytocin release from SONs in vitro from adult and post-natal day 8 (PND-8) rats, blocking L-type only had effect in PND-8 rats, while blocking other channel types had no significant effect. When oxytocin release was primed by prior exposure to thapsigargin, both N- and L-type channel blockers reduced release, while P/Q and R-type blockers were ineffective. Using confocal microscopy, we found immunoreactivity for Ca_v1.2 and 1.3 channel subunits (which both form L-type channels), 2.1 (P/Q type), 2.2 (N-type) and 2.3 (R-type) in the somata and dendrites of both oxytocin and vasopressin neurons, and the intensity of the immunofluorescence signal for different subunits differed between PND-8, adult and lactating rats. Using patch-clamp electrophysiology, the N-type Ca²⁺ current density increased after thapsigargin treatment, but did not alter the voltage sensitivity of the channel. These results suggest that the expression, location or availability of N-type Ca²⁺ channels is altered when required for high rates of somato-dendritic peptide release.

Large neurohypophysial varicosities amplify action potentials: results from numerical simulations

Axons in the neurohypophysis are known for their "beads on a string" morphology, with numerous in-line secretory swellings lined up along the axon cable. A significant fraction of these secretory swellings, called Herring bodies, is large enough to serve as an identifying feature of the neural lobe in histological sections. Little is known about the physiological role such large axonal swellings might play in neuroendocrine physiology. Using numerical simulations, we have investigated whether large in-line varicosities affect the waveform and propagation of action potentials (APs) along neurohypophysial axons. Due to the strong nonlinear dependence of calcium influx on AP waveforms, such modulation would inevitably affect neuroendocrine release. The parameters for our numerical simulations were matched to established properties of voltage-gated ion channels in neurohypophysial swellings. We find that even a single in-line varicosity can severely depress AP waveforms far upstream in the axonal cable. In contrast, AP depolarization within varicosities becomes amplified. Amplification within varicosities varies in a nontrivial manner with varicosity dimensions, and is most pronounced for diameters close to those of Herring bodies. Overall, we find that large axonal varicosities significantly modulate AP waveforms and their propagation, and do so over large distances. Varicosity size is the main determinant for the observed AP amplification, with the kinetics of voltage-gated ion channels playing a noticeable but secondary role. Our results imply that large varicosities are sites of enhanced hormone release, suggesting that small and large varicosities target different neurohypophysial structures.

Identification of the neuropeptide content of individual rat neurohypophysial terminals

The objective of this study was to develop a method that could reliably determine the arginine vasopressin (AVP) and/or oxytocin (OT) content of individual rat neurohypophysial terminals (NHT) >or=5 microm in diameter, the size used for electrophysiological recordings. We used a commercially available, highly sensitive enzyme-linked immunoassay (ELISA) kit with a sensitivity of 0.25 pg to AVP and of 1.0pg to OT. The NHT content of AVP (2.21+/-0.10 pg) was greater than OT (1.77+/-0.08 pg) and increased with terminal size. AVP-positive terminals (10.2+/-0.21 microm) were larger in diameter than OT-positive terminals (9.1+/-0.24 microm). Immunocytochemical techniques indicated that a higher percentage (58%) of smaller terminals contained OT, and that a higher percentage (42%) of larger NHTs were colabeled. Similar percentages of AVP-positive terminals were obtained between immunocytochemical (73%) and ELISA (72%) methods when NHTs were assayed for AVP alone, but there was a higher percentage of OT terminals when using immunocytochemistry (43%) compared to ELISA (26%). The percent of AVP-positive (60%) and OT-positive (18%) terminals decreased when NHT were assayed for both AVP and OT. Therefore, the best method to reliably identify AVP-positive NHTs is to assay only for AVP, since this allows the conclusion that AVP-negative terminals contain only OT.

Dehydration increases L-type Ca(2+) current in rat supraoptic neurons

The magnocellular neurosecretory cells of the hypothalamus (MNCs) regulate water balance by releasing vasopressin (VP) and oxytocin (OT) as a function of plasma osmolality. Release is determined largely by the rate and pattern of MNC firing, but sustained increases in osmolality also produce structural adaptations, such as cellular hypertrophy, that may be necessary for maintaining high levels of neuropeptide release. Since increases in Ca(2+) current could enhance exocytotic secretion, influence MNC firing patterns, and activate gene transcription and translation, we tested whether Ca(2+) currents in MNCs acutely isolated from the supraoptic nucleus (SON) of the hypothalamus are altered by 16-24 h of water deprivation. A comparison of whole-cell patch-clamp recordings demonstrated that dehydration causes a significant increase in the amplitude of current sensitive to the L-type Ca(2+) channel blocker nifedipine (from -56 +/- 6 to -99 +/- 10 pA; P < 0.001) with no apparent change in other components of Ca(2+) current. Post-recording immunocytochemical identification showed that this increase in current occurred in both OT- and VP-releasing MNCs. Radioligand binding studies of tissue from the SON showed there is also an increase in the density of binding sites for an L-type Ca(2+) channel ligand (from 51.5 +/- 4.8 to 68.1 +/- 4.1 fmol (mg protein)(-1); P < 0.05), suggesting that there was an increase in the number of L-type channels on the plasma membrane of the MNCs or some other cell type in the SON. There were no changes in the measured number of binding sites for an N-type Ca(2+) channel ligand. Dehydration was not associated with changes in the levels of mRNA coding for Ca(2+) channel alpha(1) subunits. These data are consistent with the hypothesis that a selective increase of L-type Ca(2+) current may contribute to the adaptation that occurs in the MNCs during dehydration.

Dihydropyridine receptors and type 1 ryanodine receptors constitute the molecular machinery for voltage-induced Ca2+ release in nerve terminals

Ca2+ stores were studied in a preparation of freshly dissociated terminals from hypothalamic magnocellular neurons. Depolarization from a holding level of -80 mV in the absence of extracellular Ca2+ elicited Ca2+ release from intraterminal stores, a ryanodine-sensitive process designated as voltage-induced Ca2+ release (VICaR). The release took one of two forms: an increase in the frequency but not the quantal size of Ca2+ syntillas, which are brief, focal Ca2+ transients, or an increase in global [Ca2+]. The present study provides evidence that the sensors of membrane potential for VICaR are dihydropyridine receptors (DHPRs). First, over the range of -80 to -60 mV, in which there was no detectable voltage-gated inward Ca2+ current, syntilla frequency was increased e-fold per 8.4 mV of depolarization, a value consistent with the voltage sensitivity of DHPR-mediated VICaR in skeletal muscle. Second, VICaR was blocked by the dihydropyridine antagonist nifedipine, which immobilizes the gating charge of DHPRs but not by Cd2+ or FPL 64176 (methyl 2,5 dimethyl-4[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylate), a non-dihydropyridine agonist specific for L-type Ca2+ channels, having no effect on gating charge movement. At 0 mV, the IC50 for nifedipine blockade of VICaR in the form of syntillas was 214 nM in the absence of extracellular Ca2+. Third, type 1 ryanodine receptors, the type to which DHPRs are coupled in skeletal muscle, were detected immunohistochemically at the plasma membrane of the terminals. VICaR may constitute a new link between neuronal activity, as signaled by depolarization, and a rise in intraterminal Ca2+.

Frequency-dependent potentiation of voltage-activated responses only in the intact neurohypophysis of the rat

The loose-patch-clamp technique was used with multiple-pulse protocols to study the frequency dependence of currents from the surface of the intact rat neurohypophysis (NH) and hypothalamus. In the NH, but not in the corresponding supraoptic nucleus of the hypothalamus, an initial, single pulse of 3-8 ms duration (long pulse) potentiated a secondary pulse response starting 20-50 ms after the initial pulse. Potentiation was abolished by 4-aminopyridine (4-AP), but not by tetraethylammonium (TEA) chloride or tetrandrine, indicating the participation of A-type potassium currents. Potentiation was also abolished by CdCl2, CoCl2 or 1 microM nicardipine, indicating the participation of calcium currents. The potentiation was reduced significantly in the presence of 4-6 mM extracellular CaCl2, indicating that the potentiation is not due to calcium influx. An initial train with as few as two pulses, each of 0.3-0.7 ms duration (short pulses) at 64-1,100 Hz also potentiated the secondary short pulse response significantly. We conclude that voltage-gated channels underlie this potentiation, which is due to interstitial calcium and potassium homeostasis changes induced by action potential activity and occurs only in the intact NH. A model is proposed for the participation of calcium and potassium channels in the burst patterning that is optimal for secretion from the NH.

Ca2+ clearance mechanisms in neurohypophysial terminals of the rat

The importance of intracellular calcium ([Ca2+]i) in the release of vasopressin (AVP) and oxytocin from the central nervous system neurohypopyhysial nerve terminals has been well-documented. To date, there is no clear understanding of Ca2+ clearance mechanisms and their interplay with transmembrane Ca2+ entry, intracellular [Ca2+]i transients, cytoplasmic Ca2+ stores and hence the release of AVP at the level of a single nerve terminal. Here, we studied the mechanism of Ca2+ clearance in freshly isolated nerve terminals of the rat neurohypophysis using Fura-2 Ca2+ imaging and measured the release of AVP by radioimmuno assay. An increase in the K+ concentration in the perfusion solution from 5 to 50 mM caused a rapid increase in [Ca2+]i and AVP release. Returning K+ concentration to 5 mM led to rapid restoration of both responses to basal level. The K+-evoked [Ca2+]i and AVP increase was concentration-dependent, reliable, and remained of constant amplitude and time course upon successive applications. Extracellular Ca2+ removal completely abolished the K+-evoked responses. The recovery phase was not affected upon replacement of NaCl with sucrose or drugs known to act on intracellular Ca2+ stores such as thapsigargin, cyclopiazonic acid, caffeine or a combination of caffeine and ryanodine did not affect either resting or K+-evoked [Ca2+]i or AVP release. By contrast, the plasma membrane Ca2+ pump inhibitor, La3+, markedly slowed down the recovery phase. The mitochondrial respiration uncoupler, carbonyl cyanide 3-chlorophenylhydrazone (CCCP), slightly but significantly increased the basal [Ca2+]i, and also slowed down the recovery phase of both [Ca2+]i and release responses. In conclusion, we show in nerve terminals that (i) Ca2+ extrusion through the Ca2+ pump in the plasma membrane plays a major role in the Ca2+ clearance mechanisms of (ii) Ca2+ uptake by mitochondria also contributes to the Ca2+ clearance and (iii) neither Na+/Ca2+ exchangers nor Ca2+ stores are involved in the Ca2+ clearance or in the maintenance of basal [Ca2+]i or release of AVP.

Distinct rat neurohypophysial nerve terminal populations identified by size, electrophysiological properties and neuropeptide content

Voltage-gated ion channels are critical to excitation-secretion coupling in nerve terminals. We have identified two distinct populations of rat neurohypophysial (NHP) terminals distinguished by size, neuropeptide content and electrophysiological properties, including resting membrane potential, action potential (AP) properties, and K+ current and Na+ current characteristics. In large terminals (10-16 microm diameter), resting membrane potential was more negative than in small terminals (5-9.9 microm; -61+/-4 mV vs. -55+/-3 mV; p<0.01), action potential amplitude was larger (69+/-4 mV vs. 53+/-3 mV; p<0.01), peak IK was larger (1460+/-90 pA vs. 1140+/-70 pA; p<0.05) with a more negative V1/2 for activation (-3.1 mV vs. -0.6 mV; p<0.05), and Na+ current density was greater (approximately 470 pA/pF vs. approximately 300 pA/pF; p<0.01) with more negative V1/2 values for activation from -70 or -90 mV holding potentials (-44 mV vs. -24 mV; p<0.01). A positive linear correlation between INa amplitude and terminal size showed an inflection at a diameter of 9.2 microm. Neuropeptide content was generally segregated into a population of small terminals (<10 microm diameter) containing predominantly vasopressin and a population of large terminals (> or =10 microm diameter) containing predominantly oxytocin (OT); a small fraction of terminals in each group contained both peptides. These findings suggest that electrophysiological differences between small vasopressin-containing and large oxytocin-containing neurohypophysial terminals may contribute to their observed differential firing and peptide release patterns.

Depression by isoflurane of the action potential and underlying voltage-gated ion currents in isolated rat neurohypophysial nerve terminals

We characterized the effects of the volatile anesthetic isoflurane on the ion currents that contribute to the action potential (AP) in isolated rat neurohypophysial (NHP) nerve terminals using patch-clamp electrophysiology. Mean resting membrane potential and AP amplitude were -62.3 +/- 4.1 and 69.2 +/- 2.9 mV, respectively, in NHP terminals. Two components of outward K(+) current (I(K)) were identified in voltage-clamp recordings: a transient I(K) and a sustained I(K) with minimal inactivation. Some terminals displayed a slowly activating I(K), probably the big Ca(2+)-activated K(+) current (BK). Isoflurane reversibly inhibited AP amplitude and increased AP half-width in normal extracellular Ca(2+) (2.2 mM). In high extracellular Ca(2+) (10 mM), isoflurane also reduced the afterhypolarization peak amplitude. A transient tetrodotoxin-sensitive Na(+) current (I(Na)) was the principal current mediating the depolarizing phase of the AP. A slowly inactivating Cd(2+)-sensitive current (probably a voltagegated Ca(2+) current; I(Ca)) followed the initial I(Na). Isoflurane reversibly inhibited both I(Na) and I(Ca) elicited by a voltage-stimulus based on an averaged AP waveform. The isoflurane IC(50) for AP waveform-evoked I(Na) was 0.36 mM. Isoflurane (0.84 +/- 0.04 mM) inhibited AP waveform-evoked I(Ca) by 37.5 +/- 0.16% (p < 0.05). The isoflurane IC(50) for peak I(K) was 0.83 mM and for sustained I(K) was 0.73 mM, with no effect on the voltage dependence of activation. The results indicate that multiple voltage-gated ion channels (Na(+) > K(+) > Ca(2+)) in NHP terminals, although not typical central nervous system terminals, are inhibited by the volatile general anesthetic isoflurane. The net inhibitory effects of volatile anesthetics on nerve terminal action potentials and excitability result from integrated actions on multiple voltage-gated currents.

Effects of toxic environmental contaminants on voltage-gated calcium channel function: from past to present

Voltage-gated Ca2+ channels are targets of the number of naturally occurring toxins, therapeutic agents as well as environmental toxicants. Because of similarities of their chemical structure to Ca2+ in terms of hydrated ionic radius, electron orbital configuration, or other chemical properties, polyvalent cations from aluminum to zinc variously interact with multiple types of voltage-gated Ca2+ channels. These nonphysiological metals have been used to study the structure and function of the Ca2+ channel, especially its permeability characteristics. Two nonphysiological cations, Pb2+ and Hg2+, as well as their organic derivatives, are environmental neurotoxicants which are highly potent Ca2+ channel blockers. These metals also apparently gain intracellular access in part by permeating through Ca2+ channels. In this review the history of Ca2+ channel block produced by Pb2+ and Hg2+ as well as other nonphysiological cations is traced. In particular the characteristics of Ca2+ channel block induced by these environmental neurotoxic metals and the consequences of this action for neuronal function are discussed.

Ca2+ syntillas, miniature Ca2+ release events in terminals of hypothalamic neurons, are increased in frequency by depolarization in the absence of Ca2+ influx

Localized, brief Ca2+ transients (Ca2+ syntillas) caused by release from intracellular stores were found in isolated nerve terminals from magnocellular hypothalamic neurons and examined quantitatively using a signal mass approach to Ca2+ imaging. Ca2+ syntillas (scintilla, L., spark, from a synaptic structure, a nerve terminal) are caused by release of approximately 250,000 Ca ions on average by a Ca2+ flux lasting on the order of tens of milliseconds and occur spontaneously at a membrane potential of -80 mV. Syntillas are unaffected by removal of extracellular Ca2+, are mediated by ryanodine receptors (RyRs) and are increased in frequency, in the absence of extracellular Ca2+, by physiological levels of depolarization. This represents the first direct demonstration of mobilization of Ca2+ from intracellular stores in neurons by depolarization without Ca2+ influx. The regulation of syntillas by depolarization provides a new link between neuronal activity and cytosolic [Ca2+] in nerve terminals.

Loose-patch clamp currents from the hypothalamo-neurohypophysial system of the rat

The loose-patch clamp technique was used to study voltage-activated currents from the surface of rat neurohypophysial and hypothalamic regions in situ. In the neurohypophysis, depolarizing pulses of 4-8 ms duration yielded tetrodotoxin (TTX)-sensitive sodium currents, a 4-AP-sensitive "A"-type potassium current, and a long-lasting outward TEA- and tetrandrine-sensitive Ca(2+)-activated potassium current. All of these currents were elicited during the application of the pulse. With high external calcium there were long-lasting inward currents blocked by Ni(2+) and Cd(2+), identifying them as voltage-gated calcium currents. Depolarizing pulses of 0.3-0.7 ms duration yielded fast biphasic responses, of 1-3 ms duration, composed of mostly sodium and "A"-type potassium currents. With high external calcium there were fast inward currents blocked by Ni(2+) and Cd(2+), indicating that these were voltage-gated calcium currents. These responses have the characteristics of action potential currents: they were elicited after the cessation of the applied pulse and the "A" component is eliminated together with the sodium component upon application of TTX. Similar responses to long and short pulses were obtained from the surface of the associated magnocellular somata in the supraoptic nucleus, and their projections. The explant currents are similar to those previously characterized using conventional methods from somata and terminals.

Isoflurane and propofol inhibit voltage-gated sodium channels in isolated rat neurohypophysial nerve terminals

Mounting electrophysiological evidence indicates that certain general anesthetics, volatile anesthetics in particular, depress excitatory synaptic transmission by presynaptic mechanisms. We studied the effects of representative general anesthetics on voltage-gated Na+ currents (INa) in nerve terminals isolated from rat neurohypophysis using patch-clamp electrophysiological analysis. Both isoflurane and propofol inhibited INa in a dose-dependent and reversible manner. At holding potentials of -70 or -90 mV, isoflurane inhibited peak INa with IC50 values of 0.45 and 0.56 mM, and propofol inhibited peak INa with IC50 values of 4.1 and 6.0 microM, respectively. Isoflurane (0.8 mM) did not significantly alter the V1/2 of activation; propofol caused a small positive shift. Isoflurane (0.8 mM) or propofol (5 microM) produced a negative shift in the voltage dependence of inactivation. Recovery of INa from inactivation was slower from a holding potential of -70 mV than from -90 mV; isoflurane and propofol further delayed recovery from inactivation. In conclusion, the volatile anesthetic isoflurane and the intravenous anesthetic propofol inhibit voltage-gated Na+ currents in isolated neurohypophysial nerve terminals in a concentration- and voltage-dependent manner. Marked effects on the voltage dependence and kinetics of inactivation and minimal effects on activation support preferential anesthetic interactions with the fast inactivated state of the Na+ channel. These results are consistent with direct inhibition of oxytocin and vasopressin release from the neurohypophysis by isoflurane and propofol. Inhibition of voltage-gated Na+ channels may contribute to the presynaptic effects of general anesthetics on nerve terminal excitability and neurotransmitter release.

Ionic mechanisms mediating oscillatory membrane potentials in wide-field retinal amacrine cells

Particular types of amacrine cells of the vertebrate retina show oscillatory membrane potentials (OMPs) in response to light stimulation. Historically it has been thought the oscillations arose as a result of circuit properties. In a previous study we found that in some amacrine cells, the ability to oscillate was an intrinsic property of the cell. Here we characterized the ionic mechanisms responsible for the oscillations in wide-field amacrine cells (WFACs) in an effort to better understand the functional properties of the cell. The OMPs were found to be calcium (Ca2+) dependent; blocking voltage-gated Ca2+ channels eliminated the oscillations, whereas elevating extracellular Ca2+ enhanced them. Strong intracellular Ca2+ buffering (10 mM EGTA or bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid) eliminated any attenuation in the OMPs as well as a Ca2+-dependent inactivation of the voltage-gated Ca2+ channels. Pharmacological and immunohistochemical characterization revealed that WFACs express L- and N-type voltage-sensitive Ca2+ channels. Block of the L-type channels eliminated the OMPs, but omega-conotoxin GVIA did not, suggesting a different function for the N-type channels. The L-type channels in WFACs are functionally coupled to a set of calcium-dependent potassium (K(Ca)) channels to mediate OMPs. The initiation of OMPs depended on penitrem-A-sensitive (BK) K(Ca) channels, whereas their duration is under apamin-sensitive (SK) K(Ca) channel control. The Ca2+ current is essential to evoke the OMPs and triggering the K(Ca) currents, which here act as resonant currents, enhances the resonance as an amplifying current, influences the filtering characteristics of the cell membrane, and attenuates the OMPs via CDI of the L-type Ca2+ channel.

Iberiotoxin-induced block of Ca2+-activated K+ channels induces dihydropyridine sensitivity of ACh release from mammalian motor nerve terminals

The role which Ca(2+)-activated K(+) (K(Ca)) channels play in regulating acetylcholine (ACh) release was examined at mouse motor nerve terminals. In particular, the ability of the antagonist iberiotoxin to recruit normally silent L-type Ca(2+) channels to participate in nerve-evoked release was examined using conventional intracellular electrophysiological techniques. Incubation of cut hemidiaphragm preparations with 10 microM nimodipine, a dihydropyridine L-type Ca(2+) channel antagonist, had no significant effect on quantal content of end-plate potentials. Nevertheless, 1 microM S-(-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]phenyl)-3-pyridine carboxylic acid methyl ester (Bay K 8644) enhanced quantal content to 134.7 +/- 3.5% of control. Iberiotoxin (150 nM) increased quantal content to 177.5 +/- 9.9% of control, whereas iberiotoxin plus nimodipine increased quantal content to only 145.7 +/- 10.4% of control. Coapplication of 1 microM Bay K 8644 with iberiotoxin did not significantly increase quantal content further than did treatment with iberiotoxin alone. The effects of iberiotoxin and nimodipine alone or in combination on the miniature end-plate potential (MEPP) frequency following KCl-induced depolarization were examined using uncut hemidiaphragm preparations. Nimodipine alone had no effect on MEPP frequency from preparations incubated in physiological saline containing 5 to 20 mM KCl. Moreover, iberiotoxin alone or combined with nimodipine also had no effect on MEPP frequency in physiological salines containing 5 to 15 mM KCl. At 20 mM KCl, however, iberiotoxin significantly increased MEPP frequency to 125.6% of iberiotoxin-free values; combined treatment with nimodipine and iberiotoxin prevented this increase in MEPP frequency. Thus, loss of functional K(Ca) channels unmasks normally silent L-type Ca(2+) channels to participate in ACh release from motor nerve terminals, particularly under conditions of intense nerve terminal depolarization.

Passive transfer of Lambert-Eaton syndrome to mice induces dihydropyridine sensitivity of neuromuscular transmission

Lambert-Eaton myasthenic syndrome (LEMS) is a paraneoplastic disorder in which autoantibodies apparently target the voltage-gated Ca2+ channels that regulate acetylcholine (ACh) release at motor nerve terminals. P/Q-type Ca2+ channels are primarily involved in ACh release at mammalian neuromuscular junctions. Passive transfer of LEMS to mice by repeated administration of plasma from LEMS patients reduces the amplitude of the perineurial P/Q-type current, and unmasks a dihydropyridine (DHP)-sensitive L-type Ca2+ current at the motor nerve terminal. The present study sought to determine if this DHP-sensitive component contributes to ACh release. Mice were treated for 30 days with plasma from healthy human controls or patients with LEMS. For some studies, diaphragms from naive mice were incubated with LEMS or control human plasma for 2 or 24 h. End-plate potentials (EPPs) and miniature end-plate potentials (MEPPs) were recorded from neuromuscular junctions in the hemidiaphragm. Treatment of mice with LEMS plasma evoked the characteristic electrophysiological signs of LEMS: reduced quantal content and facilitation of EPP amplitudes at high-frequency stimulation. Quantal content was also reduced in muscles incubated acutely with LEMS plasma. Nimodipine, a DHP-type blocker of L-type Ca2+ channels, did not significantly affect the quantal content of muscles treated for 2 or 24 h with either control or LEMS plasma, or following chronic treatment with control plasma. However, following 30 days treatment with LEMS plasma, nimodipine significantly reduced the remaining quantal content to 57.7 +/- 3.3 % of pre-nimodipine control levels. Thus, DHP-sensitive Ca2+ channels become involved in synaptic transmission at the mouse neuromuscular junction after chronic, but not acute treatment with LEMS plasma. However, reductions in quantal release of ACh occur even after very short periods of exposure to LEMS plasma. As such, development of the L-type Ca2+ channel contribution to ACh release during passive transfer of LEMS appears to occur only after quantal release is significantly impaired for an extended duration, suggesting that an adaptive response of the ACh release apparatus occurs in LEMS.

Adenosine inhibition via A(1) receptor of N-type Ca(2+) current and peptide release from isolated neurohypophysial terminals of the rat

Effects of adenosine on voltage-gated Ca(2+) channel currents and on arginine vasopressin (AVP) and oxytocin (OT) release from isolated neurohypophysial (NH) terminals of the rat were investigated using perforated-patch clamp recordings and hormone-specific radioimmunoassays. Adenosine, but not adenosine 5'-triphosphate (ATP), dose-dependently and reversibly inhibited the transient component of the whole-terminal Ba(2+) currents, with an IC(50) of 0.875 microM. Adenosine strongly inhibited, in a dose-dependent manner (IC(50) = 2.67 microM), depolarization-triggered AVP and OT release from isolated NH terminals. Adenosine and the N-type Ca(2+) channel blocker omega-conotoxin GVIA, but not other Ca(2+) channel-type antagonists, inhibited the same transient component of the Ba(2+) current. Other components such as the L-, Q- and R-type channels, however, were insensitive to adenosine. Similarly, only adenosine and omega-conotoxin GVIA were able to inhibit the same component of AVP release. A(1) receptor agonists, but not other purinoceptor-type agonists, inhibited the same transient component of the Ba(2+) current as adenosine. Furthermore, the A(1) receptor antagonist 8-cyclopentyltheophylline (CPT), but not the A(2) receptor antagonist 3, 7-dimethyl-1-propargylxanthine (DMPGX), reversed inhibition of this current component by adenosine. The inhibition of AVP and OT release also appeared to be via the A(1) receptor, since it was reversed by CPT. We therefore conclude that adenosine, acting via A(1) receptors, specifically blocks the terminal N-type Ca(2+) channel thus leading to inhibition of the release of both AVP and OT.

Evidence for endogenous agmatine in hypothalamo-neurohypophysial tract and its modulation on vasopressin release and Ca2+ channels

Agmatine, decarboxylated from arginine by arginine decarboxylase, is particularly prominent in the hypothalamus. The present study utilized the rat hypothalamo-neurohypophysial system to determine expression and "pre-synaptic" modulation of agmatine in the central nervous system (CNS). Under confocal-laser scanning, agmatine-like immunoreactivity (Agm-LI) was found enriched in arginine-vasopressin (AVP)-containing magnocellular neurons of the supraoptic nuclei (SON) and paraventricular nuclei (PVN). In addition, using electron microscopy, Agm-LI was found closely associated with large neurosecretory-like vesicles in neurohypophysial nerve terminals of posterior pituitary gland. Radioimmunoassay revealed that 10 and 30 microM agmatine concentration-dependently inhibited the depolarization-evoked AVP release from isolated neurohypophysial terminals. Using perforated patch-clamp, effects of agmatine on whole-terminal voltage-gated ion currents in the isolated neurohypophysial nerve terminals were examined. While it did not significantly affect either tetrodotoxin (TTX)-sensitive Na(+) or sustained Ca(2+)-activated K(+) channel currents, agmatine (1-40 microM) inhibited Ca(2+) channel currents in approximately 53% of the total nerve terminals investigated. The onset of inhibitory effect was immediate, and the inhibition was reversible and concentration-dependent with an IC(50)=4.6 microM. In the remaining (approximately 47%) neurohypophysial nerve terminals, only a higher (120 microM) concentration of agmatine could moderately inhibit Ca(2+) channel currents. The results suggest that: (1) endogenous agmatine is co-expressed in AVP-containing, hypothalamic magnocellular neurons of the SON/PVN and in neurohypophysial nerve terminals of posterior pituitary gland; (2) agmatine may serve as a physiological neuromodulator by regulating the voltage-gated Ca(2+) channel and, as a result, the release of AVP from neurohypophysial nerve terminals.

Intrinsic membrane properties and synaptic inputs regulating the firing activity of the dopamine neurons

Dopamine (DA) neurones of the ventral mesencephalon are involved in the control of reward related behaviour, cognitive functions and motor performances, and provide a critical site of action for major categories of neuropsychiatric drugs, such as antipsychotic agents, dependence producing drugs and anti-Parkinson medication. The midbrain DA neurones are mainly located in the substantia nigra pars compacta (SNPC) and the ventral tegmental area (VTA). Intrinsic membrane properties regulate the activity of these neurones. In fact, they possess several conductances that allow them to fire in a slow pacemaker-like mode. The internal set of membrane currents interact with afferent synaptic inputs which, especially in in vivo conditions, contribute to accelerate or decelerate the firing activity of the cells in accordance with the necessity to optimise the release of dopamine in the terminal fields. In particular, discrete excitatory and inhibitory inputs transform the firing from a low regular into a bursting pattern. The bursting activity promotes dopamine release being very important in cognition and motor performances. In the present paper we review electrophysiological data regarding the role of glutamatergic and cholinergic and GABAergic afferent inputs in regulating the midbrain DAergic neuronal activity.

The function of Ca(2+) channel subtypes in exocytotic secretion: new perspectives from synaptic and non-synaptic release

By mediating the Ca(2+) influx that triggers exocytotic fusion, Ca(2+) channels play a central role in a wide range of secretory processes. Ca(2+) channels consist of a complex of protein subunits, including an alpha(1) subunit that constitutes the voltage-dependent Ca(2+)-selective membrane pore, and a group of auxiliary subunits, including beta, gamma, and alpha(2)-delta subunits, which modulate channel properties such as inactivation and channel targeting. Subtypes of Ca(2+) channels are constituted by different combinations of alpha(1) subunits (of which 10 have been identified) and auxiliary subunits, particularly beta (of which 4 have been identified). Activity-secretion coupling is determined not only by the biophysical properties of the channels involved, but also by the relationship between channels and the exocytotic apparatus, which may differ between fast and slow types of secretion. Colocalization of Ca(2+) channels at sites of fast release may depend on biochemical interactions between channels and exocytotic proteins. The aim of this article is to review recent work on Ca(2+) channel structure and function in exocytotic secretion. We discuss Ca(2+) channel involvement in selected types of secretion, including central neurotransmission, endocrine and neuroendocrine secretion, and transmission at graded potential synapses. Several different Ca(2+) channel subtypes are involved in these types of secretion, and their function is likely to involve a variety of relationships with the exocytotic apparatus. Elucidating the relationship between Ca(2+) channel structure and function is central to our understanding of the fundamental process of exocytotic secretion.

Voltage-dependent membrane capacitance in rat pituitary nerve terminals due to gating currents

We investigated the voltage dependence of membrane capacitance of pituitary nerve terminals in the whole-terminal patch-clamp configuration using a lock-in amplifier. Under conditions where secretion was abolished and voltage-gated channels were blocked or completely inactivated, changes in membrane potential still produced capacitance changes. In terminals with significant sodium currents, the membrane capacitance showed a bell-shaped dependence on membrane potential with a peak at approximately -40 mV as expected for sodium channel gating currents. The voltage-dependent part of the capacitance showed a strong correlation with the amplitude of voltage-gated Na+ currents and was markedly reduced by dibucaine, which blocks sodium channel current and gating charge movement. The frequency dependence of the voltage-dependent capacitance was consistent with sodium channel kinetics. This is the first demonstration of sodium channel gating currents in single pituitary nerve terminals. The gating currents lead to a voltage- and frequency-dependent capacitance, which can be well resolved by measurements with a lock-in amplifier. The properties of the gating currents are in excellent agreement with the properties of ionic Na+ currents of pituitary nerve terminals.

Functional IP3- and ryanodine-sensitive calcium stores in presynaptic varicosities of NG108-15 (rodent neuroblastoma x glioma hybrid) cells

Presynaptic varicosities of the model neuronal cell line NG108-15, a cholinergic neuroblastoma cell x glioma cell hybrid capable of innervating striated myotubes, were examined for the presence of inositol 1,4,5-trisphosphate (IP3)-sensitive and Ca2+-activated (ryanodine-sensitive) Ca2+ stores using confocal microscopic imaging of Ca2+-sensitive fluorescent dye loaded into the cells. Initial demonstration of the presence of IP3 receptors and ryanodine receptors in the NG108-15 varicosities was obtained using immunocytochemistry. Treatment of NG108-15 cells with bradykinin (0.1 microM), whose receptor is linked to IP3 generation, and separately, caffeine (10 mM), an activator of endoplasmic reticulum ryanodine receptors, resulted in substantial increases in [Ca2+]i in the varicosities. K+-evoked changes in [Ca2+]i in the varicosities were reduced (52 %) after emptying the ryanodine-sensitive Ca2+ store using caffeine (10 mM), but were not affected by prior depletion of the IP3-sensitive Ca2+ store using thapsigargin (1 microM). Bradykinin-induced changes in [Ca2+]i were abolished following depletion of the IP3-sensitive Ca2+ store using thapsigargin (1 microM) and were reduced (72 %) by prior emptying of the ryanodine-sensitive Ca2+ store with caffeine (10 mM). The same results were obtained when the varicosities of the NG108-15 cells had formed synaptic junctions with co-cultured rat hindlimb myotubes. Taken together, the results suggest that, in the varicosities, activation of the IP3 pathway evoked the release of Ca2+ from the IP3-sensitive store, which, in turn, secondarily induced the release of Ca2+ from the ryanodine-sensitive store via Ca2+-induced Ca2+ release, and that depolarization-induced Ca2+ entry evoked Ca2+-induced Ca2+ release only from the ryanodine-sensitive store. Thus, functional internal Ca2+ stores are inherent components of presynaptic varicosities in this neural cell line.

Intracellular Ca(2+) channel immunoreactivity in neuroendocrine axon terminals

The concentration of neuroendocrine terminals in the neurohypophysis facilitates the identification and localization of Ca(2+) channel subtypes near neuroendocrine release sites. Immunoblots of rat neurohypophysial tissue identified the alpha(1)1.3, alpha(1)2.1, alpha(1)2.2, and alpha(1)2.3 Ca(2+) channel subunits. Immunofluorescence staining of axon terminal plasma membranes was weak, suggesting that Ca(2+) channels are dispersed. This contrasts with the highly punctate alpha(1)2.2 immunoreactivity in bovine chromaffin cells; the neurohypophysial terminals may therefore lack the specialized release zones found in those cells. Immunofluorescence and immunogold labeling identify dense core granule-like structures in the terminal cytoplasm containing multiple Ca(2+) channel types. Ca(2+) channels in internal membranes may play an important role in channel targeting and distribution in neuroendocrine cells.

Block of voltage-dependent calcium channels by aliphatic monoamines

We have recently identified farnesol, an intermediate in the mevalonate pathway, as a potent endogenous modulator and blocker of N-type calcium channels (Roullet, J. B., R. L. Spaetgens, T. Burlingame, and G. W. Zamponi. 1999. J. Biol. Chem. 274:25439-25446). Here, we investigate the action of structurally related compounds on various types of voltage-dependent Ca(2+) channels transiently expressed in human embryonic kidney cells. 1-Dodecanol, despite sharing the 12-carbon backbone and headgroup of farnesol, exhibited a significantly lower blocking affinity for N-type Ca(2+) channels. Among several additional 12-carbon compounds tested, dodecylamine (DDA) mediated the highest affinity inhibition of N-type channels, indicating that the functional headgroup is a critical determinant of blocking affinity. This inhibition was concentration-dependent and relatively non-discriminatory among N-, L-, P/Q-, and R-Ca(2+) channel subtypes. However, whereas L-type channels exhibited predominantly resting channel block, the non-L-type isoforms showed substantial rapid open channel block manifested by a speeding of the apparent time course of current decay and block of the inactivated state. Consistent with these findings, we observed significant frequency-dependence of block and dependence on external Ba(2+) concentration for N-type, but not L-type, channels. We also systematically investigated the drug structural requirements for N-type channel inhibition. Blocking affinity varied with carbon chain length and showed a clear maximum at C12 and C13, with shorter and longer molecules producing progressively weaker peak current block. Overall, our data indicate that aliphatic monoamines may constitute a novel class of potent inhibitors of voltage-dependent Ca(2+) channels, with block being governed by rigid structural requirements and channel-specific state dependencies.

Intracellular calcium signalling in magnocellular neurones of the rat supraoptic nucleus: understanding the autoregulatory mechanisms

Oxytocin and vasopressin, released at the soma and dendrites of neurones, bind to specific autoreceptors and induce an increase in [Ca2+]i. In oxytocin cells, the increase results from a mobilisation of Ca2+ from intracellular stores, whereas in vasopressin cells, it results mainly from an influx of Ca2+ through voltage-dependent channels. The response to vasopressin is coupled to phospholipase C and adenylyl-cyclase pathways which are activated by V1 (V1a and V1b)- and V2-type receptors respectively. Measurements of [Ca2+]i in response to V1a and V2 agonists and antagonists suggest the functional expression of these two types of receptors in vasopressin neurones. The intracellular mechanisms involved are similar to those observed for the action of the pituitary adenylyl-cyclase-activating peptide (PACAP). Isolated vasopressin neurones exhibit spontaneous [Ca2+]i oscillations and these are synchronised with phasic bursts of electrical activity. Vasopressin modulates these spontaneous [Ca2+]i oscillations in a manner that depends on the initial state of the neurone, and such varied effects of vasopressin may be related to those observed on the electrical activity of vasopressin neurones in vivo.

Implications of all-or-none synaptic transmission and short-term depression beyond vesicle depletion: a computational study

The all-or-none character of transmission at central synapses is commonly viewed as evidence that only one vesicle can be released per action potential at a single release site. This interpretation is still a matter of debate; its resolution is important for our understanding of the nature of quantal response. In this work we explore observable consequences of the univesicular release hypothesis by studying a stochastic model of synaptic transmission. We investigated several alternative mechanisms for the all-or-none response: (1) the univesicular release constraint realized through lateral inhibition across presynaptic membrane, (2) the constraint of a single releasable vesicle per active zone, and (3) the postsynaptic receptor saturation. We show that both the univesicular release constraint and the postsynaptic receptor saturation lead to a limited amount of depression by vesicle depletion, so that depletion alone cannot account for the strong paired-pulse depression observed at some cortical synapses. Although depression can be rapid if there is only one releasable vesicle per active zone, this scenario leads to a limit on the transmission probability. We evaluate additional mechanisms beyond vesicle depletion, and our results suggest that the strong paired-pulse depression may be a result of activity-dependent inactivation of the exocytosis machinery. Furthermore, we found that the statistical analysis of release events, in response to a long stimulus train, might allow one to distinguish experimentally between univesicular and multivesicular release scenarios. We show that without the univesicular release constraint, the temporal correlation between release events is always negative, whereas it is typically positive with such a constraint if the vesicle fusion probability is sufficiently large.

An R-type Ca(2+) current in neurohypophysial terminals preferentially regulates oxytocin secretion

Multiple types of voltage-dependent Ca(2+) channels are involved in the regulation of neurotransmitter release (Tsien et al., 1991; Dunlap et al., 1995). In the nerve terminals of the neurohypophysis, the roles of L-, N-, and P/Q-type Ca(2+) channels in neuropeptide release have been identified previously (Wang et al., 1997a). Although the L- and N-type Ca(2+) currents play equivalent roles in both vasopressin and oxytocin release, the P/Q-type Ca(2+) current only regulates vasopressin release. An oxytocin-release and Ca(2+) current component is resistant to the L-, N-, and P/Q-type Ca(2+) channel blockers but is inhibited by Ni(2+). A new polypeptide toxin, SNX-482, which is a specific alpha(1E)-type Ca(2+) channel blocker (Newcomb et al., 1998), was used to characterize the biophysical properties of this resistant Ca(2+) current component and its role in neuropeptide release. This resistant component was dose dependently inhibited by SNX-482, with an IC(50) of 4.1 nM. Furthermore, SNX-482 did not affect the other Ca(2+) current types in these CNS terminals. Like the N- and P/Q-type Ca(2+) currents, this SNX-482-sensitive transient Ca(2+) current is high-threshold activated and shows moderate steady-state inactivation. At the same concentrations, SNX-482 blocked the component of oxytocin, but not of vasopressin, release that was resistant to the other channel blockers, indicating a preferential role for this type of Ca(2+) current in oxytocin release from neurohypophysial terminals. Our results suggest that an alpha(1E) or "R"-type Ca(2+) channel exists in oxytocinergic nerve terminals and, thus, functions in controlling only oxytocin release from the rat neurohypophysis.

Pulsed laser imaging of Ca(2+) influx in a neuroendocrine terminal

The surge of Ca(2+) that triggers vesicle fusion is shaped by the distribution of Ca(2+) channels and the physical relationship between those channels and the exocytotic apparatus. Although channels and the release apparatus are thought to be tightly associated at fast synapses, the arrangement at neuroendocrine cells is less clear. The distribution of Ca(2+) influx near release sites is difficult to determine because of spatial and temporal limitations on Ca(2+) imaging techniques. We now present spatially resolved images of Ca(2+) influx into rat neuroendocrine terminals on a millisecond time scale. Images of voltage-dependent Ca(2+) influx into neurohypophysial terminals were captured after excitation of Ca(2+)-sensitive dyes with pulses of laser light lasting a fraction of a microsecond. Submembranous Ca(2+) increases were detected during the first millisecond of an evoked Ca(2+) tail current. Steep gradients of Ca(2+) were evident, with concentrations near the membrane reaching above 1 microM during a 30 msec depolarization. Ca(2+) influx appeared evenly distributed, even when diffusion was restricted with an exogenous Ca(2+) chelator. During longer depolarizations, mean and peak Ca(2+) concentrations reached an asymptote in parallel, suggesting that Ca(2+) binding proteins near the membrane rapidly buffer Ca(2+) and do not become saturated during prolonged influx. These data support the hypothesis that exocytosis is activated in these terminals by the summation of influx through multiple, randomly spaced Ca(2+) channels.

Lambert-Eaton myasthenic syndrome

The Lambert-Eaton myasthenic syndrome is a neuromuscular disorder characterised by defective neurotransmitter release at autonomic neurones and presynaptic terminals of the neuromuscular junction. It is caused by an IgG autoantibody formed against especially the P/Q type of voltage-gated calcium channels (VGCC) which is an essential component of the mechanism of neurotransmitter release. Many patients have an associated small cell carcinoma of the lung which appears to provide the antigenic stimulus for antibody production, although there is another group with no underlying malignancy. Both groups show an association with immunological disorders. Assay of VGCC antibody titres and electrophysiological tests help to differentiate Lambert-Eaton myasthenic syndrome from other disorders of the neuromuscular junction. Several drugs and therapeutic interventions capable of producing significant clinical improvement are currently available. Patients should also be investigated for underlying tumours, the specific treatment of which can result in remission or amelioration of symptoms.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

K+ channel modulation in rodent neurohypophysial nerve terminals by sigma receptors and not by dopamine receptors

1. Sigma receptors bind a diverse group of chemically unrelated ligands, including pentazocine, apomorphine (a dopamine receptor agonist) and haloperidol (a dopamine receptor antagonist). Although sigma binding sites are widely distributed, their physiological roles are poorly understood. Here, the whole-terminal patch-clamp technique was used to demonstrate that sigma receptors modulate K+ channels in rodent neurohypophysis. 2. Previous work suggested that dopamine type 4 (D4) receptors modulate neurohypophysial K+ current, so this study initially tested the role of dopamine receptors. Experiments using transgenic mice lacking D2, D3 or D4 receptors indicated that the reduction of K+ current by PPHT and U101958 (ligands thought to be selective for dopamine receptors) is not mediated by dopamine receptors. The sensitivity of the response to U101958 (a drug that binds to D4 receptors) was the same in both wild-type and D4 receptor-deficient mice. 3. Experiments with other ligands revealed a pharmacological signature inconsistent with any known dopamine receptor. Furthermore, dopamine itself (at 100 microM) had no effect. Thus, despite the activity of a number of putative dopamine receptor ligands, dopamine receptors play no role in the modulation of neurohypophysial K+ channels. 4. Because of the negative results regarding dopamine receptors, and because some of the dopamine receptors ligands used here are known to bind also to sigma receptors, experiments were conducted to test for the involvement of sigma receptors. In rat neurohypophysis the sigma receptor ligands SKF10047, pentazocine, and ditolylguanidine all reversibly inhibited K+ current in a concentration-dependent fashion, as did haloperidol and apomorphine (ligands that bind to both dopamine and sigma receptors). The activity of these and other ligands tested here matches the reported binding specificity for sigma receptors. 5. Fifteen candidate endogenous sigma receptor ligands, including biogenic amines (e.g dopamine and serotonin), steroids (e.g. progesterone), and peptides (e.g. neuropeptide Y), were screened for activity at the sigma receptor. All were without effect. 6. Haloperidol reduced K+ current proportionally at all voltages without shifting the voltage dependence of activation and inactivation. Sigma receptor ligands inhibited current through two distinct K+ channels, the A-channel and the Ca2+-dependent K+ channel. In rat, all drugs reduced current through both channels proportionally, suggesting that both channels are modulated by a single population of sigma receptors. In contrast, mouse peptidergic nerve terminals either have two receptors which are sensitive to these drugs, or a single receptor that is differentially coupled to ion channel function. 7. The inhibition of voltage-activated K+ current by sigma receptors would be expected to enhance the secretion of oxytocin and vasopressin from the neurohypophysis.

Presynaptic calcium-channel currents in normal and csp mutant Drosophila peptidergic terminals

The study of regulated vesicle exocytosis, which underlies neurotransmitter and neuropeptide release, has benefited from a convergence of several independent approaches. These include the use of genetically tractable organisms and model preparations that allow a direct characterization of presynaptic ionic currents. Aiming for a comprehensive analysis of release, we had already developed a Drosophila preparation in which electrophysiological recordings from peptidergic terminals are feasible. Here, we report on the characterization of the Ca2+-channel currents present in these terminals. With Ba2+ as the charge carrier, the presynaptic membrane expresses a current type with high-activation threshold and little inactivation. This current is blocked by verapamil and diltiazem at micromolar concentrations, it is relatively insensitive to nifedipine and completely resistant to non-L-type Ca2+-channel antagonists. As a comparison, we also analysed the pharmacology of high-threshold Ba+2 currents on muscle fibres. A high-activation threshold Ca2+-channel current is also present in muscle fibres, albeit with a distinct pharmacological profile. Thus, peptidergic terminals and muscle fibres exhibit different subtypes of voltage-gated Ca2+ channels. The putative role of cysteine string protein (CSP) as a neuronal Ca2+-channel modulator was tested by examining the peptidergic presynaptic current in csp null mutants. We show that CSP is expressed in peptidergic boutons and abolished in the mutant. Direct recordings, under conditions that inhibit calcium influx into glutamatergic terminals, show that Ca2+-currents in peptidergic csp terminals are entirely normal. This result indicates that CSP is not a generic Ca2+-channel modulator and it might perform different functions in fast versus slow forms of release.

Osmoregulation of vasopressin neurons: a synergy of intrinsic and synaptic processes

The release of vasopressin into the general circulation varies as a function of plasma osmolality and therefore plays a major role in systemic osmoregulation. In vivo, the secretion of this hormone in the neurohypophysis is primarily determined by the rate of action potential discharge of the magnocellular neurosecretory cells (MNCs) in the hypothalamus. Experiments done over the past 20 years have clarified much of the neurophysiological basis underlying this important osmoregulatory reflex. As discussed here, recent findings indicate that the regulation of the firing rate of MNCs during changes in systemic osmolality involves the concerted modulation of mechanosensitive ion channels in MNCs, as well as excitatory glutamatergic inputs derived from forebrain regions such as the organum vasculosum of the lamina terminalis.

Membrane excitability in the neurohypophysis

The application of patch clamp technology to the neurohypophysis has contributed significantly to our understanding of the membrane events governing neuropeptide secretion. Nerve terminals within the posterior pituitary are now known to contain three distinct K+ channel subtypes, a rapidly inactivated channel (responsible for A current), a Ca(2+)-activated K+ channel, and a delayed rectifier channel. Activation of a D2 subtype dopamine receptor reduces both the A-current and current through the Ca2+ activated K+ channels. These actions can be expected to enhance neuropeptide release. Release of nitric oxide reduces the amplitude of the A-current but enhances current through the Ca(2+)-activated K+ channel, and this would provide mechanisms for more complex modes of regulation of release. Neurohypophysial nerve terminals also express at least two types of Ca2+ channels. The first is a dihydropyridine-sensitive, "L-type" channel. The second resembles the "N-type" Ca2+ channel. Patch clamp recordings have shown that tissue culture medium conditioned by exposure to T-cells enhances this Ca2+ current. This may represent a mechanistic link between activation of the immune system and functional membrane changes within the neurohypophysis. GABA-activated Cl- channels have also been described within the neurohypophysis, and these receptors can be modulated by neuroactive steroids. One of these, the progesterone derivative allopregnanolone, changes dramatically during female reproductive transitions. Such an interaction could represent a pivotal mechanistic step in the onset of parturition, and the neurohypophysial GABA receptor may hold promise as a target of therapeutic intervention in clinical cases of preterm labor.

Electrophysiological distinctions between oxytocin and vasopressin neurons in the supraoptic nucleus

Oxytocin and vasopressin neurons can be differentiated from one another, and from neurons in the immediately adjacent perinuclear zone, by their electrophysiological properties. In both sexes, oxytocin and vasopressin neurons are characterized by a prominent transient outward rectification which is conspicuously lacking in most perinuclear neurons. In addition, perinuclear neurons, some of which project to the supraoptic nucleus, exhibit a transient depolarization which underlies short bursts of spikes. Oxytocin neurons are characterized by: 1) the presence of a sustained outward rectifier above -50 mV, active below spike threshold; 2) a rebound depolarization following deactivation of the sustained rectification which can sustain short spike trains; and 3) a smaller transient outward rectification, probably associated with the potassium current, Ia. Vasopressin neurons show little of the sustained outward rectification and rebound depolarization, but have a stronger transient outward rectification. Although both cell types exhibit depolarizing afterpotentials, in vasopressin neurons these lead to plateau potentials underlying prolonged discharges. In oxytocin neurons, the depolarizing potential usually sustains a short spike discharge, but less often leads to prolonged bursts. These data suggest that the intrinsic properties of oxytocin and vasopressin neurons lead to quantitatively different forms of burst discharges, both of which may facilitate hormone release.

mu-opioid receptor activation inhibits N- and P-type Ca2+ channel currents in magnocellular neurones of the rat supraoptic nucleus

1. The whole-cell voltage-clamp technique was used to examine opioid regulation of Ba2+ currents (IBa) through voltage-sensitive Ca2+ channels in isolated magnocellular supraoptic neurones (MNCs). The effects of local application of mu-, delta- or kappa-opioid receptor selective agonists were examined on specific components of high voltage-activated (HVA) IBa, pharmacologically isolated by use of Ca2+ channel-subtype selective antagonists. 2. The mu-opioid receptor selective agonist, DAMGO, suppressed HVA IBa (in 64/71 neurones) in a naloxone-reversible and concentration-dependent manner (EC50 = 170 nM, Emax = 19.5 %). The DAMGO-induced inhibition was rapid in onset, associated with kinetic slowing and voltage dependent, being reversed by strong depolarizing prepulses. Low-voltage activated (LVA) IBa was not modulated by DAMGO. 3. Administration of kappa- (U69 593) or delta-selective (DPDPE) opioid receptor agonists did not affect IBa. However, immunostaining of permeabilized MNCs with an antibody specific for kappa1-opioid receptors revealed the presence of this opioid receptor subtype in a large number of isolated somata. 4. mu-opioid-induced inhibition in IBa was largely abolished after blockade of N-type and P-type channel currents by omega-conotoxin GVIA (1 microM) and omega-agatoxin IVA (100 nM), respectively. Quantitation of antagonist effects on DAMGO-induced reductions in IBa revealed that N- and P-type channels contributed roughly equally to the mu-opioid sensitive portion of total IBa. 5. These results indicate that mu-opioid receptors are negatively coupled to N- and P-type Ca2+ channels in the somatodendritic regions of MNCs, possibly via a membrane-delimited G-protein-dependent pathway. They also support a scheme in which opioids may act in part to modulate cellular activity and regulate neurosecretory function by their direct action on the neuroendocrine neurones of the hypothalamic supraoptic neucleus.

L-Type calcium channels mediate a slow excitatory synaptic transmission in rat midbrain dopaminergic neurons

Patch pipettes were used to record whole-cell synaptic currents under voltage-clamp in dopaminergic neurons in slices of rat substantia nigra pars compacta and ventral tegmental area. We report that dihydropyridines (DHPs), L-type Ca2+ channel antagonists, depressed a slow EPSC (EPSCslow) evoked by a train of focally delivered electrical stimuli. In fact, the amplitude of the EPSCslow was reduced by the DHP antagonists nifedipine (1-100 microM), nimodipine (1-100 microM), and isradipine (30 nM-100 microM) in a concentration-dependent and reversible manner. On the other hand, Bay-K 8644 (1 microM), an L-type Ca2+ channel agonist, increased the EPSCslow. The DHPs depressed the EPSCslow only when the high-frequency stimulation that was used to evoke this synaptic current lasted >70 msec. On the other hand, Bay-K 8644 increased the amplitude of the EPSCslow only when it was evoked by a train <70 msec. Moreover, the DHPs did not affect the EPSCfast, the IPSCfast, and the IPSCslow. The inhibition of the EPSCslow caused by the DHPs is attributed to presynaptic mechanisms because (1) the inward current generated by exogenously administered glutamate was not affected and (2) the EPSCslow was reduced to a similar degree even when the activation state of postsynaptic L-type Ca2+ channels was changed by holding the neurons at -100, -60, and +30 mV. Finally, a DHP-sensitive component of the EPSCslow could even be detected after the blockade of N-, Q-, and P-type Ca2+ channels by the combination of omega-conotoxin GVIA, omega-agatoxin IVA, and omega-conotoxin MVIIC. Taken together, these results indicate that under certain patterns of synaptic activity, L-type Ca2+ channels regulate the synaptic release of excitatory amino acids on the dopaminergic neurons of the ventral mesencephalon.