Stimulus-secretion coupling

Electrophysiology of Hypothalamic Magnocellular Neurons In vitro: A Rhythmic Drive in Organotypic Cultures and Acute Slices

Hypothalamic neurohormones are released in a pulsatile manner. The mechanisms of this pulsatility remain poorly understood and several hypotheses are available, depending upon the neuroendocrine system considered. Among these systems, hypothalamo-neurohypophyseal magnocellular neurons have been early-considered models, as they typically display an electrical activity consisting of bursts of action potentials that is optimal for the release of boluses of the neurohormones oxytocin and vasopressin. The cellular mechanisms underlying this bursting behavior have been studied in vitro, using either acute slices of the adult hypothalamus, or organotypic cultures of neonatal hypothalamic tissue. We have recently proposed, from experiments in organotypic cultures, that specific central pattern generator networks, upstream of magnocellular neurons, determine their bursting activity. Here, we have tested whether a similar hypothesis can be derived from in vitro experiments in acute slices of the adult hypothalamus. To this aim we have screened our electrophysiological recordings of the magnocellular neurons, previously obtained from acute slices, with an analysis of autocorrelation of action potentials to detect a rhythmic drive as we recently did for organotypic cultures. This confirmed that the bursting behavior of magnocellular neurons is governed by central pattern generator networks whose rhythmic drive, and thus probably integrity, is however less satisfactorily preserved in the acute slices from adult brains.

Synaptotagmin IV: a multifunctional regulator of peptidergic nerve terminals

Many members of the synaptotagmin (Syt) protein family bind Ca(2+) and trigger exocytosis, but some Syt proteins appear to have no Ca(2+)-dependent actions and their biological functions remain obscure. Syt IV is an activity-induced brain protein with no known Ca(2+)-dependent interactions and its subcellular localization and biological functions have sparked considerable controversy. We found Syt IV on both micro- and dense-core vesicles in posterior pituitary nerve terminals in mice. In terminals from Syt IV knockout mice compared with those from wild types, low Ca(2+) entry triggered more exocytosis, high Ca(2+) entry triggered less exocytosis and endocytosis was accelerated. In Syt IV knockouts, dense-core and microvesicle fusion was enhanced in cell-attached patches and dense-core vesicle fusion pores had conductances that were half as large as those in wild types. Given the neuroendocrine functions of the posterior pituitary, changes in Syt IV levels could be involved in endocrine transitions involving alterations in the release of the neuropeptides oxytocin and vasopressin.

Biological clocks and the practice of psychiatry

Endogenous biological clocks enable living species to acquire some independence in relation to time. They improve the efficiency of biological systems, by allowing them to anticipate future constraints on major physyological systems and cell energy metabolism. The temporal organization of a giwen biological function can be impaired in its coordination with astronomical time or with other biological function. There are also external conditions that influence biological clocks. This temporal organization is complex, and it is possible that a series of psychiatric disorders and syndromes involve primary or secondary changes in biological clocks: seasonal and other mood disorders, premenstrual syndromes, social jet lag, free-running rhythms, and several sleep disorders are among them. In this review, we describe the main concepts relevant to chronobiology and explore the relevance of knowledge about biological clocks to the clinical practice of psychiatry

A mechanical spike accompanies the action potential in Mammalian nerve terminals

Large and rapid changes in light scattering accompany secretion from nerve terminals of the mammalian neurohypophysis (posterior pituitary). In the mouse, these intrinsic optical signals are intimately related to the arrival of the action potential E-wave and the release of arginine vasopressin and oxytocin (S-wave). Here we have used a high bandwidth atomic force microscope to demonstrate that these light-scattering signals are associated with changes in terminal volume that are detected as nanometer-scale movements of a cantilever positioned on top of the neurohypophysis. The most rapid mechanical response ("spike"), having a duration shorter than the action potential but comparable to that of the E-wave, represents a transient increase in terminal volume due to water movement associated with Na(+)-influx. The slower mechanical event ("dip"), on the other hand, depends upon Ca(2+)-entry as well as on intraterminal Ca(2+)-transients and, analogously to the S-wave, seems to monitor events associated with secretion.

Acute effects of prostaglandin F(2alpha) on systemic oxytocin and progesterone concentrations during the mid- or late-luteal phase in mares

The acute effects of prostaglandin F(2alpha) (PGF) on circulating oxytocin and progesterone concentrations were characterized in mares during the mid- or late-luteal phase. Pony mares were randomly assigned to the following experimental groups based on treatment with PGF (2.5mg) or saline on Day 8 or Day 13 (Day 0=ovulation): PGF-8, PGF-13, saline-8, or saline-13 (n=7/group). Mares were fitted with indwelling, jugular vein catheters and two blood samples (-5 and 0 min) were collected prior to treatment. Treatments were administered into the jugular vein (0 min) and blood collection continued thereafter at 1 min intervals until 5 min and then at 5 min intervals until 60 min. Based on the combined data of -5 and 0 min samples, mares on Day 8 had greater (P<0.05) oxytocin concentrations than mares on Day 13. On Day 8, PGF treatment resulted in a biphasic pattern of oxytocin release. Oxytocin concentrations increased (P<0.05) 1 min after PGF treatment, decreased (P<0.05) from 1 to 10 min, and increased (P<0.05) from 10 to 30 min. Oxytocin concentrations were greater (P<0.05) from 1 to 3 min in PGF-treated than saline-treated mares and at most sample times from 15 to 60 min. On Day 13, oxytocin concentrations were greater (P<0.05) in PGF-treated than in saline-treated mares for most sample times. Mares treated with PGF on Day 8 had greater (P<0.05) oxytocin concentrations at 25, 30, and 40 min than mares on Day 13. Progesterone concentrations on Day 8 also increased by 1 min after PGF, decreased toward basal concentrations by 2-3 min, and then increased to a maximum 10 min after treatment. Subsequently, circulating progesterone decreased (P<0.05) below pretreatment concentrations by 40-50 min after PGF. In conclusion, treatment with PGF resulted in an immediate and biphasic increase in progesterone concentrations prior to the expected decrease. Treatment of mares with PGF on Day 8 resulted in an overall greater increase in systemic oxytocin concentrations compared to treatment on Day 13, and the increase on Day 8 was biphasic.

Ethanol reduces the duration of single evoked spikes by a selective inhibition of voltage-gated calcium currents in acutely dissociated supraoptic neurons of the rat

The effects of ethanol were studied on single evoked spikes recorded at 20 degrees C with the perforated-patch method in acutely dissociated rat supraoptic neurons. In seven out of eight neurons, ethanol (50 mM) significantly reduced the spike duration by selectively decreasing the decay time (82+/-2% of the control), leaving the amplitude and rise time unaffected. Resting potential and threshold did not change. Similarly, CdCl2 at a concentration of 100 microM, which blocks all voltage-activated calcium current in the supraoptic neurons, reduced the decay time of single evoked spikes (76+/-3% of the control, n=10) without modifying the other above-mentioned parameters. In addition, exposure to 100 microM CdCl2 prevented any subsequent effect of 50 mM ethanol (n = 5). Exposure to apamin (10 nM) and iberiotoxin (10 nM) did not have any effect on single evoked spikes. Because these concentrations are effective in blocking, respectively, small (SK) and large (BK) conductance calcium-dependent potassium channels in these neurons, this result shows that these currents are not involved in either the shaping of single evoked spikes or the actions of ethanol on spike shape. The sustained component of whole-cell recorded calcium current measured at -10 mV (hp -60 mV) was inhibited by ethanol in a dose-dependent manner, with a significant effect detectable at 25 mM. Exposure to 50 mM ethanol significantly reduced the sustained current to 70+/-5% of the control (n=12), without any apparent shift of the current-voltage relationship. Control exposure of the neurons to either 50 mM urea or 50 mM sucrose did not affect the voltage-gated calcium currents. We conclude that ethanol reduces the duration of single evoked spikes by a specific inhibition of voltage-activated calcium currents. The results suggest that, in addition to its direct effects on release of vasopressin and oxytocin from neurohypophysial terminals, ethanol could also affect hormonal release via changes in firing patterns arising in the cell bodies.

Presynaptic excitability

Based on functional characterizations with electrophysiological techniques, the channels in nerve terminals appear to be as diverse as channels in nerve cell bodies (Table I). While most presynaptic Ca2+ channels superficially resemble either N-type or L-type channels, variations in detail have necessitated the use of subscripts and other notations to indicate a nerve terminal-specific subtype (e.g., Wang et al., 1993). Variations such as these pose a serious obstacle to the identification of presynaptic channels based solely on the effects of channel blockers on synaptic transmission. Pharmacological sensitivity alone is not likely to help in determining functional properties. Crucial details, such as voltage sensitivity and inactivation, require direct examination. It goes without saying that every nerve terminal membrane contains Ca2+ channels as an entry pathway so that Ca2+ can trigger secretion. However, there appears to be no general specification of channel type, other than the exclusion of T-type Ca2+ channels. T-type Ca2+ channels are defined functionally by strong inactivation and low threshold. Some presynaptic Ca2+ channels inactivate (posterior pituitary and Xenopus nerve terminals), and others have a somewhat reduced voltage threshold (retinal bipolar neurons and squid giant synapse). Perhaps it is just a matter of time before a nerve terminal Ca2+ channel is found with both of these properties. The high threshold and strong inactivation of T-type Ca2+ channels are thought to be adaptations for oscillations and the regulation of bursting activity in nerve cell bodies. The nerve terminals thus far examined have no endogenous electrical activity, but rather are driven by the cell body. On functional grounds, it is then reasonable to anticipate finding T-type Ca2+ channels in nerve terminals that can generate electrical activity on their own. The rarity of such behavior in nerve terminals may be associated with the rarity of presynaptic T-type Ca2+ channels. In four of the five preparations reviewed in this chapter--motor nerve, squid giant synapse, ciliary ganglion, and retina bipolar neurons--evidence was presented that supports a location for Ca2+ channels that is very close to active zones of secretion. All of these synapses secrete from clear vesicles, and the speed and specificity of transduction provided by proximity may be a common feature of these rapid synapses. In contrast, the posterior pituitary secretion apparatus may be triggered by higher-affinity Ca2+ receptors and lower concentrations of Ca2+ (Lindau et al., 1992). This would correspond with the slower performance of peptidergic secretion, but because of the large stimuli needed to evoke release from neurosecretosomes, the possibility remains that the threshold for secretion is higher than that reported. While the role of Ca2+ as a trigger of secretion dictates a requirement for voltage-activated Ca2+ channels as universal components of the presynaptic membrane, the presence of other channels is more difficult to predict. Depolarizations caused by voltage-activated Na+ channels activate the presynaptic Ca2+ channels, but whether this depolarization requires Na+ channels in the presynaptic membrane itself may depend on the electrotonic length of the nerve terminal. Variations in density between motor nerve terminals may reflect species differences in geometry. The high Na+ channel density in the posterior pituitary reflects the great electrotonic length of this terminal arbor. Whether Na+ channels are abundant or not in a presynaptic membrane, K+ channels provide the most robust mechanism for limiting depolarization-induced Ca2+ entry. K+ channel blockers enhance transmission at most synapses. In general, K+ channels are abundant in nerve terminals, although their apparent lower priority compared to Ca2+ channels in the eyes of many investigators leaves us with fewer detailed investigations in some preparations. Most nerve terminals have more than

Effects of funnel web spider toxin on Ca2+ currents in neurohypophysial terminals

Funnel web spider toxin (FTX) is reportedly a specific blocker of P-type Ca2+ channels. The effects of FTX on the Ca2+ currents of isolated neurohypophysial nerve terminals of the rat were investigated using the 'whole-cell' patch-clamp technique. Both the transient and long-lasting Ca2+ current components were maximally elicited by depolarization from a holding potential equal to the normal terminal resting potential (-90 mV). Externally applied FTX inhibited the high-voltage-threshold, transient component of the Ca2+ current in a concentration-dependent manner, with a half-maximal inhibition at a dilution of approximately 1:10000. FTX also shifted the peak current of the I-V relationship by +10 mV. The long-lasting Ca2+ current component, which is sensitive to L-type Ca2+ channel blockers, was insensitive to FTX. The transient current, which is sensitive to omega-conotoxin GVIA, was completely blocked by FTX. These results suggest that there could be a novel, inactivating Ca2+ channel in the rat neurohypophysial terminals which is affected by both N-type and P-type Ca2+ channel blockers.

Frequency dependence of vasoactive intestinal polypeptide release and vagally induced tachycardia in the canine heart

We evaluated the frequency dependence of vasoactive intestinal polypeptide (VIP) release from the parasympathetic nerves to the canine heart. In intact animals in the presence of beta-adrenergic receptor blockade (propranolol, 0.5 mg/kg), the cervical vagosympathetic trunks were stimulated at various frequencies before and after the administration of atropine (0.1 mg/kg). The stimulations before atropine produced a classical bradycardia that progressed to cardiac arrest when the stimulation frequency was raised above 10 to 15 Hz. After atropine, vagal stimulation at various frequencies increased heart rate. The heart rate reached a maximum increase of 21 +/- 3 beats per min at a stimulation frequency of 20 Hz. In an isolated atrial preparation in which the VIP outflow was measured, the tachycardia elicited after atropine had a frequency dependence similar to that obtained in vivo. The peak increase of 23 +/- 3% above the basal rate (95 +/- 8 beats per min) occurred at a stimulation frequency of 20 Hz. The VIP outflow paralleled the tachycardia response (r = 0.95); the maximum outflow of VIP was 172 +/- 54 pg/(min . 100 g wet wt) and was evoked at a stimulation frequency of 20 Hz. This suggests that the vagally induced tachycardia is mediated, at least partly, by VIP.

Three potassium channels in rat posterior pituitary nerve terminals

1. The patch clamp technique was used to investigate the K+ channels in the membranes of nerve terminals in thin slices prepared from the rat posterior pituitary. 2. Depolarization of the membrane produced a high density of K+ current. With a holding potential of -80 mV, test pulses to +50 mV activated a K+ current which was inactivated by 65% within 200 ms. Hyperpolarizing prepulses enhanced the transient K+ current, with half-maximal enhancement at -87 mV. Depolarizing prepulses reduced or eliminated the transient K+ current. 3. In cell-attached patches formed with pipettes containing 130 mM KCl, three types of K+ channel could be distinguished on the basis of single-channel properties. One channel had a conductance of 33 pS and was inactivated with a time constant of 18 ms. A second channel had a conductance of 134 pS and was inactivated with a time constant of 71 ms. A third channel had a conductance of 27 pS, was activated relatively slowly with a time constant of 65 ms, and was not inactivated during test pulses of up to one second in duration. 4. Inactivation of the whole-cell K+ current was a biphasic process with two exponential components. The fast component had a time constant of 22 ms (at +50 mV), corresponding well with the time constant of decay of average current in cell-attached patches containing only the rapidly inactivating K+ channel. The slow component of inactivation had a time constant of 104 ms (at +50 mV), which was similar to but slightly slower than the time constant of decay of the average current in cell-attached patches containing only the slowly inactivating K+ channel. Inactivation of the slow transient K+ current became more rapid with increasing depolarization. 5. The low-conductance rapidly inactivating K+ channel had a lower voltage threshold for activation than the other two K+ channels. 6. Both inactivating K+ channels were enhanced in a similar manner by prior hyperpolarization. There was no difference with regard to voltage mid-point or steepness. 7. The large-conductance slowly inactivating K+ channel was activated by Ca2+ at the inner membrane surface. The resting intracellular Ca2+ was sufficiently high to produce significant activation of this channel without depolarization-induced Ca2+ entry. 8. Removal of Ca2+ from the bathing solution produced a -10 mV shift in the voltage dependence of enhancement of both transient K+ currents by prior hyperpolarization. This could be explained as a surface charge effect.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane retrieval following exocytosis in isolated neurosecretory nerve endings

In the neurosecretory nerve endings of the neurohypophysis depolarization-induced exocytosis is followed by endocytosis of vacuole-like structures with diameter similar to that of neurosecretory granules. However, it remains unknown whether the membrane of the endocytotic vacuoles is comprised primarily of retrieved secretory granule membrane, plasma membrane or of a mixture of the granule and plasma membrane. In the present paper membrane retrieval following depolarization-induced exocytosis has been studied in isolated neurosecretory nerve endings from the rat neurohypophysis. The origin of the retrieved membrane was assessed by pre-labeling the plasma membrane with an antibody against neural cell adhesion molecule, a plasma membrane specific protein. Horseradish peroxidase was used as an index of fluid endocytosis and secretion of vasopressin was measured by radioimmunoassay. Following potassium-induced depolarization, endocytotic vacuoles showed labeling with the fluid phase marker horseradish peroxidase but never showed significant neural cell adhesion molecule labeling. The time-course of endocytosis following closely that of exocytosis as endocytotic vacuoles labeled with horseradish peroxidase were only observed when the fluid phase marker was present in the extracellular medium during the period of evoked exocytosis. Our results are consistent with a model in which in neurosecretory nerve endings, after transient exocytotic fusion of the granule membrane with the plasma membrane, the granule membrane is rapidly and selectively retrieved into the nerve endings in the form of vacuoles similar in size to that of the neurosecretory granules.

Secretion from rat neurohypophysial nerve terminals (neurosecretosomes) rapidly inactivates despite continued elevation of intracellular Ca2+

Cytoplasmic calcium concentration was measured in neurosecretory nerve terminals (neurosecretosomes) isolated from rat neurohypophyses by fura-2 fluorescence measurements and digital video microscopy. Hormone release and cytoplasmic calcium concentration were measured during depolarizations induced by elevated extracellular potassium concentration. During prolonged depolarizations with 55 mM [K+]o, the cytoplasmic calcium concentration remained elevated as long as depolarization persisted, while secretion inactivated after the initial sharp rise. The amplitude and duration of the increase in [Ca2+]i was dependent on the degree of depolarization such that upon low levels of depolarizations (12.5 mM or 25 mM [K+]o), the calcium responses were smaller and relatively transient, and with higher levels of depolarization (55 mM [K+]o) the responses were sustained and were higher in amplitude. Responses to low levels of depolarization were less sensitive to the dihydropyridine calcium channel blocker, nimodipine, while the increase in [Ca2+]i induced by 55 mM [K+]o became transient, and was significantly smaller. These observations suggest that these peptidergic nerve terminals possess at least two different types of voltage-gated calcium channels. Removal of extracellular sodium resulted in a significant increase in [Ca2+]i and secretion in the absence of depolarizing stimulus, suggesting that sodium-calcium exchange mechanism is operative in these nerve terminals. Although the [Ca2+]i increase was of similar magnitude to the depolarization-induced changes, the resultant secretion was 10-fold lower, but the rate of inactivation of secretion, however, was comparable.

Widespread release of peptides in the central nervous system: quantitation of tannic acid-captured exocytoses

Tannic acid methods have been applied to capture the exocytosis of peptide-containing granules from peptidergic neurons. The captured exocytoses have been quantitated to assess the proportion and amount of peptide released at different parts of the neuronal membrane. Examination of hypothalamic synaptic boutons shows that only about one-half of the peptidergic vesicles is exocytosed into the synaptic cleft and also that exocytosis also occurs from undilated peptidergic axons. Study of the magnocellular neurosecretory system reveals that all parts of their extensive terminal arborization appear to be equally capable to exocytose peptide. Only about one-half of their peptide is released from their nerve endings, which about the capillaries. The remainder is released much deeper in the lobules of secretory tissue where its principal target(s) could be the pituicytes and/or neurosecretory axons. Dendrites of magnocellular neurons are also capable of releasing peptide by exocytosis and dendrites could release sufficient oxytocin and vasopressin to account for the peptide known to be released into the hypothalamus. We conclude that peptidergic neurons release substantial amounts of peptides from all of their processes and that this must be taken into account when considering what functions those peptides might serve.

Involvement of non-selective cationic channels in the generation of pacemaker depolarizations and firing behaviour in cultured frog melanotrophs

The firing patterns of cultured frog melanotrophs were studied using the patch-clamp technique. In the cell-attached mode, unitary currents were frequently observed as well as biphasic waveforms which were attributed to action potentials 'leaking' through the patch membrane. An inwardly rectifying single-unit current was observed with pipette solutions containing either 100 mM K+ or 100 mM Na+. Under both conditions, these channels displayed an identical I/V relationship, yielding a unitary conductance of 110 pS. The channel opening time was extremely long (50-3000 ms) and single-channel currents showed typical relaxations, which triggered bursts of action currents. In the whole-cell configuration large (2-12 mV) fluctuations in the membrane voltage of current-clamped cells frequently occurred. The deflections appeared to result from single-channel currents. Depolarizing 'events' often led to the discharge of action potentials. Taken together, our data provide evidence for the existence of high-conductance cationic channels in frog pars intermedia cells. These channels may, at least in some cases, be responsible for the generation of pacemaker depolarizations, thereby regulating firing behaviour. It is concluded, that the current traversing a single channel can seriously affect the membrane potential and excitability of frog melanotrophs.

Activation and inactivation of oxytocin and vasopressin release from isolated nerve endings (neurosecretosomes) of the rat neurohypophysis

Neurosecretory terminals (neurosecretosomes, NSS) were isolated from rat neurohypophyses. High [K+]o or veratridine stimulated secretion of vasopressin and oxytocin by up to approximately 100-fold. Stimulated secretion was dependent on calcium and temperature, and could be elicited from NSS maintained in culture for 4 days. After overnight culture of the NSS, secretion was still inhibited by calcium channel blockers (cobalt, dihydropyridines, omega-conotoxin, D 600) and kappa opiates (dynorphin and U50488). Ionomycin evoked dose- and calcium-dependent hormone release, with a Hill coefficient for calcium of 1.74. High [K+]o enhanced the 5 microM ionomycin-induced secretion, apparently through calcium entry rather than depolarization, as the increase in secretion was abolished by 100 microM D 600. During prolonged depolarization the hormone secretion peaked within 2 min, then declined to near basal levels. Depolarization for 25 min without calcium neither activated secretion nor prevented subsequent secretion on readdition of calcium, suggesting that the decline in secretion was not due to membrane depolarization. Indeed, the rates of decline in secretion were similar for different levels of depolarization (0.070 +/- 0.003 and 0.081 +/- 0.003 min-1 for 25 and 45 mM [K+]o, respectively). Four minutes after the onset of continuous depolarization (45 mM [K+]o) in the presence of calcium, the declining secretion was still dependent on voltage-activated calcium influx through channels sensitive to D 600 and nitrendipine. The results presented here suggest that the decline in secretion during prolonged depolarizing stimuli may be due to exhaustion, inactivation, or desensitization of a calcium-triggered event.

Membrane routing during exocytosis and endocytosis in neuroendocrine neurones and endocrine cells: use of colloidal gold particles and immunocytochemical discrimination of membrane compartments

The hypothesis that the retrieval of membranes of neurohypophysial neurosecretory granules (NSG) and small electron-lucent microvesicles occurs by different routes was tested by incubating neurohypophysial neurosecretosomes with colloidal gold particles of various sizes. Neurosecretosomes derived from normal Long Evans rats and incubated in media of normal ionic composition endocytosed a few small (less than 25 nm) gold particles into 40-50 nm electron-lucent microvesicles. After depolarisation, more small gold particles were found in microvesicles, and small and large (greater than 25 nm) gold particles in vacuoles. Oxytocin-containing neurosecretosomes derived from Brattleboro rats, which contain 160 nm-diameter NSG, endocytosed gold particles in a pattern indistinguishable from that of neurosecretosomes from Long Evans rats. However, neurosecretosomes derived from defective vasopressin neurones of Brattleboro rats, which contain microvesicles, small vacuoles, and a few 100 nm dense-cored vesicles, but no 160 nm NSG, endocytosed only small colloidal gold particles. Early after depolarisation the gold particles were present only in microvesicles, but later some could be found in vacuoles and lysosome-like structures. Immunogold cytochemistry using a polyclonal antiserum raised against microvesicle-rich neurosecretosomes derived from Brattleboro rats labelled microvesicles in the posterior pituitary strongly, NSG weakly, and vacuoles to a variable extent. These data together indicate that, after exocytosis, the membranes of NSG are recaptured as large vacuoles. Microvesicles are exocytosed and endocytosed separately.

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

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

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

1. Calcium accumulation in a preparation of nerve terminals isolated from the rat neurohypophysis was measured both in rapid (10-60 s) and long-term (up to 60 min) uptake experiments, by use of 45Ca2+ as radiotracer and ion-exchange chromatography as separation method. Unless otherwise stated all experiments have been performed in the absence from the incubation media of secretagogues or depolarizing agents. 2. The uptake of 45Ca2+ in nerve terminals was linear up to 30-45 s, with an apparent initial rate of uptake of 0.98 nmol Ca2+ (mg protein)-1 min-1. 3. The level of 45Ca2+ accumulation was sensitive to manipulations of electrochemical gradient for Na+ across the plasma membrane. Alterations of extracellular concentrations of Na+ affected secretory activity to a larger extent than manipulations of internal Na+. These effects were not qualitatively dependent on the nature of the replacement for Na+. 4. Removal of extracellular Na+ induced a significant increase of both the level of 45Ca2+ accumulation and of the apparent initial rate of uptake. The concentration for half-maximal stimulatory effect was 40 mM-Na+. 5. The analysis of the stimulatory effect of high extracellular K+ on the 45Ca2+ accumulation reveals at least two components: a depolarization and an intrinsic K+ effect. 6. Sodium channel inhibitors (TTX, 1.25 microM) decreased significantly the level of 45Ca2+ accumulation, an effect which was evident from the first minute of exposure to the drug. 7. A specific L-type Ca2+ channel blocker (nicardipine) inhibited 45Ca2+ uptake, in a dose-dependent manner. Simultaneous addition of both TTX and nicardipine (20 microM) decreases the 45Ca2+ uptake up to 50%. 8. In conclusion, the uptake of Ca2+ in isolated peptidergic nerve terminals, incubated in resting conditions, is mediated by at least three pathways: a TTX-sensitive and a nicardipine (dihydropyrine)-sensitive pathway and through a Na(+)-Ca2+ exchange-dependent mechanism. The principal route of Ca2+ entry appears to be through TTX-sensitive channels.

Exocytosis in neurohypophysial nerve terminals is not coupled to protein kinase C translocation

Protein kinase C (PKC) has been implicated in the mechanism of exocytosis, although various studies have been unable to pinpoint actual translocation or activation of PKC during exocytosis. We have studied, in neurohypophysial nerve endings, intracellular Ca2+ levels, secretion of neuropeptides and PKC translocation. Neurohormone secretion was triggered by K(+)-induced or electrically induced depolarization in both the absence and the presence of phorbol esters. PKC was translocated from the cytosol to the membrane on electrical stimulation or K+ depolarization, but not to the extent obtained with phorbol ester. Data are presented clearly demonstrating that the translocation of PKC from cytosol to membrane is not required for exocytosis, nor does it alter in any way neuropeptide release from neurohypophysial nerve terminals.

Emerging concepts of structure-function dynamics in adult brain: the hypothalamo-neurohypophysial system

As the first known of the mammalian brain's neuropeptide systems, the magnocellular hypothalamo-neurohypophysial system has become a model. A great deal is known about the stimulus conditions that activate or inactivate the elements of this system, as well as about many of the actions of its peptidergic outputs upon peripheral tissues. The well-characterized actions of two of its products, oxytocin and vasopressin, on mammary, uterine, kidney and vascular tissues have facilitated the integration of newly discovered, often initially puzzling, information into the existing body of knowledge of this important regulatory system. At the same time, new conceptions of the ways in which neuropeptidergic neurons, or groups of neurons, participate in information flow have emerged from studies of the hypothalamo-neurohypophysial system. Early views of the SON and PVN nuclei, the neurons of which make up approximately one-half of this system, did not even associate these interesting, darkly staining anterior hypothalamic cells with hormone secretion from the posterior pituitary. Secretion from this part of the pituitary, it was thought, was neurally evoked from the pituicytes that made the oxytocic and antidiuretic "principles" and then released them upon command. When these views were dispelled by the demonstration that the hormones released from the posterior pituitary were synthesized in the interesting cells of the hypothalamus, the era of mammalian central neural peptidergic systems was born. Progress in developing an ever more complete structural and functional picture of this system has been closely tied to advancements in technology, specifically in the areas of radioimmunoassay, immunocytochemistry, anatomical tracing methods at the light and electron microscopic levels, and sophisticated preparations for electrophysiological investigation. Through the judicious use of these techniques, much has been learned that has led to revision of the earlier held views of this system. In a larger context, much has been learned that is likely to be of general application in understanding the fundamental processes and principles by which the mammalian nervous system works.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of anti-calmodulin agents on vasopressin release in vitro to depolarization and calcium ionophore

Calmodulin has been implicated in transducing the effects of Ca2+ on synaptic transmission and hormone release, including osmotically-stimulated vasopressin (AVP) release. If the anti-calmodulin agents block AVP release secondary to inhibition of Ca2(+)-calmodulin interactions, these drugs should inhibit AVP release to stimuli increasing Ca2+ influx via different mechanisms. Hypothalamo-neurohypophysial complexes (HNC) were exposed to ionomycin, Bay K 8644, or veratridine either alone, with any one of three distinct chemical classes of anti-calmodulin agent, or with a Ca2+ channel antagonist. All the anti-calmodulin agents impaired AVP release to ionomycin, while Ca2+ channel blockade did not. Conversely, Ca2+ channel antagonism completely blocked AVP release in response to Bay K 8644, but the anti-calmodulin agents had no effect. None of the inhibitors prevented veratridine-induced AVP release. These results are consistent with the hypothesis that the anti-calmodulin agents tested inhibit AVP release by their membrane stabilizing properties rather than by antagonizing Ca2(+)-calmodulin in HNC. Depolarization initiated by Na+ influx may stimulate Na(+)-Ca2+ exchange by a mechanism independent of slow Ca2+ channels as well.

Calcium uptake and protein phosphorylation in myenteric neurons, like the release of vasoactive intestinal polypeptide and acetylcholine, are frequency dependent

The mechanism of the electrical-to-chemical decoding involved in the preferential release of the transmitters acetylcholine and vasoactive intestinal polypeptide (VIP) by electrical field stimulation at low (5 Hz) and high (50 Hz) frequencies was studied in superfused myenteric neurons. The stimulation-induced uptake of 45Ca2+ accompanying high frequency stimulation was markedly reduced by 10 microM nifedipine, a specific blocker of L-type voltage-sensitive Ca2+ channels (VSCCs), as was also the preferential high-frequency release of VIP. By contrast, the 45Ca2+ uptake during low-frequency stimulation was somewhat lower per pulse, and neither this uptake nor the preferential release of acetylcholine occurring at this frequency was significantly reduced by nifedipine. These findings suggest that the release of acetylcholine and VIP involve different VSCCs. The pattern of in vitro protein thiophosphorylation in tissue extracts of differentially stimulated myenteric neurons involved polypeptides of 205, 173, 86, 73, 57, 54, 46, 32, 28, and 24 kDa and was also markedly stimulus and nifedipine dependent. This suggests that different phosphoproteins are involved during the frequency-dependent activation of the different Ca2+ channels and exocytotic mechanisms.

Differential effects of potassium channel blockers on neurohypophysial release of oxytocin and vasopressin. Evidence for frequency-dependent interaction with the endogenous opioid inhibition of oxytocin release

Isolated rat neurohypophyses were fixed by their stalks to a platinum wire electrode and superfused with Krebs-HEPES solution. Vasopressin and oxytocin released into the medium were determined by specific radioimmunoassays. Hormone secretion was increased by electrical stimulation of the pituitary stalk at different frequencies. The effects of several potassium channel blockers, tetraethyl-ammonium (TEA) ions, 4-aminopyridine (4-AP) and 3,4-diaminopyridine (3,4-DAP) were tested. The release of vasopressin and oxytocin evoked by electrical stimulation with 900 pulses at 15 Hz (about 900 and 1,000 microU, respectively) was about 10 times higher than that evoked by 900 pulses at 3 Hz. Both 10 and 30 mmol/l TEA enhanced the release of vasopressin evoked by stimulation at 3 and 15 Hz, by 25- and 2-fold, respectively, to attain a maximum release of about 1,800 microU per stimulation. The stimulated release of oxytocin attained a maximum of about 9,000 microU at 15 Hz in the presence of 10 mmol/l TEA or at 3 Hz with 30 mmol/l TEA. Thus, in the presence of maximally effective concentrations of TEA both stimulation frequencies (3 and 15 Hz) were equieffective in evoking release of vasopressin and oxytocin. 4-AP or 3,4-DAP enhanced the release of vasopressin evoked by 15 Hz stimulation maximally to about 1,600 microU. In the presence of 4-AP or 3,4-DAP the release of oxytocin evoked by stimulation at 15 Hz increased maximally to about 8,000 microU and that evoked by stimulation at 3 Hz to about 1,500 microU.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of raised extracellular potassium on the excitability of, and hormone release from, the isolated rat neurohypophysis

1. Single neurohypophyses from male rats were maintained in an in vitro perifusion chamber. Ion-sensitive microelectrodes were introduced into the tissue to measure changes in [K+]o and [Ca2+]o during electrical stimulation. 2. Electrical stimulation at 6 Hz for 1 min and 30 Hz for 12 s raised [K+]o by 5.4 +/- 0.4 and 13.5 +/- 0.5 mM (mean +/- S.E.M., n = 8) respectively. To investigate the effects of raised [K+]o on the excitability of the neurosecretory terminals, stimulations were repeated in media of altered K+ concentration. The increase in [K+]o evoked by 6 Hz stimulation was elevated in 10 mM-K+ medium (133% of that in 5 mM-K+ medium) and reduced in 0 mM-K+ medium and in 25 mM-K+ medium. Thus it appeared that stimulus-induced changes in [K+]o might enhance the excitability of the tissue during electrical activation. 3. To test this hypothesis, we measured the field potential responses evoked by 0.5 Hz stimulation in media of different K+ concentrations. The size of the field potential was enhanced in 10 mM-K+ medium and depressed in 0 mM-K+ medium and in 25 mM-K+ medium. 4. Electrical stimulation (6 Hz, 1 min) decreased [Ca2+]o by 10.9 +/- 1.8% (n = 6). This decrease was absent in the presence of 1 microM-tetrodotoxin or 1 mM-cadmium. Again, the [Ca2+] response to stimulation was enhanced in 10 mM-K+ medium and depressed in 0 mM-K+ medium or 25 mM-K+ medium. 5. The release of vasopressin and oxytocin evoked by stimulation at 6 or 30 Hz from isolated neurohypophyses was measured by radioimmunoassay in a separate series of experiments. Stimulation at 30 Hz for 1 min released 5- to 6-fold more hormone than stimulation at 6 Hz for 5 min. Release evoked by 6 Hz stimulation was enhanced in 15 mM-K+ medium and depressed in 25 mM-K+ medium. 6. We conclude that the rise in [K+]o that accompanies high-frequency activation of axons and terminals in the neurohypophysis contributes to the facilitation of hormone release with increasing frequencies of stimulation, and in particular to the efficiency of the milk-ejection burst discharge of oxytocin neurones for evoking oxytocin release.

Effects of tetraethylammonium ions on frequency-dependent vasopressin release from the rat neurohypophysis

1. Isolated rat neurohypophyses were fixed by their stalks to a platinum wire electrode and superfused with oxygenated Krebs-HEPES solution. Vasopressin release into the medium was determined by radioimmunoassay. Vasopressin secretion was increased by electrical stimulation at different frequencies (3-30 Hz) and different train lengths (75-900 pulses). The effects of tetraethylammonium (TEA) ions and of enhanced calcium were tested. 2. Electrical stimulation at 7.5 or 15 Hz evoked a markedly larger release of vasopressin than stimulation at 3 Hz. During continuous stimulation at 7.5 and 15 Hz the evoked vasopressin release per pulse declined rapidly, but with similar time constants for both frequencies indicating that the fatigue of the release process was strongly time dependent. The kinetic analysis showed also that the initial release per pulse was identical for 7.5 and 15 Hz stimulation. Nevertheless, with increasing duration, stimulation at 7.5 Hz became less efficient (in terms of release per total stimulus) than stimulation at 15 Hz and this was due to the time-dependent fatigue. 3. TEA (10 mM) increased the release of vasopressin evoked by 3 Hz stimulation much more than that evoked by 15 Hz stimulation resulting in an equieffective activation of release by both stimuli. On the other hand, elevation of the extracellular calcium from 1.2 to 3 mM did not alter the different efficiency of stimuli of 3 and 15 Hz. In the presence of TEA the time-dependent fatigue of the release during continuous stimulation was prevented, but an additional, slower component of the fatigue became apparent which was release or impulse dependent. 4. As prolongation of the action potential by TEA facilitates preferentially the hormone release evoked by low (ineffective) frequencies, it is suggested that a frequency-dependent broadening of action potentials which reportedly occurs on neurosecretory neurones may play an important role in the frequency-dependent facilitation of hormone release from the rat neurohypophysis.

Effects of gadolinium and cadmium on the electrically evoked release of 45calcium from the isolated rat neurohypophysis

Isolated neural lobes of the rat pituitary gland were fixed by their stalks to a platinum wire electrode. They were loaded with 45calcium and then superfused with radioactivity-free Krebs-solution. The efflux of 45calcium into the superfusion medium was determined. After 54-60 min of superfusion the spontaneous outflow of 45calcium was 2.03%/min of the tissue 45calcium. It was not affected by cadmium (Cd2+, 0.03-3 mmol/l), but reduced by 40% in the presence of 1 mmol/l gadolinium (Gd3+). Electrical stimulation with pulses of 15 Hz (3 times for 1 min with intervals of 1 min) evoked a 45calcium release of 14.4% of the tissue radioactivity. The evoked release of 45calcium was reduced by 80% in the presence of tetrodotoxin and by about 50% in the presence of gallopamil (D600, 30 mumol/l) or after omission of unlabelled calcium from the superfusion medium. Gd3+ concentration-dependently reduced the evoked release by maximally 75% at 3 mmol/l. However, it inhibited the evoked release of 45calcium less effectively than the release of vasopressin evoked by identical stimulation conditions. Cd2+ reduced the evoked release by maximally 55% at 300 mumol/l. The effect of Cd2+ on the evoked release of vasopressin was not tested because Cd2+ markedly increased the spontaneous outflow of vasopressin. When the stimulation was carried out for only 1 min at 15 Hz (i.e. 900 pulses) the evoked release of 45calcium was 10.6% of the tissue 45calcium and 100 mumol/l Cd2+ or 300 mumol/l Gd3+ caused a reduction of the evoked release similar to that observed when 3 periods of stimulation were applied.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormone release from isolated nerve endings of the rat neurohypophysis

1. Isolated neurosecretory nerve endings were prepared from rat neurohypophyses. The amount of vasopressin (AVP) and oxytocin released was measured by radioimmunoassay. 2. The amount of hormone release under resting conditions was not affected by external calcium (Ca2+o). Secretion decreased by ca. 50% when external sodium (Na+o) was replaced by choline or sucrose. 3. Ouabain did not modify the basal AVP release. 4. The Na+ ionophore monensin increased the release of AVP only in the presence of Na+o. This increase was maintained during prolonged exposure to the ionophore and occurred in the presence of Ca2+o only. 5. In the presence of Ca2+o, the amount of evoked hormone release was dependent on the external K+ concentration. Half-maximal activation was achieved with ca. 40 mM-K+. The K+-induced secretion was potentiated in Na+-free solution. 6. Prolonged 100 mM-K+-induced depolarization in the presence of Ca2+o gave rise to a large increase in hormone secretion which decreased with time (t1/2 = 2.5 min). The release could be reactivated after permeabilization of the nerve terminals in the presence of micromolar concentrations of Ca2+. 7. A stepwise paradigm in which Ko+ is incrementally increased to 25, 50, 75 and then 100 mM released more AVP than a prolonged exposure to 100 mM-K+. 8. Veratridine increased the amount of AVP released. This effect was considerably reduced in the absence of Nao+ and abolished in the presence of D600. 9. The depolarization-induced AVP release was blocked by different Ca2+-antagonists. Their effectiveness was nitrendipine = nicardipine greater than Cd2+ greater than Gd3+ greater than Co2+ = Mn2+. 10. The dihydropyridine Bay K 8644 potentiated both the basal and the K+-evoked AVP release. Its maximal effect was obtained with 25-50 mM-Ko+. 11. In conclusion, the isolated neurohypophysial terminals which have both Na+ and Ca2+ channels and release AVP and oxytocin upon depolarization might be an excellent system to study further the mechanisms leading to secretion of neurohormones.

Requirements for hormone release from permeabilized nerve endings isolated from the rat neurohypophysis

1. Isolated nerve endings from rat neurohypophyses were permeabilized with digitonin in order to gain access to the cytoplasm. Release of vasopressin (AVP), oxytocin and the neurophysins was studied under different experimental conditions. 2. Hormone release, which occurred by exocytosis, was Ca2+ dependent. Half-maximal release was observed at ca. 1.7 microM-Ca2+ in contrast to ca. 300 microM for K+-induced hormone secretion from non-permeabilized neurosecretosomes. 3. Release also occurred when the neurosecretosomes were challenged with Ca2+ 20 min after digitonin treatment. This suggests that the isolated nerve endings remain permeable after treatment with digitonin. 4. Although hormone release was potentiated in the presence of ATP, and to a lesser extent with guanosine triphosphate (GTP), secretion occurred in the absence of nucleotides. 5. Replacement of K+ as the major cation by Na+ did not modify the secretory response to a Ca2+ challenge. Release, although reduced, still occurred when KCl was replaced by sucrose. 6. Compared to glutamate, Cl-, Br- and I- did not modify the Ca2+-independent release. This release was increased in the presence of SCN-. The order of effectiveness of the anions studied in inhibiting the Ca2+-dependent release was glutamate less than Br- = Cl- = I- less than SCN-. 7. Increasing the osmolarity of the perfusate inhibited the Ca2+-dependent release of AVP and oxytocin. 8. Vincristine, which binds to microtubules, had no effect on the secretory process. 9. Ca2+ dependent AVP release was partially inhibited by the calmodulin antagonist trifluoroperazine. 10. Hormone release was potentiated by the protein kinase C activator, 4-beta-phorbol 12-myristate acetate (TPA). 11. Whereas 0.2 microM-Ca2+ induced a barely significant increase in AVP release, inositol 1,4,5-triphosphate, in the continued presence of 0.2 microM-Ca2+, produced a large secretory response. 12. 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS), an inhibitor of Cl- permeability, reduced the Ca2+-dependent AVP release. 13. Carbonyl cyanide m-chlorophenylhydrazone (CCCP), which reduces the transmembrane potential of isolated neurohypophysial granules, inhibited the Ca2+-dependent hormone secretion. 14. Maximal hormone release occurred at pH 6.6. 15. It is concluded that the permeabilized neurosecretosomes represent an excellent model for studying the minimal requirements for neurosecretion.

Isolated neurosecretory nerve endings as a tool for studying the mechanism of stimulus-secretion coupling

In the present paper we discuss the properties of a recently developed preparation of isolated neurosecretory nerve endings obtained from the rate neurohypophysis. These nerve terminals release two neurohormones, oxytocin and vasopressin, which are easily assayed by radioimmunoassay. Depolarization-induced secretion is dependent on the same parameters as those regulating release from the whole neural lobe. The isolated nerve endings can be permeabilized by means of digitonin; a treatment which gives direct access to the cytoplasm allowing the study of the minimal requirements for inducing neuropeptide release. Furthermore, some nerve endings are large enough to allow the use of the patch-clamp technique. In the present paper we present evidences which show that the isolated neurohypophysial nerve terminals represent a protent tool for studying the mechanism of stimulus-secretion.

Depolarization-induced Ca2+ increase in isolated neurosecretory nerve terminals measured with fura-2

The free Ca2+ concentration in isolated rat neurohypophysial nerve endings was measured using the Ca2+ indicator fura-2. Depolarization with high K, veratridine, or electrical stimulation induced an increase in intracellular Ca2+ concentration that was abolished by agents known to block voltage-sensitive Ca channels. Electrical stimulation of the isolated nerve endings with a pulse pattern similar to that recorded in vivo from the hypothalamic magnocellular neurons showed that the increase in intracellular Ca2+ concentration was not only a function of the applied frequency but also of the duration of the silent interburst intervals. The relationship between the cytoplasmic free Ca concentration and the release of neuropeptides is discussed.

Fine structural characteristics of neurophysin-positive perivascular plexus that develop in the rat hypothalamus following interruption of the hypothalamo-neurohypophysial tract

Transection of neurosecretory axons of the hypothalamo-neurohypophysial tract within the hypothalamus by stereotactic grafts of various tissues or knife cuts induced the development of neurophysin-positive plexus around arterioles, venules and capillaries in the vicinity of these grafts or cuts. These plexus ranged from single axons to densely woven networks and tended to increase progressively with time after experimental intervention. At the fine structural level, typical neurosecretory axon profiles were either abutting the perivascular connective tissue space or located within it. They were usually accompanied by astrocyte processes or microglial cells. Many of these axons had extensive contact with the surrounding basal lamina at which point clusters of microvesicles reminiscent of axon terminals in the neural lobe were present.

Electrical properties of axons and neurohypophysial nerve terminals and their relationship to secretion in the rat

Isolated rat pituitary stalk-neurohypophysial complexes were electrically stimulated and the evoked compound action potentials were recorded at the level of both axons and nerve terminals. The latency of the nerve terminal response increased during continuous stimulation of the stalk at frequencies as low as 1 Hz. At similar frequencies continuous stimulation of the stalk produced an increase in the latency of the response of the nerve fibres and a decrease in the amplitude of the compound action potential. The increase in the latency of the response of both axons and nerve terminals was related to the frequency and number of stimuli. The time necessary for full recovery of the response of the axons and the nerve endings, following stimulation at frequencies above 5 Hz, was not linearly related to the frequency of stimulation. Stimulation of the stalk with a pulse pattern (bursts) imitating the electrical activity of vasopressin-containing magnocellular neurones showed that the latency of the compound action potential had increased by the end of the first burst. The latency of the response of axons and nerve endings was inversely proportional to the time interval between bursts. Prolonged stimulation of the isolated neural lobe with 'vasopressin'-like bursts induced the release of vasopressin. Twelve bursts, separated by 3 min intervals, released more hormone than fifty bursts given during the same period of time, but separated by a 21 s interval. Leu-enkephalin (10(-5) M) did not modify the latency or the amplitude of the action potentials evoked with low frequency of stimulation (0.5 Hz) or with 'vasopressin'-like bursts. In conclusion, it is suggested that the electrical properties of the nerve fibres and the nerve endings goes some way to explain the pattern of hormone release observed during sustained stimulation.

Ultrastructural changes in isolated peptidergic nerve terminals induced by digitonin permeabilization and K+ stimulation in the sinus gland of the crab, Cardisoma carnifex

The preparation of isolated peptidergic nerve terminals from the sinus gland (a neurohemal organ) of the crab (Cardisoma carnifex) is described. In this species the nerve endings can have diameters up to 30 microns. They release neurosecretory material as judged by the decrease in the volumetric density of granules upon depolarization with potassium. Similar results were obtained after permeabilization of the nerve terminals with digitonin, but only in the presence of micromolar concentrations of calcium. This preparation should prove useful in correlating electrical events with other cellular processes involved in stimulus-secretion coupling.

The role of patterned burst and interburst interval on the excitation-coupling mechanism in the isolated rat neural lobe

Isolated rat neural lobes were stimulated electrically and the release of vasopressin and oxytocin was measured by radioimmunoassay. The neurohypophyses were stimulated with pulses given at a constant frequency or with a pulse pattern imitating the electrical activity, recorded in vivo, of vasopressin- or oxytocin-containing magnocellular neurones. A single burst recorded from a 'vasopressin' cell with an intraburst mean frequency of 13 Hz evoked more vasopressin release than the same number of stimuli delivered at a constant frequency of 13 Hz. The amount of vasopressin release per pulse was much higher at the beginning than at the end of the burst. Series of bursts given with interburst silent periods released more hormone than bursts delivered without silent periods. The amount of hormone released by four 'vasopressin' bursts was significantly larger with silent periods of 21 s than with shorter intervals. Four pulses were much more effective in promoting hormone release when given with 60 ms interspike intervals at the beginning of each second than when delivered at a constant frequency of 4 Hz. Prolonged stimulation with 'vasopressin' bursts had a greater effect in inducing hormone release than the same number of pulses given in burst delivered at a constant frequency of 13 Hz. After an initial increase the rate of vasopressin release declined rapidly whereas oxytocin release remained elevated for the first 20 min and only then decreased. The release of both vasopressin and oxytocin remained, however, above the release from unstimulated neurohypophyses. 45Ca uptake in the neural lobe was larger when the neurohypophyses were stimulated with vasopressin or oxytocin bursts delivered with silent intervals than when the silent periods were omitted, or when the tissue was stimulated with bursts with the same number of pulses but given at a constant frequency of 13 Hz. In conclusion, it is suggested that the interspike intervals in a burst and the silent intervals between bursts are two important determinants of the effectiveness of the burst pattern in promoting neuropeptide release.

Gadolinium ions inhibit exocytotic vasopressin release from the rat neurohypophysis

Single rat neurointermediate lobes (n.i.l.s) were fixed by their stalks to a platinum wire clip electrode and incubated in oxygenated Krebs-HEPES medium. Vasopressin release int the medium was determined by radioimmunoassay. Vasopressin secretion was increased by different stimuli and the effects of gadolinium (Gd3+) were tested. Electrical stimulation (15 Hz, three times 1 min with 1 min intervals) increased vasopressin release in a calcium-dependent manner. Gd3+ (10 microM to 3 mM) inhibited the evoked release of vasopressin in a concentration-dependent fashion; at 3 mM the inhibition was 98%. The inhibitory effect of Gd3+ up to 300 microM was antagonized by increasing the calcium concentration in the medium up to 6 mM. The effects of 1 and 3 mM-Gd3+ were unaffected by increasing the calcium concentration. Exposure of n.i.l.s to depolarizing concentrations of potassium (high K+, 60 mM, 30 min) increased the vasopressin release more than 33-fold. The elevated vasopressin release remained constant during six consecutive 5 min periods. In the initial 5 min period 300 microM-Gd3+ reduced the evoked vasopressin release by 80% but during the last 5 min period only by 30%. At 3 mM-Gd3+ vasopressin release was completely blocked during the whole time of incubation with high K+. Vasopressin release induced by exposure of n.i.l.s to cold (4 degrees C, 20 min) was completely inhibited by 3 mM-Gd3+, but reduced by only 25% in the presence of 300 microM-Gd3+. Vasopressin release induced by incubation of n.i.l.s with the ionophore X-537A (lasalocid) (10 microM, 30 min) was reduced by 90% in the presence of 300 microM-Gd3+ and completely prevented by 3 mM-Gd3+. 300 microM-Gd3+, added to the incubation medium, had no significant effect on the vasopressin release from crude synaptosomal preparations evoked by high K+. However, when 300 microM-Gd3+ was already present during the tissue homogenization, the evoked vasopressin release from the synaptosomes was completely blocked. It is concluded that Gd3+ inhibits exocytotic vasopressin release at two different sites. First, Gd3+ may block voltage-regulated calcium channels. Secondly, Gd3+ may inhibit the exocytotic release mechanism by an intracellular site of action. It is speculated that contractile proteins may be the intracellular target for Gd3+.

Large and rapid changes in light scattering accompany secretion by nerve terminals in the mammalian neurohypophysis

Large changes in the opacity of the unstained mouse neurohypophysis follow membrane potential changes known to trigger the release of peptide hormones. These intrinsic optical signals, arising in neurosecretory terminals, reflect variations in light scattering and depend upon both the frequency of stimulation and [Ca2+]o. Their magnitude is decreased in the presence of Ca2+ antagonists and by the replacement of H2O in the medium by D2O. These observations suggest a correspondence between the intrinsic optical changes and secretory activity in these nerve terminals.

Vasopressin and brain development: studies using the Brattleboro rat

Anomalies in hormonal and neurotransmitter status during early stages of brain development, can lead to lifespan alterations in the functioning of central systems. The neuropeptide vasopressin is nowadays recognized as a putative neurotransmitter, after years of study on its neurosecretory hormonal aspect in water metabolism. Since vasopressin is moreover present early in the brain, and has various mitogenic, metabolic and physiological actions, one might expect vasopressin to be of importance for normal brain development as well. Indeed, the absence of brain vasopressin in the Brattleboro mutant rat coincides with impaired brain development, and some physiological and behavioral defects of these rats are not adjusted by treatment with vasopressin. Regionally the cerebellum seems to be the most affected brain area, both morphologically and biochemically. Only when vasopressin supplementation was done prenatally, this disturbed growth could be restored, which suggests an early role for vasopressin in neurogenesis. Enhanced levels of vasopressin during the perinatal period on the other hand, have been shown to affect permanently the 'setting' of peripheral vasopressin functions in cardiovascular and renal regulatory systems. It is not excluded as yet that after such treatments central organization of vasopressin systems is not impaired as well.