STIMULUS-SECRETION COUPLING IN A NEUROSECRETORY ORGAN: THE ROLE OF CALCIUM IN THE RELEASE OF VASOPRESSIN FROM THE NEUROHYPOPHYSIS

Release of pro-thyrotropin-releasing hormone connecting peptides PS4 and PS5 from perifused rat hypothalamic slices

Thyrotropin-releasing hormone prohormone contains five copies of the thyrotropin-releasing hormone progenitor sequence Gln-His-Pro-Gly, each flanked by pairs of basic amino acids and separated by intervening sequences (connecting peptides). Using a perifusion system for rat hypothalamic slices, we have studied the ionic mechanisms underlying the release of two connecting peptides originating from the thyrotropin-releasing hormone precursor: prepro-thyrotropin-releasing hormone-(160-169) (Ps4) and prepro-thyrotropin-releasing hormone-(178-199) (Ps5). Quantification of these two peptides in the effluent fluid was performed using sensitive and highly specific radioimmunoassay procedures. Reverse phase high performance liquid chromatography analysis of the effluent perifusate showed that released peptides co-eluted with synthetic Ps4 and Ps5. The secretion of Ps4 and Ps5 was stimulated by depolarizing agents such as (i) high potassium concentrations, (ii) ouabain, an Na+/K(+)-ATPase inhibitor, and (iii) veratridine, a stimulator of voltage-operated Na+ channels. The response to potassium (70 mM) was not affected by the specific Na+ channel blocker tetrodotoxin. The K+ channel blocker tetraethylammonium did not modify K(+)-evoked release of Ps4 and Ps5. These data suggest that voltage-operated Na+ channels are not involved in the stimulatory effect of high K+ on the release of Ps4 and Ps5. The lack of effect of picrotoxin, a Cl- channel blocker, on the secretion of the connecting peptides indicates that chloride ions play a minor role in the release process. In contrast, deprivation of Ca2+ in the perifusion medium suppressed K(+)-evoked release of the two peptides, indicating that voltage-operated Ca2+ channels are implicated in the release process. Taken together, the present results show that non-thyrotropin-releasing hormone peptides originating from the thyrotropin-releasing hormone precursor are secreted by mediobasal hypothalamic fragments. The release of these peptides is stimulated by depolarization through a calcium-dependent process. These data indicate that Ps4 and Ps5 may be released at the level of the median eminence into the portal circulation, suggesting that these peptides may play a role in the control of anterior pituitary cells.

Effects of Different Opioid Receptor Antagonists on the Electrically-Evoked Release of Endogenous Dopamine from the Isolated Neural Lobe of the Rat Pituitary Gland in vitro

Abstract Isolated neural lobes of the rat pituitary gland were incubated in Krebs-HEPES solution which contained the dopamine uptake inhibitor GBR 12921 and in some experiments additionally pargyline. The release of endogenous dopamine evoked by electrical stimulation of the pituitary stalk was determined by high-performance liquid chromatography with electrochemical detection. (+/-)- Naloxone increased the evoked dopamine release maximally by 440% (EC(50) 209 nM). The (+)-enantiomer of naloxone (up to 10 muM) did not affect the release of dopamine. The preferential kappa-opioid receptor antagonist MR 2266 increased the evoked dopamine release maximally by 135% (EC(50) 7 nM). MR 2267, the inactive (+)-enantiomer of MR 2266, had no effect on dopamine release. The delta-opioid receptor selective antagonist ICI 174864 increased the release of dopamine maximally by 120% (EC(50) 10 nM). The non-selective opioid receptor agonist etorphine up to 10 muM had no effect on the evoked dopamine release. In conclusion, endogenous opioids in the neurohypophysis strongly inhibit the release of endogenous dopamine from this gland. Activation of kappa- and delta-opioid receptors appears to be involved in the inhibitory action of the endogenous opioids on the neurohypophysial release of dopamine.

Activation of neurohypophysial vasopressin release by Ca2+ influx and intracellular Ca2+ accumulation in the rat

1. Isolated rat neurohypophyses were stimulated electrically in an in vitro perifusion system. A Ca2(+)-sensitive microelectrode was placed in the centre of each neurohypophysis and [Ca2+]o decrease evoked by the stimulation were determined. 2. In neurohypophyses injected with Fura-2 AM (acetoxymethyl ester), increases and decreases in fluorescence excited at 340 and 380 nm, respectively, were evoked by stimulation. The time course of the fluorescence changes was similar to that of [Ca2+]o decreases, suggesting that the [Ca2+]o changes mirrored [Ca2+]i increases. 3. Calcium influx into neurosecretory axons and terminals was estimated as the difference in [Ca2+]o decrease rates immediately before and after train pulse stimulation. 4. Vasopressin release from the neurohypophysis, measured by specific radioimmunoassay, was facilitated by stimulation in parallel with a parameter of [Ca2+]o decrease multiplied by Ca2+ influx. 5. The O2 consumption rate, estimated as rate of PO2 decrease in the tissue, was facilitated by stimulation in parallel with [Ca2+]o decreases. 6. Possible calcium-dependent mechanisms of vasopressin release, and the energy-dependent step of the release by Ca2+, are discussed.

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.

Involvement of voltage-operated calcium channels in alpha-melanocyte-stimulating hormone (alpha-MSH) release from perifused rat hypothalamic slices

The contribution of voltage-operated calcium (VOC) channels in the mechanism of release of alpha-melanocyte-stimulating hormone (alpha-MSH) from hypothalamic neurons was investigated using perifused rat hypothalamic slices. The stimulatory effect of potassium (50 mM) on alpha-MSH release was completely blocked by cadmium (1 mM) a calcium competitor which indifferently blocks T-, L-and N-type VOC channels. To determine the nature of calcium conductances involved in K+-evoked alpha-MSH release, we have investigated the effect of a VOC channel agonist and 3 antagonists on the secretion of the neuropeptide. Administration of synthetic omega-conotoxin fraction GVIA (1 microM), a peptide toxin which blocks both N- and L-type VOC channels, reduced by 33% K+-induced alpha-MSH release. In contrast, the 1,4-dihydropyridine (DHP) antagonist nifedipine, at concentrations up to 100 microM, did not affect the response of hypothalamic alpha-MSH neurons to depolarizing concentrations of KCl. In addition, the secretion of alpha-MSH induced by high K+ concentrations was not reduced by nifedipine (10 microM) in the presence of diltiazem (1 microM), a benzothiazepine derivative which increases the affinity of the DHP antagonist for L-type VOC channels. The DHP agonist BAY K 8644 (0.1-10 microM) did not modify the early phase of the response of alpha-MSH neurons to K+-induced depolarization. In contrast BAY K 8644 (1 or 10 microM) significantly prolonged the duration of K+-induced alpha-MSH release. This sustained release of alpha-MSH induced by BAY K 8644 (10 microM) was totally suppressed by nifedipine (10 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Cholecystokinin evokes secretion of oxytocin and vasopressin from rat neural lobe independent of external calcium

Cholecystokinin (CCK) and its receptors are abundantly represented in the central nervous system. However, a specific role or mechanism of action for CCK in this context has not been established. CCK coexists with oxytocin in magnocellular neurons of the hypothalamic-neurohypophysial system, sharing common neurosecretory vesicles with oxytocin in the neural lobe of the pituitary. The neural lobe, which consists primarily of oxytocin- and vasopressin-containing axons and nerve terminals and their surrounding glia, provides a relatively simple model system allowing for the study of the regulation of neurosecretion at the nerve terminal level, free from the complex array of synaptic effects present throughout the rest of the central nervous system. In this paper, we demonstrate the presence of high-affinity CCK binding sites in the rat neural lobe and show that activation of these receptors by the sulfated octapeptide, CCK-8, and related peptides causes potent secretion of oxytocin and vasopressin from the isolated nerve terminals. The secretagogue action of CCK-8, which is blocked by a CCK receptor antagonist (L-364,718), is independent of electrical stimulation and extracellular calcium and is blocked by an inhibitor of protein kinase C. Thus, the action of CCK on the neural lobe provides an example of peptide ligand-induced neurosecretion apparently mediated by second messengers rather than depolarization-induced calcium influx.

Frequency-dependent effects of activation and inhibition of protein kinase C on neurohypophysial release of oxytocin and vasopressin

Isolated rat neurohypophyses were superfused in vitro and the release of vasopressin and oxytocin into the medium was determined by specific radioimmunoassays. Hormone secretion was increased by electrical stimulation of the pituitary stalk at different frequencies. The effects of several phorbol esters, known to activate phorbol 12,13-dibutyrate, PDB) or not to affect (4 alpha-phorbol 12,13-dideconate and phorbol 12-monoacetate) protein kinase C, and of the direct protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) were tested. Electrical stimulation with 450 pulses caused the release of about 45 microU vasopressin and 55 microU oxytocin, when a frequency of 3 Hz was applied, and of about 500 microU vasopressin and oxytocin, when a frequency of 15 Hz was used. PDB (1 mumol/l) increased the release of vasopressin evoked by 15 Hz stimulation maximally by about 40-50% and that evoked by 3 Hz stimulation by about 150%. The release of oxytocin evoked by 15 Hz stimulation was increased by about 150% and that evoked by 3 Hz stimulation by about 400-500% in the presence of PDB. Both inactive phorbol esters had no effects on the evoked release of vasopressin or oxytocin. The effect of PDB on the release of vasopressin and oxytocin was blocked by H7 (10-30 mumol/l). H7 (30 mumol/l) alone reduced the release of vasopressin evoked by stimulation at 15 Hz by 50%. The release of oxytocin was not significantly affected by H7. In the presence of naloxone (1 mumol/l) the release of oxytocin evoked by 3 and 15 Hz stimulation was increased by about 175 and 105%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-dependent and ouabain-resistant oxygen consumption in the rat neurohypophysis

To analyze rapid changes in energy metabolism in the neurohypophysis, pO2 was measured in the tissue in vitro with a miniature O2 electrode (tip diameter less than 100 microns, 90% response time less than 3 s). Electrical stimulation (20 Hz, 5 s) evoked immediate pO2 decreases by 93.4 +/- 10.5 mm Hg (mean +/- S.E.M., n = 12) which lasted for about 1 min and were blocked by tetrodotoxin (1 microM) or sodium cyanide (1 mM). Replacement of Ca2+ in the perifusing medium with Mn2+ reduced the pO2 decreases to 23.1 +/- 4.9% (n = 5) of the value before the replacement. In normal medium, ouabain application (1 mM, 3 min) suppressed the electrically evoked pO2 decreases only slightly to 82.6 +/- 6.5% (n = 5). In the Mn2+ medium, the same ouabain application suppressed the pO2 changes to 28.8 +/- 1.4%. High K+ (70 mM) evoked pO2 decreases by 175.8 +/- 14.9 mm Hg (n = 5) within 1-2 min. These pO2 changes were reduced to 35.6 +/- 3.8% in an Mn2+ medium. Veratridine (100 microM) evoked pO2 decreases by 204.8 +/- 36.3 mm Hg (n = 5). During the pO2 decreases, the effects of electrical or high K+ stimulation on pO2 were blocked. These results indicate that O2 consumption was evoked by electrical stimulation, and probably that high K+ or veratridine application in the neurohypophysis is mainly dependent on extracellular calcium and resistant to ouabain. The relationship between O2 consumption and exocytotic release is discussed.

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

The properties of the Ca2+ channels mediating transmitter release in vertebrate neurons have not yet been described with voltage-clamp techniques. Several types of voltage-dependent Ca2+ channels are known to exist on neuronal somata, but the small size and inaccessibility of most vertebrate nerve endings have precluded direct characterization of the presynaptic channels. However, large nerve endings, which release the peptides oxytocin and vasopressin in a Ca2(+)-dependent manner, can be dissociated from the rat neurohypophysis. Using both single-channel and whole-cell patch-clamp techniques, we have characterized two types of Ca2+ channels that coexist in these terminals. One is a large-conductance, high-threshold, dihydropyridine-sensitive channel that contributes a slowly inactivating current. The second is a smaller conductance channel, which is also activated at high thresholds, but underlies a rapidly inactivating, dihydropyridine-insensitive current. Both types of Ca2+ channels may participate in the peptide release process.

Ionic dependence of depolarization-mediated adipokinetic hormone release from the locust corpus cardiacum

The locust corpus cardiacum (CC) is a peripheral neurohemal organ in which are clustered a prodigious array of neurosecretory cells (NSCs), nearly all of which synthesize and release adipokinetic hormones (AKHs). We have examined the extracellular requirements for Na+ and Ca2+ in the process of AKH release following NSC depolarization by high extracellular K+ or veratridine. Na+ is not required for release mediated by high external K+ although Ca2+ is. The Ca2+ channel antagonists cobalt and lanthanum prevent release and support the hypothesis that depolarization with K+ leads to Ca2+ channel activation and subsequent AKH release. Tetrodotoxin does not block K+-mediated release suggesting that Na+ channel activation and Na+ influx are not required for K+-mediated release. The alkaloid veratridine leads to cobalt- and tetrodotoxin-sensitive release and this suggests that cell depolarization by Na+ channel activation is nevertheless capable of opening Ca2+ channels and initiating release. Release mediated by high external K+ is reduced by nifedipine but is not significantly reduced by methoxyverapamil, however veratridine-mediated release is slightly reduced by methoxyverapamil. Glandular lobes accumulated greater amounts of 45Ca2+ following high K+-mediated depolarization compared to glands incubated in normal saline and this enhanced accumulation was blocked by the Ca2+ channel antagonist lanthanum. During prolonged exposure to high K+ saline the release of AKHs and the uptake of 45Ca2+ reach a maximum and then gradually decline. The temporal pattern of the reduction in AKH release is similar to that of 45Ca2+ accumulation by the glandular lobe. This reduction in AKH release and 45Ca2+ uptake may result from inactivation of Ca2+ channels associated with the release process. These results indicate that Ca2+ influx into the NSCs by way of voltage-sensitive Ca2+ channels plays a critical role in the process of depolarization-mediated AKH release.

Calcitonin suppresses growth hormone (GH) response to growth hormone-releasing hormone (GHRH) in man

Calcitonin (CT) receptors have been found in the hypothalamus, suggesting a neuroendocrine role for this peptide. We have recently shown that, in the rat, central administration of salmon calcitonin (sCT) suppresses basal and GHRH-stimulated GH secretion. To further investigate how sCT alters GH secretion, we studied the effects of sCT (100U MRC, im) or placebo on basal and GHRH (50 micrograms, iv)-stimulated GH secretion in 6 normal men. GHRH was administered 1 h after sCT injection. Basal GH levels were not altered by sCT administration. However, GH response to GHRH was markedly suppressed by sCT (area under the curve - sCT: 574.6 +/- 69.7 vs placebo: 1057.2 +/- 284.8 micrograms. min/L; p less than 0.02). Cortisol levels were higher in sCT-treated subjects compared to controls, from 45 to 105 min after sCT injection (p less than 0.05). However, no correlation was found between GH response to GHRH and cortisol levels. No changes in glucose, calcium and PTH levels were seen. These results demonstrate that sCT inhibits GHRH-induced GH secretion in man by a mechanism apparently independent of changes in peripheral cortisol, glucose, calcium and PTH levels.

Effects of calcium availability on the release of ovine choriomammotropin from cotyledonary cells incubated in vitro

1. In this study, we examined the basal release of choriomammotropin (oCM) from monolayer cultures of cotyledonary cells obtained from ewes at different gestational ages. 2. oCM release increased with gestational age and displayed a similar profile to the concentration of oCM observed in maternal plasma. 3. Release of oCM was significantly (P less than 0.05; n = 9) increased in calcium-depleted medium, and by treatment with either phospholipase C (0.125 units/ml) or KCl (50 mM). 4. The calcium antagonist MgCl (12 mM) and the calcium channel-blocking agents verapamil (50 microM) and nefidipine (10 microM) all significantly stimulated oCM release. 5. These data are consistent with the suggestion that oCM release is inversely related to extracellular calcium concentration.

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)

Evidence for a Na+/Ca2+ exchange mechanism in the neuroendocrine cells of the locust corpus cardiacum

The glandular lobe of the locust corpus cardiacum (CC) is a peripheral neuroendocrine gland consisting primarily of neurosecretory cells (NSCs), all of which synthesize, store, and release adipokinetic hormones (AKHs). An influx of extracellular Ca2+ into the NSCs provides an essential trigger for initiating AKH release. In this study we demonstrate that the Na+ gradient across the plasma membrane of these NSCs has a significant influence on the process of AKH release and on Ca2+ fluxes across the membrane. AKHs are released from NSCs when the corpus cardiacum is incubated in Na+-free saline or in K+-free saline with ouabain. The rate of 45Ca2+ efflux from the gland is reduced by 50% when glands are incubated in Na+-free saline compared to normal saline. The amount of 45Ca2+ uptake by Na+-loaded cells is nearly 100% greater in glands exposed to Na+-free saline compared to glands in normal saline. These results are most clearly interpreted by proposing that a Na+/Ca2+ exchange system is present in the NSCs. This system may provide an essential Ca2+ buffering mechanism by extruding Ca2+ from the NSCs following a stimulus-secretion cycle.

Involvement of DHP voltage-sensitive calcium channels and protein kinase C in thyroliberin (TRH) release by developing hypothalamic neurons in culture

The intracellular mechanisms regulating the process of thyroliberin (TRH) release were studied using fetal hypothalamic neurons grown in serum-free medium. In particular, we compared the effects of dihydropyridine (DHP) derivatives, omega-conotoxin and phorbol esters on basal and K+-evoked TRH release from 12 days in vitro (DIV) neurons. BAY K 8644, a DHP calcium channel agonist increased in a dose-related manner basal and K+-evoked TRH release. PN 200-110, an antagonist of DHP-sensitive calcium channels, completely suppressed the effect of BAY K 8644, whatever the extracellular K+ concentration, but did not modify basal or K+-evoked TRH release. In contrast, omega-conotoxin partially inhibited the two latter processes. The active phorbol ester 12-O-tetradecanoyl-phorbol-beta-acetate (TPA), and to a lesser extent Sn-1,2-dioctanoylglycerol (DAG), triggered TRH release. This effect was specific, time and dose dependent and only partly dependent on extracellular calcium. Simultaneous addition of BAY K 8644 and TPA to the cells displayed a synergistic effect. The same compounds were studied on younger neurons (6-DIV cultures): BAY K 8644 stimulated TRH release whereas neither 60 mM K+ nor TPA did. These results suggest that TRH release can be mediated at least by two intracellular routes: (i) increase of intracellular calcium mediated by the opening of different types of voltage sensitive calcium channels, and (ii) activation of protein kinase C (PKC). The asynchrony in the maturation of the intracellular mechanisms underlying TRH release may be explained by different subcellular localizations of these mechanisms in neurons and is discussed in relation to synapse differentiation.

Adenylate cyclase activation is not sufficient to stimulate somatostatin release from dispersed cerebral cortical and diencephalic cells in glia-free cultures

Under conditions in which vasoactive intestinal peptide (VIP) induces somatostatin release from cortical and diencephalic neuronal cultures, VIP causes large increases in intracellular cyclic AMP. Both the release of somatostatin and the increase in cyclic AMP elicited by VIP require exogenous calcium, can be blocked by cobalt ion, and can be qualitatively mimicked by depolarizating concentrations of exogenous potassium ion. Direct activation of adenylate cyclase by forskolin causes large increases in cyclic AMP content but does not induce somatostatin release. In the absence of VIP, the calcium ionophore, ionomycin, and the phorbol ester, phorbol 12-myristate-13-acetate, also stimulate somatostatin release. These results indicate that VIP-stimulation of cyclic AMP formation and VIP-stimulation of somatostatin release are calcium-dependent and that the two phenomena are dissociatable. Cyclic AMP formation is not a necessary condition for VIP-induced somatostatin release. Nucleotide formation may be a sufficient condition for release or, possibly in association with calcium influx, it may be an event unrelated to the release process.

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)

Release of gonadotropin alpha subunit from rat pituitary cultures in response to GnRH

Gonadotropin releasing hormone (GnRH)-stimulated release of the alpha subunit common to the gonadotropins and to thyrotropin was studied in rat pituitary cell cultures. In these studies we took advantage of a recently prepared antiserum specific for the alpha subunit. We show that pituitary cells treated with GnRH released alpha subunit in a similar pattern to intact luteinizing hormone (LH) during short-term incubations (0-12 h); during prolonged incubations (12-48 h), however, release of alpha subunit did not desensitize in response to the releasing hormone and the pattern became different from that measured for intact LH. Further, we assessed the relative requirement for Ca2+ in the release of LH and alpha subunit. When pituitary cells were treated with 10(-8) M GnRH in the presence of a range of concentrations of the Ca2+ ion channel antagonist, methoxyverapamil (D-600), release of both LH and alpha subunit was inhibited in a similar and dose-dependent manner; 10(-4) M D-600 showed maximum inhibitory efficacy (IC50 = 10(-5) M). The calmodulin antagonist, pimozide, also inhibited both GnRH-stimulated LH and alpha subunit release (IC50 = 0.75 microM). These data suggested that although the Ca2+/calmodulin system appears to mediate both the release of LH and alpha subunit in response to GnRH, these processes appear differentially regulated during long-term exposure to the releasing hormone.

Visualization of secretory activities in the Xenopus neurohypophysis by a high S/N video camera

The optical turbidity of neurohypophyses of the frog Xenopus laevis was observed with the help of a high signal to noise ratio video camera and a high speed image processor. Electrical stimulation of the pituitary stalk induced a decrease in turbidity of the neurohypophysis. This response was visualized on a monitor screen as a diffuse pattern consisting of many bright spots. The diameters of these spots were similar to those of individual nerve terminals, indicating that the optical response arises from a structural change in individual nerve terminals upon secretory activation.

Extracellular potassium changes in the rat neurohypophysis during activation of the magnocellular neurosecretory system

1. Potassium-sensitive microelectrodes were used to measure extracellular [K+] in the isolated rat neurohypophysis maintained in vitro. Electrical stimulation of the neurohypophysial stalk (20 Hz 5 s) increased the inferred extracellular [K+] by 9.2 +/- 0.4 mM (mean +/- S.E. of mean; n = 21). 2. Veratridine (10 microM) enhanced the response to stalk stimulation, and at a higher concentration (50 microM) increased extracellular [K+] in the absence of stimulation. By contrast, tetrodotoxin (1 microM) blocked the [K+] increase completely and reversibly in each of five experiments, indicating that the increase was a consequence of action potential generation. 3. At the end of brief periods of stimulation, the raised extracellular [K+] returned to pre-stimulation levels within 30 s. In the presence of ouabain (100 microM), the recovery was slower: the half-decay time was extended by 150-300% in each of three experiments. 4. Replacement of calcium in the medium with cobalt, cadmium or magnesium reduced the amplitude of the [K+] increase by 26-30%, indicating that the [K+] increase was largely independent of events subsequent to evoked release of hormone and/or transmitters. 5. Potassium-sensitive microelectrodes were placed in the neurohypophysis of rats anaesthetized with urethane. Electrical stimulation of the pituitary stalk (50 Hz, 5 s) produced transient voltage increases of 7.6 +/- 0.9 mV (mean +/- S.E. of mean of seven experiments). These voltage increases were similar in magnitude to the response of the electrodes to the addition of 7.6 +/- 1.0 mM-K+ to rat plasma. 6. In seven lactating rats, the suckling of a litter of hungry pups evoked periodic reflex milk ejections, as detected by increases in intramammary pressure. Potassium-sensitive microelectrodes in the neurohypophysis recorded transient voltage increases prior to each milk ejection (0.4-5.5 mV). Each increase preceded an increase in intramammary pressure by 12-30 s. 7. Thus synchronized high-frequency activation of magnocellular neurones can produce large changes in extracellular [K+]. The implications of these findings for stimulus-secretion coupling in the neurohypophysis are discussed in the light of previous reports that hormone release from the neurohypophysis is highly dependent on the frequency and pattern of electrical stimulation.

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.

Multiple tachykinin pools in sensory nerve fibres in the rabbit iris

A population of sensory nerve fibres in the rabbit iris is known to contain calcitonin gene-related peptide and tachykinins, such as substance P and neurokinin A. In the presence of atropine and guanethidine, the isolated iris sphincter responded to electrical stimulation with a contraction that could be abolished by tachykinin antagonists. Capsaicin, known to release tachykinins from sensory fibres, evoked a long-lasting tachykinin-mediated contraction of the iris sphincter. Repeated application of capsaicin led to tachyphylaxis, possibly reflecting depletion of releasable neuronal stores of tachykinins. At this stage, electrical stimulation failed to elicit contraction. The capacity of capsaicin to release neuropeptides from sensory fibres was confirmed by determination of substance P- and calcitonin gene-related peptide-like immunoreactivity in the incubation medium and in the iris tissue. The concentrations of substance P and calcitonin gene-related peptide in the iris after capsaicin exposure were reduced by about 25%. Like capsaicin, bradykinin evoked a tachykinin-mediated contraction and tachyphylaxis. However, after development of tachyphylaxis to bradykinin, electrical stimulation or exposure to capsaicin still evoked tachykinin-mediated contraction, albeit a reduced one compared with the response before bradykinin. Hence, capsaicin completely depletes tachykinin stores releasable by prolonged electrical stimulation, whereas bradykinin exhausts only a sequestered pool. The possibility that tachykinins occur in several releasable pools in sensory nerves was investigated in yet another way: the iris sphincter muscle was stimulated electrically once every 2.5 min over several hours. The contractile response diminished gradually.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of vasopressin by calcium microinjections in the area of the paraventricular nucleus of the hypothalamus

The present experiments were designed to investigate the effect of local calcium application on the paraventricular nucleus (PVN) on arginine-vasopressin (AVP) release and blood pressure. A microinjection of 2 microliter of calcium gluconate 10% in the PVN elevated AVP levels to 71.6 +/- 8.7 pg/ml compared to 3.3 +/- 0.6 pg/ml in control rats receiving iso-osmolar solution of dextrose 11% (P less than 0.001), whereas peripheral plasma epinephrine and norepinephrine were normal. Despite the increase of plasma AVP levels, blood pressure and heart rate remained unchanged indicating that other unknown mechanisms counteract AVP's pressor effect. These data suggest that locally applied Ca2+ in the vasopressinergic neurons of the hypothalamus strongly stimulates the release of AVP.

Spontaneous neural activity of a mechanoreceptive system is undiminished by replacement of external calcium with equimolar magnesium in the presence of EGTA

Primary afferent neurons of the lateral-line mechanosensory organs, which are believed to be closely related to the auditory and vestibular organs, exhibit "spontaneous" action potentials in the absence of mechanical stimulation of the receptor cells (hair cells). Sinusoidal mechanical stimulation of the hair cells enhances the impulse rate of the afferent neurons. The spontaneous activity is found to be a decreasing function of increasing concentration of either external magnesium or calcium, when each cation is varied in the absence of the other and bath-applied to the synaptic side of the lateral-line mechanoreceptors. One mM to 6 mM magnesium with 5 mM EGTA (the latter for chelation of remaining traces of calcium) permits undiminished spontaneous afferent activity of lateral-line neurons for as long as 3 to 4 hours. With bath-applied calcium, mechanical stimulation results in evoked incremental activity--defined as total activity with stimulation minus spontaneous activity--which significantly increases with increasing calcium concentration. However, with magnesium and EGTA in the bath, mechanical stimulation produces no increase in the neural firing rate above spontaneous rate for any magnesium concentration tested. Taken together, these results suggest that spontaneous activity, in contrast to evoked incremental activity, does not require external calcium in the bath, and production of spontaneous neural action potentials may proceed via mechanisms that are modifications of those of classical stimulus-secretion coupling.

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.

Influence of nifedipine and enalapril on osmoregulation of vasopressin

To determine whether calcium fluxes and angiotensin II influence osmoregulation of vasopressin (AVP) secretion, the effects of the calcium antagonist nifedipine and of the converting enzyme inhibitor enalapril on the AVP response to an osmotic load were compared to those of a placebo in seven normal female subjects. Plasma and urinary AVP were measured before and during a 3-h infusion of 2.5% hypertonic saline. Nifedipine (10 mg orally 2 h before and 10 mg at the start of the infusion) increased heart rate but did not change blood pressure. The changes in free water clearance and in urinary AVP induced by hypertonic saline under nifedipine were greater than in the control test, but the slope and the intercept of the regression line of plasma AVP upon plasma osmolality were not significantly different. Enalapril (10 mg 3 h before the infusion) did not change heart rate or blood pressure. Free water clearance and urinary AVP did not differ from the control test, but the slope of the regression line was less steep. These slight modifications of the response to an osmotic load suggest that calcium fluxes and angiotensin II only exert a limited influence on AVP osmoregulation in normal females.

Calcium participation in thyroid function in fowl (Gallus domesticus)

The effects of the calcium antagonists ethyleneglycol-bis-(beta-aminoethylether)-N,N,N',N'-tetraace tic acid (EGTA), cobalt chloride (CoCl2), and magnesium chloride (MgCl2) on the concentrations of plasma thyroxine (T4) and triiodothyronine (T3) and on the basal and stimulated release of T4 from incubated thyroid glands have been determined in the domestic fowl. Plasma T4 levels were consistently reduced 2 hr after the administration of each calcium antagonist, although only EGTA and CoCl2 lowered the concentration of plasma total calcium. Concentrations of plasma T3 were increased following MgCl2 treatment but reduced after CoCl2 administration. The basal release of T4 by incubated thyroid glands was reduced following in vivo EGTA and CoCl2 treatment, but increased after MgCl2 injection. The addition of bovine thyroid stimulating hormone (TSH, 200 mU) to the incubation consistently stimulated in vitro T4 release, although the magnitude of the stimulation was increased following in vivo EGTA or CoCl2 treatment and reduced after MgCl2 administration. These results demonstrate the involvement of calcium dependent mechanisms in the control of T4 release in fowl.

Effects of calcium on frequency-modulated secretion of vasopressin from isolated murine neural lobes

The relationship between external calcium and frequency-facilitated arginine vasopressin (AVP) secretion from the murine neurointermediate lobe was examined in vitro. We evaluated the calcium-dependency of frequency-dependent release in this system, and found that log AVP secretion versus log external calcium plots gave slopes of 0.71, 0.92 and 1.2 for 5, 10 and 20 Hz stimulation, respectively. These slopes are considerably lower than the slopes of 3-4 Hz found at conventional synaptic junctions.

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.

Stimulation of ovine choriomammotrophin release, in vitro, by phospholipase C

Phospholipid metabolites have previously been implicated in receptor-mediated stimulation of protein hormone secretion. As the factors which regulate the release of choriomammotrophin remain to be elucidated, we investigated the potential involvement of phospholipase C-induced phospholipid metabolism in the release of this placental hormone. Phospholipase C (PLC) caused a dose-dependent release of choriomammotrophin from ovine placenta, incubated in vitro. At a concentration of 0.2 units/ml (0.25 microgram protein/ml), PLC caused the release of choriomammotrophin from placental tissue to approximately double that observed in control incubations (7.08 +/- 0.4 micrograms/50 mg/h and 3.26 +/- 0.3 micrograms/50 mg/h, respectively). PLC treatment did not significantly alter plasma membrane permeability, as indicated by the release of lactate dehydrogenase and protein. PLC-stimulated release of oCM was completely abolished by incubation in calcium-free medium or by preincubation with the inorganic calcium-channel blocking agents cobalt chloride (4 mM) and lanthanum chloride (1 mM). The effects of PLC treatment on ovine choriomammotrophin (oCM) release were also inhibited by preincubation of placental tissue with inhibitors of arachidonic acid metabolism: ibuprofen (10(-5) M), naproxen (10(-4) M) or nordihydroguaiaretic acid (NDGA) 5 X 10(-6) M). These results suggest that the effects of PLC on the release of choriomammotrophin are mediated via metabolites of arachidonic acid.

Mercuric chloride inhibition of vasopressin release from the isolated neurointermediate lobe of the rat pituitary

The effects of HgCl2 and ouabain on vasopressin release and Ca2+ uptake and distribution was examined in the neurointermediate lobe of the rat pituitary. HgCl2 (0.5 mM) inhibited vasopressin release by approx. 90% in both basal and potassium depolarized states. With 0.1 mM HgCl2 vasopressin release was inhibited by 50% in the depolarized state, but release was not effected in basal state. On the other hand, ouabain (0.5 mM) caused a 3-fold stimulation of vasopressin release in the depolarized state. Both HgCl2 (0.5 mM) and ouabain (0.5 mM) increased net 45Ca+2 uptake by about 80% in groups of neurointermediate lobes. Following 45Ca+2 uptake, HgCl2 (0.5 mM), which is absorbed by the neurointermediate lobe, produced an increase in cytosolic 45Ca+2 content and a decrease in mitochondrial 45Ca+2 content compared to control. In comparison, ouabain (0.5 mM), which does not penetrate the neurointermediate lobe, gave no change in cytosolic 45Ca+2, but an increase in mitochondrial 45Ca+2. These results suggest that HgCl2 inhibits vasopressin release from the neurointermediate lobe of the rat pituitary at a point distal to Ca+2 uptake by the gland.

Calcium levels measured in a presynaptic neurone of Aplysia under conditions that modulate transmitter release

We have utilized the Ca2+ indicator dye, Arsenazo III, to examine the role of presynaptic Ca2+ concentration in two types of synaptic plasticity observed at the synapses of cell L10 in Aplysia californica; post-tetanic potentiation (p.t.p. - the increased transmitter release which follows high frequency stimulation), and resting membrane potential modulation of release. Intracellular Ca2+ was monitored in the cell body and main neurites of L10 injected with Arsenazo III. Tetanic stimulation caused an increase in intracellular Ca2+ concentration that decayed, after tetanus, with fast and slow time constants which paralleled the time course of decay of p.t.p. When the voltage-sensitive Ca2+ current was reduced by removing external Ca2+ (0 mM-Ca2+, 4 mM-EGTA) or by blocking Ca2+ channels with divalent cation channel blocker (4 mM-Cd2+), tetanic stimulation did not cause increases in Arsenazo absorbance even when Na+ currents were not blocked. This finding suggests that Ca2+ entering the cell through voltage-dependent Ca2+ channels was the major source of Ca2+ which accumulated during the tetanus. Transmitter release is increased when L10 is maintained at a depolarized membrane potential, and is decreased when L10 is hyperpolarized. We found that the base-line Arsenazo absorbance signal in L10 increased when L10 was depolarized from -60 to -40 mV and decreased when L10 was hyperpolarized. This finding supports the idea that the steady-state Ca2+ concentration contributes to the membrane-potential modulation of transmitter release. These results support the idea that transmitter release can be modulated by the residual or resting Ca2+ concentration of the presynaptic cell.

Do opioid peptides modulate, at the level of the nerve endings, the release of neurohypophysial hormones?

Rat neurointermediate lobes and neurohypophyses separated from the pars intermedia were stimulated in vitro in the presence of either D-Ala2, D-Leu5-enkephalin (DADLE), a Leu-enkephalin stable analogue or FK 33-824 a Met-enkephalin stable analogue. Secretion of vasopressin (AVP) and oxytocin (OT) was produced by either a Ca2+-ionophore or with electrical stimulation or by K+-induced depolarization. These opioid peptides and their antagonist naloxone did not affect basal nor evoked hormone release. Furthermore, they did not affect the evoked calcium uptake induced with electrical stimulation. These findings were confirmed using a preparation of isolated neurosecretory nerve endings. Further, dopamine had no effect on the K+-induced AVP release although a crude extract of the pars intermedia abolished the electrically-evoked and reduced considerably the potassium-evoked AVP release. It is concluded that in the neurohypophysis neither Leu- and Met-enkephalin nor dopamine affect the secretion-coupling mechanism at the level of the neurosecretory nerve endings.

Effects of polyunsaturated fatty acids and hormones on synaptogenesis in serum-free medium cultures of mouse fetal hypothalamic cells

The effects of soluble factors on synaptogenesis by mouse fetal hypothalamic cells cultured in chemically defined conditions have been examined using transmission electron microscopy. Hypothalami taken on the 16th day of gestation were mechanically dissociated and cells were seeded in a minimum serum-free medium supplemented or not with the following components: triiodothyronine, corticosterone and a mixture of polyunsaturated fatty acids (arachidonic acid plus docosahexaenoic acid bound to defatted bovine serum albumin). In the minimum serum free medium synapses were found after 10 days in culture. However, the development of synaptic vesicles was very limited, whereas that of the presynaptic and postsynaptic densities was apparently normal. Supplementation of the minimum serum-free medium with triiodothyronine, corticosterone and polyunsaturated fatty acids added simultaneously, permitted a full development of synapses as attested to by the increase in number and the regular shape and diameter of synaptic vesicles as well as by the complexity and diversity of synapse configurations. Among those three factors, polyunsaturated fatty acids clearly played a key role. The ability of synapses formed in culture to respond to potassium evoked depolarization was examined on cultures grown for 12 days in the simultaneous presence of the three above mentioned supplements. Exposure for 3 min to 60 mM potassium chloride induced in synaptic boutons vesicular depletion, apposition of vesicle clusters onto the presynaptic grid, appearance of a rich filamentous network and of some coated vesicles. Return to 3mM potassium chloride induced in 3 min a massive restoration of the population of vesicles which slightly differed from synaptic vesicles in control cultures. These results show that: (1) the formation of synaptic vesicles in this system is regulated by soluble factors among which polyunsaturated fatty acids play a major role, and (2) synapses formed de novo in chemically defined conditions of culture display the same ability to respond to and to recover from potassium evoked depolarization as adult axon terminals. Thus, they offer a suitable model for analysis of the mechanisms involved in membrane traffic in central neurons.

Somatostatin in the central nervous system: physiology and pathological modifications

Since its discovery, at the beginning of 1973, somatostatin's multiple actions, in relation to its wide anatomical distribution have been widely documented. Its biochemical pathways have been elucidated with the discovery of other molecular forms as well as the mechanisms of its neuronal release. However, no definite proof is available concerning a neurotransmitter role for any peptide of the somatostatin family other than somatostatin-14. The precise determination of the roles of somatostatin in brain are still hampered by the poor pharmacology of the peptide. New tools are badly needed and in particular a true antagonist at the receptor site. The mechanisms of action of somatostatin are now well under way at least in the pituitary model. More information should come from this model and be applied to brain cells in vitro. The greatest challenge of somatostatin brain function lies in its role in the pathophysiology of neurological diseases such as Alzheimer's dementia and Huntington's disease. Nature has been using somatostatin-related molecules since inhibitory control was first needed in cell functions. Time will tell us if somatostatin is really an old peptide involved in senile dementia.

Growth hormone release: interaction of increased extracellular calcium and somatostatin

These studies were designed to examine the effects of extracellular calcium ion (Ca++) concentration upon basal and dibutyryl (db) cAMP or potassium ion (K+)-stimulated release of growth hormone (GH) and to determine whether increased extracellular Ca++ can overcome somatostatin (SRIF)-inhibited release of stored rGH in parallel with its reported effect upon SRIF inhibition of stimulated insulin and glucagon release. Experiments were performed in vitro using prelabeled rat pituitary fragments in a perifusion, specific immunoprecipitation system designed to limit observations to release of stored hormone from viable cells. Increased (up to 5.4 mM) extracellular Ca++ inhibits basal and dbcAMP-stimulated release of stored, prelabeled [3H]rGH in parallel with the effects of SRIF: post-inhibition rebound, dose responsivity, and differential effect upon early and late dbcAMP-stimulated release of stored [3H]rGH. Increased (21 mM) extracellular K+ interferes with both Ca++- and SRIF-inhibited early dbcAMP-stimulated release of stored [3H]rGH. The combination of increased extracellular Ca++ and SRIF inhibits basal release of stored [3H]rGH more than either agent alone and during dbcAMP stimulation, rebound release of stored [3H]rGH follows withdrawal of either inhibitor despite continuation of the other. This rebound release is enhanced when both inhibitors are withdrawn simultaneously.

CONCLUSIONS

(a) the inhibition of stored rGH release induced by increased extracellular Ca++ and SRIF occurs through at least partially independent mechanisms, and (b) increased extracellular Ca++ does not reverse SRIF inhibition of stimulated rGH release from prelabeled intracellular storage, in contrast with observations in the pancreatic islet.

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.

Ultrastructural changes associated with K+-evoked peptide secretion from a neurohemal organ of the crab, Cardisoma carnifex

Electron-microscopic comparison of K+-stimulated and unstimulated crab sinus glands reveals significant differences in neurosecretory terminal morphology. Sinus glands exposed to elevated K+ saline for increasing periods of time show increasing numbers of exocytotic release profiles, vacuoles, and multilamellate bodies, and a decrease in the number of microvesicles within 10 micron of release sites. These morphological changes are well correlated with secretion of red-pigment-concentrating hormone, as determined by bioassay of perfusate from the individual preparations.

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.