Are opioid peptides co-localized with vasopressin or oxytocin in the neural lobe of the rat?

Physiology of spontaneous [Ca(2+)]i oscillations in the isolated vasopressin and oxytocin neurones of the rat supraoptic nucleus

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Supraoptic fluorescent vasopressin (AVP-eGFP) and oxytocin (OT-mRFP1) neurones exhibit distinct spontaneous [Ca²⁺]_i oscillations.

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Vasopressin triggers [Ca²⁺]_i oscillations, intensifies existing oscillations, and exceptionally stops oscillations.

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Hyper- or hypo-osmotic stimuli have an intensifying or inhibitory effect on oscillations, respectively.

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Nearly 90% of neurones from 3 or 5-day-dehydrated rats exhibit oscillations.

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More than 80% of OT-mRFP1 neurones from 3 to 6-day-lactating rats are oscillatory vs. about 44% in virgins.

The magnocellular vasopressin (AVP) and oxytocin (OT) neurones exhibit specific electrophysiological behaviour, synthesise AVP and OT peptides and secrete them into the neurohypophysial system in response to various physiological stimulations. The activity of these neurones is regulated by the very same peptides released either somato-dendritically or when applied to supraoptic nucleus (SON) preparations in vitro. The AVP and OT, secreted somato-dendritically (i.e. in the SON proper) act through specific autoreceptors, induce distinct Ca²⁺ signals and regulate cellular events. Here, we demonstrate that about 70% of freshly isolated individual SON neurones from the adult non-transgenic or transgenic rats bearing AVP (AVP-eGFP) or OT (OT-mRFP1) markers, produce distinct spontaneous [Ca²⁺]_i oscillations. In the neurones identified (through specific fluorescence), about 80% of AVP neurones and about 60% of OT neurones exhibited these oscillations. Exposure to AVP triggered [Ca²⁺]_i oscillations in silent AVP neurones, or modified the oscillatory pattern in spontaneously active cells. Hyper- and hypo-osmotic stimuli (325 or 275 mOsmol/l) respectively intensified or inhibited spontaneous [Ca²⁺]_i dynamics. In rats dehydrated for 3 or 5 days almost 90% of neurones displayed spontaneous [Ca²⁺]_i oscillations. More than 80% of OT-mRFP1 neurones from 3 to 6-day-lactating rats were oscillatory vs. about 44% (OT-mRFP1 neurones) in virgins. Together, these results unveil for the first time that both AVP and OT neurones maintain, via Ca²⁺ signals, their remarkable intrinsic in vivo physiological properties in an isolated condition.

Oxytocin in the periaqueductal gray participates in pain modulation in the rat by influencing endogenous opiate peptides

Periaqueductal gray (PAG) plays a very important role in pain modulation through endogenous opiate peptides including leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek), β-endorphin (β-Ep) and dynorphin A(1-13) (DynA(1-13)). Our pervious study has demonstrated that intra-PAG injection of oxytocin (OXT) increases the pain threshold, and local administration of OXT receptor antagonist decreases the pain threshold, in which the antinociceptive role of OXT can be reversed by pre-PAG administration of OXT receptor antagonist. The experiment was designed to investigate the effect of OXT on endogenous opiate peptides in the rat PAG during the pain process. The results showed that (1) the concentrations of OXT, L-Ek, M-Ek and β-Ep, not DynA(1-13) in the PAG perfusion liquid were increased after the pain stimulation; (2) the concentrations of L-Ek, M-Ek and β-Ep, not DynA(1-13) in the PAG perfusion liquid were decreased by the OXT receptor antagonist; (3) the increased pain threshold induced by the OXT was attenuated by naloxone, an opiate receptor antagonist; and (4) the concentrations of L-Ek, M-Ek and β-Ep, not DynA(1-13) in the PAG perfusion liquid were increased by exogenous OXT administration. The data suggested that OXT in the PAG could influence the L-Ek, M-Ek and β-Ep rather than DynA(1-13) to participate in pain modulation, i.e. OXT in the PAG participate in pain modulation by influencing the L-Ek, M-Ek and β-Ep rather than DynA(1-13).

Proteolytic processing of the Aplysia egg-laying hormone prohormone

By using matrix-assisted laser desorption/ionization time-of-flight MS, individual peptidergic neurons from Aplysia are assayed. A semiquantitative method is developed for comparing single-cell profiles by using spectral normalization, and peptides are localized to specific cells by mass spectrometric cell mapping. In addition to all previously identified products of the egg-laying hormone (ELH) gene, other peptides are formed from proteolytic hydrolysis of Leu-Leu residues within ELH and acidic peptide (AP). AP exhibits further processing to yield AP1-20 and AP9-27. These peptides appear to be colocalized in vesicles with ELH, transported to specific neuronal targets, and released in a Ca2+-dependent manner. A differential peptide distribution is observed at a specific target cell, and a low-frequency variation of AP, [Thr21]AP, is detected in a single animal.

Activation of multifunctional Ca2+/calmodulin-dependent kinase in intact hippocampal slices

In vitro phosphorylation of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) converts it to a form that is independent of Ca2+. We demonstrate that significant Ca(2+)-independent CaM kinase activity is present in untreated hippocampal slices. Two manipulations that produce a long-lasting enhancement of neuronal activity in hippocampal slices, elevated extracellular Ca2+ or depolarization with high K+, generate additional Ca(2+)-independent activity. This increase is dependent on extracellular Ca2+ and is correlated with an increased phosphorylation of CaM kinase. In contrast, CaM kinase in posterior pituitary, a brain structure that is not thought to be involved in memory-related processes, is not modulated by depolarization. These results suggest that the Ca(2+)-independent form of CaM kinase may modulate neuronal activity in the hippocampus.

A fast, transient K+ current in neurohypophysial nerve terminals of the rat

1. Nerve terminals of the rat posterior pituitary were acutely dissociated and identified using a combination of morphological and immunohistochemical techniques. Macroscopic terminal membrane currents and voltages were studied using the whole-cell patch clamp technique. 2. In physiological solutions, depolarizing voltage clamp steps, from a holding potential (-80 mV) similar to the normal terminal resting potential, elicited a fast, inward followed by a fast, transient, outward current. 3. The threshold of activation for the outward current was -60 mV. The outward current quickly reached a peak and then decayed more slowly. The decay was fitted by two exponentials with time constants of 21 +/- 2.9 and 143 +/- 36 ms. These decay constants did not show a dependence on voltage. The time to peak of the outward current decreased and the amplitude increased with increasingly depolarized potential steps. 4. The outward current was blocked by the substitution of K+ with Cs+ and its reversal potential was consistent with a potassium current. 5. The transient outward current showed steady-state inactivation at more depolarized (than -80 mV) holding potentials with 50% inactivation occurring at -47.9 mV. The time course of recovery from inactivation was complex with full recovery taking greater than 16 s. 6. 4-Aminopyridine (4-AP) blocked the transient outward current in a dose-dependent manner (approximately IC50 = 3 mM), while charybdotoxin (4 micrograms/ml) and tetraethylammonium (100 mM) had no effect on the current amplitude. 7. Lowering external [Ca2+] had no effect on the fast, transient outward current nor did the calcium channel blocker Cd2+ (2 mM). 8. The neurohypophysial outward current reported here corresponds most closely to IA, and not to the delayed rectifier or Ca2(+)-activated K+ currents. Neurohypophysial IA, however, appears to be different from the outward currents found in the cell bodies in the hypothalamus which project their axons to the posterior pituitary. 9. Under current clamp, evoked action potential duration increased (122%) upon application of 5 mM-4-AP, indicating that IA is involved in neurohypophysial spike repolarization. 10. The existence of this current could help explain why maximal peptide release only occurs in response to bursts of electrical activity invading the nerve terminals.

Distribution and co-existence of Met-enkephalin-like and mesotocin-like immunoreactivity in the neural lobe of the pituitary of the frog

Content and distribution of Met-enkephalin (Met-ENK)-like immunoreactivity in the hypothalamo-hypophysial system of bovine, rat and frog were examined using specific radioimmunoassay and immunohistochemistry. Ultrastructural localization and co-existence of Met-ENK-, mesotocin (MT)- and vasotocin (VT)-like immunoreactivity in the neural lobe of the frog pituitary was examined by a method combining pre-embedding peroxidase-antiperoxidase immunostaining for Met-ENK with post-embedding immunocolloidal gold staining for MT or VT. The highest concentrations of immunoassayable Met-ENK were present in the neural lobe of the pituitary of the frog. In addition to nerve fibers showing only MT-like or Met-ENK-like immunoreactivity, nerve fibers containing neurosecretory granules showing both MT- and Met-ENK-like immunoreactivities were very rich. But VT-like and Met-ENK-like immunoreactivity was confirmed separately in different axon terminals.

Regulated release of multiple peptides from the bag cell neurons of Aplysia californica

The bag cell neurons of Aplysia californica synthesize and store large amounts of peptides derived from the egg-laying-hormone (ELH) neuropeptide precursor. Different sets of peptides derived from the amino- and carboxyl-terminal regions of the prohormone possess unique biological activities, and are packaged in distinct sets of secretory granules. We report here quantitative measurements of the amounts of the peptide products stored in and released from the bag cell neurons using high pressure liquid chromatography (HPLC) and amino acid composition analysis. These studies demonstrate that both the autocrine acting bag cell peptides (BCPs) and ELH are released coincident with electrical activity in the bag cell cluster. The composition of the released peptide mixture is similar to that stored in the bag cells. ELH and other carboxy-terminal derived peptides are most often present at 5-fold greater levels than the BCPs. These results provide further insight into the use of multiple chemical messengers by the bag cell neurons.

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)

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.

Opioid inhibition of secretion from oxytocin and vasopressin nerve terminals following selective depletion of neurohypophysial catecholamines

Opioids intrinsic to the neurohypophysis inhibit secretion from magnocellular neurosecretory terminals. This study examined whether the actions of opioids are mediated via interactions with neurohypophysial catecholamine systems. Blocking the action of intrinsic opioids in the isolated neurohypophysis with naloxone enhanced evoked secretion of oxytocin (OXT) by 150% and of vasopressin (AVP) by 30%. The enhancement of OXT secretion was not significantly altered in neurohypophyses depleted of greater than 90% of noradrenaline content by prior lesion of the ventral noradrenergic tract, or depleted of greater than 90% of both noradrenaline and dopamine content by prior reserpine treatment. Significant enhancement of AVP secretion by naloxone did not occur following depletion of catecholamines. The data suggest: (1) the majority of the influence of intrinsic opioids on secretion of OXT is not mediated via interaction with noradrenaline or dopamine systems, (2) the weaker influence of intrinsic opioids over AVP secretion may be mediated via catecholamines, (3) the majority of neurohypophysial noradrenaline is derived from projections of ascending medullary cell groups.

Dehydration reduces kappa-opiate receptor binding in the neurohypophysis of the rat

Quantitative autoradiography was used to examine the effects of dehydration on kappa-opiate receptor binding in the neural lobe of the pituitary in rats. Dehydration was produced by 5 days of water deprivation or ingestion of 2% saline. Brattleboro rats homozygous for diabetes insipidus were used as a model of chronically disturbed water balance. Slide-mounted pituitary sections were incubated with [3H]bremazocine using kappa-receptor-selective assay conditions. Binding in the neural lobe was quantified by densitometry of film autoradiographs. Specific [3H]bremazocine binding in neural lobe sections of control rats was 45.4 fmol/mg wet weight. In contrast, binding in neural lobe sections of water-deprived, saline-treated, and water-sated homozygous Brattleboro rats was lower by 50%, 35% and 37%, respectively. We suggest that chronic dehydration elevates levels of endogenous neurohypophyseal dynorphin and produces a down-regulation of kappa-opiate receptors in the neurohypophysis, a change in affinity of the receptors for kappa-agonists, or both.