Rat Neurointermediate Lobe Research Articles

Prostaglandin F2 alpha stimulates vasopressin and oxytocin release from isolated rat neurointermediate lobe.

Prostaglandin F2 alpha (1.4 X 10(-7) - 1.4 X 10(-5) M) increases vasopressin and oxytocin release from isolated rat neurointermediate lobes. Prostaglandin F2 alpha may be therefore supposed to serve as a modulatory factor for the vasopressin and oxytocin release, the respective processes being localized, at least in part, in the neurohypophysis.

Dietary sodium modulates mRNA abundance of enzymes involved in pituitary processing of proopiomelanocortin.

The messenger RNA abundance of proopiome-lanocortin (POMC) is increased in neurointermediate lobe (NIL) of rat pituitary when ingesting a high sodium diet (8%; HSD), as is the plasma concentration of the natriuretic peptide gamma-melanocyte stimulating hormone (gammay-MSH) derived from it. We examined whether the HSD also increases the mRNA abundance in rat NIL of proconvertases 1 and 2 (PC1, PC2), enzymes involved in the processing of POMC into gamma-MSH. PC1 mRNA increased by 40% after two weeks of the HSD and by 84% after three weeks. PC2 mRNA increased by 40% after two weeks and by more than 3 fold after three weeks. These results for PC2 were confined to NIL as shown by in situ hybridization at one and two weeks, and were accompanied by a significant increase in NIL PC2 protein after three weeks of the HSD as measured by immunoblotting. The increases in PC1 and PC2 mRNA abundance were paralleled by an increase in POMC mRNA level in NIL. Plasma gamma-MSH immunoreactivity averaged 35.1 +/- 3.3 fmol/ml in rats on the LSD, but increased to 70.9 +/- 4.8 fmol/ml after 3 weeks of the HSD (p < 0.002 vs LSD). These results confirm that the HSD increases the plasma concentration of gamma-MSH, consistent with a role for it as a circulating natriuretic peptide. The increased NIL expression of PC1 and PC2 in parallel with POMC in response to the HSD suggests that these changes are part of the coordinated response to states of sodium surfeit.

Characterization of Human and Rat Glucagon-Like Peptide-1 Receptors in the Neurointermediate Lobe: Lack of Coupling to Either Stimulation or Inhibition of Adenylyl Cyclase*

Glucagon-like peptide-1 (GLP-1) has been shown to bind to the posterior pituitary in the rat. We examined GLP-1 binding sites in human postmortem and rat pituitaries. Dense [125I]GLP-1 binding was seen in both human and rat posterior pituitary. In rat neurointermediate lobe membranes the binding site showed a Kd of 0.2 +/- 0.01 nM and a binding capacity of 600 +/- 33 fmol/mg protein (n = 3). In human pituitary membranes the binding site showed a Kd of 0.82 +/-0.05 nM and a binding capacity of 680 +/- 93 fmol/mg protein (n = 3). Chemical cross-linking showed a relative mol wt for the receptor-ligand complex of 73,100 +/- 1,400 (n = 3) in man and 59,300 +/- 900 (n = 3) in rat. GLP-1 (1 microM) failed to increase cAMP levels measured in rat neurointermediate lobes, whereas pituitary adenylate cyclase-activating polypeptide (100 nM) increased cAMP from a basal level of 14 +/-1 to 80 +/- 4 pmol/neurointermediate lobe 15 min (n = 5; P < 0.01). GLP-1 (up to 1 microM) did not affect the pituitary adenylate cyclase-activating polypeptide-stimulated cAMP levels. GLP-1 (up to 1 microM) also did not stimulate release of vasopressin or oxytocin from isolated rat neurointermediate lobes. The posterior pituitary shows the highest density of GLP-1-binding sites yet seen, but their function and signal transduction mechanism remain unknown.

Mu and delta opioid receptor regulation of pro-opiomelanocortin peptide secretion from the rat neurointermediate pituitary in vitro

We investigated the ability of selective opioid agonists and antagonists to influence pro-opiomelanocortin peptide secretion from the rat neurointermediate lobe in vitro. The μ-opioid agonist DAMGO ([D-Ala2, N-Me-Phe4, Gly5-ol]enkephalin) significantly stimulated β-endorphin and α-melanocyte-stimulating hormone release relative to controls early (30 min) in the incubation period. Similar effects on β-endorphin secretion were observed with the selective μ-opioid agonist dermorphin. The δ-opioid receptor agonist DPDPE ([D-Pen2,5]enkephalin) weakly inhibited β-endorphin secretion relative to controls while the κ-opioid receptor agonist U50488 had no effect. The μ-opioid selective antagonist CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2) inhibited basal β-endorphin secretion while κ- and δ-opioid receptor antagonists had no effect. Our data support a role for local μ-opioid receptor control of intermediate lobe pro-opiomelanocortin peptide secretion. Peptide secretion from melanotropes appears to be tonically stimulated by activation of μ-opioid receptors in the absence of intact neuronal innervation to the intermediate lobe.

Dynorphin A processing enzyme: tissue distribution, isolation, and characterization.

Limited proteolysis of the dynorphin precursor (prodynorphin) at dibasic and monobasic processing sites results in the generation of bioactive dynorphins. In the brain and neurointermediate lobe of the pituitary, prodynorphin is processed to produce alpha and beta neo endorphins, dynorphins (Dyn) A-17 and Dyn A-8, Dyn B-13, and leucine-enkephalin. The formation of Dyn A-8 from Dyn A-17 requires a monobasic cleavage between Ile and Arg. We have identified an enzymatic activity capable of processing at this monobasic site in the rat brain and neurointermediate lobe of the bovine pituitary; this enzyme is designated "dynorphin A-17 processing enzyme." In the rat brain and neurointermediate lobe, a majority of the Dyn A processing enzyme activity is membrane-associated and can be released by treatment with 1% Triton X-100. This enzyme has been purified to apparent homogeneity from the membrane extract of the neurointermediate lobe using preparative iso-electrofocussing in a granulated gel pH 3.5 to 10, FPLC using anion exchange chromatography, and non-denaturing electrophoresis. The Dyn A processing enzyme exhibits a pI of about 5.8 and a molecular mass of about 65 kDa under reducing conditions. The Dyn A processing enzyme is a metalloprotease and has a neutral pH optimum. It exhibits substantial sensitivity to metal chelating agents and thiol agents suggesting that this enzyme is a thiol-sensitive metalloprotease. Specific inhibitors of other metallopeptidases such as enkephalinase [EC 3.4.24.11], the enkephalin generating neutral endopeptidase [EC 3.4.24.15], or NRD convertase do not inhibit the Dyn A processing enzyme activity. In contrast, specific inhibitors of angiotensin converting enzyme inhibit the activity. The purified enzyme is able to process a number of neuropeptides at both monobasic and dibasic sites. These characteristics are consistent with a role for the Dyn A processing enzyme in the processing of Dyn A-17 and other neuropeptides in the brain.

Age-Dependent Muscarinic Stimulation of β-Endorphin Secretion From Rat Neurointermediate Lobe In Vitro

The effect of acetylcholine on the neurointermediate lobe β-endorphin secretion was studied in the neonatal and in the adult rat in vitro. Acetylcholine stimulated β-endorphin secretion from the 2-day- and 5-day-old neurointermediate lobe, the effect was dose dependent and more pronounced in the presence of the cholinesterase inhibitor eserine. The 10-day-, the 21-day-old and the adult rat neurointermediate lobes did not respond to acetylcholine, even in the presence of eserine. Basal β-endorphin secretion was elevated by the D 2 receptor antagonist sulpiride, but acetylcholine was without effect in the 10-day-old and in the adult neurointermediate lobe even after dopamine receptor blockade. The β-endorphin stimulatory response to acetylcholine was diminished by the M 1 muscarinic receptor antagonist pirenzepine and blocked by the M 3 > M 1 antagonist 4-diamino-phenyl-piperidine (4-DAMP). The selective M 2 antagonist methoctramine and nicotine had no effect. These data indicate that the neurointermediate lobe β-endorphin secretion is under special muscarinic cholinergic regulation for a relatively short time after birth. The disappearance of this stimulatory cholinergic effect in later life might be due to changes in the intracellular secretory machinery in the IL and/or to the uncoupling of the cholinergic receptors from the intracellular signal transduction system(s) responsible for the stimulated secretion in the rat melanotrope cells.

Effects of cytokines on pituitary beta-endorphin and adrenal corticosterone release in vitro.

We investigated the effects of recombinant human IL-1 alpha, -1 beta, -2, -6 and TNF on the in vitro secretion of beta-endorphin-immunoreactivity (beta E-IR) by the rat anterior and neurointermediate lobes (AL and NIL, respectively) and of B by the rat adrenal gland. Isolated AL and NIL cells were incubated for 2 h with cytokines (1 pg/m1(-1) mu g/ml), CRH (5.10(-10) M) or with cytokines in combination with CRH (AL cells), isolated adrenal cells were incubated for 2 h with cytokines, ACTH (25 pg/ml) or with cytokines in combination with ACTH. Furthermore, AL, NIL and adrenal tissue fragments were superfused for 30 or 60 min with cytokines (10 and/or 100 ng/ml). Incubation of AL, NIL and adrenal cells and superfusion of these tissues with cytokines had no significant effect on beta E-IR and B release. However, there are some exceptions: incubation of AL cells with IL-2 increased CRH-induced beta E-IR release, incubation of NIL cells with IL-2 induced an increase of basal beta E-IR release, ACTH-induced B secretion was reduced after co-incubation of adrenal cells with TNF and after prolonged (6 h) superfusion of adrenal tissue with TNF, and finally, prolonged (6 h) superfusion of adrenal fragments with IL-1 beta increased basal B release. Taken together, these data suggest that the acute activation of the pituitary-adrenal axis of rats by administration of cytokines (at least IL-1, IL-6 and TNF) in vivo is not mediated by a direct action of these cytokines at the level of the pituitary and/or adrenal gland.

Regulation of pituitary beta-endorphin secretion in aging rats: In vitro responsiveness to dopamine

The intermediate lobe of the mammalian pituitary is highly responsive to dopamine inhibition of beta-endorphin secretion. In this study, the ability of aged (12 months) intermediate lobes to respond to dopamine was compared to that of young (6 weeks) tissue, using a short-term in vitro incubation of isolated rat neurointermediate lobes, with measurement of peptide release by radioimmunoassay. Tissue from the aged rats released greater amounts of betaendorphin peptide than amounts measured from young tissue at all time periods studied. The aged lobes were also found to be significantly more sensitive to dopamine than young glands, as measured by percent change of each group compared to respective baseline release. In comparison, incubation of tissue from young animals in which the intermediate lobe had been acutely denervated by treating rats with injections of the catecholamine neurotoxin, 6-hydroxydopamine, did not differ in responsiveness to dopamine as compared to tissue from control rats. The observations suggest that aging intermediate lobe, while being hypersecretory, is supersensitive to dopamine, perhaps as the result of gradually reduced innervation.

Melanin-Concentrating Hormone in Human and Rat

Melanin-concentrating hormone (MCH) is a neuropeptide originally isolated from chum salmon pituitaries. To explore physiological roles of MCH in mammals, we studied the regional distribution of immunoreactive MCH in the rat tissues and the presence of immunoreactive MCH in human adrenal glands, adrenal tumors and plasma by radioimmunoassay, and the expression of MCH mRNA in rat tissues and human brain tissues by Northern blot analysis. Immunoreactive MCH was present in every region of rat brain and neurointermediate lobe of the pituitary gland, with the highest concentrations found in the hypothalamus (48.3 +/- 6.6 pmol/g wet weight, mean +/- SEM, n = 6). The immunoreactive MCH in rat hypothalamus, frontal lobe, and pons and medulla oblongata was eluted in the position of synthetic human/rat MCH in reverse-phase high-performance liquid chromatography. No immunoreactive MCH was detected in the rat peripheral tissues. Northern blot analysis showed that a single species of MCH mRNA (approximately 1 kb) was expressed specifically in the rat and human hypothalamus, but not detectable in other regions of brain or rat peripheral tissues. Immunoreactive MCH was not detected in human adrenal glands (< 0.5 pmol/g wet weight, n = 9) or adrenal tumors including pheochromocytomas. Immunoreactive MCH were not detected in plasma obtained from human healthy subjects (< 0.25 pmol/l) and rat (< 0.25 pmol/l).(ABSTRACT TRUNCATED AT 250 WORDS)

Dihydropyridine ligands influence the evoked release of oxytocin and vasopressin dependent on stimulation conditions

The effects of dihydropyridine ligands on the electrically evoked release of neurohypophysical hormones from isolated, rat neurointermediate lobes were investigated as a function of all combinations of two pulse widths (0.2 and 2 ms) and three stimulation frequencies (6.5, 13 and 30 Hz). The dihydropyridine agonist ( S)-(+)-202-791 potentiated concentration dependently the release of both oxytocin and vasopressin at a pulse width of 2 ms and a frequency of 6.5 Hz. This effect of ( S)-(+)-202-791 was abolished by the antagonist (−)-nitrendipine and stereospecifically by ( R)-(−)-202-791 (only vasopressin). The antagonist ( R_-(−)-202-791 alone inhibited the release of oxytocin at 13 Hz and 2 ms. The results presented show that the effects of the dihydropyridine ligands are dependent on the stimulation conditions, and thus demonstrate that the entry of Ca 2+ through the dihydropyridine sensitive L-type Ca 2+ channel is associated with electrically evoked release of neurohypophysical hormones under certain conditions.

Neuropeptide-Y potentiates the secretion of vasopressin from the neurointermediate lobe of the rat pituitary gland

Neuropeptide-Y (NPY) is colocalized with vasopressin and oxytocin in magnocellular neurons of the hypothalamo-neurohypophysial system, and a very high density of NPY-binding sites is present within the neurohypophysis. To investigate the possibility that NPY exerts a modulatory role on the release of neurohypophysial hormones, we have studied the actions of NPY on potassium-evoked release of vasopressin and oxytocin from the rat neurointermediate lobe in vitro. NPY dose-dependently potentiated vasopressin release evoked by high extracellular potassium (56 mM), with a maximal enhancement of 223% (10(-7) M NPY). A similar effect was obtained with the Y2-selective agonist NPY-(13-36). In contrast, no effect on the potassium-evoked release of oxytocin was observed at this concentration. In the absence of Ca2+ in the incubation medium, NPY did not potentiate vasopressin secretion, indicating that the effect of NPY on potassium-evoked secretion of neurohypophysial vasopressin is critically dependent on extracellular calcium ions. The number of neurohypophysial NPY-binding sites is drastically down-regulated in animals subjected to chronic osmotic stimulation. In the present study, it was observed that the potentiating effect of NPY on vasopressin secretion was completely abolished in neurointermediate lobes recovered from animals that had been drinking 2% NaCl for 12 days, reflecting the concomitant down-regulation of neurohypophysial NPY-binding sites observed during this state. Finally, it was confirmed that stimulation by high K+ significantly evoked the release of endogenous NPY from neurointermediate lobes of the pituitary gland. The present results provide evidence that NPY selectively and potently enhances evoked vasopressin secretion. Considering the coexistence of the two neuropeptides in magnocellular hypothalamo-neurohypophysial neurons, this action is likely to be part of an autostimulatory feedforward loop. NPY may be an important component in the mechanisms associated with the control of body fluid homeostasis.

Neuropeptide-Y potentiates the secretion of vasopressin from the neurointermediate lobe of the rat pituitary gland.

Neuropeptide-Y (NPY) is colocalized with vasopressin and oxytocin in magnocellular neurons of the hypothalamo-neurohypophysial system, and a very high density of NPY-binding sites is present within the neurohypophysis. To investigate the possibility that NPY exerts a modulatory role on the release of neurohypophysial hormones, we have studied the actions of NPY on potassium-evoked release of vasopressin and oxytocin from the rat neurointermediate lobe in vitro. NPY dose-dependently potentiated vasopressin release evoked by high extracellular potassium (56 mM), with a maximal enhancement of 223% (10(-7) M NPY). A similar effect was obtained with the Y2-selective agonist NPY-(13-36). In contrast, no effect on the potassium-evoked release of oxytocin was observed at this concentration. In the absence of Ca2+ in the incubation medium, NPY did not potentiate vasopressin secretion, indicating that the effect of NPY on potassium-evoked secretion of neurohypophysial vasopressin is critically dependent on extracellular calcium ions. The number of neurohypophysial NPY-binding sites is drastically down-regulated in animals subjected to chronic osmotic stimulation. In the present study, it was observed that the potentiating effect of NPY on vasopressin secretion was completely abolished in neurointermediate lobes recovered from animals that had been drinking 2% NaCl for 12 days, reflecting the concomitant down-regulation of neurohypophysial NPY-binding sites observed during this state. Finally, it was confirmed that stimulation by high K+ significantly evoked the release of endogenous NPY from neurointermediate lobes of the pituitary gland. The present results provide evidence that NPY selectively and potently enhances evoked vasopressin secretion. Considering the coexistence of the two neuropeptides in magnocellular hypothalamo-neurohypophysial neurons, this action is likely to be part of an autostimulatory feedforward loop. NPY may be an important component in the mechanisms associated with the control of body fluid homeostasis.

A prolactin release inhibiting activity isolated from neurointermediate lobe extracts is an endothelin-like peptide.

A prolactin release-inhibiting factor has been reported in extracts of rat and bovine neurointermediate lobes of the pituitary gland. This material when isolated by Sephadex G-25 chromatography or by C-8 high pressure liquid chromatography coelutes with synthetic forms of all members of the mammalian endothelin family of peptides and with their immediate precursors, big-ET-1 and big-ET-3. The endogenous material crossreacts in an endothelin radioimmunoassay. Immunohistochemical analysis of endothelin-like immunoreactivity in the neurointermediate lobe revealed a predominantly intermediate lobe localization. Individual immunopositive cells were visualized in the intermediate lobe. Preabsorption of chromatographically isolated prolactin release-inhibiting activity with an endothelin antiserum which recognizes all members of the ET family of peptides abolishes its in vitro bioactivity. These results suggest that a prolactin release-inhibiting activity present in the neurointermediate lobe is endothelin or an endothelin-like peptide.

Interaction of corticotropin-releasing factor (CRF) and alpha-helical CRF on rat neurointermediate lobes: In vitro studies

Neurointermediate lobes (NILS) of the pituitary glands of adult male Sprague-Dawley rats were incubated in media in the presence of corticotropin-releasing factor (CRF), a stimulator of proopiomelanocortin (POMC) peptide release. Alpha-helical CRF, a peptide known to inhibit CRF induced POMC peptide release from the anterior pituitary, was incubated with NILS for a period of 90 min, to study its potential ability to modulate peptide release from the intermediate lobe. The alpha-helical peptide reduced beta-endorphin release from NILS, as measured by radioimmunoassay (RIA), when added for the entire incubation, or when added 30 min after start of the incubation period, with CRF present. Alpha-helical CRF alone reduced beta-endorphin release, as compared to control or CRF-treated lobes. Ultrastructural examination of intermediate lobes fixed at the end of incubations revealed a reduction in the numbers of Golgi-associated dense granules, an indicator of new peptide synthesis, in intermediate lobe tissue treated with alpha-helical CRF alone, both peptides together, or with CRF followed by alpha-helical peptide. The in vitro studies demonstrate the effectiveness of the antagonist peptide on intermediate lobe peptide secretion, thereby extending its effects to both POMC-secreting areas of the pituitary gland.

Diethylstilbesterol- and Pregnancy-Induced Changes in Rat Neurointermediate Lobe Oxytocin, Arginine Vasopressin, Methionine Enkephalin and Dynorphin

The neurointermediate lobe of the pituitary (NIL) contains the opioid peptides methionine enkephalin (MENK) and dynorphin 1-8 (DYN) in addition to oxytocin (OT) and vasopressin (AVP). If the opioids have a functional role, such as feedback control on OT or AVP release, the content or release of the opioids might be expected to change under conditions in which OT or AVP change. This expectation was examined by studying the synthesis, storage and release of the 4 peptides under conditions in which OT and AVP dynamics are known to be altered. Diestrus, diethylstilbesterol(DES)-treated, and day 22 pregnant rats were decapitated, the hypothalamo-neurohypophysial system (HNS) excised and either superfused in oxygenated Krebs buffer at 37 degrees C or stored at -80 degrees C for measurement of paraventricular and supraoptic nuclei mRNA content by in situ hybridization analysis. Peptide content of superfusates and NIL homogenates were determined by specific RIAs. Compared with diestrus animals, DES treatment increased NIL OT but decreased MENK. In term pregnant rats, NIL OT, AVP, and DYN were increased over diestrus values, while MENK was again decreased. Release of the peptides from the isolated HNS paralleled changes in NIL content. The hypothalamic mRNA for OT was increased in DES-treated and pregnant rats while MENK mRNA was decreased. AVP and DYN mRNA was increased in pregnant animals. Although the NIL was found to contain much more immunoreactive OT and AVP than MENK or DYN, under basal conditions the release of MENK was equal to or greater than the release of OT, while the release of DYN approached that of AVP.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta‐Adrenergic Stimulation Decreases Glial and Increases Neural Contact with the Basal Lamina in Rat Neurointermediate Lobes Incubated in vitro

Abstract Physiological stimuli such as dehydration and lactation/suckling of the young have been found to induce reversible changes in the relationships of neural and glial elements with the basal lamina of the neurohypophysis. Such stimulation is associated with a decline in the extent of basal lamina occupied by glial processes and an increase in nerve terminals abutting the lamina. One possible mechanism playing a role in these changes is the activation of beta-adrenergic receptors on the neurohypophysial glia (the pituicytes), since such cells in primary culture from adult rats undergo dramatic morphological transformation when stimulated with beta-adrenergic agonists. We sought to determine if changes similar to those seen in vivo and predicted from the responses of cultured cells would occur with beta-adrenergic stimulation of the isolated neurointermediate lobe in vitro. Neurointermediate lobes from adult male rats were incubated in artificial cerebrospinal fluid containing ascorbic acid alone (control) or in the same medium with either 10(-8), 10(-7) or 10(-5) M isoproterenol for 15 min after a 30-min period of preincubation in control medium. Quantitative ultrastructural analysis revealed significant decreases in pituicyte and, corresponding increases in neural, membrane abutting the basal lamina at the lower two drug concentrations. These results support the findings of other studies suggesting a beta-adrenergic mediation of pituicyte morphology and a role for beta-receptors in control of posterior pituitary function.

Isoproterenol-stimulated release of β-endorphin and related peptides from the rat pituitary neurointermediate lobe in vitro: Evidence for preferential release of certain molecular forms of β-endorphin

The intermediate lobe of the pituitary gland synthesizes the multifactorial precursor molecule pro-opiomelanocortin (POMC), from which, through a process of post-translational enzymatic processing, β-endorphin-(1–31) (βE) and a variety of N α-acetylated and C-terminally shortened forms of this peptide are generated. Using an in vitro superfusion system, the release of these endorphins from intact rat neurointermediate lobes (NILs) was investigated under basal and isoproterenol (ISO) stimulated conditions. Superfusion of NILs with the β-adrenergic agonist ISO (30 min pulse) resulted in a rapid, sustained and concentration-dependent stimulation of the release of βE-like immunoreactivity (βE-IR) over basal as determined with an antiserum directed against the C-terminus of the βE-(1–31) sequence (10 −6M: +145%; 10 −7M: +73%; 10 −8m: +41%). The release of N α-acetylated-endorphin-like immunoreactivity (AcE-IR) was stimulated to a similar extent. These effects of ISO were antagonized by the competitive α-adrenoceptor antagonist propranolol in a concentration-dependent manner, indicating the involvement of α-adrenoceptors. The β-related peptides released from the NILs under basal and ISO-stimulated conditions were further characterized, based on their retention times in a reversed-phase HPLC system and their reactivity with specific antisera recognizing respectively the midportion of βE, the N-terminus of acetylated endorphines, the C-terminus of τ-endorphin (βE-(1–17); τE), or the C-terminus of α-endorphin (βE-(1–16); αE). In HPLC fractionated superfusates 10 peaks were resolved that reacted with the midportion βE antiserum. In superfusates collected under basal conditions, three major peaks possessed chromatographical and immunological characteristics of AcβE-(1–26), AcβE-(1–27) AcβE-(1–31). In addition, a prominent peak was found eluting around the retention time of βE-(1–31), that contained both acetylated and non-acetylated material. Six smaller peaks were observed, with the characteristics of βE-(1–26) and βE-(1–27) (these peptides were not resolved with the HPLC system used), AcτE, τE, AcαE, and des-tyrosine-αE (DTαE), respectively. In superfusates collected during superfusion of NILs with ISO (10 −6M) all peaks were increased. However, those eluting as βE-(1–31), βE-(1–26) βE-(1–27), AcβE-(1–26) and AcτE appeared to be preferentially stimulated. The results indicate, that the NIL in vitro simultaneously releases βE and a variety of highly processed βE-related peptides including τ- and α-type endorphins. In addition, the data suggest the existence of β-adrenoceptor coupled mechanisms, that govern the secretion of a preferred set of βE-related peptides from the pituitary intermediate lobe cell.

Chronic naltrexone infusion: Effects on innervation of rat neurointermediate lobe

The rat pituitary neurointermediate lobe contains nerve fibers which are sensitive to the neurotoxic effects of 6-hydroxydopamine and to acute injections of the opiate antagonist, naltrexone. In the present study, naltrexone hydrochloride was administered to adult male Sprague-Dawley rats in a chronic infusion over a period of one week, using Alza osmotic pumps to determine whether a low (10 −5 M) dose could, over a longer period, also induce toxic effects on pituitary innervation. Electron microscopy of pituitary intermediate lobe fibers revealed swollen neurovesicles and membranous structures in nerve terminals, and significantly fewer normal-appearing nerve terminals, after drug infusion. Light microscopic immunostaining of paraffin sections of pituitary glands illustrated some diminution of serotonin (5-HT)-immunofluorescence in the intermediate lobe after naltrexone treatment. Immunostaining of sections from the same animals for tyrosine hydroxylase (TH) revealed intensified fiber staining in the intermediate lobe, with some regions of terminals having a swollen appearance. High pressure liquid chromatographic-electrochemical detection (HPLC-EC) analysis of neurotransmitters in selected brain areas and pituitary indicated a significant increase in norepinephrine levels in the hypothalamus. The observations suggest that intermediate lobe innervation is sensitive to low, continuously infused doses of naltrexone, which acts as a neurotoxin to produce degenerative changes in nerve terminals. The changes appear to be reflected in alterations in staining patterns of neurotransmitters, and may also affect transmitter uptake in damaged terminals.

κ‐Opioid Receptor Agonists Differentially Affect the Release of Neurohypophysial Hormones

The influence of kappa-opioid receptor agonists and antagonists on release of oxytocin and vasopressin was examined in isolated rat neurointermediate lobes. Electrically evoked release of oxytocin and vasopressin was concentration-dependently inhibited by the specific kappa-receptor agonist U69593, whereas bremazocine only inhibited the secretion of oxytocin markedly. Treatment with naloxone enhanced the evoked release of oxytocin significantly without effect on vasopressin secretion. The U69593-mediated inhibition of oxytocin release was abolished by naloxone, whereas that of vasopression was unaffected. Naloxone did not reverse the bremazocine-induced inhibition of hormone release. The data support the theory of an inhibiting endogenous control over oxytocin secretion and show that the release of oxytocin and vasopresin is differentially affected by the two K-receptor agonists.

The Expression of the Kallikrein Gene Family in the Rat Pituitary: Oestrogen Effects and the Expression of an Additional Family Member in the Neurointermediate Lobe

Abstract Using a series of oligonucleotide probes (18 to 21 mers) specific for members of the rat kallikrein/tonin gene family (PS, S1, S2, S3, K1, P1), we have shown by Northern blot analysis that the oestrogen-dependent kallikrein gene expressed in the male and female rat anterior pituitary is true kallikrein (PS). In addition, we have demonstrated that oestrogen treatment may also induce PS gene expression in the male and female rat neurointermediate lobe. None of the other five rat arginyl-esteropeptidase genes so far described (S1, S2, S3, K1, P1) was found to be expressed in the anterior pituitary or neurointermediate lobe under these conditions. However, the demonstration of an additional hybridization signal in the male neurointermediate lobe using a relatively non-specific PS gene probe suggests the expression of another, as yet uncharacterized, kallikrein gene family member in this tissue.