Mechanisms of inactivation of hypothalamic regulatory hormones

Structure-activity studies with high-affinity inhibitors of pyroglutamyl-peptidase II

Inhibitors of PPII (pyroglutamyl-peptidase II) (EC 3.4.19.6) have potential applications as investigative and therapeutic agents. The rational design of inhibitors is hindered, however, by the lack of an experimental structure for PPII. Previous studies have demonstrated that replacement of histidine in TRH (thyrotropin-releasing hormone) with asparagine produces a competitive PPII inhibitor (Ki 17.5 microM). To gain further insight into which functional groups are significant for inhibitory activity, we investigated the effects on inhibition of structural modifications to Glp-Asn-ProNH2 (pyroglutamyl-asparaginyl-prolineamide). Synthesis and kinetic analysis of a diverse series of carboxamide and C-terminally extended Glp-Asn-ProNH2 analogues were undertaken. Extensive quantitative structure-activity relationships were generated, which indicated that key functionalities in the basic molecular structure of the inhibitors combine in a unique way to cause PPII inhibition. Data from kinetic and molecular modelling studies suggest that hydrogen bonding between the asparagine side chain and PPII may provide a basis for the inhibitory properties of the asparagine-containing peptides. Prolineamide appeared to be important for interaction with the S2' subsite, but some modifications were tolerated. Extension of Glp-Asn-ProNH2 with hydrophobic amino acids at the C-terminus led to a novel set of PPII inhibitors active in vitro at nanomolar concentrations. Such inhibitors were shown to enhance recovery of TRH released from rat brain slices. Glp-Asn-Pro-Tyr-Trp-Trp-7-amido-4-methylcoumarin displayed a Ki of 1 nM, making it the most potent competitive PPII inhibitor described to date. PPII inhibitors with this level of potency should find application in exploring the biological functions of TRH and PPII, and potentially provide a basis for development of novel therapeutics.

Characterization of [hydroxyproline9]luteinizing hormone-releasing hormone and its smallest precursor forms in immortalized luteinizing hormone-releasing hormone-secreting neurons (GT1-7), and evaluation of their mode of action on pituitary cells

[Hydroxyproline9]luteinizing hormone-releasing hormone ([Hyp9]LHRH), an endogenous hydroxylated post-translational product of the LHRH sequence, has been isolated from mammalian hypothalamus. Using the LHRH-hypothalamic cell line (GT1-7) of fetal origin, we attempted to define the substrates available for the hydroxylation process during LHRH synthesis and to characterize immunologically the [Hyp9]LHRH and pro-[Hyp9]LHRH forms with anti-LHRH antibodies of different specificities after separation by HPLC. Their biological activity and mode of action were evaluated and compared to that of LHRH and LHRH intermediate precursors in normal pituitary cells and in a gonanodotrope cell line alpha T3-1. immunoreactivity was progressively increased in cells and media during cell culture. [Hyp9]LHRH and its two smallest precursor forms ([Hyp9]LHRH-(Gly11) and -(11-13)) were detected in cells and in media. They were simultaneously detected with the homologous LHRH molecular forms indicating that the hydroxylation occurs early in the processing of pro-LHRH. [Hyp9]LHRH-like molecules were more abundant than LHRH forms in media. This predominant release may thus represent a physiological process occurring during fetal life. Free acid forms of both decapeptides were detected only in cells. Furthermore, the results obtained suggest that conversion of Gln1 in pyroGlu1 occurs before or during processing into the hydroxylated or non-hydroxylated LHRH intermediate (11-13)-precursors. The biosynthetic pathway is thus common for both decapeptides and it is not altered by the hydroxylation process. LHRH and [Hyp9]LHRH shared the same receptor for their biological activity, as assessed by measuring luteinizing hormone release and activation of phospholipase C and A2. [Hyp9]LHRH was, however, less potent than LHRH.

Suppression of food intake by apolipoprotein A-IV is mediated through the central nervous system in rats

The aim of this experiment was to investigate whether the anorectic effect of apolipoprotein A-IV (apo A-IV) after lipid feeding is mediated via the central nervous system. Infusion of 0.5 micrograms of apo A-IV into the third ventricle failed to suppress food intake. Higher doses (1 micrograms or higher) of apo A-IV infused into the third ventricle inhibited food intake in a dose-dependent manner. In contrast, when apo A-I was infused into the third ventricle it had no effect on food intake. To further test the hypothesis that apo A-IV is an important factor controlling food intake, we administered goat anti-rat apo A-IV serum into the third ventricle of rats that were allowed food and water and lib. In all rats tested, this treatment resulted in enhanced food intake. In contrast, infusion of goat anti-rat apo A-IV serum failed to elicit such a response. Lastly, we determined the apo A-IV concentration in plasma and cerebrospinal fluid before and during active lipid absorption. Apo A-IV concentration in cerebrospinal fluid was about 1/20 that of plasma. Both serum and cerebrospinal fluid apo A-IV increased markedly as a result of feeding of lipid. In conclusion, we propose that apo A-IV may act centrally to control food intake.

Degradation of thyrotropin-releasing hormone and luteinising hormone-releasing hormone by enzymes of brain tissue

In this article, the enzymes of brain and associated tissues that can degrade thyrotropin-releasing hormone (TRH) and luteinising hormone-releasing hormone (LH-RH) are reviewed. As both TRH and LH-RH are considered to act as neurotransmitters or neuromodulators in the CNS, attention is paid to the subcellular location of the enzymes described and how their topographies and substrate specificities fit them to playing roles as inactivating agents for TRH and LH-RH or as regulators of intracellular concentrations of TRH and LH-RH. Consideration is also given to enzymes involved in biotransformation of TRH to secondary metabolites that exhibit biological activity and to enzymes involved in the metabolism of secondary metabolites.

Characterization and purification of a placental protein that inactivates GnRH, TRH and angiotensin II

A protein that inactivates the immunoreactivity of GnRH, TRH and angiotensin II has been isolated from human term placentae. Only in the presence of DTT, a sulphydryl agent, are OXY and SRIF also inactivated by this protein. However, it is without effect on CRF, hCS, or hCG. It also inhibits the biological activity of GnRH, i.e. its ability to stimulate pituitary LH and FSH. The ability of this protein to inactivate GnRH, TRH or angiotensin II can be inhibited by various peptidase inhibitors. Thus, we have postulated that it is a chorionic peptidase, specific for these peptides, and herein called chorionic peptidase-1 (C-ase-1). Isolation of this protein, C-ase-1, has been effected using permeation, ion exchange and affinity chromatography. As estimated by SDS-PAGE and HPLC analyses, C-ase-1 has an apparent molecular weight of 58,000. It is proposed that C-ase-1 may be an important chorionic regulator of GnRH, TRH and angiotensin II levels during pregnancy.

Slow penetration of thyrotropin-releasing hormone across the blood-brain barrier of an in situ perfused guinea pig brain

Transport of 3H-labelled thyrotropin-releasing hormone (TRH) across the blood-brain barrier was studied in the ipsilateral perfused in situ guinea pig forebrain. The unidirectional transfer constant (Kin) calculated from the multiple time brain uptake analysis ranged from 1.14 X 10(-3) to 1.22 X 10(-3) ml min-1 g-1, in the parietal cortex, caudate nucleus, and hippocampus. Regional Kin values for [3H]TRH were significantly reduced by 43-48% in the presence of an aminopeptidase and amidase inhibitor, 2 mM bacitracin, suggesting an enzymatic degradation of tripeptide during interaction with the blood-brain barrier. In the presence of unlabelled 1 mM TRH and 2 mM bacitracin together, a reduction of [3H]TRH regional Kin values similar to that obtained with 2 mM bacitracin alone was obtained . L-Prolinamide, the N-terminal residue of tripeptide, at a 10 mM level had no effect on the kinetics of entry of [3H]TRH into the brain. The data indicate an absence of a specific saturable transport mechanism for TRH presented to the luminal side of the blood-brain barrier. It is concluded that intact TRH molecule may slowly penetrate the blood-brain barrier, the rate of transfer being some three times higher than that of D-mannitol.

Hydrolysis of the brain dipeptide N-acetyl-L-aspartyl-L-glutamate: subcellular and regional distribution, ontogeny, and the effect of lesions on N-acetylated-alpha-linked acidic dipeptidase activity

N-Acetylated-alpha-linked acidic dipeptidase (NAALADase) is a Cl- dependent, membrane bound, metallopeptidase that cleaves the endogenous neuropeptide N-acetyl-L-aspartyl-L-glutamate (NAAG) in vitro. To examine the pattern of NAALADase expression in the CNS, subcellular, regional, and developmental studies were conducted. Subcellular fractionation of lysed synaptosomal membranes revealed a substantial enrichment of the peptidase in synaptic plasma membranes as compared to mitochondrial or myelin subfractions. Regional studies reveal an apparent restriction of peptidase activity to kidney and brain. A threefold variation in specific activity was observed among brain regions, with highest specific activity in the cerebellum and lowest in telencephalic structures, a pattern that does not, in general, correlate with NAAG levels. Ontogenetic studies demonstrate a region-dependent, postnatal pattern of expression of NAALADase activity, with adult levels attained earliest in brainstem, as was previously reported for NAAG. Postnatal NAALADase expression would not appear to support a role for the peptidase in constitutive protein processing, but rather suggests that NAALADase may play a role in synaptic peptide degradation. Glutamate (Glu) excised from NAAG by NAALADase could be transported efficiently by uptake processes. Lesion studies, however, do not support a close structural association between NAALADase activity and the corticostriatal sodium-dependent, high-affinity, Glu uptake system. Similar to in vitro data documenting the route of NAAG degradation by NAALADase, after intrastriatal injection, NAAG was rapidly cleaved to two major products, N-acetyl-aspartate and Glu, with a t1/2 of approximately 10 min. Thus, the route of in vivo catabolism of NAAG parallels results from studies on NAALADase activity in vitro. These results are consistent with a role of NAALADase in the synaptic processing of NAAG. However, certain discrepancies in the regional and ontogenetic profiles of NAAG and NAALADase suggest that this relationship is not an exclusive one and may reflect a role for NAALADase on additional N-acetylated acidic peptides in vivo.

Metabolism of steroid-modifying anticancer agents

The application of steroid-modifying drugs as a strategy for the treatment of hormone-dependent cancers has gained increasing popularity during the past decade. However, it is important to point out and emphasize that very few of the agents were originally designed for their current application. Most were designed for other purposes, predominantly fertility control (e.g. LHRH agonists and the antiestrogens). Nevertheless, now it is possible to integrate their actions to design rational therapies. There are many reasons for the current interest in antisteroidal drugs. The initial euphoria over the potential ability of combination chemotherapy to cure breast and prostatic carcinoma has proved to be premature. Combination chemotherapy has many severe side-effects which limits patient acceptability, especially if the patient realizes that the likelihood of a cure is remote. In the main, antisteroidal therapies do not have many side-effects and those that do, e.g. aminoglutethimide, are the focus of increased efforts in drug design to produce increased drug specificity. Finally, there is a growing realization that hormone-dependent cancer control with a nontoxic, antisteroidal therapy may be the most acceptable approach currently available for early disease management. Chemotherapy would then be reserved as the final option for treatment. The description of drug metabolism has been central to the development of synthetic LHRH analogs and an understanding of the mode of action of nonsteroidal antiestrogens and antiandrogens. The discovery of steroid synthetic pathways has been essential for the development of the aromatase inhibitors. This whole area of endeavor has now become a major focus of attention for the medicinal chemist. A new generation of agents is entering clinical evaluation which will provide a wealth of valuable information about the successful (or unsuccessful?) methods to control hormone-dependent disease. Since the success or failure of a drug can often depend upon formulation, pharmacokinetics, bioavailability or metabolism, it is our hope that this overview might help solve some of the future problems.

Neuroendocrine mechanisms of stress ulceration: focus on thyrotropin-releasing hormone (TRH)

It is generally accepted that stress ulceration, a multifactorial or pluricausal gastrointestinal disorder, may be the result of mechanistic interrelationships between mucosal, vascular, hormonal and neurogenic factors. The relative importance of each of these independent mechanisms remains unclear. This minireview represents an attempt to interpret many recent studies on certain neurogenic mechanisms and to integrate these observations into the existing body of knowledge. A variety of in vitro techniques and animal models to manipulate actual structures, organ systems, and certain well-defined hormonal influences have been utilized. The peripheral studies have followed, for the most part, the established observation that the stomach is under reciprocal control by sympathetic inhibitory and parasympathetic excitatory autonomic fibers. As a result, several autonomic adrenergic neurotransmitter substances have been found to promote mucosal resistance. Some of these include dopamine, epinephrine, and norepinephrine. Others in contrast, appear to promote vulnerability of the mucosa, and of these, the most well-studied include acetylcholine and histamine.

Permeability of the blood-cerebrospinal fluid and blood-brain barriers to thyrotropin-releasing hormone

The permeability of the blood-cerebrospinal fluid (CSF) barrier to 3H-labelled thyrotropin-releasing hormone (TRH), was studied at the blood-tissue interface of the isolated perfused choroid plexus of the sheep, using a rapid (less than 30 s), single circulation paired-tracer dilution technique, in which D-[14C]mannitol serves as an extracellular marker. Arterio-venous loss of 14C radioactivity reflects the percentage of the D-mannitol dose that crosses the blood-CSF barrier using a non-specific pathway. This loss suggests that the choroidal epithelium is moderately leaky. Cellular uptake of TRH, estimated by directly comparing venous dilution profiles of [3H]TRH and D-[14C]mannitol was independent of this leakiness. The unidirectional transport of TRH could not be saturated with unlabelled TRH at a concentration as high as 10 mM, but was markedly reduced by 10 mM proline and by the inhibitor of amidase and aminopeptidase activity, bacitracin (2 mM). Permeability of the blood-brain barrier to [3H]TRH was studied in the adult rat, employing the intracarotid injection technique of Oldendorf in which [14C]butanol served as an 'internal standard'. Brain-uptake of 3H radioactivity corrected for residual vascular space indicated a low extraction from the blood of TRH during a 15 s period of exposure to the peptide. Self-inhibition of [3H]TRH uptake by unlabelled TRH (10 mM) could not be demonstrated, but L-proline (10 mM) and bacitracin (2 mM) strongly inhibited this uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural characterization of gonadotropin-releasing hormone (GnRH)-producing neurons

By means of preembedding immunohistochemistry, two types of gonadotropin-releasing hormone (GnRH) positive neurons in the rat could be identified and characterized in the preoptic region and in the diagonal band: (1) a "smooth" GnRH neuron with relatively even cytoplasmic contours, and (2) a "spiny" GnRH neuron with thorn-like protrusions of the perikaryon and cell processes. Both cell types contain the same organelles in similar number and distribution, but they differ in the number of synaptic contacts. In general, GnRH cell bodies have a large round or ovoid nucleus, well-developed rough endoplasmic reticulum arranged in multilayered stacks or as individual cisternae, and several Golgi complexes. Lysosomes are not numerous under the conditions studied. Specializations include kinocilia, nematosomes, and lamellar whorls. Throughout the cytoplasm, scattered dense core vesicles with a diameter of 100 nm and clear vesicles with a diameter of 30-40 nm can be seen with a preferential localization close to the cell membrane. The cell processes of smooth GnRH cells close to the perikaryal appear as extensions of the perikaryal cytoplasm with all organelles except the nucleus. The two neurites originate from the perikaryon as tapering cones over a distance of 200-300 micron, until they reach a diameter of 0.5-3 micron. Cell processes of spiny GnRH cells show bifurcations, protrusions, or invaginations and contain clear and dense core vesicles in their spines. In areas distant from the perikaryon, immunoreactive fibers with a large number of dense core and clear vesicles can occasionally be seen to terminate synaptically or asynaptically on other neurons. The GnRH neurons show postsynaptic specializations at the level of the perikaryon and at cell processes, when apposed by a presynaptic terminal. Such synaptic contacts are seen less frequently on smooth cells than on spiny cells. Large areas of the GnRH cell may be covered by a thin glial lamella, which separates the cell body from the surrounding neuropil. The results indicate the existence of two populations of GnRH cell bodies with different patterns of innervation, which suggest different integrative capacities.

GnRH secreting cultured fetal rat hypothalamic cells do not degrade GnRH

The possible degradation of GnRH in the hypothalamus was investigated. Rat fetal hypothalamic cells were kept in culture for two weeks and basal and stimulated GnRH release was measured by highly sensitive RIA. These intact hypothalamic cells did not degrade GnRH during 4 hours of incubation, but a 50% degradation occurred after 24 hours incubation followed by HPLC using specifically tritium labeled GnRH or RIA. The rate of degradation or the kinetic of degradation did not change by increasing GnRH concentration in the medium or by mechanical instead of enzymatic dispersion of the cells. For comparison GnRH degradation in homogenized hypothalamic tissue and in synaptosomal preparation was measured and rapid degradation was found. Our results suggest that intact hypothalamic cells under physiological circumstances do not degrade extracellular GnRH.

Proteases of human brain

Growing appreciation of the multiple functions of proteolytic enzymes in intracellular protein degradation and post-translational modification, in the release of biologically active macromolecules and peptides from precursors and in cellular protein regulation and quality control has stimulated interest in proteases in neurobiology and neuropathology. In this article, the proteinases and peptidases thus far studied in the human central nervous system are reviewed with respect to their enzymology, anatomical and cytological distributions and contributions to neurological and psychiatric disease states. Though information concerning brain proteases in man is fragmentary, it suffices to establish the importance of these complex systems for advancing knowledge of human cerebral function in health and disease.

Molecular identification of receptors for vasoactive intestinal peptide in rat intestinal epithelium by covalent cross-linking. Evidence for two classes of binding sites with different structural and functional properties

The cleavable cross-linking reagent dithiobis (succinimidyl propionate) or DTSP was shown to link 125I-labeled vasoactive intestinal peptide (125I-VIP) covalently to its receptors in rat intestinal epithelial membranes. DTSP treatment of 125I-VIP-labeled membranes inhibited the dissociation of VIP-receptor complexes in a way which was dependent on both time and concentration (ED50 = 200 microM). Polyacrylamide gel electrophoresis of membrane proteins revealed three 125I-VIP-protein complexes of Mr 76 000, 36 000 and 17 000. The labeling of those compounds was not observed when: (a) treatment of membranes by DTSP was omitted; (b) the reagent quench, ammonium acetate, was added together with DTSP; (c) DTSP-treated membranes were incubated with 2-mercaptoethanol which reduces the disulfide bond present within DTSP. Labeling of Mr-76 000 and Mr-36 000 complexes was specific in that it could be abolished by native VIP, while the labeling of the Mr-17 000 was not. Densitometric scanning of autoradiographs indicated that: (a) labeling of the Mr-76 000 complex was abolished by low VIP concentrations (0.03--10 nM), by VIP agonists with the relative potency VIP greater than a peptide having N-terminal histidine and C-terminal isoleucine amide greater than secretin, and by GTP (10(-5)--1 mM) but was unaffected by various other peptide hormones; (b) labeling of the Mr-36 000 complex was inhibited by high VIP concentrations (1--300 nM), by VIP agonists at high concentrations but was not affected by GTP and various peptide hormones. Assuming one molecule of 125I-VIP was bound per molecule of protein, two proteins with Mr-73 000 and 33 000 were identified as VIP binding sites. The Mr-73 000 protein displays many characteristics (affinity, specificity, discriminating power toward agonists, sensitivity to GTP regulation) of the high-affinity VIP receptors mediating adenylate cyclase activation. The Mr-33 000 protein displays the characteristics (affinity, specificity) of a low-affinity VIP binding site. This study thus shows the molecular characteristics of the VIP receptor and further argues for the molecular heterogeneity of VIP binding sites.

The effects of 5,7-dihydroxytryptamine and p-chlorophenylalanine on thyrotrophin-releasing hormone in regions of the brain and spinal cord of the rat

The distribution of thyrotrophin-releasing hormone (TRH) and 5-hydroxytryptamine (5-HT) were compared in ten regions of the rat brain and in lumbar spinal cord. After dissection, using a cutting box and tissue punches, TRH was measured by radioimmunoassay and 5-HT by HPLC with electrochemical detection. Within the brain the highest levels of TRH were found in the median eminence and the remaining hypothalamus. There were also relatively high levels in the suprachiasmatic nucleus, septal nuclei and nucleus accumbens. Highest levels of 5-HT were found in the raphe nuclei, hypothalamic nuclei, nucleus accumbens, stria terminalis, septal nuclei and hippocampus. 5,7-Dihydroxytryptamine (5,7-DHT; 200 micrograms, i.c.v.) markedly reduced levels of 5-HT in brain and spinal cord. In the ventral lumbar cord there was a comparable decrease of TRH and 5-HT (-80%) and a smaller but significant decrease in the nucleus accumbens (-55%) and septal nuclei (-38%). p-Chlorophenylalanine (PCPA; 250 mg/kg X 2) reduced levels of 5-HT (-80%), without significantly altering those of noradrenaline. p-Chlorophenylalanine also significantly reduced levels of TRH in the nucleus accumbens (-72%) but not in the other regions of brain or spinal cord taken. The results are discussed in relation to the previously described co-existence of TRH and 5-HT in the spinal cord and the possible alternative forms of interactions between amine and peptide in the nucleus accumbens.

Enzymic inactivation of hypothalamic regulatory hormones

The considerable expansion in studies on the enzymic inactivation of thyrotrophin-releasing hormone, luteinizing hormone-releasing hormone and somatostatin (growth hormone release-inhibiting hormone) has necessitated a re-evaluation of the peptidase enzymes responsible. Through the use of new methods such as high-performance liquid chromatography and the development of artificial enzyme substrates, it has been possible to clarify the mechanisms of enzyme cleavage of these hypothalamic regulatory hormones and to attempt purification of the peptidases. This has brought about a renewed interest in the physiological significance of the enzymes, as well as their role in biotransformation of the hypothalamic hormones. From such studies, the information gained may be used in the design of agonist and antagonist analogues, as well as providing details of the mechanisms of action of such analogues through their increased stability to enzymic degradation. The characterization of corticotrophin-releasing factor and growth hormone-releasing factor will provide a new field for the application of peptidase inactivation to analogue design. Similarly, future examination of the peptidases inactivating the hypothalamic hormones in certain clinical conditions may give new insight into the significance of the enzymes in pathological conditions. Identification of these enzymes, investigation of their localization, properties and functions and assessment of their contribution to the control of hormone action may yield valuable insight into the physiology and pathology of the hypothalamic regulatory hormones.

Gonadotropin releasing hormone (GnRH) is not degraded by intact pituitary tissue in vitro

The possible degradation of GnRH by intact pituitary tissue was investigated. Trypsin-, collagenase- and mechanically dispersed pituitary cells in culture and fresh pituitaries cut into eight segments were incubated with specifically tritiated GnRH or unlabeled GnRH which were detected by HPLC and RIA in incubation media. Degradation of GnRH could not be detected by either method during incubations with any of the pituitary cell cultures or fresh tissue. The results suggest that pituitary degradation is not involved in the regulation of circulating levels of GnRH.

Synthesis and biological activity of [D-Trp6] chicken luteinizing hormone-releasing hormone

An agonist of chicken hypothalamic luteinizing hormone-releasing hormone (cLH-RH), [D-Trp6] cLH-RH, was synthesized and tested for luteinizing hormone (LH)-releasing activity using dispersed chicken anterior pituitary cells, as well as for binding to rat anterior pituitary membrane receptors. cLH-RH and mammalian LH-RH (mLH-RH) gave identical dose-response curves in stimulating chicken LH release (ED50 = 1.6 and 1.8 X 10(-9) M respectively) and similar estimates of potency. The [D-Trp6] analogs of cLH-RH and mLH-RH stimulated LH release at lower doses (ED50 = 7.0 and approximately 7.0 X 10(-11) M respectively) and were approximately 20-fold more potent. In contrast to the activity in the chicken bioassay, cLH-RH bound to rat anterior pituitary membrane receptors with a much lower affinity than did mLH-RH and had a relative potency of 2%. [D-Trp6] cLH-RH was approximately 100-fold more potent than cLH-RH in the rat receptor assay while [D-Trp6] mLH-RH was 28-fold more active than mLH-RH. These data demonstrate that substitution of Gly6 of LH-RH with D-Trp enhances the LH release from chicken pituitary cells to a similar extent to that observed in mammals, and indicate that the approaches used to produce active LH-RH analogs in mammals are likely to be applicable to birds.

Thyrotrophin-releasing hormone analogues increase dopamine release from slices of rat brain

Rat brain slices were incubated with a high concentration of K+, thyrotrophin-releasing hormone (TRH), or one of two biologically stable TRH analogues (CG 3509 or RX 77368). Basal release of endogenous dopamine, measured by electrochemical detection, was increased by K+ (30 mM) from slices of hypothalamus, septum, nucleus accumbens, and striatum. CG 3509 (10-(5) -10-(3) M) increased the release of dopamine from slices of nucleus accumbens, septum, and hypothalamus in a dose-dependent fashion, whereas RX 77368 (10-(4) M) increased the release of dopamine from the septum only. Neither analogue increased the release of striatal dopamine. The results provide further evidence for specific regional interactions between TRH and dopamine in rat brain.

Mechanisms of brain inactivation of centrally-acting thyrotrophin-releasing hormone (TRH) analogues: a high-performance liquid chromatography study

High-performance liquid chromatography (HPLC) was used to investigate the degradation in vitro of several centrally-acting analogues of thyrotrophin-releasing hormone (TRH) by two subcellular fractions prepared from different areas of rat brain. Of the seven analogues studied, RX77368 (pGlu-His-(3,3'-dimethyl)-ProNH2) was the most stable analogue, showing only a small amount of degradation by the particulate fraction containing a pyroglutamyl aminopeptidase, whereas the other analogues (RX74355, CG3509, CG3703, [3MeHis]TRH, PGHPA and MK771) showed varying degrees of resistance to degradation by this enzyme and the proline endopeptidase in the soluble fraction. However, TRH was rapidly inactivated to its deamidated form, TRH-OH and the histidyl-proline diketopiperazine by both fractions. The relative stability of these TRH analogues to enzyme action may provide some explanation for their enhanced biological activity in vivo.

Induction of wet-dog shaking in rats by analogues and metabolites of thyrotrophin-releasing hormone (TRH)

The ability of thyrotrophin-releasing hormone (TRH), its metabolites and several analogues to induce wet-dog shaking (WDS) was tested by their injection into the periaqueductal grey region of male rats. TRH and its metabolite deamido-TRH (TRH-OH) both stimulated WDS, though TRH-OH gave a longer duration of response; other TRH metabolites were inactive. Of the TRH analogues studied, RX77368 (pGlu-His-3,3'-dimethyl-ProNH2) was the most potent in this behavioural test system. Both CG3509 and CG3703 were also very active in inducing WDS, as were their deamidated metabolites. The relative stability of the TRH analogues to enzymic degradation in the brain may be related to their enhanced behavioural activity over TRH. The production from these analogues of biologically-active metabolites may also explain the increased activity in stimulating WDS of the parent peptides.

Compartmentalization of luteinizing hormone pools: dynamics of gonadotropin releasing hormone action in superfused pituitary cells

Cultured rat anterior pituitary cells were continuously perfused with medium 199, and exposed to short (5 min), intermediate (30 min), or long (6 h) pulses of a maximally effective concentration of gonadotropin-releasing hormone (GnRH). Assay of the effluent by radioimmunoassay and interstitial-cell bioassay revealed a biphasic response to GnRH, and indicated that 3 pools of luteinizing hormone (LH) are present in the gonadotroph. A rapidly releasable peak of bioactive LH comprising about 2% of the total cellular LH was mobilized within 1 min of GnRH addition, lasted for 3-4 min, and was independent of the duration of stimulation. The second, larger pool of bioactive LH varied from 15 to 50% of the total LH as the duration of exposure to GnRH was increased from 5 min to 6 h. A third LH pool comprising up to 50% of the total LH could be mobilized by 50 mM potassium but not by continuous GnRH treatment, due to refractoriness of the cells to prolonged stimulation by the decapeptide. In contrast, repeated pulses of GnRH evoked a series of biphasic LH peaks with profiles similar to that observed during a single response to GnRH, indicating that continuous exposure to GnRH is necessary for densensitization. Release of LH from the perfused cells was calcium-dependent, and the bio-immuno ratio of the first and second pools of LH was similar. The in vitro secretion profile of cultured rat cells is comparable with the early and late phases of LH release observed in GnRH-infused man, but occurs much more rapidly, and demonstrates heterogeneity of the LH release process at the level of the gonadotroph. The superfusion technique provides a powerful tool to further investigate the bioactivity of GnRH and its analogs for use in fertility control.

Studies of substrate requirements, kinetic properties, and competive inhibitors of the enzymes catabolizing TRH in rat brain

The enzymes catalyzing the breakdown of TRH are soluble and dependent upon active sulfhydryl groups. One of these enzymes is a pyroglutamyl aminopeptidase (EC 3.4.11.8) (apparent molecular weight 60,000 daltons) with a specificity restricted to pyroglutamyl peptide bonds. The second enzyme is a prolyl endopeptidase (apparent molecular weight 70,000 daltons) with a specificity restricted to proline peptide bonds wherein the proline is preceded by an alpha adjacent amino acid possessing a blocked N-terminus. Substrate requirements for this latter enzyme are identical to those reported previously for the TRH deamidase of rat brain, the prolyl endopeptidase of rabbit brain and the post proline cleaving enzyme of rat brain. We conclude that this enzymatic activity variously described as TRH deamidase, post proline cleaving enzyme, and prolyl endopeptidase is that of a single enzyme best denoted as a prolyl endopeptidase (EC 3.4.22.16, proposed). The pyroglutamyl aminopeptidase has a Km for TRH of 45 micro M as compared to a Km for TRH of 2400 micro M for the prolyl endopeptidase. Total enzymatic activity of the prolyl endopeptidase, however, is approximately 3500 nmol/h/rat brain with the total enzymatic activity of the pyroglutamyl aminopeptidase being approximately 600 nmol/h/rat brain. The 5- to 6-fold higher affinity of the pyroglutamyl aminopeptidase for TRH suggests that of these two catabolic pathways, removal of the N-terminal pyroglutamyl moiety is likely to be the more important pathway for the initial catabolism of TRH in rat brain. Analysis of substrate specificity permitted the synthesis of several effective competitive inhibitors of the two enzymes. Of these, the most effective inhibitors of the pyroglutamyl aminopeptidase were p-glu-NH2 (Ki 185 micro M) and p-glu-his-OCH3 (Ki 16.5 micro M). The most effective inhibitors of the prolyl endopeptidase were Ac-gly-pro-NH2 (Ki 1143 micro M) and Z-gly-pro-NH2 (Ki 442 micro M).

Mechanism of luteinizing hormone-releasing hormone degradation by subcellular fractions of rat hypothalamus and pituitary

The pathway of LH-RH degradation by two subcellular fractions (a soluble fraction and a 25 000 X g particulate fraction) of rat hypothalamus, pituitary and cerebral cortex has been studied using high performance liquid chromatography and amino acid analysis to identify the breakdown products. The primary cleavage point in the Tyr5-Gly6 bond giving [1-5] LH-RH and [6-10] LH-RH. In the presence of dithiothreitol, cleavage of LH-RH also occurred at the Pro9-Gly10 bond giving [1-9] LH-RH. The fragment [1-5] LH-RH is further degraded sequentially from the C-terminus and [1-4] LH-RH, [1-3] LH-RH, tyrosine and tryptophan were identified. The other major fragment, [6-10] LH-RH, is rapidly broken down, the only intermediate product positively identified being Arg-Pro.

Inactivation of thyrotropin-releasing hormone (TRH) and (3Me-His) TRH by brain peptidases studied by high-performance liquid chromatography

High-performance liquid chromatography (HPLC) has been used to separate and identify the metabolites formed from thyrotrophin-releasing-hormone (TRH) and its hyperactive analogue, (3Me-His)TRH, by subcellular fractions from rat cortex, hypothalamus and thalamus. Deamidation by the proline endopeptidase and formation of histidylproline diketopiperazines by the pyroglutamyl aminopeptidase were found to be the major mechanisms of brain inactivation of both peptides; (3Me-His)TRH was slightly more stable than TRH in the presence of the brain peptidases, and with enhanced receptor binding affinity, this could explain its increased biological activity. The HPLC system used may be applicable to determining the mechanisms of brain inactivation of other TRH analogues and could also be used to define the pathways for inactivation of larger neuropeptides as well.

Degradation of luteinizing hormone-releasing hormone by serum and plasma in vitro

Luteinizing hormone-releasing hormone (LH-RH) is degraded in vitro by serum and plasma from several species (human, rat, guinea-pig and cattle). Separation of the degradation products by high-performance liquid chromatography (HPLC) followed by amino acid analysis and radioimmunoassay showed that the main sites of cleavage are the Trp3-Ser4 and Tyr5-Gly6 bonds. Two peptidases are responsible since the cleavage at Trp3-Ser4 can be selectively inhibited by EDTA. In human plasma, the peptidase responsible for Trp3-Ser4 hydrolysis has a Km of 2.9 . 10(-4) M and V of 30 nmol/h per ml plasma. The half-life in vitro of LH-RH in serum and plasma from various species ranges from 3 h (guinea-pig) to 9.8 h (human). The peptidase cleaving LH-RH at Tyr5-Gly6 is present as an impurity in some commercial bovine serum and plasma albumins. Such contamination may have important practical implications for work involving peptide assays where albumins are used as carrier proteins.

Ontogenesis of neuropeptide degrading enzymes in the mouse brain

The ontogenesis of neuropeptide degrading enzymes was studied in the mouse brain, from the 10th gestational day up to adulthood. Two activities were followed: the pyroglutamate aminopeptidase and the post-proline cleaving enzyme, using either TRH or specific fluorgenic peptides as substrates. In the hypothalamus as well as in cerebral hemispheres, the specific activities of both enzymes was highest on the 13th fetal day and decline thereafter until the 20-22nd post-natal day, with a plateau around birth. In contrast, a classical peptidase, the leucyl-arylamidase increased only in fetal life, and reached the adult level before birth.

Characterization of a nonchymotrypsin-like endopeptidase from anterior pituitary that hydrolyzes luteining hormone-releasing hormone at the tyrosyl-glycine and histidyl-tryptophan bonds

A neutral endopeptidase which degrades luteinizing hormone-releasing hormone (LH-RH, <GLu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-GLy-NH2) has been purified 900-fold from extracts of bovine anterior pituitary. This Ca2+-independent enzyme of 83 000 molecular weight (as estimated by gel filtration) cleaves LH-RH (KM = 180 microM) at the Tyr5-Gly6-His2-Trp3 bonds. Its activity is inhibited by the SH-reactive agents N-ethylmaleimide and p-(chloromercuri)benzoate but not by the OH-reactive agent diisopropyl fluorophosphate. Hydrolysis of the fluorogenic chymotrypsin substrate glutarkyl-Gly-Gly-Phe-beta-naphthylamide by this endopeptidase could not be detected. These properties differentiate the endopeptidase from chymotrypsin and from a glutaryl-Gly-Gly-Phe-beta-naphthylamide hydrolyzing activity of high molecular weight, which has been isolated from the same tissue and also hydrolyzes internal bonds of LH-RH.

Enzymic formation of TRH-OH from TRH by rat hypothalamus

Specific radioimmunoassays for thyrotrophin-releasing hormone (TRH) and deamido-TRH (TRH-OH) were used to investigate the inactivation of TRH by subcellular fractions from rat hypothalamus. TRH-OH was rapidly formed from TRH by an amidase present only in the supernatant fraction at an optimum pH of 7.38; although TRH was degraded by the particulate fraction, no TRH-OH formation was detected. Of several brain areas investigated, the cortex had the highest rate of TRH-OH production and the pituitary the lowest. Once formed in the supernatant fractions, it was found that TRH-OH was stable and did not undergo further degradation. Formation of TRH-OH from TRH appears to be an important step in the tripeptide's enzymic degradation in the brain, so the combined use of specific radioimmunoassays for TRH and TRH-OH may provide a suitable means for studies of the enzymes involved, since these enzymes may contribute to the mechanisms regulating the secretion and duration of action of TRH.