Met-enkephalin Analogue Research Articles

Effects of hexarelin and isolation stress on the Met-enkephalin system in young lambs

Koziec K., S canes C.G., Z ubel-Łojek J., Gajewska A. 2023. Effects of hexarelin and isolation stress on the Met-enkephalin system in young lambs. Folia Biologica (Kraków) 71: 195-206.Stress stands out as the primary instigator of numerous diseases – ranging from cardiovascular and gastro- intestinal to diabetes and nervous disturbances – in most developed nations. Endogenous opioid peptides (EOP), particularly Met-enkephalin, play a crucial role in mitigating the up-regulation of the hypothalamo- pituitary-adrenal axis during stress responses, consequently reducing the risk of serious diseases. Hexarelin, a synthetic analog of Met-enkephalin, has been predominantly investigated for its impact on growth hormone (GH) release in both human subjects and rodent models. This study was undertaken to evaluate the influence of isolation stress and/or hexarelin administration on various Met-enkephalin-related parameters in a novel animal model – 3-month-old lambs. Four distinct groups were established: a control group, a group intravenously injected with hexarelin, a group subjected to 60 min of isolation stress from the herd, and a group treated with both hexarelin and stress. Blood and hypothalamus samples were collected to analyze cortisol and Met-enkephalin profiles, proenkephalin (PENK) gene expression, Met-enkephalin concentration, in vitro Met-enkephalin secretion, and opioid receptor binding. The findings revealed a significant impact of stress on all assessed parameters. Hexarelin alone led to a decrease in cortisol levels and Met-enkephalin synthesis, release, and receptor binding in the hypothalamus. When administered prior to stress, hexarelin potentiated the responses of opioid parameters to isolation. These results, for the first time, demonstrate that hexarelin interacts with Met-enkephalin, modulating the stress response at both central and peripheral levels in growing lambs. It is suggested that hexarelin plays a crucial role during stress responses; however, further research on its effects should be conducted concurrently with the examination of opioid profiles.

A high‐throughput LC‐MS/MS screen for GHRP in equine and human urine, featuring peptide derivatization for improved chromatography

The growth hormone releasing peptides (GHRPs) hexarelin, ipamorelin, alexamorelin, GHRP-1, GHRP-2, GHRP-4, GHRP-5, and GHRP-6 are all synthetic met-enkephalin analogues that include unnatural D-amino acids. They were designed specifically for their ability to stimulate growth hormone release and may serve as performance enhancing drugs. To regulate the use of these peptides within the horse racing industry and by human athletes, a method is presented for the extraction, derivatization, and detection of GHRPs from equine and human urine. This method takes advantage of a highly specific solid-phase extraction combined with a novel derivatization method to improve the chromatography of basic peptides. The method was validated with respect to linearity, repeatability, intermediate precision, specificity, limits of detection, limits of confirmation, ion suppression, and stability. As proof of principle, all eight GHRPs or their metabolites could be detected in urine collected from rats after intravenous administration.

Opioid mediated activity and expression of mu and delta opioid receptors in isolated human term non-labouring myometrium

The existence of opioid receptors in mammalian myometrial tissue is now widely accepted. Previously enkephalin degrading enzymes have been shown to be elevated in pregnant rat uterus and a met-enkephalin analogue has been shown to alter spontaneous contractility of rat myometrium. Here we have undertaken studies to determine the effects of met-enkephalin on in vitro human myometrial contractility and investigate the expression of opioid receptors in pregnant myometrium. Myometrial biopsies were taken from women undergoing elective caesarean delivery at term. Organ bath experiments were used to investigate the effect of the met-enkephalin analogue [d-Ala 2, d-met 5] enkephalin (DAMEA) on spontaneous contractility. A confocal immunofluorescent technique and real time PCR were used to determine the expression of protein and mRNA, respectively for two opioid receptor subtypes, mu and delta. DAMEA had a concentration dependent inhibitory effect on contractile activity (1×10−7M–1×10−4M; 54% reduction in contractile activity, P<0.001 at 1×10−4M concentration). Mu and delta opioid receptor protein sub-types and their respective mRNA were identified in all tissues sampled. This is the first report of opioid receptor expression and of an opioid mediated uterorelaxant action in term human non-labouring myometrium in vitro.

Novel Side‐Chain Glucosylated and Adamantylated [Asp2/Glu2]Enkephalin Analogs: Synthesis and In Vitro Growth Inhibition of Human Tumor Cells

A series of new backbone-modified Leu- and Met-enkephalin analogs (13-20 a and b) were synthesized. Backbone manipulations involved the replacement of the Gly(2) residue in Tyr-Gly-Gly-Phe-Leu/Met with side-chain glucosylated or adamantylated D/L-aspartic or -glutamic acids. The in vitro antiproliferative activity of these compounds was evaluated for several cell lines and the results were compared with the effect of Met-enkephalin, the native opioid growth factor. The tested compounds modestly inhibited the growth of the tumor cells (20-50% inhibition at millimolar concentrations). Among the tested compounds, Tyr-D-Glu(AdNH)-Gly-Phe-Met (20b) showed significant antiproliferative activity, somewhat more pronounced on MCF-7 (breast carcinoma) and MOLT-4 (lymphoblastic leukemia) cells.

Improved bioavailability to the brain of glycosylated Met-enkephalin analogs

The blood–brain barrier prevents the entry of many potentially therapeutic peptide drugs to the brain. Glycosylation has shown potential as a methodology for improving delivery to the CNS. Previous studies have shown improved bioavailability and improved centrally mediated analgesia of glycosylated opioids. In this study we investigate the effect of glycosylation on the cyclic opioid peptide [ d-Cys 2,5,Ser 6,Gly 7] enkephalin. The peptide was glycosylated on the Ser 6 via an O-linkage with various sugar moieties and alignments. The peptides were then investigated for receptor binding, physiochemical attributes, in situ brain uptake in female Sprague–Dawley rats and antinociception in male ICR mice. Glycosylation resulted in a slight decrease in affinity to the δ-opioid receptor, and mixed effect on binding to the μ-opioid receptor. There was a significant decrease in lipophilicity resulting from glycosylation and a slight reduction in binding to bovine serum albumin. In situ perfusion showed that brain uptake was improved by up to 98% for several of the glycosylated peptides, and the nociceptive profiles of the peptides, in general, followed the rank order of peptide entry to the brain with up to a 39-fold increase in A.U.C.

Transport of the δ-opioid receptor agonist [D-penicillamine2,5] enkephalin across the blood–brain barrier involves transcytosis1

The delta opioid receptor antagonist [D-penicillamine2,5]-enkephalin (DPDPE) is an enzymatically stable peptide analogue of Met-enkephalin. DPDPE uses a saturable transport mechanism to cross the blood-brain barrier (BBB), though the exact mechanism is not fully understood. The aim of the present study was to identify the mechanism by which DPDPE enters the brain. The effect of phenylarsine oxide (PAO), an endocytosis inhibitor, on the transport of [3H]DPDPE was investigated using both in vitro and in situ transport studies. Two in vitro models of the BBB utilizing primary bovine brain microvascular endothelial cells (BBMEC) were studied. [3H]DPDPE permeability across monolayers of BBMEC grown on polycarbonate filters was studied. PAO significantly reduced the permeability of [3H]-DPDPE across the monolayer. PAO also reduced the uptake of [3H]-DPDPE into BBMEC cells, without affecting binding to the cells. The in situ perfusion model of the BBB was also studied, PAO reduced DPDPE uptake by the brain in a dose-dependent manner. These studies indicate that DPDPE enters the brain via an energy-dependent transcytotic mechanism.

Structure-Permeation Relations of Met-enkephalin Peptide Analogues on Absorption and Secretion Mechanisms in Caco-2 Monolayers

Due to the low effective permeabilities of peptides at many absorption sites, their structure-permeation relations are of high interest. In this work structure-permeation relations of Met-enkephalin analogues are presented using confluent Caco-2 cells as an in vitro permeation model. Four model peptides (Met-enkephalin, [D-Ala2]Met-enkephalin, [D-Ala2]Met-enkephalinamide, and metkephamid) were tested in terms of permeability, lipophilicity, charge, and molecular size. Permeability coefficients (Peff) across Caco-2 cells were low, 3.3×10-8 to 9.5×10-8 cm s-1, and were similar to typical paracellular markers. No correlation of permeability and the log(apparent octanol/buffer partition coefficient) was observed. A 40-fold increase of the permeability of metkephamid in the presence of 10mM EDTA suggested a significant contribution of paracellular transport. Independent support for this conclusion was obtained by visualizing the pathway of the fluorescein isocyanate isomer I 1–metkephamid by confocal laser scanning microscopy (CLSM). The fluorophore-labeled peptide was observed in the intercellular space only. Metkephamid permeabilities were found to be direction-specific. Permeabilities from basolateral to apical (b-to-a) were significantly higher (ca. 4-fold) than in the opposite (a-to-b) direction. The addition of verapamil equalized the permeabilities in the a-to-b and b-to-a directions, suggesting the involvement of a P-glycoprotein-mediated secretion mechanism. Similar observations were obtained with [D-Ala2]- Met-enkephalinamide, but not with Met-enkephalin and [D-Ala2]Metenkephalin. In contrast to the other analogues, metkephamid and [D-Ala2]Met-enkephalinamide are positively charged at neutral pH, as demonstrated by their isoelectric points (pl=8.6 for [D-Ala2]Metenkephalinamide and metkephamid and 5.3 for [D-Ala2]Met-enkephalin and Met-enkephalin). The data is in agreement with the literature showing that most compounds secreted by the P-glycoprotein transporter carry a positive charge.

Conformation and Orientation of Met-enkephalin Analogues in a Lyotropic Liquid Crystal Studied by the Magic-Angle- and Near-Magic-Angle-Spinning Two-Dimensional Methodology in Nuclear Magnetic Resonance: Relationships between Activities and Membrane-Associated Structures

The preferred orientation and conformation of biologically active [d-Ala2]- and inactive [l-Ala2]Met-enkephalins have been studied by liquid-crystal NMR spectroscopy employing magic-angle- and near-magic-angle-spinning (MAS/NMAS) two-dimensional methodology. The solvent employed was a CsPFO (cesium perfluorooctanoate) liquid crystal so that less proton signals interfere with the spectrum under the MAS/NMAS conditions. The conformation and orientation of [d-Ala2]- and [l-Ala2]Met-enkephalins were determined in this anisotropic environment from the analysis of the 1H−1H direct couplings obtained under the NMAS condition and of the 1H−1H ROE factors obtained under the MAS condition. These data were analyzed by the minimization based on pseudoenergy analysis. The results showed that the preferred orientations and the motional restrictions of the tyrosine residues of Met-enkephalin analogues are perpendicular to each other, although the preferred conformations are extended ones for both compounds. Also it is r...

Effect of exogenous growth hormone administration on endogenous growth hormone secretion induced by a met-enkephalin analog

Exogenous growth hormone (hGH) administration in humans attenuates the endogenous growth hormone (GH) response to some pharmacological stimuli; in particular, pretreatment with hGH completely blocks the serum GH response to growth hormone-releasing hormone. In order to evaluate the mechanism(s) whereby opiods induce GH secretion in man, we gave the following treatments to six healthy male volunteers: (a) IV saline; (b) a met-enkephalin analog G-DAMME 250 micrograms IV as a bolus at time 0'; (c) hGH 2 IU as an IV bolus at time -180'; (d) G-DAMME as above, preceded by hGH as above. In our study, G-DAMME stimulated GH secretion both basally (peak 17.9 +/- 6.0 ng/ml) and, to a lesser extent, after hGH pretreatment (6.0 +/- 2.7 ng/ml). Since in our study G-DAMME was able to partially overcome the inhibitory effect of hGH administration, it is suggested that opioids act through an inhibition of somatostatin release and not through a GHRH-dependent pathway. However, an additional direct effect of hGH on pituitary somatotrophes cannot be excluded.

The opioid peptide DAMME modulates histamine's content in gastric mucosa of the rat.

The effects of a synthetic Met-enkephalin analogue D-Ala2, MePhe4Met(O)5-ol]enkephalin (DAMME) (1 mg/kg. IP) on gastric damage produced by necrotizing agents (0.6 N HCl, ethanol 1 ml/rat, PO) were evaluated, and the correlation between histamine and opioids in stomach was investigated, Rats pretreated with DAMME bad significantly less severe lesions and lower histamine content in gastric tissue. The histamine level, expressed in mg/g of gastric tissue, changed from 0.41 +/- 0.10 of control animals to 1.33 +/- 0.12 for HCl and 1.51 +/- 0.20 for ethanol treatment, whereas in animals pretreated with DAMME the values were significantly reduced to 0.55 +/- 0.13 and 0.65 +/- 0.15. These results confirm a link between the gastroprotection produced by opioids and the modulation of histaminergic activity in the rat stomach.

The presence and effects of mammalian signal molecules in immunocytes of the insect Leucophaea maderae.

Opioid peptides activate immunocytes and opiate alkaloids inhibit this activation in the mussel, Mytilus edulis. Here we present evidence that cells of another invertebrate, Leucophaea maderae, can be influenced in a similar way by the Met-enkephalin analogue D-Ala2-Met5-enkephalin (DAMA) and morphine. Effects of different signal molecules on Leucophaea hemocytes were evaluated by computer-assisted image analysis of their conformational state. A small percentage of the untreated cells were found to display spontaneous conformational changes after 25 min of incubation without pharmacological agents which was noted as a decrease in both circularity factor and shape factor values. Activation caused the cells to become elliptical, a feature that appears to be characteristic of Leucophaea immunocytes. Administration of DAMA induced a similar activation of most of the cells. After 30 min these DAMA-activated cells started to display distinct locomotory activity not seen in the controls. alpha-Melanocyte-stimulating hormone (MSH, 10(-7)) added to the incubation medium after DAMA-activation caused the cells to return to their original "rounded" conformation. In addition, the presence of immunoreactive interleukin (IL-1), adrenocorticotropin (ACTH) and tumor necrosis factor (TNF) in the hemolymph was demonstrated. These data suggest an interaction between both vertebrate-type immunological signal molecules and neuropeptides in the regulation of immunological cells in Leucophaea.

Effects of sodium depletion on the response of rat adrenal zona glomerulosa cells to stimulation by neuropeptides: actions of vasoactive intestinal peptide, enkephalin, substance P, neuropeptide Y and corticotrophin-releasing hormone.

There are several neuropeptides, present in nerves supplying the rat adrenal zona glomerulosa, which have been shown to stimulate aldosterone secretion in the intact perfused rat adrenal preparation. The purpose of the present study was twofold: first, to determine whether these peptides acted directly on adrenocortical cells by examining their effects on collagenase-dispersed rat zona glomerulosa cells, and second, to investigate the likely physiological significance of these actions, by determining whether the responses of zona glomerulosa cells to neuropeptides were changed by prior sodium depletion. Of the peptides tested, neuropeptide Y (NPY) and substance P had only a minor effect on aldosterone secretion, which was not substantially affected by sodium depletion. Corticotrophin-releasing hormone (CRH) had a significant stimulatory effect on aldosterone secretion, but neither the threshold concentration for significant stimulation nor the maximal response to stimulation were altered by prior sodium depletion. Vasoactive intestinal peptide (VIP), on the other hand, had little effect on aldosterone secretion by cells from normal animals, but was a potent stimulus to aldosterone secretion in cells obtained from sodium-depleted animals. The response to the Met-enkephalin analogue, [D-Ala2-Met2]-enkephalinamide (DALA), was also significantly enhanced by prior sodium depletion. Experiments using the angiotensin II receptor blocker, saralasin, were carried out to determine whether the enhanced actions of DALA and VIP seen in sodium depletion may be a result of activation of angiotensin II receptors, known to be increased in sodium depletion. Saralasin did not affect the response to either peptide. These data suggest that all the peptides tested may be able to stimulate aldosterone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Novel met-enkephalin analogue: a potent systemic mu agonist antinociceptive agent

A new met-enkephalin analogue (compound 82/205) was evaluated for its opioidergic activity in mice. The compound showed antinociception (warm water tail-flick test), tolerance, cross tolerance to morphine and physical dependence. The time course of antinociptive effect of the compound was comparable to morphine. The antinociceptive ED 50 (μmol kg −1 i.p.) values for the compound and morphine base were 5.31 and 7.59, respectively. Its antinociceptive effect was blocked by naloxone, β-FNA (mu antagonist) and naloxonazine (mu 1 antagonist) but not by ICI 174,864 (delta antagonist). Naloxone precipitated withdrawal jumpings were 2.6 times less in compound 82/205 treated mice than the morphine treated group. The new analogue compound 82/205 is a potent mu agonist antinociceptive with a possible weak dependence liability.

The in vivo effects of opioid peptides on the murine immune response

We have previously reported that met-enkephalin has dual immunomodulatory properties in vitro. We have continued this investigation using an in vivo system. In this study, Alzet miniosmotic pumps were loaded with either met-enkephalin, DTLET or FK 33-824 and were surgically implanted into BAF 1/J mice. Twenty-four hours after pump implantation, mice were challenged with sub-optimal, optimal or supraoptimal immunnizing doses of antigen. The immune response was assessed 4 or 5 days after primary immunization. FK 33-824, a met-enkephalin analogue, had no effect on the response of mice challenged with a suboptimal antigen dose. However, FK 33-824, at a pump concentration of 10 −3 M, suppressed the response against optimal challenge doses of antigen. At a pump concentration of 10 −8 M, FK 33-824 suppressed, enhanced or had no effect on the supraoptimal antigen dose-induced immune response. The supressive effect of FK 33-824 in mice immunized with either optimal or supraoptimal doses of antigen was blocked by naloxone. Met-enkephalin and its delta opioid receptor specific analogue, DTLET, had no effect on the immune response to optimal antigen immunization. These results indicate that FK 33-824 has in vivo immunomodulatory activity and provide and provide evidence that opioid peptides may either upregulate or downregulate the in vivo immune response depending on the strength of the response.

Effects of the enkephalin analog ( d-Met 2 Pro 5)-enkephalinamide on α-melanocyte-stimulating hormone secretion

We used the met-enkephalin analog ( d-Met 2, Pro 5)-enkephalinamide (DMPEA) to investigate enkephalinergic control of α-melanocyte-stimulating hormone (α-MSH) secretion. Systemic (s.c.) administration of DMPEA elevated plasma titers of α-MSH in a dose- and time-related manner. Pretreatment with the opiate antagonist naltrexone had no effect on basal plasma levels of α-MSH but blocked DMPEA-induced α-MSH release. Treatment with a dose of naltrexone sufficient to block DMPEA-induced secretion of α-MSH had no effect on stress-induced secretion of α-MSH. Although pretreatment with the dopamine receptor agonist apomorphine prevented DMPEA-induced α-MSH secretion, DMPEA had no effect on the synthetic activity of tuberohypophysial dopamine neurons as gauged by measuring the accumulation of 3,4-dihydroxyphenylalanine in the neurointermediate lobe (NIL) following administration of NSD-1015. In vitro treatment of isolated NILs with DMPEA resulted in a significant increase in α-MSH release. Naltrexone completely blocked the stimulatory effects of DMPEA on α-MSH release in vitro. Our results indicate that DMPEA stimulates α-MSH secretion by acting directly through opiate receptors at the level of the NIL.

Concentrations of prolactin and LH after administration of Met-enkephalin analogue (FK 33-824) to gilts during the follicular phase

The influence of methionine-enkephalin analogue (FK 33-824) administration upon prolactin and luteinizing hormone (LH) secretion in follicular phase gilts was studied. In experiment I, gilts, selected at random, received intravenous bolus injections of FK 33-824 or saline at 4 μg kg −1 body weight (BW) on Days 16 or 19 of the estrous cycle. Peripheral blood samples were collected every 15 min from 3 h before to 6 h after administration of FK 33-824 or saline. In experiment II, gilts were infused for 4 h with saline ( N = 4) or FK 33-824 at a rate of 6.2 μg kg −1 BW h −1 ( N = 4) on Day 19 of the estrous cycle. At the onset of continuous infusion, treated gilts received a bolus intravenous injection of FK 33-824 at a dose of 4 μg kg −1 BW. On Day 19 of the cycle, four additional gilts were also infused for 4 h with FK 33-824 as in experiment II. Two of the gilts received four injections of naloxone (2 mg kg −1 BW) at 1-h intervals beginning 15 min before the infusion of FK 33-824 and the remaining two gilts were injected with 100 μg of gonadotropin-releasing hormone (GnRH) 2 h after the beginning of FK 33-824 infusion. Blood samples were collected every 15 min for 4 h before, during and 4 h after infusion. Bolus injections of FK 33-824 resulted in increased ( P < 0.05) prolactin concentrations on Day 19 of the estrous cycle but not on Day 16. Bolus injections of the opioid agonist did not influence LH profiles. Continuous infusion of FK 33-824 on Day 19 of the cycle elevated ( P < 0.05) plasma prolactin concentrations and decreased ( P < 0.05) LH concentrations and number of LH pulses. These effects of FK 33-824 were reversible by naloxone. Release of LH in response to GnRH injection during continuous infusion of FK 33-824 was observed. The results suggest an involvement of the opioid system in regulation of prolactin secretion and revealed an alteration of LH secretion when gilts were subjected to prolonged opioid influences during the late follicular phase.

Role of opioid peptides and subfornical organ in the renal function of intact and hypophysectomized rats

An inhibitory effect on water, sodium and potassium excretion occurs after both systemic and central injections of morphine, beta-endorphin and other opioid peptides. Some investigators claimed that antidiuretic hormone release could be a mechanism explaining opioid-induced oliguria. Injection into the subformical organ of a synthetic Met-enkephalin analog (FK 33824) reduced urine outflow as well as renal Na + and K + excretion. Identical effects were observed in hypophysectomized or in median eminence-lesioned rats. In addition, no changes were seen in blood pressure after FK 33824 injection into the subfornical organ. These results suggest that opioid stimulation of this structure induces an inhibitory effect on renal water, Na + and K + excretion, and that antidiuretic hormone release is probably not important to these phenomena.

Functional Interrelationships Between Adrenergic and Opioid Systems in the Neuroregulation of Growth Hormone Secretion in Infant Rats

Abstract Functional interrelationships between hypothalamic adrenergic and opioid systems were studied in 10-day-old male and female rats. Either clonidine (150 mug/kg, sc), an alpha(2)-adrenoceptor agonist, or FK 33-824 (1 mg/kg, sc), a synthetic analog of met-enkephalin, increased plasma growth hormone (GH) levels, the increment being significantly higher with FK 33-824 than with clonidine. Pharmacologic blockade of opioid receptors with naloxone (5 mg/kg, sc) did not modify either basal GH levels, or the GH response to clonidine, whereas blockade of alpha(2)-adrenoceptors with yohimbine (2.5 mg/kg, sc) reduced basal GH levels and partially counteracted the FK 33-824-induced GH rise. Clonidine (150mu/kg, sc, twice daily) administered from postnatal day 5 to 9, increased basal GH levels and pituitary GH content. In these pups, acute administration of clonidine failed to further release GH, but the GH response to acute administration of FK 33-824 was significantly enhanced. A 5-day treatment with FK 33-824 (1 mg/kg, sc, twice daily), neither modified basal GH levels, nor pituitary GH content. Under these conditions, the in vivo GH response to an FK 33-824 challenge was significantly enhanced, and the response to clonidine was preserved. Pituitaries from FK 33-824-pretreated rats were hyperresponsive to GH-releasing hormone (10(-7) M). In summary, our data indicate that in rat pups: 1) two separate components i.e. one adrenergic, the other extra-adrenergic, subserve the GH-releasing effect of opioid peptides; 2) in contrast to short-term stimulation of alpha(2)-adrenoceptors, stimulation of opioid receptors does not trigger GH synthesis or induce down-regulation or tolerance; 3) short-term opioid stimulation does not affect an alpha(2)-adrenergic challenge, but sensitizes to an opioid challenge.

Lack of modulation of the adrenocortical response to ACTH by an opioid peptide.

Eight normal male subjects received 1 mg dexamethasone at 23.00 h and 0.5 mg on wakening followed by a physiological intravenous dose of synthetic ACTH1-24 250 ng, with and without the administration of a stable met-enkephalin analogue (guanyl-DAMME, 100 micrograms) 10 minutes prior to the ACTH. The opioid analogue caused no change in the peak, incremental, or incremental area under the curve responses of plasma cortisol to the ACTH. This study does not support a role for opioid peptides in the acute modulation of the adreno-cortical response to ACTH.

Growth hormone secretion in stalk-sectioned rats

Idiopathic pituitary GH deficiency appears to result from neonatal disruption of hypophyseal portal vessels in the majority of patients. To examine the mechanism of GH deficiency associated with the disease, the effect of pituitary stalk section on GH secretion was studied in rats. Adult male rats were subjected to stalk section without inserting an impermeable membrane between the cut ends. They were studied 3 to 4 weeks after surgery. In stalk-sectioned rats, pituitary weight, body weight and hypothalamic SRIH content were significantly reduced as compared with sham-operated rats. Hypothalamic GHRH content, plasma T3, T4, corticosterone and testosterone levels, and weights of testes remove and adrenal glands were comparable in the two groups. Plasma GH profiles of sham-operated rats showed characteristic periodic pulses occurring at 2.5-3 h intervals with intervening trough period. In stalk-sectioned rats, plasma GH levels were low small fluctuations, but GH levels were significantly higher than trough levels of sham-operated rats. The amount of GH secreted during a 6-h period as measured by planimetry was significantly reduced. To ascertain the regeneration of hypophyseal portal vessels, post SRIH rebound in GH secretion, which requires the presence of endogenous GHRH, was examined. Withdrawal of exogenous SRIH infusion triggered a large rebound GH secretion whose magnitude did not differ between groups. In stalk-sectioned rats, GH response to met-enkephalin analogue, FK 33-824, was not observed, whereas prolactin response to the secretagogue was observed in the majority of rats.(ABSTRACT TRUNCATED AT 250 WORDS)