The expression of genes encoding opioid precursors and the influence of opioid receptor agonists on steroidogenesis in porcine adrenocortical cells in vitro

Endogenous opioid peptides are involved in the regulation of the HPA-axis function and stress response mechanism. However, there is a scarcity of data on opioid involvement in the regulation of the adrenocortical endocrine function. This study was performed to: 1) establish the expression of proenkephalin, POMC and prodynorphin genes in the porcine adrenal cortex and test in vitro the influence of ACTH, angiotensin II, CRH and epinephrine on this expression, and 2) determine the effects of opioid receptor agonists on basal and ACTH- or angiotensin II-affected secretion of cortisol, aldosterone and progesterone by the cultured adrenocortical cells. Our experiment has demonstrated the presence of mRNAs for opioid precursors in cells isolated from the adrenal cortex and the significant effects of ACTH and angiotensin II, but not CRH or epinephrine, on adrenocortical transcription of the analyzed genes. Angiotensin II reduced the expression of the POMC gene but stimulated that of prodynorphin. In turn, ACTH decreased the transcription of prodynorphin. The study has also demonstrated the effects of selective opioid receptor agonists - DPLPE (delta), FK33-824 (mu) and U50,488 (kappa) - on adrenal steroidogenesis in pigs. Basal secretion of cortisol was enhanced after the activation of mu or kappa receptors, whereas ACTH-stimulated cortisol output was increased only by the mu receptor agonist. Angiotensin II-treated cells significantly decreased aldosterone secretion in the presence of the kappa receptor agonist. The present results suggest that opioid peptides are synthesized in the porcine adrenal cortex, indicating their involvement in the regulation of adrenal steroidogenesis through autocrine and/or paracrine interactions.

The involvement of protein kinases in signalling of opioid agonist FK 33-824 in porcine granulosa cells

It is known that acute action of mu opioid receptor agonist, FK 33-824, results in an inhibition of oestradiol (E2) secretion by porcine granulosa cells from large follicles, but the opioid mode of action is unknown. In the present study, the involvement of two signal transduction pathways, phospholipase C/protein kinase C and adenylyl cyclase/protein kinase A, in mechanism of the opioid action was investigated. Treatment of pig granulosa cells with FK 33-824 at the dose 1 nM suppressed E2 secretion. Protein kinase C (PKC) inhibitors - staurosporine (1-100 nM), d-sphingosine (10-500 nM) and PKCi (100-2000 nM) - both alone and in combination with FK 33-824 reduced E2 release from the cells in relation to the control group. The inhibitory effect of the opioid on E2 output was also observed in PKC-deficient granulosa cells. PKC activator, PMA (10 and 100 nM) significantly attenuated the inhibitory effect of the opioid agonist. FK 33-824 also inhibited 3[H]phorbol 12,13 dibutyrate (3[H]PDBu) specific binding by granulosa cells. Adenylyl cyclase (AC) engagement in opioid signal transduction was assayed after 2-h and 4-h incubations of granulosa cells. During 2-h incubation, FK 33-824 at the dose 1 nM decreased cAMP secretion. Prolongation of the incubation up to 4 h caused disappearance of the opioid action. The addition of protein kinase A (PKC) inhibitor, PKAi (100-2000 nM), alone or together with FK 33-824, was followed by an inhibition of E2 secretion. FK 33-824 with the highest dose of PKAi (2000 nM) significantly inhibited E2 secretion by the cells in comparison to these agents tested separately. The opioid added in combination with PKA activator, 8BrcAMP (1000 microM), caused attenuation of stimulatory effect of 8BrcAMP. Collectively, these results suggest that acute action of mu opioid agonist on porcine granulosa cells leads to decrease of enzymatic activity of PKC and AC/PKA. It is not ruled out that other signal transduction pathways - not considered in this study - may also be engaged in the opioid mechanism of action in these cells.

The response of phospholipase C/protein kinase C and adenylyl cyclase/protein kinase A pathways in porcine theca interna cells to opioid agonist FK 33-824

Opioids were found as factors affecting porcine ovarian steroidogenesis. The mechanism of opioid action, however, on porcine theca interna cells is completely unknown. Therefore, the present study was designed to investigate the possible involvement of two intracellular pathways, phospholipase C/protein kinase C and adenylyl cyclase/protein kinase A, in opioid signal transduction in porcine theca cells treated with mu opioid receptor agonist, FK 33-824. Incubation of the cells for 4 h with FK 33-824 at the dose 1 nM resulted in decreases in inositol phosphate accumulation as well as androstenedione (A(4)), testosterone (T), and estradiol (E(2)) secretions. Protein kinase C (PKC) inhibitors, staurosporine (1-100 nM), D-sphingosine (10-500 nM), and PKCi (100-2000 nM), both added alone and together with the opioid agonist, depressed release of the steroid hormones. PKC activator, phorbol ester (PMA, 1-100 nM), used alone was without effect on theca cell steroidogenesis, but added in combination with FK 33-824 abolished inhibitory influence of the opioid on A(4), T, and E(2) output. The steroid hormone secretion by PKC-deficient theca cells was inhibited by the opioid agonist. FK 33-824 also suppressed PKC activity reducing [(3)H]PDBu specific binding to theca cells, whereas ionomycin (a positive control) increased labeled phorbol ester binding to the cells. In the next experiment, cAMP release from theca cells during 2 and 4 h incubations with FK 33-824 (1-100 nM), naloxone (10 microM; opioid receptor antagonist), and LH (100 ng/mL; a positive control) was examined. FK 33-824 at the dose 1 nM inhibited cAMP secretion during 2 h incubation, but had no effect during longer incubation. LH in a manner independent on incubation time multiplied cAMP release. Protein kinase A inhibitor, PKAi (100-2000 nM), alone and in combination with FK 33-824 (1 nM), inhibited A(4), T, and E(2) secretions by theca cells. PKA activator, 8BrcAMP (10-1000 microM), stimulated the steroid hormone release, but this stimulatory effect was diminished in the presence of FK 33-824. The results allow to suggest that opioid peptides affect porcine theca cell steroidogenesis and their acute action on the cells is connected with the inhibition of phospholipase C/protein kinase C and adenylyl cyclase/protein kinase A signal transduction systems.

Chronic Antidepressant Treatment Causes a Selective Reduction of μ-Opioid Receptor Binding and Functional Coupling to G Proteins in the Amygdala of Fawn-Hooded Rats

We have previously documented that chronic alcohol consumption or alcohol withdrawal affects mu-opioid receptor density and receptor-mediated G protein coupling in Fawn-Hooded (FH) rat brain, especially in mesolimbic regions. FH rats demonstrate comorbid depression and high voluntary alcohol consumption; treatment with standard antidepressants improves both facets of this phenotype. Accordingly, we sought to examine whether mu-opioid receptor binding and the receptor-mediated functional coupling to G protein is affected by this drug treatment. Using quantitative autoradiography, binding of mu-opioid receptors labeled by [125I]FK33,824 (D-Ala2,N-Me-Phe4,Met(O)5-ol enkephalin) and the coupling between receptors and G proteins determined by agonist-stimulated guanosine 5'-O -(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding was mapped throughout brain sections of FH rats after 10-day treatment with vehicle, desipramine, or sertraline. Both desipramine and sertraline produced significant decreases of [125I]FK33,824 binding in many brain regions; 13 of 20 measured regions for desipramine and 16 of 20 measured regions for sertraline. The coupling efficiency of mu-opioid receptors to G proteins was determined by an increase of [35S]GTPgammaS binding induced by stimulation with the mu-opioid receptor agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (10 microM). In contrast to the receptor binding profile, functional coupling of receptors to G proteins was only significantly reduced in the amygdala, whereas it remained unchanged in other regions compared with control. The present findings suggest that antidepressants regulate opioid systems; however, this occurs differentially, and region-specific alteration of functional coupling of mu-opioid receptors to G proteins in the amygdala suggests that opioid function within the amygdala may be modulated by antidepressants.

The Influence of Opioid Peptides on Steroidogenesis in Porcine Granulosa Cells

The present studies were undertaken to examine the influence of mu (beta-endorphin, DAMGO, FK 33-824), delta (met-enkephalin, leu-enkephalin, DPLPE) and kappa opioid receptor agonists (dynorphin A, dynorphin B, U 50488) used at different doses (1-1000 nM) alone and in combination with LH (100 ng/ml) on steroidogenesis in porcine granulosa cells derived from large follicles. The effects of mu, delta and kappa receptor agonists on both basal and LH-induced progesterone (P4) secretion were negligible. Agonists of mu opioid receptors reduced basal androstenedione (A4), testosterone (T) and oestradiol (E2) release. Co-treatment with LH entirely abolished the inhibitory effect of these agonists on A4 and E2 secretion and resulted in an increase in T release. The addition of delta receptor agonists was followed by a decrease in basal A4, T and E2 secretion. The cells incubated in the presence of LH increased the androgen production and abrogated the inhibitory effect of delta agonists on E2 output. Basal A4, T and E2 release was also suppressed by kappa receptor agonists. The presence of LH in culture media extended the inhibitory effect of these opioids on E2 output and caused either abolition of the inhibitory influence of kappa agonists or even augmentation of both androgen release in response to the opioids. In conclusion, these data support the involvement of three major types of opioid receptors in the regulation of porcine granulosa cell steroidogenesis.

The regulation of steroidogenesis by opioid peptides in porcine theca cells

The present study was designed to investigate basal and LH-induced steroidogenesis in porcine theca cells from large follicles in response to various concentrations (1-1000 nM) of mu opioid receptor agonists (beta-endorphin, DAMGO, FK 33-824), delta receptor agonists (met-enkephalin, leu-enkephalin, DPLPE) and kappa receptor agonists (dynorphin A, dynorphin B, U 50488). Agonists of mu opioid receptors suppressed basal androstenedione (A4), testosterone (T) and oestradiol-17beta (E2) secretion and enhanced LH-induced A4 and T release by theca cells. The inhibitory effect of the agonists on E2 secretion was abolished in the presence of LH. All delta receptor agonists depressed basal progesterone (P4) output. However, the influence of these agents on LH-treated cells was negligible. Among delta receptor agonist used only leu-enkephalin and DPLPE at the lowest concentrations inhibited basal A4 release. The presence of LH in culture media changed the influence of these opioids from inhibitory to stimulatory. Similarly, DPLPE reduced T secretion by non-stimulated theca cells and enhanced T secretion of stimulated cells. All of delta agonists inhibited basal E2 secretion and unaffected its release from LH-treated theca cells. Agonists of kappa receptors inhibited basal, non-stimulated, P4 secretion and two of them (dynorphin B, U 50488) potentiated LH-induced P4 output. Basal A4 and T release remained unaffected by kappa agonist treatment, but the cells cultured in the presence of LH generally increased both androgen production in response to these opioids. Basal secretion of E2 was also suppressed by kappa agonists. This inhibitory effect was not observed when the cells were additionally treated with LH. In view of these findings we suggest that opioid peptides derived from three major opioid precursors may directly participate in the regulation of porcine theca cell steroidogenesis.

Porcine theca cells produce immunoreactive beta-endorphin and change steroidogenesis in response to opioid agonist

In earlier in vitro experiments opioids affected steroidogenesis in porcine luteal and granulosa cells. The present studies were undertaken to examine the effects of FK 33-824 (opioid agonist) alone or in combination with LH, PRL or naloxone (NAL, opioid antagonist) on steroidogenesis in cultured porcine theca cells. Moreover, we have tested beta-endorphin-like immunoreactivity (beta-END-LI) concentrations in culture media under control conditions and following treatments of theca cells with LH, PRL, progesterone (P4), oestradiol (E2) or testosterone (T). FK 33-824 and NAL significantly increased P4 release by theca cells and inhibited stimulatory effect of LH on this steroid output. PRL-induced P4 secretion from the cells was blunted only by FK 33-824. Secretion of androstenedione (A4) and T was essentially elevated in the presence of FK 33-824 and this potentiation of both androgen release was completely abolished by PRL. NAL blocked stimulatory effect of the opioid agonist only in case of T. Secretion of oestradiol and oestrone was completely free from the influence of both the opioid agonist and antagonist. Pig theca cells were able to produce beta-END-LI but none of tested hormones (LH, PRL, P4, E2 and T alone or in combination) significantly affected this production. In conclusion, these data indicate that porcine theca cells may produce beta-END-LI and change their steroidogenesis in response to opioid peptides.

Constitutively active mu-opioid receptors inhibit adenylyl cyclase activity in intact cells and activate G-proteins differently than the agonist [D-Ala2,N-MePhe4,Gly-ol5]enkephalin

The most convincing evidence demonstrating constitutive activation of mu-opioid receptors is the observation that putative inverse agonists decrease basal G-protein activity in membrane preparations. However, it is not clear whether constitutively active receptors in isolated membranes have any physiological relevance in intact cells. GH3 cells expressing mu-opioid receptors (GH3MOR) exhibit higher basal G-protein activity and lower basal cAMP levels than wild-type GH3 cells, indicative of constitutively active receptors. This study determined whether alkylation of mu-opioid receptors by the irreversible antagonist beta-funaltrexamine would decrease spontaneous receptor activity in intact cells, revealing constitutive activity. GH3MOR cells were pretreated with increasing concentrations of beta-funaltrexamine followed by functional testing after removal of unbound drug. beta-Funaltrexamine pretreatment produced a concentration-dependent decrease in mu-opioid receptor binding with an IC50 of 0.98 nm and an Emax of 77%. Similar concentrations of beta-funaltrexamine pretreatment produced a half-maximal reduction in basal [35S]GTPgammaS binding, a decrease in basal photolabeling of G-proteins with azidoanilido-[alpha-32P]GTP, and an increase in basal adenylyl cyclase activity in intact cells. Therefore, mu-opioid receptors are constitutively active in intact cells, producing stimulation of G-proteins and inhibition of adenylyl cyclase. Importantly, photolabeling of Galpha-subunits with azidoanilido-[alpha-32P]GTP demonstrated that constitutively active mu-opioid receptors activate individual G-proteins differently than the agonist [d-Ala2,N-MePhe4,Gly-ol5]enkephalin.

On the mode of action of methionine enkephalin, FK 33-824 and naloxone in regulating the hemolymph glucose level in the fresh water field crab Oziotelphusa senex senex

The possible involvement of opioid system in the regulation of hemolymph glucose level in the fresh water crab Oziotelphusa senex senex Fabricius, was investigated. Opioid agonist and antagonist was also used in addition to methionine-enkephalin itself. Injection of the opioid, methionine-enkephalin and FK 33-824 significantly elevated hemolymph glucose level. In contrast, injection of naloxone in to crab resulted in decrease in hemolymph glucose level. Injection of naloxone prior to injection of methionine-enkephalin blocked the hyperglycemic action of methionine-enkephalin. Injection of methionine-enkephalin, FK 33824 and naloxone produced no significant effect on hemolymph glucose level in eyestalk-less crab. The alterations in the intact crab hemolymph glucose level hypothesised to be due to stimulation of release of hyperglycemic hormone during methionine-enkephalin and FK 33824 treatment and blocking of release of hyperglycemic hormone during naloxone treatment from the eyestalks of crab Oziotelphusa senex senex.

Gene expression and peptide localization for LH/hCG receptor in porcine small and large luteal cells: possible regulation by opioid peptides

The aim of the present studies was to investigate (1) the presence of LH receptor (LHR) in porcine separated small (SLCs) and large (LLCs) luteal cells taken from mid-luteal corpora lutea and (2) the influence of opioid agonist, FK 33-824 (FK) on LHR gene expression in these cells. Immunocytochemistry revealed intense staining for LHR in both SLCs and LLCs. Reverse transcription-polymerase chain reaction (RT-PCR) and Southern hybridization were used to check the effect of FK and hCG on LHR gene expression. The LHR gene expression was observed in non-stimulated LLCs and in both types of cells after treatment with FK or hCG. FK stimulated LHR gene expression in SLCs and inhibited the gene expression in LLCs. Moreover, FK inhibited and potentiated stimulatory influence of hCG on the gene expression in LLCs and SLCs, respectively. These results suggest that LHR gene expression in porcine luteal cells can be regulated by opioid peptides.

The physiological role of beta-endorphin in porcine ovarian follicles

Beta-endorphin-like immunoreactivity (beta-END-LI) was measured by radioimmunoassay in porcine ovarian follicular fluid (FF) from small, medium and large follicles throughout the oestrous cycle. The concentration of beta-END-LI in FF from small follicles collected on days 1-5 of the cycle was at least tenfold higher than in the fluid from any other follicles independently from their size and the period of the cycle. The level of beta-END-LI in small follicles on days 6-10 was drastically decreased. Subsequently, on days 11-16 its concentration was enhanced and reduced again in pre-ovulatory period of the cycle. Concentrations of beta-END-LI in FF from medium follicles were relatively equal throughout the cycle (days 6-21). No significant differences in beta-END-LI levels were found between small, medium and large follicles from days 17-21. However, beta-END-LI concentrations in medium follicles on days 11-13 and 14-16 were statistically lower than those in small follicles. Moreover, the effects of FSH, prolactin (PRL), progesterone (P4), testosterone (T) and 17 beta-oestradiol (E2) on beta-END-LI release by granulosa cells (GCs) from large follicles and, on the other hand, the effects of the opioid agonist FK 33-824 alone or in combination with FSH, PRL or naloxone (NAL) on follicular steroidogenesis were studied. FSH drastically increased beta-END-LI output in a dose-dependent fashion. This stimulatory effect of the gonadotrophin was inhibited by the highest dose of P4 (10(-5) M). The effect of PRL and the steroids added to the cultures on beta-END-LI release was negligible. FSH- or PRL-induced P4 secretion by GCs was essentially abolished by both FK 33-824 and NAL. However, androstenedione (A4) and testosterone output by the cells was greatly potentiated by FK 33-824. In the presence of NAL, FSH or PRL, A4 release stimulated by FK 33-824 was suppressed to the basal level. Secretion of E2 was completely free from the influence of FK 33-824 or NAL; only oestrone (E1) output was modulated by them in cultures where FSH or PRL was present. In conclusion, FSH appears to be the key regulator of beta-END-LI secretion by porcine granulosa cells. Moreover, steroidogenesis in pig granulosa cells is modulated by opioid peptides acting both alone and by way of interaction with FSH or PRL.

Action of the opioid agonist FK 33-824 on porcine small and large luteal cells from the mid-luteal phase: effect on progesterone, cAMP, cGMP and inositol phosphate release

The present study was designed to investigate the effect of the opioid agonist FK 33-824 on basal and hCG-induced progesterone (P4), cAMP and cGMP secretion and on the phosphoinositide-specific phospholipase C signalling system in separated porcine small (SLCs) and large luteal cells (LLCs). Unit gravity sedimentation was used to produce cultures of small and large luteal cells from corpora lutea (CL) on days 8-10 of the oestrous cycle. In order to examine the effect of FK 33-824 on P4 and cyclic nucleotide release, SLCs and LLCs were incubated in M199 medium at 37 degrees C in 5% CO2:95% air, for 12 h. Small and large luteal cells were treated with hCG (100 ng/ml) alone, FK 33-824 (10(-9) M) alone or were co-treated with FK 33-824 and hCG and with the opioid antagonist, naloxone (NAL, 10(-5) M). FK 33-824 alone did not influence P4 secretion by LLCs and SLCs. However, FK 33-824 completely abolished the stimulatory effect of hCG on P4 secretion by SLCs. The addition of FK 33-824 was followed by a significant increase in cAMP release (p<0.01) by LLCs and a decrease in cGMP secretion by SLCs (p<0.05). The effect of FK 33-824 was blocked by NAL, which strongly suggests that the observed influence of this opioid agonist was achieved through its binding to opioid receptors in luteal membranes. In the presence of hCG, cAMP secretion by both SLCs and LLCs was many-fold higher than in the control group. As regards cGMP output, only LLCs showed elevated secretion of this cyclic nucleotide under the influence of hCG. With the aim of examining the influence of FK 33-824 on phosphatidylinositol hydrolysis, LLCs, SLCs and mixed small and large cells were labelled with [3H]-myo-inositol (100 microCi/ml) for 3 h at 37 degrees C. The cells were then incubated in M199 medium supplemented with 10 mM LiCl, 1% BSA, and antibiotics in the presence and absence of FK 33-824 (10(-9) M) at 37 degrees C for 30 min. Liberated labelled inositol mono-, bis-, and trisphosphates (IPs) were isolated and quantified by affinity chromatography on columns of AG 1-X8 resin, followed by liquid scintillation spectroscopy. Inositol phosphate accumulation in LLCs, SLCs, and mixed small and large cells was not altered by treatment with FK 33-824 at the dose used. In view of these findings we suggest that opioid peptides affect pig corpus luteum steroid secretion, and the response is probably mediated through cyclic nucleotides, but not IPs.

Ethanol consumption by Fawn-Hooded rats following abstinence: effect of naltrexone and changes in mu-opioid receptor density

BACKGROUND

Relapse after abstinence can be modelled in rats using an alcohol deprivation effect (ADE) of enhanced ethanol consumption after a period of enforced abstinence from ethanol; however, not all rat strains display such an effect. We wanted to examine the effect of naltrexone on ethanol consumption by ethanol-preferring Fawn-Hooded (FH) rats using such a model.

METHODS

FH rats were given continual free-choice access to a 5% ethanol solution or water (4 weeks) followed by 2 weeks of water alone. At the end of this abstinence period, osmotic minipumps were implanted subcutaneously to deliver saline (n = 4) or naltrexone (n = 4; 8.4 mg/kg/day for 4 weeks). After recovery from surgery, the rats were again given access to 5% ethanol under the same free-choice conditions (4 weeks). A third group of age-matched controls drank only water during the behavioral trial. At the end of the behavioral trial, the rats were decapitated and an autoradiographic examination was made of micro-opioid receptor density through the forebrain using the ligand [125I]FK-33824.

RESULTS

First, a period of enforced abstinence from ethanol consumption caused a significant (p < 0.05) and prolonged increase in ethanol preference (+18%) and decrease in water consumption (-53%), although the volume of ethanol consumed (ml/day) did not vary, indicating an atypical ADE in this rat strain. Second, naltrexone significantly (p < 0.05) decreased ethanol consumption by the FH rats in terms of absolute amount of ethanol consumed and preference for ethanol solution, but this effect of naltrexone diminished over time, concurrent with a robust and significant elevation in micro-opioid receptor density in all brain regions examined (p < 0.05). Finally, ethanol consumption alone also upregulated micro-opioid receptor density relative to nondrinking controls in a number of brain regions, which included the nucleus accumbens (+29%) and caudate-putamen (+15%,p < 0.05), but decreased micro-opioid receptor density in other regions including the substantia nigra pars reticulata, which was suggestive of an indirect effect on micro-opioid receptors.

CONCLUSIONS

The data suggest that continual long-term naltrexone treatment may not be effective in the treatment of alcoholism, possibly because of the induced increase in micro-opioid receptor density.

Role of the neuronal histaminergic system in the regulation of somatotropic function: comparison between the neonatal and the adult rat

To study possible age-related differences in the role of neuronal histaminergic pathways in the control of GH secretion, the effects of alpha-fluoromethylhistidine (alpha-FMH), an irreversible inhibitor of histamine (HA) synthesis, were examined on basal and opioid-induced GH release in neonatal and adult rats. The mechanisms involved in such effects were evaluated by measuring pituitary GH mRNA levels and hypothalamic levels of GH-releasing hormone (GHRH) and somatostatin (SRIF) mRNAs. Daily injection of alpha-FMH (20 mg/kg, s.c.) in pups of either sex, from birth until 10 days of age, caused a significant increase in baseline plasma GH and potentiated the GH response to the [Met5]-enkephalin analog FK 33-824 (1 mg/kg, s.c.) administered 3 h after the last alpha-FMH injection. GH and SRIF mRNA levels were significantly higher in alpha-FMH-treated pups than in controls, whereas no difference was observed in GHRH mRNA levels. In young adult male rats, acute administration of alpha-FMH (100 mg/kg, s.c., 3 h before) did not change significantly basal GH levels but potentiated FK 33-824 (0.3 mg/kg, intracarotid)-induced stimulation of GH secretion. Repeated administration of alpha-FMH (200 micrograms/rat, i.c.v., for 3 days) failed to modify basal and FK 33-824-induced GH secretion, caused a significant reduction in hypothalamic GHRH mRNA levels and left SRIF and GH mRNAs unchanged. These findings indicate that HA exerts an inhibitory effect on GH secretion in both neonatal and adult rats. The different effects of short-term HA depletion on hypothalamic and pituitary indices of somatotropic function observed at the two age periods may be ascribed to the immaturity of the HA system in early postnatal life and to a different functional role of GH-regulatory factors during ontogeny.

Presynaptic inhibition of calcium-dependent and -independent release elicited with ionomycin, gadolinium, and alpha-latrotoxin in the hippocampus

1. Presynaptic inhibition of synaptic transmission in the hippocampus was investigated by comparing the effects of several agonists on miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs). 2. The Ca2+ ionophore ionomycin increased the frequency of mEPSCs and mIPSCs but did not affect their amplitude. Ionomycin-induced release required extracellular Ca2+ and was prevented by pretreatment with botulinum neurotoxin serotype F, like evoked synaptic transmission. Unlike evoked transmission, however, this increase did not involve activation of voltage-dependent Ca2+ channels because it was insensitive to Cd2+. 3. Both the lanthanide gadolinium and alpha-latrotoxin produced increases in the frequency of mEPSCs and mIPSCs, but their actions were independent of extracellular Ca2+. 4. Adenosine, the gamma-aminobutyric acid-B (GABAB) receptor agonist baclofen, and a mu-opioid receptor agonist strongly reduced the frequency of synaptic currents triggered by all three secretagogues. 5. We conclude that activation of these presynaptic receptors can reduce high frequencies of vesicular glutamate and GABA release by directly impairing transmitter exocytosis. Presynaptic inhibition of gadolinium- and alpha-latrotoxin-induced release indicates that this impairment occurs without changes in intraterminal Ca2+ homeostasis and when vesicle fusion is rendered Ca2+ independent, respectively. 6. The inhibition of ionomycin-induced release provides additional evidence for a direct, neurotransmitter receptor-mediated modulation of the proteins underlying vesicular docking or fusion as an important component of presynaptic inhibition of evoked synaptic transmission.

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.

Distribution of the mu and delta opioid binding sites in the brain of the alcohol-preferring AA and alcohol-avoiding ANA lines of rats

There is experimental evidence indicating that the positive reinforcing effects of ethanol, responsible for voluntary ethanol consumption, are in part mediated by the endogenous opioid system. Differences in some components of the endogenous opioid system have been observed between lines and strains of animals bred selectively for their high or low ethanol consumption. Our objective was to investigate the presence of differences in the density and distribution of mu and delta opioid receptors in the brain of the alcohol-preferring Alko-Alcohol and alcohol-avoiding Alko, NonAlcohol lines of rats using iodinated ligands specific for mu ([D-Ala2, MePhe4, Met(o)ol5]-Enkephalin (FK 33-824)) or for delta ([D-Ser2]-leucine enkephalin-Thr) opioid receptors. Results calculated from studies on membrane preparations of whole brain minus cerebellum indicated that the Bmax and Kd were similar between the two lines of rats; however, autoradiographic studies showed that the alko-alcohol rats presented significantly higher density of mu opioid receptors in some brain regions, including nuclei of the limbic system that are important in mediating the reinforcing properties of many drugs of abuse. Furthermore, studies on brain membrane preparations indicated that both lines of rats were deficient in high affinity delta binding sites compared to Sprague-Dawley rats. Indeed, autoradiographic studies confirmed the presence of low density of the high affinity binding sites for [D-Ser2]-leucine enkephalin-Thr in both lines of rats. However, the density of the high affinity delta binding sites was higher in some distinct brain regions of the Alko-Alcohol than Alko-NonAlcohol rats. These differences in the density of both mu and delta opioid receptors may be partially responsible for the differences in voluntary ethanol consumption exhibited by these two lines of rats.

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 BAF1/J mice. Twenty-four hours after pump implantation, mice were challenged with sub-optimal, optimal or supraoptimal immunizing 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 suppressive 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 evidence that opioid peptides may either upregulate or downregulate the in vivo immune response depending on the strength of the response.

Role of lateral hypothalamus on fluid, electrolyte, and cardiovascular responses to activation of the MSA

In this study we investigated the influence of electrolytic lesion or of opioid agonist injections into the lateral hypothalamus (LH) on the dipsogenic, natriuretic, kaliuretic, antidiuretic, pressor, and bradycardiac effects of cholinergic stimulation of the medial septal area (MSA) in rats. Sham- and LH-lesioned male Holtzman rats received a stainless steel cannula implanted into the LH. Other groups of rats had cannulas implanted simultaneously into the MSA and LH. Carbachol (2 nmol) injection into the MSA induced water intake, pressor, and bradycardic responses. LH lesion reduced all of these effects (1-3 and 15-18 days). Previous injection of synthetic opiate agonist, FK-33824 (100 ng), into the LH reduced the water intake, natriuresis, kaliuresis, and pressor responses induced by carbachol injected into the MSA. These data show that both electrolytic lesion or injection of an opiate agonist in the LH reduces the fluid-electrolyte and cardiovascular responses to cholinergic activation of the MSA. The involvement of LH with central excitatory and inhibitory mechanisms related to fluid-electrolytic and cardiovascular control is suggested.

Mechanism of mu-opioid receptor-mediated presynaptic inhibition in the rat hippocampus in vitro

1. The electrophysiological action of the mu-opioid receptor-preferring agonist D-Ala2, MePhe4, Met(O)5-ol-enkephalin (FK 33-824) on synaptic transmission has been studied in area CA3 of organotypic rat hippocampal slice cultures. 2. FK 33-824 (1 microM) had no effect on the amplitude of pharmacologically isolated N-methyl-D-aspartate (NMDA) or non-NMDA receptor-mediated EPSPs. 3. FK 33-824 (10 nM to 10 microM) reduced the amplitude of monosynaptic inhibitory postsynaptic potentials (IPSPs) that were elicited in pyramidal cells with local stimulation after pharmacological blockade of excitatory amino acid receptors. This effect was reversible, dose-dependent, and sensitive to naloxone and the mu-receptor antagonist Cys2,Tyr3,Orn5,Pen7-amide (CTOP). FK 33-824 at 1 microM caused a mean reduction in the amplitude of the monosynaptic IPSP of 70%. 4. Neither delta- nor kappa-receptor-preferring agonists had any effect on excitatory or inhibitory synaptic potentials. 5. The disinhibitory action of FK 33-824 was blocked by incubating the cultures with pertussis toxin (500 ng/ml for 48 h) or by stimulation of protein kinase C with phorbol 12,13-dibutyrate (PDBu, 0.5 microM). 6. The depression of monosynaptic IPSPs by FK 33-824 was unaffected by extracellular application of the K+ channel blockers Ba2+ or Cs+ (1 mM each). 7. FK 33-824 produced a decrease in the frequency of miniature, action potential-independent, spontaneous inhibitory synaptic currents (mIPSCs) recorded with whole-cell voltage-clamp techniques, but did not change their mean amplitude. Application of the Ca2+ channel blocker Cd2+ (100 microM) or of nominally Ca(2+)-free solutions did not alter either the frequency and amplitude of mIPSCs or the reduction of mIPSC frequency induced by FK 33-824. 8. The effect of FK 33-824 on spontaneous mIPSCs was prevented by naloxone, and by incubation of cultures with pertussis toxin. 9. These results indicate that mu-opioid receptors decrease GABA release presynaptically by a G protein-mediated inhibition of the vesicular GABA release process, and not by changes in axon terminal K+ or Ca2+ conductances that are sensitive to extracellular Ba2+, Cs+ or Cd2+.

Binding in vivo of selective mu and delta opioid agonists: localization by autoradiography

The in vivo binding properties of cerebral opioid receptors were investigated in mice after intracerebroventricular (i.c.v.) injections of iodinated FK33-824 and [D.Ala2]deltorphin-I which behave in vitro as highly selective ligands possessing high affinity for mu and delta receptors, respectively. [125I]FK33-824 and [125I][D.Ala2] deltorphin-I exhibited similar diffusion kinetics after i.c.v. injection and bound specifically to sites characterized pharmacologically as mu and delta receptors respectively. Autoradiographic analysis revealed that after i.c.v. administration, concentrations of [125I]FK33-824 and [125I][D.Ala2]deltorphin-I remained higher in the circumventricular than in the deep structure of the brain and that specific sites labelled in vivo were differently distributed from those observed after in vitro labelling. FK33-824 was 250 times more analgesic than [D.Ala2]deltorphin-I in the tail-flick test and at doses producing a similar analgesia, [D.Ala2]deltorphin-I occupied a high proportion of mu receptors. Furthermore, analgesic effect of [D.Ala2]deltorphin-I was antagonized by pretreatment with naltrexone but not by naltrindole, a selective antagonist of delta-opioid receptors. These experiments reveal the localization of mu and delta opioid receptors reached after i.c.v. injection and provide evidence to support the suggestion that delta-opioid receptors contribute little or none to the supraspinal antinociception.

A synthetic opioid peptide increases plasma growth hormone and prolactin in Holstein calves

The effect of the synthetic opioid agonist D-Ala2,N-Me-Phe4,Met(O)5-ol enkephalin (DAMME) on plasma growth hormone (GH) and prolactin (PRL) concentrations in Holstein heifer calves was investigated in this study. The possible site of action of DAMME was determined by pretreating calves with an opioid antagonist that crosses the blood-brain barrier poorly if at all (N-methyl levallorphan-methane sulphonate [MLM]) or one that crosses readily (naloxone [NAL]). All calves were assigned to one of three treatment groups: 1) pretreatment with saline, 2) pretreatment with NAL, or 3) pretreatment with MLM. All groups were injected with DAMME 30 min after pretreatments. Plasma PRL increased after injection of DAMME in calves pretreated with saline. Prolactin concentrations were not different before and after injection of DAMME in calves pretreated with either NAL or MLM. Plasma GH increased after injection of DAMME in saline- and MLM-pretreated calves but was unchanged in NAL-pretreated calves. These data show that peripherally administered DAMME increases plasma GH and PRL in Holstein heifer calves and suggest that DAMME mediates GH release through receptors located somewhere inside the blood-brain barrier, but it can induce PRL secretion at a site located outside the barrier.

Conformational analysis of mu-selective [D-Ala2,MePhe4]enkephalins

The conformational space of the potent mu-selective opioids [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) and [D-Ala2,MePhe4,Met(O)-ol5]enkephalin (FK 33-824) has been analyzed by 1H-NMR spectroscopy and theoretical calculations involving systematic conformational searching and energy minimizations. A cis-trans equilibrium of the Gly3-MePhe4 amide bond is induced by the N-methyl group, and the more energetically favoured trans isomer is proposed as the biologically relevant form. A compact interaction between the side chains of Tyr1 and D-Ala2 was demonstrated by NOE and ROE effects in both peptides in D2O and DMSO-d6, further supported by shielding of the D-Ala2 methyl protons in both solvents. Analysis of coupling constants, NOE and ROE data indicated significant restriction of the conformational freedom of the MePhe4 side-chain for both peptides in the two solvents. The NMR results and theoretical calculations point towards folded low energy conformations characterized by a beta II-type turn around Gly3-MePhe4. For the trans isomer, a Tyr1-MePhe4 phenyl ring separation between 8.5 and 12.5 A was accompanied by proximity between the D-Ala2 side chain and the C-terminal in low energy conformations. The results are in good agreement with available data on related active enkephalins. The conformational effects induced by simultaneous incorporation of D-Ala2 and MePhe4 in enkephalins is discussed in the light of the enhanced mu-opioid receptor selectivity and activity of these peptides.

The effect of a met-enkephalin analogue on growth hormone, prolactin, gonadotropins, cortisol and thyroid stimulating hormone in healthy elderly men

The aim of the study was to evaluate the effect of the guanyl derivative of the opioid analogue D-ala2,MePhe4-Met-enkephalin-(o)-ol (G-DAMME) on pituitary secretion in healthy elderly men. Nine healthy elderly men (65-88 years) and 10 young adults (20-30 years) were studied. GH, PRL, gonadotropins, cortisol (to evaluate the effect on ACTH) and TSH were measured after G-DAMME (0.25 mg iv) or placebo administration. In elderly men, the GH response to G-DAMME was reduced or absent, while prompt GH release was found in all young men. G-DAMME lowered LH levels in young men but not in elderly men. No significant variations in FSH levels after G-DAMME and placebo were noted in either group of subjects. A similar and significant rise in PRL and TSH, and a fall in cortisol, after G-DAMME was observed in both elderly and young adults. We have demonstrated that the sensitivity to opioid modulation by G-DAMME on PRL, TSH and cortisol secretion is unchanged with aging. On the other hand, the data indicate that LH and GH responsiveness to G-DAMME change with age.

Continuous intrathecal opioid treatment abolishes the regulatory effects of magnesium and guanine nucleotides on mu opioid receptor binding in rat spinal membranes

The regulatory effects of cations and guanine nucleotides on mu receptor binding after opioid drug and pertussis toxin treatment were studied in the rat spinal cord model. Continuous intrathecal (i.t.) infusion with PL017 for 5 days induced tolerance in a dose-dependent manner. Maximal tolerance was observed at day 2. A single i.t. dose (1 microgram) of pertussis toxin also induced tolerance to opioid. When mu receptor binding of the high-affinity sites was determined by 125I-FK33824, spinal membrane preparations from morphine- and pertussis toxin-induced tolerant animals demonstrated approximately 30% less binding than control membranes. Analysis of equilibrium competition binding of FK33824 against [3H]naloxone under a variety of experimental conditions (i.e., cations and guanine nucleotides) revealed differences among control and treated membranes. With Na+ (100 mM) + GDP (100 microM) pretreated membranes and binding assays conducted in the presence of Mg++, all mu receptors were observed to be in a high-affinity state in control membranes, whereas about 30% of receptors were in the low-affinity state in membranes from opioid- and pertussis toxin-treated animals. The increase in the proportion of low-affinity sites was dependent upon the infusion dose of PL017, and the increase correlated well with the degree of opioid tolerance developed. The regulatory effect of 5'-guanylylimidodiphosphate on opioid agonist binding was reduced in membranes from pertussis toxin- or opioid-treated animals. In binding assays conducted in the presence of Na+ (100 mM) + Mg++ (5 mM) + 5'-guanylylimidodiphosphate (30 microM) or Na+ (100 mM) + GDP (100 microM), all mu receptors in control membranes were in a low affinity-state, while those from opioid- or pertussis toxin-treated animals existed in both the high- and the low-affinity states. Continuous i.t. infusion with PL017 at the high dose of 1 microgram/hr for 5 days also decreased significantly (about 40%) the total number of receptors. These studies indicate that continuous opioid infusion and pertussis toxin treatment results in impairment in the receptor-G-protein coupling. This is reflected by the decreased regulatory effects of Mg++ and guanine nucleotides. Thus, in addition to receptor down-regulation, which is induced by PL017 at high doses, receptor-G-protein uncoupling may play a role in opioid tolerance induced by continuous infusion with morphine and PL017.

Neuroendocrine regulation of growth hormone secretion in sheep. V. Growth hormone releasing factor and thyrotrophin releasing hormone

The effects of intravenous (IV) and intracerebroventricular (ICV) administration of either bovine growth hormone releasing hormone (GRF) or thyrotrophin releasing hormone (TRH) on plasma growth hormone (GH) and glucose levels have been examined in sheep. Intravenous GRF 1-29NH2 at 3 and 30 micrograms stimulated an increase in GH levels in a dose-dependent fashion; administration of GRF into a lateral cerebral ventricle, however, produced a smaller GH response which was similar at these two doses. Evaluation of somatostatin levels in petrosal sinus blood (which collects pituitary effluent blood) showed that ICV administration of GRF stimulated a release of somatostatin into the blood. Furthermore, concurrent administration of GRF and a potent anti-somatostatin serum ICV resulted in a much enhanced release of GH which was similar to that obtained with a comparable dose of GRF given IV. TRH (as another putative GH-secretagogue) was also administered both IV and ICV. When given IV, 200 micrograms (but not 100 micrograms) TRH produced an elevation in GH levels. By contrast, when 5 micrograms TRH was given ICV there was a decrease in circulating GH levels, but no change in plasma somatostatin concentrations. These results indicate that the smaller GH response to ICV- compared with IV-administered GRF is due to the release of somatostatin within the brain. In addition, it would seem that TRH is not a physiological GH-secretagogue in sheep.

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 subfornical 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.

Opiate activation suppresses the drinking, pressor and natriuretic responses induced by cholinergic stimulation of the medial septal area

The present study investigates the participation and interaction between cholinergic and opiate receptors of the medial septal area (MSA) in the regulation of Na+, K+ and water excretion, drinking and blood pressure regulation. Male Holtzman rats were implanted with stainless steel cannulae opening into the MSA. Na+, K+ and water excretion, water intake and blood pressure were measured after injection of carbachol (cholinergic agonist), FK-33824 (an opiate agonist) + carbachol or naloxone (an opiate antagonist) + carbachol into MSA. Carbachol (0.5 or 2.0 nmol) induced an increase in Na+ and K+ excretion, water intake and blood pressure and reduced the urinary volume. FK-33824 reduced the urinary volume and Na+ and K+ excretion. Previous injection of FK-33824 (100 ng) into the MSA blocked the increases in Na+ and K+ excretion, water intake and blood pressure induced by carbachol. Naloxone (10 micrograms) produced no changes in the effect of 2.0 nmol carbachol, but potentiated the natriuretic effect induced by 0.5 nmol dose of carbachol. These data show an inhibitory effect of opiate receptors on the changes in cardiovascular, fluid and electrolyte balance induced by cholinergic stimulation of the MSA in rats.

Control of growth hormone secretion during the postpartum period

The postpartum period is characterized by high levels of circulating steroids which condition hypothalamo-pituitary activities. In pregnancy growth hormone (GH) levels are greatly increased and lack pulsatility. In order to investigate the behavior of GH during the postpartum period, the GH response to GH-releasing hormone stimulation (50 and 100 micrograms), to amphetamine, a dopamine receptor agonist, and to FK 33-824, an opiate receptor agonist, was investigated in women during the first 5 days after delivery. In all groups GH responses were significantly lower (p less than 0.01) than in normal women studied during the early follicular phase. FK-33-824-induced GH release was similar in postpartum and control women. These results demonstrate reduced pituitary GH response to GH-releasing hormone and to amphetamine in women during the postpartum period, confirming the peculiarity of the hypothalamopituitary component.

Exogenous opioids increase plasma prolactin in Holstein calves primarily via a dopaminergic mechanism

A study was conducted to determine whether exogenous opioids increase prolactin (PRL) secretion in Holstein heifer calves via a dopaminergic mechanism. Twenty-four Holstein heifer calves ranging in age from 5 to 7 mo were assigned to one of four treatment groups (six/treatment): 1) injection of saline (SAL); 2) injection of a synthetic enkephalin (D-Ala2, N-Me-Phe4, Met(O)5-ol enkephalin; DAMME); 3) injection of DAMME after pretreatment with the long-acting dopamine agonist 2-bromo-alpha-ergocryptine; or 4) injection of thyrotropin-releasing hormone (TRH) after pretreatment with 2-bromo-alpha-ergocryptine. Calves were equipped with indwelling jugular cannulas on d 1, and baseline plasma PRL concentrations were established. Animals receiving 2-bromo-alpha-ergocryptine were injected s.c. 3 h after the last baseline sample was drawn on d 1. On d 2, calves assigned to receive SAL, DAMME, or TRH were injected 2 h after the start of sampling, and sampling was continued for an additional 4.5 h. Basal plasma PRL was lower (P less than .01) on d 2 in calves injected with 2-bromo-alpha-ergocryptine than baseline levels on d 1. Plasma PRL was higher (P less than .01) in calves not pretreated with 2-bromo-alpha-ergocryptine after DAMME injection on d 2 but was not different after DAMME injection in calves pretreated with 2-bromo-alpha-ergocryptine. In contrast, plasma PRL increased (P less than .01) after TRH injection on d 2 in calves pretreated with 2-bromo-alpha-ergocryptine.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Analgesic effect of mu- and kappa-opioid agonists in beige and CXBK mice

The analgesic effects of mu- and kappa-opioid agonists, including morphine, FK33,824, U50,488H, tifluadom and bremazocine, have been determined in C57BL/6J-bgJ (beige) and CXBK mice which are hyporesponsive to mu-opioid receptor-mediated analgesia compared with those of control mice (C57BL/6J (C6J), C57BL/6By (C6By), BALB/cBy (BALB] using an abdominal constriction assay. The analgesic effect of subcutaneously administered morphine and FK33,824 in both beige and CXBK mice was significantly reduced compared with the controls and the analgesic effect of U50,488H and tifluadom in beige mice was significantly reduced compared with the wild strain (C6J). No reduction of analgesic effect of U50,488H and tifluadom was seen in CXBK compared with its progenitor strains, C6By and BALB, except for a reduction of the effect of tifluadom in CXBK compared with C6By. There was no strain difference in the bremazocine-induced analgesia. These results suggest that the beige mouse has a deficit in analgesia mediated by both mu- and kappa-opioid receptors, whereas the CXBK is deficient only in the mu-opioid receptor-mediated analgesia.

[The mediator spectrum of the action of morphine and the pentapeptide FK 33-824 on the neurons of the cerebral cortex]

Morphine and pentapeptide FK 33-824 were shown to reduce spontaneous and evoked neuronal discharges in the IInd somatosensory zone of the cortex on microiontophoretic and intravenous administration of cats. The neurons were identified by electric stimulation of thalamocortical fibers. The agents tested decreased neuronal responses to glutamate and acetylcholine. They failed to change responses to dopamine and enhanced responses to GABA. Morphine and pentapeptide FK 33-824 appeared to decrease slightly the inhibitory neuronal action of serotonin on microiontophoretic administration and increased it when injected intravenously. Beta-adrenoblocker propranolol decreased the inhibitory action of morphine and pentapeptide FK 33-824 on the cortical neurons.

Changes in growth hormone and luteinizing hormone following acute or chronic administration of an opioid agonist, FK33-824, in wethers

Two experiments were conducted to determine the effect of acute or chronic administration of an opioid agonist, (D-Ala2,MePhe4,Met(O)5-ol)-enkephalin (FK33-824), on concentrations of growth hormone (GH) and LH in castrate sheep (wethers). In Exp. 1, we evaluated the effect of acute administration (two injections or infusion for 2 h) of FK33-824 on GH and LH. Wethers (31 +/- 1 kg; n = 12) received FK33-824 or saline via one of the three following routes of deliver: 1) i.v. (8 micrograms/kg) at 0 and 60 min (i.v., n = 4), 2) s.c. (16 micrograms/kg) at 0 and 60 min (s.c., n = 4), or 3) i.v. infusion (8 micrograms.kg-1.h-1) for 120 min (inf., n = 4) preceded by a single injection (8 micrograms/kg). Wethers received FK33-824 and saline on d 1 and 2 or d 2 and 1, respectively. Concentrations of GH were consistently elevated (P less than .05) by FK33-824 regardless of route of delivery. Infusion of FK33-824 for 120 min maintained GH concentrations greater (P less than .05) than pretreatment or those during infusion of saline. Average LH was decreased consistently by FK33-824, independent of its route of administration; episodic release of LH was suppressed. In Exp. 2, crossbred wethers (36 +/- 2 kg; n = 12) were used to determine the effect of chronic administration (s.c.) of FK33-824 on the concentrations of GH and LH and retention of N and energy.(ABSTRACT TRUNCATED AT 250 WORDS)

D-Ala-2-Me-Phe-4-Met-(O)-ol-enkephalin in the nucleus tractus solitarius of the rat produces cardiorespiratory depression

1. The synthetic Met-enkephalin, D-Ala-2-Me-Phe-4-Met-(O)-ol-enkephalin (FK 33-824). 1 or 2 micrograms, after its injection into the nucleus tractus solitarius (NTS) of Wistar rats, anesthetized with pentobarbital and breathing spontaneously, produced a transient increase in blood pressure followed by sustained and significant (P less than 0.05) hypotension and bradycardia. This occurred in a dose dependent manner. 2. FK 33-824 in the NTS, 1 or 2 micrograms, also produced a marked respiratory depression. 3. In anesthetized rats, in which hypoventilation was prevented by mechanical ventilation, there was a definite reduction in blood pressure and heart rate that was considerably and significantly (P less than 0.05) less than that observed in spontaneously breathing rats. 4. Blood pressure fluctuations occurred after NTS injection that were more marked in spontaneously breathing animals but still occurred in animals that were ventilated mechanically. 5. FK 33-824, 1 and 2 micrograms in the NTS was fatal within 100 min for all animals but was prevented by mechanical ventilation. Higher doses of FK 33-824, 10 micrograms in the NTS, however, induced fatal ventricular arrhythmias even in the mechanically ventilated rat. 6. Thus, FK 33-824 in the NTS decreases blood pressure and heat rate in spontaneously breathing as well as mechanically ventilated rats, but much of the effect on blood pressure and heart rate is due to the profound respiratory depression in the spontaneously breathing rat.

Adrenergic stimulation of the human pituitary-adrenal axis is attenuated by an analog of met-enkephalin

It has previously been suggested that endogenous opioid peptides may suppress the pituitary-adrenal axis in man by inhibiting an excitatory alpha 1-adrenoceptor input to neural mechanisms liberating corticotrophin-releasing factor or factors (CRFs). This hypothesis has been tested here by investigating the effect of the met-enkephalin analog, DAMME (FK-33,824), on the elevation in serum cortisol induced by the catecholamine-releasing agent d-amphetamine (10 and 25 mg p.o.) and the direct alpha 1-adrenoceptor agonist methoxamine (6 micrograms/kg/min i.v.) in two groups of 6 normal male subjects. In both studies, the rise in serum cortisol was significantly attenuated by the analog of met-enkephalin. These data suggest that exogenous opioids act as a site downstream to the alpha 1-adrenoceptor input to CRF release; it appears that opioids modulate adrenocorticotrophic hormone release in man at a minimum of two distinct and separate sites.

Cardiorespiratory responses to D-Ala-2-Me-Phe-4-Met-(O)-ol-enkephalin after administration into the fourth cerebral ventricle of the rat: interaction with cholinergic mechanisms

The purpose of this study was to investigate the effect of the synthetic Met-enkephalin, D-Ala-2-Me-Phe-4-Met-(O)-ol-enkephalin (FK 33-824), on blood pressure, heart rate, respiratory rate and survival, after its injection into the 4th cerebral ventricle of Wistar rats. The animals were either anesthetized with pentobarbital and breathing spontaneously or unanesthetized. The unanesthetized rats were previously instrumented with cannulas in the 4th cerebral ventrical and a systemic artery. In anesthetized rats, intracerebroventricular administration of FK 33-824 produced a transient increase in blood pressure followed by sustained hypotension, bradycardia and respiratory depression in a dose-dependent manner. Fatalities were observed over a 150-min observation period and were a function of dose. Pretreatment with atropine sulfate (1 mg/kg i.v.) produced an accentuated response with greater hypotension, bradycardia and shorter survival. In another group of anesthetized rats, in which hypoventilation was prevented by mechanical ventilation, blood pressure and heart rate were not as reduced as in spontaneously breathing rats. Hypotension, bradycardia and hypoventilation were less marked in unanesthetized rats, compared to anesthetized rats. Thus, FK 33-824 in the 4th cerebral ventricle of the rat produces marked changes in blood pressure in anesthetized as well as unanesthetized animals, but these changes were less in the unanesthetized or mechanically ventilated animal and greater after atropine, suggesting that these effects are mediated by respiratory depression and are antagonized by the cholinergic nervous system.

Effects of a [Met]-enkephalin analogue ( [D-Ala2,N-Me-Phe4,Met-(O)5-ol]-enkephalin) on canine pituitary function

In adult healthy beagle dogs, plasma concentrations of ACTH, cortisol, alpha-MSH, GH, prolactin and arginine vasopressin (AVP) were measured after i.v. administration of [D-Ala2,N-Me-Phe4,Met-(O)5-ol]-enkephalin (DAMME) at doses of 0.1, 0.5, 1, 5 and 10 micrograms/kg body weight. Significant dose-dependent increases occurred for ACTH, cortisol and GH at dose rates of 0.5, 1, 5 and 10 micrograms/kg body weight. Increments in plasma concentrations of prolactin were significant only at 5 and 10 micrograms DAMME/kg, and there was no significant effect on plasma concentrations of alpha-MSH and AVP. Prior i.v. administration of the opiate antagonist naloxone (0.1 mg/kg) attenuated the DAMME (10 micrograms/kg)-stimulated release of ACTH and cortisol. The results demonstrate that the [Met]-enkephalin analogue DAMME stimulates the release of ACTH, cortisol, GH and prolactin in dogs, and that this stimulation is, at least in part, mediated by mu-opioid receptors. The observations for ACTH and cortisol are different from those in man, where DAMME lowers their basal concentrations.

Opiates inhibit pedunculopontine neurones in guinea pig brainstem slices

Intracellular recordings were obtained from pedunculopontine neurones in guinea-pig brainstem slices. These cells were characterized by a broad action potential, an A-like conductance and fired spontaneously in a regular manner. These neurones were inhibited by bath-application of both carbachol and serotonine at concentrations of 10(-4) M. Opioid peptides induced a dose-dependent hyperpolarization and a reduction in the spontaneous firing. These latter effects could be blocked by the opiate antagonist naloxone and were direct as they persisted in presence of tetrodotoxine or high magnesium/low calcium-containing salines. They were mediated by an opiate receptor of the mu type since they were obtained with the mu-preferring enkephalin analogues FK 33-824 and DAGO, but neither with the delta nor the kappa analogues such as DPLPE or U-50,488.

[The neurochemical characteristics of the effect of morphine and pentapeptide FK 33-824 on cerebral cortical excitability]

GABA-stimulating agents (muscimol, depakine, aminohydroxyacetic acid), serotonin stimulants (5-hydroxytryptophan, fluoxetine), catecholaminergic stimulants (L-DOPA, apomorphine) and a cholinergic inhibitor (hemicholinium-3) of the brain were found to enhance the inhibitory effect of morphine and pentapeptide FK 33-824 on the cortical excitability of free-moving cats. GABA-negative agents (thiosemicarbazide, bicuculline, picrotoxin), serotonin-negative agents (p-chlorophenylalanine, methysergide), catecholaminergic inhibitors (6-hydroxydopamine, alpha-methyl-p-tyrosine) and cholinergic stimulant (physostigmine) of the brain appeared to decrease the effects of morphine and pentapeptide FK 33-824 on the excitability of the cortex. beta-adrenoceptor blocker propranolol decreased the effects of morphine and pentapeptide FK 33-824 on the cortical excitability, whereas alpha-adrenoblockers (phentolamine, phenoxybenzamine and yochimbine) exerted no effect on their action.

Inhibition of neuronally induced relaxation of canine lower esophageal sphincter by opioid peptides

Opioid peptides have profound effects on gut motility. To assess their actions on enteric neurons regulating sphincteric smooth muscle, the ability of several opioid agonists to antagonize the neuronally induced relaxation of canine lower esophageal sphincter smooth muscle was examined. Opioid peptides selective for mu (FK 33-824) or delta [( D-Pen2,D-Pen5]enkephalin) receptors produced a concentration dependent inhibition of electrical field stimulation (EFS)-induced relaxation. In contrast, neither kappa (ketocycloclazine) or sigma (SK & F 10047) opioid agonists were potent inhibitors of EFS-induced relaxation. This inhibition was relatively selective for opioid agonists since BHT 933 (alpha 2 adrenoceptor agonist) and SK & F 89124 (D2 dopamine agonist) did not inhibit EFS-induced relaxation. Furthermore, naloxone antagonized the effects of both FK 33-824 and DPDPE. These functional data suggest that opioid receptors are present on sphincteric intrinsic inhibitory neurons and that stimulation of these neuronal receptors can regulate lower esophageal sphincter relaxation.

Attenuation by an opioid agonist of the oestradiol-induced LH surge in anoestrous ewes and its reversal by naloxone

The effects of a potent opioid peptide agonist [D-ala2-Phe4, Met(0)ol5-enkephalin (FK 33-824) on the magnitude of the oestradiol-induced LH surge and on basal plasma LH concentrations were examined in intact and chronically-ovariectomized ewes during the late-anoestrous period. In intact ewes, treatment with FK 33-824 (0.5 mg i.v. every 3 hr) for a 24 hr period commencing at the time of oestradiol-17 beta administration (25 micrograms i.m. bolus) was associated with non-significant 65% reduction in the peak plasma LH level observed and a significant (P less than 0.05) 58% reduction in the total amount of LH released during the surge (calculated from the area under the curve). Concurrent treatment with the opioid antagonist naloxone (10 mg i.v. every 3 hr) partially reversed this suppressive effect on the magnitude of the LH-surge. In ovariectomized ewes no significant effects on the oestradiol-induced LH surge of either FK 38-824 alone or FK 33-824 in combination with naloxone were observed. Administration of FK 33-824 at a 6-fold higher dose rate (0.5 mg every 30 min) failed to modify basal plasma LH concentration in intact ewes. In ovariectomized ewes, however, a significant (P less than 0.05) 25% fall in basal plasma LH was observed, an effect which was completely reversed by combined treatment with naloxone (10 mg every 30 min). These results support the conclusion that endogenous opioid peptides may contribute to the neuroendocrine mechanism through which oestradiol promotes a preovulatory-like surge in the anoestrous ewe.

Pre- and postjunctional effects of N-ethylmaleimide in the isolated mouse vas deferens

The influence of N-ethylmaleimide (NEM) on contractions due to exogenously applied noradrenaline and bethanechol and on the inhibitory effects of clonidine, of the enkephalin derivative, FK 33-824, and 2-chloroadenosine (2-CLA) on field stimulation-response curves and [3H]noradrenaline [( 3H]NA) release was studied in the isolated mouse vas deferens. Exposure to NEM (60 microM: 10 min) caused a 30% reduction of the maximal contraction due to NA but nearly abolished the response to bethanechol. NEM partially reversed the depression of the pulse width-response curves by clonidine and FK 33-824 but was without effect with 2-CLA. The contractions evoked by stimulation frequencies above 20 Hz were depressed by NEM both in presence and absence of the agonists. NEM diminished the inhibition of the stimulation-evoked release of [3H]NA by the three agonists. The prejunctional effect of NEM was markedly influenced by the stimulation parameters. These findings support the suggestion that the inhibition mediated by alpha 2-adrenoceptors, mu- and P1-receptors in the mouse vas deferens is NEM-sensitive and possibly transmitted by a pertussis toxin-sensitive G-protein.

Cardiorespiratory effects of D-Ala-2-Me-Phe-4-Met-(0)-ol enkephalin in the third ventricle, and in anterior hypothalamic and paraventricular areas of the rat brain

The purpose of this study was to investigate the cardiorespiratory effects of the synthetic Met-enkephalin, D-Ala-2-Me-Phe-4-Met-(0)-ol enkephalin (FK 33-824) after its administration into the anterior hypothalamic area, paraventricular hypothalamic nuclei and third ventricle. Wistar rats, anaesthetised with pentobarbitone and breathing spontaneously, received an injection of FK 33-824, 1 or 2 micrograms into one of these three areas. FK 33-824 produced significant (analysis of variance, p less than 0.05) and sustained hypotension of similar degree following injection into any one of these three sites. Significant (p less than 0.05) reductions in heart rate and respiratory rate were also observed. Hypotension and bradycardia occurred, but to a lesser degree, when respiratory depression was prevented by mechanical ventilation. FK 33-824 produced fatal bradyarrhythmias in spontaneously breathing and mechanically ventilated animals. When respiratory depression was prevented by mechanical ventilation, survival was lowest after third ventricular administration followed by paraventricular and anterior hypothalamic injections. Thus D-Ala-2-Me-Phe-4-Met-(0)-ol enkephalin produced marked vasodepression and respiratory depression in the rat. These effects were interrelated but reductions in heart rate and blood pressure also occurred independently of the respiratory depression.

Opioids stimulate growth hormone (GH) release in man independently of GH-releasing hormone

The Met-enkephalin analog DAMME [D-Ala2,MePhe4-Met-enkephalin-(o)-o1, FK 33-824] can stimulate GH secretion in man. In this study we investigated the effects of the guanyl derivative of DAMME (G-DAMME) on the serum GH response to an analog of GHRH in normal men. GHRH-(1-29)NH2 and G-DAMME each induced a rise in serum GH, and the increase was greater when both were given together. Since the GHRH-(1-29)NH2 dose (100 micrograms) used was a maximally stimulatory one, these results suggest that the enhancing effect of G-DAMME on GHRH-induced GH release may be mediated through inhibition of somatostatin release.

Single potassium channels opened by opioids in rat locus ceruleus neurons

Currents through single-ion channels were recorded in the cell-attached configuration from locus ceruleus neurons enzymatically dissociated from newborn rats. When the selective mu opioid receptor agonist Tyr-D-Ala-Gly-MePhe-Gly-ol was in the patch-clamp electrode, unitary inward currents were observed with conductance of approximately 45 pS (measured at zero pipette potential, with 150 mM potassium in the recording electrode). Long silences, lasting many seconds to minutes, separated periods of activity of similar durations. Within such activity periods the distribution of closed times of the channels was best fitted by the sum of two exponential functions (time constants approximately 1 and 30 ms), and the durations of channel openings were fit by a single exponential function; mean open time increased from 2 to 120 ms as agonist concentration increased. Channel activity was not seen when high concentrations of opioids were applied to the neuron outside the patch-clamp recording electrode, indicating intimate coupling between receptor and potassium channel. Unitary currents with similar properties were also seen when pipettes contained alpha 2 adrenoceptor agonists or somatostatin. Taken with previous findings, the results indicate that mu opioid receptors, alpha 2 adrenoceptors, and somatostatin receptors can couple directly to membrane potassium channels through the local intermediary action of a GTP binding protein.

Agonists of endogenous opioid peptides suppress LH, and stimulate cortisol and growth hormone during the follicular phase in heifers

We evaluated the effects of [D-Ala2,Me,Phe4,Met(0)ol]-enkephalin (FK 33-824) and morphine, opioid receptor agonists, on concentrations of LH, cortisol and GH during the follicular phase in heifers. During three trials, oestrous cycles of Angus heifers were synchronized by two injections of prostaglandin F2 alpha (PGF2 alpha) resulting in 17 induced follicular phases over an 80-day period. Treatments were administered 24 h after the second injection of PGF2 alpha. Blood samples were collected at 15-min intervals from 3 h before until 5 h after i.v. administration of 2 ml physiological saline (trials 1, 2 and 3), 0.5 mg morphine/kg (trials 1 and 2) or 1.8 micrograms FK 33-824/kg (trials 1 and 2) or 6.7 micrograms FK 33-824/kg (trial 3). Administration of both doses of FK 33-824 and morphine inhibited episodic release of LH for approximately 60 min. The concentration of cortisol was increased (P less than 0.05) after both doses of FK 33-824, but was unaffected (P greater than 0.5) by morphine or saline. The serum concentration of GH was increased (P less than 0.01) after both doses of FK 33-824 or morphine, but saline was without effect. These results provide evidence of an inhibitory effect of opioid receptor agonists on LH (FK 33-824 and morphine) and a stimulatory effect on cortisol (FK 33-824) and GH (FK 33-824 and morphine) during the follicular phase in heifers. Divergent effects of morphine and FK 33-824 on cortisol observed in this study provide evidence that opioid-induced changes in LH, GH and cortisol result from activation of different opioid receptors.

Effects of enkephalins and the analogue FK-33824 on mean arterial pressure and heart rate in conscious sheep

Experiments were designed to evaluate the central and systemic effects by enkephalins and the enkephalin analogue FK-33824 on mean arterial pressure (MAP) and heart rate (HR) in conscious sheep. Intracerebroventricular infusion of FK-33824 increased both MAP and HR in a dose-dependent manner in normal sheep. The increases in MAP and HR were attenuated by naloxone administered centrally, but not systemically. Intracerebroventricular infusion of met-enkephalin, leu-enkephalin and naloxone failed to change both MAP and HR significantly. However, intravenous infusion of met-enkephalin, leu-enkephalin and FK-33824 resulted in bradycardia. Haemorrhage alone decreased both MAP and HR. Intracerebroventricular infusion of FK-33824 blunted the reduction in MAP in response to haemorrhage. The increases in MAP and HR following FK-33824 were also accompanied by elevated levels of plasma renin concentration. It is suggested that the tachycardia and pressor effect produced by the intracerebroventricular administration of FK-33824 in normal conscious sheep may result from a combined action of both neural and chemical pathways which are involved in cardiovascular control, and are mediated via the mu-opioid receptors. Opioids may have opposite effects on cardiovascular control depending on the route of administration.